[Structure and Bonding] Non-Covalent Multi-Porphyrin Assemblies Volume 121 || Porphyrin Supramolecules by Self-Complementary Coordination

Struct Bond (2006) 121: 49–104DOI 10.1007/430_023© Springer-Verlag Berlin Heidelberg 2006Published online: 24 February 2006

Porphyrin Supramoleculesby Self-Complementary Coordination

Yoshiaki Kobuke

Graduate School of Materials Science, Nara Institute of Science and Technology,Takayama 8916-5, 630-0101 Nara, [email protected]

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

2 Characteristics of Coordination Organization . . . . . . . . . . . . . . . . 512.1 Stability Constants for Supramolecular Assemblies . . . . . . . . . . . . . . 512.2 Chlorophylls in Natural Systems . . . . . . . . . . . . . . . . . . . . . . . . 53

3 Self-Complementary Coordination of Porphyrins . . . . . . . . . . . . . . 593.1 Dimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593.2 Macrocycle and Box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643.2.1 Antenna Mimics of B850 and Box Types . . . . . . . . . . . . . . . . . . . . 643.2.2 Porphyrin Macrocycle Capable of Guest Incorporation . . . . . . . . . . . 743.2.3 Size-Tunable Giant Macrocyclic Arrays . . . . . . . . . . . . . . . . . . . . 753.3 One-Dimensional Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763.4 Two-Dimensional Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

4 Coordination Organization of Phthalocyanines . . . . . . . . . . . . . . . 84

5 Structural Characterization of Supramolecular Assemblies . . . . . . . . . 87

6 Properties of Porphyrin Assemblies . . . . . . . . . . . . . . . . . . . . . . 896.1 Photoinduced Electron Transfer in Special Pair Mimic . . . . . . . . . . . . 896.2 Energy Transfer in Photosynthetic Antenna Mimics . . . . . . . . . . . . . 916.3 Nonlinear Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 936.4 Photocurrent Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

7 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

Abstract The supramolecular organization of porphyrins using self-complementary coor-dination is reviewed. Complementarity affords large stability constants for coordinationorganization, and as a result various structural variations of porphyrin supramolecu-les were obtained. Dimeric porphyrins are of special interest as they mimic the specialpair of the photosynthetic reaction centers. A very stable slipped cofacial dimer was ob-tained and the dimer effect on photoinduced charge separation is discussed. Macrocyclicporphyrin supramolecules were prepared as photosynthetic antenna models and rapidenergy transfer rates among the components were estimated. Three-dimensional orga-nization represents another type of approach to light-harvesting antenna systems. One-and two-dimensional arrays of porphyrins have also been prepared using this method-ology. Energy and electron transfers along the array provide the basis for photonic and

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electronic applications. Introduction of molecular polarization in the array leads to thedevelopment of nonlinear optics materials. Photocurrent generation is enhanced by arrayformation on electrode surfaces.

Keywords Antenna · Charge separation · Macrocycle · Nonlinear optics · Photosynthesis

1Introduction

Light is the ultimate energy source for all living systems. Photosynthetic sys-tems utilize its energy to excite chlorophyll or bacteriochlorophyll withinorganisms [1, 2]. The electron in this photoexcited state is then transferredto the primary electron acceptors, initiating the oxidation–reduction reac-tion known as photosynthesis. After the photosynthetic organisms employedreducing agents of more negative reduction potentials, they found an evolu-tionary way to use water as a more abundant reducing agent, thus evolvingoxygen as a by-product. The subsequent accumulation of oxygen allowedthe appearance of animals, which depend totally on the products of photo-synthetic plants by consuming their component carbohydrates for food andusing the oxygen they produce for respiration.

How do natural photosynthetic systems transfer the excitation energy gen-erated by the incident light to the reaction center through multistep energy-transfer chains with minimum energy loss? How do these natural systemsavoid back-electron transfer, the most favorable side reaction resulting in theloss of the excitation energy as heat, instead converting the photoexcited stateinto a reduced acceptor and generating a hole in the original framework? Howdo they subsequently initiate cascade electron transfers and supply electronsto these holes, regenerating the original electronic state of the reaction center?It is intriguing to apply the answers found in natural systems to the prob-lem of designing artificial photosynthetic systems [3–5]. Indeed, work on thissecondary problem, which has a range of applications, may open the door toa more genuine understanding of natural photosynthesis.

This chapter focuses on supramolecular porphyrin systems that are stableenough to be characterized in solution. Assemblies coordinated only in solids(e.g., crystals) will not be included. In order to maintain these supramolecularstructures in solution, strong complementarity and multitopic coordinationsare required, rather than a simple monotopic axial coordination. Of theseconditions, this discussion will be concerned only with those necessary forthe complementary coordinations, and with the presentation of various ex-amples of these coordinations. The issue of multitopic coordination will bedealt with in another forum. Self-complementary coordination can be used toobtain dimers, rings, and one- and two-dimensional arrays. These supramo-lecular systems should not only influence structure formation, however, butalso be closely related to their functions. It is therefore natural to discuss the

Porphyrin Supramolecules by Self-Complementary Coordination 51

photophysical properties associated with these unique supramolecules fromthis viewpoint [6–9].

This review will focus on Mg and Zn ions as useful candidates from amongthe other metal ions that might be inserted into the center of porphyrinmacrocycles. Photosynthetic systems certainly employ Mg ions. However,extensive studies focusing on the Zn complexes of porphyrins have beenundertaken, since Zn(II), having a d10 structure, can be a good substitutefor Mg(II). Although the fluorescence quantum yields of Zn(II) porphyrinsare inferior to those of Mg(II) porphyrins, the former have obvious advan-tages such as permitting easy introduction of the metal into the structure,stability in acidic conditions, and durability when exposed to oxidation con-ditions. In fact, organisms living under conditions of extreme heat and aciditynear sea volcanoes have been found to employ Zn substitutes in place ofthe more common chlorophyll [10]. Most other transition metal ions quenchthe singlet excitation energy, and these metalloporphyrins may be regardedas functional dyestuffs rather than expressing photosynthetic models. Thereare many interesting examples of structure formation using these transitionmetal ions, and they can be important in the transfer of electrons and holesin the system because they are readily oxidized and reduced. With thesepoints in mind, some transition metal porphyrins are included in this reviewas well.

2Characteristics of Coordination Organization

2.1Stability Constants for Supramolecular Assemblies

Relatively strong coordination to zinc porphyrins is obtained when neutralnitrogen ligands are employed. The relation between the basicity of variousnitrogen ligands (pKa of the conjugated acid) and their stability constantswith zinc porphyrins is summarized in Fig. 1 [11]. It is reasonable for thesetypical, or d10, metal ions that a correlation should be observed between thesetwo parameters, considering the similarity between the coordination of N tometal and the bonding of N to H+ provided that steric requirements are in-significant. Among the important aromatic ligands, imidazole is preferred topyridine because of its high basicity: the pKa is more favorable by a value of2 for imidazole compared with that for pyridine. When steric requirementsbecome significant, the linearity of the relationship between the stability con-stant and the basicity drops sharply, as is observed for 2-methylpyridine or2,6-dimethylpyridine. Aliphatic amines follow the order of the stability con-stant as primary > secondary > tertiary, depending on their steric demandsrather than their pKa value. Cyclic aliphatic amines are favored under these

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Fig. 1 Stability constants of neutral nitrogen ligands with zinc tetraphenylporphyrin(ZnTPP) as a function of pKa of the conjugated acid; • pyridines, × imidazole, � ali-phatic amines, � o-substituted pyridines

conditions, since steric hindrance is properly avoided by ring closing, as isbest demonstrated for diazabicyclooctane.

Considering the general trend observed, the stability constant of imida-zole to zinc porphyrin is expected to be of the order of 104 M–1 at most.Table 1 shows calculations of the stability constants required to ensure thatmore than 90 or 99% of the initial self-assembling monomer may exist as

Table 1 Minimum stability constants for 90 and 99% initial monomers to exist as 2- to4-mers at initial concentrations of 1.0 and 10 µM

at 1.0×10–6 M at 1.0×10–5 M> 90% > 99% > 90% > 99%

Dimer 108 M–1 1010 M–1 107 M–1 109 M–1

Trimer 1015 M–2 1018 M–2 1013 M–2 1016 M–2

Tetramer 1022 M–3 1026 M–3 1019 M–3 1022 M–3


Fig. 2 Schematic illustration of ring systems made of metalloporphyrins with self-complementary coordination

dimers, trimers, or tetramers in solution at initial concentrations of 1.0 and10 µM (typical concentrations for photophysical measurements of porphyrinderivatives). It is clear that a significant enhancement in the stability constantis required to satisfy the necessary conditions.

Di- or multitopic interactions are among the most efficient methods satis-fying the above conditions, and will be treated in detail by another reviewer.An alternative method exploits the high level of self-complementarity be-tween the monomeric units to induce efficient mutual coordination. As isschematically illustrated for the cases of ring systems in Fig. 2, matching thecoordinating angle permits the formation of various macrocycles. Details ofthe molecular design required to adequately satisfy the self-complementaritycondition will be discussed thoroughly with reference to various exampleslater in the discussion.

