Synthesis, characterization, and photoluminescent studies of three-coordinate Cu(i)-NHC complexes bearing unsymmetrically-substituted dipyridylamine ligands

Article abstract of DOI:10.1039/c9ra04886c

A series of heteroleptic three-coordinate Cu(i) complexes bearing monodentate N-heterocyclic carbene (NHC) ligands of the type 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr), and bidentate N-donor ligands of the type unsymmetrically-substituted dimethyl dipyridylamine (Me₂Hdpa) and bis(mesityl)biazanaphthenequinone (mesBIAN) have been synthesized. The complexes [Cu(IPr)(3,4′-Me₂Hdpa)]PF₆, 1; [Cu(IPr)(3,5′-Me₂Hdpa)]PF₆, 2; [Cu(IPr)(3,6′-Me₂Hdpa)]PF₆, 3; [Cu(IPr)(mesBIAN)]PF₆, 6; [Cu(SIPr)(3,4′-Me₂Hdpa)]PF₆, 7; [Cu(SIPr)(3,5′-Me₂Hdpa)]PF₆, 8; and [Cu(SIPr)(3,3′-Me₂Hdpa)]PF₆, 11 have been characterized by ¹H and ¹³C NMR spectroscopies, elemental analysis, cyclic voltammetry, and photophysical studies in solid and solution phase. Single crystal X-ray structures were obtained for all complexes except 11. The crystallographic data reveal a mononuclear structure for all complexes with the copper atom ligated by one C and two N atoms. The UV-Vis absorption spectra of all dipyridylamine complexes in CH₂Cl₂ show a strong ligand-centered absorption band around 250 nm and a strong metal-to-ligand charge transfer (MLCT) band around 300 nm. When irradiated with UV light, the complexes exhibit strong emission maxima at 453-482 nm with photoluminescence quantum yields (PLQY) ranging from 0.21 to 0.87 in solid state. While the PLQY values are comparable to those of the symmetrical [Cu(IPr)(Me₂Hdpa)]PF₆ complexes, a stabilizing CH-π interaction has been reduced in the current systems. In particular, complex 3 lacks any strong CH-π interaction, but emits more efficiently than 1 and 2 wherein the interactions exist. Structural data analysis was performed to clarify the role of ligands' plane angle and the NH/CH?F interactions to the observed light interaction of unsymmetrical [Cu(NHC)(Me₂Hdpa)]PF₆ complexes. DFT calculations were performed to assist in the assignment of the electronic structure and excited state behavior of the complexes.

Full text of DOI:10.1039/c9ra04886c

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PAPER
Synthesis, characterization, and photoluminescent
studies of three-coordinate Cu(I)–NHC complexes
bearing unsymmetrically-substituted
dipyridylamine ligands†
Cite this: RSC Adv., 2019, 9, 22417
Kwame Glinton, Reza Latifi,
Bachkhoa Nguyen and Laleh Tahsini
David S. Cockrell, Matthew Bardeaux,
*
A series of heteroleptic three-coordinate Cu(I) complexes bearing monodentate N-heterocyclic carbene
NHC) ligands of the type 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3-bis(2,6-
diisopropylphenyl)imidazolidin-2-ylidene (SIPr), and bidentate N-donor ligands of the type unsymmetrically-
substituted dimethyl dipyridylamine (Me Hdpa) and bis(mesityl)biazanaphthenequinone (mesBIAN) have
been synthesized. The complexes [Cu(IPr)(3,4 -Me
(
2
0
0
0
2
Hdpa)]PF
6
, 1; [Cu(IPr)(3,5 -Me
2
Hdpa)]PF
6
, 2; [Cu(IPr)(3,6 -
0
0
Me
2
Hdpa)]PF
6
, 3; [Cu(IPr)(mesBIAN)]PF
6
, 6; [Cu(SIPr)(3,4 -Me
2
Hdpa)]PF
6
, 7; [Cu(SIPr)(3,5 -Me
2
Hdpa)]PF , 8;
6
0
1
13
and [Cu(SIPr)(3,3 -Me
2
Hdpa)]PF
6
, 11 have been characterized by H and C NMR spectroscopies, elemental
analysis, cyclic voltammetry, and photophysical studies in solid and solution phase. Single crystal X-ray
structures were obtained for all complexes except 11. The crystallographic data reveal a mononuclear
structure for all complexes with the copper atom ligated by one C and two N atoms. The UV-Vis
2 2
absorption spectra of all dipyridylamine complexes in CH Cl show a strong ligand-centered absorption
band around 250 nm and a strong metal-to-ligand charge transfer (MLCT) band around 300 nm. When
irradiated with UV light, the complexes exhibit strong emission maxima at 453–482 nm with
photoluminescence quantum yields (PLQY) ranging from 0.21 to 0.87 in solid state. While the PLQY values
2 6
are comparable to those of the symmetrical [Cu(IPr)(Me Hdpa)]PF complexes, a stabilizing CH–p
interaction has been reduced in the current systems. In particular, complex 3 lacks any strong CH–p
interaction, but emits more efficiently than 1 and 2 wherein the interactions exist. Structural data analysis
was performed to clarify the role of ligands' plane angle and the NH/CH/F interactions to the observed
Received 28th June 2019
Accepted 15th July 2019
DOI: 10.1039/c9ra04886c
2 6
light interaction of unsymmetrical [Cu(NHC)(Me Hdpa)]PF complexes. DFT calculations were performed to
rsc.li/rsc-advances
assist in the assignment of the electronic structure and excited state behavior of the complexes.
metals by more abundant alternatives is desired. In this context,
Introduction
10
d
metals such as Cu(I) are of considerable interest. Over the
Photoluminescent (PL) complexes containing precious transi- last several decades, a great variety of Cu(I) complexes have been
tion metals (Ru(II), Ir(III), Pt(II), etc.) have been extensively developed aiming to improve the emissive properties of Cu
studied due to their potential application in electroluminescent luminophores towards strong emission and long-lived excited
devices, photochemical catalysis, OLED light displays, and solar states. Majority of these efforts have focused on the four-
1
–9
energy conversion. However, owing to their limited avail- coordinate tetrahedral Cu(I) complexes with bisimine and
ability, toxicity, and relative cost, the substitution of these phosphine ligands to block their non-radiative decay pathways.
