Understanding the photochemistry of a crystalline push–pull norbornadiene photoswitch

Molecular solar thermal (MOST) materials store and release solar energy through light-induced reversible reactions involving molecular photoswitches. Solid-state crystalline MOST materials can offer higher energy densities and easier device integration than their liquid counterparts. However, their photochemical mechanisms remain poorly understood. Norbornadiene (NBD), which undergoes a [2+2]-photocycloaddition to form its photoisomer quadricyclane (QC), has been proposed as a candidate for MOST applications. We used multiconfigurational quantum mechanical calculations and non-adiabatic molecular dynamics to investigate the mechanism of a push–pull NBD-derivative, 1,3,4-trimethyl-5,6-dicyanonorbornadiene (TMDCNBD). This study demonstrates cutting-edge multiscale ONIOM(QM/QM′) nonadiabatic molecular dynamics framework in TMDCNBD crystals. The crystal packing of TMDCNBD preserves molecular flexibility, enabling ultrafast [2+2]-photocycloaddition via energetically accessible S₁/S₀ conical intersections, with negligible exciton transport. Simulations predict product quantum yields of 57% for TMDCNBD and 37% for its metastable quadricyclane (QC) form, TMDCQC, which stores 0.36 MJ kg⁻¹. This work demonstrates TMDCNBD as a promising crystalline MOST candidate and establishes a transferable computational protocol for modelling ultrafast photochemistry in the solid state.