NIR emitting new N, N-diethylaniline based NLOphoric D-π-A and D-A′-π-A dyes: Photophysical properties, viscosity sensitivity and DFT studies

Abstract

Four new N, N-diethylaniline comprising D-π-A and D-A′-π-A type dyes were synthesized and characterized by 1H NMR, 13C NMR, and elemental analysis. The dyes show positive and negative solvatochromism in absorption and emission, especially the compounds (E)− 2-Cyano-3-(5-((E)− 1-cyano-2-(4-(diethylamino)− 2-(hexyloxy) phenyl) vinyl) thiophen-2-yl) acrylic acid (D1) and (E)− 2-Cyano-3-(5-((E)− 4-(diethylamino)− 2-(hexyloxy) styryl) thiophen-2-yl) acrylic acid (D2) having cyanoacrylic acid as an acceptor group show negative emission solvatochromism, and (E)− 2-((5-(1-Cyano-2-(4-(diethylamino)− 2-(hexyloxy) phenyl) vinyl) thiophen-2-yl) methylene) malononitrile (B1) and (E)− 2-((5-(4-(Diethylamino)− 2-(hexyloxy) styryl) thiophen-2-yl) methylene) malononitrile (B2) show positive emission solvatochromism. The D-A′-π-A type dyes (B1 and D1) show viscosity sensitive emission intensity enhancement (6-fold) in far red (FR) to near infra-red (NIR) region in the electromagnetic spectrum. The charge transfer characteristics were established through solvatochromism, polarity graphs, bond length alternation (BLA), bond order alternation (BOA), natural bond order (NBO) analysis and HOMO-LUMO energy gap. Kamlet-Taft and Catalan parameter results confirmed that dipolarity and polarizability factor are responsible for solvatochromism. These dyes exhibit higher first order hyperpolarizability than the urea. Fluorescence lifetimes are more in D-A′-π-A system compared to the D-π-A system. Due to the higher non-linear optical characters, these dyes can be considered as promising candidates for organic NLOphores.