Shining light on the excited state energy cascade in kinetically inert Ln(iii) complexes of a coumarin-appended DO3A ligand

Abstract

Lanthanide based molecular probes for bioimaging relies on the antenna effect, here we are unravelling the excited state energy cascade that results in sensitized lanthanide luminescence.

A High-Throughput Method To Measure Relative Quantum Yield of Lanthanide Complexes for Bioimaging

“…4–6 This poses a specific challenge, as the efficient sensitization of lanthanide luminescence requires the incorporation of a small chromophore, commonly referred to as an antenna, for efficient energy transfer to excited f-states: the synthesis of high-quantum-yield lanthanide complexes suitable for bioimaging applications is of ongoing interest and represents a considerable challenge. 7–19 Small chemical modifications to the antenna and chelate structure can result in large changes in the corresponding lanthanide complex quantum yield. 9,12,20 A significant impediment to selection and development of suitable bifunctional lanthanide complex systems is the rapid determination of effective, lanthanide-based quantum yields (Φ Ln ).…”

“…Measurement of the triplet excited state energy of the ligand reveals a blue-shift by 1330 cm −1 , which limits efficiency of energy back-transfer to the antenna (Figures 1A and S3−S4, Table S1). 41 A recent report by Junker et al 19 details that fast energy transfer rates from T 1 to the lanthanide excited state can further enhance efficiency of the energy transfer from T 1 states of antennas, which is another plausible explanation for the high relative quantum yield of [Tb( L5 )] − .…”

Combining the Best of Two Chelating Titans: A Hydroxypyridinone‐Decorated Macrocyclic Ligand for Efficient and Concomitant Complexation and Sensitized Luminescence of f‐Elements

“…Upon decreasing the temperature to 77 K, the Tb(III)‐centered emission increased by about 30‐fold (Figure S17), providing further evidence for the BET mechanism and suggesting this complex could be a candidate for nanoscale luminescent thermometry [31] . However it is worth noting that other mechanisms have been described where the excited state kinetics of the non‐emissive 5 D 3 state deeply influences the photoemissive properties of Tb(III) complexes [26b] . The 4 F 9/2 acceptor level of Dy(III) lies even higher in energy, leading to a smaller energy gap and higher BET contribution (Figure 1).…”

“…It should be mentioned that while the absence of lanthanide emission in complexes with low‐lying T 1 has been typically ascribed to thermally‐activated back energy transfer, other non‐radiative deactivation mechanisms via the 5 D 3 state are certainly possible. In addition, Ln/An‐DOTHOPO complexes exhibit high‐energy states that are isoenergetic with ligand S 1 thus energy transfer rate constants may play a role [26b] . Hydration numbers for Tb(III) complexes can generally be calculated using the method described by Parker and co‐workers [48] .…”

π‐Expanded Thioxanthones – Engineering the Triplet Level of Thioxanthone Sensitizers for Lanthanide‐Based Luminescent Probes with Visible Excitation

“…Figure shows that naphtho[2,3‐c]thioxanthone 3 cannot be used as an antenna chromophore for terbium(III) or europium(III), since the excited triplet state is lower in energy than both the 5 D 4 electronic state of Tb(III) and 5 D 0 of Eu(III). Benzo[c]thioxanthone 2 should work only in combination with europium(III), while the parent thioxanthone 1 should work well for both europium(III) and terbium(III) when only considering energetics …”