From solution to thin films: unravelling excited-state behaviour in halogenated diradicaloids

Stable fluorine and chlorine decorated Thiele-like diradicaloids were synthesized and studied in solution and in thin films. Femtosecond transient absorption measurements showed, for both molecules, charge-separated states in solution, but only short-lived singlets in thin films, with no evidence of twisted or charge-separated intermediates. This suppression of excited-state dynamics reflects the strong intermolecular interactions in the solid state and limits their singlet fission potential, highlighting key design challenges for implementing diradicaloids in solid-state optoelectronic and photovoltaic devices.