2.2Chlorophylls in Natural Systems

In the core of all photosynthetic reaction centers, two (bacterio)chlorophyllmolecules are arranged by coordination of their imidazolyl side chains fromthe transmembrane α helices to the central magnesium(II) ions. This coordi-nation brings the two chlorophylls into a slipped cofacial orientation, wheretwo π-electronic planes are placed in an almost parallel orientation witha slippage of the central Mg centers. The interplanar distances depend on thephotosynthetic species, and range from 3.2 to 5.0A [12–15]. These coordi-nation modes are illustrated in Fig. 3. In the bacterial special pair (Fig. 3b),two (bacterio)chlorophylls interact strongly at their closest distance of 3.2 A.One of the pyrrole rings in each chlorophyll is overlapping, with a Mg · · · Mgdistance of 7.4 A [12]. Here, the close chromophore interaction is believed todrive efficient charge separation, leading to the pheophytin acting as the pri-mary electron acceptor. In photosystem I (Fig. 3c), the interplanar distanceis longer, with an intermediate value of 3.6 A. In this case, two pyrrole ringsin each chlorophyll overlap, shortening the Mg · · · Mg distance to 6.3 A [14].

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Fig. 3 Coordination modes of the special pair in photosynthetic systems. a and b photo-synthetic bacteria, c photosystem I, d photosystem II

Charge separation probably occurs mainly as a result of one of the chloro-phyll pair, with some involvement of the accessory chlorophyll. Rather thanpheophytin acting as an acceptor, in this case the electron is transferredto the chlorophyll. Since the oxidation/reduction potential of the acceptorchlorophyll is negatively shifted, it plays a role in the subsequent photosyn-thetic reduction. In the case of photosystem II (Fig. 3d), the interplanar andMg · · · Mg distances are further increased to 5.0 and 10.0 A [15], respectively,and the two chlorophylls in the pair are now regarded as almost independent.


These differences in the chlorophyll pair structures are interesting in view ofthe evolution of the photosynthetic reaction centers.

In light-harvesting antenna systems, too, X-ray crystallographic studieshave shown evidence of similar structural evolutionary effects [16–20]. Thefirst clear antenna structure was reported in 1995 for the bacterial light-

Fig. 4 Arrangement of chlorophylls in a LHI and b B850 in LHII

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harvesting complex LHII [16]. In the same year, cryomicroscopy provided animage of LHI [17], which is similar to LHII, but with a larger ring structurecontaining the reaction center in its central pore. LHII’s B850 ring is arrangedin a barrel form, and both it and the similar (quasi)ring of LHI [18] employthe same process for constructing antenna structures, as is shown in Fig. 4.Here, two (bacterio)chlorophylls are arranged almost in a slipped cofacialorientation (similar to the special pair discussed above) by the coordinationof imidazolyl side chains from the transmembrane α helices. This dimer unitis further assembled into a barrel form, thus constituting a macrocyclic ringcomposed, in the case of B850, of 18 (bacterio)chlorophylls [19]. In the case

Fig. 5 High-resolution AFM image of photosynthetic units in bacterial membrane

Fig. 6 Energy transfer rates in photosynthetic membrane of purple bacteria


of LHI, the originally reported ring is composed of 32 (bacterio)chlorophylls.In some reports, however, this structure is modified and the ring is open toafford a gate for quinone to pass through [18]. Recent developments in AFMtechniques elucidate further the closely packed structures of these antennaein the membrane (Fig. 5) [20]. These ordered structures, existing as they doin close proximity, are thought to enable the rapid, low-loss transfer of ex-citation energy to the (bacterio)chlorophyll components in the rings and tothe neighboring macrocycles. The rates estimated are much faster comparedwith the lifetime, and are of the order of femto- to picoseconds, as is shown inFig. 6 [21–26].

The ring structures observed in these bacteria systems have some obviousadvantages over other structures in terms of facilitating energy transfer. Theenergy level of every component in the ring is almost identical, and smoothenergy migration is expected to occur, since there is no large intervening en-

Fig. 7 X-ray structure of the LHC-II trimer. a Top view of trimer: chlorophylls assembleto form rings 1 and 2. b Side view in grana membrane: chlorophylls are located near themembrane surfaces along the dotted line

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ergy barrier. As the cyclic structures are densely packed in a two-dimensionalspace, each one will have many surrounding rings, so that rapid inter-ring en-ergy transfer becomes easy. Also, the reaction center (RC) complex trappedin the large LHI antenna can ultimately accept the excitation energy circulat-ing in this ring, thus starting the photoinduced oxidation–reduction reactionsequence.

Compared to the antenna systems in bacteria that we have considered sofar, those found in chloroplasts (including cyanobacteria) have quite differentproperties. Figure 7 shows the structure deduced via X-ray crystallographyof one of the antenna systems of chloroplasts [27]. This is an antenna ofouter source working mainly as the antenna system for photosystem II (PSII).Besides this antenna, many chlorophylls are also incorporated as antennaeinside both PSI and PSII. The outer antennae, containing 12 chlorophylls ineach unit, assemble to make a trimer. Many trimer units further assemblearound the PSII. The chlorophyll arrangement in this trimer unit is com-pletely different from that of the bacteria previously discussed. The principleof the assembly seems to be that the chlorophylls assembled in the innerpart (ring 1 in Fig. 7a) transfer the excitation energy to the outer assembly(ring 2 in Fig. 7a), through which the energy is further transferred to the ad-jacent trimer unit. Furthermore, it must be noted that all of these chlorophyllsare located near the outer membrane surface (i.e., along the dotted lines inFig. 7b), so that the energy may in fact be transferred across the grana mem-brane. In contrast to the structure of bacteria, this arrangement is clearlyquite complex, making it difficult to extract the structural principles of theplant antenna system.

When grown in iron-depleted conditions, even more antenna moleculesattach themselves to the PSI surface. Figure 8 shows such a situation, in

Fig. 8 Electron micrograph of antenna rings assembled further around trimeric photosys-tem I in cyanobacteria grown under iron-deficient conditions


which a further 18 outer antennae (containing 18 × 12 = 216 chlorophylls)have assembled at the outer surface of a system which already contained 270chlorophylls as components of its inner antenna. Clearly, the creation of extraantennae assists the maximum functioning of the reaction center when it isgrown under starving conditions.

3Self-Complementary Coordination of Porphyrins

3.1Dimer

On receiving light energy from the surrounding antenna complexes, the firstoxidation–reduction step in the photosynthetic process takes place via elec-tron transfer from the photoexcited special pair to pheophytin in the bacterialreaction center. The charge separation is accomplished with a time constantof 3 ps [29], which is exceptionally fast considering the edge-to-edge distanceof 10 A. The special pair is considered to play an important role, as it isknown to affect the efficiency of the charge separation process [30, 31]. Forthis reason, this configuration has long been a target of intensive research,and several porphyrin dimers mimicking the structure of the special pairhave been synthesized by both covalent [32–35] and noncovalent [36–38] ap-proaches.

In covalent approaches, two porphyrins are attached to each other throughrigid aromatic or cyclophane-type bridges, as shown in structures 1–3. (Notethat the bold numbers refer to illustrated structures throughout.) In thesemodels, two porphyrins are placed either in a face-to-face arrangement, orwith a dihedral angle of 60◦. Electronic interactions between the porphyrincomponents are related to transitions in the Soret band, and have been ex-plained using Kasha’s exciton coupling theory [39]. Face-to-face dimers shiftthe Soret band to a shorter wavelength, and the fluorescence in this case isquenched significantly due to the transfer of electronic excitation energy tothe vibrational energy levels. This is an effect that has also been observed in

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other H-type aggregates of various chromophores [40]. The arrangement oftwo porphyrins with a finite dihedral angle causes, in principle, a splitting inthe energy bands due to the existence of both face-to-face and head-to-tailorientations of the two in-plane transition moments of the porphyrin. Thisis also the case for slipped cofacial arrangements, where the existence of twosimilar transition moments produces split Soret bands.

As an alternative, we have reported a noncovalent approach to the con-struction of slipped cofacial porphyrin dimers, with close reference to theX-ray crystallography of the photosynthetic reaction center [36]. If one looksinto the formation of special pairs, it becomes apparent that the transmem-brane helices from the L and M subunits in the reaction center provide theimidazolyl ligand to the chlorophylls. This coordination structure itself couldbe cut off from the surrounding peptide moieties, and the two imidazolyl-to-chlorophyll pairs could be turned 180◦ in a disrotatory fashion along the axes,thus penetrating the chlorophylls. The imidazolyl group can be transferredby this symmetry operation to a position from which it can be connected di-rectly to the counterpart chlorophyll, providing the required complementarystructure (Fig. 9). According to this idea, porphyrin and Zn(II) ions were sub-stituted for chlorophyll and Mg(II), respectively. The employment of Zn(II)instead of Mg(II) was motivated by a consideration of the properties of mag-nesium porphyrin, which is too readily oxidized, and from which heat andacid easily remove the metal. Also, Mg(II) insertion is considerably more dif-ficult than Zn(II) insertion.

