Theoretical design, synthesis and studies on the solvatochromic behaviour of low band gap phenylenevinylene based copolymers

Abstract

A new series of low band gap phenylenevinylene based copolymers was designed by employing density functional theory in the periodic boundary condition using HSE06 exchange correlation functional using 6-31G basis set and synthesized via the Gilch polymerization. The theoretical band gap of the copolymers MD-CA-PPV, MD-FL-PPV, MD-PT-PPV and MD-AN-PPV were found to be 1.98, 2.07, 2.31 and 1.99 eV respectively using the DFT/HSE06/6-31G calculation and the optical band gap of 1.96, 2.03, 2.01 and 1.98 eV respectively, according to the onset edge of lower energy peak of the polymers in solution. The experimental results supported the theoretical prediction. Solvatochromic behaviour of the synthesized copolymers was studied thoroughly in toluene/acetonitrile mixtures of varying polarity at room temperature. The absorption spectrum of the PPV polymers was sensitive to solvent effects which showed bathochromic and hypsochromic shifts with increasing solvent polarity indicating that the transitions involved were π-π* and n-π*. The ground and excited state dipole moments were calculated using solvatochromic correlations. Onsager radius determined from ab initio calculation was used in the determination of dipole moments. The change in the dipole moment (Δμ) was calculated both from solvatochromic shift method and on the basis of microscopic empirical solvent polarity parameter (ETN) and values are compared. It was observed that the excited state dipole moment (µe) was larger than its ground state (µg) counterpart. This indicated that a substantial redistribution of the π-electron densities occurred in the more polar excited state for the selected phenylenevinylene polymers. The third-order nonlinear optical (NLO) properties of the copolymers, evaluated by Z-scan method with nanosecond laser beam at 532 nm, are reported. The nonlinear absorption coefficient of the copolymers were calculated to be 0.17 × 10−10, 2.06 × 10−10, 0.63 × 10−10 and 2.83 × 10−10 esu, respectively for MD-CA-PPV, MD-FL-PPV, MD-PT-PPV and MD-AN-PPV which certified that the polymers showed semiconductor properties.