Ultrafast time-resolved infrared (TRIR) spectra of flavin adenine dinucleotide (FAD) and the anion of lumiflavin(Lf-) are described. Ground-state recovery and excited-state decay of FAD reveal a common dominant ultrafastrelaxation and a minor slower component. The Lf- transient lacks a fast component. No intermediate speciesare observed, suggesting that the quenching mechanism is internal conversion promoted by interaction of theadenine and isoalloxazine rings in FAD. Modes are assigned, and the potential for extension of the TRIRmethod to photoactive proteins is discussed.
