Influence of polar substituent on central bending unit of bent core mesogens: Synthesis, photophysical, mesomorphism and DFT studies Manoj Kumar Paul a, * , Gayatri Kalita a , Barnali Bhattacharya b , Utpal Sarkar b a Department of Chemistry, Assam University, Silchar 788011, India b Department of Physics, Assam University, Silchar 788011, India article info Article history: Received 10 February 2016 Received in revised form 19 April 2016 Accepted 20 April 2016 Available online 23 April 2016 Keywords: Bent core molecule Photophysical Mesomorphism Computational studies abstract New five ring bent core mesogens derived from substituted 1,3-phenylenediamine (4-nitro-1,3- phenylenediamine, 4-chloro-1,3-phenylenediamine) were synthesized. Their molecular structures, photophysical properties and mesogenic behaviors were investigated. The molecular structures and the purity of the bent core molecule have been characterized by spectroscopic studies and elemental analysis respectively. Photophysical properties of bent core compounds were investigated in chloroform by using UVevisible and fluorescence spectroscopic studies. The phase transition temperatures were detected by differential scanning calorimetry analysis and the phases are confirmed by polarizing optical microscopy. The polar substituents on bent core unit of bent shaped molecule influence the mesomorphic behaviors of the bent core mesogens. The polar nitro group at 4-position of the bent core unit displays tilted smectic C phase and unknown smectic X phase whereas chloro group at 4-position exhibits orthogonal smectic A phase. The bent core mesogens are fluorescent in nature. The density functional theory calculation was carried out to obtain the stable molecular conformation and chemical reactivity of the bent core molecules. Orbitals involved in the electronic transitions and their corresponding energies together with oscillator strengths have been reported. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Since more than last two decades, achiral bent core mesogens (BCMs) paid a special attention in the area of soft materials due to their interesting functional properties such as macroscopic polar order due to restricted rotation along molecular long axis (viz., ferro- and anti-ferroelectricity) [1], macroscopic chiral structure due to spontaneous achiral symmetry breaking [2] and exhibited fascinating optical textures. The manifestation of such interesting properties and exotic optical textures in BCMs depend on the mo- lecular structural arrangement and their intermolecular in- teractions. The molecular structure and the intermolecular interactions are further governed by the factors such as bent core unit, number of the phenyl rings, length of the terminal chain, di- rection and the conjugation of the linking group between the two phenyl rings units and nature of substituents on the bending unit or outer phenyl ring of the bent core molecule. Moreover, in general, small changes in these factors led to drastic change in the mesophase behavior and their transition temperatures. Generally, the substitutions viz., fluoro, chloro, bromo, nitro, cyano, methyl etc. attached to the central bending unit change the bending angle of the molecule which in turn leads to the formation of different types of mesophase [3]. The structure-property relationship of the bent core molecules are well documented in excellent reviews [4e7]. The most of the rigid bent core unit used in mesogenic bent core molecule consists of resorcinol molecule whereas mesogenic bent core molecules obtained from 1,3-phenylene diamine rigid bent core are rare and limited [8e10]. Further, lateral substituents on the central resorcinol bending unit or the outer phenyl ring significantly influence the mesogenic behavior [11e21]. Moreover, it is established that mesophase behavior of the BCMs are much more strongly influenced by the substituent at the central bent core unit than by these at the outer phenyl ring [22e25]. The dipolar effects are more significant than the steric effects due to the pres- ence of the polar substituent on the central or outer phenyl ring of the molecule. The first symmetrical achiral bent core molecule derived from 1,3-phenylene diamine viz., N, N 0 -bis[4-(4 0 -n-alkox- ybenzoyloxy)benzylidene]-phenylene 1,3-diamine exhibited B 6 and or B 1 phase depending on the 4-n-alkoxy chain length [26]. To * Corresponding author. E-mail address: paulmanojaus@gmail.com (M.K. Paul). Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc http://dx.doi.org/10.1016/j.molstruc.2016.04.060 0022-2860/© 2016 Elsevier B.V. All rights reserved. Journal of Molecular Structure 1119 (2016) 177e187