Thermally activated delayed fluorescence (TADF) is a
phenomenon
that relies on the upconversion of triplet excitons to singlet excitons
by means of reverse intersystem crossing (rISC). It has been shown
both experimentally and theoretically that the TADF mechanism depends
on the interplay between charge transfer and local excitations. However,
the difference between the diabatic and adiabatic character of the
involved excited states is rarely discussed in the literature. Here
we develop a diabatization procedure to implement a four-state model
Hamiltonian to a set of TADF molecules. We provide physical interpretations
of the Hamiltonian elements and show their dependence on the electronic
state of the equilibrium geometry. We also demonstrate how vibrations
affect the TADF efficiency by modifying the diabatic decomposition
of the molecule. Finally, we provide a simple model that connects
the diabatic Hamiltonian to the electronic properties relevant to
TADF and show how this relationship translates into different optimization
strategies for rISC, fluorescence, and overall TADF performance.