Spectrochimica Acta Part A 78 (2011) 794–802 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa Physiochemical properties of organic nonlinear optical crystal from combined experimental and theoretical studies J. Jayabharathi ∗ , V. Thanikachalam, K. Saravanan, N. Srinivasan, M. Venkatesh Perumal Department of Chemistry, Annamalai University, Annamalai Nagar 608002, Tamil Nadu, India article info Article history: Received 6 October 2010 Received in revised form 9 November 2010 Accepted 9 December 2010 Keywords: NMR Chemosensor NLO DFT MEP abstract 2-(4-Fluorophenyl)-1-(4-methoxyphenyl)-4,5-dimethyl-1H-imidazole (fpmpdi) was synthesised and its structure was characterized by 1 H, 13 C, mass, elemental analysis and X-ray techniques. It belongs to mon- oclinic crystal system having the lattice dimensions of monoclinic with point group, P 21 /c; a = 8.5132 (1) ˚ A, b = 9.5128 (2) ˚ A, c = 19.2610 (3) ˚ A, ˇ = 96.798 (2). Since (fpmpdi) is highly sensitive to different external stimulations such as UV irradiation, heat, increasing pressure and changing the environmental pH caus- ing color change and so they can be used as a “multi-way” optically switchable material. A prominent fluorescence enhancement was found in the presence of transition metal ions such as Hg 2+ , Pb 2+ and Cu 2+ and this was suggested to result from the suppression of radiationless transitions from the n–* state in the chemosensors. The optimized geometrical parameters obtained by DFT calculation is in good agree- ment with single crystal XRD data. The Mulliken, NBO charge analysis and the HOMO–LUMO energies were also calculated. The molecular electrostatic potential surface (MEP) and first static hyperpolaris- ability have also been employed theoretically which reveal that the fpmpdi have non-linear optical (NLO) behavior with non-zero values. Ground and excited states DFT calculations were carried out in order to find out dipole moment and energy. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Recently, heterocyclic imidazole derivatives have attracted con- siderable attention because of their unique optical properties [1–3]. These compounds play very important role in chemistry as media- tors for synthetic reactions, primarily for preparing functionalized materials [4–8]. Imidazole nucleus forms the main structure of some well-known components of human organisms and also has significant analytical applications utilizing their fluorescence and chemiluminescence properties [9–11]. Some basic processes, such as excited state intermolecular pro- ton transfer (ESIPT) have been carried out for imidazole derivatives [12–14]. An important property that makes imidazole derivatives more attractive as a chelator is the appreciable change in its flu- orescence upon metal binding. Therefore, imidazole derivatives have been used to construct highly sensitive fluorescent chemosen- sors for sensing and imaging of metal ions and its chelates in particular those with Ir 3+ are major components for organic light emitting diodes [2,3] and are promising candidates for fluores- cent chemosensors for metal ions. Organic nonlinear optical (NLO) materials are very much useful in the area of second harmonic ∗ Corresponding author. Tel.: +91 9443940735. E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi). generation, optical communication, electro-optic modulation, opti- cal data storage technology etc. [15–17]. In the present paper we summarized a comprehensive study of the fpmpdi and its struc- ture was characterized by spectroscopy, elemental analysis and XRD. Photophysical and photochemical studies of fpmpdi were ana- lyzed and discussed. Theoretical calculations were carried out by using Gaussian-03 program [18,19] to supplement the experimen- tal results. 2. Experimental 2.1. Optical measurements and composition analysis NMR spectra were recorded for fpmpdi on a Bruker 400 MHz. The ultraviolet–vis (UV–vis) spectra were measured on UV–vis spectrophotometer (Perkin Elmer, Lambda 35) and corrected for background due to solvent absorption. Photoluminescence (PL) spectra were recorded on a (Perkin Elmer LS55) fluorescence spec- trometer. MS spectra were recorded on a Varian Saturn 2200 GCMS spectrometer. 2.2. Non-linear optical measurements Its second harmonic generation efficiency was assessed by Kurtz powder technique [20] at IISc., Bangalore, India. It is a well 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.12.026