Solvatochromic fluorescence properties of phenothiazine-based dyes involving thiazolo[4,5-b]quinoxaline and benzo[e]indole as strong acceptors.

The present work describes the photophysical properties of two newly synthesized compounds, namely (E)-10-butyl-3-(2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl)-10H-phenothiazine (PTQ) and (E)-10-butyl-3-(2-(1,1-dimethyl-1H-benzo[e]indol-2-yl)vinyl)-10H-phenothiazine (PBI). A strong intramolecular charge transfer (ICT) is observed in both dyes as indicated from absorption and emission studies on varying the solvent polarity. This can be concluded from the large Stokes shifts among these dyes as PTQ exhibits large Stokes shift with >270nm and PBI around 200nm. The effect of increasing polarity caused drastic increase in the charge transfer process leading to twisted intramolecular charge transfer (TICT) process in both the dyes PTQ and PBI. Time-resolved emission studies and non-radiative decay rate constant indicates that the excited states of both dyes behave differently with respect to solvent polarity. The non-radiative decay constant increases dramatically with the solvent polarity specifying change of ICT emissive states in non-polar solvent while TICT emitting states in highly polar solvent. On the other hand, PBI follows a general trend initially exhibiting higher non-radiative decay constant in non-polar solvent like cyclohexane, lowest in moderate polarity owing to the ICT emissive state but with increase in the polarity, the non-radiative decay constant again increases indicating TICT states.