Generation of Microsecond Charge-Separated Excited States in Rhenium(I) Diimine Complexes: Driving Force Is the Dominant Factor in Controlling Lifetime.

A transition-metal-based donor-(linker)-acceptor system can produce long-lived charge transfer excited states using visible excitation wavelengths. The ground- and excited-state photophysical properties of a series of [ReCl(CO)3(dppz-(linker)-TPA)] complexes, with varying donor and acceptor energies, have been systematically studied using spectroscopic techniques (both vibrational and electronic) supported by computational chemistry. The long-lived excited state is 3ILCT in nature for all complexes studied, characterized through transient absorption and emission, transient resonance Raman (TR2), and time-resolved infrared (TRIR) spectroscopy and TDDFT calculations. Modulation of the donor and acceptor energies results in changes of the 3ILCT lifetime by 1 order of magnitude, ranging from 6.1(±1) μs when a diphenylamine donor is used to 0.6(±0.2) μs when a triazole linker and triphenylamine donor is used. The excited-state lifetime may be rationalized by consideration of the driving force within the framework of Marcus theory and appears insensitive to the nature of the linker.