EXCIPLEX FORMATION DYNAMICS OF PHOTOEXCITED COPPER(II) TETRAKIS(4-N-METHYLPYRIDYL)PORPHYRIN WITH SYNTHETIC POLYNUCLEOTIDES PROBED BY TRANSIENT ABSORPTION AND RAMAN-SPECTROSCOPIC TECHNIQUES

To elucidate the interaction mechanism of water-soluble copper(II) tet rakis(4-N-methylpyridyl)porphyrin (Cu-II- (TMpy-P4)) with synthetic po lynucleotides such as poly(dA-dT)(2) and poly(dC-dC)(2), the exciplex formation dynamics of photoexcited Cu-II(TMpy-P4) with added polynucle otides have been investigated by using femtosecond transient absorptio n as well as transient Raman spectroscopic methods. The nanosecond tra nsient Raman spectra of Cu-II(TMpy-P4) mixed with poly(dA-dT)(2) clear ly demonstrate the exciplex formation between photoexcited Cu-II(TMpy- P4) and poly(dA-dT)(2). On the other hand, the exciplex formation of p hotoexcited Cu-II(TMpy-P4) with poly(dG-dC)(2) is not so manifest as c ompared with that mixed with poly(dA-dT)(2). The transient absorption of Cu-II(TMpy-P4) mixed with poly(dG-dC)(2) exhibits a rise component of 1.3 ps in addition to the very slow decay component (tau similar to 22 ns). This observation is quite different from that of Cu-II(TMpy-P 4) in aqueous solution or Cu-II(TMpy-P4) mixed with poly(dA-dT)(2), be cause the exciplex formed in poly(dG-dC)(2), if any, is so short-lived that it returns quickly back to four-coordinate Cu-II(TMpy-P4) interc alated at poly(dG-dC)(2), which blocks the axial coordination by water molecules and consequently gives rise to a long lifetime (tau similar to 22 ns).