Prediction of robustly large molecular second-order nonlinear optical properties of terpyridine-substituted hexamolybdates: Structural modelling towards a rational entry to NLO materials Muhammad Ramzan Saeed Ashraf Janjua a,1 , Wei Guan a , Likai Yan a , Zhong-Min Su a, *, Muhammad Ali b , Iftikhar Hussain Bukhari b a Institute of Functional Material Chemistry and Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, Jilin, PR China b Department of Chemistry, University of Sargodha, Sargodha 40100, Islamic Republic of Pakistan 1. Introduction Polyoxometalate (POM) chemistry has progressed dramatically during the past two decades, and various new structural types with fascinating topological beauty and important electronic, optical, and catalytic properties have been explored [1–4]. Along with this rapid progress made on these inorganic metal–oxygen cluster anions, studies on organic and polymeric conjugated materials have also exploded and flourished [5–8]. Although they are vastly different in molecular structures, POMs and organic conjugated molecules are both electrically active materials with similar electrical and optical properties such as photochromism, electro- chromism, and conductivity. The underlying mechanisms of these properties are, however, different for the two types of materials, with dp electrons responsible for the inorganic clusters, and delocalized p electrons responsible for the organic counterpart. While both areas have been enjoying significant success, there has been little success in bringing these two types of materials together through covalent bonds [9,10]. Not only will such hybrid materials combine the advantages of organic materials, such as ease in processing and structural fine tuning, with those of inorganic clusters, but the close interaction of organic delocalized p electrons with the cluster d electrons may bring striking synergistic effects. Such materials with their unique structures are extremely interesting not only to synthetic chemists and materials scientists, but also to theoreticians and experimentalists. Numerous classes of molecules have been investigated as novel molecular NLO materials during the last few decades. These molecules can be placed into three generic classes of NLO materials: inorganic salts, organic materials, and organometallic materials. Journal of Molecular Graphics and Modelling 28 (2010) 735–745 ARTICLE INFO Article history: Received 4 January 2010 Received in revised form 21 January 2010 Accepted 26 January 2010 Available online 4 February 2010 Keywords: Polyoxometalates Organic–inorganic hybrids Molybdenum Second-order polarizability TDDFT ABSTRACT We have explored an innovative, versatile, and novel molecular hybrid containing polyoxometalate (POM) cluster linked with terpyridine ligand via p-bridged donor–acceptor (D–A) configuration. The dipole polarizabilities, density of states, and second-order nonlinear optical (NLO) properties of terpyridine-substituted hexamolybdates have been investigated by using time-dependent density functional response theory (TDDFT). This class of organic–inorganic hybrid compounds possesses a robustly large molecular second-order NLO response, especially [Mo 6 O 18 (N 4 C 25 H 16 I 2 )] 2 (system 5) and [Mo 6 O 17 (N 4 C 25 H 16 (CN) 2 )(N 4 C 25 H 16 (CN) 2 )] 2 (system 10) with the static second-order polarizability (b vec ) computed to be 1209.25  10 30 esu and 1622.67  10 30 esu respectively. Thus, these systems have the possibility to be excellent second-order nonlinear optical materials. Analysis of the major contributions to the b vec value suggests that the charge transfer (CT) from POM-cluster to terpyridine ligand (D–A) along the z-axis plays the key role in the NLO response, POM-cluster (hexamolybdates) acts as a donor (D) whereas terpyridine ligand acts as an acceptor (A) in all the studied systems. The computed b vec values increase by the incorporation of electron acceptors (halogen = F, Cl, Br and I) at the terminus of terpyridine ligand. Furthermore, substitution of trifluoromethoxy (–OCF 3 ), trifluoromethyl (–CF 3 ), and cyanide (–CN) at the end of terpyridine ligand respectively enhances the optical nonlinearity. Orbital analysis shows that the degree of CT between POM and terpyridine segment was increased in 2D and organometallic/POM hybrid systems. The present investigation provides important and thought provoking insight into the robustly large NLO properties of terpyridine-substituted hexamolybdates. ß 2010 Elsevier Inc. All rights reserved. * Corresponding author. E-mail addresses: dr_janjua2010@yahoo.com (M.R.S.A. Janjua), zmsu@nenu.edu.cn (Z.-M. Su). 1 Tel.: +86 431 85099108/+92 300/321 6604948; fax: +86 431 85684009. Contents lists available at ScienceDirect Journal of Molecular Graphics and Modelling journal homepage: www.elsevier.com/locate/JMGM 1093-3263/$ – see front matter ß 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jmgm.2010.01.011