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 spin-crossover in organic molecules, while recent investigation indicated halogen-substitution can also enhance fluorescence and even lead to aggregation-induced emission (AIE). In this work, we investigated ultrafast excited state dynamics of a halogen-substituted model system, i.e. 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 a slower non-radiative S₁→S₀ decay (k_\mathrmNR^\mathrms) and subsequently enhanced fluorescence emitting. By analyzing the extracted k_\mathrmNR^\mathrms within theoretical framework of non-radiative transition in the strong coupling regime, a plausible AIE mechanism of NSQs was revealed. Our work provides a clear picture on non-radiative dynamics of halogen-substituted squaryliums, which might be useful for future development of organic dyes.