This article was downloaded by: [Mount Allison University 0Libraries]On: 10 May 2013, At: 09:35Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Molecular Physics: An International Journal at theInterface Between Chemistry and PhysicsPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tmph20

Aromatic pathways in thieno-bridged porphyrins:understanding the influence of the direction of thethiophene ring on the aromatic characterHeike Fliegl a , Nergiz Özcan b , Raul Mera-Adasme b , Fabio Pichierri c , Jonas Jusélius d &Dage Sundholm ba Center for Theoretical and Computational Chemistry (CTCC), Department of Chemistry ,University of Oslo , Oslo , Norwayb Department of Chemistry , University of Helsinki , Finlandc Department of Applied Chemistry, Graduate School of Engineering , Tohoku University ,Sendai , Japand Department of IT , University of Tromsø , Tromsø , NorwayAccepted author version posted online: 10 Apr 2013.Published online: 07 May 2013.

To cite this article: Heike Fliegl , Nergiz Özcan , Raul Mera-Adasme , Fabio Pichierri , Jonas Jusélius & Dage Sundholm(2013): Aromatic pathways in thieno-bridged porphyrins: understanding the influence of the direction of the thiophene ringon the aromatic character, Molecular Physics: An International Journal at the Interface Between Chemistry and Physics,DOI:10.1080/00268976.2013.794397

To link to this article: http://dx.doi.org/10.1080/00268976.2013.794397

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form toanyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses shouldbe independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly inconnection with or arising out of the use of this material.

Molecular Physics, 2013http://dx.doi.org/10.1080/00268976.2013.794397

RESEARCH ARTICLE

Aromatic pathways in thieno-bridged porphyrins: understanding the influence of thedirection of the thiophene ring on the aromatic character

Heike Fliegla,∗, Nergiz Ozcanb, Raul Mera-Adasmeb, Fabio Pichierric, Jonas Juseliusd and Dage Sundholmb,∗

aCenter for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, Oslo, Norway;bDepartment of Chemistry, University of Helsinki, Finland; cDepartment of Applied Chemistry, Graduate School of Engineering, Tohoku

University, Sendai, Japan; dDepartment of IT, University of Tromsø, Tromsø, Norway

(Received 28 January 2013; final version received 19 March 2013)

Magnetically induced current densities have been investigated for some recently synthesised thieno-bridged porphyrins. Theaim of the study is to understand the influence of the direction of the thieno bridge on the aromatic character of the molecules.The calculated ring-current susceptibilities for two tautomers of 2,3- and 3,4-thieno-bridged porphyrins as well as of thecorresponding Zn-containing compounds show that the molecules are all aromatic according to the ring current criterion.The ring-current susceptibilities of 16.2 nA/T and 20.2 nA/T around the porphyrin ring of the two tautomers of 2,3-thieno-bridged porphyrin are somewhat weaker than the ring-current susceptibilities of 22.8 nA/T and 23.8 nA/T obtained for thetwo tautomers of 3,4-thieno-bridged porphyrin. For 2,3-thieno-bridged porphyrin, the positions of the inner hydrogens affectthe ring-current susceptibility more than for 3,4-thieno-bridged porphyrin. The current pathways of the Zn porphyrinoids area superposition of the pathways of the two trans tautomers of the corresponding free-base porphyrinoids. The current-densitycalculations explain the different aromatic character of 2,3-thieno-bridged and 3,4-thieno-bridged porphyrins. Current-density calculations on tetra-2,3-thieno-bridged and tetra-3,4-thieno-bridged porphyrins yielded similar current pathwaysas obtained for mono-substituted thieno-bridged porphyrins. Tetra-2,3-thieno-bridged porphyrin is practically non-aromaticsince it sustains a very weak ring current of 2.6 nA/T. The calculated ring-current susceptibilities for tetra-3,4-thieno-bridgedporphyrin is 13.2 nA/T, which can be compared to the ring-current susceptibilities of 11.8 and 27.5 nA/T for benzene andfree-base porphyrin, respectively.

Keywords: gauge including magnetically induced current densities; aromaticity; aromatic pathways; porphyrins; porphyri-noids; aromatic switches

1. Introduction

The synthesis of expanded porphyrins and porphyrinoidshas undergone an enormous development in the past15 years since the appearance of Jasat and Dolphin’s re-view in 1997 [1–4]. Besides representing useful materialsfor molecular electronics, non-linear optics, biomedical ap-plications, anion recognition and transport, porphyrinoidshave become an important means to investigate the aromaticcharacter of complex multi-ring molecules [5–21]. Tailor-made porphyrinoids with purposefully designed number ofπ -electrons are thought to open the avenue towards thecontrol of the aromatic or antiaromatic character of largering-shaped molecules. The aromaticity studies also in-volve very complicated multi-ring molecules with singlyand doubly twisted Mobius structures of expanded por-phyrins, whose degree of aromaticity and preferred currentpathways are hard to assess with spectroscopic methods[8,22–35]. In this regard, computational methods based onelectronic structure calculations represent a useful tool forperforming quantitative and detailed investigations of themolecular aromaticity of planar and twisted porphyrinoids.

