Wavelength dependence of the quantum yield of fluorescence and photoionization of indoles

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Abstract
The fluorescence quantum yields φf (λx) of L‐tryptophan and indole were measured relative to that of Rhodamine B as functions of the excitation wavelength λx between 200 and 300 nm. Two different experimental geometries produced identical results, indicating that experimental artifacts were negligible. In aqueous solution at 296 K, φf (λx) increased by 45% for both solutes in going from 230 to 260 nm. Outside this wavelength region, φf (λx) was constant to within 2%. Similar results, although with a somewhat smaller shift in φf (λx), were obtained for tryptophan in an equivolume mixture of ethylene glycol and water EG/H2O at both 296 and at 113 K, the latter temperature being below the glass transition temperature of this medium. The emission spectra and the phosphorescence to fluorescence ratios were independent of λx, indicating that the shift in φf (λx) was not due to intersystem crossing from higher excited states. The shift in φf (λx) was accompanied by a considerable increase in the quantum yield of photoionization of tryptophan in aqueous solution below 260 nm. Hence, it is concluded that photoionization from higher states competes favorably with internal conversion to the fluorescent singlet state. For indole in methanol at 296 K and in EG/H2O at 113 K, φf (λx) was independent of λx to within 2%.