Excited state deactivation pathways of neutral/protonated anisole and p-fluoroanisole: a theoretical study

Abstract

The potential energy profiles of neutral and protonated anisole and p-fluoroanisole at different electronic states have been investigated extensively by the RI-MP2 and RI-CC2 methods. The calculations reveal that the relaxation dynamics in protonated anisole and p-fluoroanisole are essentially different from those of the neutral analogues. In neutral anisole/p-fluoroanisole, the ¹πσ* state plays a vital relaxation role along the O–CH₃ coordinate, yielding the CH₃ radical. For both of these molecules, the calculations indicate conical intersections (CIs) between the ground and excited state potential energy (PE) curves, hindered by a small barrier, and providing non-adiabatic gates for radiation-less deactivation to the ground state. Nevertheless, for the protonated cases, besides the prefulvenic deformation of the benzene ring, it has been predicted that the lowest ¹(σ,n)π* state along the C–O–C bond angle plays an important role in photochemistry and the relaxation dynamics. The S₁, S₀ PE profiles of protonated anisole along with the former reaction coordinate (out-of-plane deformation) show a barrierless relaxation pathway, which can be responsible for the ultrafast deactivation of excited systems to the ground state via the low-lying S₁/S₀ conical intersection. Moreover, the later reaction coordinate in protonated species (C–O–C angle from 120°–180°) is consequently accompanied with the bond cleavage of C–OCH₃ at the ¹(σ,n)π* state, hindered by a barrier of ∼0.51 eV, and can be responsible for the relaxation of excited systems with significant excess energy (hν ≥ 5 eV). Furthermore, according to the RI-CC2 calculated results, different effects on the S₁–S₀ electronic transition energy of anisole and p-fluoroanisole upon protonation have been predicted. The first electronic transitions of anisole and p-fluoroanisole shift by ∼0.3 and 1.3 eV to the red respectively due to protonation.