The attening of the photoexcited Cu(II) complexes via a Jahn–
Teller distortion promotes the formation of a ve-coordinate
Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078, USA.
E-mail: tahsini@okstate.edu
exciplex with solvent molecules or other nucleophiles that
†
Electronic supplementary information (ESI) available: Crystallographic oen leads to a radiationless deactivation of the excited
information les (CIF) for 1–3, 6–8, and 12, crystallographic data collection table,
10–16
state.
A useful strategy to prevent the excited state congu-
1
13
1
H and C{ H} NMR spectra for ligands, Cu precursors, and complexes 1–12
Fig. S16–S51), ORTEP diagrams of [Cu(SIPr)Cl] and 6 and 12 with long-distance
17
rational changes, rst examined by McMillin, is by increasing
the steric bulk of ligands at the coordination site. There has
been an extensive work on the effect of bulky substituents
(
interactions, CV of complexes 4 and 5 at varying scan rates and CV of the free
Hdpa (Fig. S5–S7), and further details of ligand and Cu complexes synthesis.
CCDC 1861502, 1865893, 1865901, 1866337, 1866344, 1866360, 1556652 and around the tetrahedral Cu(I) coordination sphere, leading to
918545. For ESI and crystallographic data in CIF or other electronic format see
1
18–24
highly emissive luminophores with long-lived excited state.
DOI: 10.1039/c9ra04886c
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Another useful approach towards blocking the four-coordinate conditions. To illustrate the effect of steric challenge and
geometry is via bridging halides or other potential linkers in rigidity on the coordination chemistry and photophysical
bi- and polynuclear complexes that will not be discussed in this properties of Cu(I) complexes, the bulky mesBIAN (L6) ligand
25–27
text.
has also been synthesized. The extent of stabilizing CH–p and
Recently, emissive three-coordinate heteroleptic Cu(I) other long-range interactions are examined by analyzing the
complexes supported by bulky phosphine ligands have been re- structural parameters from single-crystal X-ray diffraction. The
28,29
ported by groups of Peters and others.
While the possibility of electrochemical and photophysical properties of these
Jahn–Teller distortion in the excited state is eliminated in the complexes are examined and compared to the Cu(I) complexes
three-coordinate geometry, the exciplex formation can still occur bearing unsubstituted Hdpa and symmetrically substituted
3
0–33
especially in coordinating solvents. Alternatively, Thompson,
Gaillard
Me Hdpa ligands. In addition, density functional theory (DFT)
2
34–36
and their co-workers have investigated three- calculations are performed to rationalize the relationship
coordinate mononuclear Cu(I) complexes bound to sterically between the electronic structure and photophysical property in
challenged NHC and bidentate N-donor ligands. The choice of the these complexes.
ligand, guided by the high stability of Cu(I)–NHC complexes and
easy modulation of the nitrogen ligand, has led to
[
Cu(NHC)(N^N)]X complexes (N^N ¼ dipyridylamine and its
Results and discussion
Synthesis of heteroleptic Cu(I) complexes
geometrically symmetrical derivatives) with very promising lumi-
nescent properties. In their work, Gaillard and co-workers had
synthesized a series of Cu(I) complexes using symmetrically- The [Cu(NHC)(N^N)]PF
substituted Hdpa ligands bearing methyl groups on two pyridyl a single-step reaction using [Cu(NHC)Cl], the N-donor ligand,
rings. Interestingly, the position of the methyl groups plays an and KPF under an inert atmosphere at room temperature
important role in the luminescent properties of the complexes, (Scheme 1). Both [Cu(IPr)Cl] and [Cu(SIPr)Cl] precursors were
6
complexes (1–12) were synthesized in
6
0
with the highest and lowest quantum yields obtained using 3,3 - prepared according to a slightly modied Cazin–Nolan proce-
0
Me
2
Hdpa and 4,4 -Me
2
Hdpa ligands, respectively.
dure using CuCl, the imidazolium/imidazolinium salt, and
K CO as a base. Dipyridylamine ligands were obtained using
2 3
37
The strong emissive properties of these complexes have been
assigned partly to their stable excited-state geometry that a Buchwald–Hartwig coupling reaction between 2-bromopyr-
resulted from the stabilizing CH–p system interactions between idine and 2-aminopyridine derivatives. The optimal amount of
the NHC and the Hdpa ligands. It could be envisioned that Pd
2 3
(dba) catalyst for these syntheses was found to be 1.0 mol%,
disrupting such stabilizing interactions in the [Cu(NHC)(Hdpa)] half of the amount used previously for the symmetrical Me
-
2
34
X complexes would affect their emissive properties. However, Hdpa ligands. Interestingly, some ligands such as L2 and L3
knowledge about the extent of their contribution to the were obtained only at 20% or lower yields using 2.0 mol%
observed photophysical activity has yet to be developed. We catalyst loading, whereas the reduced amount of the catalyst
report herein the synthesis of a new series of cationic Cu(I) provided great to excellent yields of all ligands.
complexes bearing bulky NHC ligands (IPr, SIPr) and Me Hdpa
6
Regarding [Cu(NHC)(N^N)]PF complexes, the yield is
2
ligands with methyl groups positioned unsymmetrically on two dependent on the steric hindrance of the N-donor ligands as
pyridyl rings (Chart 1, L1–L3). Also, we have prepared Cu(I)– depicted in Table 1. However, the effect appears to be less
0
NHC complexes containing Hdpa (L4) and 3,3 -Me
for direct comparison of chemical and photophysical properties analogues as shown by a close yield of [Cu(IPr)(L1)]PF
to the unsymmetrically-substituted analogues under the same [Cu(IPr)(L2)]PF (2) complexes. A clear deviation from the ex-
pected trend based on steric hindrance was found for
Cu(NHC)(L2)]PF complexes that were isolated at a higher yield
than those with L4 and L5 ligands. Also, the formation of
Cu(NHC)(N^N)]PF complexes with bulky N-donor ligands
2
Hdpa (L5) pronounced than what is found for the symmetrical Me
2
Hdpa
(1) and
6
6
[
6
[
6
appears to be dependent on the nature of the carbene's back-
bone, i.e. saturated vs. unsaturated. While both [Cu(IPr)(L3)]PF6
(
3) and [Cu(IPr)(L6)]PF6 (6) complexes were obtained at
adequate yields, no [Cu(SIPr)(L3)]PF6 (9) and only 10%
Cu(SIPr)(L6)]PF (12) formed aer a two-day reaction as
[
6
1
conrmed by HNMR analysis. The synthesis of the heteroleptic
Cu(I) complexes is expected to follow a two-step mechanism,
Chart 1 Symmetrical and unsymmetrical Me
used in this study.
