Linkage dependence of charge separated lifetime in small donor-acceptor dyads system

Abstract

Fast charge separation (CS) process and slow charge recombination (CR) process are critical for effective utilization of solar energy in artificial photosynthesis, photocatalysis and organic photovoltaic devices. Here we demonstrated in a model donor-acceptor (D-A) dyad of styrene based triphenylamine derivative (MTPA) as an electron donor (D), triazine group (TRC) as an electron acceptor (A) (MTPA-TRC) that the CS and CR process can be modulated by the linker groups between donor and acceptor. We synthesized a series of flexibly linked D-A dyads MTPA-TRC-n with different length of methylene units (n = 1, 2, 3, n stands for the number of methylene units) as the spacer and lifetimes of charge-separated states ranging from nanoseconds to milliseconds were observed. According to the Marcus charge transfer theory, MTPA-TRC-n have similar reorganization energies (λ) but decreased electronic coupling matrix element (V). V value is the key factor to dominate the charge transfer rate and charge-separated lifetime in this D-A dyads system. Additionally, the decay coefficient (β) of length dependence of electron transfer rate was decreased from 0.4 Å−1 to 0.3 Å−1 in methanol compared to acetonitrile. It revealed that electronic coupling between the donor and acceptor could be enhanced by the increasing solvent polarity in the non-conjugated linked D-A system.