Ultrafast and Quantitative FRET Sensitization of Singlet Fission in Solution-Processable Para-Azaquinodimethane Films

Abstract

In this work, a green protocol for the synthesis of new para-Azaquinodimethane (pAQM) small molecules, functionalized with anisole (An) or bithiophene (TT) moieties on the central unit and with different alkoxy groups as lateral substituents is developed and reported. The alkoxy lateral substituents are found to strongly impact the solubility and processability of the chromophores. On the other hand, the steady–state and time-resolved spectroscopic results show completely different spectral and photophysical features for the An- and TT- derivatives. Only for the TT-compounds, the nanosecond and femtosecond transient absorption experiments reveal low efficiency of triplet production in solution as well as ultrafast (≈1 ps) and efficient (≈200%) singlet fission (SF) in thin film. The yellow An-containing molecules are instead used to sensitize the purple SF-active TT-compounds via quantitative FRET in mixed thin films, which interestingly exhibit panchromatic absorption extending in the entire visible spectral range. With this study, the synergy between FRET and SF is exploited at the intermolecular level in the solid-state for the first time by considering unconventional and stable SF chromophores, opening intriguing new possibilities for solar energy harvesting.