Electron transfer reaction of light harvestingzinc naphthalocyanine–subphthalocyanine self-assembled dyad: spectroscopic, electrochemical, computational, and photochemical studies

Abstract

Electron transfer reaction of a self-assembled donor–acceptor dyad formed by axial coordination of zinc naphthalocyanine, ZnNc, and subphthalocyanine appended with pyridine coordinating ligand, SubPc(py), was investigated in the present study. The SubPc(Py) : ZnNc self-assembled dyad absorbs the light in a wide section of the UV/Vis/NIR spectra. The formation constant of SubPc(py) : ZnNc in o-dichlorobenzene was found to be 1.2 × 10⁵ M⁻¹ from the steady-state absorption and emission measurements, suggesting stable complex formation. The geometric and electronic calculations by using ab initio B3LYP/6-311G methods showed the majority of the highest occupied frontier molecular orbital (HOMO) on the zinc naphthalocyanine entity, while the lowest unoccupied molecular orbital (LUMO) was on the subphthalocyanine entity, suggesting that the charge-separated state of the supramolecular complex is (SubPc(py))˙⁻ : ZnNc˙⁺. The electrochemical results suggest the exothermic charge-separation process via the singlet states of both SubPc(py) and ZnNc entities. Upon coordination the pyridine appended subphthalocyanine to ZnNc; the main quenching pathway involved charge separation via the singlet excited states of ZnNc and SubPc(py). A clear evidence of the intramolecular electron transfer from the singlet state of ZnNc to SubPc(py) was monitored by femtosecond laser photolysis in o-dichlorobenzene by observing the characteristic absorption band of the ZnNc radical cation in the NIR region at 960 nm. The rate of charge-separation process was found to be 1.3 × 10¹⁰ s⁻¹, indicating fast and efficient charge separation. The rate of charge recombination and the lifetime of the charge-separated state were found to be 1.0 × 10⁹ s⁻¹ and 1 ns, respectively. The absorption in a wide section of the solar spectrum and high charge-separation/charge-recombination ratio suggests the usefulness of self-assembled SubPc(Py) : ZnNc for being a photosynthetic model.