σ-Hammett parameter: A strategy to enhance both photo- and electro-luminescence features of heteroleptic copper(i) complexes

Article abstract of DOI:10.1039/c7dt00810d

This work studies the effect of the σ-Hammett parameter (σ_p)-i.e., the σ-donation effect caused by substitution at the para position of a bipyridine ligand (4,4′-R₂bipy, where R is MeO, Me, H, NO₂)-on both the photo- and electro-luminescence features of a series of heteroleptic copper(i) complexes-i.e., [Cu(N^N)(P^P)]⁺ where N^N and P^P ligands are R₂bipy and Xantphos, respectively. By virtue of a comprehensive photophysical, theoretical, and thin-film lighting device-i.e., light-emitting electrochemical cells (LECs)-investigation, we note a clear relationship between the σ_p and the photo- and electro-luminescence parameters, such as photoluminescence quantum yields, excited-state lifetimes, and emission maxima, as well as device brightness, stability, and efficacy, respectively. As the most relevant finding, the substitution with the group featuring the most negative σ_p-i.e., MeO-provides a ca. five-fold enhancement of all of the aforementioned figures-of-merit upon comparison within the series of complexes. As such, this work provides a new guideline for a device optimization through a rational ligand design for heteroleptic copper(i) complexes.

Full text of DOI:10.1039/c7dt00810d

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DOI: 10.1039/C7DT00810D
Journal Name
ARTICLE
-Hammett Parameter: a Strategy to Enhance Both Photo- and
Electro-luminescence Features of Heteroleptic Copper(I)
Complexes
Received 00th January 20xx,
Accepted 00th January 20xx
Michael D. Weber,a Marta Viciano-Chumillas,b* Donatella Armentano,c Joan Cano,b,d and Rubén D.
Costaa*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract: This work studies the effect of the -Hammett parameter (p) – i.e., the -donation effect caused by substitution
at the para position of the bipyridine ligand (4,4‘-R2bipy, where R is MeO, Me, H, NO2) – on both the photo- and electro-
luminescence features of a series of heteroleptic copper(I) complexes – i.e., [Cu(N^N)(P^P)]+ where N^N and P^P ligands are
R2bipy and Xantphos, respectively. In virtue of a comprehensive photophysical, theoretical, and thin-film lighting device –
i.e., light-emitting electrochemical cells (LEC) – investigation, we note a clear relationship between the p and the photo-
and electro-luminescence parameters, such as photoluminescence quantum yields, excited-state lifetimes, and emission
maxima, as well as device brightness, stability, and efficacy, respectively. As the most relevant finding, the substitution with
the group featuring the most negative p – i.e., MeO – provides a ca. five-fold enhancement of all of the aforementioned
figures-of-merit upon comparison within the series of complexes. As such, this work provides a new guideline for a device
optimization through a rational ligand design for heteroleptic copper(I) complexes.
a.
M. D. Weber and Dr. R. D. Costa
Department of Chemistry and Pharmacy
University of Erlangen-Nuremberg (FAU)
Egerlandstr. 3, D-91058 Erlangen, Germany.
E-mail: ruben.costa@fau.de
Dr. M. Viciano-Chumillas and Dr. J. Cano
Institut de Ciència Molecular (ICMol)
Universitat de València, 46980 Paterna, València, Spain.
E-mail: marta.viciano@uv.es
b.
c.
Dr. D. Armentano
Dipartamento di Chimica e Tecnologie Chimiche (CTC), Università della Calabria,
87030 Rende, Cosenza, Italy
Dr. J. Cano
d.
Fundació General de la Universitat de València (FGUV)
Universitat de València, 46980 Paterna, València, Spain
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Introduction
Today’s challenge in the field of thin-film lighting technologies
is to fabricate highly efficient, air-stable, low-cost, single-layer
lighting sources by means of up-scalable solution-based
techniques. In this context, the most promising technology is
the light-emitting electrochemical cell (LEC) – Figure 1.1–9 The
key conceptual aspect of LECs is the use of mobile ions in the
active layer to assist the electronic injection from air-stable
electrodes and to control the charge transport and emission
processes in a single-layer device – Figure 1. In short, the ionic
species of the active layer are randomly distributed under zero
applied bias. However, upon biasing the device, the mobile
ions move towards both electrode interfaces, forming the so-
called electric double layers (EDLs), which further assist the
charge injection process. Hence, the formation of n- and p-
doped regions takes over due to the stabilization provided by
the presence of mobile anions. At the p-n junction, the
injected charge carriers recombine, forming the p-i-n light
emitting region.1–9
Much more relevant are the recent efforts of the scientific
community in demonstrating the potential applicability of
LECs.10–15 In short, since the active layer consists of a simple
mixture of a luminescent material –
i.e., polymers, coordination complexes, small organic
molecules, nanoparticles, etc.
16–28 and a source of mobile
Figure 1. Schematic representation of a LEC device (top) and device mechanism
(middle), showing a) the random ion distribution of the active layer under zero bias
condition, b) the formation of the EDLs under applied voltage, and c) the formation of
the p-i-n light-emitting zone upon charge injection. Studied copper(I) compounds with
the p values (bottom).31
anions – i.e., ionic polyelectrolyte, ionic liquid, inorganic salt,
9,29,30
etc. –
,
they can be fabricated using inexpensive roll-to-
roll and/or spray-deposition techniques under ambient
conditions.10,12 Up to date, working LECs prepared onto 3D
substrates like kitchen forks,12 wearable fibers,14 and printing
paper15 have clearly stated that LECs might be considered as a
strong competitor to organic light-emitting diodes (OLEDs) for
applications that require moderate brightness and efficiency
performances. This is further supported by a recent review
that shows the low cost investment associated to a large scale
production of LECs.
In the field of LECs based on ionic transition-metal
complexes (iTMCs), the scientific community has recently
joined efforts to explore copper(I) complexes as alternative to
the well-known iridium(III) complexes. The root interest in this
family of compounds relies on several aspects, namely i) the
use of metal ions with d10 electronic configuration that
eliminates the presence of metal centered (MC) excited states
that are typical degradative pathways for iridium(III)
synthesis protocol and a proper ligand design are key aspects
to maximize the photoluminescence features. For instance,
this has been shown for several archetypal families of
homoleptic and heteroleptic copper(I) complexes combining
diimine (N^N) and diphosphine (P^P) ligands.36–42 Even more
relevant is that the latter achievements have recently been
anticipated to the optimization of their electroluminescence
features in LECs. For instance, the device performance can be
significantly enhanced by attaching alkyl and halogen groups
at the ortho position and the periphery of the N^N ligand,
respectively.42–48 Furthermore, several groups have applied
other copper(I) complex families, showing a promising
performance in the green and yellow regions,49–52 and even
first blue-emitting LECs using NHC-copper(I) dipyridylamine
complexes.53
complexes, ii) the high photoluminescence quantum yields ()
This work shows another concept to enhance both the
photo- and electro-luminescent features of heteroleptic
[Cu(N^N)(P^P)]+ complexes, providing a clear relationship
between ligand design and LEC performance. In particular, we
report a family of copper(I) complexes with the formula
spanning the whole visible region, iii) the emission behavior
involving a thermally activated delayed fluorescence (TADF)
mechanism under both solution and device operation
conditions, iv) the large variety of coordination geometries,
and v) the low-cost production related to the use of copper.
Concerning the design of charged copper(I) complexes, several
groups have started to demonstrate that both an optimized
[Cu(Xantphos)(R2bipy)][BF4] (
bis(diphenylphosphino)-9,9’-dimethylxanthene and R2bipy is
4,4’-R2-2,2’-bipyridine, where R is OMe, Me, H and NO2
1−4), where Xantphos is 4,5-

