Reversible coupling of radical pair spin dynamics to a locally excited electronic singlet state

Thermally assisted delayed fluorescence (TADF) in electron-donor-bridge-acceptor triads has recently been shown to provide a new way of observing the spin dynamics of charge-separated states (CSS) corresponding to linked radical pairs. In this work, we present a theoretical approach for extending standard quantum-dynamical models to describe this system. Using a representative example that combines four electronic radical-pair spin states with three nuclear spin states of a single nitrogen nucleus, we extend the Hilbert space from 12 to 15 dimensions by including the excited singlet state S₁. We derive Liouvillian operators that account for the kinetic coupling between S₁ and the CSS, including decay, charge separation, and recombination, and illustrate the resulting dynamics with numerical examples.