William R. Cherry

Selective perturbation of ligand field excited states

Abstract The photophysics of polypyridine ruthenium(II) complexes have generated intense interest recently. One controversial process involves the thermal population of a higher-lying ligand field (LF) state during the lifetime of the lowest excited triplet metal-to-ligand charge transfer (MLCT) state. In order to substantiate this thermal process, several series of closely related complexes were prepared and their photophysics examined. The results clearly suggest thermal population of an LF state. In one example, the LF state was selectively perturbed while the MLCT states were nearly unchanged energetically. This approach which involves a novel ligand, 4,5-diazafluorene, nicely complements selective perturbation of MLCT states by introduction of electron-withdrawing substituents.

Effect of chelate ring size on the photoanation reaction of polypyridine ruthenium complexes

Abstract Several new polypyridine ligands (L) with two pyridine rings linked by methylene bridges of different lengths have been synthesized. The corresponding [Ru(bpy)2L]2+ (bpy ≡ bipyridine) complexes have been prepared and their photophysical and photochemical properties examined. The low temperature (77 K) emission properties of the new complexes are similar to those of [Ru(bpy)3]2+; at higher temperatures their emission intensities are significantly decreased. In organic solvents, photoanation is efficient for all of the new complexes. In the case of [Ru(bpy)2DPE]2+ (DPE ≡ 1,2-di(2-pyridyl)ethane), substitution of the first pyridine occurs thermally in 0.01 M Cl− (solvent, CH2Cl2) and in H2SO4 so that photoanation of the second pyridine could be directly studied. For the six-membered chelate ring [Ru(bpy)2DPM]2+ (DPM ≡(2-pyridyl)methane) reclosure of the chelate ring competes with photoanation only in polar solvents where ion pairing is not significant.

Dynamics of ligand field excited states in polypyridine ruthenium(II) complexes

Abstract The temperture dependence of photosubstitution for several polypyridine ruthenium(II) complexes has been examined. In contrast to the parent tris-bipyridineruthenium(II), little temperature dependence is observed. These results, combined with quenching data, provide kinetic details on the intermediate ligand field excited state involved in these reactions.

Constrained Tryptophan Has A Monoexponential Decay

The fluorescence decay of tryptophan is complex. Various models have been proposed to account for the multiexponential decay, including ground-state rotamers, excited-state proton transfer, and lifetime distributions.1-4 In order to test the rotamer model, we are studying a series of tryptophan derivatives in which rotation about the Ca-CB and CB-Cy, bonds is constrained. The ground-state conformations are determined by X-ray diffraction, molecular and 1H NMR. Or preliminary results for a tryptophan derivative with the a-amino group incorporated in a six-member ring are described below.