Structural phase transition and nonlinear optical properties in P-Nitroaniline

A considerable number of polar organic molecules have been assessed for applications in nonlinear optics including electro optic modulation, second harmonic generation, three dimensional optical data storage, optical switching and optical limiting. Such materials generally contain donor and acceptor groups positioned at either end of a suitable conjugation path. The magnitude of the nonlinear coefficient increases with increasing length of the conjugation path and also with the charge state of the donor/acceptor groups. P-nitroaniline (C₆H₇N₂O₂ ⁺, HSO₄ ^-) is one of the simplest molecules having electron donor and acceptor moieties connected by an aromatic ring. It exhibits pronounced solvatochromism and large NLO properties. These and other photochemical features of p-nitroaniline have long been the subject of various investigations. Single-crystal diffraction experiment was carried out using the high-resolution home diffractometer equipped with an area detector. Data sets were collected at room temperature (293 K) Mo Kalpha radiation (lambda = 0.71073 Aring). The compound crystallizes in a space group P2₁2₁2₁ of orthorhombic system with cell parameters a = 6.7037(3) Aring, b = 9.1978(2) Aring, c = 14.8209(3) Aring, V= 913.85 (5) Aring³. The calorimetric behaviour of compound (1) was studied by measuring the enthalpy with differential scanning calorimetry (DSC). Upon heating and cooling the sample in the 100-400 K temperature range, the crystalline substance undergoes a reversible single crystal transition at T_c = 338 (2) K. Second harmonic generation measurements (SHG) were performed by applying the Kurtz and Perry model, to powder sample of compound (1) (lambda = 1064 nm, Nd:YAG pulsed laser). In order to measure the relative value of the SHG efficiency of 1, powdered POM (3-methyl-4-nitropyridine-1-oxide) was selected a reference for- - powder. The effective second-order nonlinear susceptibility of 1 (1.15 pmldrV^-1) equals ~9.5% of the one measured for POM. A study of the temperature dependency of the SHG signal revealed a monotonous behaviour up to 363 K (heating speed: 0.3 Kldrmin^-1) followed by an abrupt decline in intensity down to zero, which is in good agreement with a change in structure from acentric to centrosymmetric.