Structures and photoinduced electron transfer of protonated complexes of porphyrins and metallophthalocyanines

Coordination Chemistry Reviews 256 (2012) 2488–2502

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Review

Structures and photoinduced electron transfer of protonated complexes of porphyrins and metallophthalocyanines Shunichi Fukuzumi a,b,c,∗ , Tatsuhiko Honda a,b , Takahiko Kojima d,∗∗ a

Department of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan ALCA, Japan Science and Technology Agency (JST) Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Republic of Korea d Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan b c

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2488 Protonation of porphyrins and phthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2489 2.1. Structures of mono- and diprotonated porphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2489 2.2. Structures of protonated phthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2491 Photoinduced electron-transfer reactions of protonated porphyrinoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2492 3.1. Protonated porphyrin and phthalocyanine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2492 3.2. Molecular and supramolecular complexes composed of H4 DPP2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2494 3.3. Porphyrin nanochannels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2497 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2500 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2500 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2500

a r t i c l e

i n f o

Article history: Received 3 December 2011 Received in revised form 17 December 2011 Accepted 24 January 2012 Available online 2 February 2012 Keywords: Protonated porphyrin Protonated phthalocyanine Distortion Photoinduced electron transfer Supramolecular complexes

a b s t r a c t Porphyrins and phthalocyanines are planar two-dimensional ␲-compounds, which are normally difficult to protonate because of the low basicity. When many bulky substituents are introduced to porphyrins and phthalocyanines, however, the macrocyclic ␲-plane is distorted due to the steric repulsion of the bulky substituents. The ␲-plane distortion facilitates protonation to afford stable protonated porphyrins and phthalocyanines. Crystal structures of protonated porphyrins and phthalocyanines were determined to clarify the role of hydrogen bonding in the supramolecular assemblies. Protonated porphyrinoids can act as an electron acceptor rather than an electron donor in photoinduced electron-transfer reactions. The rate constants of photoinduced electron-transfer reactions of diprotonated porphyrin with different degrees of distortion were determined and they are evaluated in light of the Marcus theory of electron transfer to determine the reorganization energies of electron transfer, which are affected by the distortion of the ␲-plane. A distortion of the macrocyclic ligands also affords higher Lewis acidity at a metal center to allow facile axial coordination of ligands, due to poor overlap of the lone pair orbitals with dx2–y2 or px and py orbitals of the metal center. Thus, the distortion of the macrocyclic ligands enables one to construct various molecular and supramolecular complexes composed of porphyrins and phthalocyanines. The photodynamics of photoinduced electron-transfer reactions of various supramolecular complexes of distorted porphyrin and phthalocyanines are discussed in relation to structure and photofunction. © 2012 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. ∗∗ Corresponding author. E-mail addresses: [email protected] (S. Fukuzumi), [email protected] (T. Kojima). 0010-8545/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ccr.2012.01.011

Porphyrins (Por) and phthalocyanines (Pc) have merited significant interest because they exhibit light-harvesting efficiency for producing charge-separated states as models of the photosynthetic reaction centers [1–14] and photovoltaic cells for energy conversion [15–24]. Porphyrins and phthalocyanines contain extensively conjugated ␲-systems which enable efficient electron transfer,

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because the uptake or release of electrons usually results in minimal structural change upon electron transfer [25]. In most cases, porphyrins, phthalocyanines and their metal complexes have been employed as electron donors rather than electron acceptors in electron donor–acceptor ensembles [1–14,25], unless strongly electron-withdrawing substituents are introduced to the macrocycles [26]. In addition, cationic porphyrins having high-valent metal ions also act as electron acceptors in electron-transfer reactions [7,27]. In a similar way, a protonated porphyrin (H4 P2+ ) is expected to gain electron-accepting ability by the electronwithdrawing effect of added protons on the porphyrin ring. The protonation of porphyrin and its analogs such as phthalocyanine is the addition of proton(s) to the aromatic nitrogen atoms involved in the ␲-conjugated system and thus has the advantage of significant influence on their electronic properties as large as the synthetic modification [28,29]. Main concern in the protonation of porphyrins has been focused on the change of photophysical and structural properties [30], however, the effect of protonation on the redox properties has rarely been examined [31,32]. This is due to the low basicity of planar porphyrins, which require strongly acidic conditions for protonation [30]. On the other hand, non-planar porphyrins, which are derived from the introduction of bulky substituents to the porphyrin core, exhibit higher basicity than planar porphyrins because the lone pairs of the pyrrole nitrogen atoms direct out of the porphyrin mean plane in the non-planar porphyrins [33]. The use of a protonated porphyrinoid as an electron acceptor in porphyrinoid-based electron-transfer systems allows us to expand the variety of donor–acceptor ensembles where a porphyrin moiety normally used as an electron donor. From a structural standpoint, the construction of porphyrinoidbased donor–acceptor molecular and supramolecular systems using axial coordination and/or noncovalent interactions such as hydrogen bond attracted increasing attention because of its feasibility in preparation, compared to linking them by covalent bonds with energy- and time-consuming synthetic procedures. In this context, many light-harvesting supramolecular assemblies built up by noncovalent interaction have been reported [8–14]. These interactions, however, are usually weakened in coordinating polar solvents that can stabilize the charge-separated states resulting from photoinduced electron transfer. One of the interesting characteristics of protonated porphyrin is the formation of hydrogen bonds between NH protons attached to pyrroles and conjugated bases of Brønsted acids employed for protonation [30]. Thus, the combination of electrostatic interactions (ion pairing) and hydrogen bonding in protonated porphyrin is promising approach to construct discrete and stable supramolecular systems. This review focuses on structures and electron-transfer properties of protonated porphyrins and phthalocyanines and their molecular and supramolecular assemblies. First, the crystal structures of monoprotonated and diprotonated porphyrins are discussed to clarify the role of hydrogen bonding in the interaction with counter anions. In contrast to the protonation of the porphyrin ring, which occurs only at pyrrole-nitrogen atoms, phthalocyanines can be protonated at two different sites; the isoindole-nitrogen and meso-nitrogen atom(s). The crystal structures of both meso- and isoindole-protonated phthalocyanines are shown and the spectroscopic and electrochemical properties of these protonated phthalocyanines are discussed in relation with the electron-transfer reactions. Then, the hydrogen bonding and/or strong axial coordination of distorted porphyrinoids utilized to construct molecular and supramolecular electron donor–acceptor complexes composed of protonated porphyrinoids, is discussed. We will also discuss the electron-transfer properties and photoconductivity of a supramolecular architecture constructed by a nonplanar porphyrin called porphyrin nanochannels (PNCs), which

