Vibrational spectra of pNA

C. G. POZZI; A.C. FANTONI; G. PUNTE; A. E. GOETA; E. DE MATOS GOMES; M. BELSLEY

Buenos Aires

Workshop; At the Frontiers of Condensed Matter III; 2006

The fast and efficient response of molecular materialshave made them promising candidates for a number ofnonlinear optical (NLO) applications. NLO effects suchas second harmonic generation (SHG) are sensitive to thestructure of the molecules as well as their assembly pat-tern. Push-pull chromophores are a class of moleculesthat feature an electron-donating group and an electron-accepting group connected by a conjugated pi-system.These molecules possess large nonlinear optical responses.Two resonance forms can be written for the electronicstructure of a push-pull chromophore: a neutral structurehaving zero or small formal charges on the donor and ac-ceptor groups, and a zwitterionic form. These two struc-tures are often taken as basis states, whose linear combi-nations compose the two lowest-energy electronic eigen-states, with the ground state being predominantly the neu-tral form. p-nitroaniline is the prototypical push-pull sys-tem but the centric nature of its crystal structure inhibitsmacroscopic NLO response. Samples prepared by crystal-lization under an intense static electric field of 200 kV/mhave shown NLO response. No single crystal suitablefor X-ray diffraction has been obtained and powder X raydiffraction did not show evidence of space group modifica-tion. These findings led to a research on the possible influ-ence of the internal vibrational degrees of freedom on theobserved NLO behavior. Vibrational spectra of crystallinepowders with and without NLO response were obtainedat different temperatures. The results are analyzed on thebasis of ab-initio and DFT calculations on monomers andclusters and data of picosecond anti-Stokes resonance Ra-man spectroscopy found in the literature.