Squaraines CIS and TRANS derivatives: practical application of group theory in vibrational spectroscopy

This work shows a practical application of the group theory to a relatively simple molecular system, as well its impact in the results obtained through vibrational spectroscopic techniques. Squaraines substituted by dicyanomethylene groups have been extensively studied, and after minor approaches in the sense to facilitate the point group determination, they were classified to the C_2v (Cis) and D_2h (Trans) groups. The Trans squaraine presents lower irreducible representations when compared to the Cis derivative; alterations in the arrangements of such systems promotes very interesting conjugation effects, since different reactivity from spectroscopic particularities. Especially to the Trans squaraine, there was a clear convergence between the data suggest by the calculations and those observed in their vibrational spectra, mainly that related to the inversion operation; strong bands visualized in a vibrational technique are not identified in other, mainly in region between 1800-1000 cm^-1. The approach of local symmetry must to be adjust to better evaluation of the spectroscopic behavior of the Cis derivative, in this case, the theory suggests low symmetry, but in the spectra can be seen clearly a bigger symmetry. Then, the vibrational analysis by using different techniques was determinant to the discussion in a supramolecular level. (author)