Tetrathiafulvalene-Fused Porphyrins via Quinoxaline Linkers: Symmetric and Asymmetric Donor-Acceptor Systems

Tetrathiafulvalene-Fused Porphyrins via Quinoxaline Linkers: Symmetric and Asymmetric Donor–Acceptor Systems

Hongpeng Jia,

Belinda Schmid,

Shi-Xia Liu,*

Michael Jaggi,

Philippe Monbaron,

Sheshanath V. Bhosale ,*

Shadi Rivadehi,

Steven J. Langford,

Lionel Sanguinet,

Eric Levillain,

Mohamed E. El-Khouly,

Ysushi Morita,

Shunichi Fukuzumi ,*

and Silvio Decurtins

1. Introduction

There is considerable interest in developing multi-chromophor- ic arrays as a way to gain insight into the fundamental aspects of energy- and electron-transfer processes and to mimic multi- step charge-separation processes in natural photosynthesis and ultimately organic-based solar energy conversion technol- ogies.^[1]Porphyrins (P) are prominent molecular components of such donor–acceptor (D–A) arrays and have found widespread use in the development of artificial light-harvesting antennae,^[2]

photonic wires,^[3] redox switches,^[4]efficient sensitizers in dye- sensitized solar cells^[5] and molecular shuttles.^[6] A wide range of electron acceptor- or donor-linked porphyrins have been in- tensively studied.^{[1, 7–10]} Among them, however, only a few papers on molecular systems that include the well-known elec- tron donor tetrathiafulvalene (TTF) as an annulated moiety to the porphyrin core have appeared in the literature,^{[10, 11]} per- haps due to synthetic difficulties. In all cases, the TTF unit acts as an electron donor and the porphyrin core as an electron ac- ceptor leading to photoinduced intramolecular charge transfer from the TTF donor to the porphyrin chromophore. However, no TTF-fused porphyrin conjugates in which two porphyrin rings are directly annulated to the central TTF core throughp linkers have been reported.

Based on our experience in the functionalization of TTF^[12]

and our keen interest in molecular (opto)electronic devices, we A tetrathiafulvalene (TTF) donor is annulated to porphyrins (P) via quinoxaline linkers to form novel symmetric P–TTF–P triads 1 a–cand asymmetric P–TTF dyads 2 a,bin good yields. These planar and extended p-conjugated molecules absorb light over a wide region of the UV/Vis spectrum as a result of addi- tional charge-transfer excitations within the donor–acceptor assemblies. Quantum-chemical calculations elucidate the nature of the electronically excited states. The compounds are electrochemically amphoteric and primarily exhibit low oxida- tion potentials. Cyclic voltammetric and spectroelectrochemical studies allow differentiation between the TTF and porphyrin sites with respect to the multiple redox processes occurring within these molecular assemblies. Transient absorption meas- urements give insight into the excited-state events and deliver

corresponding kinetic data. Femtosecond transient absorption spectra in benzonitrile may suggest the occurrence of fast charge separation from TTF to porphyrin in dyads 2 a,b but not in triads1 a–c. Clear evidence for a photoinduced and rela- tively long lived charge-separated state (385 ps lifetime) is ob- tained for a supramolecular coordination compound built from the ZnP–TTF dyad and a pyridine-functionalized C₆₀ acceptor unit. This specific excited state results in a (ZnP–TTF)^·+

···(C₆₀py)^· state. The binding constant of Zn^II···py is evaluated by constructing a Benesi–Hildebrand plot based on fluores- cence data. This plot yields a binding constant K of 7.20 10⁴m¹, which is remarkably high for bonding of pyridine to ZnP.

[a]Dr. H. Jia, B. Schmid, Dr. S.-X. Liu, M. Jaggi, P. Monbaron, Prof. S. Decurtins Departement fr Chemie und Biochemie

Universitt Bern, Freiestrasse 3 3012 Bern (Switzerland) Fax: (+41) 31-631-3995 E-mail: liu@iac.unibe.ch

[b]Dr. S. V. Bhosale ,⁺S. Rivadehi, Prof. S. J. Langford School of Chemistry, Monash University Clayton, VIC-3800 (Australia) E-mail: bsheshanath@gmail.com [c] Dr. L. Sanguinet, Dr. E. Levillain

Institut des Sciences et Technologies Molculaires d’Angers Universit d’Angers, CNRS UMR 6200

2 Bd Lavoisier, 49045 Angers Cedex (France) [d]Dr. M. E. El-Khouly, Prof. S. Fukuzumi

Department of Material and Life Science Graduate School of Engineering, Osaka University Suita, Osaka 565-0871 (Japan)

E-mail: fukuzumi@chem.eng-osaka-u.ac.jp [e] Prof. Y. Morita

Department of Chemistry

Graduate School of Science, Osaka University

1-1 Machikaneyama, Toyonaka, Osaka-560-0043 (Japan) [f]Prof. S. Fukuzumi

Department of Bioinspired Science Ewha Womans University Seoul 120-750 (Korea)

[⁺] Current address: School of Applied Sciences, RMIT University GPO Box 2476V, Melbourne, VIC-3100 (Australia)

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201200350.

aimed for the synthesis and electrochemical and spectroscopic investigations of TTF-annulated free-base and metal(II) por- phyrins 1 and 2, fused via quinoxaline linker(s), as shown in Figure 1. Novel symmetric P–TTF–P triads1 a–cand asymmet- ric P–TTF dyads 2 a,b have been prepared, which exhibit almost planar molecular geometries. The reference compound quinoxalinoporphyrinR3acts as an especially interesting build- ing block for creating larger multiporphyrin arrays, proposed either as molecular wires or as functional mimics of plant and bacterial photosynthetic reaction centres.^[13] Incorporation of a strongly electron donating TTF substituent into porphyrin(s) would allow fine-tuning of electronic properties of these tar- gets. Herein, the electronic structures of1and2in the ground and electronically excited states were studied by photophysical methods (steady-state and transient), cyclic voltammetry and spectroelectrochemical experiments as well as quantum-chemi- cal calculations. In addition, a photoinduced charge-separated state of a supramolecular assembly of asymmetric P–TTF dyad 2 bwith a functionalized C₆₀acceptor unit was investigated.

