Prepared for Inorg. Chem.

The excited state properties of a series of binuclear NHetPHOS-Cu(I) complexes (NHetPHOS) have been investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT). It is shown that experimental trends observed in powder, generally explored via S and T excited state energetics and S ⇔ T intersystem crossing (ISC) efficiency, are hardly analyzed on the basis of excited state properties calculated in solution. Indeed, several local minima corresponding to various structural deformations are evident on the lowest excited state potential energy surfaces (PES) when solvent correction is applied, leading to a four-state thermally activated delayed fluorescence (TADF) mechanism. In contrast, preliminary simulations performed in the solid point to the reduction of nuclear flexibility and consequently to a rather simple two-state model.