Photophysical behavior of mono- and bis(2'-hydroxyphenyl)-6-(4'-diphenylaminophenyl)pyrimidines: Interplay of ESIPT, ICT, and vibrational nonradiative decay.

Mono- and bis(2'-hydroxyphenyl)-6-(4'-diphenylaminophenyl)pyrimidines were investigated to elucidate the interplay between excited-state intramolecular proton transfer (ESIPT), intramolecular charge transfer (ICT), and vibrational non-radiative decay processes governing their photophysical properties. Under neutral conditions, all compounds exhibit extremely low fluorescence quantum yields. Time-dependent density functional theory (TD-DFT) calculations indicate that proton transfer is energetically accessible in the excited state; however, the absence of a significant decrease in oscillator strength upon enol-keto conversion demonstrates that ESIPT does not lead to a dark excited state and is not the dominant deactivation pathway. Analysis of vibrational reorganization energies and Huang-Rhys factors reveals strong coupling between the electronic transition and low-frequency vibrational modes, suggesting that vibrationally assisted internal conversion plays a major role in fluorescence quenching. Upon protonation of the pyrimidine nitrogen atoms, marked bathochromic shifts are observed, accompanied by suppression of the ESIPT pathway and enhancement of ICT character. Nevertheless, fluorescence remains weak in polar media, consistent with efficient non-radiative decay from stabilized charge-transfer states, and increases in less polar solvents. Overall, these results highlight the complex interplay between ESIPT, ICT, and vibrational coupling in these pyrimidine-based chromophores, with vibrationally assisted non-radiative decay emerging as a dominant contribution to their excited-state deactivation.