Comparison of DODCI fluorescence depolarization in glycerol and ethylene glycol: Effect of orientational correlation on excitation transport

Solvent Effects on Nonradiative Relaxation Dynamics of Low-Energy Ligand-Field Excited States: A CuCl42- Complex

Nonradiative relaxation dynamics of CuCl₄^2- complexes photoexcited into the highest-energy ligand-field electronic state (²A₁) is studied in acetonitrile, dichloromethane, and chloroform solvents, as well as in acetonitrile-water and in acetonitrile-deuterated water mixtures. Due to ultrafast internal conversion, this excited state directly converts to the electronic ground state in dichloromethane and chloroform. The nonradiative relaxation constant is similar in anhydrous acetonitrile. Addition of water to acetonitrile solutions efficiently quenches the excited ligand-field ²A₁ state. The quenching is proposed to be due to the diffusion-controlled formation of an electronically excited pentacoordinated [CuCl₄H₂O]^2- encounter complex or a short-lived exciplex of similar structure, in which the electronic excitation energy transfers into the O-H stretch of the coordinated H₂O molecule. This is followed by the dissociation of the pentacoordinated species, resulting in the reformation of the ground-state CuCl₄^2- and free H₂O molecules.

Time-resolved fluorescence of nitrobenzoxadiazole-aminohexanoic acid: effect of intermolecular hydrogen-bonding on non-radiative decay

The fluorescence kinetics of the nitrobenzoxadiazole (NBD) chromophore were studied at low concentrations in solvents with varying polarity and hydrogen-bonding donor strength. The emission decay was essentially single exponential in all solvents studied. While the absorption and fluorescence solvatochromism is determined largely by the solvent polarity, the S1 state decay kinetics are strongly modulated by the solvent H-bonding capacity. The NBD emission lifetime, generally approximately 7-10 ns in the aprotic solvents, is reduced to 0.933 ns in water. The solvent deuterium isotope effect on the fluorescence decay is substantial in D2O and in methanol-d4, but is insignificant in DMSO-d6. These results are consistent with acceleration of S1----S0 internal conversion through an accepting vibrational mode created by intermolecular hydrogen-bonding of the NBD chromophore to an H atom-donating solvent. This work bears on the practically of using NBD as a fluorophore in assays for estrogen and progesterone receptors.