The second important point for any special pair mimic designed to repli-cate the photosynthetic model is that the central metal ion should not quenchthe singlet excitation energy. Zn(II) fulfills this requirement. Furthermore,zinc porphyrin has an additional merit from the viewpoint of coordina-tion chemistry. Since Zn(II) can accept only one axial coordination, thedimer is the only coordination product without any additional complexationevents. As was discussed in Sect. 2.1, Zn(II) gives, in general, large stabilityconstants with neutral nitrogen ligands. It should be noted here that self-complementarity results in an extraordinarily large stability constant (on

Fig. 9 C2 operation on the special pair to produce a self-complementary dimer of imida-zolyl porphyrins


the order of 1011 M–1 in toluene). Therefore, the dimer is practically thesole species existing in nonpolar solutions, even at concentrations as low asmicromoles. If coordinating solvents are added, however, the dimer struc-ture collapses and its degree and rate of formation are dependent on thenature and amount of solvent. This does not necessarily represent a draw-back, however, but can be regarded as a property allowing more sophisticatedsupramolecular structures to be constructed. Combining the processes of for-mation, breakage, and reorganization of supramolecules does indeed leadto some very useful results, and various examples of the application of thismethodology to obtain more elaborate supramolecules will be described inlater sections.

Pyridyl ligands are certainly candidates for forming stable self-comple-mentary coordination assemblies and are conventionally employed for supra-molecular structure formation. Zn and Mg complexes of meso- and β-pyr-role-substituted 2-pyridylporphyrins (4a–c) were therefore prepared [37, 38].Crystallographic analysis was used to verify that the required slipped cofa-cial arrangement had been generated, and demonstrated the overlapping ofone of the pyrrolic rings in each porphyrin. This confirmed the structuralsimilarity between the sample and the special pair. The stability constant wasreported to be 5×105 M–1 for the Mg dimer of the meso-pyridyl case 4b.

The stability constants for the monotonic coordination to zinc porphyrinby N-methyl-2-imidazole, compared to the constant for coordination by pyri-dine, is larger by one order of magnitude (∼ 104 M–1 compared to ∼ 103 M–1,respectively) [41]. Several factors contribute to this difference. One import-ant factor is certainly the difference in their basicities, since the imidazolylgroup has a larger pKa value than pyridine (see Sect. 2.1). The proton oc-cupying the alpha position relative to the nitrogen hinders the nitrogen co-ordination more appreciably for the six-membered heterocycle than is thecase for the five-membered ring. Another factor affecting the complemen-tary coordination process may be the deviation of the coordination anglefrom the perpendicular orientation, since the carbon occupying the alphaposition relative to the nitrogen is connected to the porphyrin ring. For self-complementary dimer formation, the 2-imidazolyl-substituted dimer 5 must

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experience considerable angle strain, since it deviates from the ideal 90◦ to72◦ (18◦ deviation) in the molecule. This strain energy may lower the stabilityconstant from the ideal value. However, the 2-pyridyl ligand must accommo-date an even higher strain energy in the molecule, since the angle in that casedeviates by 30◦ from the perpendicular coordination.

Such an angular strain could be avoided by connecting the pyridyl groupthrough an angle-adjusting connector, such as an arylamide spacer 6 [42].In this case, however, the distance between the two porphyrin rings becomeslonger, and little stabilizing contribution from electronic and π–π interac-tions between porphyrin chromophores is expected. In other words, the de-sired complementarity is much less likely in this case. The many rotationaldegrees of freedom of the connecting bonds decrease the stability constant,too.

X-ray crystallography was used to obtain the structure of the self-complementary dimer of N-methylimidazolyl zinc porphyrin (Fig. 10; Ina-ba Y, Nomoto A, Kobuke Y, personal communication). The angle strain ofthe imidazolyl coordination is eased by rotation of the imidazolyl ring towiden the angles of Cmeso–imidazolyl C2–imidazolyl N3 and of imidazolylC2–imidazolyl N2–Zn to 127.9 and 130.5◦, respectively, from the 126◦ ex-pected for the putative strain-free model. The slight bending (3.9◦) of theCmeso and imidazolyl–C2 bonds from the porphyrin plane may also help todecrease the strain. It is noteworthy that the interplanar distance between thetwo porphyrins is close to 3.2 A, implying strong electronic interaction be-tween the two. The zinc-to-zinc distance is 6.13 A, making the slipped cofacialstructure very similar to the special pair in the bacterial reaction center [12].

A strong exciton interaction was observed for 5 in the Soret band, whichsplit into a twin peak with a splitting energy of ∼ 1300 cm–1. Two transitionmoments, Mx and My, interact in this case, resulting in two allowed transitiondipoles (the head-to-tail and face-to-face orientations). These are observedas red- and blue-shifts of the Soret band, respectively. This split Soret band,


Fig. 10 a X-ray crystal structure of the dimer of self-complementary imidazolyl zincporphyrins. b Mode of strain release in self-complementary dimer

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along with upfield shifts in the resonances of the coordinating imidazolyl pro-tons from the porphyrin in the 1H NMR spectrum, are highly characteristicof self-complementary dimer formation. A similarly split Soret band, witha splitting energy of ∼ 1040 cm–1, was observed in the 2-pyridyl-substituteddimer 4a at a concentration of 5.2×10–5 M in CH2Cl2 [37].

3.2Macrocycle and Box

The beautiful structures of bacterial antennae have been shown to possess,within natural photosynthetic systems [19, 20], extremely important biolog-ical functions. This alone provides more than adequate motivation for exam-ining their photophysical properties, since there is no doubt that the inherentnature of energy capturing and its transfer must be associated with them.It is exciting to be able to use analyses of synthetic systems to extract thereasons why nature uses such rings for this process. By analyzing simple sys-tems, we can both understand the complexity of the natural process, andfind ways to improve the efficiency of energy transfer in general, which maylead to the use of excitation energy in various applications. Compared tothe simpler bacterial cases, it is difficult in cyanobacteria or chloroplast sys-tems to extract the principles of operation and explain why nature employsintrinsic antenna systems. In ways similar to those described for the pho-tosynthetic reaction center, evolution may have deformed a simpler originalstructure, added extra factors, and deleted others. For this reason, researchnecessarily focuses on the bacterial system. The antennae of regular macro-cyclic structures are sufficiently characterized in this system by photophysicalmeasurements based on conservation of excitation energy and its hoppingefficiency.

This part of the review is primarily concerned with supramolecular orga-nization. Covalent approaches are not described. It must be briefly noted herethat there are many reports on the synthesis and characterization of macro-cycles with artificial antennae in various forms of macrorings 7–14 [43–48]and wheels [49]. Selective macroring formation has been achieved in the pres-ence of suitable templates for macrocyclization [50–52]. These have been thesubject of various review articles [53–55].

3.2.1Antenna Mimics of B850 and Box Types

As described in Sect. 2, imidazolyl zinc porphyrin is an excellent candidate forself-complementary coordination due to its extremely large stability constant.In order to arrange these complementary coordination units into a hexam-eric macrocycle, two imidazolyl zinc porphyrins were connected throughan m-phenylene group to fix the angle at 120◦, offering a gable porphyrin


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arrangement. The initial Zn insertion, however, gave a complex mixturewhich was characterized by gel permeation chromatography (GPC) analy-sis (Fig. 11). This mixture was then dissolved in CHCl3/MeOH (1 : 1, v/v)at a dilute concentration of 3.5 µM, and the whole solvent was simply evap-orated. GPC analysis of the remaining solid showed only two peaks witha 1 : 1 ratio, excluding almost all of the oligomeric peaks of higher molecularweights. The two peaks were separated easily with a preparative GPC col-umn. The products were stable in the absence of coordinating solvents andthe first and second peaks were determined to be macrocyclic hexamer 16and pentamer 17, respectively [56, 57]. The reorganization process can be ex-plained as follows. The dissolution of the substance in a large amount ofsolvent containing MeOH resulted in significant cleaving of the coordinationbonds. During the subsequent evaporation process, complementary coordi-nation bonds regenerated. Provided the solution is highly dilute, it is unlikelythat the molecules will encounter other molecular species. Any molecularterminals with an unsaturated coordination bond try to find their counter-part in the same molecule to close the ring structure. This process followsthe principle of macrocycle preparation (Fig. 12). The pentameric ring canform by sharing the angle strain in the macrocycle components. Tetramericand heptameric rings are difficult to form because of the large strain energyassociated with these structures.

