Azo dyes of hexafluoroisopropylidene derivatives: Synthesis and nonlinear optical properties

Two series of fluorine-containing azo dyes containing hexafluoroisopropylidenebis-(4-hydroxybenzene) (B) or 4,4′-(hexafluoroisopropylidene)diphthalicimidine (C) as a backbone connecting the two D-π-A units were synthesized so as to investigate the effects of both the D-π-A units and backbones on the nonlinear optical (NLO) properties of the colourants. Hyper-Rayleigh scattering (HRS) at 1064 nm indicated that the β values of C-containing derivatives were higher than those of B-containing derivatives when the substituents were strongly electron-withdrawing groups. The first hyperpolarizability (β) and static first hyperpolarizability (β0) of the fluorine-containing dyes increased markedly when the difference between the Hammett constants (σ) [Hammett LP. J Am Chem Soc 1937;59:96. Hansch C, Leo A, Taft RW. Chem Rev 1991;91:165.] of the electron-withdrawing and electron-donating groups in the D-π-A units increased, especially when the electron-withdrawing groups in the D-π-A units were strong, such as in the case of nitro and cyan groups. In addition, UV–vis absorption maximum (λmax) of these fluorine-containing azoic dyes were not red shifted compared with the corresponding mono-D-π-A unit compounds.