[Cu(P^P)(N^N)][PF6] compounds with bis(phosphane) and 6-alkoxy, 6-alkylthio, 6-phenyloxy and 6-phenylthio-substituted 2,2′-bipyridine ligands for light-emitting electrochemical cells

Article abstract of DOI:10.1039/c8tc02882f

We report a series of [Cu(P^P)(N^N)][PF₆] complexes with P^P = bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) and N^N = 6-methoxy-2,2′-bipyridine (MeObpy), 6-ethoxy-2,2′-bipyridine (EtObpy), 6-phenyloxy-2,2′-bipyridine (PhObpy), 6-methylthio-2,2′-bipyridine (MeSbpy), 6-ethylthio-2,2′-bipyridine (EtSbpy) and 6-phenylthio-2,2′-bipyridine (PhSbpy). The single crystal structures of all twelve compounds have been determined and confirm chelating modes for each N^N and P^P ligand, and a distorted tetrahedral geometry for copper(i). For the xantphos-containing complexes, the asymmetrical bpy ligand is arranged with the 6-substituent lying over the xanthene ‘bowl'. The compounds have been characterized in solution by¹H,¹³C and³¹P NMR spectroscopies, and their photophysical and electrochemical properties are described. They are yellow emitters and solid samples show photoluminescence quantum yields in the range up to 38%, with emission lifetimes ≤10.2 μs. On going from powder to frozen Me-THF, the excited state lifetimes increase which might suggest the presence of thermally activated delayed fluorescence (TADF). All the compounds have been tested in light-emitting electrochemical cells (LECs). Bright and stable LECs are obtained with complexes containing alkoxy- or phenyloxy-substituted ligands, making this family of compounds very relevant for the future development of copper-based electroluminescent devices.

Full text of DOI:10.1039/c8tc02882f

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Materials Chemistry C
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PAPER
[Cu(P^P)(N^N)][PF6] compounds with
bis(phosphane) and 6-alkoxy, 6-alkylthio,
6-phenyloxy and 6-phenylthio-substituted
2,20-bipyridine ligands for light-emitting
electrochemical cells†
Cite this:DOI: 10.1039/c8tc02882f
a
b
Murat Alkan-Zambada,a Sarah Keller,
Laura Martınez-Sarti,
´
a
b
Alessandro Prescimone,
Edwin C. Constable,
Jose M. Junquera-Hernandez,
Henk J. Bolink,
´
´
Michele Sessolo, *b Enrique Ortı
*
a
b
b
´
a
and Catherine E. Housecroft
*
We report a series of [Cu(P^P)(N^N)][PF6] complexes with P^P = bis(2-(diphenylphosphino)phenyl)ether (POP)
or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) and N^N = 6-methoxy-2,20-bipyridine
(MeObpy), 6-ethoxy-2,20-bipyridine (EtObpy), 6-phenyloxy-2,20-bipyridine (PhObpy), 6-methylthio-2,20-
bipyridine (MeSbpy), 6-ethylthio-2,20-bipyridine (EtSbpy) and 6-phenylthio-2,20-bipyridine (PhSbpy). The
single crystal structures of all twelve compounds have been determined and confirm chelating modes for
each N^N and P^P ligand, and a distorted tetrahedral geometry for copper(I). For the xantphos-containing
complexes, the asymmetrical bpy ligand is arranged with the 6-substituent lying over the xanthene ‘bowl’.
The compounds have been characterized in solution by 1H, 13C and 31P NMR spectroscopies, and their
photophysical and electrochemical properties are described. They are yellow emitters and solid samples
show photoluminescence quantum yields in the range up to 38%, with emission lifetimes r10.2 ms. On
going from powder to frozen Me-THF, the excited state lifetimes increase which might suggest the
presence of thermally activated delayed fluorescence (TADF). All the compounds have been tested in
light-emitting electrochemical cells (LECs). Bright and stable LECs are obtained with complexes containing
alkoxy- or phenyloxy-substituted ligands, making this family of compounds very relevant for the future
development of copper-based electroluminescent devices.
Received 12th June 2018,
Accepted 18th July 2018
DOI: 10.1039/c8tc02882f
rsc.li/materials-c
level of sophistication, OLEDs are not yet fully competitive
in the lighting market and in other applications where
Introduction
Organic light-emitting diodes (OLEDs) represent a success story inexpensive light-sources are needed. Hence, simpler devices
of organic electronics, being already mass-produced for the including light-emitting electrochemical cells (LECs) are being
latest generation of portable displays. However, due to their investigated.1–3 Polymer-based LECs,4,5 and LECs containing small
molecules6–8 or ionic transition-metal complexes (iTMCs)1,9–12
a Department of Chemistry, University of Basel, BPR 1096, Mattenstrasse 24a,
as electroluminophores are now widely known. Among these,
iTMC-based devices are among the most promising because of
their high efficiencies and long lifetimes.13 The most rigorously
explored iTMCs are cyclometallated iridium(III) complexes, in
which the large spin–orbit coupling leads to singlet–triplet
mixing and harvesting of emission from both singlet and triplet
CH-4058 Basel, Switzerland. E-mail: catherine.housecroft@unibas.ch
b
´
´
´
Instituto de Ciencia Molecular, Universidad de Valencia, Catedratico Jose Beltran 2,
E-46980 Paterna, Valencia, Spain. E-mail: michele.sessolo@uv.es,
enrique.orti@uv.es
† Electronic supplementary information (ESI) available: Ligand and copper(I)
compound syntheses, including Table S1. Fig. S1–S12: 1H NMR spectra of
copper(I) complexes. Fig. S13–S24: structural figures and selected bond para- states.1 A disadvantage of iridium, however, is its low abun-
meters for all complexes. Table S2: crystallographic data. Fig. S25: isovalue
dance and high cost. In contrast, copper is Earth abundant and
frontier orbitals contour plots. Fig. S26–S29: additional absorption and emission
a potential candidate for the development of dye-sensitized
spectra; Fig. S30–S33: LEC performances and electroluminescence spectra. CCDC
solar cells14 and LECs. During the last few years, we15–18 and
1562407–1562412, 1562448, 1562449, 1562453, 1562457, 1562458 and 1562460.
others19–35 have turned attention to the use of copper(I)-based
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8tc02882f
complexes in LECs, in particular heteroleptic [Cu(P^P)(N^N)]+
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spectra were recorded at room temperature using a Bruker
Avance III-600, III-500 or III-400 NMR spectrometer. 1H and
13C NMR chemical shifts were referenced to residual solvent
peaks with respect to d(TMS) = 0 ppm and 31P NMR chemical shifts
with respect to d(85% aqueous H3PO4) = 0 ppm. Absorption and
emission spectra in solution were measured using an Agilent
8453 spectrophotometer and a Shimadzu RF-5301PC spectro-
fluorometer, respectively. Electrospray ionization (ESI) mass
spectra were recorded on a Bruker esquire 3000plus instrument.
Quantum yields in CH2Cl2 solution and powder were measured
using a Hamamatsu absolute photoluminescence (PL) quantum
yield spectrometer C11347 Quantaurus-QY. Emission lifetimes
and powder emission spectra were measured with a Hamamatsu
Compact Fluorescence lifetime Spectrometer C11367 Quantaurus-
Tau, using an LED light source with lexc = 365 nm. Lifetimes were
obtained by fitting the measured data to an exponential decay
using MATLABs; a biexponential fit was used when a single
exponential fit gave a poor fit. Quantum yields and PL emission
spectra in thin films were recorded using a Hamamatsu absolute
quantum yield C9920. Low temperature emission and lifetime
experiments were performed using an LP920-KS instrument from
Edinburgh Instruments; 410 nm excitation was obtained from
pulsed third-harmonic radiation from a Quantel Brilliant b
Nd:YAG laser equipped with a Rainbow optical parameter oscillator
(OPO). The laser pulse duration was B10 ns and the pulse frequency
10 Hz, with a typical pulse energy of 7 mJ. Detection of the spectra
occurred on an iCCD camera from Andor. Single-wavelength
kinetics were recorded using a photomultiplier tube.
