Influence of core fluorination on the phase properties of fan-like azobenzene based supramolecules, their cis-trans photoisomerization and photoluminescence dynamics

The stimuli-responsive, low molecular weight, dynamically tunable photophysical features have long been an important objective that challenges chemists in synthesizing liquid crystal (LC) compounds. Herein, the effect of core fluorination on the mesomorphic behavior of fan-like azobenzene derivatives was reported. Two new series of azobenzene derivatives which differ from each other in the length of the terminal alkoxy side chain as well as fluorine (-F) group substitution on the azobenzene moiety were synthesized (3a-3c, 4a-4c) and molecular structures of the compounds were confirmed using various analytical techniques. Absorption spectra of the LC compounds 3b and 3c are characterized by ππ∗ transitions around 350–410 nm. The self-assembly of these LC compounds was investigated using polarized optical microscope (POM) and differential scanning colorimetry (DSC). The trans-to-cis photoisomerization of the LC compounds 3b and 3c occurs in these absorption bands. The trans-to-cis photoisomerization of LC compounds 3b and 3c showed 4 h and 24.5 h whereas, thermal cis-to-trans isomerization rates were found to be 90s and 100s resulting in tuning of mesophases. Room temperature photoluminescence (RTPL) of LC compounds 3b and 3c when excited at 220 nm, 230 nm and 240 nm showed several sharp/weak emission intensity bands. Both the LC compounds (3b, 3c) showed sharp blue emission bands and yellow/green/orange colored bands correspond to weak emission spectra when excited at 220 nm. Further, steady state photoluminescence (SSPL) spectra of these both LC compounds revealed sharp near edge emission bands and broad violet emission peaks with higher Stoke's shift as well as full width half maximum (FWHM). Fluorescence lifetime decay (FLD) studies of compound 3 b unveiled an average lifetime (τ) shuttle between 17.24 ns and 103.60 ns at various excitation wavelengths. However, FLD of LC compound 3c unveiled that the τ fluctuates between 27.00 ns and 102.56 ns at various excitation wavelengths. Quantum yield (QY) decreases for both the LC compounds with an increase in excitation wavelengths. The study proved the importance of the alkoxy side chain at one end of the aromatic ring and core fluorination as a significant tool to modify the LC behavior of azobenzene derivatives. Thus, synthesized azobenzene derivatives are potentially useful for developing permanent optical storage and display devices.