A CASSCF/CASPT2 insight into excited-state intramolecular proton transfer of four imidazole derivatives

Excited-state intramolecular proton transfer (ESIPT) of four imidazole derivatives, 2-(2'-hydroxyphenyl)imidazole (HPI), 2-(2'-hydroxyphenyl)benzimidazole (HPBI), 2-(2'-hydroxyphenyl)-1H-phenanthro[9,10-d]imidazole (HPPI) and 2-(2'-hydroxyphenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (HPPPI), were studied by the sophisticated CASSCF/CASPT2 methodology. The state-averaged SA-CASSCF method was used to optimize their geometry structures of S0 and S1 electronic states, and the CASPT2 calculations were used for the calibration of all the single-point energies, including the absorption and emission spectra. A reasonable agreement is found between the theoretical predictions and the available experimental spectral data. The forward ESIPT barriers of four target compounds gradually decrease with the increase of molecular size. On the basis of the present calculations, it is a plausible speculation that the larger the size, the faster is the ESIPT rate, and eventually, HPPPI molecule can undergo a completely barrierless ESIPT to the more stable S1 keto form. Additionally, taking HPI as a representative example, the radiationless decays connecting the S0 and S1 /S0 conical intersection structures were also studied by constructing a linearly interpolated internal coordinate (LIIC) reaction path. The qualitative analysis shows that the LIIC barrier of HPI in the keto form is remarkably lower than that of its enol-form, indicating that the former has a big advantage over the latter in the nonradiative process.