Photoinduced Electron Transfer within Porphyrin-Anthra-quinone Dyads Connected by Hamilton Hydrogen Bonding

FULL PAPER

* E-mail: [email protected]; [email protected] Received July 2, 2010; revised and accepted August 20, 2010. Project supported by the National Natural Science Foundation of China (Nos. 20733007 and 20603042), the National Basic Research Program (No.

2007CB808004) and the Beijing Nova Program. † Dedicated to the 60th Anniversary of Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences.

1580 © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2010, 28, 1580—1586

Photoinduced Electron Transfer within Porphyrin-Anthra- quinone Dyads Connected by Hamilton Hydrogen Bonding†

Zhao, Xina,b(赵鑫) Chen, Jinping*,a(陈金平) Zeng, Yia(曾毅) Li, Yingyinga(李迎迎) Han, Yongbina(韩永滨) Li, Yi*,a(李嫕)

a Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China

b Graduate University of Chinese Academy of Sciences, Beijing 100039, China

Porphyrin- or zinc-porphyrin-cyanuric acid conjugants (TPP-CA or ZnTPP-CA) and anthraquinone connected with the known “Hamilton receptor” (AQ-H) were synthesized. The supramolecular dyads constructed via the AQ-H and TPP-CA/ZnTPP-CA exhibit six hydrogen bonds, which provide the association constant KAPP approxi-mately (2.8±0.3)×103 mol－1•L in toluene. Selective excitation of the porphyrin/zinc porphyrin chromophores re-sults in an electron transfer between TPP-CA/ZnTPP-CA and AQ-H within the supramolecular assemblies, leading to an efficient quenching of the TPP-CA/ZnTPP-CA fluorescence. The singlet electron transfer from porphyrin/ zinc-porphyrin to anthraquinone proceeds mainly via a ‘through space’ mechanism with efficiencies of 43%, 58%, and rate constants of 7.6×107, 7.0×108 s－1, respectively.

Keywords hydrogen bonds, electron transfer, porphyrinoids, anthraquinone

Introduction

Photoinduced electron/energy transfer process plays an important role in natural photosynthetic systems and has been inspiring intense studies.1-4 It has been demon-strated that non-covalent interactions, such as hydrogen bonds exist extensively in biological structures and play a remarkable role in many natural processes including energy transfer and electron transfer. To mimic natural processes, various supramolecular model systems as-sembled via hydrogen bonding5-10 have been developed, and electron transfer process in these systems has been extensively examined by several groups. Nocera and coworkers11 initially reported the intramolecular elec-tron transfer in a carboxyl hydrogen bonding connected porphyrin/dinitrobenzene dyad. Sessler et al.12,13 devel-oped a non-covalently assembled supramolecular sys-tem based on Watson-Crick-type nucleic acid base pairing interactions, and the hydrogen bonds provided an effective pathway for mediating the electron transfer process. The 2-ureido-4[1H]-pyrimidinone motif re-ported by Meijer et al.14 represents a fascinating self-complementary quadruple hydrogen bonding mod-ule and Tung et al.15 have shown that the triplet energy transfer can occur via a ‘through bond’ mechanism in this system. Our research group also provided an exam-ple that the hydrogen bonding and salt-bridges can me-diate the triplet energy transfer.16,17 Hamilton et al.18,19

reported a hexa-hydrogen-bonding receptor, which con-tains 2,6-diaminopyridine capable of binding cyanuric and barbituric acid derivatives through six hydrogen bonds. This hydrogen bond system possesses high binding strength and directionality, and may serve as a good model system to illustrate the fundamental princi-ples of the electron transfer process.20-26

Although several synthetic models have been pre-sented to illustrate the electron transfer process within Hamilton hydrogen bonding assemblies, more examples are required for further clarification. As we know, quinone derivatives play a key role as electron mediator in photosynthesis.27,28 Porphyrins and their metal com-plexes constitute a family of investigated molecules due to their excellent spectroscopic, photochemical and electrochemical properties. Photoinduced electron transfer between porphyrin and quinone derivative has been attracting much attention from the viewpoint of biomimetics on the initial process of photosynthetic systems. A variety of covalently29-35 and non-covalent- ly36-40 linked porphyrin-quinone derivatives have been prepared and the photoinduced electron transfer in these systems was investigated as the simple models of pho-tosynthesis. In the present work, we report the synthesis and photophysical properties of a new, non-covalent photosynthetic model that relies on Hamilton hydrogen bonding, in which anthraquinone and porphyrin (TPP)/