∗Corresponding author. Email: heike.fliegl@kjemi.uio.no

We have developed a computational method to calculategauge-independent magnetically induced current (GIMIC)densities in molecules [36,37]. Calculations of the suscep-tibility of the current density passing selected bonds yieldthe electron-delocalisation pathways that sustain the cur-rent transport around molecular rings. The obtained sus-ceptibility of the ring current can be used as aromatic-ity index for the individual rings including the porphyrinring [14,15]. The ring current pathways are not directlyexperimentally accessible. Instead nuclear magnetic reso-nance (NMR) measurements are often used for estimat-ing the aromatic pathways. GIMIC calculations on the otherhand provide detailed information about the current path-ways in complex molecules with connected molecular rings[14,15,38–42]. Current pathways and ring current suscepti-bilities (current strengths) have previously been calculatedfor porphyrins, chlorins, and bacteriochlorins. The calcu-lations provided novel insights into the aromatic characterof this kind of molecules [43]. The sign and magnitude ofthe ring currents indicate whether molecular rings are aro-matic, antiaromatic or non-aromatic, thus having diatropic,

C© 2013 Taylor & Francis

Dow

nloa

ded

by [

Mou

nt A

lliso

n U

nive

rsity

0L

ibra

ries

] at

09:

35 1

0 M

ay 2

013

2 H. Fliegl et al.

paratropic, or vanishing net ring currents, respectively [44].Diatropic ring currents are defined to generate a magneticfield opposing the applied field, whereas paratropic currentscirculate in the opposite (non-classical) direction strength-ening the external magnetic field [36,45–47]. The GIMIC

method and a variety of its applications have recently beenreviewed [48].

In this work, we employ our GIMIC method for determin-ing the strengths and pathways of the magnetically inducedcurrents of 2,3- and 3,4-thieno-bridged porphyrins, whichhave recently been synthesised by Mitsushige et al. [49].Thieno-bridged porphyrins consist of a porphyrin with athiophene ring substituted to a β carbon (Cβ) of a pyr-role ring and to its nearest meso carbon (Cmeso). A five-membered ring is formed between the thiophene and por-phyrin rings. 2,3- and 3,4-thieno-bridged porphyrins havedifferent orientations of the thiophene moiety with respectto the porphyrin ring. Based on nucleus independent chem-ical shift (NICS) calculations [50], Mitsushige et al. con-cluded that 2,3-thieno-bridged porphyrin with formally a20 π -electron circuit is characterised by a stronger antiaro-matic character than 3,4-thieno-bridged porphyrin with ananticipated 24 π -electron pathway. Our recent GIMIC studyon porphyrins, chlorins and bacteriochlorins showed thatexplicit calculations of the current density are needed to re-liably determine the current pathways in porphynoids [43].Fliegl and Sundholm unambiguously showed that neitherthe 18π [18] annulene picture [51–55], where the innerNH groups act as inert bridges, nor the more recently pro-posed 18π [16] annulene inner cross route [56,57] are thecorrect description of the aromatic pathway of porphyrins.Based on calculations of the aromatic ring current shield-ing functions, the two proposed current pathways involvingmerely 18π electrons have also previously been questionedby Juselius and Sundholm [58–60]. The GIMIC study on free-base porphyrins [43] also showed that current pathways forporphyrinoids cannot be reliably determined by using thepopular NICS method [50].

The paper is structured as follows. In Section 2, the em-ployed computational methods are briefly presented. Themolecular structures and the nomenclature are discussedin Section 3. The calculated current densities, currentstrengths and current pathways are discussed in Section 4.In Section 5, the obtained aromatic character of the indi-vidual molecular rings is discussed and compared to resultspresented in the recent study by Mitsushige et al. [49].

2. Computational methods

The molecular structures were optimised at the densityfunctional theory (DFT) level using the B3LYP functional[61,62] and the Karlsruhe split valence quality basis setsaugmented with polarisation functions (def2-SVP) [63].All the structures investigated here were confirmed to beminima on the basis of vibrational frequency calculations.

Nuclear magnetic shieldings were calculated using theB3LYP functional and triple-ζ quality basis set augmentedwith polarisation functions (def2-TZVP) [64]. The prefixdef2 is omitted hereafter. The molecular structure optimisa-tions and the calculation of the nuclear magnetic shieldingswere performed with TURBOMOLE version 6.4 [65]. Thecomputed chemical shifts agree well with the available ex-perimental data for Zn-2,3- and Zn-3,4-thieno-bridged por-phyrin (see Table S1 in the Supplementary information).

The magnetically induced current densities were calcu-lated using the GIMIC method [36,37]. GIMIC is an indepen-dent programme that uses the magnetically perturbed den-sity matrices from nuclear magnetic shielding calculations,the one-electron density matrix and basis-set informationas input data [36,37]. Accurate gauge-independent currentdensities are obtained already when using the smaller SVPbasis sets, because gauge-including atomic orbitals are em-ployed [66–69]. The ring-current susceptibility (in nA/T),which is denoted in the following as ring-current strength,can be used as a reliable measure of the degree of molecu-lar aromaticity. The current strengths and current pathwaysare obtained by numerical integration of the current den-sity passing through cut planes perpendicularly to selectedbonds of the molecules. All integration planes are placedat the midpoint of the bond of interest. The dimensions forall integration planes have carefully been checked by do-ing a contour plot analysis of the modulus of the currentdensity. The ring-current strength for benzene of 11.8 nA/Tcalculated at the B3LYP/TZVP level can be used as refer-ence value for aromatic molecules. Current-density plotsare generated with JMOL [70] and the current-pathway pic-tures are drawn with GIMP [71].

