| Abstract
| - The bimolecular rate constant, kTT, for triplet−triplet annihilation (TTA) of 2-acetonaphthone (ACN) inmethylcyclohexane (MCH) and n-hexane at pressures up to 400 MPa at 25 °C has been measured. It wasfound that kTT decreases significantly with increasing pressure, and the activation volumes for kTT, ΔVTT⧧,were determined to be 18 and 11 cm3/mol, whereas those for the solvent viscosity, ΔVη⧧, were 24 and 23cm3/mol in MCH and n-hexane, respectively. The significant difference between ΔVTT⧧ and ΔVη⧧ was attributedto the competition of the quenching with diffusion. From the analysis on the basis of the pressure dependenceof the solvent viscosity, η, on the quenching, the observed rate constant, kTT, was separated into the contributionsof the rate constant for diffusion, kdiff, and the bimolecular rate constant for quenching, kbim, in the solventcage. By using the values of kdiff thus determined, 2kTT/kdiff was found to be about 4/9 at 0.1 MPa and toapproach unity with increasing pressure. The results were interpreted by a kinetic model that involves theencounter complex pairs, i(M*M*)en (i = 1, 3, 5), with singlet, triplet, and quintet spin multiplicities. It wasconcluded that the rate constant for the dissociation, k-diff, decreases more significantly than that for theintersystem crossing, kenisc(i), between the encounter complex pairs with increasing pressure (Scheme ).
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