Tuning the inter-molecular charge transfer, second-order nonlinear optical and absorption spectra properties of a π-dimer under an external electric field

Abstract

In this work, we applied an external electric field (F) to a biphenalenyl derivative (BN-PLY₂) in the direction of the negative z-axis (F_−z) and the positive z-axis (F_+z), respectively. The influence of the two directions of F on the molecular structures and electronic properties is investigated, which gives interesting results. Density functional theory (DFT) calculations show that the application of F_−z (F_−z = 0 to −190 × 10⁻⁴) is an advantage toward improving π-dimer stability, which is attributed to an increase in bonding and attractive electrostatic interactions. Interestingly, a large amount of negative charge is induced by applying F_−z to the upper layer, resulting in an increase in the electron density in the upper layer, which is the main factor for the formation of a symmetric highest occupied molecular orbital (HOMO) at F_−z = −180 × 10⁻⁴ au (−9.26 × 10⁹ V m⁻¹). Moreover, when F_+z is applied, the HOMO and HOMO−1 undergo orbital interchange in the π-dimer at F_+z = 100/110 × 10⁻⁴ au. Significantly, the effect of the external electric field effectively regulates the first hyperpolarizabilities (β_tot). When the F_+z ranges from 0 to 140 × 10⁻⁴ au, the β_tot values slightly decrease to 0 au. Note that, upon increasing F_+z, the β_tot values sharply increase to 6.67 × 10³ au (F_+z = 190 × 10⁻⁴ au). Furthermore, the evolutions of the absorption spectra under F might well explain the trend of β_tot values. When the F_+z ranges from 0 to 140 × 10⁻⁴ au, the broad absorption spikes with a low-energy are significantly blue-shifted, while only absorption spikes with a high-energy are significantly red-shifted (F_+z = 140 to 190 × 10⁻⁴ au). The present work not only provides a deeper understanding of the relationships between the molecular structure and the electronic properties of a π-dimer system, but can also be developed for designing highly efficient nonlinear optical materials through the influence of an external electric field.