Replacing carbazole with [2.2]paracyclophane and triptycene donors: theoretical insights into the modulation of second-order nonlinear optical properties

Abstract

Functionalization of the weaker carbazole (Cz) donor and arene bridge moieties in compounds is an effective way to regulate their second-order nonlinear optical (NLO) properties. In this work, the second-order NLO responses of donor-π-acceptor molecules that consisted of [2.2]paracyclophane (CzpPhTrz)/triptycene scaffolds (TCz-Trz) incorporated into carbazole were investigated using the density functional theory method. The charge transfer (CT) characteristics of the three systems were assessed based on frontier molecular orbitals (FMOs), confirming the effective intramolecular CT (ICT) from the donor group to the diphenyltriazine (cyaphenine) acceptor moiety. A stronger electron-donating ability effectively increased the distribution of the HOMO orbital, which has an effect on the static first hyperpolarizability, β_tot. The Pcp with through-space interactions (TSIs) and TCz groups can accelerate the ICT characteristic and enhance the contributions to the axial component of the β_tot values of CzpPhTrz (38.7 × 10⁻³⁰ esu) and TCz-Trz (41.3 × 10⁻³⁰ esu) compared to that of CzPhTrz (36.3 × 10⁻³⁰ esu) using the ω*B97XD method with optimally-tuned range-separated parameters. In order to confirm its practical significance, the dynamic hyperpolarizability of the studied system was evaluated. The CT characteristics of the crucial excited state were studied based on hole–electron analysis. The reported D-index ranged from 5.634 Å to 5.995 Å, indicating the obvious charge separation in donor-substituted systems. Overall, donor substitution leads to an increase in polarization and CT characteristic, which is an effective way to regulate the second-order NLO response.