Ultrafast energy migration in platinum(II) diimine complexes bearing pyrenylacetylide chromophores

The ultrafast excited-state dynamics of three structurally related platinum(II) complexes has been investigated using femtosecond transient absorption spectrometry in 2-methyltetrahydrofuran (MTHF). Previous work has shown that Pt(dbbpy)(C C-Ph)(2) (dbbpy is 4,4'-di(tert-butyl)-2,2'-bipyridine and C C-Ph is ethynylbenzene) has a lowest metal-to-ligand charge transfer ((MLCT)-M-3) excited state, while the multichromophoric Pt(dbbpy)(C C-pyrene)2 (C C-pyrene is 1-ethynylpyrene) contains the MLCT state, but possesses a lowest intraligand ((IL)-I-3) excited state localized on one of the C C-pyrenyl units (Pomestchenko, I. E.; Luman, C. R.; Hissler, M.; Ziessel, R.; Castellano, F. N. Inorg. Chem. 2003, 42, 1394-96). trans-Pt(PBu3)(2)(C C-pyrene)(2) serves as a model system that provides a good representation of the C C-pyrene-localized (IL)-I-3 state in a Pt(II) complex lacking the MLCT excited state. Following 400 nm excitation, the formation of the (MLCT)-M-3 excited state in Pt(dbbpy)(C C-Ph)(2) is complete within 200 +/- 40 fs, and intersystem crossing to the (IL)-I-3 excited state in trans-Pt(PBu3)(2)(C C-pyrene)(2) occurs with a time constant of 5.4 +/- 0.2 ps. Selective excitation into the low-energy MLCT bands in Pt(dbbpy)(C C-pyrene)(2) (lambda(ex) = 480 nm) leads to the formation of the (IL)-I-3 excited state in 240 +/- 40 fs, suggesting ultrafast wire-like energy migration in this molecule. The kinetic data suggest that the presence of the MLCT states in Pt(dbbpy)(C C-pyrene)(2) markedly accelerates the formation of the triplet state of the pendant pyrenylacetylide ligand. In essence, the triplet sensitization process is kinetically faster than pure intersystem crossing in trans-Pt(PBu3)(2)(C C-pyrene)(2) as well as vibrational relaxation in the MLCT excited state of Pt(dbbpy)(C C-Ph)(2). These results are potentially important for the design of chromophores intended to reach their lowest excited state on subpicosecond time scales and advocate the likelihood of wire-like behavior in triplet-triplet energy transfer.