Photoinduced Electron Transfer within Porphyrin-Anthraquinone Dyads

Chin. J. Chem. 2010, 28, 1580—1586 © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 1581

zinc porphyrin (ZnTPP) groups were attached to the Hamilton receptor and the cyanuric acid, respectively (Figure 1). Upon selective excitation of the TPP/ZnTPP chromospheres, the singlet electron transfer process oc-curred efficiently within the assemblies. The efficiency and the absolute rate constant of intramolecular electron transfer were examined by steady-state and time-resolved spectroscopy. This work provides a new model for the study of the Hamilton hydrogen bonding sys-tems.

Experimental

Materials and instrumentation

Reagents were obtained from available commercial sources and used without additional purification unless otherwise indicated. Toluene was washed with oil of vitriol and distilled from Sodium. 1H NMR spectra were recorded on a Bruker 400 MHz spectrometer, and chemical shifts were reported relative to CDCl3 as in-ternal reference (δ＝7.260). IR spectra were run on a Varian Excalibur 3100 IR spectrometer. MALDI-TOF mass spectra were obtained from a Bruker BIFLEX spectrometer. High resolution ESI LC-MS was per-formed on a LCT Premier spectrometer. Melting points were determined on a XT4A apparatus and were uncor-rected. UV/vis absorption and steady-state fluorescence spectra were measured by a Shimadzu UV-1601PC spectrometer and a Hitachi F-4500 spectrometer, re-spectively. The lifetime of fluorescence was recorded with a single photon counting technique on an Edin-burgh FL900 fluorescence lifetime system.

Synthesis

The synthetic route of anthraquinone-Hamilton re-ceptor (AQ-H) is shown in Scheme 1.

Compond 2: A solution of 1 (2.0 g, 3.8 mmol) and triethylamine (0.5 g, 5.0 mmol) in dry THF (50 mL) was drop added to a solution of 4-(bromomethyl)ben- zoyl chloride (1.5 g, 6.4 mmol) in dry THF (25 mL). The mixture was stirred at 0 ℃ for 1.5 h. Then the

mixture was filtered and solvent was removed under vacuum. The residue was purified by column chroma-tography on silica gel [eluent: V(CH2Cl2)/V(ethyl ace-tate)＝8/1—2/1], to give a white solid (1.3 g, 38%). m.p. 178—181 ℃; 1H NMR (CDCl3, 400 MHz) δ: 9.24 (s, 1H, HNCO), 8.65 (s, 2H, HNCO), 8.40 (s, 2H, HNCO), 8.11 (s, 3H, Ph), 7.85—7.89 (m, 6H, Py×4 and Ph×2), 7.61 (s, 2H, Py), 7.29 (s, 2H, Ph), 4.50 (s, 2H, CH2), 1.32 (s, 18H, t-Bu); IR (KBr) ν: 3433, 2965, 1676, 1585, 1508, 1446, 1300, 1242 cm－1.

Compond 3 (AQ-H): A solution of 2 (250 mg, 0.34 mmol), 9,10-anthraquinone-2-carboxylic acid (300 mg, 1.2 mmol), potassium carbonate (400 mg, 2.8 mmol) and 18-C-6 (catalytic amount) in dry acetone (20 mL) was stirred under reflux for 4 h. Then the mixture was filtered and solvent was removed under vacuum. The residue was purified by column chromatography on sil-ica gel [eluent: V(CH2Cl2)/V(ethyl acetate)＝8/1—5/1], to afford a white solid (150 mg, 50%): m.p. 182—184 ℃; 1H NMR (CDCl3, 400 MHz) δ: 9.00 (s, 1H, AQ), 8.77 (s, 1H, HNCO), 8.58 (s, 4H, HNCO), 8.46 (d, J＝8.0 Hz, 1H, AQ), 8.40 (d, J＝8.0 Hz, 1H, AQ), 8.32—8.35 (m, 2H, AQ), 8.27 (s, 1H, Ph'), 7.98—7.96 (m, 8H, Ph×2, Py×4 and Ph'×2), 7.84—7.82 (m, 2H, AQ), 7.72 (t, J＝8.0 Hz, 2H, Py), 7.58 (d, J＝8.0 Hz, 2H, Ph), 5.50 (s, 2H, CH2), 1.35 (s, 18H, t-Bu); IR (KBr) ν: 3433, 2962, 1678, 1585, 1506, 1446, 1298, 1242 cm－1; MS (MALDI-TOF) m/z: 900.2 (M＋H＋), 938.2 (M＋K＋), calcd m/z 899.3.

