Computational study on the photophysics of protonated benzene.

The reaction paths in the lowest excited electronic states relevant for the photophysics of protonated benzene, C(6)H(7)(+), have been explored by ab initio techniques of electronic structure theory. For this purpose, the first four excited singlet electronic states of C(6)H(7)(+) have been calculated at the CC2/cc-pVTZ level of theory. The CC2 approach has been validated by CASPT2 and TD-DFT calculations. The calculated UV absorption spectrum is in good agreement with the experimental spectrum. It has been found that the out-of-plane and the in-plane ring deformation leads in the excited states in an essential barrierless manner to a low-lying conical intersection between the lowest excited states and with the ground state, providing a mechanism for efficient radiationless deactivation, which is expected to quench luminescence of the isolated molecular ion.