Modeling the Eu(III)-to-Cr(III) Energy Transfer Rates in Luminescent Bimetallic Complexes

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There is a growing interest in alternatives to lanthanide ion (Ln(III))-based luminescence sensitizing chromophores for in vivo applications, mainly in optical biological windows. Transition metals (M) are relevant candidates as chromophores as they have high absorption rates and emission bands covering a wide range of visible to near-infrared spectrum. However, despite the importance of theoretical models for the design of M–Ln(III) complexes, few contributions have devoted efforts to elucidating the energy transfer (ET) processes between M and Ln(III) ions. In this context, we adapted the intramolecular energy transfer (IET) to calculate, for the first time, the energy transfer rates for M–Ln(III) complexes. A new model was proposed that considers the assistance of phonons in the calculation of ET rates. As an example, the proposed model can estimate the ET rates between Eu(III) and Cr(III) ions in the [CrEuL3]6+ complex (where L = 2-{6-[N,N-diethylcarboxamido]pyridin-2-yl}-1,1′-dimethyl-5,5′-methylene-2′-(5-methylpyridin-2-yl)bis [1H-benzimidazole]). The calculated rates (930–1200 s−1) are in excellent agreement with the experimentally available data (750–1200 s−1) when a phonon-assisted energy transfer process is considered. Thus, this proposed model can be useful to predict and explain photophysical properties driven by the energy transfer between Ln(III) ions and transition metals.