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can include a variety of electron-donor molecules in their inner space.

2. Protonation of porphyrins and phthalocyanines 2.1. Structures of mono- and diprotonated porphyrins Generally, in the case of free-base porphyrins (H2 P) the two protonation steps are almost indistinguishable, yielding the corresponding diprotonated porphyrin (H4 P2+ ), while the monoprotonated species (H3 P+ ) is usually not detected (Scheme 1) [30,34–38]. Many attempts were made to detect and isolate monoprotonated porphyrin by protonation of porphyrins with Brønsted acids [39] or deprotonation of corresponding diprotonated porphyrins [40,41]. The existence of monoprotonated tetraphenylporphyrin was reported based on ion-transfer voltammetry at a polarized water|1,2-dichloroethane interface [42]. Although the crystal structure determination of monoprotonated porphyrin was made on ␤-substituted octaethylporphyrinium (H3 OEP+ ) [43–45], no crystal structure determination was reported on monoprotonated porphyrin having substituents at the meso positions. The effect of counteranion on the acid–base equilibrium [36,37] and the acid-induced J-aggregation of porphyrins [46,47] has been discussed in terms of the formation of hydrogen bonding among NH protons of pyrrole and conjugated base of Brønsted acid. Thus we investigated the protonation of nonplanar and highly basic dodecaphenylporphyrin (H2 DPP) [48–51] and interactions among diprotonated H2 DPP (H4 DPP2+ ) and counteranions [52]. Absorption spectroscopic titration of saddle-distorted H2 DPP with 2-anthracenesulfonic acid (2-AN-SO3 H) and 2,6anthracenedisulfonic acid (2,6-AN-(SO3 H)2 ) in benzonitrile (PhCN) indicates two-step diprotonation, whereas one-step diprotonation could be observed with 2-anthracenecarboxylic acid (2-AN-COOH) (Scheme 1, Fig. 1) [52]. In the case of the reaction of H2 DPP with 2-AN-COOH, the 1 H NMR signals due to the 1and 3-H nuclei of 2-ANCOO− are upfield-shifted even in DMSOd6 , however, such shifts are not observed for 2-AN-SO3 H and 2,6-AN-(SO3 H)2 [52]. This indicates that one-step diprotonation is governed by the formation of hydrogen bonding among NH protons of H4 DPP2+ and oxygen atoms of the carboxylate. Crystal structures of monoprotonated H2 DPP (H3 DPP+ ), H3 DPP(2-ANSO3 ), (H3 DPP)2 (2,6-AN-(SO3 )2 ), and H4 DPP2+ , H4 DPP(2-AN-COO)2 , were determined (Fig. 2) [52]. In the crystal structure of H3 DPP+ , there are hydrogen bonds between the two pyrrole NH and the oxygen atom of the sulfonate group, but there is no interaction with conjugate bases in a polar solvent. In the crystal structure of each protonated species of H2 DPP, the extent of deformation is enlarged as compared with unprotonated H2 DPP, as is usually observed for protonated porphyrin [53–56] with a few exceptions [57,58]. The redox behavior of H3 DPP+ is irreversible in the cyclic voltammograms (CV) in PhCN and DMSO [52]. In the differential pulse voltammograms (DPV) of H2 DPP, a new reduction peak appeared at −0.70 V (vs SCE) by the addition of one-equivalent of 2-AN-SO3 H in PhCN [52]. The one-electron reduction potential is higher than that of H2 DPP (−1.20 V) and lower than that of H4 DPP2+ (−0.45 V). In DMSO, only one reduction peak is observed at −0.85 V upon the addition of 2-AN-SO3 H and at −0.76 V upon addition of 2-AN-COOH. These potentials are anodically shifted as compared to that of H2 DPP in DMSO (−1.26 V). Density functional theory (DFT) calculations were conducted on H3 DPP+ and the species hydrogenbonded with 2-ANCOO− [52]. With the monoprotonated porphyrin, the dihedral angles of pyrroles relative to the H3 DPP+ mean plane have been estimated as shown in Fig. 3 [52]. The non-protonated

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Scheme 1. Concerted (upper) and stepwise (bottom) diprotonation of H2 DPP.

Fig. 1. Absorption spectroscopic changes in the course of titration of H2 DPP with (a) 2-AN-SO3 H and (b) 2-AN-COOH in PhCN. Inset shows absorbance changes at (a) 470 (black) and 490 nm (red) and (b) 470 (black) and 503 nm (red) [52].