The synthesis of porphyrin/znic porphyrin-cyanuric acid (TPP-CA/ZnTPP-CA) is shown in Scheme 2.

Compond 5: A mixture of 5-(p-hydroxyphenyl)-10, 15,20-(p-methyltriphenyl)porphyrin (4, 100 mg, 0.15 mmol), 4-(bromomethyl)benzoic acid (40 mg, 0.19 mmol), 1,3-dicyclohexylcarbodiimide (DCC, 60 mg, 0.27 mmol) and 4-dimethylaminopyridine (DMAP, catalytic amount) in 5 mL dry CH2Cl2 was stirred at r.t. for 1 h. Then the reaction mixture was diluted with CH2Cl2 and quenched with water. The aqueous layer was extracted with CH2Cl2 (3×). The combined organic

Figure 1 Structures of the complex assembled via Hamilton hydrogen bonding and the electron acceptor model compound.

Zhao et al.FULL PAPER

1582 www.cjc.wiley-vch.de © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2010, 28, 1580—1586

Scheme 1 Synthesis of anthraquinone Hamilton receptor (AQ-H)

Scheme 2 Synthesis of porphyrin/Zn-porphyrin-cyanuric acid (TPP-CA or ZnTPP-CA)



layers were dried with MgSO4 and evaporated to dry-ness. The crude product was purified by column chro-matography on silica gel (eluent: CH2Cl2), to give a purple solid (80 mg, 61%). 1H NMR (CDCl3, 400 MHz) δ: 8.89 (d, J＝8.8 Hz, 8H, porphyrin-H), 8.37 (d, J＝8.5 Hz, 2H, Ph), 8.29 (d, J＝8.4 Hz, 2H, Ph), 8.11 (d, J＝7.8 Hz, 6H, Ph), 7.61—7.65 (m, 4H, Ph), 7.57 (d, J＝7.8 Hz, 6H, Ph), 4.60 (s, 2H, CH2), 2.71 (s, 9H, CH3), －2.76 (s, 2H, NH); IR (KBr) ν: 2920, 1739, 1507, 1473, 1264, 1199, 1179, 1070 cm－1.

Compond 6 (TPP-CA): A mixture of compond 5 (70 mg, 0.08 mmol), DBU (50 mg, 0.34 mmol), cyanuric acid (50 mg, 0.39 mmol) in 3 mL dry DMF was stirred at r.t. for 1 h. The crude product was precipitated from the reaction mixture by slowly adding 15 mL water. The mixture was filtered and further purified by column chromatography on silica gel [eluent: V(CH2Cl2)/ V(methanol)＝100/1—50/1] to give a purple solid (40 mg, 54%). m.p. 252—253 ℃; 1H NMR (DMSO-d6, 400 MHz) δ: 11.63 (s, 2H, OCNHCO), 8.88 (d, J＝10.4 Hz, 8H, porphyrin-H), 8.26—8.30 (m, 4H, Ph×2 and Ph'×2), 8.10 (d, J＝7.7 Hz, 6H, Ph), 7.74 (d, J＝8.4 Hz, 2H, Ph), 7.64 (m, 8H, Ph×6 and Ph'×2), 5.01 (s, 2H, CH2), 2.67 (s, 9H, CH3), －2.92 (s, 2H, NH); IR (KBr) ν: 3024, 1747, 1705, 1464, 1265, 1200, 1182 cm－1; MS (ESI) m/z: 918.3428 (M＋H), calcd m/z 917.3326.

