TIME-RESOLVED PHOTOACOUSTICS DETERMINATION OF INTERSYSTEM CROSSING AND SINGLET OXYGEN PHOTOSENSITIZATION QUANTUM YIELDS FOR 4,5',8-TRIMETHYLPSORALEN

The interaction of 4,5',8-trimethylpsoralen (TMP) with molecular oxyge n in a variety of solvents has been investigated using pulsed, time-re solved photoacoustic calorimetry and fluorescence emission. In this wo rk we propose a methodology for the absolute determination of the quan tum yield for the photosensitized production of singlet molecular oxyg en, based on data from time resolved photoacoustics. The methodology a lso allows the determination of the quenching rate constant of the sen sitizer triplet state by oxygen as well as the intersystem crossing qu antum yield, provided the energy level of the triplet state is known. The methodology has been checked with two well-known photosensitizing agents, rose bengal and methylene blue. We found that in all of the so lvents investigated, the production of singlet oxygen by TMP is a proc ess with relatively high probability and may therefore play an importa nt role in the overall reactivity of the molecule. For comparison, pho toacoustic data for 8-methoxypsoralen (8-MOP) are reported, showing th at singlet oxygen photosensitization is low in all of the solvents exa mined.