Fig. 11 GPC charts immediately after zinc insertion (dashed curve) and after reorganiza-tion (solid curve)


Fig. 12 Reorganization into macrocyclic 6-mer 16 and 5-mer 17 from gable imidazolylzinc porphyrin 15

As we have just outlined it, supramolecular structure formation soundsvery simple. The elucidation of the structure after its formation is, however,a tougher problem. This is because the intensity of the peak associated withthe monomeric gable porphyrin unit in all the mass spectra is overwhelm-ingly high compared to the tiny peak associated with the dimer. Initially, nosign was found of such large macrocycles as 5- or 6-mers, although all thedata from the GPC and atomic force microscopy (AFM) analysis, and fromthe small-angle X-ray scattering experiments, agreed with the hypothesis thatthere was pentamer and hexamer formation. All of this latter information,however, was only suggestive, rather than being conclusive. Direct evidencewas eventually obtained using an olefin metathesis reaction (Fig. 13) [58].Two trans meso positions in each porphyrin were appended with allyloxy-carbonylethyl groups (18) and quantitatively connected to each other viaa Grubbs catalyst to give 19. Then all the complementary coordination pairsin the macrocycles of tetra(allyloxyester)-substituted gable porphyrins 20 and21 were metathesized doubly at two meso positions to yield 22 and 23, re-spectively. The parent peaks in the mass spectra were now identical to thosecalculated for 5- and 6-mers 22 and 23, respectively, thus allowing their struc-tures to be identified (Fig. 14) [59].

Hunter [60] extended the idea of self-complementary dimer formationto higher macrocycles by adjusting the coordinating angle of the appendedamidopyridyl ligands to selectively obtain the trimeric 24 and tetrameric

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Fig. 13 Covalent linking of self-complementary coordination dimer 18

Fig. 14 Fixation of the cyclic structure through covalent linking by metathesis reaction

macrocycles 25 of zinc porphyrin. In this case, the stability constants are notlarge enough to ensure the success of photophysical measurements at micro-mole concentrations because of the presence of freely rotating bonds in thesubstituents. In order to obtain larger macrocycles, some improvement in thestability constants is required.

Since Co(II) porphyrin can accept two axial ligands, the introduction oftwo meso substituents leads to a one-dimensional array or macrocycles by


adjustment of the angle between the two porphyrins in each complementarycoordination pair. When the angle was adjusted to 30◦ by the choice of twomeso-amidopyridyl substituents of different lengths (26), it has been claimedthat macrocyclic dodecamer 27 (360/30 = 12) is formed by successive comple-mentary dimer formation at concentrations lower than 0.5 mM (Fig. 15) [61].When the concentration exceeds this effective molarity for cyclization, themacrocycle opens up, resulting in mixtures of polymers with higher molecu-lar weights, as is demonstrated by their GPC behaviors. In order to addressthe ring size and its specificity or homogeneity, more direct and precise

Fig. 15 Co(II) porphyrin with two different pyridyl substituents (26) and self-assembledmacrocyclic oligomer 27

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information on the molecular weights is definitely required. The ability ofthis form of porphyrin to accept two axial coordinations certainly resultsin facile structure formation, but cobalt quenches the porphyrin fluores-cence, hampering the investigation of energy transfer events between chro-mophores. Similar features are associated with most metal ions that accepthexadentation.

Osuka et al. reported numerous porphyrin arrays of various types, mostlyobtained through covalent approaches, but also reported several tetramericsupramolecular macrorings 28 composed of mono-p-pyridyl-substituted zincporphyrins [62]. When two pyridyl zinc porphyrins are connected directlyat their meso positions, the coordination organization allows for the for-mation of porphyrin boxes 29 by two-point self-coordination. Interestingly,macrocyclization proceeds through a homochiral self-sorting assembly, as isevidenced by the successful optical separation of two enantiomers to displaystrong Cotton effects. The stability constant of the substance in CHCl3 wasincreased significantly from 1.4×1015 M–3 for monocyclization (n = 0 series)to > 1025 M–3 for the box formation described. The presence of the inter-vening phenylene groupings between the porphyrin and pyridine decreasedthe stability constants of the macrotetramerization [63]. The rigidity of theproduct contributed to an increase in the local concentration of the three-dimensional cyclization. Aida et al. reported box formation from bis- (30)and tris-pyridyl zinc porphyrins (32), both connected through butadiynyllinkages. In these cases, the porphyrins can rotate around the connectingbutadiynyl bonds, and it is proposed that the linked porphyrins are in anequilibrium between coplanar and orthogonal conformations in the resultingboxes 31 and 33 (Fig. 16) [64, 65]. A ruthenium-connected dimer of bis-(4′-


Fig. 16 Cyclotetramerization of alkynylene-bridged pyridyl zinc porphyrin dimer 30 andtrimer 32

72 Y. Kobuke

pyridyl) zinc porphyrin also underwent self-complementary coordination tomake a porphyrin tetramer 34. The structure was determined by X-ray crys-tallography in the solid state, confirming the analysis of the highly symmetricS4 structure observed in solution using NMR. Both NMR and UV/Vis spec-tra were concentration independent, implying high coordination stability ina concentration range from milli- to micromoles [66].

Balaban et al. suggested that there was preferential tetramer 35 formationfrom meso-2-aminopyrimidinyl zinc porphyrin in dry toluene at millimolarconcentrations. The Soret band shows temperature-dependent equilibriumbetween the monomer and its associated species. The associated enthalpy


change was estimated to be 220 kJ/mol, which corresponds to about fivetimes the Zn-pyrimidine ligation energy of 35–40 kJ/mol. X-ray crystallog-raphy confirms that the tetrameric structure of 35 is given by head-to-tailZn – N(aromatic) coordination bonds [67]. In this context, Aida reported thepreferential formation of cyclotetrameric structures from meso-(3-pyridyl)zinc porphyrin 36. The supramolecular pyridyl-coordinated porphyrins dis-played thermochromic change in the range 0 to 100 ◦C. The color was sensi-tive to the number of meso-alkynyl groups. Monoalkynyl-pyridyl porphyrinshowed the most distinct stepwise color change, changing from green to yel-low to red as the temperature moved from 0 to 50 to 100 ◦C [68]. We notedearlier that a Co(II) porphyrin appended with meso-5-(1-methyl)imidazolylundergoes dynamic equilibrium between cyclic trimer 37 and tetramer 38 de-pending on the concentration and the solvent employed (Fig. 17). The trimeris the predominant species in MeOH, while the tetramer is preferentially ob-tainable by raising the concentration of the CHCl3 solution [69].

Although pyrazole is inferior to imidazole as a ligand coordinating toZn porphyrins, it has an additional NH group that can assist the coordina-tion by hydrogen-bond formation. When 5-(4-pyrazolyl) zinc porphyrin isappended with a 20-(o-benzoate) group, it is automatically cyclized to the

Fig. 17 Formation of cyclic trimer 37 and tetramer 38 from imidazolyl Co(II) porphyrin

74 Y. Kobuke

trimer 39 by angle restriction on coordination with a stability constant of2.3×1015 M–2 [70].

Ru(CO) and RhCl porphyrins with a meso-(p-pyridyl) substituent giverise to the formation of macrocyclic tetramers 40 and 41, respectively. TheRu tetramer structures were converted by an excess amount (103-fold) ofpyridine into monomers. The Soret band sharpened during the change, show-ing the effect of excitonic interactions between the cofacially arranged por-phyrins [71, 72].

3.2.2Porphyrin Macrocycle Capable of Guest Incorporation

In the above gable porphyrin approach, the angle between the two imidazolylzinc porphyrins was adjusted to 120◦ to facilitate the formation of the macro-cyclic hexamer. When two porphyrins are linked through a central porphyrinvia m-phenylene connectors to adjust the dihedral angle of the two terminalimidazolyl zinc porphyrins to 60◦, the cyclic trimer 42 is the only macro-cycle formed after the dilution and reorganization process (Fig. 18) [73]. Inthis case, neither the cyclic dimer nor the tetramer are produced because oftheir high angle-strain energies. The characteristic feature of this macrocy-cle is that the central zinc porphyrin units do not participate in the structureformation by complementary coordination and can accept the axial coordi-nation, especially from the central pore side. In this way tripodal ligandscan, if properly designed for cooperative coordination, be specifically trappedwithin the central pore. UV/visible titration of the cyclic trimer 42 withthe tetrapyridyl ligand 43 gave 44 with a stability constant of 8×108 M–1 intoluene. The large stability constant enables us to construct an antenna reac-tion center composite by introducing electron acceptors into the fourth armof the tetrapodal ligand.

Fig. 18 Specific formation of macrocyclic trimer 42 and incorporation of a tripodal guestin the pore


3.2.3Size-Tunable Giant Macrocyclic Arrays

An interesting way of adjusting the size of macrocycles is to use a spacerwhich, instead of having a fixed angle, allows hingelike motion. With this

Fig. 19 Formation of giant macrocycles by self-coordination of ferrocene-bridged zinctrisporphyrin units

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in mind, ferrocene spacers were employed to connect the porphyrin unitsin the trisporphyrin building block with two imidazolyl terminals (45).Bis(imidazolyl)-trisporphyrin with two ferrocenyl spacers dimerized sponta-neously on Zn insertion by the complementary coordination of two terminalimidazolyl zinc porphyrins. Dimer 46 was dissolved in pyridine and sim-ple evaporation of the pyridine turned the compound into polymer of highmolecular weight. This phenomenon is accounted for by the fact that the con-centration exceeds the critical monomer concentration (cmc), above whichonly linear polymer increases occur [74]. When left to stand in commer-cial CHCl3, which contains 0.5% EtOH, the polymers gradually changed size.After 14 days, at least nine peaks (including the dimer 46) were observedin the GPC elution curve. All of the peaks were separated and subjectedto ring-closing metathesis reactions, and were thus identified as a series ofmacrocycles composed of three to ten trisporphyrins 47 (Fig. 19). The larg-est ring was composed of 30 porphyrins and 20 ferrocenes, where 40 mesosubstituents were metathesized pairwise. The fluorescence from these macro-cycles was quenched by rapid electron transfer from the ferrocenyl partof the ring.

