Dynamics of formation and decay of the exciplex created between excited Cu(II)-5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin and thymine CO groups in short oligothymidylates and double-stranded [poly(dA–dT)]2

Abstract Cationic water-soluble Cu(II)-5,10,15,20-tetrakis(4- N -methylpyridyl)porphyrin (CuTMpyP 4+ ) complexed with short oligothymidylates d(pT) n ( n =1, 2, 3, 4, 5, 9, 12–18) can form exciplexes simultaneously with both CO groups of thymine, forming [(CuP) * dd –CO], and with surrounding water molecules, forming [(CuP) * dd –H 2 O], where (CuP) * dd is the Cu(II)-porphyrin in its excited (d,d) state. The contribution of the CO exciplex in the complexes with d(pT) n increases from 10% for n =1 up to 100% for n =9 and 12–18. For all n , the water-exciplex rise time is of the order of 1–3 ps, its lifetime being as long as 30–160 ps and depending on n . The CO-exciplex lifetime (∼950 ps) is found to be independent of the length ( n ) of the oligothymidylate. The CO-exciplex rise time is found to be as long as ∼100 ps. This implies that the excited triplet CuTMpyP 4+ molecules, which form the CO exciplex, are protected during this time from the fast (∼1 ps) quenching by water molecules. It is assumed that the dependence of the water-exciplex lifetime on n found for CuP complexes with d(pT) n , as well as the difference of CuP-exciplex lifetimes found for various axial ligands and surrounding oligomers, may result from a dependence of the (d,d) state lifetime of five-coordinate (non-planar) CuP on the polarity of the microenvironment.