Butterfly wing type new push–pull A–π–D–π–A organic fluorophore: synthesis, photophysical, DFT and nonlinear optical property studies

Abstract

Push–pull organic molecules have aroused wide interest in photochemistry and photophysics research communities. In this report, we present the synthesis, characterisation, photophysical and nonlinear optical property studies of a new organic fluorophore (AA2) with a unique push–pull molecular configuration: acceptor–π-bridge–donor–π-bridge–acceptor (A–π–D–π–A) constituted with phenothiazine as the donor moiety. A thorough investigation to validate its structure was undertaken through ¹H, ¹³C NMR and MALDI-TOF mass spectrometry. A variety of studies were undertaken to unravel its optical properties both experimentally and theoretically. A systematic investigation probing its optical absorption and emission characteristics revealed its strong dependence on solvent polarity, which confirmed the crucial role played by the intra molecular charge transfer (ICT) process emerging from its unique molecular and electronic structure. Such a strong structure–property correlation could be exploited for various optoelectronic applications, which triggered us to explore AA2 for photonics applications driven by nonlinear optical responses. Our nonlinear optical investigations confirmed strong excited state absorption at 532 nm when excited in the nanosecond regime using the Z-scan technique. This opened up yet another avenue of exploring it for optical limiting applications. The estimated optical limiting threshold for an AA2 molecule is about 0.98 J cm⁻², which proved its high suitability for such photonics applications while outperforming many other organic fluorophores and benchmarked nanostructures under similar experimental evaluation. This is one of the very first reports on phenothiazine with an A–π–D–π–A configuration accompanied by excellent nonlinear optical properties.