The shiny side of copper: Bringing copper(i) light-emitting electrochemical cells closer to application

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The shiny side of copper: bringing copper(I) light-

emitting electrochemical cells closer to

application†

Cite this: RSC Adv., 2020, 10, 22631

a

Sarah Keller,

Alessandro Prescimone,

Maria-Grazia La Placa,^b

b

a

Jose M. Junquera-Hernandez,

Michele Sessolo,

Henk J. Bolink,

Edwin C. Constable,

´

b

a

*

Enrique Ortı

and Catherine E. Housecroft

´

Heteroleptic [Cu(P^P)(N^N)][PF₆] complexes, where N^N is 5,5⁰-dimethyl-2,2⁰-bipyridine (5,5⁰-Me₂bpy),

4,5,6-trimethyl-2,2⁰-bipyridine (4,5,6-Me₃bpy), 6-(tert-butyl)-2,2⁰-bipyridine (6-tBubpy) and 2-ethyl-1,10-

phenanthroline (2-Etphen) and P^P is either bis(2-(diphenylphosphino)phenyl)ether (POP, PIN [oxydi(2,1-

phenylene)]bis(diphenylphosphane)) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos, PIN

(9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane)) have been synthesized and their NMR

spectroscopic, mass spectrometric, structural, electrochemical and photophysical properties were

investigated. The single-crystal structures of [Cu(POP)(5,5⁰-Me₂bpy)][PF₆], [Cu(xantphos)(5,5⁰-Me₂bpy)]

[PF₆], [Cu(POP)(6-tBubpy)][PF₆], [Cu(POP)(4,5,6-Me₃bpy)][PF₆]$1.5Et₂O, [Cu(xantphos)(4,5,6-Me₃bpy)]

[PF₆]$2.33CH₂Cl₂, [Cu(POP)(2-Etphen)][PF₆] and [Cu(xantphos)(2-Etphen)][PF₆] are described. While alkyl

substituents in general exhibit electron-donating properties, variation in the nature and substitution-

position of the alkyl group in the N^N chelate leads to diﬀerent eﬀects in the photophysical properties

of the [Cu(P^P)(N^N)][PF₆] complexes. In the solid state, the complexes are yellow to green emitters

with emission maxima between 518 and 602 nm, and photoluminescence quantum yields (PLQYs)

ranging from 1.1 to 58.8%. All complexes show thermally activated delayed ﬂuorescence (TADF). The

complexes were employed in the active layer of light-emitting electrochemical cells (LECs). The device

performance properties are among the best reported for copper-based LECs, with maximum luminance

values of up to 462 cd m^ꢀ2and device half-lifetimes of up to 98 hours.

Received 28th April 2020

Accepted 31st May 2020

DOI: 10.1039/d0ra03824e

rsc.li/rsc-advances

Energy Information Administration estimated that some 7% of

the total U.S. energy usage was accounted for by lighting.²The

Introduction

transition from energy-ineﬃcient incandescent bulbs to light-

emitting diodes (LEDs) is almost complete in the European

sector; organic light-emitting diodes (OLEDs) represent an

emerging technology that is currently being deployed

worldwide.³

In 2015, the United Nations Member States adopted the 2030

Agenda for Sustainable Development, which identied 17

sustainable development goals (SDGs).¹Many of these SDGs

pose challenges that the materials science community must

address with better, smarter and more sustainable materials. In

OLEDs are devices in which light generation occurs in an

electroluminescent organic layer and the phosphorescent

organic light-emitting diodes are of particular interest.⁴In

OLEDs, electrons and holes are injected at the cathode and

anode, respectively, and combine to form excitons in either

a singlet or a triplet state in a 1 : 3 ratio; exciton decay generates

light through spontaneous emission. Typical organic molecules

and semiconducting polymers used in OLEDs are uorescent

and the decay of the triplet exciton is spin forbidden. As a result,

uorescent OLEDs can only harvest the singlet excitons giving

a limit to the internal quantum eﬃciency of 25%. In phospho-

rescent organic light-emitting diodes, the triplet excitons may

also be harvested by adding a phosphorescent sensitizer, which

facilitates the radiative decay of both singlet and triplet states.

The commonest strategy is the addition of a heavy transition

particular, SDG

7 “Aﬀordable and clean energy” places

emphasis not only upon the generation of energy in clean and

sustainable manners but also upon the need for new eﬃcient

technologies associated with energy usage. In 2017, the U.S.

^aDepartment of Chemistry, University of Basel, BPR 1096, Mattenstrasse 24a, CH-4058

Basel, Switzerland. E-mail: catherine.housecro@unibas.ch

^bInstituto de Ciencia Molecular, Universidad de Valencia, 46980 Paterna, Valencia,

Spain. E-mail: michele.sessolo@uv.es; enrique.orti@uv.es

† Electronic supplementary information (ESI) available: Experimental procedures;

Fig. S1–S21: NMR and ESI-MS spectra; cyclic voltammograms (Fig. S22–S29);

computational results (Table S1 and Fig. S30); absorption and emission spectra

and photophysical properties (Tables S2 and S3 and Fig. S31–S36); LEC device:

Fig. S37; device performance parameters (Table S4 and Fig. S38–S41). CCDC

1978436–1978442. For ESI and crystallographic data in CIF or other electronic

format see DOI: 10.1039/d0ra03824e

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metal complex in which the large spin–orbit coupling facilitates

the intersystem crossing, a process which mixes the singlet and

triplet character of excited states.³

Although OLEDs can have high conversion eﬃciencies, they

also have a number of disadvantages in terms of SDG 7. Typical

sensitizers are iridium(^III) complexes, which impacts on

sustainability as iridium is one of the rarest elements, present

in sub-ppb quantities in the Earth's crust.⁵Furthermore, OLEDS

are dependent on the work function of the electrodes and

electron injection requires highly electropositive metals, which

in turn predicates protection from air and water in fabrication

and operation.³An alternative approach is the light-emitting

electrochemical cell (LEC), which uses mobile charged species

as electroluminescent material. The ionic character of the light-

emitting species removes the dependence on the work function

of the electrode and makes the device architecture simpler. The

simplicity of LECs and their processing from solution has

motivated that the research activity on LECs has considerably

increased in the last years.^6–10Much interest has centred upon

LECs incorporating ionic transition-metal complexes (iTMCs),

which are inherently more eﬃcient than those based upon

charged light-emitting polymers as the emission is based on

phosphorescence rather than on uorescence.^11–13The colour-

tuning of iTMC-based LECs is well-established in the case of

iridium complexes, although the use of this element makes

again the sustainability questionable. We^14–28and others^29–38

have investigated copper(I) complexes of the type

[Cu(P^P)(N^N)]⁺(P^P ¼ chelating diphosphane, N^N ¼ diimine

or 1,10-phenanthroline) as alternative electroluminescent

materials for LECs. These complexes are of particular interest as

they exhibit the phenomenon of thermally activated delayed

uorescence (TADF), in which the rst excited singlet (S₁) and

triplet (T₁) states are strongly coupled, allowing intersystem

crossing between the two levels without the need for a heavy

metal. The energy gap between the singlet and triplet states is

Scheme 1 Structures of ligands with ring and atom labels for NMR

spectroscopic assignments.

6,6⁰-Me₂bpy (which led to LECs with high luminance values) or

4,4⁰-Me₂bpy; (c) an unsymmetrical bpy with three electron-

donating Me groups, 4,5,6-Me₃bpy; and (d) the replacement of

the bpy by a 1,10-phenanthroline, here with 2-Etphen instead of

6-Etbpy (which led to LECs with long device lifetimes).

The results of this study contribute to the rational design of

emissive [Cu(P^P)(N^N)]⁺complexes and reiterate the sensi-

tivity of these systems to minor electronic and steric changes

induced by ligand substitution patterns.