Scheme 1 Structures of the POP and xantphos P^P ligands and of
the bpy N^N ligands substituted with alkoxy, alkylthio, phenyloxy and
phenylthio groups in the 6-position.
cations (P^P and N^N = chelating bis(phosphino) and diimine
ligands, respectively). One of the exciting features of this family
of coordination compounds is the potential for thermally
activated delayed fluorescence (TADF), which accesses emis-
sion from both the triplet and singlet states.36–38
Our initial investigations of [Cu(POP)(N^N)]+ cations (POP =
bis(2-(diphenylphosphino)phenyl)ether) showed that the lumines-
cence efficiency improved upon going from N^N = 2,20-bipyridine
(bpy) to 6-methyl-2,20-bipyridine (Mebpy) and 6,60-dimethyl-2,20-
bipyridine (Me2bpy).15,16 This trend reflects the enhancement
of the emission of [Cu(POP)(phen)]+ observed by McMillin and
coworkers upon increasing the number of substituents in the
2,9-positions of 1,10-phenanthroline (phen).39 Subsequently, we
extended our investigations to a series of [Cu(POP)(N^N)]+ and
[Cu(xantphos)(N^N)]+ (xantphos = 4,5-bis(diphenylphosphino)-9,9-
dimethylxanthene) emitters in which the N^N ligand was Mebpy,
Me2bpy, 6-ethyl-2,20-bipyridine (Etbpy) and 6-phenyl-2,20-bipyridine
(Phbpy). Pleasingly, a LEC with [Cu(xantphos)(Mebpy)][PF6] in the
active layer had a lifetime of 415 h (t1/2 defined as the time for the
maximum luminance, Lmax, to decay to Lmax/2); the value of Lmax
was 90 cd mꢀ2. The introduction of a second methyl substituent in
[Cu(xantphos)(Me2bpy)][PF6] resulted in a shorter lifetime (0.8 h)
but a brighter device (Lmax = 145 cd mꢀ2).17 Replacing the methyl
group in [Cu(xantphos)(Mebpy)][PF6] by an ethyl substituent
resulted in a significant gain in device lifetime but with a lower
luminance (Lmax = 77 cd mꢀ2). This series of LECs were operated
under pulsed current driving.40,41 The introduction of aryl groups
in the 6-position of bpy proved to be detrimental to the emission
behaviour.17,42 We now describe the effects of incorporating alkoxy,
alkylthio, phenyloxy and phenylthio substituents into the 6-position
of bpy (Scheme 1). While our studies were in progress, Weber et al.
demonstrated that, for a series of [Cu(xantphos)(4,40-R2bpy)]+ com-
plexes in which R = MeO, Me, H or NO2, the substituent with the
most negative s-Hammett parameter in the 4,40-positions of the
bpy ligand (4,40-(MeO)2bpy) led to the best-performing LECs; these
devices were also driven using a pulsed current mode.43
Electrochemical measurements were made using a CH Instru-
ments 900B potentiostat with glassy carbon, platinum wire and
silver wire as the working, counter and reference electrodes,
respectively. The compound for study was dissolved in HPLC
grade CH2Cl2 (10ꢀ4 to 10ꢀ5 M) containing 0.1 M [nBu4N][PF6]
as supporting electrolyte. Solutions were degassed with argon.
Cp2Fe/Cp2Fe+ was used as internal reference.
POP and xantphos were purchased from Acros and Fluorochem,
respectively. [Cu(MeCN)4][PF6] was prepared by the published
method.44
Syntheses and characterization of copper(I) compounds
Details of syntheses, 1H, 13C and 31P NMR characterization and
assignments, electrospray mass spectrometric data, and elemental
analyses are given in the ESI.†
Crystallography
Data were collected on a Bruker Kappa Apex2 diffractometer with
data reduction, solution and refinement using the programs APEX45
and CRYSTALS.46 Structural analysis was carried out using Mercury
v. 3.5.1.47,48 Crystallographic data for the complexes are given in
Table S2 (ESI†). For [Cu(POP)(EtSbpy)][PF6]ꢁEt2O, SQUEEZE49 was
used to treat the solvent region and electron density removed
equated to one Et2O molecule per formula unit. SQUEEZE was also
used for the solvent region in [Cu(xantphos)(PhSbpy)][PF6]ꢁ0.5CH2Cl2
and electron density removed equated to 0.5CH2Cl2 per formula
Experimental
General
Reactions under microwave conditions were carried out in a unit. In 2{[Cu(POP)(PhObpy)][PF6]}ꢁ1.5CH2Cl2, half a CH2Cl2
Biotage Initiator 8 microwave reactor. 1H, 13C and 31P NMR molecule per two formula units was refined and SQUEEZE
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was then used to treat the solvent region; the electron density The active area of the devices was 4 mm2. LECs were not
removed equated to one extra CH2Cl2 per two formula units. In encapsulated and were characterized inside the glovebox
[Cu(xantphos)(EtSbpy)][PF6]ꢁ1.5Et2O, one Et2O molecule was at room temperature. The device lifetime was measured by
refined and the other 0.5Et2O was deduced based from the applying a pulsed current and monitoring the voltage and
SQUEEZE output.
luminance versus time by a True Colour Sensor MAZeT
(MTCSiCT Sensor) with a Botest OLT OLED Lifetime-Test
System. The electroluminescent (EL) spectra were measured
Computational details
Density functional theory (DFT) calculations were performed for using an Avantes AvaSpec-2048 Fiber Optic Spectrometer during
the [Cu(P^P)(N^N)]+ cations (where P^P = POP and xantphos, device lifetime measurement.
and N^N corresponds to the six bpy ligands shown in Scheme 1)
using the A.03 revision of the Gaussian 16 program package.50
The Becke’s three-parameter B3LYP exchange–correlation
Results and discussion
Ligand synthesis
functional51,52 was used together with the ‘‘double-z’’ quality
def2svp basis set for C, H, N, P, S and O atoms and the ‘‘triple-z’’
quality def2tzvp basis set for Cu atoms.53,54 The D3 Grimme’s
The compounds MeObpy, EtObpy, PhObpy, MeSbpy, EtSbpy
and PhSbpy (Scheme 1) have previously been synthesized by
dispersion term with Becke–Johnson damping was added to the
various routes.63–67 However, we found the reaction of 6-bromo-
B3LYP functional (B3LYP-D3) to get a better description of the
2,20-bipyridine (prepared by Negishi coupling as described by
intramolecular non-covalent interactions.55,56 These interactions
Fang and Hanan68) with NaOR or NaSR (R = Me, Et, Ph) under
are expected to be of relevance in the determination of the
microwave conditions to be convenient. Synthetic details are
molecular geometries of the studied complexes, which feature
bulky POP and xantphos ligands. The geometries of all the
complexes in both their singlet ground electronic state (S0) and
their lowest-energy triplet excited state (T1) were optimized
without imposing any symmetry restriction. The T1 state was
optimized using the spin-unrestricted UB3LYP approximation
given in the ESI.† 1H and 13C NMR spectroscopic data for
EtObpy63 have not, to our knowledge, been reported and are
given in the ESI.† The optimized reaction conditions and
purification methods for the syntheses of the bipyridine
ligands are summarized in Table S1 (ESI†).
with a spin multiplicity of three. The lowest-lying excited states
of each complex, both singlets and triplets, were computed at the
minimum-energy geometry optimized for S0 using the time-
Synthesis and characterization of the [Cu(P^P)(N^N)][PF6]
complexes
dependent DFT (TD-DFT) approach.57–59 The S1 and T1 states As previously detailed,42 different strategies can be used to prepare
were also optimized for a selected group of systems using the heteroleptic [Cu(POP)(N^N)][PF6] and [Cu(xantphos)(N^N)][PF6] com-
TD-DFT approach to obtain a better estimate of the adiabatic plexes. To ensure the formation of hetereoleptic rather than homo-
energy difference separating the minima of these two states. All leptic complexes, the optimized route to [Cu(POP)(N^N)][PF6] is the
the calculations were performed in the presence of the solvent addition of 1–1.2 equivalents of POP to [Cu(MeCN)4][PF6] followed,
(CH2Cl2). Solvent effects were considered within the self- after a period of time (30 minutes to 2 hours), by addition of the N^N
consistent reaction field (SCRF) theory using the polarized ligand. Excess of POP, if added, can be removed by layer crystal-
continuum model (PCM) approach.60–62
lization of the complex with Et2O and subsequent washing of the
microcrystalline solid with hexanes. For [Cu(xantphos)(N^N)][PF6],
a solution containing both the N^N ligand and xantphos are added
Device preparation and characterization
LECs were prepared on top of patterned indium tin oxide (ITO, to [Cu(MeCN)4][PF6]. These procedures were followed to afford
15 O sqꢀ1) coated glass substrates previously cleaned by sub- [Cu(POP)(N^N)][PF6] and [Cu(xantphos)(N^N)][PF6] (N^N = MeObpy,
sequent sonication in water with soap, deionized water and EtObpy, PhObpy, MeSbpy, EtSbpy or PhSbpy) as yellow solids in
2-propanol baths. After drying with nitrogen, the substrates yields ranging between 61% and 98%. The electrospray mass
were placed in a UV Ozone cleaner for 20 minutes. Prior to spectrum of each compound exhibited a peak envelope arising
the deposition of the emitting layer, 80 nm thick films of poly- from the [Cu(POP)(N^N)]+ or [Cu(xantphos)(N^N)]+ cation (see
(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) experimental section in ESI†).
