Fig. 5: The role of intermolecular excitations in delayed fluorescence.
a A schematic of a representative BF2 dimer and the most relevant electronic excitations (state energies not to scale), demonstrating how the electron-hole separation in the inter-CT state can be less than the intra-CT exciton. This renders the inter-CT states more stable than the intra-1CT, meaning intermolecular charge transfer following photoexcitation is thermodynamically favourable. The loosely bound inter-CT states can then undergo HFI-ISC processes, enabling efficient spin mixing in BF2, followed by recombination to the intra-1CT for light emission. b A schematic of a representative 4CzIPN dimer and the most relevant electronic excitations (state energies not to scale), where the electron-hole separation in the inter-CT state is significantly larger than the intra-CT excitons. Consequently, the inter-1CT state is >0.12 eV higher than the intra-1CT excitons and intra-1CT to inter-1CT interconversion does not readily occur. As a result, the ISC processes in 4CzIPN are promoted by spin-orbit coupling, SOC, likely involving the intra-1CT, intra-3CT and 3LE states. c The photophysical processes occurring in a neat BF2 film that enable efficient spin state interconversion via the HFI. Following photoexcitation, the intra-1CT dissociates to form an inter-1CT state with the neighbouring molecule in the dimer. Subsequently, the inter-CT state can readily obtain longer-range separation, likely enabled by the shallow dependence of the inter-CT state energy on distance, to form loosely bound inter-CT states where the exchange energy is small enough for the HFI to mediate the ISC processes.