Results and discussion

Ligand synthesis

small, allowing reverse intersystem crossing (T₁/ S₁) to occur. The ligands 6-tBubpy, 4,5,6-Me₃bpy and 2-Etphen (Scheme 1)

This eﬀectively allows the harvesting of both singlet and triplet were selected on the basis of their various steric and electronic

states through repopulation of the singlet state and uores- eﬀects to compare with previous studies on 6- and 6,6⁰-

cence to the ground state.^39–43

substituted bpy ligands. Transition metal complexes bearing

Although a wide variety of diﬀerent substituents in the N^N the 6-tBubpy ligand have been widely investigated,⁴⁴in partic-

chelating ligand in [Cu(P^P)(N^N)]⁺complexes has been ular because of insertions into a C–H bond of the tert-butyl

studied so far, we note that relatively simple substitution, group to form cyclometallated complexes.^45–506-tBubpy has

especially with alkyl groups, oen gives the most promising previously been prepared from the reaction of bpy with tert-

photophysical and device properties. Encouraged by our results butyllithium⁴⁴or from tert-butylcarbonitrile.^45–50However, we

with [Cu(P^P)(N^N)][PF₆] complexes, where P^P is either POP found it more convenient to use a modied Negishi procedure

bis(2-(diphenylphosphano)phenyl)ether; IUPAC PIN [oxydi(2,1- introduced by Hanan,⁵¹and the reaction of 2-tert-butyl-6-

phenylene)]bis(diphenylphosphane)

bis(diphenylphosphano)-9,9-dimethylxanthene; IUPAC PIN of [Pd(PPh₃)₄] gave 6-tBubpy in 51% yield. A single report on the

(9,9-dimethyl-9H-xanthene-4,5-diyl)(bis(diphenylphosphane))

synthesis of 4,5,6-Me₃bpy is found in the patent literature,⁵²but

or

xantphos

(4,5- chloropyridine with 2-pyridinylzinc bromide in the presence

and N^N is a 6-methyl, 6-ethyl- or 6,6⁰-dimethyl-substituted we also prepared this compound from the Negishi reaction of 2-

bpy,^24,27we decided to investigate the eﬀect of further alkyl chloro-4,5,6-trimethylpyridine and 2-pyridinylzinc bromide in

substitution patterns. In this study, we focus on the four 4% yield. The low yield is most likely due to the fact that the

following modications (Scheme 1): (a) a diﬀerent alkyl reaction mixture was only stirred at room temperature, whereas

substituent in the 6-position of the bpy ligand, in this case 6- for the synthesis of 6-tBubpy microwave conditions were

tBubpy instead of 6-Mebpy or 6-Etbpy (because complexes of the employed (see ESI† for details). It can be expected that micro-

latter led to LECs with long device lifetimes); (b) disubstitution wave conditions should lead to a similar yield for 4,5,6-Me₃bpy

in diﬀerent positions in the bpy, here 5,5⁰-Me₂bpy instead of as for 6-tBubpy. It has been reported that 2-Etphen can be

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prepared in low yield from the direct reaction of 2-ethyllithium

with 1,10-phenanthroline⁵³or in 76% yield from the reaction of

8-aminoquinoline with 2-methyl-3-ethoxycyclobutanone.⁵⁴In

our hands, the reaction of 2-ethyllithium with 1,10-phenan-

throline gave 2-Etphen in 5% yield. Each of the ligands was fully

characterized by routine spectroscopic and analytical methods

(see the ESI†).

Synthesis and characterization of [Cu(P^P)(N^N)][PF₆]

complexes

The synthesis of the [Cu(P^P)(N^N)][PF₆] complexes was carried

out by addition of the respective P^P and N^N ligand (Scheme 1)

to a solution of [Cu(MeCN)₄][PF₆] in CH₂Cl₂. The synthetic

procedure for the POP-containing complexes used the estab-

lished sequential addition of the two ligands, where the bpy

ligand was added aer stirring POP and [Cu(MeCN)₄][PF₆] for

two hours.²⁸For the preparation of the [Cu(xantphos)(N^N)]

[PF₆] complexes, the two ligands were simultaneously added.

The compounds [Cu(POP/xantphos)(N^N)][PF₆] with N^N ¼

5,5⁰-Me₂bpy, 4,5,6-Me₃bpy, 6-tBubpy and 2-Etphen were isolated

as yellow solids in yields of 69 to 96%. All compounds were

characterized by elemental analysis, mass spectrometry and

multinuclear NMR spectroscopies including 2D methods to

assign the spectra. Detailed procedures and characterization

can be found in the ESI,† together with ¹H and ³¹P NMR spectra

of the compounds (Fig. S1–S12†) as well as mass spectra

(Fig. S13–S21†). In the ³¹P NMR spectra ((CD₃)₂CO or CD₂Cl₂),

all compounds give broad signals between d ꢀ11 and ꢀ15 ppm

(Fig. S1†).

Structural characterization

Layer crystallisation (diﬀusion of Et₂O into CH₂Cl₂solutions of the

respective compound) yielded X-ray quality crystals of [Cu(POP)-

(5,5⁰-Me₂bpy)][PF₆], [Cu(xantphos)(5,5⁰-Me₂bpy)][PF₆], [Cu(POP)(6-

tBubpy)][PF₆], [Cu(POP)(4,5,6-Me₃bpy)][PF₆], [Cu(xantphos)(4,5,6-

Me₃bpy)][PF₆], [Cu(POP)(2-Etphen)][PF₆] and [Cu(xantphos)-

(2-Etphen)][PF₆]. The geometry of the complexes is tetrahedral, with

varying degrees of distortion; see Table 1 for a summary of the most

important structural parameters dening the coordination sphere

of copper, and Fig. 1–4 for illustrations of the complex cations. The

planes dened through P–Cu–P and N–Cu–N of the P^P and N^N

chelating ligand range from being almost orthogonal in [Cu(x-

antphos)(4,5,6-Me₃bpy)]⁺, [Cu(POP)(4,5,6-Me₃bpy)]⁺and [Cu(POP)(2-

Etphen)]⁺(89.77, 88.44 and 88.05^ꢁ, respectively) to exhibiting

a greater distortion in [Cu(POP)(6-tBubpy)]⁺(74.66^ꢁ). The Cu–P

distances are relatively similar and in the expected range, from

˚

2.2396(4) to 2.3004(6) A. The Cu–N distances are found between

˚

2.0421(11) and 2.134(2) A, with one exception: in the [Cu(POP)(t-

Bubpy)]⁺cation (Fig. 2a), one Cu–N bond is within the normal range

˚

with 2.0648(18) A, however the Cu–N bond to the pyridine ring that

˚

bears the bulky CMe₃group is elongated to 2.3949(19) A, which is

˚

around 0.26 A longer than the second longest Cu–N distance of this

complex series. Such a lengthening of the Cu–N bond has not been

observed for other [Cu(POP/xantphos)(N^N)][PF₆] complexes (survey

of 179 structures containing chelating bisphosphane with dibenzyl

ether backbone and any N^N chelating residue comprising two

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be expected to be considerably weaker, which may have an impact

on the photophysical properties (see later discussion).

Another relevant parameter is the N–C–C–N torsion angle of the

two pyridine rings in the bpy or 2-Etphen ligand; once again, the

[Cu(POP)(6-tBubpy)]⁺cation is an outlier with a torsion of

ꢀ28.8(3)^ꢁ, while the other complexes exhibit angles from as low as

ꢀ1.4(2)^ꢁ

for

[Cu(POP)(2-Etphen)]⁺

to

15.7(2)^ꢁ

for

[Cu(xantphos)(5,5⁰-Me₂bpy)]⁺. The extreme torsion angle of the bpy

ligand together with the elongation of the Cu–N bond for the tBu-

bearing pyridyl ring in [Cu(POP)(6-tBubpy)]⁺might be the best

option to accommodate the bulky tBu group while maintaining

the tetracoordinated geometry at the copper centre. A possible

correlation between planar coordination of the rings within the

N^N chelating ligand and emissive properties of the complex will

be discussed in the photophysical section of the paper.

Fig. 1 (a) Structure of the [Cu(POP)(5,5⁰-Me₂bpy)]⁺cation with ellip-

soids plotted at 50% probability, H atoms are omitted for clarity. Face

to face p-stacking between the phenyl ring containing C15 and the

ring of the POP backbone at P2 (angle between the ring planes 13.94^ꢁ,

centroid–centroid distance 3.641 A˚). (b) Structure of the

[Cu(xantphos)(5,5⁰-Me₂bpy)]⁺cation with ellipsoids plotted at 50%

probability, H atoms are omitted for clarity. Weak p-interaction

between two of the phenyl rings on each P atom (involving C23 and

C46, respectively) (angle between the ring planes 21.65^ꢁ, centroid–

centroid distance 3.843 A˚) and no intramolecular p-interaction

involving the xanthene backbone.

The complexes show eﬀective intramolecular p–p interac-

tions between the aromatic rings,⁵⁷with relatively low oﬀsets

between the stacking rings and centroid–centroid distances

˚

between 3.641 A and 3.924 A. For the complexes with POP, a p–

p interaction takes place between a phenyl ring at a phosphorus

atom and a ring of the backbone of the bisphosphane ligand

(Fig. 1a, 2a, b and 3b). In the case of the xantphos complexes,

the interaction occurs between two phenyl rings on diﬀerent

phosphorus atoms, and does not involve the xanthene back-

bone (Fig. 1b, 3a and 4).

