Extended π-Systems in Diimine Ligands in [Cu(P^P)(N^N)][PF6] Complexes: From 2,2′-Bipyridine to 2-(Pyridin-2-yl)Quinoline

Sarah Keller,Edwin C Constable,Alessandro Prescimone,Murat Alkan-Zambada,Catherine E Housecroft

doi:10.3390/cryst10040255

Sarah Keller, Edwin C Constable + Show 3 more

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https://doi.org/10.3390/cryst10040255

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Journal: Crystals	Publication Date: Mar 27, 2020
Citations: 20	License type: CC BY 4.0

Affiliation: University of Basel

Abstract

We describe the synthesis and characterization of [Cu(POP)(1)][PF6], [Cu(POP)(2)][PF6], [Cu(xantphos)(1)][PF6], and [Cu(xantphos)(2)][PF6] in which ligands 1 and 2 are 2-(pyridin-2-yl)quinoline and 2-(6-methylpyridin-2-yl)quinoline, respectively. With 2,2'-bipyridine (bpy) as a benchmark, we assess the impact of the extended π-system on structural and solid-state photophysical properties. The single crystal structures of [Cu(POP)(2)][PF6], [Cu(xantphos)(1)][PF6], and [Cu(xantphos)(2)][PF6] were determined and confirmed a distorted tetrahedral copper(I) coordination environment in each [Cu(P^P)(N^N)]+ cation. The xanthene unit in [Cu(xantphos)(1)][PF6] and [Cu(xantphos)(2)][PF6] hosts the quinoline unit of 1, and the 6-methylpyridine group of 2. 1H NMR spectroscopic data indicate that these different ligand orientations are also observed in acetone solution. In their crystal structures, the [Cu(POP)(2)]+, [Cu(xantphos)(1)]+, and [Cu(xantphos)(2)]+ cations exhibit different edge-to-face and face-to-face π-interactions, but in all cases, the copper(I) centre is effectively protected by a ligand sheath. In [Cu(POP)(2)][PF6], pairs of cations engage in an efficient face-to-face π-stacking embrace, and we suggest that this may contribute to this compound having the highest photoluminescence quantum yield (PLQY = 21%) of the series. With reference to data from the Cambridge Structural Database, we compare packing effects and PLQY data for the complexes incorporating 2-(pyridin-2-yl)quinoline and 2-(6-methylpyridin-2-yl)quinoline, with those of the benchmark bpy-containing compounds. We also assess the effect that Cu⋯O distances in the {Cu(POP)} and {Cu(xantphos)} domains of [Cu(P^P)(N^N)][X] compounds have on solid-state PLQY values.

Highlights

Heteroleptic [Cu(PP)(NN)]+ coordination complexes in which PP is a wide bite-angle bisphosphane [1] and NN is a derivative of 2,2'-bipyridine or 1,10-phenanthroline are gaining in popularity as the emissive component in the active layer in light-emitting electrochemical cells (LECs) [2,3]
In [Cu(POP)(2)][PF6 ], pairs of cations engage in an efficient face-to-face π-stacking embrace, and we suggest that this may contribute to this compound having the highest photoluminescence quantum yield (PLQY = 21%) of the series
McMillin and coworkers first recognized that copper(I) coordination compounds with PPh3 or PP ligands and phen or bpy possess low-lying metal-to-ligand charge transfer (MLCT)

Summary

Introduction

Heteroleptic [Cu(PP)(NN)]+ coordination complexes in which PP is a wide bite-angle bisphosphane [1] and NN is a derivative of 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen) are gaining in popularity as the emissive component in the active layer in light-emitting electrochemical cells (LECs) [2,3]. It has been demonstrated that many [Cu(PP)(NN)]+ complexes exhibit thermally activated delayed fluorescence (TADF) [6,7]. Fast intersystem crossing from the lowest-lying singlet excited state to the triplet excited state The long lifetime of the excited triplet state and its relatively slow phosphorescence permits thermal repopulation of the singlet excited state with concomitant fluorescence. Harvesting of the emission from both the excited singlet and triplet states significantly increases the photoluminescent quantum yield (PLQY).

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