(CLEVIOSt P VP AI 4083, Heraeus) were coated in order to flatten
1H, 13C and 31P NMR spectra were recorded in acetone-d6
the ITO electrode and to increase its work function. The emitting solutions, and H and 13C spectra were assigned using COSY,
layer (100 nm thick) was prepared by spin-coating a MeCN NOESY, HMQC and HMBC techniques; atom labelling used for
solution of the emitting compound with the addition of the NMR assignments are given in Scheme 2. Fig. 1 shows the aromatic
ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate region of the 1H NMR spectrum of [Cu(POP)(MeObpy)][PF6]
[Emim][PF6] (498.5%, Sigma-Aldrich), in a 4 to 1 molar ratio. as a representative example (see Fig. S1, ESI† for the full
The devices were then transferred to an inert atmosphere glovebox spectrum) and Fig. S2–S12 (ESI†) depict the spectra of the
(o0.1 ppm O2 and H2O), where the aluminium cathode (100 nm) remaining eleven complexes. In the NOESY spectrum of each
1
was thermally deposited in high vacuum using an Edwards [Cu(xantphos)(N^N)][PF6] compound, cross peaks between
0
Auto500 chamber integrated in the glovebox. The thickness of signals for Ha/HC5 and Ha /HC5 distinguished the resonances
all films was determined with an Ambios XP-1 profilometer. for HC5 and HC3
.
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variable, exhibits a range of P–Cu–P bond angles from 108.64(3)
to 118.99(3)1, compared to 115.00(3)1 in [Cu(POP)(bpy)]+. In
most cases, the angle between the PCuP and NCuN planes
is close to (or is) 901. The values of 79.6 and 74.91 observed in
the benchmark [Cu(xantphos)(bpy)]+ and one independent
[Cu(POP)(PhObpy)]+ cation, respectively, are associated with
significant twisting of the bpy backbone (torsion angles of
20.5(2) and 15.9(3)1, respectively).
For each of the xantphos-containing complexes, the asym-
metrical bpy ligand is oriented with the 6-substituent lying
over the xanthene ‘bowl’ (Fig. 2 and 3). We have previously
reported that in solution, [Cu(xantphos)(N^N)]+ complexes may
exist as a mixture of conformers which interconvert through
inversion of the xanthene bowl-shaped unit, and that a prefer-
ence for a given orientation is influenced by steric factors.17,42
In the solid-state structure of [Cu(xantphos)(6-CF3bpy)][PF6]
(6-CF3bpy = 6-trifluoromethyl-2,20-bipyridine), the 6-CF3bpy ligand
is orientationally disordered with the CF3 group facing the
xanthene unit (modelled with 75% occupancy) or away from it
(25% occupancy).69
In contrast to the POP-containing compounds, each of the
complexes with xantphos features a face-to-face p-stacking inter-
action between phenyl rings of the two PPh2 units. Fig. 3 shows
the similarity between the orientations of the phenyl rings in all
six [Cu(xantphos)(N^N)]+ cations. In [Cu(xantphos)(MeObpy)]+,
the angle between the planes of the p-stacked phenyl
rings is 7.11, the centroidꢁ ꢁ ꢁplane distance is 3.66 Å and the
centroidꢁ ꢁ ꢁcentroid distance is 3.82 Å. These parameters are 6.01,
3.70 Å and 3.80 Å for [Cu(xantphos)(EtObpy)]+, 10.01, 3.73 Å
and 3.83 Å for [Cu(xantphos)(PhObpy)]+, 4.81, 3.72 Å and
3.80 Å for [Cu(xantphos)(MeSbpy)]+, 6.51, 3.65 Å and 3.72 Å
for [Cu(xantphos)(EtSbpy)]+ and 6.21, 3.65 Å and 4.00 Å for
[Cu(xantphos)(PhSbpy)]+. Thus, all these p-stacking interactions
are efficient.70
Intramolecular p-stacking features are only present in some
of the [Cu(POP)(N^N)]+ cations, and the aromatic rings involved
vary. Stacking of one phenyl ring of a PPh2 unit and one arene
ring of the POP backbone is seen in [Cu(POP)(MeObpy)]+,
[Cu(POP)(EtObpy)]+ and [Cu(POP)(EtSbpy)]+, but the interactions
are not ideal, with angles between the ring planes of 19.4, 23.5
and 16.61, respectively. In contrast, [Cu(POP)(MeSbpy)]+ and one
of the crystallographically independent [Cu(POP)(PhObpy)]+
cations exhibit p-stacking between one PPh2 phenyl ring and
one ring of the bpy unit (centroidꢁ ꢁ ꢁcentroid separations of 3.79
and 3.61 Å, respectively, and centroidꢁ ꢁ ꢁring plane distances of
3.56 and 3.40 Å, respectively). No intramolecular p-stacking
contacts are present in [Cu(POP)(PhSbpy)]+ and the second
[Cu(POP)(PhObpy)]+ cation. The variation presumably arises
from the flexibility of the POP ligand, and is in sharp contrast
to the consistency of the phenylꢁ ꢁ ꢁphenyl interactions among the
xantphos-containing compounds (Fig. 3).
Scheme 2 Structures of the [Cu(POP)(N^N)]+ and [Cu(xantphos)(N^N)]+
complexes with ring and atom labelling for NMR spectroscopic data. The
OPh or SPh ring is labelled E.
Structural characterizations
X-ray quality single crystals of [Cu(POP)(MeObpy)][PF6], [Cu(POP)-
(EtObpy)][PF6], 2{[Cu(POP)(PhObpy)][PF6]}ꢁ1.5CH2Cl2, [Cu(POP)-
(MeSbpy)][PF6], [Cu(POP)(EtSbpy)][PF6]ꢁEt2O, [Cu(POP)(PhSbpy)]-
[PF6], [Cu(xantphos)(MeObpy)][PF6]ꢁCH2Cl2ꢁ0.5Et2O, [Cu(xantphos)-
(EtObpy)][PF6]ꢁ0.5CH2Cl2ꢁ0.5H2O, [Cu(xantphos)(PhObpy)][PF6]ꢁ
3CH2Cl2, [Cu(xantphos)(MeSbpy)][PF6]ꢁ0.5CH2Cl2ꢁ0.5Et2O, [Cu(xant-
phos)(EtSbpy)][PF6]ꢁ1.5Et2O and [Cu(xantphos)(PhSbpy)][PF6]ꢁ
0.5CH2Cl2 were grown by slow diffusion of Et2O into CH2Cl2
solutions of the complexes. Crystallographic data for the compounds
are summarized in Table S2 (ESI†). The structure determina-
tions confirmed the expected chelating mode of each N^N and
P^P ligand, and the distorted tetrahedral coordination geo-
metry of each copper(I) centre. The structures are shown in ORTEP
style in Fig. S13–S24 (ESI†), and selected bond parameters are
given in the figure captions. The six POP-containing compounds
exhibit similar gross structural features, as do members of the
xantphos series. The Cu–P and Cu–N bond distances in the twelve
compounds lie in the expected ranges 2.2259(6)–2.2913(7) and
2.026(4)–2.138(2) Å, respectively. Table 1 gives values of the P–Cu–P
and N–Cu–N angles, the angles between the PCuP and NCuN planes,
and the bpy N–C–C–N torsion angles, and also includes data
for [Cu(POP)(bpy)][PF6]15 and [Cu(xantphos)(bpy)][PF6]69 as
benchmarks. The chelate angle of the bpy ligands is essentially
fixed (Table 1). There is only a small variation in the xantphos
P–Cu–P chelate angle, and the range of 113.29(3)–115.71(2)1
compares to 113.816(14)1 in [Cu(xantphos)(bpy)]+. In contrast,
the more flexible POP ligand, in which the Pꢁ ꢁ ꢁP distance is
As with the O-atom of the POP and xantphos ligands, the
O- and S-atoms of the 6-substituents of the bpy ligands are
potential donors, but all Cuꢁ ꢁ ꢁO and Cuꢁ ꢁ ꢁS separations are too
long to be considered as meaningful interactions. For POP,
Cuꢁ ꢁ ꢁO separations are in the range 2.977(2)–3.257(2) Å, and for
Fig. 1 Part of the 500 MHz 1H NMR spectrum of [Cu(POP)(MeObpy)][PF6]
in acetone-d6. Chemical shifts in d/ppm. See Fig. S1 (ESI†) for the complete
spectrum. Atom labels are defined in Scheme 2.
