Density functional theory predictions of the nonlinear optical (NLO) properties in triphenylamine based α-cyanocinnamic acid compounds: Effect of fluorine on NLO response

In this study, the energy gaps, second-order nonlinear optical (NLO) properties and dipole polarizabilities of triphenylamine based α-cyanocinnamic acid acetylene derivatives have been investigated by employing time-dependent density functional theory (TD-DFT). These compounds were designed theoretically by substitution of fluorine (F) atom at different positions of phenyl ring of the α-cyanocinnamic acid segment. The results have indicated that the systems substituted by fluorine show remarkable NLO response, especially D4 system with static second-order polarizability (β _tot ) computed to be 70537.95 (a.u). Hence, these compounds have the likelihood to be an excellent second-order nonlinear optical compounds. The β _tot value suggests that along the x-axis the charge transfer (CT) from triphenylamine to α-cyanocinnamic acid (D-A) plays a key role in NLO response; whereas α-cyanocinnamic acid acts as an acceptor (A) and triphenylamine acts as a donor (D) in all the studied compounds (D0-D4). Incorporation of an electron acceptor (F) at the end phenyl ring of the α-cyanocinnamic acid segment increases the β value. The LUMO-HOMO energy gaps of all dyes have been found smaller which results in large NLO response. The present investigation therefore provides an important insight into the remarkably greater NLO properties of α-cyanocinnamic acid and triphenylamine attached via acetylene.