Figure 1. The chosen synthesis strategy is based on well-known
protocols in concert with the recently reported precautions to
minimize the formation of diimine and diphosphine
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homoleptic copper(I) compounds.39,40 In short, Xantphos was 747(s), 695(s), 517(s), 501(s), 461(m), 4D3O6I(:w10)..10E3S9I-/CM7SDT(0m08/z10):D
selected as P^P ligand, since its bite angle stabilizes the
tetrahedral coordination geometry, while the formation of
857 [Cu(Xantphos)(4,4’-(MeO)2-bipy)]+, 641 [Cu(Xantphos)]+.
[Cu(Xantphos)(4,4’-Me2-bipy)][BF4] (2). Yield: 148 mg (90%).
homoleptic and/or dinuclear copper(I) complexes is sterically Yellow crystals. IR (
unfavorable in the presence of suitable N^N ligands. On the 1435(s), 1401(s), 1308(w), 1219(s), 1156(w), 1082(s), 1050(s),
other hand, the R2bipy ligand controls the luminescence 1033(s), 917(w), 872(w), 827(m), 799(w), 768(w), 750(s),
features of [Cu(N^N)(P^P)]+ complexes.32–35 Therefore, we 696(s), 535(m), 516(s), 507(s), 465(w). Anal. Calcd for
carried out a comparison attaching different R groups i.e., (C51H44BCuF4N2OP2): C, 67.08; H, 4.86; N, 3.07. Found: C,
OMe, Me, H, and NO2
at the para position of the bipyridine 67.10; H, 4.72; N, 3.10.54
ligand. This pattern substitution allows to investigate the [Cu(Xantphos)(bipy)][BF4] (3). Yield: 103 mg (68%). Yellow
effect of the -Hammett parameter ( p) on both the photo- crystals. IR (
max/cm-1): 3448(br), 1595(m), 1561(m), 1481(m),

max/cm−1): 3447(br), 1613(s), 1480(w),
2




and electro-luminescent features, such as photoluminescence 1469(w), 1437(s), 1405(s), 1310(w), 1231(m), 1158(w),
quantum yields, excited-state lifetimes, and emission maxima, 1096(m), 1059(s), 999(m), 878(w), 795(w), 768(m), 752(s),
as well as device brightness, stability, and efficacy,
respectively. The -Hammett parameter relates to the (C49H40BCuF4N2OP2): C, 66.49; H, 4.56; N, 3.16. Found: C,
electronic effect of the substituents
i.e., electron 66.65; H, 4.80; N, 3.05.54
742(s), 693(s), 533(s), 511(s), 483(m), 470(w). Anal. Calcd for 3


donating/withdrawing ability on the rates and equilibria of [Cu(Xantphos)(4,4’-(O2N)2-bipy)][BF4]∙CH2Cl2 (4). Yield: 161
1
organic reactions.31
mg (95%). Red crystals. H NMR (300 MHz, CDCl3, 300 K):

=
As the most relevant finding, we noticed a linear
9.07 (d, 2H, J = 3 Hz, CH bipy), 8.50 (d, 2H, J = 6 Hz, CH bipy),
relationship between all these parameters above with respect 8.16 (dd, 2H, J = 3 Hz, J = 6 Hz, CH bipy), 7.69 (dd, 2H, J = 3 Hz,
to the p. Here, the substitution with MeO groups – i.e., the 9 Hz, CH arom), 7.29 (m, 4H, CH arom), 7.18 (m, 10H, arom),
group with the most negative p value – maximizes all the 6.93 (m, 8H, arom), 6.61 (m, 2H, CH arom), 1.79 (s, 6H. Ar-CH3).
aforementioned parameters within the series of complexes, Anal. Calcd for (−CH2Cl2) (C49H38BCuF4N4O5P2): C, 60.35; H,
achieving up to a five-fold enhancement. Overall, this work 3.93; N, 5.75. Found: C, 60.80; H, 3.42; N, 6.14. IR (
max/cm−1):
states that the influence of the p must be taken into account 3448(br), 3101(w), 3061(w), 2981(w), 1605(w), 1590(w)