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Table 1 Comparison of important structural parameters in the cations of the [Cu(P^P)(N^N)][PF6] compounds. Reference [Cu(P^P)(bpy)][PF6]
complexes are included for comparison
P–Cu–P chelating
angle/1
N–Cu–N chelating
angle/1
Pꢁ ꢁ ꢁP
Angle between PCuP
and NCuN planes/1
N–C–C–N torsion
angle/1
Cation
distance/Å
[Cu(POP)(bpy)]+ a
115.00(3)
113.83(5)
116.48(2)
110.83(3)
108.64(3)
118.99(3)
113.86(7)
112.49(2)
113.816(14)
114.42(4)
114.15(4)
113.29(3)
113.33(3)
113.65(3)
115.71(2)
79.66(7)
79.49(17)
79.65(7)
79.43(8)
79.04(9)
78.09(9)
79.8(2)
78.86(7)
79.32(5)
79.67(13)
79.91(17)
78.76(11)
79.70(11)
79.48(9)
79.40(8)
3.790(1)
3.779(1)
3.8236(7)
3.7039(9)
3.6755(9)
3.894(1)
3.778(2)
3.7844(8)
3.8010(5)
3.778(1)
3.768(1)
3.7746(9)
3.771(1)
3.7844(9)
3.8256(8)
88.5
89.7
87.8
74.9
86.7
90.0
84.9
87.5
79.6
87.2
88.8
89.4
87.4
88.6
86.8
ꢀ2.8(3)
2.2(7)
ꢀ0.9(3)
15.9(3)
ꢀ2.7(3)
ꢀ3.6(4)
4.1(8)
ꢀ3.2(3)
20.5(2)
ꢀ1.6(5)
0.4(7)
0.8(5)
1.2(5)
2.8(4)
ꢀ0.0(3)
[Cu(POP)(MeObpy)]+
[Cu(POP)(EtObpy)]+
[Cu(POP)(PhObpy)]+ b
[Cu(POP)(MeSbpy)]+
[Cu(POP)(EtSbpy)]+
[Cu(POP)(PhSbpy)]+
[Cu(xantphos)(bpy)]+ c
[Cu(xantphos)(MeObpy)]+
[Cu(xantphos)(EtObpy)]+
[Cu(xantphos)(PhObpy)]+
[Cu(xantphos)(MeSbpy)]+
[Cu(xantphos)(EtSbpy)]+
[Cu(xantphos)(PhSbpy)]+
a
c
Data for [Cu(POP)(bpy)][PF6]ꢁCHCl3.15 b Two crystallographically independent cations. Data for [Cu(xantphos)(bpy)][PF6].69
Fig. 2 Structure of the [Cu(xantphos)(PhObpy)]+ cation (H atoms omitted
for clarity) showing the orientation of the 6-substituted bpy ligand with
respect to the xanthene ‘bowl’ of the xantphos ligand.
xantphos the range is 3.225(4)–3.257(2) Å. For the OR substi-
tuents, Cuꢁ ꢁ ꢁO separations are between 3.035(5) and 3.204(2) Å,
and for the sulfur-containing ligands, the Cuꢁ ꢁ ꢁS separations lie
in the range 3.201(1)–3.2655(8) Å.
Electrochemistry
Cyclic voltammetry was used to investigate the redox activity
of the copper(I) complexes in CH2Cl2 solution. Each complex
showed a quasi-reversible copper-based oxidation (Table 2)
and the range of values (+0.74 to +0.86 V) is similar to
other [Cu(POP)(N^N)][PF6] and [Cu(xantphos)(N^N)][PF6]
complexes in which the N^N ligand is a 6-substituted
bpy.17,42 Ligand-based reduction processes within the solvent
accessible window were poorly defined. The Cu+/Cu2+
processes are all at higher potentials than those of the
benc‘hmark compounds [Cu(POP)(bpy)][PF6] (+0.72 V) and
Fig. 3 The structures of the xantphos-containing cations showing
face-to-face p-stacking between phenyl rings of two PPh2 units:
(a) [Cu(xantphos)(MeObpy)]+, (b) [Cu(xantphos)(EtObpy)]+, (c) [Cu(xantphos)-
(PhObpy)]+, (d) [Cu(xantphos)(MeSbpy)]+, (e) [Cu(xantphos)(EtSbpy)]+,
(f) [Cu(xantphos)(PhSbpy)]+.
[Cu(xantphos)(bpy)][PF6] (+0.76 V),69 indicating that the Cu+/Cu2+ process more positive by stabilizing the tetrahedral
substituents in the 6-position of the bpy ligand render the geometry of the copper(I) cation.
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Table 3 Angles defined by the PCuP and NCuN planes computed for
both S0 and T1 states, and frontier molecular orbital energies calculated
for S0 for all the [Cu(P^P)(N^N)]+ complexes at the B3LYP-D3/(def2svp +
def2tzvp) level
DFT calculations
The geometries of all the [Cu(P^P)(N^N)]+ cations in their
electronic ground state (S0) were optimized without imposing
any symmetry restriction at the DFT B3LYP-D3/(def2svp +
def2tzvp) level in the presence of solvent (CH2Cl2). A selection
of calculated geometrical parameters regarding the coordina-
tion sphere of Cu(I) is given in Table S3 (ESI†). As previously
reported for related Cu-iTMCs,71 the theoretical approach
employed here reproduces the distorted tetrahedral structures
observed in the single crystal X-ray diffraction with accuracies
of 0.04 Å for the Cu–P and Cu–N bond distances and of 41 for
the P–Cu–P and N–Cu–N chelating angles.
The geometry of the first triplet excited state (T1) was also
optimized at the UB3LYP-D3 level for all the [Cu(P^P)(N^N)]+
cations (Table S3, ESI†). As discussed below, the T1 state implies
a charge transfer from a d orbital of the Cu atom (d10) to a
molecular orbital centered on the bpy ligand. Consequently,
the metal atom features a partial oxidation and deviates from
the tetrahedral geometry obtained for S0, tending to adopt the
square-planar coordination sphere expected for four-fold coordi-
nated d9 Cu complexes. The angle defined by the PCuP and NCuN
planes has a value of 901 for a perfect tetrahedral structure and
can be used as an estimation of the deviation from the orthogonal
disposition of the P^P and the N^N ligands. The values computed
for this angle are collected in Table 3, which also includes those
previously reported for the reference complexes [Cu(POP)(bpy)]+
and [Cu(xantphos)(bpy)]+,71 and give us information about the
extent of the flattening experienced by the complex upon excita-
tion from S0 to T1. The angle is calculated to be greater than 801
for all the complexes in the S0 state, being slightly higher and
more uniform for the xantphos-containing complexes (86.6–89.71)
than for the POP complexes (80.4–86.01) due to the higher rigidity
of the xantphos ligand. The values for the xantphos complexes are
indeed very similar to the experimental X-ray values (86.8–89.41)
reported in Table 1. In the excited T1 state, the angle formed by
the PCuP and NCuN planes drastically changes showing a
reduction in the range 20–241 for the two families of complexes.
This reduction is significantly smaller than that found for the
Compound
Angle S0/1 Angle T1/1 HOMO/eV LUMO/eV
[Cu(POP)(bpy)]+ a
80.4
81.1
82.6
86.0
82.0
82.3
84.4
86.9
59.7
60.7
60.8
63.8
62.1
62.8
65.3
57.5
61.0
65.0
65.6
64.1
65.0
69.4
ꢀ5.94
ꢀ5.87
ꢀ5.90
ꢀ5.92
ꢀ5.94
ꢀ5.93
ꢀ5.98
ꢀ6.00
ꢀ5.92
ꢀ5.91
ꢀ5.93
ꢀ5.90
ꢀ5.93
ꢀ5.92
ꢀ2.46
ꢀ2.46
ꢀ2.47
ꢀ2.55
ꢀ2.52
ꢀ2.52
ꢀ2.55
ꢀ2.56
ꢀ2.53
ꢀ2.52
ꢀ2.57
ꢀ2.53
ꢀ2.55
ꢀ2.52
[Cu(POP)(MeObpy)]+ a
[Cu(POP)(EtObpy)]+
[Cu(POP)(PhObpy)]+
[Cu(POP)(MeSbpy)]+
[Cu(POP)(EtSbpy)]+
[Cu(POP)(PhSbpy)]+
[Cu(xantphos)(bpy)]+
[Cu(xantphos)(MeObpy)]+ 87.8
[Cu(xantphos)(EtObpy)]+ 88.8
[Cu(xantphos)(PhObpy)]+ 89.7
[Cu(xantphos)(MeSbpy)]+ 86.6
[Cu(xantphos)(EtSbpy)]+ 88.0
[Cu(xantphos)(PhSbpy)]+ 89.4
a
Values from ref. 71.
reference [Cu(xanthpos)(bpy)]+ complex (B301), and is similar to
that calculated for the [Cu(xanthpos)(6-Mebpy)]+ complex, for
which a decrease of 211 (from 88.3 to 67.31) is obtained. This
reflects the fact that the geometrical flattening of the complex is
hindered by the presence of substituents in 6-position of the bpy
ligand in agreement with the results reported in previous
works69,71 and fully supports the trends observed experimentally
for the Cu+/Cu2+ processes (Table 2). Substituents in the 6-position
impede the movement of the ligand towards a more planar cation
geometry thus limiting the distortion from the tetrahedral struc-
ture on going from S0 to T1. The distortion is indeed slightly
smaller for the complexes bearing the bulkier PhO- and PhS-
groups (see Table 3). As discussed below, this limited geometry
relaxation in T1 induces a blue shift of the emission with respect
to that expected based on electronic considerations.
