Third-order Nonlinear Optical Properties Of The Azine Dyes With Resonance Structures

Molecules owning diradical characters or potential electron fluctionality abilities attracted increasing attention in the third-order nonlinear optical (NLO) materials research in recent years, and suitable molecules with above characteristics are urgently needed in this field.Besides the valence bond theory, we try to find further potential candidates by examining materials with resonance structures. Experiments were performed to verify and optimize the mentioned structures. Herein, we focused on the azine dyes bearing resonance structures serving for the third-order NLO materials. Series of phenothiazinium, phenoxazinium, phenazinium and xanthylium containing different substituted amine in side groups were designed, synthesized and characterized. The obtained compounds were examined for third-order NLO properties. The work can be summarized as the following:(1) Series of high-purity organic azine dyes with resonance structures were designed and synthesized. The compounds owning good solubility in most polar solvents were obtained after passing through a chloride resin exchange column, and for the first time we got the good NMR spectra of phenothiazinium chlorides. The linear optical absorption characteristics of triazine dyes, such as UV-vis absorption spectra and fluorescence emission spectra, were measured. The series of compounds also have high thermal stability, which lay a good foundation for their device researches.(2) The electric charge distributions of phenoxaziniums were uncovered by theoretical calculation using Gaussian 98 program package and the molecular orbitals energies were also obtained. It is found that the energy gaps between HOMO and LUMO are quite small, which may result in the excellent nonlinear properties.(3) The compounds were evaluated in solution for third-order nonlinear optical properties at 532 nm using a nanosecond and picosecond Z-scan technique. The phenothiazinium and phenoxazinium exhibit strong reverse saturable absorption and nonlinear refraction under irradiation with either a nanosecond or picosecond laser. To gain an insight into the nonlinear origin of these compounds, a picosecond time resolved pump-probe experiments have been performed at 532 nm. The picosecond pump-probe response implies that the nonlinear mechanism is excited‐state nonlinearity.(4) The phenothiazinium chlorides were evaluated for in vitro antiprotozoal activities. It is found that 3,7-di(piperidinyl)phenothiazinium chloride and 3,7-di (benzyl(methyl)amino)phenothiazinium chloride have good inhibitory ability against T. cruzi and L. donovani, respectively. Moreover, the N-Aromatic and N-benzyl substituted phenothiazinium chlorides are less toxic than the N-alkyl substituted compounds.