Assignment of the electronic transition of phenothiazine radical cation in the visible region– a resonance Raman spectroscopy and theoretical calculation investigation

Phenothiazine (PTZ) derivatives represent an important class of molecules, being studied in a number of fields of chemical and pharmaceutical research. The interesting chemical and biological properties of PTZs are considered to be related to their relative stable radical cations, whose formation and reactivity have been extensively studied. In this work, the radical cation of phenothiazine (PTZ+) was generated by the reaction with the mordenite solid acid, an environment in which this species is stabilized. The most intense electronic transition of PTZ+ is located at 516 nm. In order to characterize the electronic and molecular structure of this radical species, a resonance Raman and theoretical calculations of the structure, PTZ+ vibrational frequencies and electronic spectrum were calculated employing DFT B3LYP/6-31G(d,p) and TDDFT methods. In the ground state, PTZ+ is planar, in contrast to the folded structure of neutral PTZ. The strong Raman band at 476 cm−1 was assigned to a CSC bending mode. The reason for its preferential enhancement was investigated using TDDFT, in order to obtain the molecular orbitals involved in the electronic transition at 516 nm. The TDDFT calculations show that this transition is from SOMO-1 and SOMO-2, both double occupied molecular orbitals, localized at the aromatic rings and goes toward SOMO (single occupied molecular orbital). This SOMO has contributions of sulphur and nitrogen atomic orbitals, but with predominance of sulphur atomic orbitals, which are absent in SOMO-1 and SOMO-2. The predominance of sulphur atomic orbitals in the SOMO explains the preferential resonance Raman enhancement of a CSC vibrational mode.