The energies calculated for the highest-occupied (HOMO) and
lowest-unoccupied molecular orbital (LUMO) of the studied com-
plexes are summarized in Table 3, and the isovalue contour plots
for the HOMO and LUMO of two representative examples are
shown in Fig. S25 (ESI†). As previously reported for this type of
complex,7,15,17,69,71 the HOMO is mainly centred over the metal
and the phosphorus atoms, with small contributions of the
phenyl rings, and the LUMO is exclusively located on the N^N
ligand. As the HOMO is situated on a region that undergoes no
structural change along the series, its energy only features small
variations within the two family of complexes, in good agreement
with the similar values registered for the Cu+/Cu2+ processes
(Table 2). The LUMO shows almost no contribution of the
substituents in 6-position of the bpy ligand and its energy lies
in a range of 0.1 eV. The HOMO–LUMO energy gap therefore
oscillates between 3.36 and 3.43 eV, and it is expected that the
excited states described by the LUMO’HOMO monoexcitation
appear at similar energies for all the complexes studied.
Table 2 Cyclic voltammetry data for [Cu(P^P)(N^N)][PF6] complexes
in CH2Cl2 (vs. Fc+/Fc, [nBu4N][PF6] as supporting electrolyte, scan rate =
0.1 V sꢀ1). Data for [Cu(P^P)(bpy)][PF6] are included for comparison
Compound
Eo1/x2/V
(Epc ꢀ Epa/mV)
[Cu(POP)(MeObpy)][PF6]
[Cu(POP)(EtObpy)][PF6]
[Cu(POP)(PhObpy)][PF6]
[Cu(POP)(MeSbpy)][PF6]
[Cu(POP)(EtSbpy)][PF6]
+0.74
+0.76
+0.80
+0.84
+0.84
+0.84
+0.72
+0.81
+0.82
+0.86
+0.80
+0.82
+0.80
+0.76
90
100
130
110
100
100
110
90
110
110
100
100
110
110
[Cu(POP)(PhSbpy)][PF6]
[Cu(POP)(bpy)][PF6]a
[Cu(xantphos)(MeObpy)][PF6]
[Cu(xantphos)(EtObpy)][PF6]
[Cu(xantphos)(PhObpy)][PF6]
[Cu(xantphos)(MeSbpy)][PF6]
[Cu(xantphos)(EtSbpy)][PF6]
[Cu(xantphos)(PhSbpy)][PF6]
[Cu(xantphos)(bpy)][PF6]a
Photophysical properties
The absorption spectroscopic data for CH2Cl2 solutions of the
[Cu(P^P)(N^N)][PF6] complexes are given in Table 4, and Fig. 4
a
Values from ref. 69.
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Table 4 Absorption spectroscopic data for CH2Cl2 solutions of the [Cu(P^P)(N^N)][PF6] complexes
l
max/nm (e/(dm3 molꢀ1 cmꢀ1))
Compound
Ligand-based
MLCT
[Cu(POP)(MeObpy)][PF6]
[Cu(POP)(EtObpy)][PF6]
[Cu(POP)(PhObpy)][PF6]
[Cu(POP)(MeSbpy)][PF6]
[Cu(POP)(EtSbpy)][PF6]
306 (22 600)
305 (20 600)
307 (23 500)
268 (27 000), 333sh (9800)
269 (25 000), 320sh (9584)
267 (28 200), 330sh (10 900)
285 (24 400), 307sh (17 700), 326sh (11 800)
284 (26 300), 305sh (19 000), 322sh (13 900)
282 (27 700), 307sh (20 000), 322sh (15 700)
270 (32 400), 336 (6800)
393 (3300)
393 (3300)
398 (3100)
398 (2600)
393 (2800)
399 (2833)
389 (3100)
388 (3900)
393 (3800)
393 (3000)
391 (3200)
393 (3000)
[Cu(POP)(PhSbpy)][PF6]
[Cu(xantphos)(MeObpy)][PF6]
[Cu(xantphos)(EtObpy)][PF6]
[Cu(xantphos)(PhObpy)][PF6]
[Cu(xantphos)(MeSbpy)][PF6]
[Cu(xantphos)(EtSbpy)][PF6]
[Cu(xantphos)(PhSbpy)][PF6]
271 (35 800), 338 (7900)
273 (33 700), 333 (8500)
Table 5 Vertical excitation energies calculated at the TD-DFT B3LYP-D3/
(def2svp + def2tzvp) level for the lowest singlet (S1) and triplet (T1) excited
states of complexes [Cu(P^P)(N^N)]+ in CH2Cl2 solution. S0 - S1 oscillator
strengths (f) are given within parentheses
Compound
S1/eV
T1/eV
[Cu(POP)(MeObpy)]+
[Cu(POP)(EtObpy)]+
2.78 (0.09)
2.79 (0.09)
2.75 (0.08)
2.77 (0.11)
2.77 (0.10)
2.77 (0.10)
2.77 (0.07)
2.77 (0.07)
2.75 (0.06)
2.73 (0.08)
2.76 (0.08)
2.75 (0.07)
2.52
2.54
2.51
2.55
2.55
2.57
2.52
2.53
2.50
2.50
2.53
2.53
[Cu(POP)(PhObpy)]+
[Cu(POP)(MeSbpy)]+
[Cu(POP)(EtSbpy)]+
[Cu(POP)(PhSbpy)]+
[Cu(xantphos)(MeObpy)]+
[Cu(xantphos)(EtObpy)]+
[Cu(xantphos)(PhObpy)]+
[Cu(xantphos)(MeSbpy)]+
[Cu(xantphos)(EtSbpy)]+
[Cu(xantphos)(PhSbpy)]+
Fig. 4 Absorption spectra of CH2Cl2 solutions of [Cu(xantphos)-
(N^N)][PF6] complexes (concentration = 2.5 ꢂ 10ꢀ5 mol dmꢀ3).
contribution (495%) to both states comes from the HOMO -
compares the spectra of the compounds containing xantphos. LUMO monoexcitation. This excitation implies an electron
Fig. S26 (ESI†) shows the spectra of the POP-containing com- transfer from the Cu(P^P) environment to the bpy ligand, and
plexes. The intense high-energy absorptions between 250 and therefore confirms the MLCT character of the S1 and T1 states.
350 nm are assigned to ligand-based p - p* and n - p* The transition to S1 accounts for the low-energy band observed
transitions, and the shift to lower energy on replacing oxygen by in the 400 nm region in the absorption spectra (Fig. 4), as no
sulfur is consistent with the trends observed in 2-substituted other singlet state with relevant oscillator strength is calculated
pyridines, for example on going from 2-methoxypyridine to in this energy range. The almost identical energies predicted for
2-methylthiopyridine.72 The broad absorption around 390 nm the S0 - S1 transition are in good agreement with the very
arises from a metal-to-ligand charge transfer (MLCT). The similar values calculated for the HOMO–LUMO gaps, and
position of this band is essentially independent of the reproduce the trend observed experimentally for the absorption
6-substituent and of the P^P ligand. However, the maxima maxima of the MLCT band, which lie in a narrow range of only
of the MLCT bands of the complexes with 6-OR or 6-SR 11 nm for the entire set of complexes (Table 4).
substituents are all at lower energies than the corresponding
complexes with bpy, namely [Cu(POP)(bpy)][PF6] with lmax
Photoluminescence (PL) spectra of the complexes were
=
recorded for deaerated CH2Cl2 solutions and powder samples.
max
388 nm and [Cu(xantphos)(bpy)][PF6] with lmax = 383 nm.69 The Table
6 presents PL maxima (l ), photoluminescence
em
OR/SR substituents lead to a smaller HOMO–LUMO gap by quantum yields (PLQY) and PL lifetimes (t), and includes
stabilizing the LUMO which is mainly located on the bpy ligand previously reported data for [Cu(POP)(bpy)][PF6] and
(Fig. S25, ESI†).
