Synthesis and structural properties of 2-((10-alkyl-10H-phenothiazin-3-yl)methylene) malononitrile derivatives; a combined experimental and theoretical insight

Background: Donor acceptor moieties connected through p-conjugated bridges i.e. D-pi-A, in order to facilitate the electron/charge transfer phenomenon, have wide range of applications. Many classes of organic compounds, such as cyanine, coumarin carbazole, indoline, perylene, phenothiazine, triphenylamine, tetrahydroquinoline and pyrrole can act as charge transfer materials. Phenothiazines have been extensively studied as electron donor candidates due to their potential applications as electrochemical, photovoltaic, photo-physical and DSSC materials. Results: Two phenothiazine derivatives, 2-((10-hexyl-10H-phenothiazin-3-yl)methylene) malononitrile (3a) and 2-((10-octyl-10H-phenothiazin-3-yl) methylene) malononitrile (3b) have been synthesized in good yields and characterized by various spectroscopic techniques like FT-IR, UV-vis, H-1-NMR, C-13-NMR, and finally confirmed by single crystal X-ray diffraction studies. Density functional theory (DFT) calculations have been performed to compare the theoretical results with the experimental and to probe structural properties. In order to investigate the excited state stabilities the absorption studies have been carried out experimentally as well as theoretically. Conclusions: Compound 3a crystallises as monoclinic, P2 (1)/a and 3b as P-1. The X-ray crystal structures reveal that asymmetric unit contains one independent molecule in 3a, whereas 3b exhibits a very interesting behavior in having a higher Z value of 8 and four independent molecules in its asymmetric unit. The molecular electrostatic potential (MEP) mapped over the entire stabilized geometries of the molecules indicates the potential sites for chemical reactivities. Furthermore, high first hyperpolarizability values entitle these compounds as potential candidates in photonic applications.