Exploring Multiexciton Generation in an Asymmetric Aza-BODIPY Dimer

We investigate the photophysics of the aza-BODIPY dimer D[1,3], formed by linking two aza-BODIPY monomers by a C-C single bond at positions 1 and 3, using multireference electronic structure calculations and quantum nuclear dynamics. Our findings reveal that the dimer exhibits strong electronic couplings and weakly endoergic singlet fission (SF) energetics, making SF energetically feasible. Superexchange-like adiabatic states are obtained, giving rise to cross-correlation contributions in the simulated absorption spectrum from a localized diabatic state. Re-expressing the diabatic Hamiltonian from a local basis to a superexchange basis allowed accurate simulation of the absorption spectrum. Quantum nuclear dynamics simulations show rapid formation of a superexchange-like state on a 50 fs time scale. The formed superexchange-like state is composed of charge transfer (CT) (30%), multiexcitonic (20%), and local excitation (20%) configuration state functions (CSFs). The limited multiexcitonic character renders SF inefficient. Instead, the charge-transfer manifold becomes substantially populated, activating an alternative route for triplet formation. Spin-orbit coupling calculations and rates of intersystem crossing (10⁷ s^-1) reveal that the spin-orbit charge-transfer intersystem crossing (SOCT-ISC) mechanism provides a more effective pathway for triplet-state generation in the dimer D[1,3].