[Cu(xantphos)(bpy)][PF6]69 for comparison. The PL spectra of
To gain greater insight into the nature of the low-lying the xantphos- and POP-containing complexes in solution are
electronic states, the lowest-energy singlet and triplet excited shown in Fig. 5 and Fig. S27 (ESI†), respectively. All compounds
states were calculated using the time-dependent DFT (TD-DFT) are yellow emitters both in solution and in the solid state, with
approach. The vertical B3LYP-D3/(def2svp + def2tzvp) excitation emissions in solution being very weak (PLQY r 1%). The
energies computed for the S1 and T1 states at the optimized emission bands are broad and exhibit two maxima around
geometry of S0 are collected in Table 5. In all cases, the main 610 and 635 nm (Table 6), which are similar to those of
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Table 6 Photoluminescence characterization for solutions, powder, frozen solution at 77 K and thin films of the [Cu(N^N)(P^P)][PF6] complexes
Compound
Solution
Powder
Solution at 77 K
Film
max
em
max a
em
max a
Cu(P^P)(N^N)[PF6]
l
/nm
PLQY (deaerated)/% t (deaerated)/ns
l
/nm PLQY/% t/ms
l
em
/nm PLQY/% t/ms PLQY/%
POP
MeObpy 613, 640b o1
204
253
277
334
259
258
46
565
570
585
549
564
566
580
17
9
5
30
22
20
3
3.3
2.7
599
596
5
6
3
10
9
7
21
28
16
48
31
33
16
4
5
5
5
5
7
—
EtObpy
613, 641c o1
PhObpy 610, 641d
1
1.5b 613
MeSbpy 613, 639b o1
10.2
563
598
600
610
EtSbpy
PhSbpy
bpy f
605, 635b o1
6.5
6.2
1.5
606, 630e o1
618, 649
o1
6
Xantphos MeObpy 609, 637e o1
231
279
313
315
280
303
104
566
566
572
557
552
552
587
19
22
12
21
32
38
2
4.7
4.0
2.7
6.0
6.5
9.1
1.3
593
594
610
588
575
576
613
11
15
11
20
20
23
3
23
23
13
38
38
44
11
6
7
6
5
6
7
—
EtObpy
610, 633d
1
PhObpy 608, 635b o1
MeSbpy 608, 632b
1
EtSbpy
PhSbpy
bpy f
605, 630b o1
603, 629b
620, 650
1
o1
a
b
c
d
e
f
lexc = 365 nm. lexc = 420 nm. lexc = 400 nm. lexc = 410 nm. lexc = 390 nm. Data from ref. 69.
Fig. 5 Emission spectra of solutions (CH2Cl2, 2.5 ꢂ 10ꢀ5 mol dmꢀ3) of
[Cu(xantphos)(N^N)][PF6] complexes (see Table 6 for lexc).
Fig. 6 Emission spectra of powder samples of [Cu(xantphos)(N^N)][PF6]
complexes (lexc = 365 nm).
[Cu(POP)(Mebpy)][PF6]16 and [Cu(xantphos)(Mebpy)][PF6],17 higher PLQYs as compared with POP-based compounds. This is
and are blue-shifted with respect to emission maxima of most likely due to the more rigid structure of the xantphos
[Cu(POP)(bpy)][PF6] and [Cu(xantphos)(bpy)][PF6].69 Across the ligand, partially hindering the planarization of the complex
series of compounds, there is some variation in the relative in the excited state as discussed above. At the same time,
intensities of the two emission bands. This is most noticeable the PLQYs of the complexes bearing the RSbpy ligands are
for [Cu(xantphos)(MeObpy)][PF6] (blue curve in Fig. 5) and higher than those obtained for the complexes with RObpy
[Cu(POP)(MeObpy)][PF6] (Fig. S27, ESI†) where the high energy ligands. Also, within each class of compounds, the PLQY
band is less intense. The PL lifetimes in solution for the series increases when the size of the 6-substituent on the bpy is
of compounds are relatively short (between 204 and 334 ns), augmented, i.e. going from MeXbpy to EtXbpy and to PhXbpy
with no evident influence of the P^P ligands. We have pre- (X = O and S). We propose that with the larger substituents, the
viously observed that the introduction of the 6-substituents on copper centre is better protected against quenchers such
the bpy results in a longer lifetime16,17,69 (Table 6), and similar as oxygen or water. Hence, the highest PLQY (38%) is observed
trends are observed here moving from [Cu(P^P)(bpy)][PF6] to for [Cu(xantphos)(PhSbpy)][PF6], which is comparable to the
[Cu(P^P)(RObpy)][PF6] or [Cu(P^P)(RSbpy)][PF6].
best-in-class [Cu(P^P)(N^N)][PF6] complexes (N^N = 6-Mebpy,
The PL spectra for the powders are broad and, for each 6,60-Mebpy) reported to date.16,17
complex, a single maximum is observed (Fig. 6 and Fig. S28,
As mentioned above, TD-DFT calculations predict that the
ESI†). Moving from solution to solid, the emissions are con- emitting HOMO - LUMO T1 triplet has an MLCT character
sistently blue-shifted17 (Table 6). At the same time, the PLQY is and implies a charge transfer from the Cu(P^P) domain, where
substantially enhanced and longer PL lifetimes t are observed. the HOMO lies, to the bpy ligand, where the LUMO is located
In general, [Cu(P^P)(N^N)][PF6] complexes with xantphos have (Fig. S25, ESI†). As the flattening of the tetrahedral structure of
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Table 7 Main device parameters obtained from LECs driven at 50 A mꢀ2
P^P
N^N
ton/h Lummax/cd mꢀ2 t1/2/h Eff./cd Aꢀ1
Xantphos MeObpy
EtObpy
4.2
0.5
1.8
0.9
0.7
3.1
0.7
45
79
80
44
39
56
77
48
54
47
20
14
80
51
0.9
1.6
1.6
0.9
0.8
1.1
1.5
PhObpy
MeSbpy
EtSbpy
PhSbpy
6-Etbpya
POP
MeObpy
EtObpy
PhObpy
MeSbpy
EtSbpy
15.9
1.0
3.4
0.4
0.3
3.9
4.3
17
63
37
32
14
22
53
200
102
104
14
6
60
0.3
1.3
0.7
0.6
0.2
0.4
1.1
Fig. 7 Emission spectra of [Cu(xantphos)(N^N)][PF6] complexes in frozen
Me-THF (lexc = 410 nm).
PhSbpy
6-Etbpya
82
a
Data from ref. 17.
the complex in the T1 state occurs to a similar extent for all the
complexes (see Table 3), similar energies in the 1.8–1.9 eV
range are predicted for T1 after full relaxation of the molecular
geometry. The close energies calculated for T1 both at the TD-DFT
level (Table 5), determined by the similar HOMO–LUMO gaps,
and after full geometry relaxation explain the similarity of the
experimentally observed emission maxima (Table 6).
Low temperature emission spectra were recorded for frozen
(77 K) solutions of the complexes in Me-THF. The emission
spectra of the [Cu(xantphos)(N^N)][PF6] and [Cu(POP)(N^N)][PF6]
complex in the presence of [Emim][PF6] (4 : 1 molar ratio) and
an aluminium cathode. Devices were driven with a pulsed
current (50 A mꢀ2 average, 50% duty cycle, 1 kHz) and the
electroluminescence and voltage were monitored over time.
The main device parameters obtained for the entire samples
series are reported in Table 7.
The time evolution of efficacy and luminance for devices
employing RObpy emitters are reported in Fig. 8, while the data
for RSbpy compounds can be found in Fig. S30 (ESI†). The
evolution of the voltage with time for the complete device series
is reported in Fig. S31 (ESI†). The spectral shape and position
of the electroluminescence (EL) signals were found to be
extremely similar for the entire series of materials, independent
of the choice of P^P and N^N ligands (Fig. S32, ESI†). The EL
maxima are asymmetric with maximum intensities at 585 nm,
and can be approximated well with the sum of two Gaussian
functions centred at 575 nm and 639 nm (Fig. S33, ESI†).
Interestingly, these wavelengths are very similar to the PL
maxima observed for the series of complexes in the solid state
and in solution, respectively.
LECs employing xantphos-based complexes were found to be
generally more efficient than the POP analogues, in agreement
with the PLQY trend described above for samples in powder.
However, the trends in device lifetimes are the reverse. For
the complexes with 6-OR substituents on the bpy, the devices
containing [Cu(POP)(N^N)]+ exhibit longer lifetimes than those
with [Cu(xantphos)(N^N)]+, whereas for the SR analogues, the
device lifetimes are longer for the [Cu(xantphos)(N^N)]+ com-
plexes. We must note, however, that the brightest and more
efficient LECs were obtained for complexes including alkoxy-bpy
ligands, which showed in general a lower PLQY compared to the
alkylthio-derivatives. Within each family of compounds, we also
observed (with few exceptions) that the maximum luminance and
hence the efficiency increase with increasing steric hindrance
of the substituent on the bpy domain, which is probably due to
a better stabilization of the tetrahedral complex geometry. We
have obtained the brightest and most efficient devices with
compounds are shown in Fig. 7 and Fig. S29 (ESI†), respectively.
max
em
Values of l , lifetimes and PLQYs are given in Table 6. The
frozen solution approaches the solid state, with PLQYs being
intermediate between values observed in solution and in powder.
