Abstracts

The widespread application of photonic materials has spurred chemists to enhance their photophysical properties. In this sense, trivalent lanthanide ions (Ln³⁺) coordinated with organic ligands feature unique luminescence due to 4f↔4f transitions. Owing to their parity-forbidden nature, the primary sensitization mechanism hinges upon ligand-to-Ln³⁺ energy transfer (ET) from the singlet (S₁) and/or triplet (T₁) ligand's excited states, the latter being the most important for most of Ln³⁺. Thus, efficient ET requires the T₁ state population, mediated by intersystem crossing (ISC) induced by the presence of Ln³⁺. However, ISC predominantly occurs within the ligand counterpart and is intrinsically linked to the overlapping vibrational levels of the two excited states, a factor often overlooked in existing literature. Hence, this study aims to introduce a model for elucidating the vibronic coupling effect while rationalizing ISC rates in three Eu³⁺ complexes: [Eu(tta)₃(H₂O)₂] (1), [Eu(tta)₄]⁻ (2) and [Eu(PyrCF₃)₃(phen)] (3). tta = 2-thenoyltrifluoroacetate, phen = 1,10-phenantroline, and PyrCF₃ = (1-(1-methyl-1H-4-pyrazolyl)-4,4,4-trifluorobutane-1,3-dionate). The adopted strategy entails considering vibronic coupling in ISC through the path-integral (PI) approach. Subsequently, the nature of normal vibrations was identified by decomposing them into local vibrations. This methodology yielded ISC rates of 8.4×10⁷, 1.3×10⁸, and 4.2×10¹⁰ s⁻¹ for complexes 1, 2, and 3, respectively. Standard models such as the Marcus-Levich (ML) approach fail to explain the two-orders of magnitude difference between 1 and 3. These values closely align with experimental measurements (2.6×10⁷, 4.4×10⁸, and 4.2×10¹⁰ s⁻¹, for 1, 2, and 3, respectively) rather than ML predictions, which underestimated the rates by almost one order of magnitude. This outcome highlights the primary importance of vibronic coupling in ISC. In this context, for complexes 1 and 2, the coupling stems from high-energy vibrations (above 3000 cm^-1), whereas for complex 3, vibrations in the low-to-intermediate energy range (750 – 1400 cm^-1) dominate. This elegant result implies that vibronic coupling could induce higher ISC rates and augment the T₁ population. However, the energies of these vibrations are crucial for emission dynamics, as high frequencies are associated with multiphonon relaxation, which diminishes the emission quantum yield. Localizing these vibrations within the molecule reveals that O−H and C−H fragments are primarily responsible for vibronic coupling in complexes 1 and 2, respectively. Conversely, for complex 3, the most important vibrations were delocalized across several molecular fragments, reducing the probability of multiphonon quenching. Thus, confining vibronic coupling to low-energy vibrations while dispersing them throughout the molecule enhances the ISC and mitigates the Eu^{3+ 5}D₀ quenching. Combining both outcomes opens a pathway to reach faster ISC and tune the ET by tailoring the ligand scaffold of novel complexes.