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) tetrakis(4-N-methylpyridyl)porphyrin (CuII(TMpy-P4)) with synthetic polynucleotides such as poly(dA-dT)2 and poly(dG-dC)2, the exciplex formation dynamics of photoexcited CuII(TMpy-P4) with added polynucleotides have been investigated by using femtosecond transient absorption as well as transient Raman spectroscopic methods. The nanosecond transient Raman spectra of CuII(TMpy-P4) mixed with poly(dA-dT)2 clearly demonstrate the exciplex formation between photoexcited CuII(TMpy-P4) and poly(dA-dT)2. On the other hand, the exciplex formation of photoexcited CuII(TMpy-P4) with poly(dG-dC)2 is not so manifest as compared with that mixed with poly(dA-dT)2. The transient absorption of CuII(TMpy-P4) mixed with poly(dG-dC)2 exhibits a rise component of 1.3 ps in addition to the very slow decay component (τ ∼ 22 ns). This observation is quite different from that of CuII(TMpy-P4) in aqueous solution or CuII(TMpy-P4) mixed with poly(dA-dT)2, because the exciplex formed in poly(dG-dC)2, if any, is so short-lived that it returns quickly back to four-coordinate CuII(TMpy-P4) intercalated at poly(dG-dC)2, which blocks the axial coordination by water molecules and consequently gives rise to a long lifetime (τ ∼ 22 ns).