3. Molecular structures and nomenclature

The DFT-optimised structures of the investigated thieno-bridged porphyrins and the labelling of the sub-rings areshown in Figure 1. The porphyrinoids have either the 2,3or 3,4 carbon–carbon bond of thiophene fused to a β car-bon (Cβ) and its nearest meso carbon of the porphyrin.The fused C–C bond shared by sub-rings 5 and 6 of 2,3-thieno-bridged porphyrin (Figure 1(a)) is slightly elongated(1.40 A) with respect to the 2,3 bond of thiophene (1.37 A),while the fusion does not introduce any significant changesin the lengths of the 1,2 and 3,4 bonds of the thiophene ring.For 3,4-thieno-bridged porphyrin, the C–C bond shared bysub-rings 5 and 6 (Figure 1(b)) is slightly elongated (1.46A) with respect to the same bond in thiophene (1.42 A). Thelongest C–C bonds are those connecting the thiophene ringsto the porphyrin ring. For 2,3-thieno-bridged porphyrin, theC2–C and C3–C bonds are 1.45 and 1.49 A, respectively,and for 3,4-thieno-bridged porphyrin the C3–C and C4–Cbonds are 1.46 and 1.48 A, respectively.

The molecular structures of the two other trans tau-tomers (not shown) were also optimised. The energy

Dow

nloa

ded

by [

Mou

nt A

lliso

n U

nive

rsity

0L

ibra

ries

] at

09:

35 1

0 M

ay 2

013

Molecular Physics 3

Figure 1. The molecular structures of (a) 2,3-thieno-bridged porphyrin, (b) 3,4-thieno-bridged porphyrin, (c) Zn-2,3-thieno-bridgedporphyrin, (d) Zn-3,4-thieno-bridged porphyrin, (e) 4,5-thieno-bridged porphyrin, (f) tetra-2,3-thieno-bridged porphyrin and (g) tetra-3,4-thieno-bridged porphyrin including the labelling of the individual rings.

Dow

nloa

ded

by [

Mou

nt A

lliso

n U

nive

rsity

0L

ibra

ries

] at

09:

35 1

0 M

ay 2

013

4 H. Fliegl et al.

differences between the two tautomers of 13.0 and10.9 kJ/mol for 2,3- and 3,4-thieno-bridged porphyrins, re-spectively, are quite small. Replacing the two inner hydro-gens with a Zn2 + ion yields the metal porphynoids shownin Figure 1(c) and (d), where the Zn–N bond distances rangefrom 2.01 to 2.11 A. As a result of the four coordinationbonds with the metal, the separation between N atoms ofcontiguous pyrrole rings are more uniform and range from2.82 to 2.95 A in comparison to the free-base porphyri-noids, where the N/N separations range from 2.78 to 3.01A. All the DFT-optimised structures are planar. The calcu-lated bond distances display a reasonably good agreementwith those deduced from the crystal structures [49].

We have also investigated the structures of three por-phyrinoids that were not synthesised in the study by Mit-sushige et al. [49]. Figure 1(e) shows the DFT-optimisedstructure for 4,5-thieno-bridged porphyrin, where the sul-phur atom is oriented in the direction opposite to that of2,3-thieno-bridged porphyrin (Figure 1(a)). For 4,5-thieno-bridged porphyrin, the length of the C–C bond shared bysub-rings 5 and 6 is 1.40 A, which is the same as for2,3-thieno-bridged porphyrin. Figure 1(f) and (g) showsthe DFT-optimised structures for tetra-2,3- and tetra-3,4-thieno-bridged porphyrins. The molecular structures oftetra-thieno-bridged porphyrins belong to the C2h pointgroup. The tetra-thieno-bridged porphyrins are investigatedto explore the effect of the number of fused thiophene ringson the magnetically induced currents.

4. Current-density calculations

Current-density calculations were performed on both thio-phene and thieno-bridged porphyrins. Thiophene with 6 π

electrons is known to be aromatic. Current-density calcula-tions at the B3LYP/TZVP level on an isolated thiophenemolecule yielded a ring-current strength of 11.6 nA/T,which can be compared with the ring-current strength of11.8 nA/T for benzene [44]. The current-density plot forthieno-bridged porphyrins shows that diatropic currentsflow at the exterior edge of the molecule. A paratropic cur-rent flows along the interior part of the molecule. For theZn porphyrinoids, a weak diatropic current also circulatesaround the Zn2 + ion.

4.1. 2,3-Thieno-bridged porphyrin

For the first trans tautomer with the inner hydrogens con-nected to the pyrrole rings 1 and 3, the net current strengthcircling around the porphyrin ring is 16.2 nA/T, showingthat 2,3-thieno-bridged porphyrin is globally aromatic. Thecurrent pathway is split into an outer and an inner routeat each of the pyrrole rings. For rings 1 and 3, the cur-rent strengths of 2.8 and 3.7 nA/T along the inner routesare much weaker than the ones of 14.0 and 13.2 nA/T forthe corresponding outer pathways. For the unsubstituted

pyrrole ring without an inner hydrogen (ring 2), the ring cur-rent is almost equally split into current strengths of 7.9 and8.3 nA/T along the inner and outer pathways, respectively.The current pathways at the three pyrrole rings are very sim-ilar to those we recently obtained for free-base porphyrins[43]. The pyrrole ring with the thiophene substituent be-tween Cβ and Cmeso (ring 4) is antiaromatic with a paratropicring current of −9.6 nA/T. The five-membered ring (ring5) formed between the thiophene substituent and the por-phyrin ring sustains an even stronger paratropic ring currentof −23.6 nA/T. The substituted thiophene molecule (ring6) is practically non-aromatic with a weak diatropic ringcurrent of 0.5 nA/T. The current strength of 16.2 nA/T cir-culating around the porphyrin ring is significantly smallerthan the ring-current strength of 27.5 nA/T for free-baseporphyrin [43]. The current pathways and current strengthsare depicted in Figure 2(a). Diatropic currents are marked inwhite and the paratropic pathways are grey. The calculationsshow that the substitution of the 2,3-thieno ring strongly af-fects the current pathways and the current strengths, sincethe substitution significantly reduces the aromaticity of theporphyrin ring.

