Spectral Heterogeneity of Phenol Blue in Protic Solvents Revealed by Ultrafast Nonradiative Decay Dynamics.

Phenol blue (PhB) is a dye that exhibits positive solvatochromism which is also nonfluorescent due to its ultrashort excited state lifetime. We have conducted femtosecond time‐resolved transient absorption (TA) spectroscopic measurements of a PhB solution using two distinct excitation wavelengths, at shorter and longer wavelength sides of the visible absorption band. The results revealed that a long‐lived heterogeneity exists in protic solvents whereas, in aprotic solvents, the system rapidly returns to the original thermal equilibrium on the ultrafast timescale. Interestingly, the lifetime of the heterogeneity is longer for methanol (MeOH) compared to ethanol (EtOH) solution suggesting stronger interaction between PhB and MeOH. Additionally, since the excited state lifetime of PhB was still in the sub‐picosecond domain even in a polymer matrix, we suggest a nonradiative decay mechanism through high‐frequency vibrational modes without involving large structural reconfiguration such as twisting of the benzoquinone (BzQ) moiety. [ABSTRACT FROM AUTHOR]