2. Results and Discussion

2.1. Synthesis and Characterization

In brief, target compounds 1 and 2 were obtained by direct condensation reaction of TTF precursors3^[14]and4^[15]with the corresponding porphyrin-2,3-diones 5^[16c] and 6^[16] (Scheme 1).

Compounds 1 a and 1 c were produced in the presence of acetic acid at reflux as brownish red powders in 74 and 77 % yield, respectively. Compound 2 a was synthesized in CHCl₃/ pyridine at 658C for 2 h as a greenish brown crystalline solid in 96 % yield. Finally, zinc complexes1 band2 bwere obtained in

good yields by metallation of the corresponding 1 a and 2 a with Zn(OAc)₂.

All new compounds were purified by chromatographic sepa- ration on silica gel and fully characterized. IR spectra of the products showed no carbonyl stretches at about 1725 and 1738 cm¹, indicative of the absence of any remaining carbonyl groups of the a-dione groups of 5^[17] and 6,^[16c] respectively.

1H NMR and mass spectra as well as elemental analyses unam- biguously indicate that the synthesized compounds are in ac- cordance with the predicted molecular structures.

In case of the asymmetric P–TTF dyad, crystals suitable for X-ray crystallography were grown by diffusion of hexane into a solution of 2 a in CHCl₃. The molecular structure of 2 a is shown in Figure 2. The molecules are slightly corrugated along the long axis of thep-conjugated skeleton. In the crystal struc- ture, they stack in an almost perfect alignment along the P–TTF axis in an alternating head-to-tail fashion with the TTF group of one molecule overlying the porphyrin unit of another one (Supporting Information Figure S1).

2.2. Electrochemistry

The electrochemical properties of compounds1and2were in- vestigated by cyclic voltammetry in CH₂Cl₂. Their electrochemi- cal data are collected in Table 1 together with those of the ref- erence compoundsR–R3for comparison.

Compounds 1 a–c undergo several reversible multi-electron oxidation processes for successive oxidation of both the cen- tral TTF core and two porphyrin rings. As shown in Figure 3, such complex and broad patterns of redox waves are indica- tive of sequential overlapping oxidations. By simple inspection alone, a comprehensive interpretation of these oxidation pro- cesses is not straightforward. Therefore, thin-layer cyclic vol-

Figure 1.Structures of investigated compounds1and2and reference compoundsR–R3.

tammetry (TLCV) experiments were performed. However, only the TLCV of 1 c(Figure 4) provides clear evidence of the ratio of number of electrons involved in each oxidation step. Clearly, 1 c exhibits two two-electron and one single-electron oxida- tions, which are assigned to the two stable successive oxida- tion states of the two porphyrin rings at 0.44 and 0.75 V and one oxidation state of the TTF core at 0.61 V, respectively, by comparison with the redox data of reported porphyrin deriva- tives such asR2^[18]andR3as well as TTF reference compound R1.^[14]In the cases of1 aand1 b, the instability of the radicals at low scan rates does not allow us to observe well-defined redox processes. Since insertion of metal(II) ions into the por- phyrin core leads to negative shifts of redox potentials,^{[18, 19]}

the first oxidation process for1 aand1 bis anticipated for si-

multaneous oxidation of the two porphyrin rings to the radical cation species. The broadness of the second waves suggests that the first oxidation of the central TTF core to generate the radical cation overlaps with the second simultaneous oxidation of two prophyrin radical cations to their dication states. Com- pared to reference compoundR3, the first oxidation potential of1 ais shifted by 120 mV. This finding can be attributed to the stabilizing effect of the extended p conjugation, which offers the possibility of delocalizing the free spin over the entire conjugatedpsystem, and, importantly, also to the elec- Scheme 1.Synthesis of TTF-annulated porphyrins1and2.

Figure 2.Molecular structure of2 awith 50 % thermal ellipsoids and hydro- gen atoms as spheres of arbitrary size.

Table 1.Redox potentials [V vs. Fc/Fc⁺] of compounds1and2in CH2Cl2

and of reference compoundsR1,^[14]R2,^[18a]R3andR^[11](2 awith decyl in- stead of propyl chains).

Compound E₁₌₂^ox1 E₁₌₂^ox2 E^ox3₁₌₂ E^ox4₁₌₂ E₁₌₂^red1 E^red2₁₌₂

1 a 0.47^[a] 0.75^[b] 1.62 1.79

1 b 0.34^[a] 0.56^[b] 1.71 2.06

1 c 0.44^[a] 0.61 0.75^[a] 1.64 2.06

2 a 0.15 0.60^[a] 0.85 1.58 1.77

2 b 0.19 0.48 0.64^[a] 1.03 1.76^[c]

R^[d] 0.11 0.50 0.80 1.04 1.44 1.77

R1 0.57 0.89 1.93

R2a^[e] 0.53^[a] 1.59 1.79

R2b^[e] 0.26 0.64^[c] 1.77 2.21

R3 0.59 0.74 1.61 1.81

[a] Two overlapping one-electron redox processes. [b] Broad peak. [c] Irre- versible. [d] Reported potentials relative to Ag/AgCl in CH₂Cl₂, which are converted to Fc/Fc⁺ scale by subtracting 0.51 V from values relative to Ag/AgCl. [e] Reported potentials relative to SCE in CH2Cl2, which are con- verted to Fc/Fc⁺scale by subtracting 0.46 V from SCE values.^[20]

tron-donating effect of the TTF moiety. Additional strong sup- port for these assignments is given by spectroelectrochemical data (see below).

In contrast, the first oxidation process of compounds2 aand 2 b corresponds to oxidation of the TTF unit to the radical cation species, by comparison withRandR3(see Table 1). As illustrated in Figure 5, differences clearly exist between metal- free2 aand metallated2 bin the broadness of the subsequent oxidations. For2 a, the overlapping oxidations of the TTF radi- cal cation and the porphyrin core to its trication form give rise to a broad redox wave, followed by the third one-electron oxi- dation of the porphyrin radical to generate its tetracation spe- cies at 0.85 V. For2 b, upon metallation, a negative shift in the oxidation potential centred at the porphyrin core leads to a one-electron oxidation at 0.48 V. The sequential oxidations of both TTF and porphyrin radical cations are not separated in

their potentials, and thus show an apparent one-step conver- sion of the dication to its tetracation species.

In the negative potential direction, all of them exhibit two reduction processes, corresponding to reduction of the por- phyrin core(s) in analogy to reference compoundsR–R3.

