An investigation of electronic structure and mesomorphic properties of cyano-containing liquid crystals by molecular mechanical and quantum mechanical methods

ABSTRACTThe synthesis of new liquid crystalline compounds and the determination of the properties ofthese mesogen molecules have gained considerable attention in the last decade due to thewidespread use of such materials in display devices. Although, most of the newly synthesizedliquid crystal molecules are structurally very similar, each has its own specific combination ofstructural moieties which determine their physical properties. However, there is very limitedexperimental data on the properties of mesogen molecules. Therefore, theoreticalcomputations are needed to predict and explain the behaviour of liquid crystal molecules interms of their molecular structures.In this study, with the intention of determining the effects of core structures and alkyl chainlengths, the structures of 5 sets of cyano-containing mesogen molecules with different rigidcores and variable alkyl chain lengths from 1 to 10 carbon atoms have been investigatedtheoretically. The most stable conformer for each of the molecules has been determined bycarrying out a conformer analysis by means of molecular mechanics MMFF calculations. Inorder to determine the most suitable quantum chemical method for the mesogen moleculesunder investigation, geometry optimizations have been performed with different methods andbasis sets namely, semi-empirical AM1 and PM3, ab initio Hartree-Fock HF/3-21G and HF/6-31G* and Density Functional Theory DFT/B3LYP/6-31G*. The optimum geometricparameters, thermodynamic and electronic properties of the molecules have been calculated.The results show that mesomorphic behaviour of the cyano-containing mesogen moleculescan be explained in terms of the properties calculated by quantum chemical methods. DFTcalculations have given better results than either ab initio or semi empirical HF methods.However, it may be concluded that semi-empirical PM3 method can be used to calculate theproperties of such molecules since it reproduces the experimental results with good accuracy.Thus, a considerable amount of computer time could be saved and more detailed informationcould be obtained.Keywords: Semi-empirical, ab initio, DFT, liquid crystals, ciyano-containning mesogens