Theoretical and Experimental Chemistry, Vol. 32, No. 4, 1996 INFLUENCE OF TEMPERATURE ON QUENCHING OF FLUORESCENCE IN COMPOUNDS OF THE COUMARIN SERIES T. V. Sakhno, I. V. Korotkova, and O. A. Khakhel UDC 535.37 A study has been made of the influence of temperature on the quantum yield of fluorescence of 3,4- benzocoumarin in ethanol or n-hexane, in the temperature interval 77-300 K. A mechanism of fluorescence quenclu'ng is proposed, consisting of thermal "activation" of intercombination conversion between neighboring energy levels that differ in orbital nature. The question of the influence of temperature on the quantum yield of fluorescence of a number of organic compounds has been discussed rather extensively in the literature in the examples of anthracene derivatives [1, 2], phthalimid [3], a number of amino acids and proteins, rhodamine [4], and coumarin and its derivatives [5]. However, there is no agreement on the mechanism of thermal quenching of fluorescence of these compounds. We have carried out measurements of the temperature dependence of the quantum yield of fluorescence for derivatives of ot-pyrones, and are proposing a mechanism of thermal quenching. The spectral characteristics (absorption and fluorescence spectra) were registered at various temperatures (77-300 K) by a procedure described in [6]; the preparation of samples for the luminescence studies was also described in [6]. At room temperature, in ethanol or n-hexane, the molecules of 3,4-benzocoumarin and 4,9-dioxapyrene-5,10-dione do not fluoresce. When the temperature is lowered, intense fluorescence and phosphorescence are observed under analogous conditions. In Fig. 1 we present curves reflecting the temperature dependence of the quantum yield of fluorescence for 3,4- benzocoumarin in different solvents. As can be seen, the quantum yield of fluorescence in-ethanol or n-hexane drops from 0.36 to very nearly zero as the temperature is raised from 150 to 300 K. When the temperature is lowered below 150 K, all the way down to 77 K, there is hardly any change in quantum yield. For solutions of coumarin and its derivatives in n-hexane, the change in quantum yield of fluorescence can be followed down to the freezing point of the solvent. For planar molecules such as 3,4-benzoeoumarin, two mechanisms can be proposed for the quenching of fluorescence as the temperature is changed: ~>'tt . -x-I O & & ~ ._~'ar'.2"~.T* £ - Poltava Branch, Academy of Sciences of Engineering Cybernetics of the Ukraine, 3 Koval' Street, Poltava 314601, Ukraine. Translated from Teoreticheskaya i l~ksperimental'naya Khimiya, Vol. 32, No. 4, pp. 247-250, July-August, 1996. Original article submitted September 26, 1995. 0040-5760/96/3204-0217515.00 01997 Plenum Publishing Corporation 217