Compond 7 (ZnTPP-CA): A mixture of compond 6 (21 mg, 0.023 mmol), zinc acetate (20 mg, 0.095 mmol) in 5 mL CH2Cl2 and 1 mL methanol was stirred at r.t. overnight. The solvent was removed under vacuum. The residue was purified by column chromatography on sil-ica gel [eluent: V(CH2Cl2)/V(methanol)＝50/1] to give a carmine solid (19 mg, 85%). m.p.＞270 ℃; 1H NMR (DMSO-d6, 400 MHz) δ: 11.62 (s, 2H, OCNHCO), 8.81 (d, J＝11.5 Hz, 8H, porphyrin-H), 8.24—8.28 (m, 4H, Ph×2 and Ph'×2), 8.07 (d, J＝7.8 Hz, 6H, Ph), 7.71 (d, J＝8.4 Hz, 2H, Ph), 7.59—7.64 (m, 8H, Ph×6 and Ph'×2), 5.01 (s, 2H, CH2), 2.67 (s, 9H, CH3). IR (KBr) ν: 3440, 1751, 1701, 1458, 1408, 1267, 1203, 1165, 1072, 999 cm－1; MS (ESI) m/z: 978.258 (M－H), calcd m/z 979.246.

Results and discussion

Synthesis and characterization of the compounds

Anthraquinone-Hamilton receptor (AQ-H) was syn-thesized following the reported procedure by De Cola et al.26 The porphyrin-cyanuric acid (TPP-CA) and zinc porphyrin-cyanuric acid (ZnTPP-CA) were prepared via the method reported by Hirsch et al.20 The details of all synthesis and characterization are described in the ex-perimental section. The target compounds, TPP-CA, ZnTPP-CA, AQ-H, were characterized by 1H NMR, IR, and mass spectrometry (MALDI-TOF or ESI).

Steady-state absorption and fluorescence spectros-copy

Figure 2 illustrates the absorption spectrum of the

mixture of TPP-CA and AQ-H with the molar ratio of 1∶140 in toluene, together with those of TPP-CA and AQ-H. No measurable ground state electronic interac-tion between the anthraquinone and porphyrin chromo-phores can be observed from the absorption spectra. Significantly, the absorption of the porphyrin chromo-phore extends to a longer wavelength than that of the anthraquinone group, which suggests that the singlet- singlet energy transfer from the excited porphyrin chromophore to the anthraquinone group is endothermic, and consequently, unlikely. Furthermore, this fact per-mits selective excitation of the porphyrin moiety above 500 nm in the assembled system. The absorption spec-trum of the mixture of ZnTPP-CA and AQ-H in the same ratio exhibits similar characteristics to that of the mixture of TPP-CA and AQ-H.

Figure 2 Absorption spectra of TPP-CA, AQ-H and the mix-ture of TPP-CA∶AQ-H＝1∶140 (molar ratio) in toluene.

Figure 3 shows the fluorescence spectra of TPP-CA in toluene with variable concentrations of anthraquinone-Hamilton receptor (AQ-H). All of them exhibit a fluorescence characteristic of the porphyrin chromo-phore with maxima at 653 and 719 nm, while the over-all intensities of the fluorescence decrease significantly with the addition of AQ-H. This finding indicates that the fluorescence of TPP-CA is quenched by the an-thraquinone group. As the ratio of AQ-H to TPP-CA increases from 0 to 140, the fluorescence intensity of TPP-CA decrease 32%. The quenching experiments indicate that ZnTPP-CA shows similar results but with 45% decrease in fluorescence intensity. For comparison, the fluorescence quenching experiment of TPP-CA by 9,10-anthraquinone-2-carboxylate (AQM), which could not form hydrogen bonds with TPP-CA, was also per-formed. There was almost no change for the fluores-cence of TPP-CA (c＝1×10－5 mol/L) upon addition of 200 equiv. AQM, suggesting that the hydrogen bonding between TPP-CA and AQ-H is crucial for the fluores-cence quenching, rather than diffusion encounter. Fur-thermore, the fluorescence quenching of TPP-CA by AQ-H disappeared when ca. 1% methanol was added to the assembled system to dissociate the hydrogen bond-



ing, manifesting that the fluorescence quenching occurs only in the forming supramolcular system assembled via hydrogen bonding.

Figure 3 Fluorescence spectra of TPP-CA with different con-centration of AQ-H in toluene. λex＝516 nm, [TPP-CA]＝1.0×10－5 mol/L, [AQ-H]＝0—1.4×10－3 mol/L. The inset is the plot of fluorescence emission at 653 nm vs. the concentration of AQ-H.

