Photoinduced redox cycle of riboflavin at a water/oil interface

A photoinduced redox reaction cycle of Riboflavin (RF) at a water/CCl4 interface was studied directly by means of both steady-state and time-resolved total internal reflection (TIR) fluorescence spectroscopies. The TIR fluorescence spectrum of RF observed at the water/CCl4 interface with the maximum wavelength of 517 nm was assigned to the pi-pi* transition from the excited singlet-state of the isoalloxazine chromophore in RF. Upon prolonged laser irradiation (400 nm) in the presence of N,N-dioctadecyl-[1,3,5]triazine-2,4,6-triamine (DTT) as a guest for RF in the CCl4 phase, on the other hand, a new TIR fluorescence band appeared at around 480 nm. Furthermore, the fluorescence intensity at around 480 nm increased in the presence of acetic acid in the water phase. Detailed studies demonstrated that the new fluorescence band should be ascribed to 1,5-dihydoroflavin (RFH2). The present results indicated that RFH2 was produced through the photoreaction of the RF-DTT hydrogen-bonded complex formed at the water/CCl4 interface, whose reaction mechanisms were discussed on the basis of the results observed by fluorescence spectra/dynamics measurements under the TIR conditions as well as by transient absorption spectroscopy.