2
Hdpa and NHC ligands Scheme 1 The formation of heteroleptic Cu(I) complexes via a mono-
coordinate Cu–NHC intermediate.
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Table 1 Isolated yields of heteroleptic Cu(I) complexes
tetrahydrofuran and hexane solution and report its crystal
structure here (Fig. S1†). ORTEP diagrams of IPr- and SIPr-
based complexes are shown in Fig. 1. A collection of distances
and angles are presented in Table 2 with crystallographic data
collection parameters summarized in Tables S1 and S2.†
As expected for the heteroleptic Cu(I) complexes, each Cu
center is bound to the NHC and the bidentate N-donor ligand in
a three-coordinate geometry. The dipyridylamine ligation in
complexes 1–3, 7, and 8 results in a 6-membered chelate unlike
mesBIAN that forms a 5-membered ring with Cu in 6 and 12. A
common feature among the heteroleptic Cu(I) complexes in this
study is the presence of long-distance interactions in their solid
state. This includes, but is not limited to, the non-bonding
CH/p interactions between the hydrogen atom(s) at the
a position of the pyridyl rings and the p system of the diiso-
propylphenyl groups on the NHC ligands. The structures in
which the calculated distance between the a hydrogen atom and
a
b
Entry
Complex
Yield
1
2
3
4
5
6
7
8
9
[Cu(IPr)(L1)]PF
[Cu(IPr)(L2)]PF
[Cu(IPr)(L3)]PF
[Cu(IPr)(L4)]PF
[Cu(IPr)(L5)]PF
[Cu(IPr)(L6)]PF
[Cu(SIPr)(L1)]PF
[Cu(SIPr)(L2)]PF
[Cu(SIPr)(L3)]PF
[Cu(SIPr)(L4)]PF
[Cu(SIPr)(L5)]PF
[Cu(SIPr)(L6)]PF
6
6
6
6
6
6
(1)
(2)
(3)
(4)
(5)
(6)
77
83
62
70
53
68
69
85
0
6
(7)
6
6
6
6
6
(8)
(9)
(10)
(11)
(12)
1
1
1
0
1
2
63
78
ꢁ10
c
a
Reaction conditions: [Cu(NHC)Cl] (1 equiv.), N^N ligand (1 equiv.),
b
c
KPF
6
(10 equiv.), THF, stir at r.t. Isolated yield. Calculated from the
1
H NMR spectrum of the reaction mixture containing the product and
Cu(IPr)Cl].
[
˚
the center of the NHC aromatic ring is shorter than 3.05 A are
3
8
identied to possess this interaction. As shown in Table 2, all
the dipyridylamine complexes present this interaction, except 3
wherein a mono-coordinate [Cu(NHC)]PF
6
forms initially from
the removal of Cl with tenfold excess KPF (Scheme 1). This
was conrmed by a white solid isolated from the [Cu(IPr)Cl] and
ꢀ
6
that has a much longer CH/p distance than the cut-off. The
KPF6 reaction that shows a different composition from the sp -CH/p interactions were considered crucial for the forma-
2
starting material by elemental analysis (ESI†).
tion of complexes with symmetrical Me
2
Hdpa ligands to an
0
extent that [Cu(IPr)(6,6 -Me
2
Hdpa)]PF
6
could not form due to
34
the lack of a hydrogen atoms on the Hdpa ligand. Our ndings
support the signicance of CH/p interactions to the stability
X-ray crystallography of Cu(I) complexes
Single crystals of complexes 1–3, 6–8, and 12 were obtained by of the Cu(I) complexes bearing unsymmetrical Me
Hdpa ligands
2
0
slow diffusion of hexane into a solution of copper complexes in as well. Nonetheless, the synthesis of [Cu(IPr)(3,6 -Me
Hdpa)]
2
2
dichloromethane at room temperature. The red crystals of 12 PF
were isolated carefully from a mixture of the complex and indicates the contribution of other parameters to its stability.
starting precursor, [Cu(SIPr)Cl], to further verify its formation
Not surprisingly, due to the lack of a hydrogen atom with
and structure by X-ray crystallography. Also, we obtained single respect to nitrogen in the mesBIAN ligand, the interligand sp -
crystals of [Cu(SIPr)Cl] precursor from
mixture of CH/p interactions do not exist in complexes 6 and 12.
6
(3) with no sp -CH/p interaction at a good yield (Table 1)
2
a
6 6 6 6 6 6
Fig. 1 ORTEP diagrams of [Cu(IPr)(L1)]PF (1), [Cu(IPr)(L2)]PF (2), [Cu(IPr)(L3)]PF (3), [Cu(IPr)(L6)]PF (6), [Cu(SIPr)(L1)]PF (7), [Cu(SIPr)(L2)]PF
(
8), and [Cu(SIPr)(L6)]PF (12). Only one of the cationic complexes in the unit cell are shown for 1, 7, and 12. Anions and some hydrogens are
6
omitted for clarity. Ellipsoids are shown at the 55% probability level.
