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-(N-methylpyridyl)]porphyrin (Cu(TMpy-P4)) and the DNA model compound poly(dA-dT) have been studied in 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 containing thymine (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 between Cu(TMpy-P4) and poly(dA-dT) were carried out by using both picosecond transient absorption pump-probe technique (10-ps time resolution) and two-color time-resolved RR technique (100-ps time resolution). A comparative nanosecond Raman study of this exciplex and of the excited (d,d) state of copper meso-tetraphenylporphyrin (CuTPP) model compound dissolved in a number of oxygen-containing solvents has also been performed, to clarify the excited electronic state which is at the origin of this process. It has been found that the binding of one of the CO-groups of T or U to Cu(TMpy-P4) in its lowest excited triplet state results in a shortening of the triplet-state lifetime to 35 +/- 7 ps. In addition, a population of an excited (2)[d(z2),d(x2-y2)] state, i.e., the most low-lying and long-lived excited state for the five-coordinated Cu(TMpy-P4) (exciplex state), occurs in the process of excitation relaxation. Large wavenumber shifts of structure-sensitive vibrational marker lines from the porphyrin skeleton reveal the promotion of one of the copper d electrons into the half-filled d(x2-y2) orbital and the expansion of the porphyrin core to accommodate the occupation of this d orbital. The exciplex deactivation process (excited (d,d) state decay) has a time constant of 3.2 +/- 0.5 ns and is accompanied by the CO-group deattachment with a disruption of the exciplex into initial components.