Kofod Nicolaj, Nielsen Lea Gundorff, Sørensen Thomas Just
Nano-Science Center and Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 København Ø, Denmark.
J Phys Chem A. 2021 Sep 30;125(38):8347-8357. doi: 10.1021/acs.jpca.1c04994. Epub 2021 Sep 21.
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-)] and [Eu(MeOH-)]-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.
三价镧系离子在4f壳层内的能级之间表现出光学跃迁。根据分子光物理中使用的量子力学选择规则，所有这些跃迁在形式上都是禁阻的。然而，可以制备出高发光的配合物，其中铽（III）和铕（III）离子是特别有效的发光体。本报告始于文献中明显缺乏数据促使我们重新审视铕（III）的基本光物理性质。在五个不同温度下，于氘代甲醇中研究了两种配合物——[Eu·DOTA(MeOH-)]和[Eu(MeOH-)]的光物理性质。吸收光谱表明，随着温度升高，吸光度降低。发光光谱和时间分辨发射衰减曲线表明，随着温度升高，强度和寿命均降低。在对发射光谱进行吸收光子实际数量以及非辐射途径差异的校正后，揭示了相对发射概率。发现这些概率随温度升高而增加。发光的跃迁概率随温度升高而增加，而光吸收的跃迁概率降低。跃迁概率的变化与吸收体或发射体对称性的变化相关，对称性平均降低会降低吸光度，而形成更不对称的结构会增加发射速率常数。测定发光量子产率和自发发射的爱因斯坦系数使我们得出结论，对称性降低会使二者均增加。此外，发现当使用高浓度时，碰撞自猝灭是镧系发光的一个问题。最后，详细分析表明，用于计算辐射寿命和本征量子产率的所谓“韦茨方法”基于的假设在此处研究的两个体系中并不成立。我们得出结论，我们缺乏对组态内f-f跃迁的良好理论描述，并且镧系基本光物理的某些方面仍有待探索。