The second trans tautomer with an inner hydrogen atrings 2 and 4 is slightly more aromatic than the other one,since it sustains a diatropic ring current of 20.2 nA/T aroundthe porphyrin ring. At a first glance, this result does not seemto agree with the trends obtained in our recent porphyrinstudy, where we found that the inner pathways via pyrrolenitrogens with an inner hydrogen have a larger resistancethan when the pyrrole nitrogen does not have any hydro-gen bonded to it [43]. However, the strength of the currentthat passes the pyrrole nitrogen of ring 4 is indeed almost10 nA/T smaller when ring 4 has an inner hydrogen thanwithout it. The resistance of the NH moiety of ring 4 directsthe current to flow along the outer route which results innet diatropic currents along all bonds of ring 4. The ob-tained current pathways are depicted in Figure 2(b). Thering current is split into diatropic inner and outer routesat each of the pyrrole rings similarly as obtained for theunsubstituted free-base porphyrins [43]. The current pass-ing the nitrogen atom of ring 1 is 11.0 nA/T. A currentof 8.8 nA/T passes the outer Cβ=Cβ bond of ring 1. Forring 2, which has an inner hydrogen, the current strengthsalong the inner and outer routes are 5.2 and 14.8 nA/T,respectively. For ring 3, which has no inner hydrogen, thecorresponding current strengths are 9.2 and 10.0 nA/T. Forring 4, a current of 15.9 nA/T passes along the inner routeand 3.6 nA/T takes the outer one via the Cβ=Cβ bond. Thethiophene ring is almost non-aromatic with a diatropic ringcurrent of 1.8 nA/T. The five-membered ring between thepyrrole and thiophene rings is strongly antiaromatic, sinceit sustains a paratropic ring current of −19.1 nA/T. Calcu-lations on 4,5-thieno-bridged porphyrin yielded almost thesame current pathways and current strengths as obtainedfor 2,3-thieno-bridged porphyrin.

Dow

nloa

ded

by [

Mou

nt A

lliso

n U

nive

rsity

0L

ibra

ries

] at

09:

35 1

0 M

ay 2

013

Molecular Physics 5

Figure 2. The calculated current pathways of (a) 2,3-thieno-bridged porphyrin, (b) 2,3-thieno-bridged porphyrin tautomer, (c) 3,4-thieno-bridged porphyrin and (d) 3,4-thieno-bridged porphyrin tautomer. The numbers in black denote integrated current strengths for selectedbonds. The total current strength for the macrorings are (a) 16.2 nA/T, (b) 20.2 nA/T, (c) 22.8 nA/T and (d) 23.8 nA/T. The net diatropiccurrent pathways are coloured white. The pathways of the net paratropic currents are shown in grey. The black arrows indicate the directionof the current flow.

The energy barrier between porphyrin tautomers isknown to be small [72] implying that a thermal averageof the tautomer properties is observed in spectroscopicmeasurements at room temperature. The comparison withexperiment is easier for the corresponding Zn porphyrins,because they have only one tautomer form for the inner partof the porphyrin.

4.2. 3,4-Thieno-bridged porphyrin

The current pathways and ring-current strengths of 3,4-thieno-bridged porphyrin differ from those of 2,3-thieno-bridged porphyrin. However, both molecules sustain diat-ropic ring currents around the porphyrin ring. For the transtautomer of 3,4-thieno-bridged porphyrin with the innerhydrogens at rings 1 and 3, the strength of the ring currentaround the porphyrin ring is 22.8 nA/T. The ring currentis 6.6 nA/T stronger than for 2,3-thieno-bridged porphyrin

and comparable to the ring-current strengths of the trans-chlorin and trans-bacteriochlorin tautomers that sustain thestrongest ring current among the chlorins and bacteriochlo-rins [43]. The second trans tautomer of 3,4-thieno-bridgedporphyrin sustains a diatropic ring current of 23.8 nA/Taround the porphyrin ring. Thus, the two 3,4-thieno-bridgedporphyrin tautomers sustain almost the same ring currentaround the porphyrin ring. Substitution of the thiophenering does not affect the strength of the ring current aroundthe porphyrin ring of 3,4-thieno-bridged porphyrin as muchas it does for 2,3-thieno-bridged porphyrin. The aromaticcharacter of 3,4-thieno-bridged porphyrin is reminiscent ofthat for chlorin with a saturated Cβ=Cβ bond that preventsthe current to take that route. The substituted thiophene alsorestricts the current flow via the Cβ=Cβ bond of ring 4.

The ring current around the macroring of 3,4-thieno-bridged porphyrin is split into inner and outer routes atthe four pyrroles, which also has been obtained for free-

Dow

nloa

ded

by [

Mou

nt A

lliso

n U

nive

rsity

0L

ibra

ries

] at

09:

35 1

0 M

ay 2

013

6 H. Fliegl et al.

Figure 3. The calculated current pathways of (a) Zn-2,3-thieno-bridged porphyrin and (b) Zn-3,4-thieno-bridged porphyrin. The numbersin black denote integrated current strengths for selected bonds. The total current strength for the macrorings are (a) 18.1 nA/T and(b) 23.2 nA/T. The net diatropic current pathways are coloured white. The pathways of the net paratropic currents are shown in grey. Theblack arrows indicate the direction of the current flow.

base porphyrin and other unsubstituted porphyrins that wehave investigated [43]. For the pyrrole rings without aninner hydrogen, the current flow is split into two pathways,the current strength of which are practically equal. Theonly exception is ring 4, which has a strong current of19.2 nA/T along the inner branch and only 3.3 nA/T takesthe outer route. The current strength along the outer pathwayis generally stronger than the inner one, when a hydrogen isattached to the pyrrole nitrogen. For ring 1 and 3, the ringcurrent along the outer pathway is about 10 nA/T strongerthan via the inner one when they have an inner hydrogen.For ring 2 with an inner hydrogen, the outer route sustains acurrent that is 7.3 nA/T stronger than for the inner branch.

