Photoindued Electron Transfer in Hydrogen-Bonded Supramolecular Assemblies Using a Diprotonated Saddle-Distorted Porphyrin
Authors: Kojima Takahiko 
Published: February 2020

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 (H2DPP) and its derivatives, which can be easily protonated to afford stable diprotonated species, such as H4DPP2+.2 The diprotonated species can act as electron acceptors3 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 H4DPP2+ as a electron acceptor and electron donors with a carboxylate group.5

                For example, a hydrogen-bonded supramolecular assembly can be formed using H2DPP 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 H2DPP 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(ClO4)2 (TPA = tris(2-pyridylmethyl)amine)7 reacted with PyCOOH in MeOH to give [RuCl(TPA)(PyCOOH)]ClO4 (1) in 38% yield. The complex 1 was reacted with H2DPP in acetone to form a 2:1 assembly, RuIITPA•••H4DPP2+•••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 H2DPP, H3DPP+, with 1(Scheme 1).

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Scheme 1. Formation of supramolecular assemblies composed of 1 and H­2DPP in acetone.

                The association constant for the 2:1 assembly was determined to be (1.6 ± 0.7) × 1012 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) × 106 M–1 for the first step forming the 1:1 assembly and (5.0 ± 0.2) × 104 M–1 for the second step affording the 2:1 assembly. Since the MeOH molecule has been demonstrated to form hydrogen bonding with H3DPP+,4 it is plausible that the MeOH binding on the opposite side of the H3DPP+can stabilize the 1:1 assembly.

                The redox potentials of the H4DPP2+ moiety of the 2:1 assembly in acetone was determined to be –0.53 V (vs SCE) and the RuII/RuIII couple of the RuII-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 H4DPP•+, which should be derived from an ET State generated by photoinduced ET from the RuII-TPA component to the H4DPP2+ moiety. The lifetime of the ET state was determined to be 172 ps (k
BET = 5.8 × 106 s–1).

                In this presentation, details of formation of the supramolecular assemblies and the photodynamics upon visible light excitation will be described.

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Figure 1