The dependency of emission on the concentration of AQ-H was used to determine the association constants for the formation of the assembled dyads of TPP-CA/ AQ-H. The data were taken at the emission maximum of 653 nm for TPP-CA, and plotted as a function of the AQ-H concentration (Figure 3, inset). A nonlinear curve fitting according to Eq. (1) allowed estimation of the association constants.20

IF＝I0－I0/(2cD)[1/KAPP＋c0＋cD－sqrt(1/KAPP＋c0

＋cD－4c0cD)] (1)

In Eq. (1), I0 refers to the initial fluorescence intensity, c0 is the total porphyrin concentration, cD is the total concentration of the added AQ-H, and KAPP is the asso-ciation constant. The obtained association constants are (2.8±0.3)×103 mol－1•L and (2.9±0.3)×103 mol－1•L for AQ-H/TPP-CA and AQ-H/ZnTPP-CA, respectively, which are in good agreement with those obtained pre-viously by Hirsch et al.20

To clarify the reason for the fluorescence quenching of porphyrin/zinc porphyrin by the anthraquinone group in the assembled systems, we estimated the free energy change involved in an electron transfer process, which can be calculated by the Rehm-Weller equation.41

∆G (kJ•mol－1)＝96.44[E(D•＋/D)－E(A/A•－)－e2/rε]

－E00 (kJ•mol－1) (2)

E00 is the excited state energy and in this study repre-sents the singlet state energy of the porphyrin and zinc porphyrin, which is 183.3 and 198.7 kJ/mol (estimated from the emission spectra), respectively. E(D•＋/D) and E(A/A•－) are the redox potentials of the donor and the acceptor, determined as ＋0.96 V (vs. SCE)30 for por-

phyrin, ＋0.81 V (vs. SCE)32 for zinc porphyrin, and －0.78 V (vs. SCE)30 for 9,10-anthraquinone-2-carboxylate, respectively. The e2/rε represents the Coulom-bic energy associated with bringing separated radical ions at a distance r in a solvent of dielectric constant ε. Since the assembled system via hydrogen bonding is not fully rigid, the distance between donor and acceptor (r, center-to-center) was estimated from its lowest-energy conformation, which is computed by using MM＋, the Polak-Ribiere algorithm (HyperChem 6.0 program), giving r to be 25 Ǻ. Calculation according to Eq. (2) reveals that the electron transfer process from the singlet excited porphyrin/znic porphyrin chromophore to the anthraquinone group is exothermic by 21.3/51.0 kJ/mol. suggesting that the electron transfer process could occur efficiently in these supramolecular assemblies. There-fore, we infer that the quenching of the fluorescence in the TPP-CA/AQ-H and ZnTPP-CA/AQ-H assemblies is due to the intramolecular electron transfer from the porphyrin/zinc porphyrin chromophore to the an-thraquinone group.

Time-resolved fluorescence spectroscopy

Fluorescence lifetimes were examined by using the single photon counting technique. The excitation wave-length was the absorption maximum at 420 nm, and the fluorescence decay of TPP-CA was monitored at the emission maximum 653 nm, which could be well fitted by a monoexponential function, giving a lifetime of 9.8 ns. On the other hand, the decay of the mixture of TPP-CA (1×10－ 5 mol－ 1•L) and AQ-H (1×10－ 3 mol－1•L) at the same conditions can only be fitted dou-ble-exponentially with lifetimes of 9.8 and 5.6 ns. The longer lifetime component with the same lifetime of TPP-CA is assigned to the dissociative donor compound TPP-CA, and the shorter one is assigned to TPP-CA associating with AQ-H in the Hamilton hydrogen bond-ing assembly in which the electron transfer occurs. The similar spectroscopy behavior was observed in the Hamilton hydrogen bonding assembly constructed by ZnTPP-CA and AQ-H, with lifetimes of 1.9 and 0.8 ns. All the obtained lifetimes are summarized in Table 1.

Table 1 Fluorescence lifetimes, rate constants and efficiencies of electron transfer in the Hamilton hydrogen bonding assem-blies.

