A Time Resolved Laser Spectroscopy Study of Resonance Energy Transfer between Tb(III) and an Organic Dye: 6-Hydroxy-5-[(2-methoxy-5-methyl-4-sulfophenyl)azo]-2-naphthalenesulfonic acid disodium salt

Intermolecular energy transfer is a process of crucial importance to photochemistry and photophysics. Complexed lanthanide ions are well suited as energy transfer donors because they display dependable absorption and luminescence qualities with narrow rare-earthion emission in addition to relatively long excited-state lifetimes. This project used a pulsed nitrogen laser and high speed digital techniques to study energy transfer dynamics between excited state Tb(acac)3 and an ionic organic dye called Red 40. Luminescence decay profiles of excited terbium atoms in the presence of the quencher were measured in both high and low viscosity solvents. The effects of solvent viscosity and radiative path length on wavelength and time resolved spectra have shown that both radiative energy transfer and resonance (Förster) energy transfer were the principal quenching mechanisms in this system. The excited state lifetime of terbium in the absence of Red 40 has been determined in cold glycerol and in room temperature ethanol as 1083(10) and 796(5) μs respectively. These lifetimes are similar to those of other reported terbium systems. The Förster critical radius has been determined in cold glycerol and in room temperature ethanol as 81(5) and 85(5) Å respectively. These values are larger than any previously reported critical transfer radii. Experimentally measured energy transfer efficiencies in cold glycerol were found to be in good agreement with those predicted by Förster theory. Measured diffusion enhanced energy transfer efficiencies in room temperature ethanol solution were also in agreement with theoretical predictions. This study has revealed trends in excited state lifetimes, diffusion coefficients and activity coefficients that dramatically deviate from those of ideal systems. These trends suggest that strong ion associations occur in ethanol solutions of divalent Red 40 at concentrations above 10-6 M resulting in greatly diminished activity, ionic mobility and diffusion coefficients. Diffusion data were used to estimate mean activity coefficients for Na2Red 40. The concentration dependences of electrical conductance, diffusion and activity coefficients were found to resemble those of other divalent electrolytes. Differences between electrolyte behavior in ethanol and water were shown to be consistent with their respective dielectric constants