Physiochemical properties of organic nonlinear optical crystal from combined experimental and theoretical studies

2-(4-fluorophenyl)-1-(4-methoxyphenyl)-4,5-dimethyl-1H-imidazole (fpmpdi) was synthesised and its structure was characterized by 1H, 13C, mass, elemental analysis and X-ray techniques. It belongs to monoclinic crystal system having the lattice dimensions of monoclinic with point group, P21/c; a=8.5132 (1) Å, b=9.5128 (2) Å, c=19.2610 (3) Å, β=96.798 (2). Since (fpmpdi) is highly sensitive to different external stimulations such as UV irradiation, heat, increasing pressure and changing the environmental pH causing 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 Hg2+, Pb2+ and Cu2+ and this was suggested to result from the suppression of radiationless transitions from the n-π* state in the chemosensors. The optimized geometrical parameters obtained by DFT calculation is in good agreement 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 hyperpolarisability 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.