Enabling Singlet Fission Exothermicity by Tuning the Solvent in a High Energy Triplet Formyl‐Fluorene Derivative

In this investigation, solvent polarity and viscosity effects coupled with the concentration effect on the photophysical behavior of a formyl fluorene derivative ( AF ) have been explored by stationary and ultrafast spectroscopies. The femtosecond spectroscopic results pointed out the formation of a double triplet state with a short lifetime characteristic of a singlet and spectral features of the triplet state. Moreover, efficient triplet exciton separation was observed with a triplet yield in a trend with solution concentration that reaches values higher than 100% and up to 140% in viscous medium at high concentration. The latter finding definitively corroborated the presence of an unconventional INTRA + INTERmolecular singlet fission (SF) mechanism: INTRAmolecular formation of the correlated triplet pair 1 TT, followed by INTERmolecular triplet separation via a “super‐diffusional” triplet–triplet energy transfer process. Interestingly, small changes in solvent dielectric constant/polarizability have been found to be very effective in modulating the S 1 ‐ 1 TT energy gap pointing to a slightly endothermic SF in toluene and an exothermic SF in the less polar cyclohexane and decahydronaphthalene. Moreover, femtosecond fluorescence up‐conversion measurements of AF in Tol unveiled delayed fluorescence from S 1 and proper emission of 1 TT and allowed the quantum yield for these processes to be separated and roughly estimated (∼0.01 for both of them).