Ring 5, which lies between ring 4 and the thiophenering, sustains paratropic currents of −8.1 and −6.2 nA/Tfor 3,4-thieno-bridged porphyrin tautomers. The currentstrengths are much smaller than the local paratropic ring-current strengths of −23.6 and −19.0 nA/T in ring 5 of thetwo 2,3-thieno-bridged porphyrin tautomers. In 3,4-thieno-bridged porphyrins, the substituted thiophene ring (ring 6)is practically non-aromatic for both tautomers with currentstrengths of 1.0 and 2.3 nA/T, respectively. The calculatedring-current flow and the strength of the currents passingchemical bonds are shown in Figure 2(c) and (d). Diat-ropic currents are marked in red and the paratropic ones ingreen.

4.3. Zn-2,3-thieno-bridged porphyrinand Zn-3,4-thieno-bridged porphyrin

The metal substitution of the free-base porphyrins doesnot significantly affect the current pathways and cur-rent strengths shown in Figure 3. For Zn-2,3-thieno-bridged porphyrin, the ring current around the porphyrin

ring is 18.1 nA/T as compared to the average value of18.2 nA/T for the corresponding free-base porphyrinoids.Ring 5 is strongly antiaromatic, since it sustains a current of−20.8 nA/T, which leads to a weakening of the diatropiccurrents circling around the porphyrin ring. For 2,3-thieno-bridged porphyrin, the paratropic current circling aroundring 4 decreases when Zn or a hydrogen is bound to thepyrrole nitrogen of ring 4. For Zn-2,3-thieno-bridged por-phyrin, ring 4 is weakly antiaromatic as it sustains a parat-ropic current of −1.7 nA/T.

Zn-3,4-thieno-bridged porphyrin sustains a ring cur-rent of 23.2 nA/T around the macroring, which can becompared to the average value of 23.3 nA/T for the twotrans-3,4-thieno-bridged porphyrins. The diatropic currentflow around ring 4 of 8.5 nA/T is not affected by the Zn sub-stitution. A paratropic current of −8.3 nA/T circles aroundring 5 and weakens the current of the porphyrin ring. ForZn-3,4-thieno-bridged porphyrin, insertion of the thiophenemoiety leads to formation of an antiaromatic ring 5, whosecurrent strength of −8.3 nA/T is much weaker than theone of −20.8 nA/T for Zn-2,3-thieno-bridged porphyrin.The current pathways for 3,4-thieno-bridged porphyrin arealmost unaffected by Zn substitution. Comparison of thecurrent pathways and current strengths of Zn-complexedthieno-bridged porphyrins and the corresponding free-baseporphyrinoids shows that the currents of the Zn-complexesbehave as a superposition of the two free-base tautomers.The small deviations can to some extent be explained bythe diatropic current that circulates around the metal ion.

A very accurate determination of the current strengthsfor the Zn-complexes is difficult due to the congested cur-rent flows in the vicinity of the Zn atom. For example, asmall current of 0.5–1.3 nA/T that circulates around the Znion might explain the discrepancy of 1.5 nA/T between the

Dow

nloa

ded

by [

Mou

nt A

lliso

n U

nive

rsity

0L

ibra

ries

] at

09:

35 1

0 M

ay 2

013

Molecular Physics 7

Figure 4. The calculated current pathways of (a) tetra-2,3-thieno-bridged porphyrin and (b) tetra-3,4-thieno-bridged porphyrin. Thenumbers in black denote integrated current strengths for selected bonds. The total current strength for the macrorings are (a) 2.6 nA/T and(b) 13.2 nA/T. The net diatropic current pathways are coloured white. The pathways of the net paratropic currents are shown in grey. Theblack arrows indicate the direction of the current flow.

current strengths of the porphyrin ring that passes the mesocarbons.

4.4. Tetra-2,3-thieno-bridged porphyrin

Current-density calculations on thieno-bridged porphyrinssuggest that tetra-substituted thieno-bridged porphyrinsmight have interesting aromatic characters. At a first glance,the current pathways of tetra-2,3-thieno-bridged porphyrinis very reminiscent of the ones for 2,3-thieno-bridgedporphyrin tautomers. However, by also considering thestrengths of the currents that pass through chemical bonds,a more complete picture is obtained. The ring-currentstrength around the macroring of tetra-2,3-thieno-bridgedporphyrin is only 2.6 nA/T, which is 14 nA/T smaller thanfor 2,3-thieno-bridged porphyrin. Thus, the porphyrin ringof tetra-substituted 2,3-thieno-bridged porphyrin is almostnon-aromatic. The aromatic character of the pyrrole ringwith the thiophene substituent depends on whether the ni-trogen has an inner hydrogen or not as also obtained forthe two tautomers of 2,3-thieno-bridged porphyrin. For thetetra-substituted porphyrinoids, the current strengths aregenerally weaker than for the mono-substituted ones. Theonly exception is thiophene, which are more aromatic intetra-substituted porphyrinoids. The substitution of four2,3-thieno moieties does not lead to changes in the aro-matic character of the porphyrin ring. It merely introducesantiaromatic sub-rings and weakens the diatropic ring cur-rent around the porphyrin ring. The calculated current path-

ways of the tetra-substituted thieno-bridged porphyrins aredepicted in Figure 4.