Fig. 2. X-ray crystal structures of (a) H4 DPP(2-AN-SO3 )2 , (b) (H4 DPP)2 (2,6-AN-(SO3 )2 ) and (c) H4 DPP(2-AN-COO)2 [52]. Gray, carbon; blue, nitrogen; red, oxygen; yellow, sulfur. Hydrogen atoms and solvent molecules of crystallization are omitted for clarity.

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Fig. 3. Schematic description of DFT-optimized structures of (a) H3 DPP+ and (b) that hydrogen-bonded with 2-AN-COO− . Values described in the figure are dihedral angles relative to each porphyrin mean plane [52]. The phenyl groups of H3 DPP+ are omitted for clarity. R in (b) stands the 2-anthryl moiety.

pyrrole shows a dihedral angle of 36.1◦ (Fig. 3a), whereas the hydrogen-bonded H3 DPP+ exhibits a larger dihedral angle (44.9◦ , Fig. 3b). This suggests that the non-protonated pyrrole in the hydrogen-bonded H3 DPP+ is more subject to protonation than that in H3 DPP+ without a hydrogen-bonding carboxylate group. Thus, the second protonation should be enhanced to give the corresponding diprotonated porphyrin. Recently, the detection of monoprotonated porphyrin using a nonplanar porphyrin dendrimer was reported by Thyagarajan et al. and they concluded, based on the DFT calculations [59], the detectability of the monoprotonated porphyrin is largely related to the non-planarity of porphyrin ring. Their conclusion agrees well with the results described above. 2.2. Structures of protonated phthalocyanines Phthalocyanines are highly developed heteromacrocycles especially used in dyes and pigments [60–62]. However, the properties

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of protonated phthalocyanines resulting from the acid-base reaction, because of their lower basicity [63–71], are little known compared with the protonation of porphyrins. So far, the protonation of phthalocyanines has been utilized as a method to solubilize the insoluble phthalocyanines [63–65]. In sharp contrast to the protonation of porphyrin ring, phthalocyanines undergo protonation at two different sites: the isoindole-nitrogen and meso-nitrogen (Scheme 2). Although the electronic density of the isoindole-nitrogen atom is higher than that of the meso-nitrogen atom [71], the protonation of phthalocyanine generally occurs at the meso-nitrogen atom [66–70] probably due to the difficulties of structural distortion associated with the protonation at the isoindole-nitrogen atom as is observed in the protonation of porphyrins [53–56]. The formation and crystal structure determination of two different types of protonated phthalocyanine (i.e., isoindoleand meso-nitrogen protonation) were investigated by employing the free base and a zinc complex of saddle-distorted ␣-octaphenylphthalocyanine (H2 Ph8 Pc and Zn(Ph8 Pc), respectively) [72–74]. Compared with distorted porphyrins such as DPP that shows conformational flexibility in solution [48c,49], the non-planarity of H2 Ph8 Pc stems from the inevitable steric repulsion among phenyl groups and thus is expected to maintain their distorted structure in solution. Single crystals of protonated phthalocyanines have been obtained by recrystallization of H2 Ph8 Pc with HBr and Zn(Ph8 Pc) with HCl, respectively [74]. The crystal structure of a bromide salt of isoindole-nitrogen diprotonated H2 Ph8 Pc, H4 Ph8 PcBr2 , shows a significant increase in the out-of-plane distortion of the phthalocyanine ring (Fig. 4a), while structural deformations in meso-nitrogen protonated Zn(Ph8 Pc), [ZnCl(Ph8 PcH)] (Fig. 4b), are nearly the same as those in Zn(Ph8 Pc) [74]. In the crystal structure, H4 Ph8 PcBr2 forms hydrogen bonds

Scheme 2. Isoindole-nitrogen (upper) and meso-nitrogen protonation (bottom) of Ph8 Pcs.

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Fig. 4. Crystal structures of protonated phthalocyanines: (a) H4 Ph8 PcBr2 and (b) [ZnCl(Ph8 PcH)] [74]. Dark gray, carbon; blue, nitrogen; red, oxygen; brown, bromine; pink, zinc; light green, chlorine. Hydrogen atoms and solvent molecules are omitted for clarity.

among two of the N H protons of the isoindoles and a bromide ion [74]. On the other hand, in the crystal structure of [ZnCl(Ph8 PcH)], the chloride ion coordinates to the central zinc(II) ion. Absorption spectroscopic titration of H2 Ph8 Pc with trifluoroacetic acid (TFA) in PhCN reveals one-step diprotonation, whereas, in the case of Zn(Ph8 Pc), two-step diprotonation is observed, which coincide with the observation in the crystal structures (Fig. 5). In the 1 H NMR spectrum of the single crystals of H4 Ph8 PcBr2 in CDCl3 , a singlet at ı = 6.4 ppm is assigned to the isoindole NH protons, since the peak disappeared upon addition of an aliquot of D2 O [74]. Upon protonation, the signal of the isoindole NH protons shifted downfield (ı = 4.4 ppm), consistent with the observation in diprotonated porphyrins [31]. The 1 H NMR spectrum of [ZnCl(Ph8 PcH)] shows more complex signals than that of Zn(Ph8 Pc) probably because of the lowering symmetry induced by the proton at a mesonitrogen atom. This situation is confirmed by the observation of an exchangeable proton with a signal at ı = 12.3 ppm, which was assigned to the proton bound to the meso-nitrogen atom [74]. From electrochemical measurements in CH2 Cl2 , we could observe large

positive shifts of the reduction potentials of H4 Ph8 PcBr2 (+0.58 V) and [ZnCl(Ph8 PcH)] (+0.38 V), which correspond to the energetic stabilization of the LUMO upon protonation.

