Temperature Dependence of Fundamental Photophysical Properties of [Eu(MeOH-d4)9]3+ Solvates and [Eu·DOTA(MeOH-d4)]- Complexes.

The trivalent lanthanide ions show optical transitions between energy levels within the 4f shell. All these transitions are formally forbidden according to the quantum mechanical selection rules used in molecular photophysics. Nevertheless, highly luminescent complexes can be achieved, and terbium(III) and europium(III) ions are particularly efficient emitters. This report started when an apparent lack of data in the literature led us to revisit the fundamental photophysics of europium(III). The photophysical properties of two complexes-[Eu·DOTA(MeOH-d4)]- and [Eu(MeOH-d4)9]3+-were investigated in deuterated methanol at five different temperatures. Absorption spectra showed decreased absorbance as the temperature was increased. Luminescence spectra and time-resolved emission decay profiles showed a decrease in intensity and lifetime as the temperature was increased. Having corrected the emission spectra for the actual number of absorbed photons and differences in the non-radiative pathways, the relative emission probability was revealed. These were found to increase with increasing temperature. The transition probability for luminescence was shown to increase with temperature, while the transition probability for light absorption decreased. The changes in transition probabilities were correlated with a change in the symmetry of the absorber or emitter, with an average increase in symmetry lowering absorbance and access to more asymmetric structures increasing the emission rate constant. Determining luminescence quantum yields and the Einstein coefficient for spontaneous emission allowed us to conclude that lowering symmetry increases both. Furthermore, it was found that collisional self-quenching is an issue for lanthanide luminescence, when high concentrations are used. Finally, detailed analysis revealed results that show the so-called "Werts' method" for calculating radiative lifetimes and intrinsic quantum yields is based on assumptions that do not hold for the two systems investigated here. We conclude that we are lacking a good theoretical description of the intraconfigurational f-f transitions, and that there are still aspects of fundamental lanthanide photophysics to be explored.