Elucidating the non-radiative losses encountered in intramolecular charge transfer compounds with benzodithiophene-4,8-dione acceptors

A new yellow-emitting quadrupolar donor–π–acceptor–π–donor (D–π–A–π–D) molecule compound has been synthesised featuring benzo-[1,2-c:4,5-c′]dithiophene-4,8-dione as the acceptor. This molecule was prepared for the purpose of elucidating the origins of the very low photoluminescence quantum yield encountered in its thermally activated delayed fluorescent (TADF) red-emitting isomer which used benzo-[1,2-b:4,5-b′]dithiophene-4,8-dione as the acceptor. The molecule was designed to circumvent the energy gap law, by having a wider HOMO–LUMO gap, while retaining a comparable singlet–triplet gap but ultimately demonstrates even weaker photoluminescence than the red isomer. It shows extremely fast intersystem crossing followed by rapid non-radiative decay and no observable TADF. The electronic structure of this new molecule has been studied using cyclic voltammatery alongside steady-state and transient optical spectroscopy, with observations underpinned by computational insights. To identify whether the observations made from the experimental results might be general properties of benzodithiophene-4,8-dione containing emitters, a computational study is extended to the four isomers of benzodithiophene-4,8-dione in comparison with 9,10-anthraquinone. The results suggest that the singlet and triplet manifolds of these systems are strongly coupled via spin–orbit interactions, and explain how the relative electron-accepting strength of these quinones arises from an interplay between the resonance gains or losses of the central benzene and fused thiophene rings upon photoexcitation. This provides valuable insights into the design principles required for efficient organic light-emitting materials.