Extended π -Systems in Diimine Ligands in [Cu(P^P)(N^N)][PF 6 ] Complexes: From 2,2'-Bipyridine to 2-(Pyridin-2-yl)Quinoline

: We describe the synthesis and characterization of [Cu(POP)( 1 )][PF 6 ], [Cu(POP)( 2 )][PF 6 ], [Cu(xantphos)( 1 )][PF 6 ], and [Cu(xantphos)( 2 )][PF 6 ] 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 )][PF 6 ], [Cu(xantphos)( 1 )][PF 6 ], and [Cu(xantphos)( 2 )][PF 6 ] 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 )][PF 6 ] and [Cu(xantphos)( 2 )][PF 6 ] hosts the quinoline unit of 1 , and the 6-methylpyridine group of 2 . 1 H 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 )][PF 6 ], pairs of cations engage in an efficient face-to-face  -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.


Introduction
Heteroleptic [Cu(P^P)(N^N)] + coordination complexes in which P^P is a wide bite-angle bisphosphane [1] and N^N 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]. McMillin and coworkers first recognized that copper(I) coordination compounds with PPh3 or P^P ligands and phen or bpy possess low-lying metal-to-ligand charge transfer (MLCT) excited states [4,5]. More recently, 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 (which is at only slightly lower energy) occurs. The long lifetime of the excited triplet state and its relatively slow phosphorescence permits 13 C{ 1 H} and 31 P{ 1 H} NMR spectra were recorded at 298 K on a Bruker Avance III-500 NMR spectrometer (Bruker BioSpin AG, Fällanden, Switzerland), and 1 H and 13 C NMR chemical shifts were referenced to residual solvent peaks with respect to (TMS) = 0 ppm and 31 P NMR chemical shifts with respect to (85% aqueous H3PO4) = 0 ppm. Shimadzu LCMS-2020 (Shimadzu Schweiz GmbH, 4153 Reinach, Switzerland) and Bruker esquire 3000plus instruments (Bruker BioSpin AG, Fällanden, Switzerland) were used to record electrospray ionization (ESI) mass spectra with samples introduced. Solution absorption and emission spectra were recorded using Shimadzu UV-2600 spectrophotometer and Shimadzu RF-5301PC spectrofluorometer (Shimadzu Schweiz GmbH, 4153 Reinach, Switzerland), respectively.

[Cu(xantphos)(2)][PF6]
A solution of [Cu(MeCN)4][PF6] (73 mg, 0.197 mmol, 1.0 eq) in CH2Cl2 (10 mL) was stirred for 5 min and was then added to a solution of xantphos (114 mg, 0.197 mmol, 1.0 eq) and 2 (52 mg, 0.236 mmol, 1.2 eq) in CH2Cl2 (20 mL) which had previously been stirred for 15 min. The mixture was stirred for 5.5 h at room temperature, after which time it was filtered. The solvent was removed from the filtrate under reduced pressure and the crude product was redissolved in CH2Cl2 and. The product was precipitated by layering with Et2O and the solid was washed with n-hexane and after filtration, was redissolved in CH2Cl2. Layering with Et2O yielded [Cu(xantphos) (2)

Crystallography
Single crystal data were collected on a Bruker APEX-II diffractometer (CuK radiation) with data reduction, solution and refinement using the programs APEX [21] and CRYSTALS [22], or using a STOE StadiVari diffractometer equipped with a Pilatus300K detector and with a Metaljet D2 source (GaK radiation) and solving the structure using Superflip [23,24] and Olex2 [25]. Structure analysis including the ORTEP representations, used Mercury CSD v. 4.1.1 [26,27].

Characterization of Copper(I) Complexes
The base peak in the ESI mass spectrum of each [Cu(P^P)(N^N)][PF6] compound corresponded to the [M-PF6] + ion, and mass spectra are presented in Supplementary Figures S1-S4. In the 31 P{ 1 H} NMR spectrum of each complex, a broadened peak around δ −12 ppm (see Sections 2.3-2.6) was observed for the P^P ligand in addition to the septet for the [PF6] -counterion. The 1 H and 13 C{ 1 H} NMR spectra were assigned using 2D methods and the HMQC and HMBC spectra are shown in Figures S5-S12 in the Supporting Information. , and a similar comparison is made for the xantphoscontaining compounds in Figure 2. In each complex, signals for H B3 and H B4 were distinguished using the NOESY cross-peak between H A3 and H B3 . The phenyl rings of the PPh2 groups split into two sets, labelled D and D'. In the xantphos-containing compounds, the rigidity of the xanthene backbone leads to a ligand conformation in which two phenyl rings face towards the N^N ligand and two phenyls point away. A NOESY cross-peak between H A6 (on the N^N ligand) and H D2 (on xantphos) allows the D and D' rings to be distinguished. In contrast, in the POP-containing complexes, the NOESY spectrum evidences conformational exchange (EXSY) peaks between pairs of signals for H D2/D2' , H D3/D3' , and H D4/D4' (Figure 3). This is readily rationalized in terms of the conformational flexibility of the POP ligand.