2.3. Steady-State Optical Absorption Measurements

Structurally symmetric compound 1 a, which forms a red- purple solution, exhibits intense electronic absorptions over the whole visible part of the optical spectrum at all energies higher than 16 000 cm¹ (below 625 nm, Figure 6 a). This ab- sorption pattern does not correspond solely to the sum of the optical absorptions of the constituents of this molecule. In- stead of the distinct and narrow Soret band of porphyrin sys- tems, which typically appears around 24 000 cm¹ (400–

450 nm), multiple and extensively broadened absorption bands show up. This observation is apparently an outcome of the annulation ofp-conjugated molecular units with different electron donor and acceptor characteristics into the predomi- nantly planar structure of 1 a, which finally contributes to the occurrence of additional electronic charge-transfer (CT) transi- tions. As is discussed below, the electronic structure of1 acan thus be represented as an A–A’–D–A’–A system, which is a useful description for analysis of the different CT characteris- tics. The electronic absorption spectrum of the structurally asymmetric2 a (Figure 6 b), now an electronic A–A’–D system, exhibits an analogous pattern, but the dominant contributions from charge-transfer transitions do not show up all that much.

The same trend holds also for the next even smaller fragment, namely, the quinoxalino[2,3-b]tetraphenylporphyrin R3 (Fig- ure 6 c).

2.4. Quantum-Chemical Calculations

Quantum-chemical calculations were carried out to determine energies, intensities and type of electronic excitations of 1 a, 2 aand reference compoundR3for comparison. The molecular structure of quinoxalino[2,3-b]tetraphenylporphyrinR3was cal- Figure 3.Cyclic voltammograms of1 a(solid line),1 b(dashed line) and1 c

(dotted line) in CH2Cl2(0.1mBu4NPF6; Pt working electrode; scan rate 100 mV s¹).

Figure 4.TLCV of1 c(4.3 10⁵m) in CH2Cl2(0.1mBu4NPF6; Pt working elec- trode ; scan rate 10 mV s¹).

Figure 5.Cyclic voltammograms of2 a(solid line) and2 b(dashed line) in CH2Cl2(0.1mBu4NPF6; Pt disc working electrode; scan rate 100 mV s¹).

Figure 6.Electronic absorption spectra of1 a(a),2 a(b) andR3(c) in benzo- nitrile solution, together with the calculated S₀!Sntransitions (energies and oscillator strengths) at the TD-B3LYP/TZVP level of theory.

culated in C₂ symmetry, and the molecular structures of 1 a and 2 a were determined without symmetry constraints. The calculated geometry of2 ais in good agreement with its X-ray structure (see Supporting Information). The molecular skele- tons of the p-conjugated systems were virtually planar (Sup- porting Information Figure S2), with the sole exception of the well-known soft structural bending within the neutral TTF frag- ments (ca. 138) and, as expected, the out-of-plane rotations of the pendant phenyl groups (70–1118). However, calculations of the MOs were approximated while applying D_2h or C_2h point group symmetries to the molecules. Some important frontier MOs and their calculated energies are shown in Figures 7 and 8. The frontier MOs of1 abasically result from linear combina- tions of orbitals from the central TTF core with the plus/minus linear combinations of orbitals from the pendant quinoxaline and porphyrin units. In 1 a, the HOMO (a_u) electron density is mainly located on the central TTF unit with clear inclusion of both pendant quinoxaline units. This can be best compared

with the case of an analogous electronic pentad system show- ing the same core fragment.^[14] Next, the energetically close lying HOMO1 and HOMO2 as well as HOMO3 and HOMO4 are essentially both pairs of plus/minus combina- tions (b_g, a_u) of “left and right” of what would be HOMO and HOMO1 of a single porphyrin molecule alone. Analogously, LUMO and LUMO+1 of a single porphyrin now combine to LUMO and LUMO+1 as well as LUMO+2 and LUMO+3 for the symmetric molecule 1 a. The LUMO+4 and LUMO+5 fi- nally represent the quinoxaline LUMOs. The dividing line be- tween HOMO and HOMO1/2 is opposite to what is expect- ed from the experimental CV data ; however, the calculations are for isolated molecules with symmetry constraints and, even more importantly, the energies of their highest occupied MOs are in a quite narrow energy range anyway. The interpretation of the MO scheme of2 afollows analogously, although corre- sponding linear combinations are no longer needed. For R3, since no TTF donor is built in, the HOMO is mainly localized on the porphyrin core, but signifi- cantly decoupled from the fused quinoxaline fragment. In all of these cases, the LUMO and LUMO+1 extend across the qui- noxaline group(s), lowering their energies. These results are in ac- cordance with those of reported analogues.^[19]

The calculated vertical elec- tronic transitions for 1 a,2 aand R3 are shown by sticks in the spectra in Figure 6, and their cal- culated energies and oscillator strengths are given in Tables 2–

4. For1 a, the DT-DFT calculation (D_2h constraint) predicts S₀!S₁ and S₀!S2 excitations with only very weak intensities, and their electronic character corresponds basically to the Q-band-type transitions of porphyrin systems.

However, the energetically close lying S₀!S3 transition describes the expected symmetry-allowed HOMO (a_u)!LUMO (bg) promo- tion (93 %) and thus exhibits CT character (A ! ! D!!A). The calculated energy and oscillator strength compare fairly well with the first absorption band ap- pearing around 16 500 cm¹ (605 nm). The S₀!S11 excitation reflects the A’ ! D!A’ type of charge-transfer absorption, and the S₀!S20 excitation reveals contributions of the A!A’–D–

A’ ! A type of charge-transfer transition. The following S₀!S29

Figure 7.Frontier MOs of thep-conjugated skeleton of1 a.

excitation with the highest oscil- lator strength corresponds to the Soret-band character of the porphyrins. Altogether, these latter excitations lead to the broad and intense absorption profile from 18 000 cm¹ up- wards in energy (below 555 nm) ; however, a specific assignment of the calculated transitions within this broad profile can not be made at this stage.

For 2 a, as expected, the HOMO!LUMO promotion (98 %) significantly shows up with the excitation S₀!S₁ at a similar energy as in the case of 1 a, and it matches the lowest energy absorption band quite well; this transition describes an A ! ! D charge transfer. The pro- nounced S₀!S₅ excitation corre- sponds to the expected A’ ! D type of charge transfer. And lastly, the S₀!S₁₁ excitation re- flects again the corresponding porphyrin Soret band. Overall, it is clear that the Soret-type ab- sorption dominates the spec- trum, and the different charge- transfer transitions contribute to broadening of the absorption profile towards lower energies.

