Excited state conjugation breaking in rigid, planar molecules leading to charge-transfer states and thermally activated delayed fluorescence

TADF efficiency relies in part on minimizing the energy gap between the lowest singlet and triplet states, commonly achieved by orthogonally oriented D and A charge transfer molecules, i.e. twisted DA molecules. Fully rigid DA structures could result in far more stable molecules, offering high rates of fluorescence and short CT lifetime and negligible energy dispersion, but the main obstacle to a fully rigid molecular structure is the inherent planarity and conjugation which is believed to make intermolecular charge transfer (ICT) improbable. Here I will report on the first planar rigidized molecule showing a stable ICT state with moderate TADF. From time resolved spectroscopy and theoretical studies we propose a mechanism where conjugation is broken in the excited state leading to stabalization of the planar ICT state. This new molecule offers a radically different direction for future TADF molecules.