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6
Table 2 Selected bond lengths and angles for [Cu(NHC)(N^N)]PF complexes
C
NHC
–
Plane angle
q (deg)
a
ꢂ
˚
˚
CH/p ( A˚ )
˚
Complex
Cu–CNHC (A)
Cu–NN^N (A)
NH/CH/F (A)
Cu–NN^N ( )
[
Cu(IPr)(L1)]PF
6
(1)
Cu1
Cu2
1.891(2)
1.906(2)
Cu1
Cu2
2.019(2)
2.038(2)
2.015(2)
Cu1
Cu2
2.78
2.45
2.70
Cu1
Cu2
2.25 (NH)
2.48, 2.54 (CH)
2.25 (NH)
2.31(CH)
2.43 (NH)
2.43 (CH)
2.68 (CH)
Cu1
Cu2
138.23(7)
Cu1
Cu2
35.52
59.76
131.94(7)
134.08(7)
135.24(7)
136.22(7)
134.21(7)
134.44(7)
134.02(7)
133.86(6)
147.70(6)
2
.044(2)
2.032(2)
.041(2)
2.020(2)
.029(2)
2.231(3)
.062(2)
[
[
[
Cu(IPr)(L2)]PF
Cu(IPr)(L3)]PF
Cu(IPr)(L6)]PF
6
6
6
(2)
(3)
(6)
1.900(2)
1.887(2)
1.922(3)
2.58
2.95
47.13
76.06
45.89
2
2
2.53 (CHiPr
2.74 (CHMe)
)
2.32 (CHNHC
2.47 (CHNHC
)
)
2
2
2
2
.77 (CHiPr
.94 (CHace
.98 (CHace
)
)
)
[
Cu(SIPr)(L1)]PF
6
(7)
Cu1
Cu2
1.893(2)
1.917(2)
Cu1
Cu2
2.026(2)
2.020(2)
2.021(2)
Cu1
Cu2
2.74
2.42
2.74
2.26 (NH)
2.50 (CH)
2.28 (CH)
Cu1
130.70(8)
139.25(8)
134.27(9)
135.30(9)
134.13(7)
135.57(7)
138.4(1)
142.5(1)
138.4(1)
142.7(1)
Cu1
Cu2
38.77
59.59
Cu2
Cu2
2
.052(2)
2.045(2)
.046(2)
[
Cu(SIPr)(L2)]PF
6
6
(8)
1.918(2)
2.46
2.94
2.65 (CHiPr
2.71 (CHMe
2.24 (NH)
2.61 (CH)
2.26 (NH)
2.65 (CHNHC
45.04
2
b
[
Cu(SIPr)(L6)]PF
(12)
Cu1
Cu2
1.914(4)
1.914(4)
Cu1
Cu2
2.126(3)
2.137(3)
2.087(3)
)
Cu1
Cu2
Cu1
Cu2
48.46
49.74
)
)
2.76 (CHiPr
2.87 (CHace
.92 (CHace
)
2.155(3)
)
)
2
a
b
The distances have been determined only for the asymmetric unit. The CH/p distances are shown only for one of the cationic complexes (Cu1)
in the asymmetric unit.
However, the presence of several aromatic rings may facilitate complexes will be elucidated via structural analysis and pho-
3
the sp -CH/p interactions between the aliphatic hydrogens tophysical studies (vide infra). The q parameter in the table
and the p systems of the NHC and diimine ligands in these describes the angle between the NHC ring and the plane formed
complexes. Furthermore, 12 benets from an intermolecular by three nitrogen atoms in the Hdpa ligand or the plane of the
2
sp -CH/p interaction between the CH groups at position 6 of mesBIAN ligand (Fig. S4†). There seem to be no direct correla-
the BIAN ring and the p system of the phenyl ring in SIPr tion between q and the ligand type, e.g. saturated NHC vs.
3
(
Fig. S2†). The sp -CH/p interactions in 6 occur between the unsaturated NHC or substituted Hdpa vs. unsubstituted
isopropyl groups of IPr and the aromatic mesityl substituents of derivatives.
mesBIAN as well as the methyl groups of mesBIAN and aromatic
phenyl ring of IPr (Fig. S3†). The complexes 6 and 12, also,
Electrochemical study of Cu(I) complexes
2
benet from an intraligand interaction between the sp -CH at
The cyclic voltammetry (CV) data were measured to investigate
the electrochemical features and to estimate the highest occu-
pied molecular orbital (HOMO) and lowest unoccupied molec-
ular orbital (LUMO) energy levels of the complexes 1–11. The
electrochemical data are summarized in Table 3. As shown in
Fig. 2a, all Cu–IPr complexes bearing Hdpa and its derivatives
display an electrochemically quasi-reversible/reversible oxida-
positions 5 and 10 of the BIAN ring and the aromatic ring of the
mesityl substituents (Fig. S2 and S3†). Additionally, the effect of
NH/F and CH/F hydrogen-bonding interactions on the
stability of the current heteroleptic Cu(I) systems should not be
disregarded. For all the complexes in Table 2, the H/F distance
ꢀ
2
between the F atoms of PF
groups of Hdpa falls within the range 2.24–2.72 A. Since the
6
anion and the NH or sp -CH
˚
+
tion at 0.61–0.65 V (vs. Fc /Fc), assigned to the Cu(II)/Cu(I)
distance is shorter than the sum of van der Waals radii of H and
˚
couple. The reversibility of this event is more prevalent for 4 and
its unsymmetrical analogs shown by a linear plot of current
versus the square root of scan rate (Fig. S5†). The partial
reversibility was conrmed by a shi of the reduction or
oxidation potentials at varying scan rates for all the Cu–IPr
complexes, but is more distinct for complex 5 (Fig. S6†). On the
other hand, all the Cu–SIPr complexes containing Hdpa ligand
and its derivatives reveal a predominantly quasi-reversible
Cu(II)/Cu(I) couple (Fig. 2b). The shape of the rst oxidation
peak of 4, 5, and 10 appears different in our work from that
F (2.76 A), the interaction can be classied as a legitimate
39
hydrogen bond. It should also be noted that 3 is the only
Hdpa-based complex in the table in which no NH/F interac-
tion was found. The signicance of hydrogen-bonding to the
shaping, stability, and self-assembly of organic molecules has
39–41
been extensively investigated,
however, their effect on the
transition metal-containing structures is far less understood. In
this work, the interplay between the long-range interactions, the
