The intersystem crossing mechanism of an ultrapure blue organoboron emitter

Quantum dynamics and a spin-vibronic Hamiltonian are used to investigate the intersystem crossing (ISC) mechanism of a narrow organoboron molecular emitter, 5,9-diphenyl-5,9-diaza-13 b-boranaphtho [3,2,1-de]anthracene (DABNA-1). We find a rate of ISC (kISC) in good agreement with experiment and which operates via a second-order spin-vibronic coupling mechanism. The nonadiabatic coupling activating this mechanism occurs between the lowest singlet (S1) state and higher lying singlet states promoting ISC into the T2 state. The large S1-T1 energy gap, combined with the slow ISC arising from small spin-orbit coupling and the rigidity of the molecule is the reason for the slow rISC observed experimentally. The importance of the spin-vibronic mechanism, even for narrow Thermally Activated Delayed Fluorescence (TADF) emitters illustrates the importance of identifying the effect of key vibrational modes and their action, when attempting to design molecular emitters combining narrow TADF with efficient rISC.