4.5. Tetra-3,4-thieno-bridged porphyrin

The current pathways in tetra-3,4-thieno-bridged por-phyrin do not significantly differ from those obtainedfor tetra-2,3-thieno-bridged porphyrin tautomers. However,the strength of the ring-current around the macrocycleis five times larger for tetra-3,4-thieno-bridged porphyrinthan for tetra-2,3-thieno-bridged porphyrin, but the currentstrengths are generally weaker than for the correspond-ing mono-substituted compounds. Tetra-3,4-thieno-bridgedporphyrin is aromatic, since the strength of the current cir-culating around the porphyrin ring is 13.2 nA/T, which is10 nA/T smaller than for mono-substituted 3,4-thieno-bridged porphyrin but of the same size as for benzene.

5. Discussion and conclusions

Mitsushige et al. recently synthesised and spectroscopicallycharacterised thieno-bridged porphyrins [49]. The experi-mental study was supplemented by calculations of NICSsand the anisotropy of the magnetically induced current den-sity using the gauge-dependent approach of Herges et al.[50,73]. Based on the results obtained in the combined com-putational and experimental study, Mitsushige et al. con-cluded that 2,3-thieno-bridged porphyrin shows a largerantiaromatic contribution than 3,4-thieno-bridged por-phyrin [49]. Their explanation was that the aromaticity of

Dow

nloa

ded

by [

Mou

nt A

lliso

n U

nive

rsity

0L

ibra

ries

] at

09:

35 1

0 M

ay 2

013

8 H. Fliegl et al.

thieno-bridged porphyrins can be divided into global aro-matic and antiaromatic contributions, whose mutual extentcan be adjusted by the direction of the substituted thio-phene ring. The explanation is based on an idea that waspreviously proposed to hold for pyracylene [74].

To assess the mechanism proposed by Mitsushige et al.,we calculated gauge-independent magnetically inducedcurrent densities for the thieno-bridged porphyrins with ourGIMIC method. Calculations of current pathways and currentstrengths provide a deep understanding of the mechanismsbehind the difference in the aromatic character and the de-gree of aromaticity of the thieno-bridged porphyrins. Thecalculations yield accurate information about the currentpathways and quantitative values for the current strengthspassing selected bonds. Thus, the mechanism that emergesfrom the present GIMIC analysis is somehow more complexthan the one proposed by Mitsushige et al. [49].

The current-density calculations show that the five-membered ring between the thiophene and porphyrin ringssustains a much stronger paratropic ring current in 2,3-thieno-bridged porphyrin than in 3,4-thieno-bridged por-phyrin. The large bond-length alternation (BLA) in ring 5of 2,3-thieno-bridged porphyrin implies that ring 5 can beexpected to sustain a stronger paratropic ring current thanin 3,4-thieno-bridged porphyrin, according to the harmonicoscillator model of aromaticity [75]. The BLA of ring 5can be traced back to the orientation of the thiophene ring,because the joint bond between ring 5 and ring 6 of 2,3-thieno-bridged porphyrin is the short formal double bondof thiophene, whereas for 3,4-thieno-bridged porphyrin thecommon bond is the significantly longer C3–C4 bond ofthiophene. The C–C bonds between the porphyrin and thio-phene rings are of the same length as the C3–C4 bondof thiophene. For 3,4-thieno-bridge porphyrin, the BLAof ring 5 is thus much smaller than for 2,3-thieno-bridgeporphyrin.

The current pathways obtained in the calculations can beunderstood when one assumes that the currents can be seenas a stream of electrons. The strong paratropic stream flow-ing around ring 5 can incorporate ring 4, when the diatropiccurrent of the porphyrin ring along the outer pathway of ring4 is weakened. For the tautomer of 2,3-thieno-bridged por-phyrin that lacks an inner hydrogen at ring 4, the diatropiccurrent of the porphyrin ring prefers to flow along the in-ner route. The paratropic current from ring 5 is, therefore,able to continue and incorporate ring 4. For the other tau-tomer, the inner hydrogen reduces the diatropic flow alongthe inner route as for free-base porphyrin, implying that thediatropic current along outer pathway becomes strongerthan the paratropic current originating from ring 5. For 3,4-thieno-bridged porphyrin, the paratropic current of ring 5is too weak to overcome the diatropic current flow alongthe outer route of ring 4. The insertion of the Zn2 + ioninto the free-base 2,3- and 3,4-thieno-bridged porphyrinsdoes not lead to a significant change of the current path-

ways and current strengths. Thus, the same explanation aspresented for the free-base porphyrinoids also holds for theZn-substituted ones.

The strong paratropic current of ring 5 prevents thering-current of the porphyrin ring to take the outer path-way at ring 4. Therefore, thiophene substitution reducesthe strength of the ring-current of the porphyrin ring anal-ogously as saturation of the Cβ=Cβ double bond does forporphyrins [43].

Supporting information

Electronic supplementary information (ESI) available:Cartesian coordinates, nuclear magnetic shieldings of thestudied molecules and some representative current densityplots.