plan angle q, and the light-emitting properties of the copper
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Table 3 Photophysical and electrochemical data for [Cu(NHC)(N^N)]PF
6
complexes
abs a
em b
ox
1/2, DE
c
d
d,g
e
e
Complex
l
max (nm)
l
max (nm)
E
p
(V)
E
Pc, Epa (V)
HOMO/LUMO (eV)
PLQY/FPL
TPL (ms)
f
[
[
[
[
[
[
[
[
[
[
Cu(IPr)(L1)]PF
Cu(IPr)(L2)]PF
Cu(IPr)(L3)]PF
Cu(IPr)(L4)]PF
Cu(IPr)(L5)]PF
Cu(IPr)(L6)]PF
Cu(SIPr)(L1)]PF
Cu(SIPr)(L2)]PF
Cu(SIPr)(L4)]PF
Cu(SIPr)(L5)]PF
6
6
6
6
6
6
(1)
(2)
(3)
(4)
(5)
(6)
253, 302
254, 306
254, 307
252, 305
251, 303
241, 321, 336
254, 307
254, 309
254, 307
254, 304
468
475
482
453
458
ꢁ490
472
471
466
473
0.62, 0.276
0.63, 0.278
0.63, 0.266
0.65, 0.244
0.61, 0.267
1.07, 0.364
0.61, 0.425
0.65, 0.294
0.67, 0.298
0.68, 0.376
ꢀ2.29, 0.524
ꢀ2.20, 0.530
ꢀ2.22, 0.502
ꢀ2.35, 0.546
ꢀ2.35, 0.514
ꢀ1.27, 1.03
ꢀ2.15, 0.603
ꢀ2.16, 0.562
ꢀ2.29, 0.629
ꢀ2.25, 0.601
ꢀ5.32/ꢀ2.51 (ꢀ2.04)
ꢀ5.33/ꢀ2.60 (ꢀ2.04)
ꢀ5.30/ꢀ2.58 (2.03)
ꢀ5.35/ꢀ2.45 (ꢀ2.03)
ꢀ5.31/ꢀ2.45 (ꢀ1.99)
ꢀ5.83/ꢀ3.53 (ꢀ3.3)
ꢀ5.40/ꢀ2.65 (ꢀ2.20)
ꢀ5.36/ꢀ2.64 (ꢀ2.19)
ꢀ5.43/ꢀ2.51 (ꢀ2.13)
ꢀ5.40/ꢀ2.55 (ꢀ2.25)
0.48
0.49
0.64
0.21
0.87
0
0.55
0.33
0.85
0.83
18
19
23
15
35
6
(7)
24
19
15
25
6
6
6
(8)
(10)
(11)
a
b
c
Data obtained in CH
complexes and 0.1 M TBAPF
wave potential of 0 and DE
¼ Epc ꢀ Epa ¼ 0.174 V was determined for ferrocene under the same conditions. The values in () were calculated by the
difference between the HOMO levels and the HOMO–LUMO gap obtained from the absorption edge of the UV/Vis spectra.
2
Cl
2
at room temperature. Data obtained in solid state at room temperature. Recorded in CH
2
Cl
2
containing 2 mM Cu(I)
ꢀ1
ꢂ
d
ꢀ1
ꢂ
e
f
6
at 50 mV s scan rate and 22 C. Recorded at 100 mV s scan rate and 22 C. Estimated errors ꢃ5%. The half-
g
p
34
reported earlier. While this event was irreversible and attrib- levels were also calculated by considering the difference
uted to the oxidation of NH group of Hdpa ligands, we have between their HOMO levels and the corresponding HOMO–
determined a reversible/quasi-reversible couple and assigned it LUMO gap determined from the absorption edge of their UV-Vis
to a metal-centered oxidation feature. In addition to the pre- spectra. The calculated HOMO and LUMO values for complexes
sented data in Fig. S5 and S6,† the comparative cyclic voltam- 1–11 in solution are listed in Table 3.
mograms of the solvent and the Hdpa ligand support the
assignment (Fig. S7†). A metal-based oxidation is also more
Photophysical study of Cu(I) complexes
consistent with a similar feature observed in the CV studies of
2
+
The UV-Vis absorption spectra, recorded in CH Cl , and the
solid-state emission spectra of complexes 1–11 are shown in
Fig. 3, S9,† and Table 3. The intense absorption bands around
[
Ru(Hdpa) (bpy)
]
(n
¼
3–0) and [Ni(Hdpa) (salo)]Cl
2
2
n
3ꢀn
2
complexes and with the HOMO composition in the DFT calcu-
42,43
lations (vide infra).
Nonetheless, our assessment of the
irreversible reduction features at around ꢀ2.3 V and ꢀ2.5 V vs.
+
Fc /Fc in the CV of the Hdpa/Me
2
Hdpa complexes is in line with
the previous assignments to the Cu(I)/Cu(0) and Hdpa
34,44
reduction.
As expected the electron-transfer behavior of complex 6 is
different from the Hdpa-containing analogs. The CV of this
complex (Fig. S8†) presents a quasi-reversible Cu(II)/Cu(I) couple
+
at 1.30 V (vs. Fc /Fc) resembling the same oxidation event in the
CV of Cu(I)–(aryl-BIAN) complexes bearing nitrile and phos-
45
phine co-ligands. The quite large difference (0.4 V) between
the oxidation and reduction waves of this couple and the
substantial decrease in the cathodic peak current is likely due to
some rearrangements or chemical processes during the oxida-
tion. When continuing the cathodic scan of complex 6, two
quasi-reversible reduction waves appear at ꢀ1.41 and ꢀ2.03 V
+
(
vs. Fc /Fc) assigned to the Cu(I)/Cu(0) and mesBIAN/
ꢀ
mesBIAN c couples. The reduction potential of the ligand to its
radical anion form is reasonably close to that of the free ligand
45
reported under similar conditions. While the authors did not
observe this reduction couple in the cathodic scan of the bis-
chelate Cu(I)–aryl-BIAN complexes, this could be due to
a shorter potential window utilized than that for 6 in this study.
The onset oxidation and reduction potentials of complexes
1–11 were used to calculate the HOMO and LUMO levels
according to the equation EHOMO/LUMO ¼ ꢀ(E[onset, ox/red vs. Fc
+
/Fc]
46,47
+
4.8) eV.
Since the reduction potential of the dipyridylamine Fig. 2 Cyclic voltammogram of (a) [Cu(IPr)(Me Hdpa)]PF and (b)
2
6
complexes is close to the onset of solvent reduction, erroneous [Cu(SIPr)(Me
2
Hdpa)]PF
6
complexes (2.0 mM) in deaerated CH
2
Cl
2
/-
n
ꢀ1
Bu
4
NPF
6
(0.1 M) at a scan rate of 100 mV s indicating Cu(II)/Cu(I)
reading of the onset potential might occur. Thus, the LUMO
redox couple.