Compd. τ1/ns τ2/ns kET/s－1 ФET

TPP-CA 9.8

TPP-CA/AQ-H 9.8 5.6 7.6×107 0.43

ZnTPP-CA 1.9

ZnTPP-CA/AQ-H 1.9 0.8 7.0×108 0.58

The rate constant (kET) and the efficiency (ΦET) for the electron transfer from the porphyrin or zinc porphy-rin chromophore to the anthraquinone group in the TPP-CA/AQ-H or ZnTPP-CA/AQ-H assemblies are



calculated to be 7.6×107, 7.0×108 s－1, and 0.43, 0.58, according to Eqs. (3) and (4), respectively.42 The esti-mated kET values are compatible with that in dendritic examples previously reported by Modarelli et al.31,32 The time-resolved fluorescence experiments validates the occurrence of the electron transfer process in the Hamilton hydrogen bonding assemblies.

kET＝1/τ2－1/τ1 (3)

ФET＝1－τ2/τ1 (4)

Mechanism of electron transfer in Hamilton hydro-gen bonding assemblies

The photophysical studies reveal that the electron transfer from the porphyrin chromophore to the an-thraquinone group can occur efficiently in the formed Hamilton hydrogen bonding assemblies. Generally, the electron transfer requires strong donor-acceptor orbital overlap, and the rate constant decreases exponentially with the increasing of the distance between donor and acceptor. The electron transfer will become inefficient when the donor-accepter distance increases beyond the sum of their Van de Waals radii (5—10 Å) except it occurs via a ‘through-bond’ mechanism in a conjugated or a rigid system.43 The control experiment reveals that the electron transfer process only occur in the Hamilton hydrogen bonding assemblies. Computation from the lowest energy conformation shows that the separation of the porphyrin/zinc porphyrin chromophore and the an-thraquinone group in the Hamilton hydrogen bonding assemblies is about 25 Å, which is much larger than the sum of van der Waals radii of the porphyrin chromo-phore (3.5 Å) and the anthraquinone group (3.1 Å). At such a separation, the electron transfer from the porphy-rin/zinc porphyrin chromophore to the anthraquinone group via a ‘through space’ mechanism would be very inefficient and this is obviously contrary to the experi-ment results. Since the assemblies are not fully rigid, the assemblies could have many conformations in solution. From the molecular modeling shown in Figure 4, we noticed that the separation of the porphyrin chromo-phore and the anthraquinone group (center-to-center) changes from 25 to 5 Å as the assemblies transform from an extended conformation to a folded one. Within the distance of folded conformation, the electron trans-fer could occur, which is in accordance with the photo-physical experiment results. Therefore, the electron transfer in Hamilton hydrogen bonding assemblies might proceed via a ‘through space’ mechanism. When the excitation happens on the porphyrin chromophore in proximity of the anthraquinone group within a folded conformation, the electron transfer occurs directly. If the distance between the donor and acceptor is out of the range for the effective electron transfer, a movement of the excited porphyrin chromophore toward the an-thraquinone within its lifetime will make the electron transfer take place. The rate constant magnitude for the Hamilton hydrogen bonding assemblies are about 107

—

108 s－1, which are compatible with that in other flexible linked porphyrin-anthraquinone systems31,32 via a ‘through space’ mechanism.

Figure 4 Molecular modeling for the ZnTPP-CA/AQ-H assem-bly with extended and folded conformations.

The ‘through bond’ mechanism can be excluded in our designed Hamilton hydrogen bonding assemblies, since it required a rigid system.23 In fact, we tried to clarify whether the hydrogen bonds mediate the electron transfer by switching the donor and acceptor positions at the Hamilton hydrogen bonding interface. Unfortunately, the solvability of the anthraquinone modified cyanuric acid is too low (＜10－5 mol•L－1) in nonpolar solvents for further investigation. On the basis of experimental results mentioned above and the discussion, we propose that the electron transfer in Hamilton hydrogen bonding assemblies TPP-CA/AQ-H and ZnTPP-CA/AQ-H pro-ceeds mainly via a ‘through space’ mechanism.

Conclusions

The selfassembly and the photophysical properties of the supramolecular donor-acceptor dyads, connected via a Hamilton hydrogen bonding motif, TPP-CA/AQ-H and ZnTPP-CA/AQ-H, were investigated. The corre-sponding association constants for both supramolecular assemblies are about (2.8±0.3)×103 mol－1•L deter-mined by fluorescence titration experiments. The singlet electron transfer occurs between the donor and the ac-ceptor within the supramolecular assemblies with rate constants of 7.6×107, 7.0×108 s－1, and efficiencies of 43%, 58% for TPP-CA/AQ-H and ZnTPP-CA/AQ-H, respectively. The intramolecular electron transfer pro-ceeds mainly via a ‘through space’ mechanism. These findings provide a new model for the study of the Ham-ilton hydrogen bonding system, and will contribute to a greater insight in the electron transfer processes occur-ring in biological systems.

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