Nonplanar Donor–Acceptor Chiral Molecules with Large Second-Order Optical Nonlinearities: 1,1,4,4-Tetracyanobuta-1,3-diene Derivatives

Abstract

We have investigated the chiroptical, linear, and second-order nonlinear optical (NLO) properties of five 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) derivatives and elucidated structure-property relationships from the micromechanism. The experimental UV-vis absorption and circular dichroism (CD) spectra were well reproduced by our calculations at TDB3LYP/6-31+G* level of theory. The electron transition property and chiroptical origin have been assigned and analyzed. The results show that the studied compounds possess large molecular first hyperpolarizabilities, especially for compound 5 which has a value of 35 × 10(-30) esu, which is comparable with the measured value for highly π-delocalized phenyliminomethyl ferrocene complex and about 200 times larger than the average first hyperpolarizability of the organic urea molecule. Despite the nonplanarity of these compounds, efficient intramolecular charge transfer (CT) from electron donor to electron acceptor moieties was observed, which plays the key role in determining the NLO response. The intramolecular charge transfer cooperativity was also probed. In view of the first hyperpolarizability values, intrinsic noncentrosymmetric electronic structure, and high stability, the studied compounds have the possibility to be excellent second-order NLO materials.