Second Order Hyperpolarizabilities Dependence on the Benzenic Ring position of Organic and Organometallic Benzo[c]thiophene species: an assessment by DFT methods

Abstract

Nonlinear optical (NLO) active organic chromophores find application in a wide range of technological applications, from molecular electronics to optical sensors and switches. Thus, the establishment of methods that can predict a priori the optical properties of organic or organometallic chromophores is indeed a crucial topic of current research. Density Functional Theory (DFT) methods are by far the most used computational methods for estimation of optical properties of molecules due to their low computational effort and accurate results. [1] Furthermore, they allow the study of structure/activity correlations in molecules without the need to synthesize all of them. DFT calculations can be used for a screening of NLO chromophores prior to the synthetic labor. Among all the available DFT functionals, B3LYP is by far the most common. Others, like CAM-B3LYP are now being evaluated and they show similar or improved results concerning the prediction of optical properties. [2] In the present work a combination of DFT with the Finite Field (FF) method was used to investigate the influence on the benzenic ring position on the magnitude of the static first hyperpolarizibility of several nitro acetylene organic fragments based on benzo[c]thiophenes. Time Dependent – DFT methods (TD-DFT) were employed to determine the spectral data of all the compounds. After the screening of the organic chromophores was complete, coordination to an iron (II) and ruthenium (II) monocyclopentadienyl fragment was investigated. The chromophores for coordination were chosen according to available procedures in literature to further synthetic achievement. Results show that better NLO responses should be obtained when the benzene ring is close to the electron donor group, whilst the presence of such aromatic ring close to the acceptor group should lead to a decreasing on the NLO response. This behavior is justified, among other possibilities, in terms of HOMO-LUMO band gaps by correlation of the DFT and TD-DFT methodologies.