Resonance Raman and UV-vis characterization of charge transfer complexes of TCNQ and aromatic amines

Although the molecular charge transfer complexes formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) and several different donors have been widely investigated in the past, in this paper it is shown for the first time the vibrational spectroscopic characterization of the complexes formed by TCNQ and the simplest series of substituted anilines in solution. The UV-vis spectra indicate the formation of TCNQ complexes stabilized by two aromatic amines in a sandwich π type complex. It was observed that the charge transfer transition energies of the complexes follow a linear correlation with the ionization potential of the amines. The resonance Raman spectra revealed, by the analysis of the ν(CC) and ν(CN) stretching modes of TCNQ, that the complexes keep their neutral character in the ground electronic state, and not as a TCNQ2− dianion species as reported before. The observed enhancement of the TCNQ bands as the amine donor capacity increases, confirmed the charge transfer nature of the electronic transitions. The DFT and TDDFT results were obtained and the theoretical Raman and electronic spectra data supported the experimental findings. The results of the model systems presented herein can contribute to a deep understanding of the photoinduced charge transfer process in complexes of TCNQ, which is a key step in the designing of organic molecular devices.