The effect of intramolecular and intermolecular charge transfers on the third order nonlinear optical properties of the self-assemble chromophores

Self-assembled chromophores were widely used in nonlinear optical devices due to their unique charge transfer properties. The structures, intermolecular interactions, absorption spectra, charge transfer (CT) and third order nonlinear optical (NLO) responses (gamma) of monomers, dimers and trimers based on N,N '-butyl-3,4,9,10-perylenediimide (PDIB) have been studied using density functional theory. It is found that with the addition of phenyl substituents, the Egap values between HOMO and LUMO of monomers as well as dimers and trimers decreases gradually, their main absorption peaks of the absorption spectra were redshifted, and intramolecular CT was more pronounced, thus their gamma values increased gradually. Moreover, the intermolecular interaction energies (Eint) of dimers increases with the addition of phenyl substituents, indicating that the enhancement of Eint values increases the degree of intermolecular electron coupling and enhances intermolecular CT. On the other hand, in order to explore the effect of increasing number of layers on the optical properties of dimers and trimers, we divide the gamma values by the number of layers (n = 1, 2 and 3). It is found that the average gamma/n values decreases as n increases, which was due to that the CT in homomolecular self-assembly was non-directional, even isotropic and cancelling. We hope that this work will lead to the careful design of self-assembled NLO materials.

Automated summary

In this study, density functional theory was used to investigate the structures, absorption spectra, charge transfer, and nonlinear optical properties of monomers, dimers, and trimers based on PDIB. The introduction of phenyl substituents resulted in decreased energy gaps, redshifted absorption peaks, enhanced intramolecular charge transfer, and increased intermolecular electron coupling and intermolecular charge transfer. Furthermore, increasing the number of layers led to a decrease in the average nonlinear optical response.