Azo-azulene derivatives as second-order nonlinear optical chromophores

The molecular and solid state nonlinear optical (NLO) properties of several (phenylazo)-azulenes are investigated. In particular, (4-nitrophenylazo)-a zulene (2b) exhibits a quadratic hyperpolarizability (beta(vec)) of 80 x 10 (-30) cm(5) esu recorded at 1.907 mu m by the electric field-induced second -harmonic (EFISH) technique. This molecular material, which crystallizes in the monoclinic noncentrosymmetric space group Pc, exhibits an efficiency 4 20 times that of urea in second-harmonic generation. The origin of the opti cal nonlinearity in azo-azulene is discussed in relation with crystal struc tures and semiempirical calculations within the INDO/SOS formalism, and com pared with that of the well known disperse red one (DR1) organic dye.