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 (1)πσ* 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.

Computational simulation and interpretation of the low-lying excited electronic states and electronic spectrum of thioanisole

“…5,6,7,8,9,10,11,12,13 For thioanisole, the existence and nature of the 1 nσ* state has been identified and discussed in previous work. Among these, phenol and thiophenol, as well as their derivatives anisole and thioanisole, exhibit qualitatively similar potential energy profiles that may be considered prototypes of this behavior.…”

“…Among these, phenol and thiophenol, as well as their derivatives anisole and thioanisole, exhibit qualitatively similar potential energy profiles that may be considered prototypes of this behavior. [5][6][7][8][9][10][11][12][13] For thioanisole, the existence and nature of the 1 ns* state has been identified and discussed in previous work. 13,14 It was conjectured, based on complete active space self-consistent field (CASSCF) calculations and spectroscopic studies, that the dynamics of the photodissociation of thioanisole into thiophenoxyl and methyl radicals is facilitated by the coupling of the optically accessible 1 pp* state to the dark 1 ns* state.…”

Excited state dipole moments of anisole in gas phase and solution

“…The direction of the dipole moment in anisole is only slightly altered upon electronic excitation. Furthermore, the energetically following excited singlet state (S 2 ) is 9200 cm −1 apart [55], minimizing the problem of perturbative state mixing. In subsequent studies, we will systematically vary the molecules according to these limitations in order to allow for more exact dipole moment determinations of excited state in the condensed phase.…”

Tunneling dynamics dictated by the multidimensional conical intersection seam in the πσ*‐mediated photochemistry of heteroaromatic molecules

“…The πσ*‐mediated photochemistry, in this aspect, is quite unique as it gives the rational explanation for the ultrafast nonradiative transitions of many heteroaromatic molecular systems upon the electronic excitations, which are often relevant to the photochemical or photobiological activities 6,19–38 . As a matter of fact, spectroscopic and dynamic studies on heteroaromatic systems including phenols, 9,12,39–75 thiophenols, 10–12,76–97 anisoles, 98–102 thioanisoles, 5,103–115 or many others are vast in terms of their diversities and detailed dynamic features. Herein, we focus on the H‐atom detachment reactions of phenols and thiophenols especially in terms of the state‐specific tunneling rate constants.…”