Comparison of PL maxima reveals a bathochromic shift of
between 14 and 34 nm when going from powder to frozen
Me-THF. The excited state lifetimes increase, which might
suggest the presence of TADF37,38 for these compounds. At room
temperature, there is emission from the singlet S1 state after
thermal population from the long-lived triplet excited state T1. At
77 K, TADF is less likely to occur and, as a result, the PL is
dominated by phosphorescence from T1 rather than fluores-
cence from S1. The former occurs at lower energies and explains
the red-shift of the emission maxima at 77 K. As observed for the
solid state, the sulfur-containing compounds exhibit longer life-
times and higher PLQY values as compared to their oxygen-
containing analogues. The energy difference between the S1 and
T1 states is predicted in the range 0.22–0.25 eV at the optimized
geometry of S0 (Table 4), which is small enough to allow efficient
TADF.37,38 To confirm this prediction, the geometries of the S1
and T1 states were further optimized at the TD-DFT level for
selected complexes. After full relaxation, the energy difference
between S1 and T1 is 0.23 eV for [Cu(POP)(MeObpy)]+, 0.21 eV for
[Cu(POP)(MeSbpy)]+ and 0.21 eV for [Cu(xantphos)(MeSbpy)]+.
These energies are slightly smaller than the values obtained
from the vertical excitation energies, and support the feasibility
of TADF in these systems.
LECs
The series of compounds was tested in LECs using ITO/ [Cu(xantphos)(N^N)][PF6] with N^N = EtObpy and PhObpy;
PEDOT:PSS as the anode, an emitting layer consisting of the these LECs have similar performances of around 80 cd mꢀ2
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Journal of Materials Chemistry C
Fig. 8 Efficacy (top) and luminance (bottom) versus time for LECs employing Cu(I) complexes with RObpy and either (a) xantphos or (b) POP ligands.
LECs were driven at an average current density of 50 A mꢀ2
.
and 1.6 cd Aꢀ1 (Fig. 8a). For comparison, the optoelectronic PLQY values up to 38%, with emission lifetimes r10.2 ms. All
parameters of complexes with 6-Etbpy are included in Table 7. On the compounds have been tested in LECs. Bright and stable LECs
going from [Cu(P^P)(Etbpy)][PF6]17 to [Cu(P^P)(EtObpy)][PF6], are obtained with complexes containing alkoxy- or phenyloxy-
whereas there is no substantial difference among xantphos com- substituted ligands. Compared with the state-of-the-art copper(I)-
plexes, there is a slight enhancement of the LEC parameters for based LECs which contain [Cu(P^P)(Etbpy)][PF6],17 the LECs with
compounds involving POP. The turn-on time (ton, defined here as [Cu(P^P)(EtObpy)][PF6] exhibit enhanced values of ton, Lummax, t1/2
the time to reach the maximum luminance) shortens from 4.3 to and efficacy. These observations point towards [Cu(P^P)(N^N)][PF6]
1.0 h. Lummax increases from 53 to 63 cd mꢀ2, the device lifetime compounds with N^N being 6-alkoxy- or 6-phenyloxy-bpy
t1/2 goes from 82 to 102 h, and efficacy from 1.1 to 1.3 cd Aꢀ1
,
ligands being very relevant for the future development of
respectively. The next best complexes are those with a PhO copper-based electroluminescent devices.
substituent. Interestingly, whereas [Cu(POP)(MeObpy)][PF6] gave
the longest device lifetime (4200 h, Fig. 8b), it came at the expense
of ton (15.9 h) and the efficacy (0.3 cd Aꢀ1), with the device only
reaching a brightness of 17 cd mꢀ2. This is in agreement with our
earlier observations that there is a trade-off between brightness
and device lifetimes.17 However, the introduction of the EtO or
PhO substituents leads to improved device parameters with
respect to the best of our previously reported LECs.17
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Financial support from the Swiss National Science Foundation
(grant number 162631), the University of Basel, the Spanish
Ministry of Economy and Competitiveness (MINECO, CTQ2015-
71154-P, MAT2017-88821-R, PCIN-2015-255, PCIN-2017-014 and
Conclusions
We have prepared and fully characterized a series of twelve
[Cu(P^P)(N^N)][PF6] complexes with a bisphosphane ligand
P^P = POP or xantphos and chelating ligand N^N = MeObpy,
EtObpy, PhObpy, MeSbpy, EtSbpy or PhSbpy. The single crystal
structure of each compound confirms chelating modes for each
N^N and P^P ligand, and a distorted tetrahedral geometry for
copper(I). The xantphos-containing compounds all exhibit face-
to-face p-stacking between phenyl rings of two PPh2 units. In
contrast, intramolecular p-stacking in the POP-containing com-
pounds does not show a consistent pattern. In the xantphos-
containing complexes, the asymmetrical bpy ligand is oriented
with the 6-OR or SR substituent lying over the xanthene ‘bowl’.
The compounds are yellow emitters and powder samples show
´
Unidad de Excelencia Marıa de Maeztu MDM-2015-0538), the
Generalitat Valenciana (PROMETEO/2016/135), and European
FEDER funds (CTQ2015-71154-P) is gratefully acknowledged.
M. S. thanks the MINECO for his RyC contract. Fabian Brunner
is acknowledged for valuable laboratory assistance. Prof. Oliver
S. Wenger is thanked for use of the LP920-KS instrument from
Edinburgh Instruments.
References
´
1 R. D. Costa, E. Ortı, H. J. Bolink, F. Monti, G. Accorsi and
N. Armaroli, Angew. Chem., Int. Ed., 2012, 51, 8178.
J. Mater. Chem. C
This journal is ©The Royal Society of Chemistry 2018

View Article Online
Journal of Materials Chemistry C
Paper
2 Q. Pei, G. Yu, C. Zhang, Y. Yang and A. J. Heeger, Science, 25 K. Zhang and D. Zhang, Spectrochim. Acta, Part A, 2014,
1995, 269, 1086. 124, 341.
3 S. B. Meier, D. Tordera, A. Pertegas, C. Roldan-Carmona, 26 L. Bergmann, J. Friedrichs, M. Mydlak, T. Baumann,
´
´
´
¨
M. Nieger and S. Brase, Chem. Commun., 2013, 49, 6501.
E. Ortı and H. J. Bolink, Mater. Today, 2014, 17, 217.
´
4 Y. Zhang and J. Gao, J. Appl. Phys., 2006, 100, 084501.
5 N. Kaihovirta, C. Larsen and L. Edman, ACS Appl. Mater.
Interfaces, 2014, 6, 2940.
27 E. Mejıa, S.-P. Luo, M. Karnahl, A. Friedrich, S. Tschierlei,
A.-E. Surkus, H. Junge, S. Gladiali, S. Lochbrunner and
M. Beller, Chem. – Eur. J., 2013, 19, 15972.
6 S. Tang, W.-Y. Tan, X.-H. Zhu and L. Edman, Chem. Commun., 28 X.-L. Chen, R. Yu, Q.-K. Zhang, L.-J. Zhou, X.-Y. Wu,
2013, 49, 4926. Q. Zhang and C.-Z. Lu, Chem. Mater., 2013, 25, 3910.
7 M. D. Weber, M. Adam, R. R. Tykwinski and R. D. Costa, 29 A. Kaeser, M. Mohankumar, J. Mohanraj, F. Monti, M. Holler,
Adv. Funct. Mater., 2015, 25, 5066.
8 M. Y. Wong, G. J. Hedley, G. Xie, L. S. Kolln, I. D. W. Samuel,
J.-J. Cid, O. Moudam, I. Nierengarten, L. Karmazin-Brelot,
C. Duhayon, B. Delavaux-Nicot, N. Armaroli and J.-F.
Nierengarten, Inorg. Chem., 2013, 52, 12140.
´
A. Pertegas, H. J. Bolink and E. Zysman-Colman, Chem.
Mater., 2015, 27, 6535.
30 C. Femoni, S. Muzzioli, A. Palazzi, S. Stagni, S. Zacchini,
F. Monti, G. Accorsi, M. Bolognesi, N. Armaroli, M. Massi,
G. Valenti and M. Marcaccio, Dalton Trans., 2013, 42, 997.
9 J. D. Slinker, A. A. Gorodetsky, M. S. Lowry, J. Wang,
S. Parker, R. Rohl, S. Bernhard and G. G. Malliaras, J. Am.
Chem. Soc., 2004, 126, 2763.
10 J.-K. Lee, D. S. Yoo, E. S. Handy and M. F. Rubner, Appl.
Phys. Lett., 1996, 69, 1686.
11 F. G. Gao and A. J. Bard, J. Am. Chem. Soc., 2000, 122, 7426.
12 C. E. Housecroft and E. C. Constable, Coord. Chem. Rev.,
2017, 350, 155.
13 T. Hu, L. He, L. Duan and Y. Qiu, J. Mater. Chem., 2012,
22, 4206.
14 See for example: C. E. Housecroft and E. C. Constable,
Chem. Soc. Rev., 2015, 44, 8386.
