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 PI-PI DIMERIZATION

UV-VIS Absorption spectroscopy of the unusually stable and structurall y characterized radical cations 1,4-diethyl-1,4-dihydropyrazinium (1.), 1,4-diethyl-1,4-dihydroquinoxalinium monomer (2,+) and pi-pi dimer [(2.+)2], 1,4,6,7-tetramethyl-1,4-dihydroquinoxalinium (3.+) and 5,10- diethyl-5,10-dihydrophenazinium (4.+) reveals low-energy pi-->pi tran sitions with different intensities and partial vibrational structuring . The spectra of the radical cations and of the corresponding dication s (1(2+)-4(2+)) have been analysed with the help of INDO/S calculation s; the resulting assignments could also be rationalized using Huckel M O theory. Both higher symmetric systems 1.+ and 4.+ exhibit vibrationa lly split long-wavelength bands. The additional long-wavelength absorp tion of the pi-pi dimer (2.+)2 (K(D) ca. 4 X 10(3)) is assigned to a c harge transfer transition, similar to that of methylviologen radical d imer.