Single molecular images of the cross-linked macrocycles were obtainedusing scanning tunneling microscopy (STM) [75] under ultrahigh vacuumconditions with a pulse injection technique [76, 77]. Clear circular arrange-ments of ten and five bright spots, corresponding to the coordinated dimersin the respective supramolecules, were observed. These coordinating dimerunits seem to be standing perpendicular to the substrate, with the centralporphyrin units in the trisporphyrin lying horizontally.

3.3One-Dimensional Arrays

A one-dimensional porphyrin array is the most basic product to come fromthe connection of porphyrin units. The preparation of one-dimensional por-phyrin arrays has been approached in various ways, using not only cova-lent but also noncovalent approaches. There are various ways to connectπ-conjugated molecular systems ∼ 1 nm2 in size. Planar, conjugated, fused,orthogonal, and helical array connections are all available. Depending on themode of connection, these arrays may be used as materials for molecularwires, photocurrent generation, optoelectronics, and so on. Therefore, manystudies have been reported on the preparation of one-dimensional porphyrinarrays.

Meso–meso coupling of porphyrins has opened the way to orthogonallylinked arrays, where a meso-free Zn porphyrin is treated successively untilthe number of porphyrins in the array reaches 2n after n steps of Ag(I)-promoted oxidative couplings (Fig. 20) [78, 79]. Ten such repetitions makeavailable a 1024 (= 210)-mer (48) as a monodispersed porphyrin array. Com-


Fig. 20 Stepwise syntheses of meso–meso covalently linked linear multi-porphyrin arrays

bined interactions of coordination and hydrogen bonding convert the lineararray into a helical one. The structure and photophysical properties of theseone-dimensional arrays have been characterized by 1H NMR, AFM, STM, andtime-resolved spectroscopies. Porphyrin tapes highly conjugated by triplyfused rings were prepared; their Q-bands were shifted to dramatically longerwavelengths, with those of the porphyrin 12-mer 49 appearing in the IRregion [80, 81]. Anderson has reported a conjugated, butadiyne-connectedpolymeric zinc porphyrin array that transforms into a porphyrin ladder 50upon titration with a bridging ligand such as 4,4′-bipyridyl [82, 83]. Theselong π-conjugated porphyrin systems are interesting in view of their applica-bility in the field of nonlinear optics [84–86] and as two-photon absorptionmaterials [87, 88].

Self-complementary dimer formation of imidazolyl zinc porphyrin al-lowed two such units to be connected directly at their meso positions togive 51. Since the two coordination assemblies grow in opposite directions inthis case, successive self-complementary coordination leads to a linear multi-

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Fig. 21 Formation of one-dimensional linear multi-porphyrin supramolecular arrays 52

porphyrin array (52) with a large stability constant (Fig. 21) [89]. Figure 22shows a GPC elution curve of the product along with the curves for standardpolystyrene mixtures. The molecular weights at the maximum and distribu-tion peaks were estimated to be 5×105 and 1×105, respectively, based ona calibration against the polystyrene mixtures. This corresponds to arrays of800 and 160 porphyrin units. The AFM image of this sample (Fig. 23) showsa molecule that is a few hundreds of nanometers in length.

The absorption spectrum for this array clearly demonstrates how the self-complementary coordination is exploited during the array formation. TheSoret band at the longer wavelength is now shifted to 490 nm from theposition of the dimer signal (450 nm), the red-shift occurring due to suc-cessive head-to-tail excitation interactions as described above. On the otherhand, the Soret band at the shorter wavelength does not shift significantlyfrom the dimer signal position because face-to-face interactions in the meso–meso dimer are orthogonal to each other and cannot be transferred throughthe linkage. Although the self-complementary coordination is very stable innonpolar solvents, it is adversely affected by the addition of coordinatingsolvents. The addition of MeOH decreases the splitting width of the Soretbands. The face-to-face interaction is cleaved, inducing a small red-shift ofthe shorter wavelength Soret band. On the other hand, the collapse of thehead-to-tail interaction results in a large blue-shift of the longer wavelengthband, which passes through a clear isosbestic point. From this it can be con-cluded that the choice of solvents or other external ligands can be used tocontrol the length of the linear multi-porphyrin array produced.


Fig. 22 GPC charts of 52 (solid line) and a mixture of polystyrene standards (dotted line)eluted by CHCl3. The exclusion limit of the column was 5×105 Da

Fig. 23 AFM image of linear multi-porphyrin arrays 52 dispersed on a mica plate

These formation and dissociation equilibria can be used to manipulate themulti-porphyrin assemblies, as was demonstrated in the following experi-ments. A mixture of coordination polymer 52 and dimer 53 was dissociatedby the addition of MeOH followed by simple evaporation. The GPC elutioncurves before (Fig. 24a) and after (Fig. 24b) the evaporation were completelydifferent. The new peaks (1, 2, and 3) in Fig. 24b were separated and identi-fied as oligomers terminated by monomeric porphyrins 54 (n = 0, 1, 2, and3). This result demonstrates the possibility of controlling the length of theporphyrin array and terminating it with other imidazolyl zinc porphyrins

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Fig. 24 GPC chart of a mixture of 52 and 53 in a 5 : 1 molar ratio (dotted line) andthe same sample mixture when dissolved in CHCl3/MeOH (1 : 1), then evaporated andredissolved in CHCl3 (solid line)

(Fig. 25). Alternatively, one can “reverse” the process. Porphyrin array for-mation can start with imidazolyl zinc porphyrin, which can be immobilizedusing, for example, thiolate attachment on the electrode surface [90, 91]. Inthis case the porphyrin attached to the electrode can serve as a molecular sol-der connecting the electrode to polymers made of hundreds of meso–meso-or bis(acetylene)-linked bis(imidazolyl zinc porphyrins) 55 [92]. Molecularwires or light-energy conversion systems could be built on the basis of suchtechnologies.

If we encourage hexa-coordination by using a central metal ion, even a sin-gle porphyrin unit can produce an array structure by having two coordinatingligands attached to it. The Mg(II) ion is typically chosen for this process.However, the sixth coordination is rather weak and gives a long array struc-ture. Bis(imidazolyl) Mg porphyrin, for example, has been found to produceonly one-dimensional oligomers (56) up to the heptamer level, as detected by

Fig. 25 Porphyrin oligomers 54 terminated by dimer porphyrin 53


electrospray ionization mass spectrometry [93]. When imidazolyl rather thanN-methylimidazolyl substituents are appended at two meso positions of theporphyrin, and the ion inserted is Ga(III), the resulting Ga(III) porphyrin canaccept both an imidazolide anion and a neutral imidazolyl ligand. As a result,a porphyrin array with a staircase structure (57) can be obtained [94]. AFMmeasurements suggest that this results in the formation of rodlike assem-blies a few hundreds of nanometers in length. This type of one-dimensionalmulti-porphyrin array is a possible candidate for not only a one-dimensionalantenna, but also a molecular wire for molecular electronic applications. Thephotoirradiation of a film of such arrays has been shown to enhance theelectric conduction across a coated electrode. Co(III) can form a long lin-ear array (58) by accepting a much stronger coordination from imidazolyl,although the singlet excitation energy is quenched in this case [95]. As isshown in Fig. 26, the introduction of two N-methylimidazolyl substituents re-sulted in a widely split Soret band at 404 and 474 nm. This was in contrastto the Co(III) monoimidazolyl porphyrin dimer signal (dashed), which in-dicates the development of extensive exciton coupling. The results of a GPCanalysis suggested a molecular weight (MW) of over 30 000, and AFM imagesshowed many rodlike assemblies. These were mostly 50–100 nm in length,which corresponds to 90- to 170-mers. The maximum observed length was1.1 µm (a 1900-mer).

Hunter and Michelsen developed a methodology to obtain linear as wellas macrocyclic arrangements of porphyrins by using the properties of Co(II)porphyrin, which can accept two axial ligands. Thus, a cobalt porphyrin with

Fig. 26 UV/visible absorption spectra of self-complementary Co(III) dimer (dotted line)and polymer 58 (solid line)

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two identical amidopyridyl substituents appended to it was shown to giverise to self-complementary dimers successively arranged in a linear fashion(59) [96]. (Note: These authors do not exclude the possibility of oxidationto the Co(III) state.) The molecular size, as deduced from GPC analysis, wasfound to grow depending on the concentration of the species. For this test, theconcentration was varied from 55 to 7000 µM.

