Exploiting structure/property relationships in organic non-linear optical materials: developing strategies to realize the potential of TCNQ derivatives

Abstract

The intrinsic relationship between molecular structure and non-linear optical (NLO) properties is exploited with regard to materials for use in second-harmonic generation (SHG). A bond-length-alternation (BLA) study is performed on all available crystal structures of tetracyanoquinodimethane (TCNQ) derivatives in order to predict better SHG contenders. This builds upon a previous bond-length-alternation study that was carried out on a series of TCNQ derivatives. There, a formalism was derived to calculate the relative level of delocalisation within the central TCNQ fragment and relate this to the molecular hyperpolarizability, β, the measure of the SHG coefficient on the molecular scale. This formalism is based upon the ‘strength–length’ relationship in bonds that has been validated via a charge-density study on one of these TCNQ derivatives using a topological approach. The results of the study are grouped according to the relative levels of delocalisation present in each molecule and compared with known SHG contenders. The promise of TCNQ derivatives for non-linear optics lies partly in their typically high molecular dipole moments, μ. However, large values of μ usually force the molecules to pack centrosymmetrically in a crystalline environment since the dipoles oppose each other in close proximity, and centrosymmetry precludes any SHG effect since this property is a third-rank tensor. A strategy for forming these materials in a solid-state head-to-tail arrangement that is regular but not crystalline is therefore highly desirable. A feasibility study on the adsorption of a TCNQ derivative into zeolite, ZSM-5, shows that this objective may be achievable in this class of materials, thus providing a mechanism for realizing the potential of highly SHG-active materials identified in the bond-length-alternation study.