Photoindued Electron Transfer in Hydrogen-Bonded Supramolecular Assemblies Using a Diprotonated Saddle-Distorted Porphyrin

Abstract

Photoinduced electron transfer is an essential reaction to gain charge separation for photosynthesis in bacteria and plants, solar cells, and photoconductive materials. Among light-absorbing molecules and materials for photofunctionality, porphyrins showing strong absorption bands in the visible region are attractive molecules and have been utilized to envision the charge separation toward energy conversion. A number of photofunctional systems performing charge separation have been developed using porphyrins and the derivatives as electron donors in photoinduced electron transfer. 1 On the other hand, we have developed a new concept by using saddle-distorted dodecaphenylporphyrin (H 2 DPP) and its derivatives, which can be easily protonated to afford stable diprotonated species, such as H 4 DPP 2+ . 2 The diprotonated species can act as electron acceptors 3 and also form strong intermolecular hydrogen bonding with molecular entities having carboxylate groups to afford stable supramolecular assemblies. 4 Based on the hydrogen bonding, we can construct photofunctional supramolecular assemblies composed of H 4 DPP 2+ as a electron acceptor and electron donors with a carboxylate group. 5 For example, a hydrogen-bonded supramolecular assembly can be formed using H 2 DPP and a Zn(II) complex of a saddle-distorted phthalocyanine as an electron donor with nitrogen-bound 4-carboxyl pyridine (PyCOOH) as an axial ligand of the Zn(II) center. 6 The usage of PyCOOH may expand the possibility to attain a variety of hydrogen-bonded supramolecules using stable metal complexes, which can hold N -bound PyCOOH as a ligand and also act as electron donors. Herein, we will report construction of a supramolecular assembly using H 2 DPP and a Ru(II)-pyridylamine complex with N -bound PyCOOH as a ligand and photodynamics of photochemical reactions to afford an electron-transfer (ET) state. Dimeric [RuCl(TPA)] 2 (ClO 4 ) 2 (TPA = tris(2-pyridylmethyl)amine) 7 reacted with PyCOOH in MeOH to give [RuCl(TPA)(PyCOOH)]ClO 4 ( 1 ) in 38% yield. The complex 1 was reacted with H 2 DPP in acetone to form a 2:1 assembly, Ru II TPA•••H 4 DPP 2+ •••RuTPA (Scheme 1). Addition of MeOH (3%) to the acetone solution allowed us to obtain a unique 1:1 hydrogen-bonded supramolecular assembly of the monoprotonated H 2 DPP, H 3 DPP + , with 1 (Scheme 1). (insert Image) Scheme 1. Formation of supramolecular assemblies composed of 1 and H­ 2 DPP in acetone. The association constant for the 2:1 assembly was determined to be (1.6 ± 0.7) × 10 12 M –2 in acetone at 298 K. In the presence of MeOH (3%) in acetone, we could observe two-step association to form 1:1 and 2:1 assemblies and the binding constants were determined to be (3.2 ± 0.5) × 10 6 M –1 for the first step forming the 1:1 assembly and (5.0 ± 0.2) × 10 4 M –1 for the second step affording the 2:1 assembly. Since the MeOH molecule has been demonstrated to form hydrogen bonding with H 3 DPP + , 4 it is plausible that the MeOH binding on the opposite side of the H 3 DPP + can stabilize the 1:1 assembly. The redox potentials of the H 4 DPP 2+ moiety of the 2:1 assembly in acetone was determined to be –0.53 V (vs SCE) and the Ru II /Ru III couple of the Ru II -TPA moiety was done to be +0.59 V (vs SCE). Thus, the energy level of an electron-transfer state of the assembly was estimated to be 1.12 eV. Femtosecond laser flash photolysis of the 2:1 assembly with photoexcitation at 500 nm in acetone/MeOH (3%) allowed us to observe the formation of H 4 DPP •+ , which should be derived from an ET State generated by photoinduced ET from the Ru II -TPA component to the H 4 DPP 2+ moiety. The lifetime of the ET state was determined to be 172 ps ( k BET = 5.8 × 10 6 s –1 ). In this presentation, details of formation of the supramolecular assemblies and the photodynamics upon visible light excitation will be described. References S. Fukuzumi, T. Kojima, J. Mater. Chem. 2008 , 18 , 1427. R. Harada, T. Kojima, Chem. Commun. 2005 , 716. (a) T. Kojima et al. , Chem.–Eur. J. 2007 , 13 , 8714. (b) T. Nakanishi et al. , J. Am. Chem. Soc. 2009 , 131 , 577. (c) T. Honda et al. J. Phys. Chem. C 2010 , 114 , 14290. (d) S. Fukuzumi, T. Honda, T. Kojima, Coord. Chem. Rev. 2012 , 256 , 2488. T. Honda, T. Kojima, S. Fukuzumi, Chem. Commun. 2009 , 4994. T. Honda et al. , J. Am. Chem. Soc. 2010 , 132 , 10155. T. Kojima et al. , Angew. Chem. Int. Ed. 2008 , 47 , 6712. T. Kojima et al. , Inorg. Chem. 1998 , 37 , 4076. Figure 1