3.4Two-Dimensional Arrays

The introduction of supramolecular multi-porphyrin assemblies is expectedto contribute significantly to the field of molecular electronic and photonicdevices because of their excellent levels of light absorption and charge sep-


aration, and their ability to facilitate the transfer of the resulting excitons,holes, and electrons. As has been demonstrated in the previous sections, self-complementary coordination is a powerful technique for connecting a largenumber of porphyrins into one-dimensional arrays.

This methodology can be extended further to create two-dimensional net-works by self-coordination of the cross-shaped zinc porphyrin pentamer60, which has imidazolyl porphyrins at four meso positions. This arraycan propagate in four directions to form a planar porphyrin aggregate 61(Fig. 27) [97]. When organized in a concentrated solution, only insoluble ma-terial was obtained. Well-organized species could be prepared, however, byperforming a reorganization procedure. The compound was dissolved firstin pyridine/nitrobenzene (1 : 10, v/v), after which gentle evaporation of thepyridine was allowed to take place. An absorption spectrum of a film of 61on a glass plate shows split Soret bands with a peak separation of 930 cm–1,indicating the presence of complementary self-coordination. AFM images ofthis substance on a mica substrate show a series of spots spread over 350 nmwith a height distribution of 2.0–2.4 nm. Given a zinc-to-zinc diagonal dis-tance of 3 nm, the spot length corresponds to an assembly of ∼ 130 porphyrinpentamer units. The thickness of the sample suggests a filmlike assemblyformation of six to seven porphyrin layers, which accords well with the sug-gestion that there is a random approach from the top and bottom sides ofthe sample during coordination. Two-dimensional network formations of 62using covalent approaches have also been achieved after multistep synthe-sis [98].

Two-dimensional propagation of self-complementary coordination pro-vides a more robust network system for electrical conduction, since the net-work is not fully disrupted by defect formation in the structure. This property

Fig. 27 Formation of two-dimensional supramolecular porphyrin array 61

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is of the utmost importance if these substances are to be used to build con-ductive molecular devices.

4Coordination Organization of Phthalocyanines

Phthalocyanines are typically characterized by wide color variation alongwith high levels of stability in response to chemical reactions, heat, and il-lumination with light. They are often used as functional dyes of extremedurability [99]. Exploitation of their excellent electronic and photoconduc-tive properties has led to their widespread use as materials for xerography,organic electroluminescence, solar energy conversion systems, and manyothers. Their expanded π-electronic plane is ideal for electron–hole delo-calization due to the stabilization of anion or cation radicals in the largeπ-electronic system [100]. Phthalocyanines have large extinction coefficientsin the Q-band region and high fluorescence quantum yields. Their possibleapplication in the field of nonlinear optics [101], as optical data storage de-vices, and in the production of molecular-scale devices is also of extremeinterest.

In spite of extensive research, supramolecular methodologies for the or-ganization of phthalocyanines through coordination processes have not beenvery successful. Various sandwich complexes have been prepared, and their


electronic and electrochromic properties have been extensively studied. How-ever, other examples of organization through coordination are rather limited.The pyrido-substitution of one of the benzo groupings in zinc phthalocya-nine, where an edge-to-face monotopic complexation (63) was suggestedby NMR and GPC analyses, has been reported [102, 103]. Kobayashi alsoreports the connection of titanium bis-phthalocyanine 64 to octahydroxy-phthalocyanine through chelating or rather covalent bondings [104].

The incorporation of phthalocyanines into a multichromophore supra-molecular system was attempted by attachment to imidazolyl zinc porphyrin65 (Fig. 28). A combination of strong absorption of the Soret band of por-phyrin and of the Q-band of phthalocyanine makes compound 66 an at-tractive dyestuff similar to chlorophylls and covering the whole visible re-

Fig. 28 Formation of self-complementary dimer 66 from phthalocyanine-substituted imi-dazolyl zinc porphyrin 65

86 Y. Kobuke

gion [105]. The porphyrin part exists as a self-complementary dimer and actsas an excellent energy donor for the phthalocyanine, because the fluorescencewavelength of porphyrin overlaps with the absorption band of phthalocya-nine. Strong fluorescence, with a high quantum yield of 0.71, was observedfrom the free base phthalocyanine part of the system upon excitation of thewhole absorption area, since fast and efficient energy transfer was observedwhen the porphyrin part was excited.

Given these results, it seemed that the methodology developed in thefield of porphyrin supramolecules could be extended to phthalocyanine ifappropriate modifications were made. The introduction of an imidazolyl sub-stituent at one of the benzo positions of metallo(Zn or Mg)phthalocyanine67 spontaneously gave the self-complementary dimer 68 in a way similar tothe behavior of the porphyrins discussed earlier (Fig. 29) [106]. The Q-bandswere split into a twin peak and converged into a single band through a clearisosbestic point by the addition of a large excess of N-methylimidazole. Fromthis competitive titration curve, the stability constants for the complementarycoordination were estimated to be of the order of 1011–1012 M–1, depend-ing on the metal ion species (Zn or Mg) and ring substituents ((t-Bu)4 or

Fig. 29 Self-assembled imidazolyl phthalocyanine dimer 68


(OBu)8). These extremely large stability constants are certainly a consequenceof the nature of complementary self-coordination itself, a fact which wasestablished by investigation of the 1H NMR spectra of the systems. Thesespectra demonstrated a characteristic shielding of the imidazolyl and half ofthe phthalocyanine ring protons. Since the substituent at one of the benzopositions of phthalocyanine does not lie along the molecular symmetry axis,imidazolyl coordination gives rise to the formation of two geometrical iso-mers. The NMR analysis established that there is a 1 : 1 mixture of specieswith parallel and oblique orientations. These isomers behave similarly and donot introduce any additional complexity into the spectroscopic characteriza-tion of the sample.

Measurements of the oxidation potentials yielded results which reflectedthe close proximity of the two phthalocyanine planes. The first two oxi-dation potentials (one-electron oxidations from each phthalocyanine ring)were ∼ 100 mV apart, suggesting the delocalization of the cation radicalover the two phthalocyanines’ π-electronic framework. This behavior showssome similarity to that of the porphyrin dimers, and is expected to fa-vor energy- and electron-transfer reactions. Another characteristic featureof this phthalocyanine dimer is that its fluorescence quantum yields are al-most the same as those of the corresponding monomers (0.45, 0.26, and0.76 for Zn(OBu), Zn(t-Bu), and Mg(t-Bu) phthalocyanines, respectively).Such a highly fluorescent phthalocyanine dimer has never been reportedbefore.

In this way, self-complementary coordination developed in the field of por-phyrin chemistry was successfully transferred to a related field, and has beenshown to be a successful first step toward similar advances in phthalocyaninechemistry. Since phthalocyanine has several advantages over other materi-als in a range of important applications, this development should open upthe field of supramolecular chemistry in phthalocyanines as an area of muchmore active research.

5Structural Characterization of Supramolecular Assemblies

As has now been demonstrated through various examples, supramolecularmethodology is a useful technique for generating unique structures spon-taneously, provided that the initial building blocks are properly designed.When successful, it certainly saves considerable time and effort compared toother, more tedious synthetic processes. In the approach using only covalentbonds, identification of the final product is, in principle, accomplished rela-tively easily. The identification is generally based on information about thespecific molecular weight, which is found using mass spectroscopy measure-ments, and on other spectroscopic evidence, most notably on the results of

88 Y. Kobuke

NMR spectroscopy. In the case of the supramolecular sciences, synthesis ofthe target unit molecules which are to be identified is only the start of thechemistry. The supramolecular structure depends on the concentrations ofthe substances, the solvents employed, and many other factors, and in generalis not readily verifiable. A detailed list of structure elucidation techniques hasbeen described in a previous review [9]. Brief comments on these techniquesare made below.

GPC (gel permeation chromatography): GPC is very powerful not only foranalytical but also for preparative purposes, if the stability constants of thesupramolecules are large enough for them to be subjected to the process. Onlynonpolar solvents are allowed for certain gels, but polystyrene-based gels al-lowing the use of polar solvents are also available. Difficulties may occur dueto adsorption of the sample to the resin, in which case it may be worthwhileto try the addition of LiBr salt to the eluent.

Mass Spectrometry: Modern organic chemistry depends on the devel-opment of technologies, such as soft ionization of molecules without bondbreaking, which are now an integral part of mass spectrometry. ESI-TOF massspectrometry via the cold ion-spray method is very powerful. In our limitedexperience, however, this was not a valid method for our compounds. Almostall of the species analyzed in this way gave only the dissociated assembly unit,even though the supramolecules belong to a class with amongst the largeststability constants. The best solution to this problem was to use metathe-sis to link the self-complementary coordination pair. For example, owing tothe almost quantitative nature of the reaction, 40 allylic meso substituentswere successfully connected pairwise in a decamer of ferrocene-connectedtrisporphyrin units (see Sect. 3.2.3). After the metathesis linking, the parentpeak was observed at the calculated mass number of 21 783.72 (M + 1) witha sufficient peak intensity for it to be properly identified.