Electrochemistry

The electrochemical behaviour of the eight heteroleptic

[Cu(P^P)(N^N)][PF₆] complexes was investigated using cyclic

voltammetry (CV) and their oxidation potentials E_1/2are

compared to those measured for complexes with N^N ¼ bpy, 6-

Mebpy and 6,6⁰-Me₂bpy (Table 2 and Fig. S22–S29†). The rst

quasi-reversible oxidation in the positive mode is due to the

oxidation of Cu⁺to Cu²⁺, whereas the second oxidation peak is

Fig. 2 (a) Structure of the [Cu(POP)(6-tBubpy)]⁺cation with ellipsoids

plotted at 50% probability, H atoms are omitted. There is a p-stacking

interaction between the phenyl ring containing C45 and the ring of the

POP backbone attached to P18 (angle between the ring planes 24.27^ꢁ,

centroid–centroid distance 3.889 A˚). (b) Structure of the

[Cu(POP)(4,5,6-Me₃bpy)]⁺cation with ellipsoids plotted at 50% prob-

ability. H atoms and solvent molecules are omitted. p-Stacking

between the phenyl ring containing C35 and the ring of the POP

backbone connected to P2 (angle between the ring planes 13.52^ꢁ,

centroid–centroid distance 3.710 A˚).

connected pyridyl rings as substructure in the Cambridge Structural

Database (CSD),⁵⁵online search conducted 22.03.2020), not even for

[Cu(POP/xantphos)(N^N)][PF₆] complexes with large aromatic

substituents next to the coordinating nitrogen such as for example

pyrene.²³The sum of the ionic radius of Cu⁺(coordination number

Fig. 3 (a) Structure of the [Cu(xantphos)(4,5,6-Me₃bpy)]⁺cation with

ellipsoids plotted at 30% probability. H atoms and solvent molecules

are omitted for clarity. p-Stacking between the phenyl ring containing

C25 and the phenyl ring bonded to P2 (angle between the ring planes

7.77^ꢁ, centroid–centroid distance 3.754 A˚). (b) Structure of the

[Cu(POP)(2-Etphen)]⁺cation with ellipsoids plotted at 50% probability,

H atoms are omitted for clarity. Oﬀset p-interaction between the

phenyl ring containing C21 and the ring of the POP backbone at P2

(angle between the ring planes 10.93^ꢁ, centroid–centroid distance

3.733 A˚).

˚

4) of 0.74 A and the van der Waals radius of nitrogen (1.6 A) is 2.34

˚

56

˚

A. Although the elongated Cu–N distance of 2.3949(19) A in the

[Cu(POP)(6-tBubpy)]⁺cation is found to be slightly longer than that,

˚

if we consider the van der Waals radius of 1.4 A for Cu, the distance

is still in a range where interaction can be assumed to take place.

However, the coordination of this nitrogen atom to the copper could

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attributed to non-reversible oxidation of the phosphane ligand. TZVP) level in the presence of the solvent (CH₂Cl₂) using

The oxidation potentials for the copper(^I) complexes with an a continuum model. The values obtained for the most repre-

alkyl group next to one coordinating nitrogen atom of the bpy sentative geometrical parameters dening the coordination

ligand are shied to slightly higher potentials (+0.77 to +0.87 V) sphere of the copper atom are summarized in Table S1.† The

compared to complexes with unmodied bpy (+0.72 V for calculations reproduce the main features discussed above for

[Cu(POP)(bpy)][PF₆] and +0.76 V for [Cu(xantphos)(bpy)][PF₆]).²¹the X-ray geometries. The atoms forming the coordination

This eﬀect of slightly impeded copper oxidation has also been sphere of Cu(^I) dene a distorted tetrahedral structure with

observed

complexes

for

the

analogous

alkyl-substituted angles between the P–Cu–P and N–Cu–N planes in the range of

(+0.85

V),²¹79.3–86.9^ꢁin good agreement with those observed experimen-

[Cu(xantphos)(6-Mebpy)][PF₆]

[Cu(POP)(6,6⁰-Me₂bpy)][PF₆] (+0.92 V)⁵⁸and [Cu(xantphos)(6,6⁰- tally (Table 1). This angle can be used as an indication of the

Me₂bpy)][PF₆] (+0.89 V),²¹and for complexes with halogen deviation from the orthogonal disposition of the P^P and N^N

substituents adjacent to one or both nitrogen atoms of the bpy ligands in the tetrahedral structure. Calculations also reproduce

ligand.⁵⁹The eﬀect is mainly attributed to the stabilization of the p–p interaction between a phenyl ring of a phosphorus

the tetrahedral complex geometry, independent from the nature atom and a ring of the phosphane ligand in the POP complexes

˚

of the substituent in 6-position, which results in a stabilization (centroid–centroid distances around 3.65 A), and the less eﬃ-

of the copper(^I) state and consequently higher Cu⁺/Cu²⁺oxida- cient p–p interactions between phenyl rings on diﬀerent

tion potentials. This is further supported by the observation phosphorus atoms that take place in the xantphos complexes.

that the oxidation potentials for the complexes [Cu(POP)(5,5⁰- The xanthene unit in the xantphos complexes adopts a “bowl-

Me₂bpy)][PF₆] (+0.70 V) and [Cu(xantphos)(5,5-Me₂bpy)][PF₆] like” conformation in which the 6-alkyl substituent in the bpy

(+0.75 V), in which the geometric stabilization eﬀect is much complexes and the 2-ethyl group in the phen complex lie.

less eﬀective, are even slightly lower than for the respective

The calculated Cu–P and Cu–N distances are in the ranges of

˚

complexes with unsubstituted bpy, which means that the 2.25–2.29 and 2.07–2.11 A (Table S1†), respectively, which is in

oxidation from Cu⁺to Cu²⁺is facilitated. The oxidation good accord with the experimental values (Table 1). In the case

processes are quasi-reversible and dened reduction processes of the [Cu(POP)(6-tBubpy)]⁺cation, calculations reproduce the

were not observed for any of the complexes, which is typical for unusual elongation of the Cu–N bond adjacent to the tert-butyl

˚

compounds with simple alkyl substituted N^N chelating substituent, although the calculated bond length (2.21 A) is not

˚

ligands, although this has been observed for complexes with as long as the experimentally determined X-ray value (2.39 A).

CF₃substituents.²¹

This elongation is also observed in the [Cu(xantphos)(6-

tBubpy)]⁺complex (Table S1†), for which unfortunately no X-ray

structure could be obtained. This indicates that the observed

elongation is a general consequence of the presence of the bulky

6-tBubpy ligand and not a singularity of the interaction of it

with POP. Therefore, a similar weakening of the Cu–N coordi-

nation should be expected for complexes bearing this ligand.

DFT calculations

The

geometries

of

the

new

[Cu(POP)(N^N)]⁺

and [Cu(xantphos)(N^N)]⁺cations in their electronic ground

state (S₀) were optimized at the DFT B3LYP-D3/(def2-SVP + def2-

Table 2 Cyclic voltammetric data for [Cu(P^P)(N^N)][PF₆] complexes

referenced to internal Fc/Fc⁺¼ 0.0 V; CH₂Cl₂(dry) solutions with

[ⁿBu₄N][PF₆] as supporting electrolyte and scan rate of 0.1 V s^ꢀ1

Processes are quasi-reversible

.

Complex cation

E_1/2/V

(E_pcꢀ E_pa/mV)

[Cu(POP)(bpy)]^+a

+0.72

+0.76

+0.69

+0.85

+0.92

+0.89

+0.70

+0.75

+0.83

+0.87

+0.77

+0.82

+0.80

+0.86

110

125

100

183

145

127

131

136

99

[Cu(xantphos)(bpy)]^+a

[Cu(POP)(6-Mebpy)]^+b

[Cu(xantphos)(6-Mebpy)]^+a

[Cu(POP)(6,6⁰-Me₂bpy)]^+c

[Cu(xantphos)(6,6⁰-Me₂bpy)]^+a

[Cu(POP)(5,5⁰-Me₂bpy)]⁺

[Cu(xantphos)(5,5⁰-Me₂bpy)]⁺

[Cu(POP)(6-tBubpy)]⁺

[Cu(xantphos)(6-tBubpy)]⁺

[Cu(POP)(4,5,6-Me₃bpy)]⁺

[Cu(xantphos)(4,5,6-Me₃bpy)]⁺

[Cu(POP)(2-Etphen)]⁺

114

122

92

Fig. 4 Structure of the [Cu(xantphos)(2-Etphen)]⁺cation with ellip-

soids plotted at 50% probability, H atoms are omitted for clarity.

Negligible p-interaction between two of the phenyl rings on the

diﬀerent phosphorus atoms (involving C34 and C39, respectively)

(angle between the ring planes 27.10^ꢁ, centroid–centroid distance

3.924 A˚), but none involving the xanthene backbone.

[Cu(xantphos)(2-Etphen)]⁺

91

a

Data from ref. 21. ^bData from ref. 27. ^cData from ref. 58.

