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The influence of temperatur and pressure on the fluorescence quantum yield of N-methylacridone (9,10-dihydro-9-oxo-10-methyl-acridine) in toluene in the range of 283-313 K and 1 bar to 2.5 kbar, respectively, has been investigated. Treatment of the data in terms of the Eyring transition-state theory leads to a consistent interpretation of the observed effect. The unusually large increase of the quantum yield with increasing pressure is attributed to a positive volume of activation, ⊿V ≠ , for the thermally activated S 1 -T 2  intersystem crossing which is known to be the only deactivation process (of the Si-state) competing with fluorescence. Comparison of the values for ⊿H ≠ , the activation enthalpy of this process, determined at various pressures, indicates a decrease in ⊿H ≠  at elevated pressures. Since ⊿H ≠  can be associated with the S 1 -T 2  energy gap involved in intersystem crossing, this result further confirms the conclusion that the change in Franck-Condon factors alone cannot account for the decrease in the intersystem crossing rate with increasing pressure.

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Effects of Pressure and Temperature on the Fluorescence Quantum Yield of N-Methylacridone in Solution Interpreted in Terms of the Eyring Transition-StateTheory