Straight Z and twisted E isomers from triphenylamine derivatives: Intramolecular charge transfer and second-order nonlinear optical response

Intramolecular charge transfer (CT) in push-pull systems plays an important role in enhancing nonlinear optical (NLO) response. The main purpose of this work is to explore the effect of straight Z and twisted E isomers on the second-order NLO properties, two types of push-pull systems (Z(E)-A and D-Z(E)-A) based on the triphenylamine are designed and investigated. Significantly, the first hyperpolarizability (βtot) results show that the βtot value of straight Z-A is larger than that of twisted E-A. It is noteworthy that Z conformation modified by acceptor units can effectively adjust the second-order NLO properties. Further, D-Z-A and D-E-A systems combining multiple donor and acceptor units exhibit large second-order NLO properties. Obviously, D3-Z-A3 shows the largest βtot values of 2.46 × 104 au. Moreover, the trend of the transition energy (ΔE) value is inconsistent with the trend of βtot value, which explains the variation tendency of the βtot value. The presented correlation between conformation and the second-order NLO properties can provide a simple predictive tool to design highly NLO material based on triphenylamine derivatives.