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The calculations do not reproduce the higher N–C–C–N Me₂bpy)]⁺complexes bearing no substituent in 6,6⁰-positions,

torsion observed experimentally for the bpy ligand of for which the angle between the N–Cu–N and P–Cu–P planes

[Cu(POP)(6-tBubpy)]⁺(28.8(3)^ꢁ). The computed value of 12.0^ꢁis decreases from 79.5 and 86.0^ꢁin S₀to 59.0 and 57.0^ꢁin T₁,

similar to those of the other members of the series, ranging respectively, similarly to what happens for the unsubstituted

from 12.0 to 15.8^ꢁ, with the exception of the complexes with [Cu(POP)(bpy)]⁺and [Cu(xantphos)(bpy)]⁺complexes (Table

ligand 2-Etphen, where the higher rigidity of the phenanthro- S1†). As discussed previously,^17,21the distortion from the tetra-

line moiety leads to torsion angles of 2.1^ꢁ(POP complex) and hedral structure is limited by the introduction of alkyl groups in

0.5^ꢁ(xantphos complex). The discrepancies in the N–C–C–N the 6-positions of the bpy ligand, because substituents in these

torsional angle are attributed to the eﬀects of packing and positions impede the movement of the ligands towards more

interaction with neighbouring molecules that are not consid- planar dispositions. For instance, in the [Cu(POP)(6-tBubpy)]⁺

ered in the calculations.

and [Cu(xantphos)(6-tBubpy)]⁺complexes, the angle between

The geometry of the rst triplet excited state (T₁) was also the N–Cu–N and P–Cu–P planes decreases in a lower degree

optimized at the UB3LYP level for all the complexes. In the T₁from 80.6 and 85.6^ꢁin S₀to 68.4 and 76.7^ꢁin T₁, respectively.

˚

state, the Cu–P bonds are lengthened to 2.30–2.40 A and the Cu– Thus, the presence of alkyl groups in the 6-positions aﬀects the

˚

N bonds are shortened to 1.95–2.00 A. As a consequence, the P– degree of geometrical relaxation of the T₁excited state and

Cu–P angles are as small as 103–104^ꢁin POP complexes (105– thereby its energy position relative to S₀.

106^ꢁin xantphos complexes) and the N–Cu–N angles are

The energy and topology calculated for the frontier molec-

widened to 83–84^ꢁ. Although an elongation of the Cu–N bond in ular orbitals of the new [Cu(POP)(N^N)]⁺complexes are dis-

complexes bearing the 6-tBubpy ligand is not observed in the T₁played in Fig. 5 (see Fig. S30† for the [Cu(xantphos)(N^N)]⁺

state, it must be remembered that the ground-state calculations complexes). The atomic orbital composition of the molecular

underestimate the elongation observed in the crystal geometry. orbitals remains almost unchanged along both series and only

Indeed, the most relevant geometrical change on going from S₀the contour plots computed for the reference compound

to T₁is the reduction of the angle formed by the N–Cu–N and P– [Cu(POP)(bpy)]⁺and the [Cu(POP)(2-Etphen)]⁺complex are

Cu–P planes (Table S1†). As discussed below, upon excitation to drawn in Fig. 5 as representative examples. As previously

T₁the metal atom is partially oxidized and tends to adopt the described,^17,19,21,24the highest-occupied (HOMO) and the lowest-

square-planar coordination sphere expected for four-coordinate unoccupied molecular orbital (LUMO) in these kind of

d⁹Cu(II) complexes, instead of the tetrahedral conformation complexes are mostly orthogonal. Whereas the HOMO is mainly

typical of d¹⁰Cu(I) complexes. This eﬀect is more pronounced centred on the metal with small contribution from the vicinal

for the [Cu(POP)(5,5⁰-Me₂bpy)]⁺and [Cu(xantphos)(5,5⁰- phosphorus atoms, the LUMO fully resides on the N^N ligand.

Fig. 5 Energy diagram showing the energies calculated for the HOMOꢀ1, HOMO, LUMO and LUMO+1 of the [Cu(POP)(N^N)]⁺complexes. The

HOMO–LUMO energy gap is also quoted. Isovalue contour plots (ꢂ0.03 a.u.) are shown for [Cu(POP)(bpy)]⁺and [Cu(POP)(2-Etphen)]⁺.

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As shown in Fig. 5, the HOMO features close energies in a small

A closer look at the absorption MLCT bands of the complexes

range between ꢀ5.89 eV for [Cu(POP)(5,5⁰-Me₂bpy)]⁺and (Fig. S32 and S33† for POP and xantphos complexes, respec-

ꢀ6.03 eV for [Cu(POP)(6-tBubpy)]⁺, which is in good correlation tively) shows that the highest-energy MLCT bands belong to the

with the experimental values of the rst oxidation potential complexes with 4,5,6-Me₃bpy, 6,6⁰-Me₂bpy, 5,5⁰-Me₂bpy and 6-

comprised between +0.70 V for [Cu(POP)(5,5⁰-Me₂bpy)]⁺and tBubpy, whereas the ones with bpy and 2-Etphen are at lower

+0.83 eV for [Cu(POP)(6-tBubpy)]⁺(Table 2). The electron- energies. As alkyl groups have electron donating character and

donating character of the methyl groups attached to the bpy lead to a destabilization of the LUMO, which is mainly located

ligand signicantly destabilizes the LUMO of [Cu(POP)(5,5⁰- on the N^N chelating ligand, a larger HOMO–LUMO gap is

Me₂bpy)]⁺(ꢀ2.28 eV) and [Cu(POP)(4,5,6-Me₃bpy)]⁺(ꢀ2.32 eV) obtained for the complexes with alkyl-substituted diimine

with respect to the unsubstituted complex [Cu(POP)(bpy)]⁺ligands. This then results in higher energies (shorter wave-

(ꢀ2.46 eV). In contrast, the complexes incorporating the 2- lengths) of the absorbed light: the more donating or larger

Etphen and 6-tBubpy ligands feature energies very close to that number of alkyl groups at the bpy, the more blue-shied the

of the reference complex. For the complexes with N^N ¼ 2- absorption. The lower energy of the MLCT bands of the

Etphen, the LUMO+1, also spreading over the phenanthroline complexes with 2-Etphen can be explained by the extension of

ligand, is largely stabilized and lies close to the LUMO. In these the conjugated p-system. It is worth mentioning that the

complexes the energy diﬀerence between the HOMO and complex solutions with 6-tBubpy exhibit less intense MLCT

LUMO+1 is close to the HOMO–LUMO energy gap, and excited bands and both the solutions as well as the solid compounds

states described by the HOMO / LUMO+1 monoexcitation are considerably paler by eye.

would appear close in energy to the expected HOMO / LUMO

The rst singlet and triplet excited states were computed for

transition and could become competitive in the emission all the complexes at the optimized geometry of the ground

process.

electronic state using TD-DFT calculations. The energies of the

rst singlet (S₁) and two triplet (T₁and T₂) excited states are

summarized in Table 3 together with those calculated for the

reference complexes [Cu(POP)(bpy)]⁺and [Cu(xantphos)(bpy)]⁺.

In all the cases, the lowest-energy singlet S₁results from the

HOMO / LUMO monoexcitation, which implies an electron

transfer from the Cu(P^P) environment of the complex to the

N^N ligand, and therefore supports the MLCT character of S₁

and thereby of the lowest-energy band around 400 nm (3.0 eV).

The transition S₀/ S₁is calculated at slightly higher energies

compared with the reference complexes. This is in good agree-

ment with the absorption spectra showing the MLCT maxima

for all complexes displaced to bluer wavelengths with respect to

those of the reference complexes. The oscillator strengths

calculated for the S₀/ S₁transitions are similar for all the

complexes except for the complexes with the 6-tBubpy ligand,

for which the signicantly smaller oscillator strengths are in

good agreement with the smaller intensities of the experimen-

tally observed MLCT bands (Fig. S32 and S33†).

Photophysical properties and excited states

The complexes were subjected to detailed photophysical studies

and the results compared to those of the previously studied

analogous complexes with bpy, 6-Mebpy and 6,6⁰-Me₂bpy. The

absorption spectra of CH₂Cl₂solutions of the xantphos

complexes are presented in Fig. 6, and the spectra for the

analogous POP complexes in Fig. S31.† The intense, high-

energy bands at wavelengths shorter than 330 nm are typically

a result of ligand-based p / p* transitions. In contrast to the

complexes with bpy N^N ligands, those with 2-Etphen show

a strong band at 272 nm, but lack the band at around 283 nm

and the typically observed shoulders in the area between 300

and 320 nm. The broad and less intense bands between 330 and

450 nm are assigned to metal-to-ligand charge transfer (MLCT)

transitions and are responsible for the yellow colour of the

complexes.

