Theoretical design, synthesis, third-order nonlinear optical properties and DNA binding studies of benzothiadiazole based conjugated polymer

Benzothiadiazole based copolymer P(BTZ-BT) was theoretically designed by density functional theory methods employing B3LYP/6-31 G (d,p) and HSE06/6-31 G (d,p) basis sets. A low band gap of 1.31 eV was observed for the copolymer by HSE06/6-31 G (d,p) basis set. TG analysis of the polymer showed high thermal stability at 500 degrees C. Experimentally observed band gap for the copolymer was 1.66 eV by cyclic voltammetry. Fluorescence lifetime of the polymer P(BTZ-BT) was studied and it exhibited an average lifetime of 2.59 ns. The copolymer exhibited positive solvatochromism with a Stoke's shift value ranging from 3082 cm-1 to 4569 cm-1. Open -aperture Z-scan traces of the copolymer in DMSO at 91 mu J, 165 mu J, 285 mu J and 450 mu J showed positive NLO absorption coefficient beta in the range of 10-10 m/W and exhibited optical threshold value of 0.03 GW/cm2, 0.05 GW/cm2 and 0.12 GW/cm2 respectively. No linear transmission was observed at 450 mu J for the copolymer during the study of optical limiting property. DNA binding studies of the copolymer with double stranded calfthymus DNA was carried out. Fluorescence observed for the copolymer was quenched by 85% on interaction with the ct-DNA and gradual decrease was observed in the lifetime of the copolymer from 2.59 ns to 1.8 ns.