THE PHOTOCHEMISTRY OF RIBOFLAVIN—VI. THE PHOTOPHYSICAL PROPERTIES OF ISOALLOXAZINES

Abstract— Many of the photophysical properties of riboflavin and several other N-10 substituted isoal-loxazines have been measured and these include: quantum yields of fluorescence at 77 K and 298 K, φ, quantum yields of phosphorescence at 77 K, φ_P, lifetimes of the triplet state by electron spin resonance and phosphorescence at 77 K, φp,. and the quantum yield of intersystem crossing. φ_isc. For riboflavin in an alcoholic matrix at 77 K the limiting values were: φ _J = 0.32, φ_P= 0.007, φp = 0.20s and φ_isc= 0.7. At 298 K, φ _f for riboflavin in water and alcohol were 0.25 and 0.32, respectively. The results for the photophysical processes are compared with several photochemical processes known to involve the triplet state of riboflavin in aqueous solution. It is concluded that the φ _isc decreases from 0.7 in alcohol to 0.6 in water at 298 K.