Stimulating intra- and intermolecular charge transfer and nonlinear optical response for biphenalenyl biradicaloid dimer under an external electric field

An interesting biphenalenyl biradicaloid (IDPL) dimer consisting of both-middle superimposed phenalenyls and both-end nonsuperimposed phenalenyls has been synthesized, and has attracted intensive research interest due to its intra- and intermolecular interactions and semiconductive characteristics. It is significant that under regulation of the external electric field the directional charge transfer (CT) can produce attractive properties. In the present work, the structure and electronic properties of the IDPL dimer under an external electric field (along the horizontal Fx or the vertical Fz directions) are explored, and the following properties determined: (i) as the horizontal Fx increases, the intramolecular CT becomes larger, which induces the intermolecular CT of the IDPL dimer. (ii) In contrast, as the vertical Fz increases, the large intermolecular CT gives rise to the intramolecular CT of the IDPL dimer. (iii) More importantly, the external electric field effectively regulates and controls the first hyperpolarizability (βtot) of the IDPL dimer. Compared with the vertical Fz, the horizontal Fx induces a larger first hyperpolarizability (βtot = 5.48 × 105 a.u.). Furthermore, the application of a uniform external electric field (Fx,y,z) to the IDPL dimer was investigated to define the external electric field direction of the material application. The βtot values were increased with increasing of the uniform Fx,y,z. Our study provides an effective strategy for developing high-performance NLO materials by tuning the external electric field, and could be of significance for application in switch devices.