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

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, Ru^IITPA•••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 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 H₄DPP^•+, which should be derived from an ET State generated by photoinduced ET from the Ru^II-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.

References

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