4


for a rational ligand design towards device optimization.
1537(s), 1479(m), 1435(s), 1408(s), 1354(s), 1272(w), 1241(s),
1182(w), 1154(w), 1097(s), 1054(s), 999(m), 910(w), 887(m),
867(w), 850(w), 794(m), 776(w), 748(s), 696(s), 532(w), 517(s),
506(s), 484 (w), 466(m). ESI-MS (m/z): 889 [Cu(Xantphos)(4,4’-
(O2N)2-bipy)]+, 641 [Cu(Xantphos)]+.
Experimental section
Materials. All chemicals were obtained from commercial
sources and used as received. Solvents were reagent grade
and stored over activated 3 Å molecular sieves. All reactions
were carried out under an inert argon atmosphere using
standard vacuum line techniques.
X-ray Crystallography. Single-crystal X-ray diffraction data of
1
−
4
were collected on a Bruker-Nonius X8APEXII CCD area
detector diffractometer using graphite-monochromated Mo-
Kα radiation at T = 296 ( and ) and 90 K ( and ). All
2
3
1
4
calculations for data reduction, structure solution, and
refinement were done through the SAINT55 and SADABS56
programs. The structures were solved with the ShelXS
structure solution program, using the Patterson method. The
models were refined with version 2013/4 of ShelXL against F2
on all data by full-matrix least squares.57 All non-hydrogen
atoms were refined with anisotropic displacement
parameters. The hydrogen atoms on the Xantphos ligand and
Syntheses. [Cu(Xantphos)(R2bipy)][BF4]. [Cu(CH3CN)4][BF4]
(0.18 mmol) and Xantphos (0.18 mmol) were dissolved in dry
dichloromethane (5 mL) and stirred in the absence of light at
room temperature for 30 minutes. Then, the corresponding
R2bipy (0.18 mmol) was added leading to a yellow or red
solution. Addition of diethyl ether yielded a yellow or red
precipitate, which was isolated by filtration, washed with
diethyl ether and dried. Single crystals suitable for X-ray
diffraction were grown by slow diffusion of diethyl ether into
a dichloromethane solution at room temperature.
on the lattice solvent molecules (3 and 4) were included at
geometrically calculated positions and refined using a riding
model. The final geometrical calculations and the graphical
manipulations were carried out with the PLATON package.58
Crystallographic data for compounds 1−4 is given in Table S1.
Crystallographic data for the structures reported in this paper
have been deposited with the Cambridge Crystallographic
[Cu(Xantphos)(4,4’-(MeO)2bipy)][BF4]∙CH2Cl2∙Et2O (1). Yield:
1
158 mg (92%). Yellow crystals. H NMR (300 MHz, CDCl3, 300
K):  = 8.16 (d, 2H, J = 3 Hz, CH bipy), 7.70 (d, 2H, J = 6 Hz, CH
arom), 7.63 (dd, 2H, J = 3 Hz, 9 Hz, CH bipy), 7.26 (m, 4H, CH
arom), 7.13 (m, 10H, CH arom), 6.92 (m, 8H, CH arom), 6.70
(dd, 2H, J = 3 Hz, 6 Hz, CH bipy), 6.53 (m, 2H, CH arom), 4.17
Data
Centre
as
supplementary
publication
CCDC−1522132−1522135 (1−4). Copies of the data can be
(s, 6H, OCH3), 1.76 (s, 6H. Ar-CH3). Anal. Calcd for
(−CH2Cl2∙Et2O) (C51H44BCuF4N2O3P2): C, 64.81; H, 4.69; N, 2.96.
Found: C, 64.60; H, 4.42; N, 3.10. IR (
max/cm−1): 3444(br),
1
obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (fax: (+44) 1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).

1606(s), 1561(m), 1496(m), 1473(m), 1435(s), 1405(s),
1336(m), 1315(m), 1282(m), 1242(m), 1224(s), 1151(w),
1054(s), 1033(s), 916(w), 877(w), 838(m), 791(w), 776(w),
Spectroscopic characterization. 1H NMR spectra were
recorded at room temperature on
a Bruker AC 300
spectrometer. Chemical shifts are reported in ppm. CDCl3 was
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used as solvent. Infrared spectra (4000−300 cm−1) were several shells of solvent molecules. This model must be
recorded on a Nicolet 5700 spectrophotometer as KBr pellets.
High-resolution electrospray mass spectra (ESI-MS) in parameters. Once the geometries are optimized and the
methanol solution were obtained on AB SCIEX TripleTOF™ vibrational frequencies are computed for the starting,
inserted in a dielectric cavity with the used solvent
5600 LC/MS/MS System. Steady-state absorption spectra oxidized, or reduced species, the free energy of the redox
were recorded with a Perkin Elmer Lambda 35. Steady-state process at the chosen temperature is evaluated from the
emission spectra were recorded with
a Fluoromax-P- rotational, translational and, mainly, electronic and vibrational
spectrometer from HORIBA Jobin Yvon. Lifetimes were
measured using nanosecond laser flash photolysis transient
contributions. This methodology must be also applied to the
redox pair used as reference (Fc/Fc+). The interaction between
absorption measurements, which were carried out with a solvent molecules and the aromatic rings in the outside of 1−4
Nd/YAG laser (third harmonic, 355 nm, 10 mJ) from Brilliant B,
Quantel. The optical detection is based on a pulsed (pulser
MSP 05 – Müller Elektronik Optik) xenon lamp (XBO 450, relative oxidation potentials of
is weak, and therefore the shells of solvent molecules were
not considered in a semi-quantitative evaluation. The first
were evaluated in
1−4
Osram), monochromator (Spectra Pro 2300i, Acton acetonitrile and at room temperature.
a
Research), a fast InGaAs photodiode (Nano 5, Coherent) with
300 MHz amplification, and a 1 GHz digital oscilloscope
Device fabrication and analysis. Double layer LECs were
fabricated as follows. ITO coated glass plates were patterned
(WavePro7100, LeCroy). Data was fitted using a biexponential by conventional photolithography (Naranjo Substrates). The
fitting function. Quantum yield measurements were substrates were cleaned by using sequential ultrasonic baths,
performed on a quantum yield measurement system from namely in water-soap, water, ethanol, and propan-2-ol
Hamamatsu, model C9920-01 at an excitation wavelength of solvents. After drying, the substrates were placed in a UV–
376 nm.
ozone cleaner (Jetlight 42-220) for 8 min. An 80 nm layer of
Electrochemical characterization. Cyclic voltammetry polyethylene dioxythiophene:polystyrene sulfonic acid
measurements were carried out at 25°C using an AUTOLAB (PEDOT:PSS; 4500 Heraeus) was doctor-bladed from the
PGSTAT 204 potentiostat. A three-electrode system was used, commercial aqueous dispersion onto the ITO-glass substrate
consisting of a static glassy carbon working electrode, a to increase the device preparation yield (400
platinum wire auxiliary electrode and an Ag/AgCl reference distance and a speed of 10 mm/s). The luminescent layer was
electrode. Acetonitrile solutions of 1x10−3 M of compounds entirely prepared with complexes
. The active layer was
1−4 with 0.10 M of [nBu4N][PF6] as supporting electrolyte and deposited by means of doctor blading technique (600
ferrocene as internal standard were measured. substrate distance and a speed of 20 mm/s) reaching a
m substrate
1−4

