Molecular Luminescence Studies of Flavins, II: Interactions Involving the Excited States

The effects of experimental geometry on the theoretical polarizations of the S₁←S₀and S₂ ← S₀bands of π→π* type in riboflavin have been examined. Polarizations of these two bands are characterized by an angle between them in the range of 20 — 28° and are relatively insensitive to the input geometry. Thus the predicted polarizations are generally in agreement with fluorescence polarization spectrum of riboflavin at 77 °K. Alloxazine forms a strong complex with KI, and the fluorescence and phosphorescence from the charge transfer states have been characterized by means of luminescence and photoselection measurements. Riboflavin did not form a strong complex with KI, but it forms aggregates (dimer) more readily than alloxazine. The excited states of flavins can be populated by the weak dipole-dipole coupling mechanism of energy transfer from ¹L_a states of indoles to the S₂ state of flavins. The measured critical distances estimated from the fluorescence depolarization experiments range from 31 A for indole to 40 A for indole-2-carboxylic acid in glycerol-methanol mixture (9:1) at 263 °K.