ENTROPY-DRIVEN DIMERIZATION OF XANTHENE DYES IN NONPOLAR SOLUTION AND TEMPERATURE-DEPENDENT FLUORESCENCE DECAY OF DIMERS

Entropy-driven dimerization has been observed in the systems of rhodamine 3B, rhodamine 6G, and pyronine B in toluene, 1-chloronaphthalene, and phenanthrene in the temperature range 22-230-degrees-C. The equilibrium constant of dimerization increases with temperature and can be as high as 3 x 10(8) M-1 at 230-degrees-C in case of rhodamine 6G chloride in toluene. The dimer fluorescence lifetimes in the above systems display activated behavior and decrease from about 4 ns at room temperature to 100 ps at 230-degrees-C. Strongly fluorescent dimers of oblique geometry (J dimers) of rhodamine 6G chloride, which are detected by emission kinetics, are found to be in equilibrium with nonfluorescent dimers of sandwich-type structure (H dimers), which are detected by absorption spectroscopy. A strong dependence of J dimerization on the counteranion has been observed in rhodamine 6G solutions, where the chloride exhibits an equilibrium constant 1000 times larger than that of the perchlorate. The tendency for dimerization is less pronounced for rhodamine B and is substantially smaller for rhodamine 101. The lifetimes of the dimer fluorescence in the latter zwitterionic systems are considerably longer and only weakly dependent on temperature.