Resolving the Ambiguity of Thermal Reversion in a Nonconjugated Monocyclic Diene-Based Photoswitch for Rechargeable Solar Thermal Batteries.

Abstract

We report nonconjugated monocyclic dienes (nCMDs) as unique photoswitchable molecules that hold promise for harvesting substantial solar energy and storing it for extended durations. Herein, cyclohepta-1,4-diene 1a and its N-heterocyclic analogue 2a have been considered as prototypical models for investigating photoswitching behavior in nCMDs. Initially, the nonradiative deactivation pathway of nCMD from the low-lying excited state to the [2 + 2]-cycloadduct has been evaluated. This work resolves the ambiguity and rationalizes the kinetically preferred route for the regeneration of parent diene from a photoisomer that enables the thermo-reversible photoswitching cycle. Moreover, it sheds light on how the syn-anti-isomerism upon thermolysis facilitates the long-term storage of harvested solar energy. Extensive electronic structure analysis reveals that charge transfer from a lone pair on N restricts syn-anti-isomerism while promoting direct reversal of the photoproduct into diene and is highlighted as a pivotal factor for relatively short-term energy storage. The critical physical insights gained in this work are crucial for achieving efficient and sustainable solutions for solar thermal energy storage using novel photoswitches which can contribute to the global transition toward renewable energy sources.