Influence of solvent viscosity on T ? T energy transfer from chlorophyll to?-carotene

Abstract

5. 6. L. I. Kononenko, S. V. Bel'tyukova, S. A. Gava, et al., "Change in the intensity ratio of bands of the fluorescence spectra of terbium and dysprosium ions in solutions of certain complexes, " Zh. Prikl. Spektrosk., 2_33, 851-854 (1975). N. S. Polu~ktov, L. A. Alalmeva, and M. Tishchenko, "Intensity of "ultrasensitive" transitions Nd 3+, Ho 3+, and Er 3+ ions in certain complexes with different numbers of ligands,' Zh. Prikl. Spektrosk., i__77, 819-822 (1972). INFLUENCE OF SOLVENT VISCOSITY ON T - T ENERGY TRANSFER FROM CHLOROPHYLL TO -CAROTENE E. I. Sagun, N. D. Kochubeeva, and A. P. Losev UDC 535.35 According to contemporary ideas, the transfer of energy among triplet levels occurs via direct contact between the interacting molecules and by overlap of their peripheral electronic shells [i, 2]. It is assumed that this process is so effective in liquid solutions that it occurs on practically every collision of donor (D) and aceeptor (A) molecules. In this case the rate of exothermic triplet-triplet energy transfer, Ktr, should be equal to the rate constant of the reaction limiting the diffusion in a given solvent, Kd, and the range Of action for such transfer will be close to the sum of the kinetic radii of the interacting molecules. The presently available experimental data [3, 4] indicate a satisfactory correlation between the rate constant Ktr and the solvent viscosity ~?. At the same time, in several cases significant differences in the values of Ktr and K d have been explained as the result of steric factors [5], of the nature and lifetime of the electronic states of the interacting molecules [6], or of the reversibility of triplet-triplet energy transfer [7]. However, even under the most favorable conditions for energy transfer the limiting rate constant Ktr in low-viscosity solvents does not approach the rate constant K d calculated from the formula K d = 8RT/3000~, being smaller by a fac- tor of 2-10 [i]. This question was first studied in detail in [8], where the transfer rate constant Ktr was stu- died as a function of the fluidity (7 -I) of various solvents. These authors have shown that for exothermic energy transfer (AE = EDT'EA T _> I000 cm -I) in solvents with viscosity ~ > 3.10 -3 Pa- sec the transfer rate constant Ktr increases linearly with the fluidity, while in solvents with viscosity ~ < 3.10 -3 Pa. sec it increases signifi- cantly more slowly than ~-I, leading to a significant decrease in the efficiency of the transfer. Analogous re- sults were obtained in [9, i0], where it was shown that triplet-triplet energy transfer in low viscosity solvents is not strongly diffusion controlled. However, it should be noted that the transfer rate constant Ktr was cal- culated in [8] from data on the decay of the rate of chemical process taking place in various solvents and pro- ceeding via a triplet state of the donor. Therefore, regardless of the deep theoretical analysis of the problem, these results, obtained by an indirect technique in terms of several assumptions, required further development and the use of more direct measurement techniques. In the present work we report an investigation of the influence of the macroscopic characteristics of the solvent on the efficiency of triplet-triplet energy transfer between photosynthetic pigments using the method of pulsed photoexcitation. Chlorophyll A was used as the donor, and the acceptor was fi-carotene, as first studied in [ii]. This donor--acceptor pair is characterized by the fact that the difference in the energy of their triplet levels is about 2000 cm -I, and the intrinsic triplet state lifetime of the acceptor does not exceed 5.10 -6 sec [12] in liquid solution, so that the triplet-triplet transfer has an exothermic character. Chlorophyll A was extracted from nettle and the fl-carotene, from carrots, and they were purified by magnesium oxide chromatography. The absorption spectra of the solutions studied were recorded on SF-10 and Unicam SP-800 spectrophotometers. Diethyl ether, pyridine, benzene, isopropyl alcohol, and cyclo- hexanol purified by the techniques described in [13] were utilized as solvents. The solvent viscosity at various temperatures was found graphically from the data reported in [14]. All the pigment solutions studied were placed in a Dewar filled with petroleum ether, cooled to-90~ or with distilled water heated to +70 ~ C. The triplet-triplet absorption of the Chlorophyll A was measured at intermediate temperatures, Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 36, No. 3, pp. 434-441, March, 1982. Original article submitted March 27, 1981. 312 0021-9037/82/3603-0312507.50 9 1982 Plenum Publishing Corporation