m
Theoretical characterization. Calculations were performed thickness of 90−100 nm. These conditions resulted in
through the Gaussian09 package using the CAM-B3LYP homogenous thin films with a roughness less than 5%, having
functional and the quadratic convergence approach.59–63 no apparent optical defects. The latter was determined using
Double-
 and triple- all electron basis sets proposed by the profilometer DektakxT from Bruker – see supporting
Ahlrichs et al. were used for nonmetallic and metallic atoms,
respectively.64,65 Molecular geometry of the ground state was
information. Once the active layer was deposited, the samples
were transferred into an inert atmosphere glovebox (<0.1 ppm
optimized in all systems in the gas phase – see supporting O2 and H2O, Innovative Technology). Aluminum cathode
information. Electronic spectra of these compounds were electrode (90 nm) was thermally evaporated using a shadow
theoretically simulated from calculations based on the time- mask under high vacuum (<1 x 10–6 mbar) using an Angstrom
dependent (TD) formalism.66,67 To test the influence of the Covap evaporator integrated into the inert atmosphere
solvent, geometry optimizations and subsequent TDDFT
glovebox. The device statistics involve up to five different
i.e., a total number of 20 pixels. Time dependence of
calculations for were also done using a polarizable devices
1

continuum model with the parameters of the acetonitrile and
dichloromethane solvents.68 Similar quantitative results were
found to those where the optimization was done in the gas
luminance, voltage, and current was measured by applying
constant and/or pulsed voltage and current by monitoring the
desired parameters simultaneously by using Avantes
phase, for which reason only the last ones are discussed in the spectrophotometer (Avaspec-ULS2048L-USB2) in conjunction
text. Molecular geometries optimization on the lowest singlet with a calibrated integrated sphere Avasphere 30-Irrad and
and triplet excited states were carried out following the time- Botest OLT OLED Lifetime-Test System. Electroluminescence
dependent excitations promoted from the ground singlet
state. Vibrational frequencies were calculated to verify that
optimized molecular geometries corresponded with a real
minimum in the potential curve. Fluorescence and
phosphorescence emission spectra were simulated on the
optimized geometries of the excited singlet and triplet states,
respectively, from TDDFT calculations on the singlet ground
state.69 The quantitative evaluation of the redox potential
from the quantum calculations is a hard and painstaking task
that requires a model with the complex encircled by one or
spectra were recorded using the above mentioned
spectrophotometer.
Results and Discussion
Syntheses and X-ray structure description: Compounds 1−4
were obtained in good yields by reaction in dichloromethane
solution of [Cu(CH3CN)4][BF4] with one equivalent of the
Xantphos ligand followed by one equivalent of the substituted
R2bipy ligand – Figure 1. Stable single crystals for all
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compounds were obtained by slow diffusion of diethyl ether compounds
into a solution of the complexes in dichloromethane under
aerobic conditions. Signals in the 1H NMR spectra point to the fold angle between the mean aromatic planes. While in the
1
−4
, respectively. In the same way, the Xantphos
ligand is bended on the central non-aromatic ring leading to a
formation of only the heteroleptic copper(I) complexes with
free ligand this value is 156°,70 in
137° ( ), 149° ( ), and 153° ( ). The dihedral angles between
the phenyl rings of the same phosphorous atom are 76.07°
and 74.74° for , 79.41° and 89.39° for , 67.28° and 80.10° for
, and 75.76° and 74.11° for . Intramolecular π−π interactions
involve the phenyl rings from the two different phosphorous
atoms of the Xantphos ligand for , and ‒ Table S3 and
Figure S1. In contrast, shows intramolecular π−π interactions
1−4 the angles are 150° (1),
2
3
4
1
2
3
4
1
,
3
4
2
between the phenyl rings of the bipy and Xantphos ligands,
probably forcing to a large folding of the latter ‒ Table S3 and
Figure S1. Moreover, intermolecular π−π interactions are only
present in
2 between the phenyl rings of the R2bipy ligand of
different molecules. Crystallized solvent molecules are just
present for
distances in
3
and
4. The shortest intermolecular Cu∙∙∙Cu
1−4 vary in the range of 9.157−10.365 Å.
Cyclic voltammetry study: Cyclic voltammetry experiments of
compounds were performed in solution – Figure 3 and
Table 1. Compounds show three irreversible oxidation
1−4
1−3
waves at ca. +0.70, +0.98, and +1.35 V, as well as a quasi-
reversible reduction wave at around –2.10 V. In stark contrast,
4
shows only one irreversible oxidation at around +1.06 V and
Figure 2. Platon projection of the [Cu(Xantphos)(R2bipy)]+ cations (1−4). Hydrogen
atoms are omitted for clarity.
three quasi-reversible reductions at –1.22, –1.36, and –1.89 V.
For comparison purposes, we also provided an energy diagram,
in which the HOMO and LUMO levels of the complexes are
shown along with those of the electrodes used for device
studies – Figure 3.
no evidence of either the presence of homoleptic species or
equilibrium between them at room temperature. 1H NMR
spectra of compounds
reported earlier.54
2 and 3 are in agreement with the ones
The first oxidation potential value of compounds 1−3 is
typically ascribed to the metal ion oxidation from Cu(I) to
Cu(II). The other two oxidations should occur at the Xantphos
moiety, since similar redox behavior has been observed for the
Compounds 1−4 consist of a copper(I) ion with one N^N
and one P^P environment formed by R2bipy and Xantphos
−
ligands, and a BF4 as counterion. The molecular structure of
free ligand. In the case of 4, the only observed oxidation process
the cationic complexes
Crystallographic data for compounds
Whereas and crystallize in the triclinic space group P−1,
and does in the monoclinic space group P21/n and P21/c.
1
−4
are depicted in Figure 2.
might occur at the Cu(I) ion, following the trends reported in
literature related to similar transion-metal complexes, as well as
the theoretical study below.71,72
1−4
is given in Table S1.
1
2
3
4
Next, the reduction potentials shift to high values going from
Selected angles and distances are listed in Table S2. In all
complexes, the Cu(I) ion shows a distorted tetrahedral
geometry with no coordination of the O1 atom of the
Xantphos ligand, varying the Cu∙∙∙O1 distances in the range of
3.082(2)−3.198(2) Å. The Cu–P distances are in the range
2.2460(10)−2.3031(3) Å and the Cu–N distances lies in the
range of 2.023(3)−2.131(5) Å. These values are in agreement
with the ones found in the literature. The N1–Cu–N2 angles
are similar in all compounds (79.82(12)°, 79.86(7)°, 79.26(9)°
3