In the case ofR3, the S₀!S1and S₀!S₂ excitations bear the Q- type character of the porphyrin unit, and the S₀!S3 and partly the S₀!S₅excitations have a por- phyrin-to-quinoxaline charge- transfer character. Finally, the transitions to the S₇ and S₈ states show mixtures of Soret- band and charge-transfer charac- ters. Compared to the corre- sponding meso-tetraphenylpor- phyrin, direct fusion of the qui- noxaline unit to the porphyrin core gives rise to broadening and red shifts of the spectrum due to the extended p conjuga- tion. Again, the calculated transi- tions show up at slightly higher energies than the observed ones. Furthermore, one can also conclude that for all three com- pounds the theoretical HOMO–

LUMO gaps (HLG) compare fa- Figure 8.Frontier MOs of thep-conjugated skeletons of2 a(a) andR3(b).

Table 2.Energies, oscillator strengths and dominant contributions of the respective molecular orbitals for S0!Snof1 a.

State Excitation

energy [cm¹]

Oscillator strength

Dominant contributions [%]

S1 17 122 0.0002 H2!L+1 (43), H1!L (23), H1!L+2 (13)

S2 17 170 0.0000 H2!L+3 (34), H2!L+2 (16), H3!L+1 (14)

S₃ 17 274 0.6276 H!L(93)

S11 20 748 1.3026 H!L+4 (88)

S20 22 233 0.2617 H3!L+4 (31), H4!L+5 (23), H4!L+1 (18)

S29 24 991 3.8195 H4!L+3 (22), H3!L+2 (17)

vourably with the experimental CV data and similarly well with the optical HLG determined from the onset of the respective absorption profiles; for example, these values are in the range of 2.0–2.4 eV for1 a, and 2.0–2.3 eV for2 a.

2.5. Spectroelectrochemical Studies

To gain insight into the potential evolution of the redox behav- iours of1and2and also to investigate in more detail the site of electron transfer, systematic changes in the optical spectra upon oxidation of the neutral species were examined by spec- troelectrochemistry, as shown in Figures 9 and 10 (see also the Supporting Information, Figures S3–S5).

As illustrated in Figure 9 a, chemical oxidation of 2 a with less than 1 equiv of [Fe(bpy)₃](PF₆)₃(bpy=2,2’-bipyridine) leads to a gradual decrease of the absorption band around 600 nm (16 670 cm¹) and concomitant emergence of a new broad ab- sorption band peaking at 830 nm (12 050 cm¹), characteristic for formation of the TTF radical cation species. However, the Soret band is intensified, sharpened and slightly moves to higher frequency. All of these observations suggest minor par- ticipation of the porphyrin ring in the first oxidation process, which occurs mainly on the TTF moiety. Furthermore, beyond 1.4 equiv of [Fe(bpy)₃](PF₆)₃(Figure 9 b), the TTF radical absorp- tion band decreases quickly, while new broad absorption bands grow in the 600–1000 nm (16 670–10 000 cm¹) region along with a rapid decrease in the intensity of the Soret band, a spectral feature that is conceivably attributable to formation of a porphyrin radical cation during the second oxidation pro- cess. The sequential oxidations are also borne out by the simi- larity to the thin-layer UV/Vis/NIR spectra of 2 a and 2 b ob- tained in situ by successively applying oxidation potentials (Supporting Information Figures S3 and S4). All of these results are in good agreement with the previous discussion from

cyclic voltammetry, in that the first oxidation occurs at the TTF unit and the second at the por- phyrin core.

Figure 10 shows the variation of the absorbance spectra of 1 c as the voltage is stepped from 0.2 to+0.5 V. As the potential shifts positively, the intensities in the higher energy range (with Soret-band character) significant- ly decrease and a new broad ab- sorption band grows in the 600–

800 nm (16 670–12 500 cm¹) region. These spectral changes suggest that the electron-trans- fer site is porphyrin ring centred and leads to the formation of porphyrin cation radicals during the first oxidation process. The same holds for the thin-layer UV/Vis/NIR spectral change of 1 b (Supporting Information Fig- Table 3.Energies, oscillator strengths and dominant contributions of the respective molecular orbitals for

S0!Snof2 a.

State Excitation

energy [cm¹]

Oscillator strength

Dominant contributions [%]

S1 16 513 0.1524 H!L (98)

S2 17 166 0.0000 H2!L (48), H1!L+1 (47)

S4 18 132 0.0023 H1!L (63), H2!L+1 (35)

S5 20 307 0.3757 H!L+2 (96)

S₆ 21 942 0.0563 H1!L+2 (83), H2!L+1 (11)

S7 22 340 0.1600 H2!L+2 (58), H2!L (23), H1!L+1 (16)

S11 25 279 2.0773 H2!L+1 (43), H1!L (23), H1!L+2 (13)

Table 4.Energies, oscillator strengths and dominant contributions of the respective molecular orbitals for S0!SnofR3.

State Excitation

energy [cm¹]

Oscillator strength

Dominant contributions [%]

S1 17 230 0.0000 H!L+1 (50), H1!L (49)

S2 18 169 0.0131 H!L (64), H1!L+1 (35)

S₃ 22 541 0.0739 H!L+2 (84), H1!L+1 (10)

S5 22 897 0.2196 H1!L+2 (54), H1!L (23), H!L+1 (17)

S7 25 817 1.7913 H1!L+1 (48), H!L (27), H!L+2 (14)

S8 25 818 0.5522 H1!L+2 (37), H3!L+1 (19), H!L+1 (18)

Figure 9.Variation of the UV/Vis/NIR absorption spectra of2 a(7.16 10⁶m) in CH2Cl2upon successive addition of aliquots of [Fe(bpy)3](PF6)3.

ure S5) upon the first oxidation. These observations are in qualitative agreement with the aforementioned interpretation of the electrochemical data.