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4
ꢀ1
ꢀ1
2
51–254 nm (3 ¼ (2.4–4.3) ꢄ 10 L mol cm ) in the Hdpa- range 15–35 ms (Table 3). In this series, complex 4 exhibits the
based complexes are attributed to the p–p* ligand-centered lowest yield and complexes 5, 10, and 11 show the highest
(
3
LC) transitions. The relatively weaker bands around 302– quantum yields with 87% (T ¼ 35 ms), 85% (T ¼ 15 ms), and 83%
4
ꢀ1
ꢀ1
09 nm (3 ¼ (1.4–1.9) ꢄ 10 L mol cm ) in these complexes (T ¼ 25 ms), respectively. The calculated QY values for 4, 5, and
34
are ascribed to the metal-to-ligand charge transfer (MLCT) 10 are consistent with the previous report, however, the
transitions. In complex 6, the p–p* LC transitions result in an emission lifetime of 10 is substantially different.
34
intense shoulder below 250 nm beside the two equal-intensity
4
ꢀ1
ꢀ1
bands at 321 nm and 336 nm (3 ¼ 1.8 ꢄ 10 L mol cm
)
48
Structure and light interaction relationship
due to the spin-allowed MLCT transitions (Fig. S9a†). Addi-
tionally, the broad band between 350 and 480 nm could be Regarding the structure effect on the emission prole, different
assigned to the intraligand p–p* charge transfer (ILCT) from trends were found for the Cu–IPr and Cu–SIPr complexes
the aryl groups to the naphthyl backbone and/or a low-energy bearing the same Hdpa ligands. While 4 is the weakest emitter
48,49
+
MLCT band.
among the [Cu(IPr)(N^N)] complexes, 10 has the highest QY in
+
The PL spectra of the complexes were recorded in solid state the [Cu(SIPr)(N^N)] series. The difference cannot be simply
at room temperature using an excitation of 340 nm. As shown in attributed to the greater electron-donating ability of SIPr than
Fig. 3, the Hdpa-bound complexes display broad emission IPr given the reverse order of the quantum yields for 2 vs. 8 and
spectra with PL maxima in the range of 453–482 nm. In 5 vs. 11 (Table 3). Given the PL prole of these complexes in
contrast, the PL spectrum of complex 6 shows a very weak solid state, close attention should be paid to the long-range
emission around 490 nm (Fig. S9†). Furthermore, the methyl interactions within the solid matrix. Three structural parame-
substitution of the Hdpa ligands leads to the red-shied emis- ters relevant to the emissive properties of the Hdpa-based
sion maxima in Cu–IPr complexes in the order L4 < L5 < L1 < L2 copper complexes are listed in Table S3.† Of these parame-
<
L3. In contrast, the emission pattern and PL maxima of the ters, the NH/CH/F interactions are detrimental to PL, most
substituted Cu–SIPr complexes are very similar indicating that likely due to inhibiting the planarization of the central nitrogen
3
2
the position of methyl groups has a minimal effect on the atom and change of its hybridization from sp to sp during the
emissive energy of these complexes. Exploring the emission and photoexcitation. This change is necessary to access a stable
absorption proles of 1–11, the PL quantum yields (PLQY) were triplet state and an effective emissive system. On the other
estimated between 21% and 87% with emission lifetimes in the hand, the CH/p interactions are in favor of the PL property,
Fig. 3 (a and b) Absorption spectra of complexes 1–11 (except 6) in CH
Cl
2 2
solution at room temperature. (c and d) Photoluminescent (PL)
spectra of the complexes in the solid state.
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possibly due to enhancing the p–p* character of electronic functionals and basis sets indicating the sensitivity of this
transitions, e.g. T1 / S0 via the interligand CT (vide infra). interaction to the matrix effect in solid state (Fig. S10†). The
Finally, the large plane angle q has a positive impact on the CH/F hydrogen-bonding interactions between the phenyl
emission yield by breaking the symmetry in the solid matrix and rings of IPr or the b hydrogen atoms of pyridyl rings and F
allowing the symmetry-forbidden transitions to intensify. This atoms seem to contribute greatly to the matrix effect and
parameter is mostly prominent in 3, with no favorable CH/p lengthening of the CH/p distance in complex 3 (Fig. S10†).
interaction and with relatively long CH/F distances. Complex 3 These interactions are disregarded in the gas-phase DFT
displays PL with a QY of 0.64 (T ¼ 23 s), making it the third calculations, thus leading to a closer CH/p distances than
0
highly emissive [Cu(IPr)(N^N)]PF
6
complex aer [Cu(IPr)(3,3 - those determined experimentally.
Me
OMe
Me Hdp)]PF and [Cu( IPr
)(Hdpa)]PF reported by Gaillard
Electronic structure. The ground-state electronic structures
It is also worth noting that 3 possesses the of complexes 1–8, and 10 were calculated to determine the
largest plane angle q among the reported [Cu(NHC)(N^N)]PF6 energies and composition of HOMO and LUMO. The HOMO–
2
6
6
34,36
and coworkers.
complexes and those presented here (Table S3†).
LUMO levels and gaps are greatly affected by the DFT methods,
with hybrid functionals, UB3LYP and M06, providing smaller
gaps than those obtained with dispersion-including methods
such as wB97XD (Fig. S11†). The orbital energies, the HOMO–
Computational study of Cu(I) complexes
Density functional theory (DFT) and time-dependent DFT (TD- LUMO gaps, and the composition of frontier and other close
DFT) were used to rationalize the fundamental properties of lying orbitals in copper complexes are listed in Tables S5 and
these copper complexes and provide a theoretical foundation to S6.† Different contributions from copper and Hdpa ligand to
explain the experimentally detected luminesce behavior. The the HOMO orbitals of complexes were predicted using the three
details of the theoretical calculations are described in the ESI.† DFT methods. While UB3LYP predicts a great contribution (61–
Despite the diverse range of functionals and basis sets used for 73%) from the d orbitals of copper to the HOMO, this orbital is
copper complexes in literature that oen provide good largely distributed (58–80%) over the p* orbitals of the Hdpa
50–53
results,
CT systems, requires their careful screening in the lumines- of the complexes, 1, 4, and 10, the Hdpa orbitals contribute
cence studies.