3. Photoinduced electron-transfer reactions of protonated porphyrinoids 3.1. Protonated porphyrin and phthalocyanine As expected from the results described above, the diprotonated porphyrin can act as electron acceptors due to the elevated reduction potentials [75], which are comparable to those of fullerenes and quinones [76,77], whereas unprotonated porphyrins usually act as electron donors rather than electron acceptors [1–14]. There are some studies on electron-transfer reduction of metalloporphyrins; however, the site of electron transfer is not at the porphyrin ring but at the redox-active metal center [78–80]. Thus, the electron transfer properties of such diprotonated porphyrins

Fig. 5. Absorption spectroscopic changes upon addition of TFA to the solution of (a) H2 Ph8 Pc and (b) Zn(Ph8 Pc) in CH2 Cl2 [74].

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without high-valent metal ions acting as electron transfers in the electron-transfer reaction have yet to be exploited. The kinetics of photoinduced intermolecular electron transfer from a series of electron donors to the triplet excited states of a series of porphyrin hydrochloric acid salts including H2 DPP, tetraphenylporphyrin (H2 TPP), octaphenylporphyrin (H2 OPP), tetrakis(2,4,6-trimethylphenyl)porphyrin (H2 TMP) were investigated (Fig. 6) [81]. Among such distorted diprotonated porphyrins, H2 DPP with intrinsic saddle-distortion can form stable diprotonated species in sharp contrast to H2 TMP, H2 TPP and H2 OPP which can afford such species only under strongly acidic conditions; a minimal amount of HCl (ca. 30 equiv.) was required to retain formation of the diprotonated form of H2 DPP. The reduction potentials of H4 TPPCl2 and H4 OPPCl2 in acetonitrile (MeCN) were determined by CV to be −0.49 and −0.34 V (vs SCE), respectively, and that of H4 DPPCl2 to be −0.37 V [81]. Although the reduction potentials of H4 DPPCl2 and H4 OPPCl2 are less negative than that of H4 TPPCl2 , the peak separations of the former (190 and 160 mV) are larger than the latter (100 mV) at the same scan rate (100 mV s−1 ) [81]. In each case, the peak separation increases with increasing scan rate. These results indicate that the electron-transfer reduction of H4 DPPCl2 and H4 OPPCl2 is thermodynamically more favorable but that the intrinsic barrier of electron transfer is higher as compared with that of H4 TPPCl2 . The rate constants of photoinduced intermolecular electron transfer (ket ) from a variety of electron-donor molecules to triplet excited states of diprotonated porphyrins were examined by nanosecond laser flash photolysis (Scheme 3) [81]. As shown in Fig. 7a, the reorganization energies of photoinduced electron transfer of H4 TMPCl2 , H4 TPPCl2 , H4 OPPCl2 and H4 DPPCl2 are 1.21, 1.29, 1.45 and 1.69 eV, respectively [81]. In the photoinduced electron transfer, the reorganization energy of diprotonated porphyrin is

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Scheme 3. Scheme of intermolecular photoinduced electron transfer. H4 PCl2 represents the hydrochloric salts of porphyrins in the scheme.

regulated by the intramolecular reorganization energy, which is governed by large structural change of the distorted porphyrin in the course of electron transfer. This is supported by a linear relationship between the out-of-plane displacements (RMS) of 24 atoms in the porphyrin core and the reorganization energies () of electron transfer as represented in Fig. 7b: the RMS values of H4 TMPCl2 , H4 TPPCl2 , H4 OPPCl2 , and H4 DPPCl2 have been estimated by DFT calculations as 0.39, 0.46, 0.65 and 0.81, respectively (Fig. 6) [81]. The clarification of such electron-transfer properties of non-planar diprotonated porphyrins provides invaluable information to establish a new aspect of porphyrins and to develop their novel functionality based on the saddle distortion of the porphyrin core. The advantage of the protonation of phthalocyanine as compared with porphyrin is the occurrence of the protonation of metallophthalocyanine at the meso-nitrogen without demetallation (vide supra) [66–70]. In order to facilitate the protonation of phthalocyanines at the meso-positions, it is useful to introduce functional groups at peripheral positions to form intramolecular hydrogen bonds with protons trapped at the meso-nitrogen atoms. Thus, we have focused on ␣-alkoxy substituted phthalocyanines,

Fig. 6. Optimized structures of (a) H4 DPPCl2 , (b) H4 OPPCl2 , (c) H4 TPPCl2 and (d) H4 TMPCl2 obtained by DFT calculations at the B3LYP/3-21G level of theory [81].

Fig. 7. (a) Driving force dependence of log ket for electron transfer from electron donors to 3 [H4 DPP2+ ]* (black), 3 [H4 OPP2+ ]* (red), 3 [H4 TPP2+ ]* (blue) and 3 [H4 TMP2+ ]* (green) in MeCN at 298 K and the fit of the curve based on the Marcus theory of electron transfer is shown by the solid line with  = 1.69, 1.45, 1.29 and 1.21 eV [81]. (b) A relationship between root-mean-square out-of-plane displacements (RMS) and  values in photoreduction of diprotonated porphyrins in MeCN.

