Photobleaching of nonlinear organic co-crystals 2,6-diaminopyridine-4-nitrophenol-4-nitrophenolate

The processes occurring during photobleaching of a new nonlinear optical co-crystal 2,6-diaminopyridine-4-nitrophenol-4 nitrophenolate are investigated. Using infrared spectroscopy of co-crystals, destruction of an aminopyridine-nitrophenol molecular complex by photoactinic radiation without breakage of aminopyridine and nitrophenol molecules integrity is shown. Differential scanning calorimetry and thermogravimetric analysis of the co-crystals showed a decrease in temperature of transitions of exposed co-crystals due to structural changes. Intensities of the second harmonic of the co-crystals under study were measured before and after photoactinic exposure; the results showed the decrease in the intensities of the second harmonic during prolonged illumination due to the structural changes in volume of the co-crystals confirming the breakage of the aminopyridine-nitrophenol molecular complex with preservance of the integrity of the molecules forming the molecular complex. Methods of terahertz absorption spectroscopy showed changes in absorption and a refractive index of a co-crystal in the range of 0.3-2.5 THz after the exposure to the photoactinic radiation, which can be interpreted as a result of destruction of the crystal structure. All the results obtained clearly showed that a co-crystal structure is disordered and the aminopyridine-nitrophenol bond is broken on the exposure to the photoactinic radiation, while the molecules of both components do not collapse, which leads mainly to disappearance of nonlinear optical properties. The investigated process is suitable for creating nonlinear optical lattices in the co-crystals using the photolithography method.