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Alkali metal complexes of an enantiopure iminophosphonamide ligand with bright delayed fluorescence

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Abstract

The enantiomerically pure ligand P,P-diphenyl-N,N′-bis((R)-1-phenylethyl)phosphinimidic amide (1; (R)-HPEPIA) was synthesized and subsequently deprotonated with alkali metal precursors to yield dimeric complexes [M2{(R)-PEPIA}2] (M = Li (2), Na (3), K (4), Rb (5)). The cesium compound [M{(R)-PEPIA}] (6) crystallized as a cocrystal composed of dimeric ([Cs2{(R)-PEPIA}2] (6d) and 1D-polymeric ([Cs{(R)-PEPIA}]n) (6p) species in a 1 : 1 ratio. The coordination polymer 6p features a unique sinus-shaped configuration of repeating –Cs–N–P–N–Cs–N–P–N– units. Unusual photoluminescence (PL) properties were found for solid 1–6: in contrast to the fluorescent ligand 1, the alkali metal complexes show phosphorescence at low temperatures (<100 K) and thermally activated delayed fluorescence (TADF) above ∼150 K. The latter provides for PL quantum yields up to 36% (3) at ambient temperature. DFT calculations support that both 1 and 2–6d have similar singlet and triplet excited states with energy separations of a few tens of meV. The strongly enhanced intersystem crossing (ISC) in the metal complexes, resulting in TADF, is attributed to their dimeric structure. This suggests that the fluorophore dimerization may serve as a tool to effect ISC for the design of TADF emitters.

Graphical abstract: Alkali metal complexes of an enantiopure iminophosphonamide ligand with bright delayed fluorescence

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Publication details

The article was received on 04 Feb 2019, accepted on 11 Mar 2019 and first published on 09 Apr 2019


Article type: Edge Article
DOI: 10.1039/C9SC00629J
Citation: Chem. Sci., 2019, Advance Article
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    Alkali metal complexes of an enantiopure iminophosphonamide ligand with bright delayed fluorescence

    T. J. Feuerstein, B. Goswami, P. Rauthe, R. Köppe, S. Lebedkin, M. M. Kappes and P. W. Roesky, Chem. Sci., 2019, Advance Article , DOI: 10.1039/C9SC00629J

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