Vibrational study of push–pull chromophores for second-order non-linear optics derived from rigidified thiophene π-conjugating spacers

Abstract

The solid-state infrared absorption and Raman scattering spectra of 5-(2,2-dicyanoethenyl)-5′-[(E)- N, N′-dimethylaminobenzylidene)methyl] bithiophene which can be viewed as a π-conjugated bridge (bithiophene) substituted with an electron-donor group ((E)- N, N′-dimethylaminobenzylidene)methyl), and with an electron-acceptor group (2,2-dicyanoethenyl), are reported. The main infrared and Raman features have been analyzed and assigned to precise normal modes with the help of accurate Density Functional Theory calculations. Vibrational bands associated with the donor, with the acceptor and with the oligothienyl bridge have been identified. The peak position of the IR-active stretching vibration associated with the dicyano groups has been related to the degree of charge transfer from the donor to the acceptor in the molecule. The great ressemblance between the infrared and Raman spectra reveals that this molecule behaves as an effective push–pull system. Theory full accounts for the vibrational properties of this type of push–pull system in relationship with its electronic structure.