Theoretical design, synthesis and third-order non-linear optical properties of thiophene and tetrafluorobenzene based low band gap conducting polymers

Abstract

Low band gap donor-acceptor type π-conjugated conducting polymers P(BT-4FPH), P(3METH-4FPH) and P(TH-4FPH) were theoretically designed using Gaussian 09 with two hybrid functionals B3LYP, HSE06/6–31 G (d,p) basis set. Oscillator strength and excitation energy of the monomers were determined by TD-DFT/B3LYP/6–31 G (d,p) method. Copolymers were synthesized by the direct arylation polymerization method. Structural characterization was done by FTIR and 1H NMR spectra. Thermal stabilities of the copolymers were evaluated by TG. The molecular weights of the copolymers were obtained by Gel permeation chromatography. Cyclic voltammetry (CV) was used to observe the electrochemical band gaps of the copolymers. Theoretical observations from HSE06/6–31 G (d,p) method supported the experimental results. Third-order non-linear optical properties and optical limiting effect of the copolymers were studied at different laser intensities using the open-aperture Z-scan technique at 532 nm in DMSO. P(BT-4FPH) exhibited strong optical non-linearity with reverse saturable absorption when compared with P(3METH-4FPH) and P(TH-4FPH) with non-linear absorption coefficient β = 1.20 × 10−8 m/W at 20 µJ. Optical threshold values obtained for the copolymers were low which indicated that the polymers could be used for optical power limiting devices. Among the three synthesized copolymers, P(BT-4FPH) exhibited higher molecular weight, lowest electrochemical band gap and showed strong non-linear absorption with lowest optical threshold value of 0.009 GW/cm2 at 10 µJ due to the increased delocalization of π- electrons over the conjugated backbone of donor-acceptor units.