Highly efficient cycloreversion of photochromic dithienylethene compounds using visible light-driven photoredox catalysis

Photochromic cis-1,2-dithienylethene (DTE) compounds are the most suitable to the application in reversible molecular memories and switches, but imbalance in the quantum yields for the chromic interconversion limits the full potentials. We have demonstrated and investigated photoelectrocatalytic cycloreversion of DTE compounds. A series of cyclometalated Ir(III) complexes served as photoredox catalysts to achieve one order of magnitude enhancement in the cycloreversion quantum yields. The mechanism, involving photoinduced oxidation of DTE, electrocatalytic ring opening and reductive termination, has been thoroughly investigated. Nanosecond transient spectroscopic techniques were employed to directly monitor bidirectional electron transfer between DTE and the photoredox catalyst. It was found that the oxidative photoinduced electron transfer was diffusion-controlled and located in the Marcus-normal region, whereas the competing back electron transfer occurred in the Marcus-inverted region. This novel discovery establishes that synthetic control over back electron transfer, rather than photoinduced electron transfer, can improve the performance of the photoelectrocatalysis. Combined studies, including the kinetic investigations with the use of variable-temperature stopped-flow UV-vis absorption spectroscopy and quantum chemical calculations based on time-dependent density functional theory, further enabled identification of the radical intermediate that underwent thermal, electrocatalytic cycloreversion. Finally, analyses based on the Marcus theory of electron transfer suggested regeneration of the excited-state catalyst in the termination step to initiate dark-state electrocatalytic cycloreversion. The results obtained in this work established novel principles to maximizing quantum yields for photoinduced cycloreversion of DTEs. It is envisioned that our findings will provide novel guidance to the future application of the truly reversible photochromism to a broad range of molecular photonic systems.