Exciting the complexes in solution (CH₂Cl₂, 2.5 ꢃ 10^ꢀ5mol

dm^ꢀ3) with wavelengths in the area of the MLCT band leads to

green-yellow to orange emission, with maxima between 582 and

650 nm (Fig. 7, S34† and Table 4). All bands are structured with

two emission maxima, which is a typical feature of these

complexes.^21,24,32,60

For all the complexes, any alkyl substitution in the bipyridine

ligand leads to a blue shi of the emission with respect to

[Cu(POP/xantphos)(bpy)][PF₆], and also the solution emission of

max

[Cu(POP)(2-Etphen)][PF₆] (l

¼ 597, 629 nm) is blue shied

em

max

em

with respect to [Cu(POP)(phen)][PF₆] (l

¼ 648 nm).²⁸For the

[Cu(xantphos)(N^N)][PF₆] complex emission spectra in solu-

tion, the most hypsochromically shied are the ones with 2-

Etphen and 4,5,6-Me₃bpy, where the wavelengths and proles of

the emission bands are very similar (Fig. 7). Slightly less blue-

shied and also very similar are the emission proles of the

complexes with 6-Mebpy and 6,6⁰-Me₂bpy, then again very

similar those with 6-tBubpy and 5,5⁰-Me₂bpy and the most

Fig. 6 Solution absorption spectra of the [Cu(xantphos)(N^N)][PF₆]

complexes (CH₂Cl₂, 2.5 ꢃ 10^ꢀ5mol dm^ꢀ3).

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Table 3 Vertical excitation energies (E) calculated at the TD-DFT

B3LYP/(def2-SVP + def2-TZVP) level for the lowest singlet (S₁) and

triplet (T₁and T₂) excited states of complexes [Cu(P^P)(N^N)]⁺in

CH₂Cl₂solution. S₀/ S₁oscillator strengths (f) are given within

parentheses

POP backbone, two methyl groups (or other substituents) next

to the nitrogen atoms of the bpy are necessary to eﬃciently

stabilize the tetrahedral complex geometry, whereas for the

more rigid xantphos a single substituent on one side of the N^N

chelating ligand is suﬃcient and the total number of alkyl

groups (three in 4,5,6-Me₃bpy versus two in 6,6⁰-Me₂bpy) has

a stronger impact on the HOMO–LUMO gap than their position

on the N^N chelating ligand. The high sensitivity of these type

of complexes towards number, position and type of alkyl groups

on the ligands is also reected in the quantum yield and life-

time values.

Deaeration by 20 minute argon ow through the complex

solutions leads to increased PLQY and lifetime values (Table 4).

By far, the best values are obtained for the complexes with 6,6⁰-

Me₂bpy, which was to be expected because this ligand has the

highest steric hindrance and works best in stabilizing the

tetrahedral complex geometry. Following up are the complexes

with 2-Etphen. It is interesting to see that the complexes with

4,5,6-Me₃bpy also give better emissive parameters than with 6-

Mebpy, especially for the xantphos compounds: 1.8 vs. 3.3%

PLQY and 784 vs. 1595 ns lifetime on going from 6-Mebpy to

4,5,6-Me₃bpy (deaerated values). Another interesting observa-

tion is the unexpected decrease in the emissive properties on

going from the complexes with 6-Mebpy to 6-tBubpy, for the

xantphos complexes the PLQY is reduced from 1.8 to 0.5% and

the lifetime from 784 to 93 ns (deaerated values).

Complex cation

S₁E (eV) (f)

T₁E (eV)

T₂E (eV)

[Cu(POP)(bpy)]⁺

2.79 (0.08)

2.82 (0.10)

2.93 (0.08)

2.93 (0.10)

2.81 (0.03)

2.84 (0.06)

2.93 (0.08)

2.94 (0.10)

2.86 (0.09)

2.86 (0.11)

2.54

2.57

2.66

2.67

2.66

2.67

2.71

2.61

2.60

2.92

2.74

3.01

2.82

2.91

2.84

3.00

2.93

2.69

2.68

[Cu(xantphos)(bpy)]⁺

[Cu(POP)(5,5⁰-Me₂bpy)]⁺

[Cu(xantphos)(5,5⁰-Me₂bpy)]⁺

[Cu(POP)(6-tBubpy)]⁺

[Cu(xantphos)(6-tBubpy)]⁺

[Cu(POP)(4,5,6-Me₃bpy)]⁺

[Cu(xantphos)(4,5,6-Me₃bpy)]⁺

[Cu(POP)(2-Etphen)]⁺

[Cu(xantphos)(2-Etphen)]⁺

Table 3 summarizes the vertical excitation energies calculated

for the T₁and T₂states at the minimum-energy geometry of S₀.

Although those values should not be assimilated to emission

energies, they follow the trend observed for the maxima of the

emission spectra, being in all cases displaced to higher energies

with respect to the reference complexes. Similarly to S₁, the T₁state

mainly originates from the HOMO / LUMO monoexcitation,

which implies an electron transfer from the Cu(P^P) environment

to the N^N ligand and therefore determines a partial oxidation of

the Cu(^I) atom upon excitation to T₁. As discussed above, this

partial oxidation leads to a attening of the tetrahedral coordina-

tion that is hindered by the attachment of alkyl groups to the 6-

positions of the bpy ligand. The relaxation of the T₁triplet is

therefore more restricted for the complexes bearing the 4,5,6-

Me₃bpy and 6-tBubpy ligands (Table S1†) supporting the more

blue-shied emission energies experimentally observed in the

former case for both xantphos and POP complexes (Table 4). The 6-

tBubpy ligand produces a less signicant eﬀect because the bulky

tert-butyl group provokes other changes in the geometry of the

complex such as the increase of the Cu–N distances. In agreement

with the experimental results, the emission energies calculated at

the optimized geometry of T₁are signicantly higher

for [Cu(xantphos)(4,5,6-Me₃bpy)]⁺(1.62 eV) and [Cu(xantphos)(2-

Etphen)]⁺(1.68 eV) than for [Cu(xantphos)(6-tBubpy)]⁺(1.53 eV),

and a similar trend is found for the equivalent POP complexes. It

should be mentioned that for the complexes with 2-Etphen the T₂

state is close in energy to the T₁state, as expected from the MO

study, but the HOMO / LUMO T₁state remains to be the lowest

state upon relaxation and then phosphorescent emission takes

place from the same MLCT triplet in all the complexes.

Fig.

7

Normalized

solution

emission

spectra

of

the [Cu(xantphos)(N^N)][PF₆] complexes (CH₂Cl₂, 2.5 ꢃ 10^ꢀ5mol

dm^ꢀ3, for complexes with 6-tBubpy 5 ꢃ 10^ꢀ5mol dm^ꢀ3). For l_excsee

Table 4.

bathochromic emission is by the complex with bpy. In contrast

to the absorption spectra, for the emissive properties the exact

position of the alkyl group on the bpy N^N ligand appears to

play a more crucial role. Substituents in position next to the

coordinating nitrogen atom(s) are proven to be especially

benecial and lead to more blue-shied emission, higher

photoluminescence quantum yield (PLQY) values and longer

excited-state lifetimes s (Table 4). This is a typical observation

for tetrahedral [Cu(N^N)(P^P)]⁺complexes⁶¹and can be

explained by the increased rigidity of the complex and therefore

reduced Jahn–Teller distortion in solution.

In the case of POP (Fig. S34† and Table 4), the situation is

diﬀerent in that 6,6⁰-Me₂bpy gives the complex with consider-

ably the most hypsochromic emission. Then, as in the xantphos

analogues, the complexes with 2-Etphen and 4,5,6-Me₃bpy are

next and give similar emission bands, then 6-Mebpy, and

aerwards 6-tBubpy and 5,5⁰-Me₂bpy with similar maxima.

Finally, the complex with bpy is again the most red-shied one.

