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
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.