AFM (atomic force microscopy): Recently, a number of AFM images havebeen published which disclose the assembled structure of both natural andreconstituted photosynthetic systems [20]. The scanning is accomplished bythe application of smaller cantilever amplitudes and weaker forces than arenormally used, the measurements being made with an oxide-sharpened Si3N4tip with a 2–5-nm edge. Just as elucidation of the photosynthetic reactioncenter and the antenna by X-ray crystallography in the 1980s–1990s stimu-lated the construction of artificial photosynthetic unit structures, similarlytheir total assembled structures (which can now be viewed for the first time)have become interesting targets for construction.

STM (scanning tunneling microscopy): Pulse injection of the sample solu-tion onto a metal surface under ultrahigh vacuum gives excellent images ofsingle molecules within the sample [76, 77]. This contributes greatly to the ob-servation of nonsublimable supramolecules of chemical significance. Whileit must be remembered that these images result from a strong interactionbetween the target molecule and the substrate, this technique permits the ac-


Fig. 30 STM images of macrocyclic 10-mer (left) and 5-mer (right) from ferrocene-bridged zinc trisporphyrin unit 45

quisition of unique information about single molecules that would otherwisebe unobtainable (Fig. 30).

VPO (vapor pressure osmometry): Unfortunately, this classical techniqueis becoming more and more difficult to employ due to the limited availabilityof ultrahigh-precision thermostats. Machines produced recently lack the highsensitivity required. Moreover, the molecular weights of recent interest havebecome in general too high to apply the concentrations required for detectionwith any meaningful level of precision.

6Properties of Porphyrin Assemblies

6.1Photoinduced Electron Transfer in Special Pair Mimic

It is interesting to examine how the special pair mimic which has been cre-ated behaves in a photoinduced charge separation reaction. First, the redoxproperties of imidazolyl porphyrin dimer 5 were investigated. Differentialpulse voltammetry of the dimer showed four one-electron oxidation peaksat 414, 632, 968, and 1120 mV vs Ag/Ag+ in chloroform. The first oxida-tion potential is close to or higher than the corresponding potential for themonomer (381 mV), which was obtained by the addition of an excess amount(220 equiv) of N-methylimidazole. It is important to note that dimer forma-tion never favors the first oxidation of the porphyrin. However, the secondoxidation potential, corresponding to one-electron oxidation of the secondporphyrin unit in the pair, was shifted up by 228 mV. It is clear that the cationradical, once generated on one of the porphyrin units, must be subsequentlybe delocalized over the π-electronic system of the strongly interacting por-phyrin dimer.

Attachment of pyromellitdiimide as the electron acceptor to this dimerand to the dissociated monomer porphyrins with two different separation

90 Y. Kobuke

distances, 69, 70 and 71, 72, respectively, quenched the porphyrin fluores-cence efficiently. In all cases, the fluorescence was quenched more efficientlyin the dimer than in the monomer (as can be seen in the last column ofTable 2), which suggests that electron transfer is occurring more efficiently inthe dimer [107]. In addition, the time constants for both photoinduced chargeseparation (CS) and charge recombination (CR) were determined by measur-ing the fluorescence lifetime and the transient absorption of the porphyrincation radicals and the pyromellitdiimide anion radicals. Unfortunately, inthe case of the monomers, the CR rates were faster than the CS rates. In con-trast, dimer 69 showed an accelerated CS rate (with the lifetime decreasingfrom 10 to 2 ps) and a decelerated CR rate (with the time taken increasingfrom 3 to 12 ps). The same is true for the 70 series, where the CS life-time decreased from 1000 to 120 ps and the CR time increased from < 100to 160 ps.

As discussed above with reference to the oxidation behavior of the system,the cation radical is delocalized over two closely interacting porphyrins inthe dimer, reducing the charge density of the dimer compared with that of

Table 2 Kinetic parameters of charge separation in dimers 69 and 70, and the correspond-ing monomers 71 and 72 in CHCl3

Compound τCS (ps) a τCR (ps) a τf (ps) b Qeff (%) c

71 10 3 10 99.169 2 12 2 99.572 1000 < 100 920 70.3

70 120 160 120 94.6

a Time constants obtained by time-resolved transient absorptionb Fluorescence lifetimec Quenching efficiencies defined as Qeff = (1 – Icompound/Ireference) × 100, where I is fluor-escence intensity.


Fig. 31 Schematic drawings of the Marcus parabolas for dimer and monomer

the monomer. This leads to a smaller solvent reorganization energy for thedimer, shifting the Marcus parabola [108–111] to the left. Since the CS rate islocated in the normal region, this shift induces the observed increase in therate, while the CR rate in the inverted region decreases (Fig. 31). It is clearthat dimer formation accelerates the CS rate and decelerates the CR rate, facil-itating the total photoinduced charge separation efficiency. This is a result ofthe dimer having a smaller reorganization energy than the monomer due tocharge delocalization over the enlarged π-electronic framework. This simplemodel system thus clearly demonstrates the vital role which the special pairarrangement plays in the electron-transfer process during photosynthesis.

6.2Energy Transfer in Photosynthetic Antenna Mimics

Supramolecular porphyrin rings have been designed to mimic the antennafunction of bacterial photosynthesis, taking into account the fact that therandom assembly formation usually loses its singlet excitation energy dueto the formation of energy trapping sites. The antenna rings investigated,macrocyclic pentamer 17 and hexamer 16, both behaved ideally. Almost nofluorescence quenching was observed, and they maintained a constant fluor-escence lifetime of 2.2 ns both before and after the assembly formation. Thehopping rate of the excitation energy was estimated via anisotropy depolar-ization and exciton–exciton annihilation dynamics (Fig. 32) [112]. Althoughthe latter technique does not yield a unique time constant, the slowest of thelaser power-dependent decay processes can be considered representational ofthe energy hopping process. This energy hopping is caused by exciton an-

92 Y. Kobuke

Fig. 32 Energy hopping rates in cyclic antenna complexes 16 and 17

nihilation due to the disproportionation of two excitons. As for anisotropydepolarization, one can extract the real hopping rate from the other decayprocesses by observing the rate of in-phase anisotropy decay and molecu-lar rotation. Finally, this rate must be verified by considering the coincidenttime constant from two decay processes. According to the analysis, the energyhopping rates were estimated as 8.0 and 5.3 ps for the 5-mer and 6-mer sys-tems, respectively. A larger exciton–exciton interaction level for the latter, asdemonstrated by the larger splitting of its Soret band, may be responsible forthe faster energy hopping rate. The combined evidence clearly demonstratesthe fact that these self-assembled cyclic multi-porphyrin arrays are excellentmimics of the light-harvesting antenna of B850.

The energy hopping rates analyzed for three-dimensional box-type assem-blies are schematically shown in Fig. 33 [63]. In these cases, the rates wereanalyzed as if they were occurring in an xy plane without any anisotropyalong the z direction. The rate was found to be strongly dependent on thecenter-to-center distance, indicating that the excitation energy migration pro-cesses are well described by a Förster-type incoherent energy hopping mode.Similar analyses have been made for covalently connected light-harvestingmodels [113].

A linear assembly formed through successive self-complementary coordi-nation of meso–meso-coupled bis(imidazolyl zinc porphyrin) units can alsotransfer excitation energy along the one-dimensional array. In order to es-timate the efficiency of this energy transfer, imidazolyl Mn porphyrin wasadded to the linear array of imidazolyl zinc porphyrins 52. The terminalsof the array can accept complementary coordination with imidazolyl Mnporphyrin [114]. As a result, the fluorescence of Zn porphyrin (φ = 0.053)in the one-dimensional array is quenched. This quenching never happens


Fig. 33 Energy hopping rates in antenna boxes 29

Fig. 34 Energy hopping along linear antenna complex and quenching at terminal Mnporphyrins

when Mn tetraphenylporphyrin is added without an imidazolyl substituent.It can therefore be concluded that efficient quenching occurs through a fastenergy hopping process along the one-dimensional array to the terminalMn porphyrin coordinated by complementary coordination. GPC analysis ofthe mixture of the bis(imidazolyl zinc porphyrin) array and imidazolyl Mnporphyrin indicates that the energy hopping quenching occurs through 130porphyrin units in an array containing 200 porphyrin units in total (Fig. 34).

6.3Nonlinear Optical Properties

Nonlinear optical (NLO) materials are interesting in view of their pos-sible photonic applications, such as ultrafast optical switching and modula-tion [115–117]. The general principle for obtaining large third-order suscep-tibility χ(3) values, which are required if nonlinear optical effects are to beobserved, requires molecules composed of a π-conjugated core in association

94 Y. Kobuke

with polarizing donor/acceptor terminals. The coordination reorganizationprocess introduced in Sect. 3.1 seems to afford an ideal tool for obtainingsuch an assembly for porphyrin [118]. A mixture of bis(imidazolyl zinc por-phyrin) and monozinc-bis(imidazolyl porphyrin) was therefore reorganizedfrom a CHCl3/MeOH solution, and a series of oligomeric compounds withand without free base porphyrin terminals (73–75 and 76–78, respectively)were isolated.

