THE EMISSION OF PYRIDINE AND POLYPYRIDINECHROMIUM(III) COMPLEXES IN RIGID AND FLUID MEDIA.

Abstract

The excited state emissions and lifetimes of [Cr(bipy)₃]³⁺, cis – [Cr(bipy)₂Cl₂]⁺, trans – [Cr(py)₄FBr]⁺, cis – [Cr(phen)₂F₂]⁺, and trans – [Cr(py)₄F₂]⁺ were studied in hydroxylic and nonhydroxylic solvents at 77 K and as a function of temperature. The emission characteristics are determined by the relative disposition of ²E and one component of ²T₁. The energy of these two levels is influenced by intramolecular and environmental factors, and solvent-induced level inversion occurs in cis – [Cr(phen)₂F₂]⁺. The influence of temperature and solvent viscosity were studied in several glasses with variable melting ranges. The emission of all of the complexes, except [Cr(bipy)₃]³₊, were found to be influenced by solvent viscosity. This influence is large when ²E and the component of ²T₁ are close in energy, and level inversion upon melting of the solvent occurs in cis –[Cr(phen)₂F₂]⁺. The decay of the emission of Cr(III) complexes normally shows small temperature dependence at lower temperatures and large temperature dependence at higher temperatures. The threshold for the appearance of strong temperature dependence depends on intramolecular and environmental factors. The decays of all the complexes except [Cr(bipy)₃]³⁺ show strong discontinuities in the rigid-fluid transition regions of the solvent glasses which are accompanied by risetimes in the decay profiles when the emission intensity is monitored at long wavelengths. This is attributed to solvent relaxation during the excited state lifetime.