Y-shape pyridine-based D-it-A chromophores for improved nonlinear optical properties and optical limiting in polyurethanes

In this work, we have synthesized Y-shape D-it-A conjugated chromophores namely 4,4 '-(4-(4-nitrophenyl)pyridine-2,6-diyl)-diphenol (VL) and 4,4 '-(4-(2,4-dinitrophenyl)-pyridine-2,6-diyl)-diphenol (VM) and polymerized with 1,4-diisocyanatobenzene and 2,4-diisocyanato-1-methylbenzene to get four different polyurethanes. The structural characterization of chromophores and polymers were confirmed through various spectroscopic and analytical techniques such as FTIR, NMR, LCMS and MALDI-TOF. The dipole moment and energy bandgap between HOMO and LUMO of the optimized chromophores were theoretically calculated using DFT method. These polymers showed high glass transition temperatures ranging from 259 degrees C to 390 degrees C. The nonlinear optical properties of these polymers were measured by using open-aperture Z-scan (Nd:YAG) at wavelength of 532 nm and a laser pulse of 9 ns. The polymers exhibited reverse saturable absorption due to two-photon absorption process. Among these polymers, VM2 showed the highest nonlinear absorption coefficient of 11.89 x10-11 m/W and lowest optical limiting threshold of 0.43 x1012 W/m2. The developed polymers were found be a suitable candidate for optical limiting applications.