AcknowledgementsThis research has been supported by the Academy of Fin-land through its Computational Science Research Programme(LASTU). We thank Tohoku University and Magnus Ehrnroothfoundation for financial support. CSC – the Finnish IT Centerfor Science – is acknowledged for computer time. H. F. thanksfor support by the Norwegian Research Council through the CoECentre for Theoretical and Computational Chemistry (Grant No.179568/V30). This work has received support from the NorwegianSupercomputing Program (NOTUR) through a grant of computertime (Grant No. NN4654K).

References[1] A. Jasat and D. Dolphin, Chem. Rev. 97, 2267 (1997).[2] T.K. Chandrashekar and S. Venkatraman, Acc. Chem. Res.

36(9), 676 (2003).[3] J.L. Sessler and E. Tomat, Acc. Chem. Res. 40(5), 371

(2007).[4] R. Misra and T.K. Chandrashekar, Acc. Chem. Res. 41(2),

265 (2008).[5] A. Osuka and S. Saito, Chem. Commun. 47, 4330 (2011).[6] T. Higashino, J.M. Lim, T. Miura, S. Saito, J.Y. Shin, D. Kim,

and A. Osuka, Angew. Chem. Int. Ed. 49, 1 (2010).[7] J.Y. Shin, K.S. Kim, M.C. Yoon, J.M. Lim, Z.S. Yoon, A.

Osuka, and D. Kim, Chem. Soc. Rev. 39, 2751 (2010).[8] Z.S. Yoon, A. Osuka, and D. Kim, Nat. Chem. 1, 113 (2009).[9] J. Sankar, S. Mori, S. Saito, H. Rath, M. Suzuki, Y. Inokuma,

H. Shinokubo, K.S. Kim, Z.S. Yoon, J.Y. Shin, J.M. Lim, Y.Matsuzaki, O. Matsushita, A. Muranaka, N. Kobayashi, D.Kim, and A. Osuka, J. Am. Chem. Soc. 130, 13568 (2008).

[10] T.K. Ahn, J.H. Kwon, D.Y. Kim, D.W. Cho, D.H. Jeong, S.K.Kim, M. Suzuki, S. Shimizu, A. Osuka, and D. Kim, J. Am.Chem. Soc. 127, 12856 (2005).

[11] S. Saito, J.Y. Shin, J.M. Lim, K.S. Kim, D. Kim, and A.Osuka, Angew. Chem. Int. Ed. 47, 9657 (2008).

[12] Y. Tanaka, S. Saito, S. Mori, N. Aratani, H. Shinokubo, N.Shibata, Y. Higuchi, Z.S. Yoon, K.S. Kim, S.B. Noh, J.K.Park, D. Kim, and A. Osuka, Angew. Chem. Int. Ed. 47, 681(2008).

[13] J.Y. Shin, K.S. Kim, M.C. Yoon, J.M. Lim, Z.S. Yoon, A.Osuka, and D. Kim, Chem. Soc. Rev. 39, 2751 (2010).

[14] H. Fliegl, D. Sundholm, S. Taubert, and F. Pichierri, J. Phys.Chem. A 114, 7153 (2010).

[15] H. Fliegl, D. Sundholm, and F. Pichierri, Phys. Chem. Chem.Phys. 13, 20659 (2011).

Dow

nloa

ded

by [

Mou

nt A

lliso

n U

nive

rsity

0L

ibra

ries

] at

09:

35 1

0 M

ay 2

013

Molecular Physics 9

[16] H.S. Rzepa, Org. Lett. 10, 949 (2008).[17] C.S.M. Allan and H.S. Rzepa, J. Org. Chem. 73, 6615

(2008).[18] E. Steiner and P.W. Fowler, Org. Biomol. Chem. 4, 2473

(2006).[19] E. Steiner and P.W. Fowler, Org. Biomol. Chem. 2, 34 (2004).[20] J. Aihara, Y. Nakagami, R. Sekine and M. Makino, J. Phys.

Chem. A 116, 11718 (2012).[21] M. Makino and J. Aihara, J. Phys. Chem. A 116, 8704 (2012).[22] J. Setsune and S. Maeda, J. Am. Chem. Soc. 122, 12405

(2000).[23] J.Y. Shin, H. Furuta, S. Igarashi, and A. Osuka, J. Am. Chem.

Soc. 123, 7190 (2001).[24] N. Sprutta and L. Latos-Grazynski,Chem. Eur. J. 7, 5099

(2001).[25] J. Setsune, Y. Katakami, and N. Iizuna, J. Am. Chem. Soc.

121, 8957 (1999).[26] M. St

↪epien, B. Szyszko, and L. Latos-Grazynski,Org. Lett.

11, 3930 (2009).[27] M. St

↪epien, L. Latos-Grazynski, N. Sprutta, P. Chwalisz, and

L. Szterenberg, Angew. Chem. Int. Ed. 46, 7869 (2007).[28] M. St

↪epien, B. Szyszko, and L. Latos-Grazynski,J. Am.

Chem. Soc. 132, 3140 (2010).[29] M. St

↪epien, N. Sprutta, and L. Latos-Grazynski,Angew.

Chem. Int. Ed. 50, 4288 (2011).[30] S.M. Rappaport and H.S. Rzepa, J. Am. Chem. Soc. 130,

7613 (2008).[31] N. Jux, Angew. Chem. Int. Ed. 47, 2543 (2008).[32] C.S. Wannere, H.S. Rzepa, B.C. Rinderspacher, A. Paul,

C.S.M. Allan, H.F. Schaefer, III, and P. von Rague Schleyer,J. Phys. Chem. A 113, 11619 (2009).

