Absorption spectra and photophysical properties of a series of polypyridine ligands containing appended pyrenyl and anthryl chromophores and of their ruthenium(II) and osmium(II) complexes

The absorption spectra and luminescence properties (both in fluid solution at room temperature and in rigid matrix at 77 K) of four polypyridine ligands substituted with pyrenyl and anthryl chromophores have been investigated, together with the same properties of 15 Ru(II) and Os(II) complexes prepared using the same ligands. Absorption spectroscopy revealed that the various chromophores contribute to the overall absorption spectra of the mixed-chromophore species in a roughly additive way. The photophysical investigations allowed us to obtain information on several aspects. For example, in the mixed pyrene-anthracene bipyridine-based systems, efficient singlet-singlet pyrene-to-anthracene energy transfer takes place, whereas the same process involving the triplet states is apparently not efficient, unless a metal-to-ligand charge-transfer (MLCT) excited-state mediates the process. The luminescence properties of the Ru(II) complexes are dominated by (MLCT)-M-3 levels at room temperature (with a few exceptions, that is, the anthracene-containing species) and by pyrene- and/or anthracene-based triplet states at 77 K. However, at room-temperature, equilibration between (MLCT)-M-3 and pyrene triplets levels is established, and as a consequence, the luminescence lifetimes of the complexes are significantly prolonged (up to 18 mus for the species containing six pyrenyl chromophores). This excited-state lifetime prolongation is linearly related to the number of pyrenyl chromophores and is independent of connectivity. The photophysical properties of the Os(II) complexes are dominated by (MLCT)-M-3 levels both at room temperature and at 77 K. However, the anthryl chromophores clearly influence the excited-state lifetime of these complexes at room temperature. Indeed, although the anthracene-based triplet states lie at higher energy than the (MLCT)-M-3 levels, an excited-state equilibration process is also established, as clearly demonstrated by transient absorption spectroscopy, and the luminescence lifetime is (slightly) extended.