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 (Cu^II-(TMpy-P4)) with synthetic polynucleotides such as Poly(dA-dT)₂ and Poly(dG-dC)₂, the exciplex formation dynamics of photoexcited Cu^II(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 Cu^II(TMpy-P4) mixed with Poly(dA-dT)₂ clearly demonstrate the exciplex formation between photoexcited Cu^II(TMPy-P4) and poly(dA-dT)₂. On the other hand, the exciplex formation of photoexcited Cu^II(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 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 (τ ∼ 22 ns). This observation is quite different from that of Cu^II(TMpy-P4) in aqueous solution or Cu^II(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 Cu^II(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).