The first hyperpolarizabilities of hemicyanine cationic derivatives studied by finite-field (FF) calculations

In this paper, we have designed 26 hemicyanine cationic derivatives by selecting π-conjugated bond and increasing alkyl chain length of electron donor (D) group and of electron acceptor (A) in order to describe their structure-property relationship. The geometries and molecular frontier orbitals are obtained from semi-empirical PM3 method. The finite-field (FF) method is performed to calculate molecular first hyperpolarizability (β). It is concluded that hemicyanine molecular second-order nonlinear optical (NLO) properties are not dependent on σ-chain (abbreviation for the alkyl chain connected to cationic pyridinium ring) but determined by donor and acceptor. With the π-conjugation length increasing, the magnitudes of β increase and approach saturation, while the HOMO-LUMO energy gap ΔEHL values decrease resulting in red shift of absorption spectra peak. Moreover, it is interesting to note that the first hyperpolarizabilities of hemicyanine derivatives are linearly related to the HOMO-LUMO energy gap ΔEHL. It may be helpful to further understand the structure-property relationship of the organic NLO materials.