Theoretical Insights of the Non‐Rigid Behavior of Benzophenone by Franck‐Condon Factors Approach

ABSTRACT Benzophenone is a molecule with several extremely relevant characteristics, widely used as a type‐2 photoinitiator due to its unique electronic properties and a very efficient intersystem crossing. In general, benzophenone can absorb directly to S 1 or S 2 states, but S 0 → S 1 transition is weak. Also, benzophenone has symmetric activity of the torsional modes of the phenyl groups, suggesting that is a non‐rigid molecule. This work has two fundamental purposes. The first is to examine the ground state (S 0 ) and first singlet excited state (S 1 ) of benzophenone using TD‐DFT methodology to generate the potential energy surface (PES) to understand its non‐rigid behavior; and the second, to examine the Franck‐Condon factors (FC factors) between the transition S 0 → S 1 . From our results, the most accurate was the hybrid functional PBE0. From the PES analysis of S 0 and S 1 states, we observe that several minima were located and that they are separated by relative low energy barriers. The global minimum of S 0 is found at θ 1 / θ 2 = 28.15° and for S 1 at θ 1 / θ 2 = 20.71°. Interestingly, the PES of S 1 state shows a very extensive area of minimum energy and a local minimum located at θ 1 = 90.71°/ θ 2 = 0.71°. From the vibrational spectra, we observe two intense signals that correspond to the symmetric phenyl twisting of normal mode 2 (2 3 and 2 4 ), and a combination between the symmetric hydrogen scissoring of 44 1 and 2 3 . As the vibronic spectrum tells, this transition is forbidden by the orbital theory but it is electronically allowed. Also, from the Duschinksy matrix, we observe a high mixing of vibrational modes.