Structural characterization of photoluminescent, light-driven fluorescence and phosphorescence decay properties of new azobenzene dyes containing alkoxy side chain and their DFT studies

Abstract

In the present research work, a new series of azobenzene dyes (3a-3c, 4a-4c) with terminal alkoxy side chains (butyl, pentyl, hexyl, octyl) have been synthesized and are characterized by ^1HNMR (proton nuclear magnetic resonance), ^13CNMR (carbon nuclear magnetic resonance), ¹⁹F-NMR (Fluorine 19 nuclear magnetic resonance), and LCMS (liquid crystal mass spectroscopy) spectroscopies. Optical studies of azobenzene dyes 3a-3c, 4a-4c revealed shift in the absorption maxima towards visible region of the spectrum and optical band gap (E_g) of 3a-3c, 4a-4c varies between 2.08 eV to 2.56 eV. Polarizing optical microscope (POM) studies revealed that the synthesized azobenzene dyes 3a-3c, 4a-4c does not possesses birefringence/liquid crystalline mesophases. The steady state photoluminescence (SSPL) studies of azobenzene dyes 3a-3c, 4a-4b unveiled sharp/broad emission peaks located at ∼ 420 nm (violet) and E_g increases with an increase in the excitation wavelengths. Room temperature photoluminescence (RTPL) studies of azobenzene dyes 3a-3c, 4a-4b displayed blue and red emission peaks at various excitation wavelengths. The fluorescence decay (FLD) studies of azobenzene dyes 3a-3c when excited at 320 nm and 375 nm unveiled slow decay characteristics and average lifetime varies between 0.037 ns to 0.124 ns. However, azobenzene dyes 4a-4b showed a comparatively faster decay profile with an average lifetime in dwell between 0.0398 ns to 0.0670 ns. Quantum yield (QY) of azobenzene dyes 3a-3c, 4a-4c increases monotonically with increase in the excitation wavelengths. The phosphorescence decay (PD) profile of azobenzene dyes 3a-3c, 4a-4b showed comparatively fast decay with average lifetime varies between 114.3 ns to 118 ns. For further investigations density functional theory (DFT) studies were employed to explain the nature of ground state photophysical properties, extensive HOMO-LUMO orbital interactions, chemical reactivity, photochemistry, photoluminescence, and electrostatic potentials of azobenzene dyes 3a-3c, 4a-4c respectively. The synthesized new azobenzene dyes can be used as potential candidates for fluorescent devices, advanced photonics, and as luminescent materials.