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

• Published in: Physical chemistry chemical physics : PCCP Vol. 19; no. 47; pp. 31958 - 31964
• Main Authors: Gao, Feng-Wei, Xu, Hong-Liang, Su, Zhong-Min
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 2017
• Subjects: Absorption spectra; Charge transfer; Chemical bonds; Density functional theory; Electric fields; Electron density; Molecular orbitals; Molecular structure; Nonlinear optics; Optical materials; Optical properties; Optics; Spikes
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c7cp06412h

In this work, we applied an external electric field (F) to a biphenalenyl derivative (BN-PLY ) in the direction of the negative z-axis (F ) and the positive z-axis (F ), 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 (F = 0 to -190 × 10 ) 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 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 = -180 × 10 au (-9.26 × 10 V m ). Moreover, when F is applied, the HOMO and HOMO-1 undergo orbital interchange in the π-dimer at F = 100/110 × 10 au. Significantly, the effect of the external electric field effectively regulates the first hyperpolarizabilities (β ). When the F ranges from 0 to 140 × 10 au, the β values slightly decrease to 0 au. Note that, upon increasing F , the β values sharply increase to 6.67 × 10 au (F = 190 × 10 au). Furthermore, the evolutions of the absorption spectra under F might well explain the trend of β values. When the F ranges from 0 to 140 × 10 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 = 140 to 190 × 10 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.