Temperature-Controlled Locally Excited and Twisted Intramolecular Charge-Transfer State-Dependent Fluorescence Switching in Triphenylamine–Benzothiazole Derivatives

Triphenylamine-benzothiazole derivatives, N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylbenzenamine (1), N-(4-(benzo[d]thiazol-2-yl)-3-methoxyphenyl)-N-phenylbenzenamine (2), and 2-(benzo[d]thiazol-2-yl)-5-(diphenylamino)phenol (3), showed unusual temperature-controlled locally excited (LE) and twisted intramolecular charge-transfer (TICT) state fluorescence switching in polar solvents. The detailed photophysical studies (absorption, fluorescence, lifetime, and quantum yield) in various solvents confirmed polarity-dependent LE and TICT state formation and fluorescence tuning. 1 and 2 exhibited strong fluorescence with short lifetime in nonpolar solvents compared to polar solvents. 1, 2, and 3 in dimethylformamide (DMF) at room temperature showed low-energy weak TICT state fluorescence, whereas high-energy strong LE state fluorescence was observed at -196 °C. Interestingly, further increasing the temperature from 20 to 100 °C, the DMF solution of 1 and 2 exhibited rare fluorescence enhancement with a slight blue shift of λmax via activating more vibrational bands of the TICT state. Thus, 1 and 2 showed weak TICT state fluorescence at room temperature, strong LE state fluorescence at -196 °C, and activation of TICT state at 100 °C. Moreover, molecular conformation and aggregation in the solid state influenced strongly on the fluorescence properties of 1, 2, and 3. Solid-state fluorescence and pH-responsive imidazole nitrogen have been exploited for demonstrating halochromism-induced fluorescence switching. Computational studies provided further insights into the fluorescence tuning and switching. The present studies provide understanding and opportunity to make use of D-A organic molecules in the LE and TICT states for achieving fluorescence switching and tuning.