Theoretical study on photophysical properties of double helicenes entwined with two perylene diimides†

Double helicenes, with their notable chirality and intriguing properties, have captivated the great interest of chemists. Comprehending the relationship between their structure and properties is fundamentally significant for enhancing performance and facilitating practical applications. In this study, we have explored the photophysical properties of a series of double helicenes, composed of two perylene diimides (PDIs) entwined via C–C single bonds, using density functional theory (DFT) calculations. These properties include electronic transition characteristics, absorption spectra, and second-order nonlinear optical (NLO) responses. The simulated electronic absorption spectra of the parent compound (compound 1) closely resemble the experimental results. Compound 1 also exhibits an intrinsic second-order NLO response. Compounds 2–7, designed by substituting the terminal CH₃ groups in compound 1 with electron acceptors (NO₂, 7,7,8,8-tetracyanoquinodimethane (TCNQ)), electron donors (NH₂, tetrathiafulvalene (TTF)), and their combinations, demonstrate tunable HOMO/LUMO distributions, band gaps, absorption intensities, and electron transition properties. Notably, the maximum β_HRS value reaches 8.71 × 10⁻³⁰ esu, approximately 95 times greater than that of the organic molecule urea. Our findings provide insights for the development of double helicenes with PDI subunits.