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The dynamics and mechanism of the photoinduced reversible process of formation and decay of an exciplex species created between the water-soluble cationic metalloporphyrin copper 5, 10, 15, 20-tetrakis [4-(iV-methylpyridyl)] porphyrin (Cu (TMpy-P4)) and the DNA model compound poly (dA-dT) havebeen studiedin detail. Such a photoinduced process had been previously observed in transient resonance Raman (RR) spectra under high-power laser irradiation of complexes of Cu (TMpy-P4) with calf thymus DNA and some oligo-and polynucleotides containingthymine (T) or uracile (U) residues. It was found that the interaction of excited Cu (TMpy-P4) with carbonyl groups of T or U involved in polymers having an appropriate secondary structure was responsible for the new transient species detected in high-power Raman spectra. In the present work, direct kinetic measurements of the exciplex formation …