Alkali metal complexes of an enantiopure iminophosphonamide ligand with bright delayed fluorescence. Issue 18 (9th April 2019)

• Record Type: Journal Article
• Authors: Feuerstein, Thomas J.; Goswami, Bhupendra; Rauthe, Pascal; Köppe, Ralf; Lebedkin, Sergei; Kappes, Manfred M.; Roesky, Peter W.
• Is Part Of: Chemical science. Volume 10:Issue 18(2019)
• Page Start: 4742
• Page End: 4749
• Publication Date: 2019-04-09
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c9sc00629j
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : Alkali metal complexes of an enantiopure iminophosphonamide bearing chiral centers at both nitrogen atoms are described. They show bright phosphorescence and thermally activated delayed fluorescence (TADF). 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 polymer6p features a unique sinus-shaped configuration of repeating –Cs–N–P–N–Cs–N–P–N– units. Unusual photoluminescence (PL) properties were found for solid1–6 : in contrast to the fluorescent ligand1, 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 both1 and2–6d 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.