Theoretical study on second-order nonlinear optical response in S-shaped double helicenes incorporating thiophene and...

Recently, S-shaped double helicenes embedded with thiophene and thiopyran units were experimentally prepared, which exhibit fascinating structural features and distinctive optical properties. Nevertheless, the exploration of their application potential in the field of second-order nonlinear optical (NLO) materials is still in its infancy. In this work, we present the first systematic theoretical investigation of the second-order NLO responses and electronic transition properties of a series of S-shaped double helicenes, including the parent compound (compound 4) and four designed derivatives (compounds 1-3 and 5) with donor substituents -C₄H₉, -CH₃, and -NH₂. The results reveal that donor substituents combined with geometrical modulation significantly enhance the second-order NLO response. Specifically, compound 4 with -C₄H₉ and -CH₃ substituents exhibits a considerable total first hyperpolarizability (β_tot) of 1122.97 a.u., significantly exceeding that of compounds 1-3. More importantly, replacing the -CH₃ group with the stronger electron-donor -NH₂ group in compound 5 further boosts β_tot to 1555.58 a.u. This enhancement is attributed to the reduced excitation energy, and the increased oscillator strength and dipole moment variation in crucial excited states. Furthermore, frequency-dependent calculations at incident wavelengths of 1907, 1460, and 1340 nm demonstrate pronounced resonance enhancement effects, yielding dynamic first hyperpolarizability (β_HRS) substantially larger than that of their static counterparts. Solvent effects further amplify the NLO responses, with β_HRS values in toluene and dichloromethane consistently exceeding those obtained in the gas phase. Our findings highlight the critical role of donor engineering in boosting the second-order NLO activity in S-shaped double helicenes and unveil their promising potential as high-performance second-order NLO materials.