EXOTHERMIC TRIPLET-TRIPLET EXCITATION TRANSFER IN SOLUTION UNDER HIGH-PRESSURE

The rate constants, k(q), for the exothermic energy transfer from the triplet states of benzophenone ((3)BZP) and triphenylene ((TPh)-T-3*) to naphthalene (N), and also from the singlet sate of triphenylene (( TPh)-T-1) to benzophenone (BZP), were measured in polar and nonpolar solvents as a function of pressure at 25 degrees C. For all the system s of the donor-acceptor pairs, the plots of k(q) against l/eta showed downward curvature. For (3)BZP/N in acetonitrile, methanol, and n-hex ane, the plots of ln k(q) against ln eta were linear with the slopes l arger than -1, while for (3)BZP/N in toluene and for (TPh)-T-1*/BZP i n n-hexane they showed downward curvature. However, the plots of 1/k(q ) against eta were linear with positive intercepts for all the systems examined. It was also found that the plot of 1/k(q) against eta/T, in which k(q) was measured for (3)BZP/N as a function of temperature an d pressure in n-hexane, is linear. These results were interpreted by t he energy transfer mechanism via the formation of an encounter complex between the donor and acceptor molecules, and it was concluded that t he exothermic energy transfer examined in the present study is not ful ly diffusion-controlled but competes with a diffusion process that is expressed by a modified Debye equation. The bimolecular rate constants for the energy transfer, k(bim) (=k(diff)k(et)/k(-diff)), were in the range of (1-10) x 10(10) M-1 s(-1) depending on solvent. The pressure dependence of k(et) is discussed from the experimental fact that k(bi m) is independent of solvent viscosity changed by varying pressure and temperature in all the systems examined.