Linear and non-linear optical properties of hetero-cyclic aromatic polymers

We investigated linear and non-linear optical properties of non-fused and fully fused hetero-cyclic thiophene, selenophene and pyrrole oligomers. We found that these oligomers show relatively better environmental stability in their fused form than non-fused geometry. Linear extrapolation of calculated energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in the polymer limit exhibits extremely good agreement with experimentally reported band gap for thiophene, selenophene and pyrrole oligomers in their non-fused form. The lowest singlet dipole allowed excited state was calculated using time-dependent density functional theory and extrapolated optical gap estimated were in good agreement with experimental observation. Static linear and first hyperpolarisabilities were also studied as a function of chain length for donor–acceptor substituted polymers. The (hyper)polarisabilities are increasing as a function of chain length in both forms although the non-fused pyrrole oligomers show slow variation. Our study on hyperpolarisability also shows that donor–acceptor substituted non-fused selenophene oligomers are most promising candidates to contribute significantly in non-linear photonics and all fully fused hetero-cyclic oligomers show strong NLO responses.