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Reversible luminescence thermochromism of dibenzoyl(methanato)boron difluoride was found. A substantial hypsochromic shift of the luminescence band maximum (60 nm) with a temperature decrease from 300 to 77 K was observed, and the luminescence color changed from green to blue. A considerable decrease (by 0.386 Å) in the unit cell parameters along the c axis was detected by X ray diffractometry when the temperature of the crystal decreased from 293 to 123 K. Shortening of the distances between the planes of adjacent molecules (stacking factor) weakens the interaction between the dimers and enhances the monomeric and excimeric luminescence.Development and synthesis of reversible luminescent thermochromes are urgent from both practical and theo retical standpoints. 1,2 One of the promising lines of search for new luminescent thermochromes is a study of mo lecular organized organic systems, including dimers, tri mers, and aggregates of conjugated molecules, whose supramolecular architecture produces different intermo lecular interactions: dimeric (excimeric), interdimeric, and stacking interactions. 3,4Continuing our works on studying the interrelation of the structure and luminescence and photochemical prop erties of boron difluoride β diketonates, 5-7 in this work we studied a change in the luminescence of dibenzo yl(methanato)boron difluoride (1) at different tempera tures and revealed reversible luminescence thermo chromism of this compound, namely, a substantial hypso chromic shift of the fluorescence band maximum (60 nm) and a change in the luminescence color from green to blue when the temperature decreases from 300 to 77 K. ExperimentalCompound 1 was synthesized according to a previously pub lished procedure. 8 Crystals were prepared by recrystallization from an acetonitrile-toluene (1 : 1) mixture. Fluorescence spec tra of solutions were obtained on a Shimadzu RF 5000 in strument. Luminescence spectra of crystalline compounds at 77-300 K were recorded on an SDL 1 spectrometer, using a DRSh 250 lamp as an excitation source and an UFS 6 filter (λ exc = 365 nm). To measure a temperature dependence of the luminescence spectra, a sample was placed in a quartz Dewar flask above the liquid nitrogen surface. The temperature of the sample was measured by a platinum thermocouple. The X ray diffraction study of compound 1 was carried out at 293, 203, and 123 K in a SMART 1000 CCD Bruker system using Мо Кα radiation. Results and DiscussionAs can be seen from the data in Fig. 1, a decrease in the temperature of the crystal from 300 to 77 K produces a substantial hypsochromic shift of the fluorescence band maximum of compound 1 (60 nm). For a more detailed analysis of the evolution of the luminescence spectra with a temperature change, the spectra were deconvoluted by the Gaussian expansion using the Origin 6.1 program; the results of deconvolution are presented in Fig. 1.It turned out that the fluorescence spectrum of com pound 1 at each temperature consists of three compo nents with maxima at 470, 500, and 560 nm, and o...

Zinc and silver vapor homogeneous nucleations are studied experimentally at the temperature from 600 to 725 and 870 K, respectively, in a laminar flow diffusion chamber with Ar as a carrier gas at atmospheric pressure. The size, shape, and concentration of aerosol particles outcoming the diffusion chamber are analyzed by a transmission electron microscope and an automatic diffusion battery. The wall deposit is studied by a scanning electron microscope (SEM). Using SEM data the nucleation rate for both Zn and Ag is estimated as 10(10) cm(-3) s(-1). The dependence of critical supersaturation on temperature for Zn and Ag measured in this paper as well as Li, Na, Cs, Ag, Mg, and Hg measured elsewhere is analyzed. To this aim the classical nucleation theory is extended by the dependence of surface tension on the nucleus radius. The preexponent in the formula for the vapor nucleation rate is derived using the formula for the work of formation of noncritical embryo [obtained by Nishioka and Kusaka [J. Chem. Phys. 96, 5370 (1992)] and later by Debenedetti and Reiss [J. Chem. Phys. 108, 5498 (1998)]] and Reiss replacement factor. Using this preexponent and the Gibbs formula for the work of formation of critical nucleus the dependence of surface tension on the radius R(S) of the surface of tension is evaluated from the nucleation data for above-mentioned metals. For the alkali metals and Ag the surface tension was determined to be a strong function of R(S). For the bivalent metals (Zn, Hg, and Mg) the surface tension was independent of radius in the experimental range. A new formula for the Tolman length delta as a function of surface tension and radius R(S) is derived by integration of Gibbs-Tolman-Koenig equation assuming that delta is a monotonic function of radius. The formula derived is more correct than the Tolman formula and convenient for the elaboration of experimental data. Using this formula the values of delta are determined as a function of R(S) from the experimental nucleation data. It is determined that all the metals considered are characterized by strong dependence of delta on radius; for the bivalent metals delta changes sign.

Essential differences in the luminescence properties of dibenzoylmethanatoboron difluoride (1) bulk and microcrystals were detected. Analysis of the spectral data showed that in every case the fluorescence spectrum of 1 consisted of three components: monomer fluorescence, excimer fluorescence, and fluorescence related with interdimer interaction. Evolution of luminescence spectra observed with decrease of the crystal size are bound with decrease of monomer fluorescence intensity and considerable growth of intensity of fluorescence related to interdimer interaction. The presence of reversible luminescence thermochromism was discovered for 1: lowering the temperature from 300 to 77 K resulted in essential hypsochromic shift of the luminescence band maximum ( approximately 60 nm) connected with considerable decrease of the distances between the planes of the neighboring molecules ( approximately 0.1 Angstrom) that lead to the weakening of interdimer interaction and intensification of monomer luminescence.

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