A possible explanation is that in the case of the more exible

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Table 4 Emission maxima, photoluminescence quantum yields (PLQY) and lifetimes (s) for [Cu(P^P)(N^N)][PF₆] complexes

Thin lm^c

CH₂Cl₂solution^a

l_exc[nm] l^m_em^ax[nm] deaerated)^b[%]

Powder^b

PLQY (non-deaerated/ s (non-deaerated/

max

Complex cation

deaerated)^b[ns]

l

[nm] PLQY [%] s [ms] PLQY [%]

em

[Cu(POP)(bpy)]^+d

390

378

379

372

379

390

390*

380

618, 649

620, 650

610, 639

605, 635

564, 645

606, 635

622, 643

616, 642

614, 648* 0.5/0.5

615, 632* 0.4/0.5

598, 630

582, 627

597, 629

583, 626

0.4/0.5

0.5/0.5

0.6/1.2

1.0/1.8

1.3/13.8

1.6/10.0

0.5/0.7

0.4/0.9

43/46

75/104

580

587

567

547

535

539

585

571

602

556

518

529

558

550

3.0

1.7

9.5

33.8

43.2

37.3

2.7

6.3

1.1

9.6

1.5

1.3

2.6

2*

—

11

10

38

22

5

[Cu(xantphos)(bpy)]^+d

[Cu(POP)(6-Mebpy)]^+e

126/172

272/784

310/4032

451/3406

57/108

153/338

39/45

76/93

202/730

228/1595

240/2401

262/4987

[Cu(xantphos)(6-Mebpy)]^+f

[Cu(POP)(6,6⁰-Me₂bpy)]^+e

[Cu(xantphos)(6,6⁰-Me₂bpy)]^+f

[Cu(POP)(5,5⁰-Me₂bpy)]⁺

[Cu(xantphos)(5,5⁰-Me₂bpy)]⁺

[Cu(POP)(6-tBubpy)]⁺

9.7

10.5

11.4

2.3

5.1

0.4

3.3

9.3

9.8

8.7

5

<1

16

19

26

10

[Cu(xantphos)(6-tBubpy)]⁺

[Cu(POP)(4,5,6-Me₃bpy)]⁺

1.0/1.5

0.9/3.3

0.8/6.0

0.9/9.6

42.7

58.8

27.5

9.8

[Cu(xantphos)(4,5,6-Me₃bpy)]⁺380

[Cu(POP)(2-Etphen)]⁺

390

[Cu(xantphos)(2-Etphen)]⁺

10.2

a

Solution concentration ¼ 2.5 ꢃ 10^ꢀ5mol dm^ꢀ3, except where labelled with an asterisk * (5.0 ꢃ 10^ꢀ5mol dm^ꢀ3). ^bl_exc¼ 365 nm. ^cl_exc¼ 365 nm.

Thin lms compose of spin-coated [Cu(P^P)(N^N)][PF₆] : [Emim][PF₆] in a 4 : 1 molar ratio, except where marked with an asterisk * (1 : 1 molar

f

ratio). ^dData from ref. 21. ^eData from ref. 27. Data from ref. 24. Radiative and non-radiative decay rates k_rand k_nrare given in Table S3.

The solid compounds emit in the green to yellow region, with of the emission energies between the respective POP and xant-

the powder emission maxima between 518 and 602 nm (Fig. 8 and phos complex is in the range of 138 to 645 cm^ꢀ1(17 to 80 meV)

S35,† Table 4). The signicant blue shi with respect to the solu- with one outlier: the emission of [Cu(POP)(6-tBubpy)][PF₆] (602

tion emission maxima is typically observed for these types of nm) is 1374 cm^ꢀ1(170 meV) lower in energy than the one from

complexes. For the xantphos series (Fig. 8), alkyl substitution on its xantphos analogue (556 nm).

the bpy ligand always leads to shorter wavelengths of the powder

A possible explanation for the considerably less eﬃcient

emission (Table 4) as well as to an increase in PLQY and lifetime, emissive properties of the [Cu(POP)(6-tBubpy)][PF₆] complex

with the highest PLQY of 59% being found for [Cu(xantphos)(4,5,6- might be the impaired coordination of the nitrogen atom in the

Me₃bpy)][PF₆], followed by 37% for [Cu(xantphos)(6,6⁰-Me₂bpy)] pyridine ring bearing the bulky (C(CH₃)₃) group. As discussed in

[PF₆]. The excited state lifetimes are in the range between 1.3 and the crystallography section, due to the steric hindrance of the

11.4 ms, with the shortest for [Cu(xantphos)(bpy)][PF₆] and the tert-butyl substituent next to the nitrogen atom, the Cu–N

longest for [Cu(xantphos)(6,6⁰-Me₂bpy)][PF₆]. The most hyp- distance is elongated to 2.3949(19) A, which is around 0.26 A

sochromic emission in powder of the complexes with xantphos is longer than the typical distance range for these types of

the one from [Cu(xantphos)(4,5,6-Me₃bpy)][PF₆] (529 nm), which is complexes. In addition, the N–C–C–N torsion angle of the bpy

58 nm lower (232 meV higher) than for [Cu(xantphos)(bpy)][PF₆] ligand is as large as ꢀ28.8(3)^ꢁ, which might also impact the

(587 nm).

˚

For the analogous complex series with POP (Fig. S35† and

Table 4), also [Cu(POP)(4,5,6-Me₃bpy)][PF₆] is the most blue-

shied complex, 62 nm lower (256 meV higher) than

[Cu(POP)(bpy)][PF₆]. Similar to the observation in solution, the

PLQY values increase on going from 6-Mebpy (Table 4) to 6-

Etbpy (24% for [Cu(POP)(6-Etbpy)][PF₆] and 37% for [Cu(POP)(6-

Etbpy)][PF₆]),¹⁴but then decrease again with the larger alkyl

group in 6-tBubpy (Table 4). The highest PLQY is for

[Cu(POP)(6,6⁰-Me₂bpy)][PF₆] (43.2%), followed closely by

[Cu(POP)(4,5,6-Me₃bpy)][PF₆] (42.7%). Surprisingly, it is not the

[Cu(POP)(bpy)][PF₆] complex that gives the lowest PLQY, but

[Cu(POP)(6-tBubpy)][PF₆] (1.1%), which also exhibits the most

bathochromic emission and a considerably shorter excited state

lifetime (0.4 ms) than the other complexes (1.5–10.5 ms for the

series with POP).

Upon comparison between the POP and xantphos series, it is

sometimes the POP and sometimes the xantphos complex that

shows the more blue-shied emission (Table 4). The diﬀerence

complexes, l_exc¼ 365 nm.

Fig. 8 Normalized emission spectra of solid [Cu(xantphos)(bpy)][PF₆]

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Table 5 Comparison between the emission maxima of the [Cu(P^P)(N^N)][PF₆] complexes in powder at room temperature and in a frozen

solution of Me-THF at 77 K

max

em

Complex cation

l^m_em^ax[nm] powder

l

[nm] Me-THF^b77 K

s^b[ms] 77 K

D wavenumber [cm^ꢀ1

]

DE [meV]

[Cu(POP)(bpy)]^+a

580

587

585

571

602

556

518

529

558

550

610

613

591

594

588

578

566

559

555

557

16

11

63

44

14

25

81

75

27

14

847

722

173

678

105

89

21

[Cu(xantphos)(bpy)]^+a

[Cu(POP)(5,5⁰-Me₂bpy)]⁺

[Cu(xantphos)(5,5⁰-Me₂bpy)]⁺

[Cu(POP)(6-tBubpy)]⁺

84

ꢀ395

ꢀ49

[Cu(xantphos)(6-tBubpy)]⁺

[Cu(POP)(4,5,6-Me₃bpy)]⁺

[Cu(xantphos)(4,5,6-Me₃bpy)]⁺

[Cu(POP)(2-Etphen)]⁺

684

84

1637

1014

ꢀ96

228

202

125

ꢀ12

28

[Cu(xantphos)(2-Etphen)]⁺

a

Data from ref. 21. ^bl_exc¼ 410 nm.

overlap of the aromatic systems of the two pyridyl rings, which to 11.4 ms, Table 4)), which is a strong indication for TADF. The

in return aﬀects the photophysical properties. Unfortunately, emission spectra at 77 K are displayed in Fig. 9 for xantphos

attempts for suitable X-ray quality crystals of [Cu(xantphos)(6- complexes (Fig. S36† for POP complexes). However, when it

tBubpy)][PF₆] were not successful, so there are no structural comes to the emission maxima, two complexes unexpectedly

data to support this hypothesis. We note, however, that the give a blue shi instead of a red shi upon cooling: for

theoretical results predict a similar elongation of the C–N bond [Cu(POP)(6-tBubpy)][PF₆] and [Cu(POP)(2-Etphen)][PF₆] the

for both [Cu(POP)(6-tBubpy)][PF₆] and [Cu(xantphos)(6-tBubpy)] emissions at 77 K are higher in energy by 396 cm^ꢀ1(49 meV)

[PF₆] complexes. In addition, it is also possible that the nine and 97 cm^ꢀ1(12 meV), respectively. Both complexes show

C–H bonds of the tert-butyl group in vicinity of the copper signicantly longer lifetimes upon cooling, consistent with

centre facilitate vibronic non-radiative deactivation of the a TADF mechanism, and the observed blue shi can be attrib-

excited states. Similar complexes where the phenyl groups at the uted to intermolecular matrix eﬀects in the Me–THF glass. It is

coordinating phosphorus were replaced by tert-butyl groups noteworthy that the complexes bearing the 6-tBubpy and 2-

also showed very weak emissive properties.⁶²Also, both Etphen ligands present energy diﬀerences between T₁and S₁of