The optical Kerr effect (OKE), which is an NLO property, was assessed forthese compounds. The measurement was made under off-resonant conditionsat 800 nm using a femtosecond laser with a CS2 reference. The results are shownschematically in Fig. 35. The molecular second-order hyperpolarizabilities|γyyyy|of oligomers 73–75 were extremely large compared with those of the por-phyrin polymers separated by a phenylene or an acetylene spacer. The valuesbelong to the class of molecules showing the largest γ value, as is evident whenone considers the value for a donor–acceptor pair of 2,7-diethynylfluorene andN,N,N′,N′-tetrakis(4-phenyl)-4,4′-diamino-1,1′-biphenyl under similar con-ditions (4.5×10–30 esu) [119]. These large |γyyyy| values were observed onlyfor those compounds having free base porphyrins at the molecular terminals.The combination of terminal free base porphyrins and a core of coordination-linked zinc porphyrins can therefore be seen to be effective for enhancing the|γyyyy| value through efficient molecular polarization.

The bisporphyrin discussed above is connected directly at meso–meso pos-itions, and the two π-electronic systems are almost orthogonal to each other.

Fig. 35 Third-order nonlinearity of porphyrin self-assemblies from femtosecond OKEmeasurements


In order to obtain a better conjugation, two porphyrins were then connectedby a butadiyne grouping and organized in a similar way, so that a free base-terminated bisporphyrin dimer was generated. Two-photon absorption (2PA)cross section values (σ (2)) were then measured using a femtosecond openaperture Z-scan method in the off-resonant region [120].

In contrast to the small σ (2) value (370 GM at 964 nm) measured formonozinc meso–meso-connected bisporphyrin 73, butadiyne-linked bis-porphyrin 79 (Fig. 36) gave an extremely large value (7600 GM at 887 nm),suggesting that the increased π conjugation had a significant effect on theabsorption rate. This value belongs to the largest class of such measure-ments for organic compounds in the femtosecond timescale [121–135]. Incontrast, when the monomer had been dissociated by the addition of ex-cess N-methylimidazole, the result was a value of 1800 GM. Monomericbiszinc complex 81 and free base 82 gave values of 1200 and 1000 GM, re-spectively. Comparison of these values indicates that the complementarycoordination and polarization of the asymmetric zinc-free base structurealso contributes to the enhancement of the 2PA cross section. Furthermore,

96 Y. Kobuke

Fig. 36 Two-photon absorption spectra of 73 (filled triangles) and 79 (filled circles) meas-ured by the femtosecond Z-scan method

the self-assembled imidazolyl porphyrin polymer 52, with a mean molecularweight of Mn = 1.5×105, had an extremely large σ (2) value of 4.4×105 GM at873 nm [136].

The two-photon absorption cross section of any material depends on thesquare of the incident light intensity and occurs only at the focal point ofthe laser light. Therefore, much attention has been paid to this process as itapplies to such developments as two-photon photodynamic therapy, three-dimensional optical data storage, optical limiting, three-dimensional micro-fabrication, and fluorescence microscopy. Porphyrins and phthalocyanineshave been candidates for 2PA materials because of their highly conjugatedπ systems. Although monomeric porphyrin derivatives have been shown tohave relatively small σ (2) values (less than 102 GM), the examples shown heresuggest possibilities of further improvements.

6.4Photocurrent Generation

Photocurrent generation is one of the most interesting direct applicationsof photosynthetic studies. The adsorption of sensitizers onto semiconduc-tor surfaces has been found to be an efficient way to generate photocurrentsand has been studied extensively. Ruthenium bipyridyl complexes, in particu-lar, have been the focus of recent research [137–139]. In these cases, onlythe first layer of molecules, which is in direct contact with the surface, isactive. A highly porous semiconductor material was therefore employed tocompensate for the low level of absorption of the single molecular layer. Othervarieties of chromophores, semiconductor materials, and electron carriers fortotally solid systems have been the subjects of extensive studies. The present


review addresses specifically the application of self-complementary coordina-tion to this system.

Photocurrent generation systems on flat electrode surfaces may becomeimportant in the development of applications involving flexible materialssuch as plastic electrodes. A simple self-assembled monolayer (SAM) of pho-tosensitizers (including porphyrins) cannot generate a large enough pho-tocurrent density to be practicable. This is because the absorbance of a singlemolecular layer is small even with chromophores having a strong Soret band,and its sharp absorption band without exciton interactions is too narrow tocover the whole visible range. In order to overcome these inherent problems,antenna layers can be induced to grow on the SAM from the electrode.

Our approach [90, 91] involved the use of an imidazolyl porphyrin freebase to which ω-mercaptoalkyl substituent 83 had been appended. This was

Fig. 37 Scheme of photocurrent generation. A porphyrin antenna was grown from a SAMon a gold surface by self-complementary coordination followed by covalent immobiliza-tion by a ring-closing metathesis reaction

98 Y. Kobuke

attached to a gold surface by a simple soaking process [140]. After theintroduction of Zn(II), the SAM substrate was immersed in a solution ofmeso–meso-coupled imidazolyl zinc porphyrin dimer 84, which was disso-ciated partially in a CH2Cl2/MeOH solution. This was followed by a rinsewith CH2Cl2 to facilitate the imidazolyl-to-zinc coordination. After the bis-porphyrins were coordinated complementarily, the coordination-organizedpair was immobilized covalently through a metathesis reaction by simplydipping the plate into a solution of Grubbs catalyst. Alternating coordina-tion deposition and metathesis reaction cycles were repeated until multi-porphyrin layers were obtained (Fig. 37). The successive accumulation ofmulti-porphyrin arrays was clearly indicated by the gradual increase in theUV/Vis absorbance of the sample over a wide range (Fig. 38). Both the com-plementary imidazolyl-to-zinc coordination and the covalent linking of theallyl side chains are indispensable to the stable growth of the array structure.As a result of these processes, the covalently immobilized porphyrin arraysare resistant to dissociation when the next organizing solution is added,even though it contains a dissociation-inducing solvent. The optimum spacerlength for the covalent linking was employed to allow a 95% yield in the solu-tion, and the dimer connection proceeded efficiently on the solid substrate.

Cathodic photocurrent generation was observed using an aqueous elec-trolyte solution containing viologen as the electron carrier. Figure 39 showsa plot of the light-harvesting effect by integrating the total absorbance in thewhole range and the total current values at all the excitation wavelengths inthe action spectrum. The sharp increase in both the total photocurrent and

Fig. 38 Photocurrent-action spectra normalized to a constant light intensity at174.6 W cm–2 at – 200 mV (Ag/AgCl)


Fig. 39 Plot of integrated total current values vs total absorbance for different arraylengths. Numbers in the figure correspond to the number of the accumulation cycle

the current intensity emphasizes the efficient “light-harvesting” function ofthe surface-grafted multi-porphyrin arrays.

7Concluding Remarks

Self-complementary coordination is a powerful tool for supramolecular or-ganization of various porphyrins. Molecules equipped with a set of com-plementary coordination (i.e., ligands and metalloporphyrins) spontaneouslyorganize into their self-complementary structures, if they are appropriatelydesigned. Due to this self-complementarity, the stability constants of thesesubstances are far greater than those of the same ligand–metalloporphyrincombinations without complementarity. The structures must be rigid by thecomplementarity requirement. Therefore, this is the process of choice fordesigning specific arrangements of molecular systems with large stabilityconstants. The smallest product is the coordination dimer, where the por-phyrin π planes must be slip-stacked by the requirement of mutual self-coordination. In other words, the dimer is essentially a J-type aggregate. Thisresults in some of the key advantages of this coordination organization, sincethe fluorescence of the substance is not quenched and the absorption band isred-shifted. When the coordination angle is adjusted, larger rings or box-typeorganizations become possible.

The employment of hexa-coordinating metal ions in the porphyrin centeris a way to extend the supramolecular structure from the dimer to various

100 Y. Kobuke

macrocycles. In such cases, however, photonic application is limited due tothe quenching of singlet excited states by these hexa-coordinating metal ions(mostly transition metal ions). An alternative way to obtain higher-level orga-nized structures is through the connection of self-complementary sets. Variousconnection methods are associated with new organized structures. Direct con-nection of two zinc porphyrins at the meso–meso position gives rise to theformation of linear arrays, along which excitons, electrons, or holes can betransferred. Connection via acetylenic bonds allows the highly conjugated zincporphyrin arrays to develop improved electronic conduction levels. The attach-ment of energy (or electron) donor or acceptor substituents provides materialsfor nonlinear optics by increasing the polarization levels of the material. meta-Phenylene linkage of two porphyrins provides a method for obtaining artificiallight-harvesting complexes. Not only two but also three or more porphyrins canbe connected to yield other interesting organized structures.

The methodology reviewed in this chapter will be further improved anddeveloped to provide interesting products, not only in the field of scientificresearch, but also in the realm of technological applications.

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Documents

[Structure and Bonding] Non-Covalent Multi-Porphyrin Assemblies Volume 121 || Porphyrin Supramolecules by Self-Complementary Coordination