2.6. Femtosecond and Nanosecond Absorption Spectral Studies

Femtosecond transient absorption spectroscopy was used to obtain insight into the excited-state events of triads1, dyads 2, and reference compoundR3. The compounds were probed with 400/430 nm excitation to selectively excite the porphyrin fluorophores. The femtosecond transient absorption spectra of R3in benzonitrile (Figure 11) reveal instantaneous formation of the singlet porphyrin features. Here, the transient absorption spectra exhibit the absorption bands in the visible region with a maximum at 475 nm, which decayed slowly (1.00 10⁸s¹) to populate the corresponding triplet manifold.

The spectral features in the visible region, which were seen immediately upon excitation of triads 1 a–c in benzonitrile (Supporting Information Figures S6–S8), reveal transient ab- sorption bands of the singlet-excited state of porphyrin, with rate constants of 1.18 10⁸, 1.80 10⁸and 2.23 10⁹s¹, respec- tively. Therefore, one can state clearly that electron transfer from TTF to the excited state of porphyrin is not detected in these triads. This observation is in a good agreement with the electrochemical studies, which suggest that the electron trans- fer is thermodynamically not favoured.

Upon excitation of dyad 2 a in benzonitrile (Figure 12), the femtosecond spectrum recorded at 7 ps showed formation of

the singlet excited state of porphyrin. A study on the kinetics of 2 a in the initial 200 ps suggests that the singlet excited state of porphyrin decays much faster (1.0 10¹⁰s¹) than that of1 a. The transient spectra of2 b(Supporting Information Fig- ure S9) exhibit the same features as1 bwith a rate constant of

1ZnP* (7.0 10¹⁰s¹). These results indicate electron transfer from TTF to the singlet porphyrin, considering that energy transfer from the singlet-excited porphyrin to TTF is not feasi- ble. This observation is consistent with the electrochemical studies, which show that electron transfer is thermodynamical- ly favoured for2 aand2 bin polar benzonitrile. The difference in the rates of charge separation from TTF to the singlet excit- ed porphyrin of dyad 2 a (k_CS=9.80 10⁹s¹) and 2 b (k_CS= 6.98 10¹⁰s¹) can be explained by the difference in the free- energy changes of charge-separation processes (DGCS) of 2 a (0.17 eV) and2 b(0.86 eV).^[21]

Upon exciting referenceR3with a 430 nm laser, the comple- mentary nanosecond transient absorption spectra of R3 in benzonitrile showed the absorption bands of the triplet excit- ed porphyrin in the visible region with a maximum at 470 nm (Figure 13). The triplet excited state of porphyrin decays with Figure 10.Variation of UV/Vis/NIR spectroelectrochemistry of1 c

(4.6 10⁴m) in CH2Cl2(with 0.1mBu4NPF6) upon variation of the electric po- tential.

Figure 11.Differential absorption spectra obtained upon femtosecond flash photolysis (430 nm) of referenceR3in benzonitrile at the indicated time in- tervals. Inset : Decay profile of the singlet porphyrin at 475 nm.

Figure 12.Differential absorption spectra obtained upon femtosecond flash photolysis (400 nm) of2 ain benzonitrile at the indicated time intervals.

Inset : Decay profile of the singlet porphyrin at 480 nm.

a rate constant of 6.4 10³s¹. Similar spectra were observed for 1 a and 2 a in benzonitrile (Supporting Information Figur- es S10 and S11). These observations suggest the absence of electron transfer via the triplet state of porphyrin, which is thermodynamically not favoured. The short-lived triplet state of triad1 cwas not observed in the transient spectra (Support- ing Information Figure S12).

2.7. Complex Formation and Photochemical Studies of Supramolecular Triad 2b:C₆₀py

Dyad 2 b was utilized to build, by an axial-coordination ap- proach, a novel supramolecular architecture with fullerene functionalized by a pyridine entity (Figure 14). The optical ab- sorption changes observed during increasing addition of N- pyridyl-3,4-fulleropyrrolidine (C₆₀py)^[22]to a solution of2 bino- dichlorobenzene, a non-coordinating solvent, are shown in Figure 15. During the titration, the ZnP-type absorption band of2 blocated at 422 nm diminished in intensity with red shifts of the absorption band to 432 nm, characteristic of axial coor- dination of the Zn^IIion.

Formation of the supramolecular triad 2 b:C₆₀py was also confirmed by steady-state emission studies. As shown in Figure 16, the fluorescence spectrum of dyad2 bino-dichloro- benzene reveals an emission band at 640 nm corresponding to the singlet (ZnP–TTF)*. An increasing concentration of C₆₀py in the solution results in a decrease in emission intensity. This suggests the occurrence of a photochemical processes; an electron transfer from the singlet state of 2 bto C₆₀could be envisioned. The Zn^II···py binding constant was evaluated by constructing a Benesi–Hildebrand plot, as shown in Figure 16 (inset), which yields a binding constant K of 7.20 10⁴m¹, nearly two orders of magnitude higher than that reported for C₆₀py binding to ZnP.^[23]

To probe the redox properties of the2 b:C₆₀py, cyclic voltam- metric studies were performed in o-dichlorobenzene contain- ing 0.10m Bu₄N(PF₆) as supporting electrolyte (Supporting In- formation Figure S13). The first reduction potential of the Figure 13.Nanosecond transient absorption spectra of referenceR3in ben-

zonitrile at the indicated time intervals. Inset : Decay profile of the triplet porphyrin at 470 nm.

Figure 14.Molecular structure of supramolecular triad2 b:C60py.

Figure 15.UV/Vis spectral changes observed upon increasing addition of C60py to dyad2 bino-dichlorobenzene.

Figure 16.Fluorescence spectral changes observed upon increasing addition of C60py to2 bino-dichlorobenzene;lex=420 nm. Inset : Benesi–Hildebrand plot.

C₆₀py entity was located at0.67 V versus SCE, while the oxi- dation potentials of 2 b were located at 0.64, 0.86 and 1.10 V versus SCE. The energetics for a photoinduced electron trans- fer from¹ZnP*–TTF to C₆₀were evaluated by using the redox and spectral data discussed above. The driving forces for charge separation (DG_CS) and charge recombination (DG_CR) are given via the excited singlet state ZnP*–TTF and were found to be 1.31 and 0.77 eV, respectively. These results suggest that the electron transfer from the singlet ZnP*–TTF to C₆₀is ther- modynamically possible.

