Resonance Raman investigation of free-radical cations of the donor–aromatic-acceptor molecules p-(CH3)2NC6H4R, R = CN, COOH, COOEt, in ethanol glasses at 77 K

Abstract

A newly designed, indium metal clad Raman cell in a special optical system is described which enables measurements to be made of the resonance Raman spectra of photogenerated free radicals in low-temperature glasses. By photoionization of the donor–aromatic-acceptor molecules p-(CH₃)₂NC₆H₄R, R = CN (I), COOH (II), COOEt (III), in glassy EtOH at 77 K the corresponding radical cations (I^+˙)-(III^+˙) were generated, from which the resonance Raman spectra were obtained. From these resonance Raman spectra, by comparison with the vibrational analysis of the parent molecules (I)-(III), it is shown that p-(CH₃)₂[graphic omitted]NĊ₆H₄CN (I^+˙) possesses a partially semiquinoidal structure. The conjugated system in (I^+˙) includes only the N(CH₃)₂ group and the benzene ring but not the nitrile group. The stretching mode ν(CN) in (I^+˙) shows very little resonance enhancement.