[Cu(POP/xantphos)(6-tBubpy)][PF₆] complexes have lower PLQY around 0.16 eV, that are signicantly smaller than those found

and shorter lifetime values than [Cu(POP/xantphos)(6-Mebpy)] for the rest of the complexes lying between 0.22 and 0.27 eV

[PF₆]. We conclude that, in addition to the negative eﬀect of (Table 3).

large aromatic substituents,²³large alkyl groups next to the

coordinating nitrogen at the N^N chelating ligand are not

benecial for the emissive properties of these type of copper

Device properties

complexes. However, small substituents such as methyl, ethyl,

CF₃or chloro next to the nitrogen lead to an improvement of the

All complexes were tested as emitters in the active layers of

LECs. Details of the device fabrication are given in the ESI,† and

photophysical properties. Furthermore, an increased number of a schematic representation of a LEC is shown in Fig. S37.†

methyl groups at the N^N chelating ligand results in a positive Devices were prepared on ITO-coated glass slides, using

accumulating eﬀect, which was observed for the [Cu(POP/ PEDOT:PSS as the hole injection layer and a thin lm (about

xantphos)(N^N)][PF₆] complexes on going from bpy to 6-Mebpy 100 nm thick) of the complex deposited by spin-coating of its

to 4,5,6-Me₃bpy.

dichloromethane solutions, and nished with a thermally

deposited aluminium lm (100 nm) as the reective cathode.

The ionic liquid [Emim][PF₆] was added to the solutions of the

complexes in a 4 : 1/[Cu(P^P)(N^N)][PF₆] : [Emim][PF₆] molar

[Cu(P^P)(N^N)]⁺complexes have been shown to be good

candidates for TADF,^40,41a mechanism that allows the repopu-

lation of S₁from T₁via the available thermal energy at room

temperature and thus results in a combined singlet and triplet ratio to enhance the ionic conductivity of the resulting layers.

emission. Upon decreasing the temperature, singlet state LECs were tested by monitoring over time the average lumi-

repopulation takes place in a lower degree and the contribution nance and voltage upon application of a pulsed current (average

of the triplet emission or phosphorescence increases at the current density 50 or 100 A m^ꢀ2, 1 kHz, 50% duty cycle, block

expense of the singlet emission or uorescence. This is experi- wave). In view of the good performance with the Cu(^I) complexes

mentally observable in the red shi of the emission (because

the triplet excited state is energetically lower than the singlet

excited state) and a signicant elongation of the excited state

with the ligands 4,5,6-Me₃bpy and 2-Etphen (main performance

parameters reported in Table 6 and dynamic optoelectronic

behavior in Fig. 10 and S37–S40†), we mainly focus our

lifetime on going from powder at room temperature to a frozen discussion on these devices. The device performance data for

solution of Me–THF at 77 K. As detailed in Table 5, all the the remaining complexes can be found in the ESI (Table S4†).

complexes show signicantly extended lifetimes at 77 K (11 to The LECs with the four best-performing complexes show elec-

81 ms) in comparison to their powders at room temperature ((0.4 troluminescence (EL) spectra (Fig. S38†) which are blue-shied

22640 | RSC Adv., 2020, 10, 22631–22644

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minutes). The analogous complex employing the xantphos

ligand showed a diﬀerent optoelectronic behaviour. While EL

was not initially observed, again due to a saturation of the

average voltage at 9 V, LECs based on [Cu(xantphos)(4,5,6-

Me₃bpy)][PF₆] showed a sudden and fast increase of the EL

signal up to a remarkable Lum_max¼ 462 cd m^ꢀ2in only 13

minutes. Unfortunately, as is oen observed in LECs using

[Cu(P^P)(N^N)]⁺cationic complexes, such high EL intensity was

observed to decay quickly (t_1/2¼ 3 hours). The maximum

external quantum eﬃciency (EQE_max) was found to be 1.7%,

which is signicant but still limited by substantial non-radiative

recombination losses. In fact, considering the PLQY in thin lm

(19%), an outcoupling eﬃciency of 20% and neglecting charge

carrier balance and exciton generation yield, the maximum

achievable EQE for this compound is 3.8%. It is also reasonable

to assume that [Cu(xantphos)(4,5,6-Me₃bpy)][PF₆] has improved

charge transport/injection properties compared to the analo-

gous POP complex, as the voltage decreased rapidly to stabilize

at a lower value of about 6 V. The high eﬃciency and the moderate

voltage resulted in a peak power conversion eﬃciency (PCE_max) of

2.0 lm W^ꢀ1(Fig. S40†). LECs employing Cu(I) complexes with the 2-

ethyl-1,10-phenanthroline ligand showed a very diﬀerent behav-

iour compared to those based on substituted bipyridines. Devices

employing [Cu(POP)(2-Etphen)][PF₆] displayed instantaneous and

intense electroluminescence Lum₀¼ 273 cd m^ꢀ2, which increased

Fig. 9 Normalized emission spectra of the [Cu(xantphos)(N^N)][PF₆]

complexes in Me-THF at 77 K. l_exc¼ 410 nm.

with respect to the PL spectra of the compounds in solution and

red-shied in comparison with the PL in the solid state (Table

4). This is expected in view of the intermediate degree of

conformational freedom of the complex in the (mainly amor-

phous) lms and in presence of ionic liquids. LECs based on

[Cu(POP)(4,5,6-Me₃bpy)][PF₆] turned on instantaneously aer

biasing (initial luminance Lum₀¼ 64 cd m^ꢀ2), and were

observed to slowly increase the EL intensity up to a maximum

luminance (Lum_max) of 111 cd m^ꢀ2in about 18 min (time to

reach the maximum luminance, t_on). The slow turn-on and the

limited EL intensity are likely due to a hindered charge

injection/transport eﬃciency, as the voltage at the beginning of

the test was found to exceed the limit of our setup (9 V,

Fig. S39†). Upon ionic redistribution, the voltage only slightly

decreased to approximately 7.5 V aer 3 hours (indicating

improved charge injection/transport) when, however, electro-

rapidly (t_on¼ 25 min) to a maximum luminance of 451 cd m^ꢀ2

.

However, the EL decay was also rather fast, with a lifetime t_1/2¼ 5.7

hours, although the device continued to emit for 24 hours,

reaching a luminance of approximately 100 cd m^ꢀ2. The corre-

sponding EQE_maxis 1.8%, the highest for the device series reported

here but still far from the theoretical maximum (5.2%) which can

be estimated from the PLQY in thin lm (26%). Interestingly, LECs

with [Cu(POP)(2-Etphen)][PF₆] showed a fast drop of the average

applied voltage to less than 6 V in about an hour, resulting in

a PCE_maxof 2.3 lm W^ꢀ1. Distinct performance was observed for

luminescence was no longer observed (the device lifetime t_1/2

,

which is the time to decay to half of Lum_max, was only 42

Table 6 Performance of ITO/PEDOT:PSS/[Cu(P^P)(N^N)][PF₆] : [Emim][PF₆] 4 : 1 molar ratio/Al LECs measured using a pulsed current driving

(average current density 50 or 100 A m^ꢀ2, 1 kHz, 50% duty cycle, block wave). Device data for the other four complexes is summarized in Table

S4. Data for previously published complexes with similar performances are added for comparison

a

b

c

d

e

f

g

J_avg

t_on

Lum₀

Lum_max

t_1/2

EQE_max

[%]

PCE_max

Eﬃcacy_max

Complex

[A m^ꢀ2

]

[min]

[cd m^ꢀ2

]

[cd m^ꢀ2

]

[hours]

[lm W^ꢀ1

]

[cd A^ꢀ1

]

l^m_EL^{ax h}[nm]

571

[Cu(POP)(4,5,6-Me₃bpy)]⁺

[Cu(xantphos)(4,5,6-Me₃bpy)]⁺

[Cu(POP)(2-Etphen)]⁺

[Cu(xantphos)(2-Etphen)]⁺

50

100

50

100

50

100

50

100

50

100

11

18

27

13

55

25

319

122

9540

60

260

137

82

64

12

0

76

273

0

92

0.6

0.7

7.1

3.1

8.4

5.7

98.3

34.0

200

102

82

0.6

0.4

1.4

1.7

1.1

1.8

1.1

0.6

—

0.6

0.4

1.8

2.0

1.4

2.3

0.7

0.9

—

1.8

1.1

3.8

4.6

2.8

4.5

1.7

1.5

0.3

1.3

0.6

1.1

111

190

462

140

451

87

153

17

63

570

582

580

0

[Cu(POP)(6-MeObpy)]⁺ⁱ

[Cu(POP)(6-EtObpy)]⁺ⁱ

[Cu(POP)(6-Etbpy)]^+j

—

25

5

585

582

589

—

0.2

0.5

—

0.2

0.4

53

109

[Cu(xantphos)(6-CF₃bpy)]^+k

31

a

b

c

d

Time to reach the maximum luminance. Initial luminance. Maximum luminance. Time to reach half of the maximum luminance.

e

f

g

h

Maximum external quantum eﬃciency. Maximum power conversion eﬃciency. Maximum current eﬃciency. Wavelength at the maximum

intensity of the electroluminescence spectrum. ⁱData from ref. 18. ^jData from ref. 24. ^kData from ref. 21.

