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

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)-H2O], 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 it = 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 ( similar to 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 similar to 100 ps. This implies that the excited triplet CuTMpyP(4 +) molecules, which form the CO exciplex, are protected during this time from the fast ( similar to 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. (C) 1999 Elsevier Science S.A. All rights reserved.