Syntheses of novel 1,3-diazaazulene derivatives and their nonlinear optical characterization

Abstract

We have synthesized three new 1,3-diazaazulene derivatives, namely, 2-(4′-N,N-dimethylaminophenyl)-6-nitro-1,3-diazaazulene (18, DMAPNA), 2-(4′-aminophenyl)-6-nitro-1,3-diazaazulene (19, APNA) and 2-[4′-N-(2-hydroxyethyl)aminophenyl]-6-nitro-1,3-diazaazulene (20, HEAPNA), each of them contains an amino substitute as the electron donor (D), 2-phenyl-1,3-diazaazulene as the π-conjugated bridge and a nitro as the electron acceptor (A). Our theoretical results have predicted that these D–π–A type chromophores possess low ground-state dipole moment (μ_g) and large first-order hyperpolarizability (β), which may facilitate a low degree of aggregation for the chromophores dispersed in a polymeric matrix as well as a large nonlinear optical (NLO) response. The expected NLO performance has been confirmed by the experimental β and μ_g values, e.g., for 18, 407.8 × 10⁻³⁰ esu and 4.7 D, respectively. The origins of large β and low μ_g are explained in terms of a two-state quantum model. The DMAPNA (18)-doped and poled polymethylmethacrylate film exhibits a large second harmonic generation (SHG) coefficient of d₃₃ = 10.9 pm V⁻¹ with excellent thermal stability (above 70% of the maximal SHG coefficient remains at ∼100 °C).