Fluorosolvatochromism and hyperpolarizability of one-arm and two-arms nitro-compounds bearing heterocyclic rings

We report here the spectral characterization and hyperpolazizatin coefficinet of all-trans isomers of two-bran- ched push-pull systems of interest as new efficient organic materials for Non Linear Optics, compared with the corresponding mono-branched analogues. These molecules are characterized by Acceptor-π-Het-π-Acceptor and Acceptor-π-Het structures, respectively, where the strong electron acceptor is the nitro group and the electron rich portion is a heteroaromatic ring (pyridine, furan and thiophene). These compounds exhibit strong fluor- osolvatochromism, in agreement with the substantial photoinduced intramolecular charge transfer (from the heteroaromatics to the nitro groups) undisclosed by quantum mechanical calculations. The hyperpolarizability of these molecules, here estimated by the solvatochromic method, reaches significant values in all cases: en- hanced for the two-branched systems with respect to their mono-branched analogues and higher for the furan/ thiophene derivatives with respect to the pyridine ones. Surprisingly, hyperpolarizabilities of the symmetrical Acceptor-π-Het-π-Acceptor structures are found to be similar or higher than those of the asymmetrical Acceptor- π-Het-π-Donor counterparts.