To unravel the electron-transfer route and obtain kinetic in- formation, further transient spectral studies on a femtosecond timescale were performed. The 2 b:C₆₀py supramolecular triad was probed with 430 nm laser excitation to selectively excite the porphyrin fluorophore in toluene solution (Figure 17). The

transient absorption spectrum measured 5 ps after femtosec- ond laser excitation exhibits the transient absorption of the ZnP–TTF singlet excited state (¹ZnP*–TTF). With time, the

1ZnP*–TTF absorption peak at 540 nm diminishes in intensity while a concomitant increase in absorbance at 1000 nm, due to the C₆₀ radical anion (C60^·), and similarly at 620–700 nm due to the ZnP radical cation (ZnP^·+), is observed. These spec- tra provide direct evidence for electron transfer from the pho- toexcited ZnP*–TTF unit to C₆₀. The kinetics of this electron transfer from¹ZnP*–TTF to C₆₀was analyzed by exponential fit- ting of the rise profile of the radical C₆₀^· (at 1000 nm; k_CS= 2.84 10¹²s¹ and k_CR=2.60 10⁹s¹). Based on the k_CR value, the lifetime of the (ZnP–TTF)^·+···(C₆₀py)^· charge-separated state was determined to be 385 ps.

3. Conclusions

Novel symmetric porphyrin–tetrathiafulvalene–porphyrin triads annulated through quinoxaline linkers into planar and largely

extendedp-conjugated molecules have been synthesized. Sim- ilarly, asymmetric porphyrin–tetrathiafulvalene dyads have been prepared as well. These electrochemically amphoteric compounds have been investigated by cyclic voltammetry, thin-layer cyclic voltammetry and spectroelectrochemical methods in order to elucidate the sites of the redox processes.

The data suggest that in the triads initial oxidation occurs at the peripheral porphyrin sites, and is directly followed within a narrow potential range by formation of the TTF radicals. This order is reversed for the dyad systems and other reported TTF- annulated porphyrins. Ab initio calculations demonstrate that the TTF-type and porphyrin-type HOMOs show up at quite similar energies. Photophysical experiments reveal electronic excitations which can be traced back to the specific type of in- tramolecular charge-transfer character; these additional elec- tronic transitions are a direct consequence of the synthetic ap- proach to annulate donor and acceptor chromophores into rigid and planar p-conjugated molecular systems. The poten- tial to build up further supramolecular assemblies was demon- strated by using a fullerene acceptor unit.

Experimental Section

General: Air- and/or water-sensitive reactions were conducted under argon in dry, freshly distilled solvents. Elemental analyses were performed on an EA 1110 Elemental Analyzer CHN Carlo Erba Instruments. FTIR spectra were recorded on a PerkinElmer One FTIR spectrometer. ¹H and ¹³C NMR spectra were measured on a Bruker spectrometer with tetramethylsilane as internal standard.

Mass spectra were recorded on an FTMS 4.7T BioAPEX II with the MALDI ionization method.

Materials: Unless otherwise stated, all reagents were purchased from commercial sources and used without additional purification.

2-[5,6-Diamino-4,7-bis(4-pentylphenoxy)-1,3-benzodithiol-2-yli- dene]-4,7-bis(4-pentylphenoxy)-1,3-benzodithiole-5,6-diamine (3),^[14]

5,6-diamino-2-(4,5-bis(propylthio)-1,3-dithio-2-ylidene)-benzo[d]- [1,3]dithiole (4),^[15] 2,3-dioxo-5,10,15,20-tetrakis(3’,5’-di-tert-butyl- phenyl)chlorin (5),^[16c] 2,3-dioxo-5,10,15,20-tetrakisphenylchlorin (6)^[16] and N-pyridyl-3,4-fulleropyrrolidine (C60py)^[22] were prepared according to literature procedures.

Cyclic Voltammetry: Cyclic voltammetry (CV) was performed for 1 a–cin a three-electrode cell equipped with a platinum millielec- trode, a platinum wire counter-electrode and a silver wire as quasi- reference electrode. The electrochemical experiments were carried out under dry and oxygen-free atmosphere (H2O<1 ppm, O2<

1 ppm) in CH2Cl2(0.8 mm) with 0.1mBu4NPF6 as supporting elec- trolyte at 100 mV s¹. The voltammograms were recorded on an EGG PAR 273A potentiostat with positive feedback compensation.

Based on repetitive measurements, absolute errors on potentials were estimated to be around5 mV. The number of electrons was determined by recording the voltammograms under thin-layer conditions (TLCV), and dichloronaphthoquinone was used as an in- ternal reference for the number of exchanged electrons. The exper- imental voltammograms were deconvoluted with the Condecon software.

Cyclic voltammetry (CV) was carried out for 2 a, 2 b and R1 in a three-electrode cell equipped with a Pt disc working electrode, a glassy carbon counter-electrode and Ag/AgCl reference elec- trode. The electrochemical experiments were carried out under dry and an oxygen-free atmosphere in CH2Cl2 with 0.1m Bu4NPF6 as Figure 17.Differential absorption spectra obtained upon femtosecond flash

photolysis (430 nm) of2 b:C₆₀py at the indicated time intervals in toluene.

Inset : Rise–decay profile of the C60radical anion monitored at 1000 nm to evaluate the charge-separation and charge-recombination kinetics.

supporting electrolyte. The voltammograms were recorded on a PGSTAT 101 potentiostat.

Cyclic voltammograms of supramolecular triad 2 b:C60py were re- corded on a BAS CV-50W Voltammetric Analyzer. A platinum disc electrode was used as working electrode, while a platinum wire served as a counter-electrode. An SCE electrode was used as refer- ence electrode. All measurements were carried out ino-dichloro- benzene containing Bu4NPF6(0.1m) as supporting electrolyte. The scan rate was 50 mV s¹.

Spectroelectrochemistry: The setup used for the UV/Vis spectro- electrochemical experiments has been described previously.^[24]The evolution of UV/Vis/NIR spectra after successive additions of [Fe- (bpy)3](PF6)3 aliquots was followed on a PerkinElmer Lambda 10 spectrophotometer in a 1 cm quartz cell with a solution of 2 a (7.16 10⁶m) in CH2Cl2 and a solution of [Fe(bpy)3](PF6)3 (1.43 10³m) in CH3CN.

