Spectral manifestations of aggregates structure of heteroaromatic molecules at low temperatures

The results of investigation of temperature influence on spectral-luminesce nce properties of ethanol solution of N,N-dibutyl-3,8-diazapyran-5,10-dione (DPD) in an interval of temperatures of 300-77 K and concentrations of 1.6 x 10(-3)-8.0 x 10(-6) mol/l are discussed. The room-temperature absorption and fluorescence spectra of DPD remained constant in a broad interval of c oncentration. The lowering of temperature of a solution to 148 K leads to t he reduction of the long wavelength band and the formation of two maxima (a t lambda = 374 and 377.5 nm). It was found that their intensities' ratio de pend on concentration as well as on the method of freezing the solution. Th e low-temperature fluorescence spectra are short-wave shifted at 35 cm(-1). The spectral modifications of rigid ethanol solutions of DPD were attribut ed to the formation of the associated forms of molecules due to van der Waa ls forces. Absorption spectra analysed at different cooling rates revealed a "sandwich" structure of associates at a fast rate of freezing and "foot" structure tangle 90 degrees) at a slow rate of cooling. The distance betwee n the associated molecules of DPD was estimated to be about 11 A. The natur e of spectral modifications due to associations of DPD molecules is discuss ed. (C) 2000 Elsevier Science B.V. All rights reserved.