New trans-stilbene derivatives with large two-photon absorption cross-section and non-linear optical susceptibility values--a theoretical investigation.

A detailed theoretical study of linear and non-linear optical susceptibilities (NLOS), one- and two-photon absorption (OPA and TPA) properties for a series of push-pull trans-stilbene (TSB) derivatives with introduction of different electron donor (D) and acceptor (A) groups on either side of the TSB ring system is presented. The objective of the work is to design new TSB derivatives with large TPA cross-section values and to explore their linear and non-linear optical susceptibilities, OPA and TPA properties. We have used linear and quadratic response theory methods and CAM-B3LYP functional in conjunction with the 6-31+G* basis set for all property calculations. We have explained the results of the first hyperpolarizability and TP transition probability using two-state model (2SM) calculations, the results of which are in excellent agreement with the response theory methods. The TP tensor elements have been analysed to explain the large TP activity of molecules. Orbitals involved in the transition processes have been studied both qualitatively (molecular orbital pictures) and quantitatively (Λ-values) in order to explain the nature of charge transfer in different TSB derivatives. The study reveals that the novel derivatives TSBD-10, TSBD-11, TSBD-12 and TSBD-13 have large non-linear optical susceptibilities and TPA cross-section values, the largest being found for TSBD-13 (5560 G.M.).