RSC Adv., 2020, 10, 22631–22644 | 22641

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Fig. 10 Time evolution of luminance for ITO/PEDOT:PSS/

[Cu(P^P)(N^N)][PF₆] : [Emim][PF₆] 4 : 1 molar ratio/Al LECs measured

using a pulsed current driving and under continuous operation

(average current density 100 A m^ꢀ2). Inset shows the behaviour over

the ﬁrst ﬁve hours. For time evolution of further parameters (average

voltage, power conversion eﬃciency, external quantum eﬃciency and

electroluminescence spectra) see Fig. S38–S41.†

LECs based on the [Cu(xantphos)(2-Etphen)][PF₆] complex, which

turned on slowly (t_on¼ 122 min) to reach a peak luminance

Lum_max¼ 153 cd m^ꢀ2, corresponding to an EQE_maxof 0.6%. The

less intense EL compared to the analogous complex with the POP

ligand correlates with the lower PLQY in thin lm (10%). Never-

theless, this compound exhibited a very stable operation (t_1/2¼ 34

hours) with the lowest average voltage among this device series,

below 5 V aer 6 hours driving. This low power consumption is

Fig. 11 Comparison of device properties (maximum luminance and

device lifetime) for the complexes described in this work (round col-

reected in a moderate but stable PCE_maxof 0.9 lm W^ꢀ1. In oured dots), compared to devices with previously tested complexes

prepared in our group (black squares).^{15,17–19,21,24,25}Top: average current

general, the LEC performance observed for this series of

compounds is among the highest reported for similar cationic

[Cu(P^P)(N^N)]⁺complexes.

density 50 A m^ꢀ2; bottom: average current density 100 A m^ꢀ2

.

In order to better compare the device performance of this

complex series with previously prepared LECs based on cop-

per(^I) complexes from our laboratories, we illustrated the two

crucial device parameters that we are constantly trying to

Conclusions

We have described a series of eight [Cu(POP)(N^N)][PF₆] and

[Cu(xantphos)(N^N)][PF₆] complexes with the alkyl-substituted

N^N chelating ligands 5,5⁰-Me₂bpy, 6-tBubpy, 4,5,6-Me₃bpy and

2-Etphen. The study provides meaningful insights into the variety

of eﬀects that alkyl substitution has on the photophysical and

device properties of the complexes. A combination of electronic

and steric eﬀects can lead to a signicant improvement of PLQY

values as well as the maximum luminance in the device, as was

the case for the [Cu(P^P)(4,5,6-Me₃bpy)][PF₆] complexes. On the

other hand, we have demonstrated that while an alkyl group in

the 6-position of the bpy ligand is benecial in the case of methyl

or ethyl groups, a tBu substitution has a detrimental eﬀect and

leads to lower PLQY and shorter excited state lifetimes of the

complexes. Another nding is that incorporation of 5,5⁰-Me₂bpy

results in less eﬃcient emitters than 6-Mebpy, which underlines

again that the stabilization of the tetrahedral complex geometry

with a substitution in 6-position of the bpy is more benecial

than the electron-donating eﬀect of two methyl groups. As the

complexes with 6-Etbpy gave some of the longest living devices,

we decided to replace the ligand with the analogous phenan-

throline, 2-Etphen. While the resulting devices gave higher

improve, namely brightness (Lum_max) and device lifetime (t_1/2

)

in two summarizing graphs which include all tested devices

(Fig. 11). This overview clearly shows that three of the here-

presented complexes are among the best performing copper(^I)

emitters that we ever tested in a LEC. Brightness records go to

[Cu(xantphos)(4,5,6-Me₃bpy)][PF₆] and [Cu(POP)(2-Etphen)]

[PF₆] (462 and 451 cd m^ꢀ2, respectively). At an average current

density of 100 A m^ꢀ2, [Cu(xantphos)(2-Etphen)][PF₆] is the new

champion in terms of lifetime with almost 34.0 hours. And at 50

A m^ꢀ2, the device with this complex is still among the most

longest living ones, and by far the brightest of those with

a lifetime of over three days. Compared to copper-based LECs in

the literature, our devices play in the league of the best per-

forming ones and are, to our knowledge, among the very

brightest reported for copper LECs and are the devices with the

longest reported lifetime.^{7,8,38,60,63–67}

Compared to LECs, state-of-the-art OLEDs (including

solution-processed OLEDs) using copper emitters are still about

hundred orders of magnitudes brighter, with Lum_maxvalues in

the four- and even ve-digit range.^68–71

22642 | RSC Adv., 2020, 10, 22631–22644

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RSC Advances

luminance values, only the complex with xantphos lead to longer 12 C. E. Housecro and E. C. Constable, in Light-Emitting

device lifetimes, and the device with the analogous POP complex

was considerably less stable with 2-Etphen than with 6-Etbpy.

Electrochemical Cells. Concepts, Advances and Challenges, ed.

R. Costa, Springer, Cham, 2017, p. 167.

This highlights once more the sensitivity of the interplay between 13 L. He, in Light-Emitting Electrochemical Cells. Concepts,

the P^P and N^N chelating ligands and the challenge and relative

unpredictability when it comes to the complex composition of

Advances and Challenges, ed. R. Costa, Springer, Cham,

2017, p. 203.

stable and eﬃcient copper emitters. However, with a systematic 14 S. Keller, M. Alkan-Zambada, A. Prescimone, E. C. Constable

approach, we have been able to improve the design of the Cu– and C. E. Housecro, Crystals, 2020, 10, 255.

iTMC, thereby increasing both brightness and lifetime of the 15 N. Arnosti, F. Brunner, I. Susic, S. Keller, J. M. Junquera-

´

Hernandez, A. Prescimone, H. J. Bolink, M. Sessolo,

devices, which are among the best reported in copper–LEC

´

literature.

E. Ortı, C. E. Housecro and E. C. Constable, Adv. Opt.

Mater., 2020, 1901689.

16 S. Keller, M. Bantle, A. Prescimone, E. C. Constable and

C. E. Housecro, Molecules, 2019, 24, 3934.

Conﬂicts of interest

´

17 F. Brunner, A. Babaei, A. Pertegas, J. M. Junquera-

There are no conicts to declare.

´

Hernandez, A. Prescimone, E. C. Constable, H. J. Bolink,

´

M. Sessolo, E. Ortı and C. E. Housecro, Dalton Trans.,

2019, 48, 446.

Acknowledgements

´

18 M. Alkan-Zambada, S. Keller, L. Martınez-Sarti,

´

A. Prescimone, J. M. Junquera-Hernandez, E. C. Constable,

Financial support from the Swiss National Science Foundation

(grant number 200020_162631), the University of Basel, the

MCIU of Spain (MAT2017-88821-R, RTI2018-095362-A-I00,

PGC2018-099568-B-I00, PCIN-2017-014 and Unidad de Exce-

´

H. J. Bolink, M. Sessolo, E. Ortı and C. E. Housecro, J.

Mater. Chem. C, 2018, 6, 8460.

19 S. Keller, A. Prescimone, H. J. Bolink, M. Sessolo, G. Longo,

´

Junquera-Hernandez,

L.

Martınez-Sarti,

J.

M.

´

lencia Marıa de Maeztu MDM-2015-0538), the Generalitat

´

E. C. Constable, E. Ortı and C. E. Housecro, Dalton

Valenciana (PROMETEO/2016/135) and European FEDER funds

(PGC2018-099568-B-I00) is gratefully acknowledged. We thank

Marco Meyer for recording some of the ESI-MS experiments. M.

S. acknowledges the MCIU for his RyC contract.

Trans., 2018, 47, 14263.

20 S. Keller, A. Prescimone, E. C. Constable and

C. E. Housecro, Photochem. Photobiol. Sci., 2018, 17, 375.

21 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. Housecro, ChemPlusChem, 2018, 83, 217.

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22644 | RSC Adv., 2020, 10, 22631–22644

Article Doi

The shiny side of copper: Bringing copper(i) light-emitting electrochemical cells closer to application

DOI: 10.1039/d0ra03824e

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