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Scheme 4. Protonation equilibrium of Zn((OBu)8 Pc).

which are commercially available and are widely studied [82–87]. Although ␣-alkoxy substituted phthalocyanines are well-known compounds, their protonation has been misunderstood as dimerization [85]. Facile protonation of ␣-octabutoxyphthalocyaninato zinc(II) (Zn((OBu)8 Pc)) occurs to afford up to tetra-protonated species (Scheme 4 and Fig. 8) [88]. The higher proton-accepting ability of Zn((OBu)8 Pc) is well explained by the formation of the intramolecular hydrogen bonds: the distance between a meso-nitrogen atom and an oxygen atom of the alkoxy side chain is close enough ˚ to form hydrogen bonds between a proton attached to (3.0 A) the meso-nitrogen and the ether oxygen [86,87]. The formation of hydrogen bonds is further demonstrated by the observation of a 1 H NMR signal assigned to the meso-NH protons at 14.7 ppm, compared with 12.3 ppm in the case of Zn(Ph8 Pc) without hydrogen bonds [88]. The IR spectrum of the acetic acid salt of Zn((OBu)8 Pc), [Zn((OBu)8 PcH)](CH3 COO), in a KBr pellet displays sharp absorption assigned to the N H stretching at 3253 cm−1 , which is not observed in that of Zn((OBu)8 Pc) [88]. The reduction potentials of mono-, di-, tri-, and tetra-protonated Zn((OBu)8 Pc) are positively shifted from unprotonated Zn((OBu)8 Pc) (Ered = −1.17 V vs SCE) to −0.60, −0.15, 0.14, and 0.52 V, respectively (Fig. 8b) [88]. This indicates that the one-electron reduction potential of Zn((OBu)8 Pc) is raised around 0.41 V by addition of a proton, which is nearly the same value obtained by the stepwise protonation of mesosubstituted corroles (0.35 V) [32]. The enhanced electron-accepting ability of protonated Zn((OBu)8 Pc) enables the photoinduced electron transfer between unprotonated Zn((OBu)8 Pc) (Eox = 0.26 V vs SCE) and monoprotonated [Zn((OBu)8 PcH)]+ (Ered = −0.60 V) [88]. As shown in Scheme 5, the photoexcitation of a mixture of equimolar concentration of Zn((OBu)8 Pc) and [Zn((OBu)8 PcH)]+

Scheme 5. Energy diagrams of intermolecular photoinduced electron transfer and back electron transfer in a mixture of Zn((OBu)8 Pc) and [Zn((OBu)8 PcH)]+ in PhCN [88]. ZnPc represents Zn((OBu)8 Pc) in the scheme.

in PhCN resulted in photoinduced intermolecular electron transfer from 3 [Zn((OBu)8 Pc)]* (1.04 eV) to Zn((OBu)8 PcH)+ and from Zn((OBu)8 Pc) to 3 [Zn((OBu)8 PcH+ )]* (1.08 eV) to produce Zn((OBu)8 Pc)•+ and Zn((OBu)8 PcH)• [88]. Thus, the presence and absence of a proton at the meso-nitrogen can switch the characteristics of phthalocyanine to act as an electron acceptor or an electron donor in photoinduced electron transfer. 3.2. Molecular and supramolecular complexes composed of H4 DPP2+ Assembling porphyrins and other molecules by noncovalent interactions such as hydrogen bonding and ␲–␲ interactions provides more elegant and convenient methods as compared with the use of covalent bonds [8–14,89–93]. A combination of electrostatic interaction (ion pairing) with hydrogen bonding is a promising approach to construct discrete and stable supramolecular systems. The non-planarity of porphyrins enable the formation of hydrogen

Scheme 6. Chemical structures of H4 DPP2+ and electron-donor molecules having carboxyl group [94].

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bonding with counteranion when carboxylic acid is used as a protonating agent as is observed in the case of 2-AN-COOH (Fig. 2c) [52]. The excited-state photodynamics of intrasupramolecular photoinduced electron transfer were investigated in a series of hydrogen-bonded supramolecular complexes composed of H4 DPP2+ and electron donors bearing a carboxylate group (Scheme 6) [94]. The crystal structure of the supramolecular assembly using ferrocenecarboxylate (FcCOO− ) revealed a new structural motif involving two-point and single-point hydrogen bonding among saddle-distorted H4 DPP2+ dication and two FcCOO− anions (Fig. 9). The formation of supramolecular complexes in solution was examined by spectroscopic measurements. The binding constants obtained by spectroscopic titration indicate the strong binding (108 –1010 M−2 ) even in a polar and coordinating solvent, benzonitrile (PhCN). 1 H NMR titrations have also been conducted in CDCl3 to investigate formation of the supramolecular assemblies [94]. By adding up to 2 equiv. of FcCOOH, peaks assigned to meso-phenyl protons of H4 DPP2+ are all downfield-shifted, indicating formation of H4 DPP2+ . On the contrary, signals of the ferrocene moiety exhibits upfield shifts due to shielding by the ring current of the porphyrin ring upon addition of increasing amounts of FcCOOH. The largest upfield shift is observed on the proton adjacent to the carboxylate, indicating formation of hydrogen bonds between the pyrrole N H moieties of H4 DPP2+ and the carboxylate group of FcCOO− in CDCl3 [94]. The reduction potentials of H4 DPP2+ in the presence of excess amount of the electron-donor molecules

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Scheme 7. Energy diagrams of photodynamics of supramolecular complexes of H4 DPP2+ with hydrogen-bonded electron donors [94]. D denotes an electron-donor moiety in the scheme.

having the carboxyl groups vary in the range −0.52 to −0.71 V, indicating that the strength of the hydrogen bonding in solution affects the reduction potentials of H4 DPP2+ . The energy diagram of intrasupramolecular photoinduced electron transfer and back electron transfer in hydrogen-bonded supramolecular complexes is depicted in Scheme 7. Intrasupramolecular photoinduced electron transfer occurs from an electron-donor unit in the ground state to 1 [H4 DPP2+ ]* and the back electron transfer affords the ground state of H4 DPP2+ rather than the triplet excited states (3 [H4 DPP2+ ]*), because the energy level of the electron-transfer state (0.85–1.29 eV) is lower than that of 3 [H4 DPP2+ ]* (1.43 eV) [94]. The rate constants of photoinduced electron transfer and back electron transfer determined by femtosecond laser flash photolysis have been evaluated in light of the Marcus theory of electron transfer, allowing

Fig. 8. (a) Absorption spectra and (b) cyclic voltammograms of [Zn((OBu)8 PcHn )]n+ (n = 0–4) in PhCN [88].

