Room temperature phosphorescence from ruthenium(II) complexes bearing conjugated pyrenylethynylene subunits

We describe the synthesis, electrochemistry, and photophysical properties of several Ru(II) complexes bearing different numbers of pyrenylethynylene substituents in either the 5- or 5,5'-positions of 2,2'-bipyridine, along with the appropriate Ru(II) model complexes bearing either bromo- or ethynyltoluene functionalities. In addition, we prepared and studied the photophysical behavior of the diimine ligands 5-pyrenylethynylene-2,2'-bipyridine and 5,5'-dipyrenylethynylene-2,2'-bipyridine. Static and dynamic absorption and luminescence measurements reveal the nature of the lowest excited states in each molecule. All model Ru(II) complexes are photoluminescent at room temperature and exhibit excited-state behavior consistent with metal-to-ligand charge transfer (MLCT) characteristics. In the three Ru(II) molecules bearing multiple pyrenylethynylene substituents, there is clear evidence that the lowest excited state is triplet intraligand (3IL)-based, yielding long-lived room temperature phosphorescence in the red and near IR. This phosphorescence emanates from either 5-pyrenylethynylene-2,2'-bipyridine or 5,5'-dipyrenylethynylene-2,2'-bipyridine, depending upon the composition of the coordination compound. In the former case, the excited-state absorption difference spectra that were measured for the free ligand are easily superimposed with those obtained for the metal complexes coordinated to either one or two of these species. The latter instance is slightly complicated since coordination of the 5,5'-ligand to the Ru(II) center planarizes the diimine structure, leading to an extended conjugation on the long axis with a concomitant red shift of the singlet pi-pi absorption transitions and the observed room temperature phosphorescence. As a result, transient absorption measurements obtained using free 5,5'-dipyrenylethynylene-2,2'-bipyridine show a marked blue shift relative to its Ru(II) complex, and this extended pi-conjugation effect was confirmed by coordinating this ligand to Zn(II) at room temperature. In essence, all three pyrenylethynylene-containing Ru(II) complexes are unique in this genre of chromophores since the lowest excited state is 3IL-based at room temperature and at 77 K, and there is no compelling evidence of interacting or equilibrated excited states.