Photophysics of Coumarin and Carbostyril-Sensitized Luminescent Lanthanide Complexes: Implications for Complex Design in Multiplex Detection

Abstract

Luminescent lanthanide (Ln(III)) complexes with coumarin or carbostyril antennae were synthesized and their photophysical properties evaluated using steady-state and time-resolved UV-vis spectroscopy. Ligands bearing distant hydroxycoumarin-derived antennae attached through triazole linkers were modest sensitizers for Eu(III) and Tb(III), whereas ligands with 7-amidocarbostyrils directly linked to the coordination site could reach good quantum yields for multiple Ln(III), including the visible emitters Sm(III) and Dy(III), and the near-infrared emitters Nd(III) and Yb(III). The highest lanthanide-centered luminescence quantum yields were 35% (Tb), 7.9% (Eu), 0.67% (Dy), and 0.18% (Sm). Antennae providing similar luminescence intensities with 2-4 Ln-emitters were identified. Photoredox quenching of the carbostyril antenna excited states was observed for all Eu(III)-complexes and should be sensitizing in the case of Yb(III); the scope of the process extends to Ln(III) for which it has not been seen previously, specifically Dy(III) and Sm(III). The proposed process is supported by photophysical and electrochemical data. A FRET-type mechanism was identified in architectures with both distant and close antennae for all of the Lns. This mechanism seems to be the only sensitizing one at long distance and probably contributes to the sensitization at shorter distances along with the triplet pathway. The complexes were nontoxic to either bacterial or mammalian cells. Complexes of an ester-functionalized ligand were taken up by bacteria in a concentration-dependent manner. Our results suggest that the effects of FRET and photoredox quenching should be taken into consideration when designing luminescent Ln complexes. These results also establish these Ln(III)-complexes for multiplex detection beyond the available two-color systems.

Highly luminescent lanthanide complexes sensitised by tertiary amide-linked carbostyril antennae

“…S11 †). The observed lifetimes and the intrinsic quantum yields are identical within experimental error within the group of Eu1a-d. Interestingly, in a previous study, an Eu complex with the same Ln binding site and a tertiary amide-linked 7-amidocoumarin antenna had a very similar observed lifetime, 0.65 ms, while a non-alkylated analogue had τ obs ∼ 0.6 ms. 39 Thus, N-alkylation indeed increases the Eu lifetime and the intrinsic quantum yield, probably because of the removal of the N-H oscillator.…”

“…These conditions were chosen because previously we observed that Ln complexes with trifluoromethylated carbostyril antennae showed a reversible loss in Ln emission at pH > 7. 39 Analysis of the spectral shape of such Eu complexes showed no changes in the coordination environment, suggesting that deprotonation occurred in a non-coordinated group. As we could not exclude the loss of the core N-H proton, we have decided to do our experiments at a pH where deprotonation is not significant.…”

“…In Dy9d (Fig. 3, synthesised previously, crystal structure not reported 39 ) the amide and the heterocycle are more co-planar, which should be beneficial for the charge transfer excited state. 43 The more efficiently electrondonating substituent of Ln9a-d yields a more polar emitting state in Ln9a-d than in Ln1a-d.…”

“…38,40 Many were evaluated as antennae, even though in-depth photophysical characterizations are rare. 39 Most of the structural variations were limited to the peripheral substituents, usually in the 3 and 4 (R 1 and R 2 , respectively in Fig. 1) positions.…”

“…[29][30][31][32][33][34][35][36][37] Some are even effective for Sm and Dy, 38 as well as the near infrared (NIR) emitting Yb and Nd. 39 A variety of substituted carbostyrils have been reported (Fig. 1a).…”