Theoretical study on thiophene-based double helicenes with intrinsic large second-order nonlinear optical response

Abstract

Helicenes have attracted great attention due to their inherent chirality and distinctive optical features. Moreover, the intrinsic chirality of helicene can meet non-centrosymmetric electronic structure requirements of second-order optical materials. Thiophene or polythiophene plays an important role in organic photoelectric materials. For organic molecular materials, several electronic behaviors could be combined into a single molecular entity. By utilizing density functional theory calculations, we here for the first time investigated the electronic structures and optical properties of thiophene-based double helicenes. The calculated electronic excitation energies are in good agreement with the experimental ones. The simulated ECD spectra nicely reproduce the experimental ECD spectra in both excitation energy and rotational strength, which can be used to assign the absolute configurations of thiophene-based double helicenes with high confidence. The electronic transition properties and chiroptical origins have been assigned and analyzed. The studied compounds possess remarkably large molecular first hyperpolarizabilities, especially compound a-5 which has a value of 29.09 × 10⁻³⁰ esu, which is about 300 times larger than the first hyperpolarizabilities of an organic urea molecule and 7 times larger than that measured for a highly π-delocalized phenyliminomethyl ferrocene complex. The effect of single/double helicenes on first hyperpolarizabilities is compared and discussed. We also probed the intramolecular charge transfer cooperativity, which is important for further experimental investigation. Based on the intrinsic non-centrosymmetric electronic structures, high transparency, and the larger first hyperpolarizability values, the studied compounds are expected to be excellent second-order nonlinear optical materials.