EXCITED-STATE PROTON-TRANSFER OF METHYL-SUBSTITUTED AND CYANO-SUBSTITUTED ALLOXAZINES IN THE PRESENCE OF ACETIC-ACID

The photoinduced excited state double proton transfer reaction of meth yl- and cyano-substituted alloxazines was examined by steady state and time-resolved methods. The fluorescence decay times of the alloxazini c forms in 1,2-dichloroethane and 1,2-dichloroethane in the presence o f acetic acid are similar and of the order of hundreds of picoseconds. The fluorescence decay times of the isoalloxazinic forms, created by excited state proton transfer, are of the order of nanoseconds. On the basis of the function describing the emission decay of the bands corr esponding to the isoalloxazinic and alloxazinic forms and the lifetime s obtained, it was found that the excited alloxazinic form is a precur sor of the excited isoalloxazinic form, and the rate constant of the p roton transfer process is lower than 10(9) s(-1). These results and th ose of stationary studies, indicating that the isoalloxazinic and allo xazinic species derive from different forms in the ground state, have enabled a model of the proton transfer reaction of alloxazines to be p roposed. In this model, the possible formation of alloxazine-acetic ac id complexes in a conformation permitting proton transfer, in both the ground and excited states, was assumed.