Subphthalocyanines as light-harvesting electron donor and electron acceptor in artificial photosynthetic systems

Light-harvesting subphthalocyanine-ferrocenophane (SubPc-Fc; 1) and subphthalocyanine-naphthalenediimide (SubPc-NDI; 2) dyads have been synthesized, characterized, and probed by femtosecond laser photolysis. In dyads 1 and 2, both the electron-donating ferrocenophane and the electron-accepting naphthalenediimide are axially linked with the functional O-Ph groups (at the para position) in the axial positions of SubPc. Electrochemical data show that the SubPcs can act as both electron donors and electron acceptors. The geometric and electronic structures of dyads 1 and 2 were calculated by ab initio B3LYP/6-311G methods. The optimized structures showed that the Fc and NDI entities are separated from SubPc by 8.32 Å (for dyad 1) and 8.85 Å (For dyad 2). The distribution of HOMOs and LUMOs suggests the formation of SubPc ^•--Fc ⁺ and SubPc ^•+-NDI ^•- as charge-separated states for dyads 1 and 2, respectively. Upon photoexcitation of the subphthalocyanine unit, these arrays undergo photoinduced electron transfer to form the corresponding charge-separated species, SubPc ^•--Fc ⁺ and SubPc ^•+ -NDI ^•-, in which SubPc acts as an electron acceptor and an electron donor, respectively, as expected from their redox potentials determined by cyclic voltammetry. Femtosecond transient spectroscopic studies have revealed that a fast charge separation (10 ¹¹∼10 ¹² s ^-1) occurs for dyads 1 and 2. From the kinetic studies, the rate of charge recombination and the lifetime of the charge-separated state (SubPc ^•+-NDI ^•-) were found to be 2.9 × 10 ⁹ s ^-1 and 345 ps, respectively.