Interpretation of the infrared and Raman spectra of zwitterionic push–pull dyes based on quinoidal thiazole

The infrared and Raman spectra of a donor (dithiole)–acceptor (dicyano) compound having a quinoidal thiazole bridge connecting the two electro-active groups have been recorded and the vibrational properties described on the basis of MP2 and DFT theoretical approaches. The spectroscopic data have been interpreted accounting for: (i) charge polarization or donor–acceptor charge separation in the S0 ground electronic state from the infrared spectrum; (ii) π-conjugation on the donor-(π-spacer)–acceptor sequence from the non-resonant Raman spectrum; and (iii) intramolecular donor-to-acceptor charge transfer character of the S0 → S1 excitation from the resonant Raman spectrum; and (iv) dependence of the non-resonant and resonant Raman spectra varying the spacer and the donor group. This paper highlights the interesting combination of vibrational spectroscopy and theoretical modeling in order to get insight on the relevant electro-optical and molecular properties on these compounds which are items of interest to develop organic molecules for non-linear optical applications.