Circularly Polarized Luminescence from Enantiopure C₂-Symmetrical Tetrakis(2-pyridylmethyl)-1,2-diaminocyclohexane Lanthanide Complexes

Abstract

We report the synthesis and characterization of C 2 -symmetrical lanthanide complexes supported by enantiopure hexadentate ligands derived from 1,2-diaminocyclohexane. Coordination of (R,R)-or (S,S)-N,N,N′,N′-tetrakis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane (tpdac) to samarium, europium, terbium, and dysprosium generates the corresponding C 2 -symmetrical (tpdac)Ln(OTf) 3 complexes in high yields. The tpdac ligands are competent sensitizers for lanthanide luminescence, yielding modest emissions (Φ of ≤28%). Additionally, the complexes exhibit strong circularly polarized luminescence (|g lum | values of up to 0.13, 0.09, 0.22, and 0.15 for Sm, Eu, Tb, and Dy, respectively) in solution. We also observed that some transitions typically associated with small dissymmetry factors exhibit unusually high |g lum | values and, therefore, should not be overlooked in future studies.

Circularly polarized lanthanide luminescence for advanced security inks

“…[H2] Dy III complexes Dy III complexes emit from the blue to the NIR, with luminescence typically appearing as blue/green to the eye in solution. 39,40,119 We found only four studies reporting CPL-active Dy III complexes and CPB or CPBi have not yet been quantified for any of them. 39,40,74,118,119 Dy III complexes exhibit emission lifetimes in the range of ~10 µs.…”

“…Eu III exhibits overall intense red emission, Tb III exhibits overall intense green emission, Sm III exhibits a less intense overall purple emission and Dy III exhibits less intense blue/green emission. 11,[37][38][39][40] In contrast, Yb III and Nd III complexes emit in the NIR region. 41,42 The key energy levels involved in the emission of circularly polarised photons from Ln III are summarized in Dieke diagrams, one of which is available for Eu III , Tb III , Sm III , Dy III and Yb III complexes, 43 with a more comprehensive diagram covering all trivalent lanthanides.…”

“…117 An important and somewhat detrimental feature of Tb III CPL is that multiple CPL sign inversions of individual electronic transitions within emission bands is common and may reduce CPBi for otherwise promising complexes. 40,[118][119][120] Another notable complex is [TbL 3 ] + (L 3 is an (O8) 4--donor ligand), which is UV-A excitable (350 or 373 nm) and combines very good quantum yields (63%) with weak CPL emission (gem ± 0.04), leading to an estimated CPBi of 195 M −1 cm −1 (λex = 350 nm). 39,74 Despite the aforementioned challenges, chiral Tb III complexes may yet be viable as green CPL-active luminescent security inks.…”

“…104,121,122 and others require UV-B excitation. 40,119 It is fair to say that Sm III complexes are not particularly well developed and often exhibit poor quantum efficiencies, which severely limits the CPB values achievable. However, there have been relatively few studies of CPL-active Sm III complexes, so future work may yet improve excitation wavelengths, quantum yields and CPL emission properties in order to maximise CPBi.…”

“…118 More newly-developed complexes have been excited with unfavourable UV-B light and combine poor quantum yield (<0.5%) and weak-to-modest CPL properties. 40,119 Overall, CPL-active Dy III complexes are in general not well developed. Whilst they could serve to provide blue/green luminescence with intermediate lifetimes (~10 µs), they are likely to be overlooked in favour of Tb III and Eu III complexes with better and more well-studied photophysical properties.…”