, to
p is, the higher the reduction potential becomes. 71,72 Thus, the
reduction process might involve the R2bipy ligand. Finally, the
distinct reduction behavior of compared to the other
2, and to 1, in line with the p i.e. the more negative the
4
complexes should be related to the reduction processes
involving the NO2 group.73 The differences of
transfered to other physical properties ‒ vide infra.
Next, theoretical calculations based on density functional
theory (DFT) approach were perfomed – see experimetal
section for details. As shown in Figure S2, the spin density
4 can be
and 79.41(7)° for 1−4, respectively). Despite the bite P1–Cu–
P2 angles are closer to the ideal Td symmetry, they differ
slightly for all compounds (117.42(4)°, 119.72(2)°, 113.61(3)°,
maps on the one-electron oxidized species for 1−4 show that
the oxidation occurs on the copper atom. Whereas the
optimized geometries of the ground state can be seen as
slighly distorted tetrahedra with a dihedral angle (θ) between
CuN2 and CuP2 planes of around 89°, this angle is smaller for
the oxidized forms (ca. 60°) ‒ Table S4. This structural change
stabilizes a copper(II) configuration, even if they are still far
from a planar square geometry, disfavouring a reversible
oxidation process. In line with the experimental results, the
and 114.61(2)° for
between P1–Cu–P2 and N1–Cu–N2 planes are 86.39°, 79.45°,
78.28°, and 81.08° for compounds respectively,
1−4, respectively). The dihedral angles
1−4,
corresponding to a distorted tetrahedron. This indicates a
strong distortion of the coordination sphere. Moreover, the
R2bipy ligand is not planar and the angles between their
aromatic rings are 4.54°, 17.62°, 23.04°, and 12.57° for
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relative first oxidation potentials taking
1
as reference (E1 =
0.00 V) were +0.09, +0.11 and +0.44 V for
Therefore, the oxidation behavior is similar for compounds
, while the unique oxidation wave at +1.06 V of should
2−4, respectively.
1
−3
4
also be ascribed to occur at the Cu(I) ion – Figure S2. Indeed,
the NO2 derivative can be reduced more easily than the other
three R2bipy ligands, whereas it is harder to oxidize.71,72
Electronic absorption spectra: The absorption spectra of 1−4
in solution are displayed in Figure 4, while the absorption
maxima are summarized in Table 1. The absorption features
encompass an intense band located at 275−330 nm that is
followed by a less intense and broad band centered at around
380 nm for
slightly blue-shifted following the increased σ-donation
highlighted by the p parameter of the attached groups to the
1−3 and 500 nm for 4. The low-energy bands are