Crystal Structures of [Cu(xantphos)(1)][PF6], [Cu(xantphos)(2)][PF6] and [Cu(POP)(2)][PF6]
Yellow single crystals of [Cu(xantphos) (1) Figure 5 and selected bond lengths and angles are summarized in Table 1. Table 1 also includes values of Houser's 4 parameter which is used to quantify distortion at the copper centre along a path from Td (τ4 = 1.00) towards C3v symmetry (τ4 = 0.85) [29]. The τ4 values in Table 1 Table 1) that contributes to the value of τ4 = 0.79 and this is associated with a phenyl-2-(pyridin-2-yl)quinoline stacking contact (see later discussion).   Figure 6). Interestingly, within this dataset, the deviation from planarity is quite severe in some cations, with the largest angle of 31.6° occurring in [Cu(xantphos)(N^N)][PF6] in which N^N is 6,6'-dimethyl-4,4'-diphenyl-2,2'-bipyridine [31]. The biquinoline (biq) ligand may also undergo significant twisting, with angles of 15 [34]. Thus, the small twist angles in coordinated ligands 1 and 2 are not a consequence of the extended -system on going from bpy to 2-(pyridin-2-yl)quinoline.  (Figure 7a and 7c). In [Cu(xantphos)(2)] + , ligand 2 is tilted so as to engage in a -stacking interaction with one phenyl ring (Figure 7d); the angle between the planes of the rings containing C21 and N2 is 16.5° and the centroid⋯centroid distance is 3.90 Å. In [Cu(xantphos)(1)] + , a face-to-face -stacking contact is observed between the phenyl rings containing C21 and C42 (Figures 5a and 7a).  The POP-containing complex exhibits only one -stacking interaction for which the arrangement of the participating phenyl and pyridine rings (Figure 8a,b) is not optimal. The angle between the ring planes is 20.4°, and the centroid⋯centroid distance is 4.03 Å. However, this weak contact is augmented by inter-cation packing interactions involving a centrosymmetric -stacking embrace between the rings in ligands 2 containing N2 and N2i (symmetry code i = -x, 2-y, 1-z) (Figure 8c). The separation of the ring planes is 3.24 Å and the centroid⋯centroid distance is 3.73 Å. These parameters are consistent with an efficient interaction [36].   Figure 9 illustrates the solution absorption spectra of the four copper(I) compounds and absorption maxima are given in Table 2. Bands below ca. 350 nm arise from ligand-centred π*π transitions, and the broad absorption band at ca. 420-430 nm is assigned to MLCT. The latter shifts to higher energy ( Table 2) on introducing the 6-methyl substituent into the pyridyl unit in keeping with observations for analogous compounds containing bpy and 6-Mebpy ligands [8,9,16] [9], consistent with the extended π-system in ligands 1 and 2. Figure 9. Solution absorption spectra of the copper(I) compounds in CH2Cl2 (2.5  10 -5 mol dm -3 for complexes with ligand 1 and 2.1  10 -5 mol dm -3 for those containing 2.). Table 2. Solution absorption maxima for the copper(I) compounds (CH2Cl2, 2.5  10 -5 or 2.1  10 -5 mol dm -3 ).

Compound
λmax / nm ( / dm 3 mol -1 cm -1 ) Ligand-Centred π*π MLCT In CH2Cl2 solution, the compounds are weak emitters and we focus only on the solid-state properties. Emission data for powdered samples of the compounds are summarized in Table 3 and normalized emission spectra are depicted in Figure 10. The introduction of the 6-methyl group into the pyridine ring on going from 1 to 2 leads to a blue-shift in the emission maximum. Similar trends in values of λ em max are observed on going from [Cu(POP)(bpy)][PF6] (581 nm) [16] [9]. Introducing the methyl group to the N^N ligand leads to an enhancement of the PLQY (Table 3), and follows trends previously reported for bpy-based ligands [8,9] [16], with the data in Table 3

Comments on Structure-Property Relationships
One of the key challenges in improving the photoluminescence properties of [Cu(P^P)(N^N)] + species is the design of the N^N ligands, and selecting an appropriate combination of N^N and P^P ligands. In a light-emitting device such as a LEC, electroluminescence (EL), rather than photoluminescence (PL), becomes the critical phenomenon. Although it does not necessarily follow that high PLQY leads to high EL, it is usually true that low PL indicates a poor candidate for EL.  (Figure 11a). This has not previously been described [8], and analysis of the packing (CSD refcode BOSVAI) reveals an inter-plane separation of  (Figure 11b, top) are striking, and it is interesting to note that the solidstate PLQY values are of the same orders of magnitude (3.0% and 9.5%, respectively [8,16]).   [35], was used to generate the results given in Table S1 in the supporting information. Only compounds for which solid-state PLQY data are available are included. Figure 12 presents a scatter plot of the data, with POP-and xantphos-containing compounds delineated by blue and orange markers, respectively. Data for [Cu(xantphos) (1) Interestingly, however, the highest PLQYs are for complexes containing the POP ligand in which N^N = 4,4'-bis(4-fluorophenyl)-6,6'dimethyl-2,2'-bipyridine (CSD refcode EVAFAK and PLQY = 74% [31]), 4,4',6,6'-tetramethyl-2,2'bipyridine (CSD refcode COYHEF and PLQY = 55% [38]), and 6,6'-dimethyl-2,2'-bipyridine (CSD refcode BOSYUF and PLQY = 43.2% [8]).   Table S1: summary of data from the CSD used for Figure 12.