Fig. 9. Crystal structure of H4 DPP(FcCOO)2 from different directions [94]. Gray, carbon; blue, nitrogen; red, oxygen; orange, iron. Hydrogen atoms and solvent molecules of crystallization are omitted for clarity.

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Fig. 10. (a) Driving force dependence of log kET (䊉) or kBET () for intrasupramolecular photoinduced electron transfer and back electron transfer in supramolecular complexes of H4 DPP2+ with hydrogen-bonded electron donors (the numbers correspond to those in Scheme 6) in PhCN at room temperature [94]. Fitting to the Marcus theory of the electron transfer is shown by the solid line with  = 0.68 eV and V = 43 cm−1 and the dotted lines with  = 0.65 and 0.71 eV and V = 43 cm−1 , respectively. (b) Distance dependence of ln kET for intrasupramolecular electron transfer from electron donors (FcCOO− , FcPhCOO− , and FcbphCOO− ) to 1 [H4 DPP2+ ]*, where the distance between an electron donor and an electron acceptor is defined as that between the Fe atom of the ferrocene moiety and the center of a mean plane of the porphyrin ring.

one to determine the reorganization energy () and the electronic coupling matrix constant of non-adiabatic photoinduced electron transfer and back electron transfer to be 0.68 eV and 43 cm−1 , respectively (Fig. 10a). The  value is comparable to those of covalently linked donor–acceptor systems consisting of electrically neutral porphyrins as electron donors (0.41–0.66 eV) [5–7,95–99]. The large electronic coupling matrix element may result from the strong interaction between H4 DPP2+ and electrondonor molecules because the HOMO of FcCOO− extends to the carboxylate moiety. The distance dependence of electron-transfer rate constants has also been examined using a series of ferrocenecarboxylate derivatives connected by linear phenylene linkers. The distance dependence of the rate constant of electron transfer (kET ) is kET = k0 exp(−ˇr), where ˇ = 0.64 A˚ −1 (Fig. 10b) [94]. This indicates that intrasupramolecular photoinduced electron transfer and back electron transfer in supramolecular complexes of H4 DPP2+ with hydrogen-bonded electron donors proceed as efficiently as those in covalently bonded donor–acceptor systems [1–14]. Thus, the combination of a variety of electron donors with carboxyl groups acting as hydrogen bonding sites with a diprotonated porphyrin (H4 DPP2+ ) provides a versatile strategy to construct stable supramolecular electron donor–acceptor systems for further development of supramolecular photofunctional materials. The saddle-distortion of porphyrin and phthalocyanine cooperatively stabilizes the supramolecular assembly by virtue of the facile protonation of the porphyrin to form strong hydrogen bonds with carboxyl groups and also by the enhanced Lewis acidity of a Zn(II) center in the phthalocyanine to strengthen the axial coordination bond to the metal center [100,101]. On the basis of this concept, we prepared a supramolecular assembly (8) composed of H4 DPP(4-PyCOO− )2 (4-PyCOO− = 4-pyridinecarboxylate) and Zn(Ph8 Pc) [102]. As shown in Fig. 11, in the crystal structure of 8, the linker, 4-PyCOO− , forms hydrogen bonds through the carboxyl group with the N H protons of H4 DPP2+ and coordination bond through the nitrogen atom with the Zn(II) center [102]. 1 H NMR spectrum of 8 in CDCl3 is different from those of H4 DPP(4-PyCOO− )2 and Zn(Ph8 Pc), exhibiting line-broadening probably due to restricted flexibility [102]. In order to confirm the presence of the supramolecular assembly 8 in solution, 1 H diffusion ordered spectroscopy (1 H DOSY) experiments have been conducted in CDCl3 to determine their diffusion constants to estimate exclude

volumes [102]. The molecular volume of 8 obtained by 1 H DOSY (4849 A˚ 3 ) shows a good agreement with that calculated on the basis of its crystal structure (5096 A˚ 3 ) [102]. These results strongly suggest that the supramolecular structure of 8 can be maintained even in solution. Photoexcitation of 8 in PhCN clearly gives rise to the generation of an intrasupramolecular electron-transfer state (8-ET) involving one-electron reduced H4 DPP2+ (H4 DPP•+ ) and one-electron oxidized Zn(Ph8 Pc) (Zn(Ph8 Pc)•+ ) via the singlet excited states of both the H4 DPP2+ and the Zn(Ph8 Pc) moieties (Scheme 8) [102]. This strategy can be applied to construction of novel supramolecular assemblies consisting of porphyrins and phthalocyanines to develop photofunctional materials, which can utilize a wide range of visible light effectively. The enhanced electron-accepting ability of protonated porphyrin enables the photoinduced electron transfer from an unprotonated porphyrin to a protonated porphyrin, which is closely related to the electron-transfer reaction from the excited chlorophyll to pheophytin in the photosynthetic reaction center [103,104]. So far, the introduction of electron-withdrawing groups such as fluorine groups to the porphyrin ring was utilized to enhance the electron-accepting ability of porphyrin [105], however, such methods may encounter difficulties in the synthesis as compared to simple protonation. Some examples have been reported to demonstrate the photoinduced electron transfer in diporphyrins covalently linked by a phenylene bridge or an amide linkage; however, the lifetimes of the charge-separated state were limited within the pico- to nanosecond range [26,75,106,107].

