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

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.