Ultrafast Excited-state Dynamics of Halogen-Substituted Squarylium: Absence of the Heavy-atom Effect

The heavy-atom effect of halogen (Br and I) has been widely employed for boosting transition between singlet and triplet excited state of organic molecules, while recent investigation indicated halogen-substitution can also enhance fluorescence even lead to aggregation-induced emission (AIE). In this work, we investigated ultrafast excited state dynamics of halogen-substituted model system, <i>i.e.</i> squaryliums NSQ-R (R = H, Cl, Br, I), by using femtosecond spectroscopy and theoretical approach. Fast external reorganization (Ex-re, ~3 ps) and slow internal reorganization (In-re, 5–20 ps) were observed, while quantitative fitting indicated halogen-substitution leads to slower non-radiative S₁→S₀ decay (<i>k</i>_NR^S) and subsequently enhanced fluorescence emitting. By analyzing the extracted <i>k</i>_NR^S within theoretical framework of non-radiative transition in strong coupling regime, plausible AIE mechanism of NSQs was revealed. Our work provided a clear picture on non-radiative dynamics of halogen-substituted squaryliums, which might be useful for future development of organic dyes.