Abstract
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•Symmetrical disubstituted compounds with D-A-D-A-D configuration was synthesized.•Exhibited excellent NLO properties with promising optical limiting threshold values under nanosecond pulse excitation.•A detailed energy level mechanism is proposed for the NLO activity of the molecules.•Formation of charge separation state by photo-induced charge transfer is discussed.
Organic materials figure among the most widely investigated nonlinear optical (NLO) materials towards the realization of optical limiters. Among the organic materials, donor–acceptor systems are getting special consideration due to their excellent NLO activities arising from the intramolecular charge transfer (ICT). But NLO activities of many of these systems are restricted by photo-bleaching and limited thermal stability. This deficiency can be overcome by introducing more than one donor–acceptor groups into the framework. Herein we report NLO activities of two symmetrical disubstituted compounds with D-A-D-A-D configuration, containing bis-1,3,4-oxadiazole with phenothiazine backbone in which two different electron donor systems namely naphthalene (NOPON)/4-bromophenyl (BOPOB) were bonded with the phenothiazine-1,3,4-oxadiazole core unit. Solution phase absorption spectrum of these compounds exhibited two peaks, one located in the 300–350 nm regions and a comparatively weak and broad band located at the longer wavelength region attributed to ICT state. Photophysical, electrochemical and excited state life time analysis of the compounds were carried out. HOMO-LUMO energies of the system obtained from DFT calculation are comparable with the experimental values obtained from voltammetric study. NLO and optical limiting behaviour of the compounds were analysed by Z scan technique using a Q switched Nd: YAG laser at 532 nm. Both the compounds exhibited promising limiting threshold values (2.43 J/cm2 for NOPON and 3.80 J/cm2 for BOPOB) comparable with many of the reported values, suggesting their ability to perform as good optical limiters.