substantially (35–40%) to the HOMO frontier orbital despite the
the functional sensitivity of TD-DFT, especially for ligand in complexes 2, 3, 5, 7, and 8 at wB97XD level. For the rest
54
Geometry optimization. The prediction of structures is copper dominance. The composition of the molecular orbitals
always the rst and most important step in computational predicted at M06 level are generally close to that obtained at
studies, since the subsequent steps depend on its accuracy. The UB3LYP except for 2 and 8, wherein copper contribution has
full geometrical optimization of complexes 1–8 and 10 was reduced signicantly. Additionally, the LUMO frontier orbitals
performed using three levels of theory, UB3LYP, M06, and of all complexes are mostly localized on the p orbitals of the
wB97XD, and two basis sets, LANL2DZ and 6-31+G(d,p). We Hdpa ligand (>95%) as predicted by all methods.
have avoided using the combination of effective core potential
For the HOMO of complex 6, a major contribution (68–78%)
basis sets, e.g. SDD on the metal and 6-31+G(d,p) on the other from the p* orbitals of mesBIAN and a much smaller distri-
53
elements due to a possible overestimation of binding energy.
bution (20–25%) over copper orbitals were calculated at three
To assess the reliability of the theoretical methods, the calcu- levels. Fig. 4 depicts the electron density distributions of the
lated structural parameters were compared to those obtained HOMO and LUMO of complexes 1 and 6 at wB97XD/6-31+G(d,p)
experimentally from X-ray crystallography. It should be noted level.
that in comparing the crystal structure data to the gas-phase
Regarding the effect of methyl groups on the HOMO–LUMO
optimized DFT results the matrix effect of the solid state is gap, there are marginal differences between unsubstituted and
neglected. This includes the long-range interactions with the substituted Hdpa complexes at three theory levels. Further-
PF
6
anion, possible p–p stacking, and the interactions with the more, all saturated SIPr complexes have a slightly larger
solvent molecule trapped in the solid form. The quality of HOMO–LUMO gap than their unsaturated IPr analog due to the
describing the complex's structure at a certain DFT level was more electron-donating carbene and a more stabilized HOMO
evaluated by subtracting the theoretically determined bond level. A comparative HOMO–LUMO level energy diagram of all
parameter (b.p.) from its experimental counterpart using the complexes is shown in Fig. S12.†
(
b.p.calc ꢀ b.p.exp)/b.p.exp ꢄ 100 (Table S4†). The bond lengths
Excited state. TD-DFT calculations were performed to ratio-
and angles of the dipyridylamine-linked Cu–IPr and Cu–SIPr nalize the nature of the electronic transitions in the absorption
complexes are estimated reasonably within 5.0% using all the and emission spectroscopy of the copper complexes. Given the
six functionals/basis sets combinations. The lowest over- or different HOMO–LUMO gap and MO composition of the fron-
underestimation of the Cu–CNHC bonds was obtained with tier orbitals obtained with hybrid and non-hybrid functionals,
wB97XD/6-31+G(d,p) with values between 0.2 and 1.9%, while we have performed TD-DFT at both wB97XD and M06 levels. In
the lowest Cu–NN^N bond's deviation was calculated between these calculations, both singlet–singlet and singlet–triplet
0
.1 and 1.7% using M06/LANL2DZ. Remarkably, the CH/p transitions were considered due to the experimentally detected
distances were estimated with deviations ranging from 0.2 to emissive properties and relatively long emission lifetimes (from
1.9% for all the complex ions except 3. This distance in 3 is 15 to 35 ms). Assignment of the character for each excited state
underestimated with values between 20.0 to 34.5% using all the was based on the compositions of the occupied and virtual MO
1
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(CH
2
Cl
2
) within M06 and wB97XD methods. To simulate the
absorption spectra, a superposition of Gaussian peaks centered
at the excitation energies were considered, with height equal to
the oscillator strength and a width of 0.25 eV. The theoretically
predicted UV-Vis spectrum of complex 4 at M06 level shows
three peaks at 297, 250, and 216 nm, in good agreement with
the experimental data. When simulated at wB97XD level, the
absorption peaks are shied to shorter wavelengths at 275, 234,
and 186 nm, making the calculations with M06 method a better

t to the experimental data (Fig. 5). It is worth noting that the
simulated spectrum in the gas phase resembles the same
features but at slightly red-shied areas at both DFT levels
(Fig. 5). Furthermore, a good agreement with the experimental
data was found for the simulated UV-Vis spectra of 1 and 10 at
M06 level, as examples of methyl-substituted dipyridylamine
and saturated NHC complexes (Fig. S13†). According to TD-DFT,
the lowest-energy absorption at 294 nm in 1 arises predomi-
nantly from the HOMO ꢀ 1 / LUMO electronic transition
which can be described as a mixture of MLCT and LCCT tran-
sitions (Fig. S14†). Interestingly, no shi was observed in the
location of this peak when the spectrum was recorded in
3
CH CN, a more polar solvent (Fig. S15†). This can be rational-
0
+
2
Fig. 4 (a) HOMO and (b) LUMO of [Cu(IPr)(3,4 -Me Hdpa)] (1), and (c)
+
ized by the sum of two contrary solvent effects, hypsochromic
and bathochromic, on the MLCT and LCCT transitions that
results in no change in the energy of the band arising from their
mixture. This is further validated by the red shi of the
HOMO and (d) LUMO of [Cu(IPr)(mesBIAN)] (6) calculated at
wB97XD/6-31+G (d,p) level.
absorption band at 250 nm in CH Cl to 260 nm in CH CN,
consistent with the p / p* character of this electronic transi-
tion. The TD-DFT calculations describes a predominantly
of the dominant excitation of the excited state. Thus, if the
occupied orbital is metal-based, and the virtual orbital is mainly
localized on the N^N or NHC ligand, the transition is desig-
nated as metal-to-ligand charge transfer (MLCT). In this
2
2
3
interligand transition character for this band arising from the p
0
orbitals of IPr to the p* orbitals of 3,4 -Me
LUMO).
