Tuning of Second-Order Nonlinear Optical Response Properties of Aryl-Substituted Boron-Dipyrromethene Dyes: Unidirectional Charge Transfer Coupled with Structural Tailoring

Abstract

Controlling the intramolecular charge transfer (ICT) process at the molecular level could be a key to develop strong nonlinear optical (NLO) active materials for technological applications. In this paper, we report quantum chemical investigation of NLO response properties of select aryl-substituted boron-dipyrromethene (BODIPY) dyes, a class of ICT molecules. Density functional theory (DFT) with long-range corrected CAM-B3LYP functional is employed to compute the electronic structures and NLO response properties of the aforesaid molecules. The effects of basis sets on the NLO response properties of these molecules are also considered. Calculations at the second-order Møller–Plesset perturbation theory (MP2) are performed for comparison. The results suggest that the charge transfer process in these molecules is mostly unidirectional, and the total first hyperpolarizability (βtotal) values of these molecules are dominantly shaped by the response in the direction of charge transfer. Donor/acceptor substitution as well as twisting of the phenyl ring due to incorporation of methyl groups to BODIPY moiety affects the charge transfer process of these molecules, ultimately altering their NLO response properties. The ratios of vector components of first hyperpolarizability (βvec) to βtotal of the probe molecules also support the unidirectional charge transfer process, except for some molecules with acceptor group substitutions either alone or in conjunction with structural changes. The results presented here are expected to shed light on the origin of NLO response of several aryl-substituted BODIPY dyes and provide means to optimize it for future technological applications.