R2bipy ligand – Table 1.
Such electronic changes depending on the nature of the R
support the trend described in the cyclic voltammetry studies.
To determine the nature of the absorption bands, we
performed theoretical calculations based on the time-
dependent formalism applied to the density functional theory
(TDDFT) – see experimental section. Details on more intense
theoretical absorptions in 1−4 – i.e., singlet-singlet transitions
–are shown in Table S5. For all the compounds, the theoretical
and experimental spectra are in good agreement, showing a
slight shift to higher energies for the former. This is more
pronounced when a solvation model with acetonitrile or
dichloromethane as solvent is considered
 Figure S3. From
Figure 3. Top: Energy diagram showing the energy levels of the copper(I) complexes
along with those of the devices. Bottom: cyclic voltammograms of compounds 1−4 in
solution. Potentials are referenced relative to internal Fc/Fc+ (defined as 0). Scan rate
is 100 mV/s.
the natural transition orbitals (NTOs),74,75 which provide easier
visualization of the electronic transitions than that from the
molecular orbitals, the experimental bands at 372 and 285 nm
in 3 can be assigned to metal-ligand charge transfer (MLCT)
Table 1. Photophysical and electrochemical features of compounds 1−4.
Compound
Absorption
λmax [nm]a
[ε [103 L·mol-1·cm-1]
Redox potentials
Emission
b
b
d
Eoxd
Ered
c
[nm]
λem
e
[%]
f
Fluor.
[nm]g
Phosphor.
[nm]g
[V]
[V]
[cm-1]
[s]
0.68;
0.99;
1.37
0.74;
0.98;
1.34
0.68;
0.97;
1.33
261 (12.7), 285 (7.65), 372
(0.85)
1
2
3
4
–2.18
545
570
600
-
461
391
303
-
18.9
9.7
1.8
-
2.26
1.35
0.51
-
623
636
653
926
719
742
281 (12.6), 376 (1.46)
282 (7.89), 384 (0.87)
–2.16
–2.06
768
–1.22;
–1.37;
–1.89
281 (9.90), 321 (3.50), 423
(1.25), 503 (1.78)
1.06
1553
a Measured in dichloromethane (298 K, ~1x10–4 M). b Potentials are given vs. Fc/Fc+ couplec Emission in solid-state upon excitation at 376 nm. d Energetic difference
f
between emission maxima at room and 77 K temperature conditions. e Photoluminescence quantum yield measured in solid-state upon excitation at 376 nm.
Excited-state lifetimes measured in solid-state upon excitation at 370 nm. g Fluorescence and phosphorescence values obtained by theoretical calculations with the
optimized geometries employing a solvation model.
 Figure 5. Bands located at the high-energy region are attributed to * electronic
excited triplet states. Here, the lowest excited triplet state (T1) is located below the
transitions. This kind of calculations also provides information on the nature of the
first excited singlet state (S1), but the next triplet excited states are placed among S1 
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values consist of two componDeOnIt:s1,0.1w0h39ic/Ch7DcTa0n08b10eD
Table S5 and Figure 6. Although the electronic excitations involving triplet excited
The

averaged using the formula given in Figure S6. The obtained
Figure 5. Perspective views of the natural transition orbitals (NTOs) involved in the
theoretical electronic excitations of 3. These transitions and their correspondence with
the experimental absorption spectra are shown in Table S5 for 1−4. The isodensity
surfaces correspond to a cut-off value of 0.05 e bohr-3. In the picture, one electron is
promoted from the orbital at the left side to the other one at the right side. The
wavelengths for each transitions are those found from a TDDFT calculation in gas
phase.
Figure 4. Top: Absorption spectra of 1−4 in solution. Bottom: Emission spectra of 1−3
in solid-state (exc = 376 nm). Inset: Relationship between wavelength maxima and p.
averaged lifetimes <
in good agreement to values noted for similar copper(I)
complexes
Table 1 and Figure S6.42–52,53
> are in the range of 0.5 - 2.3 µs. This is
states occur in the UV-Vis region, they are masked by the less
forbidden singlet-singlet transitions. The nature of the first

electronic transitions are similar for
experimental absorption spectra shown in Figure 4. In stark
contrast, the bands at 503 and 423 nm of are assigned to
combinations of d-d and MLCT and intraligand transitions,
respectively Table S5 and Figure S4. This suggests that the
1‒3, as noted in the
To further study the nature of the emission mechanism, we
compared the emission in solid-state at both room
temperature and 77 K – Figure S5. This is a qualitative way to
determine if a TADF process is involved.33-39 In particular, an
emission behavior involving one excited state typically
features a bathochromic shift of the maximum emission upon
freezing, while TADF materials show a red-shifted emission
due to the change of the fluorescence and phosphorescence
contribution at room temperature and 77 K, respectively. As
such, the emission mechanism involves only the triplet excited
state at low temperature, while at room temperature a two
excited states model, in which the emitting S1 is thermally
repopulated from the lowest triplet excited state (T1), is
present. Similar to other copper(I) complexes,33-39 a clear red-
shifted emission of several hundreds of wavenumbers was
4

nature of the emitting excited states, involving either
fluorescence or phosphorescence processes, could be
different in
4 compared to those of 1−3, pointing to a very
different photoluminescence behavior ‒ vide infra.
Photoluminescence studies: The photoluminescence features
in both solution and solid-state were investigated for all the
compounds – Figures 4 and S5, as well as Table 1. All the
copper(I) complexes are non-emissive in solution, while only
1
−
3
show intense and featureless emission spectra in solid-
state. As shown in Figure 4 inset, the wavelength maxima are
linearly red-shifted upon increasing p from to . A divergent
behaviour was noted for , which is not emissive in both
solution and solid-state. Next, the photoluminescence
quantum yields ( ) were also determined. The values
decrease linearly going from to – Table 1 and Figure S5.
This trend is also noted for the excited-state lifetime ( ) values,

1
3
noted for
1. Interestingly, similar to the linear relationship between the
emission maxima and the p, the em values linearly decrease
going from to – Figure S5. The latter indicates that the
1−3 upon cooling – i.e., em as summarized in Table
4



