Low band gap donor-acceptor phenothiazine copolymer with triazine segment: Design, synthesis and application for optical limiting devices

Soluble conjugated donor-acceptor phenothiazine-N-piperidine substituted triazine copolymer (P(PZ-TN)) has been designed and synthesized via Suzuki coupling reaction. To investigate the variation in band structure of the copolymer, quantum-chemical calculation using DFT theory was carried out in the periodic boundary condition (PBC) formalism at HSE06 and B3LYP correlation function using 6-31 G basis set. The insertion of triazine unit as an alternating monomer in the copolymer of phenothiazine and triazine lowers the HOMO and LUMO energy levels. The optical band gap of the copolymer was calculated to be 2.5 and 2.3 eV in THF solution and as thin film, respectively from the onset of low energy optical transition. In thin film, the energy gap tends to narrow and the absorption and emission peaks are red shifted owing to the better interaction and increase in planarity of the copolymer in thin film. Theoretical studies along with photophysical and electrochemical studies confirmed that the copolymer exhibited relatively low band gap than that of homopolymer. The absorption and emission spectra of the copolymer, in solvents of varying polarity showed positive solvatochromism. The third-order nonlinear optical properties of copolymer, P(PZ-TN) were investigated by Z-scan technique at 532 nm. The copolymer showed strong third-order nonlinear optical susceptibility and low optical limiting threshold values of 1.27 × 10−11 esu and 0.22 GW/cm2, respectively.