Exploring the quinoidal oligothiophenes to their robust limit for efficient linear and nonlinear optical response properties

The possibility of tuning the chemical structures of oligothiophenes offers various applications in designing of optoelectronics and nonlinear optical (NLO) materials. In the current study, we report a computational study of quinoidal oligothiophenes to explore the influence of type of π -conjugation chains and nature of terminal acceptor groups to push their linear ( α ) and third-order nonlinear optical ( γ ) polarizability to a robust limit. The ground-state molecular geometries of designed compounds 1 – 8 are optimized using DFT at the M06/6-311G** level of theory. Among all the designed compounds, the largest linear isotropic ( α iso ) and anisotropic ( α aniso ) polarizability values of 342.7 × 10 –36 and 765.6 × 10 –36 esu are shown by compound 8 owing to its excellent electronic connection between central thiophene rings and terminal acceptor groups. Similarly, the highest average static third-order nonlinear optical (NLO) polarizability amplitude of 7448 × 10 –36 esu is shown by compound 8 which is ~ 1319, ~ 103, ~ 21, ~ 77, ~ 40, ~ 28, and ~ 13 times greater as compared to compounds 1 – 7 , respectively. Strikingly, comparative analysis of between compound 8 and p -NA (prototype NLO molecule) reveals that amplitude of compound 8 is 1021 times more than that of p -NA at the same M06/6-311G** methodology as calculated in current investigation. Remarkably, the large difference between values of compound 8 and p -NA designates significant potential of our designed compound for various NLO applications. Furthermore, TD-DFT computations indicate that the greater NLO response of compound 8 is because of its higher oscillator strength and lower transition energy in contrast to other designed molecular systems. Moreover, TD-DFT calculations are also used to investigate structure-NLO property relations in terms of frontier molecular orbitals (FMOs), the density of states (DOS), molecular electrostatic potential (MEP), and transition density matrix (TDM) parameters. We believe the current design will instigate the materials science community for such novel motifs with efficient optoelectronic and NLO properties. Graphical abstract