Tuning the inter-molecular charge transfer, second-order nonlinear optical and absorption spectra properties of a π-dimer under an external electric field†
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 (βtot). When the F+z ranges from 0 to 140 × 10−4 au, the βtot values slightly decrease to 0 au. Note that, upon increasing F+z, the βtot 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 βtot 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.