15 R. D. Costa, D. Tordera, E. Ortı, H. J. Bolink, J. Schonle,
S. Graber, C. E. Housecroft, E. C. Constable and
J. A. Zampese, J. Mater. Chem., 2011, 21, 16108.
´
´
31 I. Andres-Tome, J. Fyson, F. Baiao Dias, A. P. Monkman,
G. Iacobellis and P. Coppo, Dalton Trans., 2012, 41, 8669.
32 C. L. Linfoot, M. J. Leitl, P. Richardson, A. F. Rausch,
O. Chepelin, F. J. White, H. Yersin and N. Robertson, Inorg.
Chem., 2014, 53, 10854.
33 S.-M. Kuang, D. G. Cuttell, D. R. McMillin, P. E. Fanwick and
R. A. Walton, Inorg. Chem., 2002, 41, 3313.
34 K. Chen, J. Shearer and V. J. Catalano, Inorg. Chem., 2015,
54, 6245.
35 M. D. Weber, C. Garino, G. Volpi, E. Casamassa,
M. Milanesio, C. Barolo and R. D. Costa, Dalton Trans.,
2016, 45, 8984.
36 C. L. Linfoot, M. J. Leitl, P. Richardson, A. F. Rausch,
O. Chepelin, F. J. White, H. Yerson and N. Robertson, Inorg.
Chem., 2014, 53, 10854.
´
¨
16 S. Keller, E. C. Constable, C. E. Housecroft, M. Neuburger,
´
A. Prescimone, G. Longo, A. Pertegas, M. Sessolo and
H. J. Bolink, Dalton Trans., 2014, 43, 16593.
37 R. Czerwieniec, M. J. Leitl, H. H. H. Homeier and H. Yersin,
Coord. Chem. Rev., 2016, 325, 2.
´
´
´
17 S. Keller, A. Pertegas, G. Longo, L. Martınez, J. Cerda,
´
J. M. Junquera-Hernandez, A. Prescimone, E. C. Constable, 38 Highly Efficient OLEDS: Materials Based on Thermally
´
C. E. Housecroft, E. Ortı and H. J. Bolink, J. Mater. Chem. C,
Activated Delayed Fluorescence, ed. H. Yersin, Wiley-VCH,
2016, 4, 3857.
Weinheim, 2018.
´
´
18 F. Brunner, L. Martınez-Sarti, S. Keller, A. Pertegas, 39 D. G. Cuttell, S.-M. Kuang, P. E. Fanwick, D. R. McMillin and
A. Prescimone, E. C. Constable, H. J. Bolink and
R. A. Walton, J. Am. Chem. Soc., 2002, 124, 6.
´
C. E. Housecroft, Dalton Trans., 2016, 45, 15180.
40 D. Tordera, S. Meier, M. Lenes, R. D. Costa, E. Ortı,
´
19 A. Kaeser, O. Moudam, G. Accorsi, I. Seguy, J. Navarro,
W. Sarfert and H. J. Bolink, Adv. Mater., 2012, 24, 897.
¨
A. Belbakra, C. Duhayon, N. Armaroli, B. Delavaux-Nicot 41 N. M. Shavaleev, R. Scopelliti, M. Gratzel, M. K. Nazeeruddin,
´
and J.-F. Nierengarten, Eur. J. Inorg. Chem., 2014, 1345.
A. Pertegas, C. Roldan-Carmona, D. Tordera and H. J. Bolink,
20 C. Bizzarri, C. Strabler, J. Prock, B. Trettenbrein,
J. Mater. Chem. C, 2013, 1, 2241.
¨
M. Ruggenthaler, C.-H. Yang, F. Polo, A. Iordache, 42 F. Brunner, S. Graber, Y. Baumgartner, D. Haussinger,
P. Bru¨ggeller and L. De Cola, Inorg. Chem., 2014, 53, 10944.
A. Prescimone, E. C. Constable and C. E. Housecroft, Dalton
Trans., 2017, 46, 6379.
¨
21 D. Volz, M. Wallesch, S. L. Grage, J. Gottlicher, R. Steininger,
D. Batchelor, T. Vitova, A. S. Ulrich, C. Heske, L. Weinhardt, 43 M. D. Weber, M. Viciano-Chumillas, D. Armentano, J. Cano
¨
T. Baumann and S. Brase, Inorg. Chem., 2014, 53, 7837.
and R. D. Costa, Dalton Trans., 2017, 46, 6312.
22 J.-J. Cid, J. Mohanraj, M. Mohankumar, M. Holler, F. Monti, 44 G. J. Kubas, Inorg. Synth., 1979, 19, 90.
G. Accorsi, L. Karmazin-Brelot, I. Nierengarten, J. M. Malicka, 45 APEX2, version 2 User Manual, M86-E01078, Bruker Analytical
M. Cocchi, B. Delavaux-Nicot, N. Armaroli and J.-F. Nierengarten,
Polyhedron, 2014, 82, 158.
23 N. Armaroli, G. Accorsi, M. Holler, O. Moudam, J. F.
X-ray Systems, Inc., Madison, WI, 2006.
46 P. W. Betteridge, J. R. Carruthers, R. I. Cooper, K. Prout and
D. J. Watkin, J. Appl. Crystallogr., 2003, 36, 1487.
Nierengarten, Z. Zhou, R. T. Wegh and R. Welter, Adv. 47 I. J. Bruno, J. C. Cole, P. R. Edgington, M. K. Kessler,
Mater., 2006, 18, 1313.
24 R. Czerwieniec and H. Yersin, Inorg. Chem., 2015, 54, 4322.
C. F. Macrae, P. McCabe, J. Pearson and R. Taylor, Acta
Crystallogr., Sect. B: Struct. Sci., 2002, 58, 389.
This journal is ©The Royal Society of Chemistry 2018
J. Mater. Chem. C

View Article Online
Paper
Journal of Materials Chemistry C
48 C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington, 56 S. Grimme, S. Ehrlich and L. Goerigk, J. Comput. Chem.,
P. McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor, J. van 2011, 32, 1456.
de Streek and P. A. Wood, J. Appl. Crystallogr., 2008, 41, 466. 57 M. Petersilka, U. J. Gossmann and E. K. U. Gross, Phys. Rev.
49 A. L. Spek, Acta Crystallogr., Sect. C: Struct. Chem., 2015,
C71, 9.
50 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
Lett., 1996, 76, 1212.
58 C. Jamorski, M. E. Casida and D. R. Salahub, J. Chem. Phys.,
1996, 104, 5134.
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, 59 M. E. Casida, C. Jamorski, K. C. Casida and D. R. Salahub,
G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, J. Chem. Phys., 1998, 108, 4439.
A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, 60 J. Tomasi and M. Persico, Chem. Rev., 1994, 94, 2027.
B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, 61 C. J. Cramer and D. G. Truhlar, in Solvent Effects and
´
J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini,
F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson,
Chemical Reactivity, ed. O. Tapia and J. Bertran, Kluwer,
1996, pp. 1–80.
D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, 62 J. Tomasi, B. Mennucci and R. Cammi, Chem. Rev., 2005,
G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, 105, 2999.
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, 63 P. M. Pojer and L. A. Summers, J. Heterocycl. Chem., 1974,
Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. 11, 303.
Montgomery Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, 64 S.-H. Kim and R. D. Rieke, Tetrahedron Lett., 2009, 50, 5329.
´
J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, 65 J. F. M. da Silva, A. F. Yepes Perez and N. P. de Almeida,
T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, Synth. Commun., 2015, 45, 1995.
A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, 66 P. Gros and Y. Fort, J. Chem. Soc., Perkin Trans. 1, 1998, 3515.
J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. 67 J. Carroll, H. G. Woolard, R. Mroz, C. A. Nason and S. Huo,
Ochterski, R. L. Martin, K. Morokuma, O. Farkas,
Organometallics, 2016, 35, 131.
J. B. Foresman and D. J. Fox, Gaussian 16, Revision A.03, 68 Y.-Q. Fang and G. S. Hanan, Synlett, 2003, 852.
´
´
Gaussian, Inc., Wallingford CT, 2016.
51 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter
Mater. Phys., 1988, 37, 785.
69 S. Keller, F. Brunner, J. M. Junquera-Hernandez, A. Pertegas,
M.-G. La-Placa, A. Prescimone, E. C. Constable, H. J. Bolink,
E. Ortı and C. E. Housecroft, ChemPlusChem, 2018, 83, 217.
´
52 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
70 C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885.
53 F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005, 71 S. Keller, A. Prescimone, H. Bolink, M. Sessolo, G. Longo,
´ ´
L. Martınez-Sarti, J. M. Junquera-Hernandez, E. C. Constable,
7, 3297.
´
54 F. Weigend, Phys. Chem. Chem. Phys., 2006, 8, 1057.
55 S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys.,
2010, 132, 154104.
E. Ortı and C. E. Housecroft, Dalton Trans., 2018, DOI:
10.1039/c8dt01338a.
72 S. F. Mason, J. Chem. Soc., 1959, 1253.
J. Mater. Chem. C
This journal is ©The Royal Society of Chemistry 2018