Photophysical Measurements: Steady-state absorption spectra were recorded on a Shimadzu UV-3100PC spectrometer or a Hew- lett Packard 8453 diode-array spectrophotometer at room temper- ature. Fluorescence measurements were carried out on a Shimadzu spectrofluorophotometer (RF-5300PC). Fluorescence spectra of the 2 b:C60py supramolecular triad were monitored by using a Varian Eclipse spectrometer. A right-angle detection method was used.

Femtosecond transient absorption spectroscopic experiments were conducted by using an Integra-C ultrafast source (Quantronix Corp.), a TOPAS optical parametric amplifier (Light Conversion Ltd.) and a commercially available optical detection system (Helios) pro- vided by Ultrafast Systems LLC. The sources for the pump and probe pulses were derived from the fundamental output of Inte- gra-C (780 nm, 2 mJ pulse¹and fwhm=130 fs) at a repetition rate of 1 kHz. 75 % of the fundamental output of the laser was intro- duced into TOPAS, which has optical frequency mixers resulting in tuneable range from 285 to 1660 nm, while the rest of the output was used for white light generation. Typically, 2500 excitation pulses were averaged for 5 s to obtain the transient spectrum at a set delay time. Kinetic traces at appropriate wavelengths were as- sembled from the time-resolved spectral data. All measurements were conducted at 298 K. The transient spectra were recorded by using fresh solutions in each laser excitation.

For nanosecond transient absorption measurements deaerated sol- utions of the compounds were excited by a Panther OPO equipped with a Nd:YAG laser (Continuum, SLII-10, 4–6 ns fwhm) with a power of 10–15 mJ pulse¹. The photochemical reactions were monitored by continuous exposure to an Xe lamp (150 W) as probe light and a photomultiplier tube (Hamamatsu 2949) as de- tector. Solutions were deoxygenated by argon purging for 15 min prior to the measurements.

Ab Initio Calculations: DFT and time-dependent DFT calculations of porphyrin–quinoxaline, TTF–porphyrin and TTF–diporphyrin were performed with the B3LYP hybrid functional and the SVP basis set.

All calculations were carried out with the TURBOMOLE V6.0 pro- gram package.^[25] The molecular ground-state geometries of por- phyrin–quinoxaline, TTF–porphyrin and TTF–diporphyrin were opti- mized at the B3LYP/SVP level of theory. Porphyrin–quinoxaline was calculated inC2symmetry and the other two without symmetry re- strictions. The electronic excitation spectra were calculated with the SVP basis set.

Synthesis of Triad 1 a: A suspension of 5 a (19 mg, 0.017 mmol) and3(8.3 mg, 0.008 mmol) in glacial acetic acid (6 mL) was heated to reflux for 5 h under Ar. After cooling to room temperature, the solvent was evaporated. The residue was purified by column chro-

matography (SiO2, petroleum ether (b.p. 40–608C)/CH2Cl2 3/2) to afford a crude product. Subsequent reprecipitation from a solution in CH2Cl2with methanol gave1 a(19 mg, 74 %) as a brownish red powder.¹H NMR (300 MHz, CDCl3):d=8.90 (d,J=5.1 Hz, 4 H), 8.78 (d,J=5.1 Hz, 4 H), 8.75 (s, 4 H), 8.06 (d,J=1.7 Hz, 8 H), 7.92 (d,J=

1.7 Hz, 8 H), 7.78–7.77 (dd,J=1.7 Hz,J=1.9 Hz, 4 H), 7.66–7.65 (dd, J=1.7 Hz, 4 H), 7.04 (d,J=8.6 Hz, 8 H), 6.73 (d,J=8.6 Hz, 8 H), 2.57 (t, J=7.5 Hz, 8 H), 1.53, 1.50 (2 s, 152 H, tert-butyl protons and CH2CH2(CH2)2CH3are overlapped), 1.35 (m, 16 H), 0.88 (t,J=6.8 Hz, 12 H),2.52 ppm (br s, 4 H); FTIR (KBr pellet):n˜=3437, 2960, 2923, 2856, 1626, 1593, 1504, 1475, 1384, 1362, 1208, 1167, 1154, 1109, 922, 800, 720 cm¹; MALDI-TOF MS: m/z 3125.91 [M]⁺; calcd for C210H244N12O4S4 3125.81; elemental analysis calcd (%) for C210H244N12O4S4·2 CH3OH: C 79.76, H 7.96, N 5.26; found: C 80.16, H 8.30, N 4.78.

Synthesis of Triad 1 b: A solution of Zn(OAc)2·2 H2O (9 mg, 0.041 mmol) in CH3OH (5 mL) was added to a solution of com- pound1 a(19 mg, 6mmol) in CH2Cl2(15 mL). The resulting mixture was heated to 508C and stirred for 3.5 h. After cooling to room temperature, the solvent was evaporated. The residue was purified by column chromatography (SiO2, petroleum ether (b.p. 40–608C)/

CH2Cl2 3/2) to afford a crude product. Subsequent reprecipitation from a solution in CH2Cl2with methanol gave 1 b(13.7 mg, 70 %) as a brownish red powder. ¹H NMR (300 MHz, CDCl3):d=8.87 (d, J=4.7 Hz, 4 H), 8.82 (s, 4 H), 8.68 (d, J=4.7 Hz, 4 H), 8.02 (d, J=

1.7 Hz, 8 H), 7.86 (d, J=1.9 Hz, 8 H), 7.75–7.74 (dd, J=1.7 Hz, J=

1.9 Hz, 4 H), 7.62–7.60 (dd,J=1.7 Hz, 4 H), 7.05 (d,J=8.7 Hz, 8 H), 6.75 (d,J=8.7 Hz, 8 H), 2.58 (t,J=7.5 Hz, 8 H), 1.49, 1.53 (2 s, 152 H, tert-butyl protons and CH2CH2(CH2)2CH3are overlapped), 1.36 (m, 16 H), 0.91 ppm (t, J=7.2 Hz, 12 H); FTIR (KBr pellet): n˜=3437, 2958, 2923, 2854, 1625, 1592, 1504, 1464, 1384, 1361, 1218, 1170, 1112, 939, 812, 798, 711 cm¹; MALDI-TOF MS: m/z: 3250.76 [M+H]⁺; calcd for C210H241N12O4S4Zn2: 3250.65; elemental analysis calcd (%) for C210H240N12O4S4Zn2: C 77.48, H 7.43, N 5.16; found: C 77.15, H 7.83, N 4.63.

