Symmetry forbidden vibronic spectra and internal conversion in benzene

Abstract

The spectra of symmetry-forbidden transitions and internal conversion were investigated in the present work. Temperature dependence was taken into account for the spectra simulation. The vibronic coupling, essential in the two processes, was calculated based on the Herzberg-Teller theory within the Born-Oppenheimer approximation. The approach was employed for the symmetry-forbidden absorption/fluorescence, and internal conversion between 1 1 A 1g and 1 1 B 2u states in benzene. Vibrational frequencies, normal coordinates, electronic transition dipole moments, and non-adiabatic coupling matrix elements were obtained by ab initio quantum chemical methods. The main peaks, along with the weak peaks, were in good agreement with the observed ones. The rate constant of the 1 1 A 1g ' 1 1 B 2u internal conversion was estimated within the order of 10 3 s À1 . This could be regarded as the lower limit (about 4.8 Â 10 3 s À1 ) of the internal conversion. It is stressed that the distortion effect was taken into account both in the symmetry-forbidden absorption/fluorescence, and the rate constants of internal conversion in the present work. The distortion effects complicate the spectra and increase the rate constants of internal conversion.

First-principles method for calculating the rate constants of internal-conversion and intersystem-crossing transitions

“…13,14 The rate constants for the radiative transitions can be calculated quantum chemically [15][16][17] employing nonadiabatic molecular dynamics simulations at ab initio levels of theory [18][19][20][21][22] or by using specific approximations considering empirical corrections. 1,15,23 Molecular dynamics simulations are computationally expensive, which limits their application to smaller molecular systems. The semiempirical approach by Plotnikov, Artyukhov and Maier, which is based on the incomplete neglected differential overlap (INDO) method with a spectroscopic parameterization, can be used for calculating the photophysical properties of large organic molecules.…”

On the Problem of Multiple Minima on the S2 Excited Potential Energy Surface of Benzene: A Restricted Active Space Self Consistent Field Study

“…The study of benzene's excited states has been the subject of intense experimental and theoretical research and during the past years its vertical transition energies S 0 → T n and S 0 → S n , n=1‐4, have been studied in great detail . The first suggestions about a planar D 6h structure for the state S 2 constituting the so‐called spectroscopic minimum were given in the 1980’s and it was later confirmed by Roos and his collaborators using advanced ab‐initio calculations .…”

Ab Initio Semiclassical Evaluation of Vibrationally Resolved Electronic Spectra With Thawed Gaussians

“…Typically, the nonadiabatic couplings with the ground state (β = 1) can be neglected at the ground-state optimized geometry, around which the transition dipole moment is expanded. This leads to an expression (Li et al, 2010)…”

“…An extreme case of Herzberg-Teller spectra occurs in molecules, in which the electronic transition is symmetry-forbidden, but vibronically allowed due to the nonzero elements of the gradient of the transition dipole moment (Herzberg, 1966). A prototypical example is the absorption spectrum of benzeneà (Herzberg, 1966;Li et al, 2010), in which the nonzero elements, corresponding to the doubly degenerate e 2g vibrational modes, arise due to the nonadiabatic coupling between the first and higher excited electronic states (as discussed in Section V D and in Begušić et al (2018a); Li et al (2010); Quack and Merkt (2011)). The absorption spectrum contains a single progression, attributed to the totally symmetric ring-breathing mode, and a number of weak hot bands.…”