Scheme 8. Energy diagram of photodynamics of 8 in PhCN. Por2+ and Pc represent the H4 DPP2+ and Zn(Ph8 Pc) units, respectively [102].

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Fig. 11. Views of the crystal structure of [(H4 DPP){Zn(Ph8 Pc)(␬1 -N-4-PyCOO)}2 ] (8) from different orientations [102]. Dark gray, carbon; blue, nitrogen; red, oxygen; pink, zinc. Hydrogen atoms are omitted for clarity.

Sn(IV)-porphyrins can form two robust axial coordination bonds with carboxylates or phenolates because of the oxophilic nature of the Sn(IV) center even under acidic conditions [108]. Thus, the use of a Sn(IV)-porphyrin as the scaffold to organize a multiporphyrin array is advantageous in terms of its stability as compared with Zn(II) porphyrin. In addition to the structural aspect, the longer excited-state lifetime of Sn(IV)-porphyrins than Ni(II)-porphyrins have the merit to construct such electron-transfer systems. Holten and co-workers have reported the photodynamics of free-base and Ni(II) complexes of distorted porphyrins; the short lifetimes of singlet-excited states of the Ni(II)-porphyrin complexes are derived from the deactivation pathway via metal-centered excited states [110]. The combination of a Sn(IV)-porphyrin complex with a H4 DPP2+ derivative bearing a carboxyl group via two axial coordination bonds of carboxylates to the Sn(IV) center would afford a robust electron donor–acceptor triad composed of only porphyrin derivatives (Fig. 12) [109]. The reaction of Sn(DPP)(OH)2 with H2 F16 DPPCOOH (2,3,7,8,12, 13,17,18-octakis(3,5-difluorophenyl)-5-(4-carboxyphenyl)-10,15, 20-triphenylporphyrin) afforded a porphyrin triad, Sn(DPP) (H2 F16 DPPCOO)2 (9) in which the Sn(DPP) unit is linked with the two H2 F16 DPPCOO− units by strong coordination bonds [109]. The H2 F16 DPPCOO− unit of Sn(DPP)(H2 F16 DPPCOO)2 was diprotonated by the reaction with trifluoroacetic acid (CF3 COOH) to afford a robust electron acceptor-donor–acceptor porphyrin triad, Sn(DPP){(H4 F16 DPPCOO)(CF3 COO)2 }2 (10), without demetallation. The Sn(DPP) and H4 F16 DPP2+ COO− (H4 F16 DPPCOO+ ) units in 10 act as an electron donor and an acceptor, respectively (Fig. 12) [109]. The photodynamics of 9 have been investigated by femtosecond laser flash photolysis measurements in PhCN to reveal that the energy transfer occurs from the singlet excited state of the Sn(DPP) unit to the H2 F16 DPPCOO− unit to generate the singlet excited state of H2 F16 DPPCOO− [109]. In contrast to the case of 9, femtosecond transient absorption spectroscopy on 10 that contains the diprotonated form (H4 F16 DPPCOO+ ) clearly indicated the

occurrence of fast electron transfer from the singlet excited state of the Sn(DPP) unit to the H4 F16 DPPCOO+ unit [109]. The resulting singlet electron-transfer state (1 10-ET) composed of Sn(DPP)•+ and (H4 F16 DPPCOO)• decays to the ground state with the rate constant of 1.4 × 1010 s−1 in competition with the generation of the triplet ET states (3 10-ET), which has also been detected by nanosecond transient absorption spectroscopy [109]. The lifetime of the triplet ET state (50 ␮s) is much longer than that of the singlet ET state (71 ps) due to the spin-forbidden character of the back electron-transfer process (Scheme 9) [109]. 3.3. Porphyrin nanochannels Self-assembly or aggregation of porphyrin arrays were extensively studied and developed in terms of specific properties and reactivities [9–11]. For example, J-aggregation of ionic porphyrins such as diprotonated tetrakis(4-sulfonatophenyl)porphyrin (H4 TSPP2− ) is widely studied because of its intriguing photophysical and structural properties [27,43,44,111–116]. In regard to the molecular recognition, utilization of non-planar porphyrins can be a useful strategy to create a specific ␲-space constructed by curved surface of distorted porphyrins, which cannot be achieved by a planar porphyrin scaffold (Scheme 10). The hydrochloric acid salt of H2 DPP, H4 DPPCl2 , forms a porphyrin nanochannel (PNC) by its recrystallization from chloroform and acetonitrile (Scheme 10) [117,118]. In the absence of guest molecules, the PNC architecture includes a cluster of four water molecules in a cavity formed inside the channel (Fig. 13a) [117]. In the presence of excess amount of guest molecules, however, the cavity is occupied by the guest molecules such as tetrafluorohydroquinone (Fig. 13b) and tetrathiafulvalene [118] in place of water molecules. Since the PNC architecture consists of positively charged and electron-accepting H4 DPP2+ , encapsulation of the guest molecules is primarily made based on the electronic characteristics and secondly based on the shape and size of the guest molecules.

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Fig. 12. (a) Schematic descriptions of porphyrin triad (10). (b) The optimized structure of 10 calculated by the AM1 method [109].

