Absolute fluorescence quantum yields of large molecules in supersonic expansions

In this paper we report on the determination of absolute quantum yields from the electronic origin, S₁(0), of the first electronically excited singlet state of isolated large molecules cooled in pulsed, planar, supersonic expansions. We have measured the absorption spectra, the fluorescence excitation spectra, and the fluorescence quantum yields from the photoselected S₁(0) states for pairs of large molecules seeded in planar supersonic expansions by using a pulsed xenon lamp and a monochromator. The relative quantum yields thus obtained were calibrated by using a reference molecule to give absolute quantum yields, Y, from the vibrationless S₁ states. Y values from the S₁(0) state of 13 aromatic molecules and their derivatives were determined. Information is inferred on the mechanism of intersystem crossing mediated by a nearby triplet state and on solvent effects on this process.