Reversible luminescence thermochromism and phase transition in crystals of thiophenylacetylacetonatoboron difluoride

The low-temperature phase transition (-22.4 degrees C) is accompanied by marked change of the optical properties, as was first revealed for 3-phenylthiopentan-2,4-dionatoboron difluoride (1) in the series of beta-diketonatoboron difluorides. This transition was investigated by luminescence and reflection spectroscopy, differential scanning calorimetry (DSL), X-ray analysis and atomic force microscopy methods. It was observed by the DSL method that at the room temperature the complex existed in its high-temperature modification and transformed into the stable one with Delta H = 0.981 kcal/mol below -22.4 degrees C. The reverse transition takes place at 43.4 degrees C with Delta H = 0.59 kcal/mol. Cooling of 1 results in a noticeable change of its morphology: the crystal cracked along the layer borders and nanometer particles were formed. Having been cooled the light-yellow crystals turned into white powder and luminescence colour changed from aquamarine to white. Heating of the cooled I to the 47.9 degrees C (the second phase transition) results in partial coincidence of the reflection and luminescence spectra, as well as those of initial single crystals that indicates the existence of size-dependent optic properties. (C) 2008 Elsevier B.V. All rights reserved.