Positron annihilation studies on chalcone chromophore doped PVA

A novel organic non-linear optical material 1-(4-methylphenyl)-3-(4- N, N dimethyl amino phenyl)-2-propen-1-one (MPDMAPP) chalcone chromophore has been synthesized by standard method. Pure and MPDMAPP doped Poly(vinyl alcohol) films are prepared using solution casting method and characterized using UV-Visible absorption and Positron Annihilation Lifetime Spectroscopy. The optical spectroscopic study shows three absorption bands, 196-202 nm assigned to localized n→π* transitions, 205-320 nm to n→π* inter-band and 385-428 nm assigned to π→π* transition and arises due to the charge transfer complex. Using observed UV-Vis spectra, three optical energy gaps, E_g1 (4.96-4.25 eV), E_g2 (3.47-3.32 eV) and E_g3 (2.33-2.24 eV) have been estimated. The observed change in E_g upon doping is understood based on the formation of charge transfer complex arising from the - HC=CH- structure present in the composite and due to the interactions of dopant and OH group of PVA. The PALS results shows that the o-Ps lifetime decreases continuously as doping concentration increases and I₃ initially increases (from 18.82% to 20.95%) from 0 wt% to 0.05 wt% and then decreases gradually up to 16.76% for higher dopant concentrations with decrease in optical band gaps. The variation of positron data with E_g is understood by correlating the probability of Ps formation with the complex.