Electronically excited states of protonated phenol and para-substituted phenol

The low-lying electronic excited states of protonated phenol and para-Fluorophenol have been investigated extensively by RI-MP2/RI-CC2 methods. Although, protonation of phenol leads to a small red-shift-effect on the S1–S0 (ππ∗) electronic transition in respect to its neutral homologue, a large red-shift-effect, on the same electronic transitions of para-substituted phenol has been predicted. The ππ∗ excited state of protonated phenol stays in the UV range (4.34 eV), while its πσ∗ state lies in the VUV region (8.3 eV). The S1 excited-state geometry optimization of protonated phenol predicted unstable S1 state owing to the strong out-of-plane deformation in the benzene ring.