Asymmetric planar-to-Dewar isomerisation in BN-doped naphthalene: mechanistic implications for molecular solar thermal storage.

The planar-to-Dewar valence isomerisation of 4a,8a-azaboranaphthalene (BNNaph), a π-extended BN-doped analogue of azaborine, is investigated to evaluate how BN incorporation reshapes the minimum-energy pathway on the ground state. This process is, for example, relevant in the context of molecular solar thermal (MOST) energy storage, where absorbed sunlight is converted into chemical energy through reversible photoisomerisation. Structures and vertical excitations were computed using DFT and TD-DFT, minimum-energy pathways were mapped with nudged elastic band (NEB) calculations and pathway energetics were refined with state-averaged XMS-CASPT2. In addition, azaborine was examined as a comparison system, with particular emphasis on whether substituents at nitrogen and boron promote Dewar formation. Compared with the carbon analogue, the conversion pathway becomes asymmetric with a metastable intermediate stabilised by a transient boron-carbon contact. The transition structure closely resembles an S0/S1 conical intersection, which is consistent with a vibrationally activated nonradiative funnel. For tuning MOST properties, screening of single substituents across the whole molecule reveals predominantly red-shifted S1 energies together with increased oscillator strengths and indicates that appropriate substitution can improve Dewar formation in azaborine derivatives.