Synthesis of Triad 1 c: A suspension of 5 b (19 mg, 0.016 mmol) and3(8.3 mg, 0.008 mmol) in glacial acetic acid (4 mL) was heated to reflux for 3 h under Ar. After cooling to room temperature, the solvent was evaporated. The residue was purified by column chro- matography (SiO2, petroleum ether (b.p. 40–608C)/CH2Cl2 1/1) to afford a crude product. Subsequent reprecipitation from a solution in CH2Cl2with methanol gave1 c(20 mg, 77 %) as a brownish red powder. FTIR (KBr pellet):n˜=3435, 2960, 2924, 2856, 1593, 1504, 1460, 1393, 1362, 1217, 1173, 1009, 939, 815, 799 cm¹; MALDI-TOF MS:m/z3248.69 [M+H]⁺; calcd for C210H241N12O4S4Cu2: 3248.65; el- emental analysis calcd (%) for C210H240N12O4S4Cu2: C 77.57, H 7.44, N 5.17; found: C 77.97, H 8.15, N 4.55.

Synthesis of Dyad 2 a: A mixture of porphyrin dione 6 (100 mg, 1.55 10⁴ mol) and 4 (0.0735 mg, 1.70 10⁴mol) in CHCl3/pyri- dine (10/1 v/v) was heated at 658C for 2 h. Colour change was ob- served from yellowish to dark green and completion of the reac- tion was monitored by TLC. After completion pyridine was re- moved on a rotary evaporator, and the residue was purified by flash column chromatography on silica with CH2Cl2/CH3OH (100/1) as eluent to afford2 a(154.9 mg, 96 %) as a dark greenish brown crystalline solid. ¹H NMR (400 MHz, CDCl3): d=8.92 (d, J=8.2 Hz, 4 H), 8.71 (s, 2 H), 8.23–8.21 (d, J=8.0 Hz, 4 H), 8.13–8.12 (d, J=

8.0 Hz, 4 H), 7.79–7.75 (m, 10 H), 7.63 (s, 2 H), 7.62 (s, 2 H), 2.86–2.82 (t,J=6.4 Hz, 4 H), 1.76–1.66 (m, 4 H), 1.07–1.03 (t, J=7.2 Hz, 6 H), 2.75 ppm (s, 2 H); ¹³C NMR (125 MHz, CDCl3): d=161.76, 155.01, 152.53, 145.40, 144.93, 141.86, 141.78, 140.74, 139.58, 139.55, 138.02, 134.46, 133.84, 128.14, 127.99, 127.63, 126.91, 121.70,

121.29, 117.13, 38.38, 23.20, 13.18 ppm; FTIR (KBr pellet):n˜=3389, 3318, 2965, 2969, 1716, 1669, 1509, 1478, 1432, 1384, 1287, 1169, 1174, 1072, 1002, 917, 908, 793, 865 cm¹; elemental analysis calcd (%) for C60H44N6S6: C 69.20, H 4.26, N 8.07; found: C 69.18, H 4.26, N 8.09.

Synthesis of Dyad2 b: A solution of Zn(OAc)2·2 H2O (30 mg, excess) in MeOH (1 mL) was added to a solution of 2 a (25 mg, 2.15 10⁵mol) in CHCl3 (5 mL). The resulting solution was heated at 658C for 2 h. After total conversion (confirmed by TLC), the solu- tion was washed with water and dried over anhydrous sodium sul- fate. The compound was purified on a silica gel column with CHCl3

as eluent to obtain 2 b (22 mg, 93 %) as a purple solid. ¹H NMR (300 MHz, CDCl3):d=8.64 (d,J=8.2 Hz, 4 H), 7.99 (s, 2 H), 7.98–7.97 (d,J=8.0 Hz, 4 H), 7.88–7.86 (d,J=8.0 Hz, 4 H), 7.77–7.76 (m, 10 H), 7.70 (s, 2 H), 7.63 (s, 2 H), 2.87–2.85 (t,J=6.4 Hz, 4 H), 1.74–1.69 (m, 4 H), 1.07–1.03 ppm (t, J=7.2 Hz, 6 H); ¹³C NMR (125 MHz, CDCl3) d=160.05, 155.03, 148.45, 144.10, 142.93, 140.90, 140.23, 140.13, 139.75, 139.45, 138.12, 133.986, 131.94, 130.24, 127.94, 127.67 126.89, 121.76, 121.31, 117.43, 38.37, 23.36, 13.20 ppm; FTIR (KBr pellet): n˜=3389, 3318, 2965, 2969, 1716, 1669, 1509, 1478, 1432, 1384, 1287, 1169, 1174, 1072, 1002, 917, 908, 793, 865 cm¹; ele- mental analysis calcd (%) for C60H42N6S6Zn: C 65.23, H 3.83, N 7.61;

found: C 65.21, H 3.85, N, 7.59.

X-Ray Structure Determination: Crystal data for2 a: C60H44N6S6,M= 1040.20, a=15.833(2), b=19.591(3), c=16.824(2) , b= 107.918(4)8, monoclinic P21/n, Z=4, V=4965.5(13) ³, 1calcd= 1.393 g cm³, F000=2168, l=0.71073 , T=123(2) K, m= 0.232 mm¹, Bruker X8 Apex II CCD diffractometer,fscan, 28 599 data collected, corrected for Lorenz and polarization effects, 8733 unique (Rint=0.0569) and 8733 observed [I>2s(I)], 694 refined pa- rameters, R=0.0985, Rw=0.2265, w=[s²(F)]¹. CCDC 869800 (2 a) contains the supplementary crystallographic data for this paper.

These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re- quest/cif.

Acknowledgements

This work was supported by the Swiss National Science Founda- tion (grant No. 200020-130266/1), the EU-project (FUNMOLS FP7- 212942-1), the Australian Research Council for Future Fellowship award (FT110100152) and the ARC Discovery Grant Program (DP1093337) as well as by the Global COE (center of excellence) program “Global Education and Research Center for Bio-Environ- mental Chemistry” of Osaka University from Ministry of Educa- tion, Culture, Sports, Science and Technology, Japan, and KOSEF/

MEST through WCU project (R31-2008-000–10010-0) from Korea.

Keywords: density functional calculations · donor–acceptor systems·photophysics·porphyrinoids·redox chemistry

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Received: April 23, 2012 Revised: June 1, 2012

Published online on June 29, 2012