1
3
1
3
TADF process might be affected using groups with different p
values. This aspect needs to be corroborated monitoring the
excited state lifetime changes upon varying the
temperature.32–35 Much more relevant is the lack of emission

which can be fitted with a biexponential function – Figure S6.
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features noted for
4
independently of the temperature. This is
in their optimized geometries. However, the energy order, ES0 < ET1
< ES1, remains invariable in the three optimized geometries – Figure
6. There is a good agreement between the experimental and
in agreement with the different nature of the lowest excited
state of as derived from the electrochemical and
4
spectroscopic studies.
computed emission shifts induced by the different
character of the substituents of the R2bipy ligand, stabilizing
both the S1 and T1 states along the series . This effect is
especially significant for , which could explain the lack of
emission behavior. Indeed, the theoretical results support that
both the fluorescence and phosphorescence of should either
-donor
1−4
4
4
appear in the infrared region or should not evolve. In addition,
the phosphorescence emission calculated from the direct DFT
moved towards lower energies than that obtained from the
time-dependent formalism. Some difficulties to optimize the
geometry in
4 were revealed probably due to problems
inherent to time-dependent formalism to correctly describe
low-energy electronic excitations. In brief, geometry
optimizations for the T1 excited state can be carried out using
the TDDFT formalism or the less expensive direct DFT, but the
phosphorescence emission should only be calculated applying
the time-dependent formalism on the S0 state in the optimized
geometry of T1 – Figure 6.
It is accessible and appropriate visualizing the electronic
configuration of S1 state from the NTOs that define the
electronic transition between the S0 and S1 states – Figure 6.
However, the electronic nature of T1 is better described by its
spin density map – Figure S7. Both, S1 and T1 states, are
characterized by the magnetic copper(II) ion/R2bipy radical
pair generated from promoting one electron from the
copper(I) ion to the R2bipy ligand – Figure 6. This electronic
feature is translated to the change in the molecular structure
of both states. Then, the optimized geometries for S1 and T1
states show a similar θ dihedral angle of R2bipy than that of
the oxidized species, supporting the presence of a Cu(II) ion –
Table S4. On the other hand, whereas the C−N and C−C bond
lengths in the phenyl rings of the bipy ligands are invariant in
S0 and the oxidized species, they show an alternating topology
that is the footprint of a radical species – Table S7. Moreover,
the inter-phenyl ring distance in the bipy derivative is much
Figure 6. Schematic view of the potential curves of the S0, S1, and T1 states. The dashed
lines shows the optimized geometries for the S0, S1, and T1 states. Natural transition
orbitals (NTOs) involved in the theoretical electronic excitations from S0 to S1 and
calculated spin density of the excited T1 state are displayed to indirectly visualize the
electronic structures of S1 and T1, respectively. The isodensity surfaces correspond to
a cut-off value of 0.05 and 0.005 e bohr–3 for NTOs and spin density, respectively. In
the last case, blue and yellow isosurfaces represent the positive and negative regions
of spin density, respectively.
Theoretical study of the excited states: The optimization of shorter in S1 and T1 than in S0 and the oxidized species. The
the geometries of S1 and T1 states is essential to study the
nature of the fluorescence and phosphorescence processes.
strict orthogonality between the magnetic orbitals localized
on the metal ion and organic radical lead to a ferromagnetic T1
Practically, the only way to accurately and non-expensively state with the spin momenta parallel aligned, which is more
study the S1 state is following its energy from TDDFT stable than the antiferromagnetic S1 state. This shared
calculations on the S0. The energy of the T1 state could be electronic nature of T1 and S1 is reflected in the quasi-total
calculated from a DFT calculation, which we labeled as direct concordance between their optimized geometries – Figure S8.
DFT approach; however, its relative energy from the S0 ground Despite the similarity in the electronic structure of T1 in
state will be largely underestimated (ET1). Some more specific the NO2 is the only substituent that strongly delocalizes the
and elaborated methodologies can be envisaged as acceptable spin density of the R2bipy radical – Figure S7. Two effects are
approaches.76–78 In this family with a singlet as ground state observed. On one hand, this additional spin delocalization in
1−4,
4
and a triplet as excited state to be visualized, the approach
used for S1 is also satisfactory for T1. Thus, we have indirectly
followed the energy of T1 (ET1) in its geometry optimization from a
TDDFT calculation on the S0 state. According to the results for the
four compounds –Table S6 – and even though the energy found
from direct DFT is not correct (ET1), the optimized geometry for the
T1 state is very similar to that obtained from the TDDFT formalism.
As expected, each one of these states (S0, S1, and T1) is more stable
favours significantly the stabilization of T1. On the other hand,
since the spin density on the R2bipy radical is placed further
from the copper center than in , the return of an additional
electron on the R2bipy radical towards the Cu(II) ion is less
favoured in , avoiding a phosphorescence emission even at
77 K – vide supra.
1−3
4
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the fluorescence emission. HoweveDr,OI: 10.1039/C7DT00810D
a fast thermal
conversion between T1 and S1 should be necessary. This is
possible owing to the small energy gap between these states
and the insignificant geometrical reorganization upon
intersystem crossing (ISC). Atomic spin densities on the C4 and
C4’ carbon and the nitrogen atoms of the R2bipy radical in the
T1 state increase along the series 1-4, making the magnetic
coupling stronger between the copper(II) ion and the R2bipy
radical, and promoting a larger splitting between the S1 and T1
states
Electroluminescence studies: The electroluminescent
properties of were studied in thin-film devices based on
 Tables S6 and S8.
1−4
the LEC concept. In short, the active layer of around 90 nm
consists of only the complexes without using any additive to
provide a fair comparison. This layer is sandwiched in between
an Indium-Tin-Oxide (ITO) bottom electrode, which is
previously modified with a thin PEDOT:PSS layer (80 nm) to
ensure the device reproducibility, and an evaporated
aluminium top electrode – see Figure 10. The devices were
driven using a pulsed current of 5 mA scheme based on a
block-wave at 1000 Hz and a duty cycle of 50%. While the
electroluminescence behavior of the best devices is displayed
in Figure 7 and the figures-of-merit are summarized in Table 2,
the relationship between p and the mean values of the most
relevant device parameters is shown in Figure 8. Details
concerning the device preparation, analysis, and statistics can
be found in the experimental section.
All the devices showed the same behavior over time –
Figure 7. In particular, the average voltage profile shows a
similar initial voltage peak when the device is switched on
followed by a slow decay up to a similar steady-state voltage
regime for all the devices. During this time the luminance and
efficacy values reach the maximum values and slowly
decrease. Importantly, devices prepared with
4 did not show
any electroluminescence response, even though charge
injection takes place. As shown in Figure 7, the
electroluminescence response is associated to a broad
electroluminescence (EL) spectra centered at ca. 580 (
1), 595
(2
), 605 ( ) nm with x/y Commission International d’Eclairage
3
1931 (CIE) coordinates of ca. 0.49/0.39, 0.53/0.46, and
0.55/0.43, respectively. The maxima of the EL spectra also
follow a linear relationship with p  Figure 7, inset.
Since the applied voltage remains practically constant, the
low stability of the devices should be ascribed to the fast
reduction of the p-i-n light-emitting zone – Figure 1. Indeed, a
severe degradation would have led to a gradual increase of the
applied voltage due to over oxidation/reduction processes
Concerning the experimentally proposed TADF emission
process, the magnetic coupling between the copper(II) ion and
the R2bipy radical is the reason for the small energy gap
and/or formation of black spots as recently noted by Gao et al.
81,82
Figure 7. Luminance (top) and applied voltage (central) responses vs. time, as well as
the electroluminescence spectra of devices with 1−3 driven at applied pulsed current
of 5 mA. Inset: relationship between the electroluminescence wavelength maxima and
p.
Table 2. Device figures-of-merit of the best devices with 1, 2, and 3.
c
d
Compound
Luminancea
Efficacyb
[cd/A]
0.35
0.20
0.08
Utot
em
[cd/m2]
54
[µJ/cm2]
1027
353
[nm]
580
595
605
between S0 and T1 states.38,79,80 Calculations support that,
in other words, both emitting states are involved together in a
radiative process, and their contributions depend on their
population, which can be thermally modified. Since T1 is more
stable than S1, an increase of the temperature should favour
1
2
3
34
14
209
a Flux of light emitted by the device measured in candela per surface.b Emitted
light per electric flux, measured in candela per ampere. c The device stability is
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studied with the Utot parameter that refers to the total emitted energy density
taking the time needed to reach one fifth of the maximum luminance as the
Conclusions
upper limit. This approach was followed since the devices show different
d
We show a new concept for a rational ligand design based on
the effect that the -Hammett parameter ( p) exerts on both
the photo- and electro-luminescence features of a series of
heteroleptic copper(I) complexes
i.e. [Cu(N^N)(P^P)]+ where
N^N and P^P ligands are R2bipy and Xantphos, respectively.
We have noticed clear relationship between the
luminance values to allow
a clear comparison using lifetime values.
Electroluminescence wavelength maxima