[33] M. St↪epien and L. Latos-Grazynski, in Topics in Hetero-

cyclic Chemistry, Vol. 19, Aromaticity in Heterocyclic Com-pounds, edited by Tadeusz Krygowski and Michal Cyranski(Springer Berlin, Heidelberg, 2009), pp. 83–153.

[34] M. Broring, Angew. Chem. Int. Ed. 50, 2436 (2011).[35] J. Aihara and M. Makino, Org. Biomol. Chem. 8, 261 (2010).[36] J. Juselius, D. Sundholm, and J. Gauss, J. Chem. Phys. 121,

3952 (2004).[37] S. Taubert, D. Sundholm, and J. Juselius, J. Chem. Phys.

134, 054123:1 (2011).[38] M.P. Johansson, J. Juselius, and D. Sundholm, Angew. Chem.

Int. Ed. 44, 1843 (2005).[39] J. Juselius and D. Sundholm, Phys. Chem. Chem. Phys. 10,

6630 (2008).[40] Y.C. Lin, J. Juselius, D. Sundholm, and J. Gauss, J. Chem.

Phys. 122, 214308:1 (2005).[41] S. Taubert, D. Sundholm, and F. Pichierri, J. Org. Chem. 74,

6495 (2009).[42] M. Kaipio, M. Patzschke, H. Fliegl, F. Pichierri, and D.

Sundholm, J. Phys. Chem. A 116, 10257 (2012).[43] H. Fliegl and D. Sundholm, J. Org. Chem. 77, 3408 (2012).[44] H. Fliegl, D. Sundholm, S. Taubert, J. Juselius, and W. Klop-

per, J. Phys. Chem. A 113, 8668 (2009).[45] R. Zanasi, P. Lazzeretti, M. Malagoli, and F. Piccinini, J.

Chem. Phys. 102, 7150 (1995).

[46] P. Lazzeretti, Prog. Nucl. Magn. Reson. Spectrosc. 36(1), 1(2000).

[47] J.A.N.F. Gomes and R.B. Mallion, Chem. Rev. 101, 1349(2001).

[48] H. Fliegl, S. Taubert, O. Lehtonen, and D. Sundholm, Phys.Chem. Chem. Phys. 13, 20500 (2011).

[49] Y. Mitsushige, S. Yamaguchi, B.S. Lee, Y.M. Sung, S. Kuhri,C.A. Schierl, D.M. Guldi, D. Kim, and Y. Matsuo, J. Am.Chem. Soc. 134, 16540 (2012).

[50] P. von Rague Schleyer, C. Maerker, A. Dransfeld, H. Jiao,and N.J.R. van Eikema Hommes, J. Am. Chem. Soc. 118,6317 (1996).

[51] E. Vogel, W. Haas, B. Knipp, J. Lex, and H. Schmickler,Angew. Chem. Int. Ed. 27, 406 (1988).

[52] E. Vogel, J. Heterocycl. Chem. 33, 1461 (1996).[53] T.D. Lash and S.T. Chaney, Chem. Eur. J. 2, 944 (1996).[54] T.D. Lash, J.L. Romanic, J. Hayes, and J.D. Spence, Chem.

Comm. 819 (1999).[55] E. Steiner and P.W. Fowler, Chem. Phys. Chem. 3, 114

(2002).[56] M.K. Cyranski, T.M. Krygowski, M. Wisiorowski, N.J.R.

van Eikema Hommes, and P. von Rague Schleyer, Angew.Chem. Int. Ed. 37, 177 (1998).

[57] J.I. Wu, I. Fernandez, and P. von Rague Schleyer, J. Am.Chem. Soc. 135, 315 (2013).

[58] J. Juselius and D. Sundholm, Phys. Chem. Chem. Phys. 2,2145 (2000).

[59] J. Juselius and D. Sundholm, Phys. Chem. Chem. Phys. 1,3429 (1999).

[60] J. Juselius and D. Sundholm, J. Org. Chem. 65, 5233(2000).

[61] A.D. Becke, J. Chem. Phys. 98, 5648 (1993).[62] C. Lee, W. Yang, and R.G. Parr, Phys. Rev. B 37, 785 (1988).[63] A. Schafer, H. Horn, and R. Ahlrichs, J. Chem. Phys. 97,

2571 (1992).[64] F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys. 7,

3297 (2005).[65] R. Ahlrichs, M. Bar, M. Haser, H. Horn, and C. Kolmel,

Chem. Phys. Lett. 162, 165 (1989), current version: see<http://www.turbomole.com>.

[66] F. London, J. Phys. Radium 8, 397 (1937).[67] H.F. Hameka, Mol. Phys. 1, 203 (1958).[68] R. Ditchfield, Mol. Phys. 27, 789 (1974).[69] K. Wolinski, J.F. Hinton, and P. Pulay, J. Am. Chem. Soc.

112, 8251 (1990).[70] JMOL: An Open-source Java Viewer for Chemical Structures

in 3D. <http://www.jmol.org>.[71] GIMP: GNU Image Manipulation Program.

<http://www.gimp.org>.[72] D. Sundholm, H. Konschin, and M. Haser, Chem. Eur. J. 5,

267 (1999).[73] D. Geuenich, K. Hess, F. Kohler, and R. Herges, Chem. Rev.

105, 3758 (2005).[74] E. Steiner and P.W. Fowler, J. Phys. Chem. A 105, 9553

(2001).[75] T.M. Krygowski, Chem. Inf. Comput. Sci. 33, 70 (1993).

Dow

nloa

ded

by [

Mou

nt A

lliso

n U

nive

rsity

0L

ibra

ries

] at

09:

35 1

0 M

ay 2

013