Electronic absorption spectroscopy of some exceptionally stable 1,4-dialkyl-1,4-dihydro-1,4-diazinium radical cations: assignment of transitions, vibrational structure and effects of π–π dimerization

Abstract

UV–VIS Absorption spectroscopy of the unusually stable and structurally characterized radical cations 1,4-diethyl-1,4-dihydropyrazinium (1˙⁺), 1,4-diethyl-1,4-dihydroquinoxalinium monomer (2˙⁺) and π–π dimer [(2˙⁺)₂], 1,4,6,7-tetramethyl-1,4-dihydroquinoxalinium (3˙⁺) and 5,10-diethyl-5,10-dihydrophenazinium (4˙⁺) reveals low-energy π→π* transitions with different intensities and partial vibrational structuring. The spectra of the radical cations and of the corresponding dications (1²⁺–4²⁺) have been analysed with the help of INDO/S calculations; the resulting assignments could also be rationalized using Hückel MO theory. Both higher symmetric systems 1˙⁺ and 4˙⁺ exhibit vibrationally split long-wavelength bands. The additional long-wavelength absorption of the π–π dimer (2˙⁺)₂(K_Dca. 4 × 10³) is assigned to a charge transfer transition, similar to that of methylviologen radical dimer.