Molecular geometry and TADF photophysics: the strange case of DMAC-py-TRZ

We present the synthesis, optoelectronic characterization, and a detailed theoretical study of DMAC-py-TRZ, a novel, efficient TADF emitter. This compound is a structural relative of the well-known TADF compound DMAC-TRZ, substituting the bridging phenylene for a pyridyl group. This marginal change has an enormous impact on the structure and hence on the photophysics as the steric interactions between the DMAC donor and the bridge that force DMAC-TRZ into an orthogonal conformation are attenuated and permit DMAC-py-TRZ to adopt a planar and slightly bent structure in the ground state. The large degree of conjugation in the bent DMAC-py-TRZ structure, demonstrated by the strong intensity of the lowest excitation with CT character, is responsible for a large singlet triplet gap, hence hindering TADF of this bent conformer. The computational analysis predicts that emission occurs, however, from a relaxed orthogonal excited-state geometry, as confirmed by the huge Stokes shift observed in non-polar solvents. In this relaxed orthogonal geometry TADF is indeed observed. Emission from the unrelaxed state is recovered in glassy frozen solvents, where the emission band is largely blue-shifted compared with measurements in liquid solvent, and TADF is quenched. In amorphous matrices, structural disorder leads to the coexistence of both conformers, even if, depending on the emitter concentration, dual fluorescence may disappear due to a fast energy transfer from the bent to the orthogonal conformers. We maintain that this efficient energy transfer is responsible for the good efficiency of DMAC-py-TRZ devices, because of the presence in the matrix of a sizable proportion of compounds that adopt the bent structure, favorable to act as the host for the orthogonal TADF conformer of DMAC-py-TRZ.