Scheme 9. Energy diagrams of photodynamics of (a) 9 and (b) 10 in PhCN. 1 10-ET and 3 10-ET represent singlet- and triplet-electron transfer states of 10, respectively [109].

Scheme 10. Schematic representation of the formation of the PNCs.

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Fig. 13. Crystal structures of (a) PNC-water and (b) PNC-H2 QF4 [117b]. Dark gray, carbon; blue, nitrogen; red, oxygen; yellow, fluorine; light green, chlorine. H4 DPP2+ is described by a gray wireframe. Hydrogen atoms are omitted for clarity.

Scheme 11. Summary of photochemical events in PNC-H2 Q as a representative [117b].

The PNC supramolecules including electron-donating molecules allow us to observe photoinduced electron transfer from the guest molecules to the singlet excited state of H4 DPP2+ in the crystal as detected by solid-state femtosecond laser flash photolysis in a KBr pellet [117b]. The one-electron reduced species of H4 DPP2+ undergoes fast disproportionation to give the original H4 DPP2+ species and the two-electron reduced species of H4 DPP2+ (H4 DPP) via electron hopping in the solid state owing to close contacts with other H4 DPP2+ ions through intermolecular ␲–␲ interactions (Scheme 11) [117b]. In addition, H4 DPP•+ undergoes back electron transfer from the one-electron oxidized guest molecule to form 3 [H4 DPP2+ ]*, the energy level of which is lower than that of the electron-transfer state ([H4 DPP]•+ -guest•+ ). The occurrence of photoinduced electron transfer was also confirmed by EPR measurements; under photoirradiation, EPR signals derived

Fig. 14. A view of the crystal structure of PNC-TTF [118]. Dark gray, carbon; blue, nitrogen; red, oxygen; yellow, sulfur; light green, chlorine. H4 DPP2+ is described by a gray wireframe. Hydrogen atoms are omitted for clarity.

from one-electron-oxidized guest molecules can be observed with hyperfine structures [117b]. The photoconductivity of porphyrin nanostructures in the solid state has been a key factor toward the development of photovoltaic devices such as dye-sensitized solar cells [119–121]. The guest-included PNC supramolecules can be recognized as electron donor–acceptor organic materials and thus are expected to exhibit photoconductivity in the solid state. Electron hopping in the PNC supramolecules in the solid state allowed us to observe photocurrent under visible-light irradiation [118]. As a representative of photoconducting PNC-guest

Fig. 15. (a) A view of sample manipulation for photoconductivity measurements of a single crystal of PNC-TTF [118]. (b) Photocurrent response for a single crystal of PNC-TTF obtained by photoexcitation at 633 nm with He–Ne laser (in the direction of the crystallographic c axis) under various electrical field strengths: (a) 3.5 × 104 V cm−1 ; (b) 3.1 × 104 V cm−1 ; (c) 2.6 × 104 V cm−1 ; (d) 2.1 × 104 V cm−1 [118].

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Fig. 16. (a) Schematic presentation of a photoelectrochemical cell with OTE/SnO2 /PNC-TTF photoanode [118]. (b) IPCE plots for OTE/SnO2 /PNC-TTF (circle) and OTE/SnO2 /PNCwater (square).

supramolecules, the crystal structure of PNC-TTF (TTF, tetrathiafulvalene) is depicted in Fig. 14. The TTF molecule is included in the guest-inclusion site of the PNC architecture in the same way as other guest molecules as shown in Fig. 13b. Rapid photocurrent response was observed upon turn-on and turn-off of photoirradiation with He–Ne laser (5 mW, 633 nm) on a single crystal of PNC-TTF (Fig. 15a) [118]. The photocurrent in the direction of the crystallographic c axis increased in accordance with increase in the electrical field strength in the range of 2.1 × 104 to 3.5 × 104 V cm−1 , gaining 0.7 nA of photocurrent at the maximum value at the field strength of 3.5 × 104 V cm−1 (Fig. 15b). The photocurrent is 10-fold larger than that observed in the directions perpendicular to the crystallographic c axis [118]. In order to evaluate the photoelectrochemical performance of the PNC supramolecules, various OTE/SnO2 /PNC electrodes were prepared as photoanodes in photoelectrochemical cells (see Fig. 16a) [118]. The photoactive materials in the photoanodes are PNC supramolecular assemblies including various guest molecules, such as TTF, p-aminophenol (p-AP), p-hydroquinone (p-H2 Q), and p-tetrafluoro-hydroquinone (p-H2 QF4 ). Plots of incident photon-to-current efficiency (IPCE) vs the irradiation wavelength are shown in Fig. 16b for OTE/SnO2 /PNC-TTF (circle) and OTE/SnO2 /PNC-water (square). The maximum IPCE value of 10.1% was obtained for the OTE/SnO2 /PNC-TTF electrode at 460 nm, which corresponds to the absorption maximum of the OTE/SnO2 /PNC-TTF electrode [118]. 4. Concluding remarks The distortion of porphyrinoids enhances their intrinsic protonaccepting nature to expand the possibility for the formation of supramolecules and enables the control of redox potentials. The hydrogen bonding ability of protonated porphyrins offers us the opportunity to modify the surface of metal oxide by self-assembly. The construction of molecular and supramolecular assemblies composed of protonated porphyrin and phthalocyanine acting as electron acceptors provides a new avenue toward supramolecular photofunctional materials such as photovoltaic materials. Acknowledgments The author gratefully acknowledges the contributions of their collaborators and coworkers mentioned in the references. The

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