-0.3
-0.2
-0.1
0.0

60
40
20
a
photoluminescence features and the molecular and electronic
structural changes caused by attaching R groups featuring
different p values. Much more relevant, we have also
demonstrated how this strategy can be used to predict
changes related to the electroluminescence behavior of
copper(I) complexes in LECs. Indeed, the device performance
has been enhanced by a factor of five using a copper(I)
complex with a strong electron-donating group – i.e., the most
negative p – compared to devices prepared with the
unsubstituted copper(I) complex. Overall, this work provides a
clear guideline for a device optimization through a rational
ligand design for heteroleptic copper(I) complexes. This could
also be extrapolated to other families of complexes featuring
different N^N ligands.
0.30
0.15
0.00
1200
800
400
0
Acknowledgements
RDC and MDW acknowledge funding from DFG Excellence
‘Engineering of Advanced Materials’ (EAM) and ‘Fonds der
Chemischen Industrie’ (FCI) in the Liebig grant framework. This
work was financially supported by the MICIIN (Spain) (Project
CTQ2013-44844-P), the Generalitat Valenciana (Spain)
(Project PROMETEOII/2014/070) and ‘Maria de Maeztu’
Excellence Unit by MINECO (MDM-2015-0538). M.V-C. thanks
the MICIIN for the contract. Additionally, we would like to
thank Dr. Axel Kahnt for helping us with excited-state lifetime
measurements.
-0.3
-0.2
-0.1
0.0
p
Figure 8. Statistical change of the device figures-of-merit in function of p
However, we cannot rule out the degradation of the emitters
under operation conditions owing to their unfavorable
electrochemical behavior. As such, the moderate stability of
the devices is quite likely related to the reduction of the
emitting non-doped region, as a result of the intrinsic
mechanism of this type of devices.1-9 In addition, since the
initial voltage is similar for all devices we can also rule out
effects due the changes in the ionic conductivity. Much more
interesting is the direct comparison of the luminance, efficacy,
and stability values – Table 2. In detail, devices prepared with
p=-0.27) outperform those with
luminance, efficacy, and stability values of around 50 cd/m2,
0.35 cd/A, and 1000
J/cm2, respectively. This represents a
five-fold enhancement considering the performance of
devices with the unsubstituted complex p=0), which can,
Keywords: Hammett parameter • copper (I) complexes •
electroluminescence • light-emitting electrochemical cells •
thin-film lighting devices
Notes and references
1
(

2 and 3, featuring
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the
matter under discussion, limited experimental and spectral
data, and crystallographic data.

3 (
therefore, be taken as a reference. Similar to the relationship
noted between the photoluminescence parameters and the
1
2
3
4
5
6
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photoluminescence
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for
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Table of Contents: -Hammett Parameter: A
strategy to Enhance Both Photo- and Electro-
luminescence Features of Heteroleptic Copper(I)
Complexes
Michael D. Weber, Marta Viciano-Chumillas,* Donatella
Armentano, Joan Cano, and Rubén D. Costa*
Hammett was also right for devices. Heteroleptic copper(I)
complexes bearing N^N and P^P ligands show very promising emission
properties for thin-film lighting applications. This work directly links both
the enhancement of the photo- and electro-luminescent properties of a
family of copper(I) complexes with a rational ligand design using the
-Hammett parameter.
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