2
Hdpa (HOMO ꢀ 5 /
context, three singlet–singlet transitions, S
/ S , and the rst singlet–triplet transition (S
copper complexes are listed in Tables S7 and S8,† calculated at
0
/ S
1
, S
0
/ S
2
, and
S
0
3
0
/ T
1
) in
wB97XD and M06 level, respectively. The very weak oscillator Experimental methods
strength (f) of the rst two singlet–singlet transitions has been
General procedure
attributed to them being inhibited by symmetry in the gas-
34
The Cu(I) complex syntheses were performed using standard
Schlenk techniques or in an MBraun glove box under Ar
atmosphere. Imidazolium salts, IPr$HCl and SIPr$HCl were
purchased from TCI America. [Cu(IPr)Cl] and [Cu(SIPr)Cl]
precursor complexes were prepared according to our modied
phase DFT calculations. In solid state, the symmetry breaks
down due to the counter anion allowing these transitions to
happen.
Regardless of the different excitation energy and the main
MO contributor to the S / S transition, both wB97XD and
0 1
M06 exhibit comparable emission energy and transition weight
37
2
version of Cazin–Nolan's procedure. The ligands, Me Hdpa
and mesBAIN, were synthesized according to the modied
literature procedure (ESI†). The solvents used for the syntheses
and analytical studies of copper complexes were puried and
for the S / T transition (Table 4). Furthermore, both methods
describe distinct characters for these transitions in the Hdpa-
based complexes from those in complex 6. For 1–5 and 7–10,
0
1
distilled under N atmosphere before being stored over acti-
the S
/ T
based charge transfer (LCCT). In contrast, both S
/ T in 6 are spatially identical and have predominantly
0
/ S
transition is a complex mixture of MLCT and ligand-
/ S and
1
transition is primarily MLCT in nature, whereas the
2
˚
vated molecular sieves (4 A) in the glovebox. Tetrahydrofuran
S
0
1
and diethylether were puried by distillation over Na/
0
1
benzophenone under N
methane, and hexane were distilled from CaH
2
atmosphere. Acetonitrile, dichloro-
2
under N .
S
0
1
LCCT character due to the H / L transition being the main MO
contributor (Tables S9 and S10†).
2
Potassium tert-butoxide (97%) was obtained from Sigma-
Aldrich and crystallized from THF. Tetrabutylammonium hex-
auorophosphate for electrochemical studies was doubly
recrystallized from dichloromethane and diethylether. NMR
Comparison with experimental data. To further clarify the
agreement between the theoretical calculations and experi-
ments, the absorption spectra of 1, 4, and 10, as exemplary
complexes, were simulated with TD-DFT. The calculations were
performed both in gas-phase and in the presence of a solvent
samples were prepared under Ar in CDCl , containing 0.05%
3
TMS as internal standard. NMR spectra were measured using
a 400 MHz Bruker Avance III spectrometer. Chemical shis (d)
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Table 4 Vertical transition energies, absorption wavelength (l in nm), oscillator strengths (f), and main MO contribution for the first singlet–
singlet (S
0
/ S
1
) and singlet–triplet (S
0
/ T
1
) transitions calculated with wB97XD/6-31+G(d,p) and M06/6-31+G(d,p)
a
S
0
/ S
1
0 1
S / T
b
Complex
Transition type (weight, %)
E (eV)
l (nm)
f
Transition type (weight, %)
E (eV)
l (nm)
Method
+
[
[
[
[
[
[
[
[
[
Cu(IPr)(L1)] (1)
H / L + 1 (45)
H / L (88)
4.25
3.80
4.23
3.76
4.29
3.83
4.24
3.77
4.23
3.77
2.42
2.04
4.14
3.83
4.15
3.86
4.15
3.80
291.8
326.5
293.0
329.8
289.2
323.9
292.4
328.9
292.8
328.7
511.4
608.8
299.3
324.1
299.1
320.9
298.7
326.6
0.003
0.003
0.002
0.001
0.012
0.004
0.001
0.000
0.000
0.002
0.001
0.001
0.001
0.002
0.002
0.004
0.001
0.001
H ꢀ 1 / L (40)
H ꢀ 1 / L (46)
H / L (43)
3.52
3.56
3.49
3.31
3.51
3.36
3.55
3.39
3.49
3.31
2.23
1.90
3.53
3.35
3.49
3.30
3.56
3.40
351.8
369.5
355.7
375.2
353.7
369.2
349.7
365.8
355.5
374.8
556.5
654.0
351.8
370.1
354.8
375.8
348.5
364.9
wB97XD
M06
wB97XD
M06
wB97XD
M06
wB97XD
M06
wB97XD
M06
wB97XD
M06
wB97XD
M06
wB97XD
M06
+
Cu(IPr)(L2)] (2)
H ꢀ 1 / L (29)
H / L (78)
H ꢀ 1 / L (38)
+
Cu(IPr)(L3)] (3)
H ꢀ 1 / L (32)
H / L (86)
H / L + 1 (58)
H / L (97)
H / L (38)
H ꢀ 1 / L (46)
H ꢀ 1 / L (37)
H ꢀ 1 / L (54)
H / L (39)
+
Cu(IPr)(L4)] (4)
+
Cu(IPr)(L5)] (5)
H ꢀ 1 / L + 1 (41)
H / L (97)
H ꢀ 1 / L (54)
+
Cu(IPr)(L6)] (6)
H / L (87)
H / L (95)
H / L (85)
H / L (93)
H / L (34)
+
Cu(SIPr)(L1)] (7)
H ꢀ 1 / L + 1 (36)
H / L (60)
H ꢀ 1 / L (44)
+
Cu(SIPr)(L2)] (8)
H ꢀ 1 / L + 1 (40)
H ꢀ 1 / L + 1 (56)
H / L + 1 (49)
H / L (98)
H / L (50)
H / L (52)
+
Cu(SIPr)(L4)] (10)
H ꢀ 1 / L (36)
wB97XD
M06
H ꢀ 1 / L (53)
a
b
The oscillator strength for the singlet to triplet transitions is 0 due to them being forbidden by the spin selection rule. For each excitation energy,
the corresponding most relevant molecular orbital transitions are shown with the relative weight.
1
13
for H and C NMR spectra were referenced to the resonance of state emission spectra and emission lifetimes were recorded in
TMS or the residual protio solvent. Elemental analyses were solid state on a CARY ECLIPSE uorescence spectrophotometer.
performed by Atlantic Microlabs, Inc. (Norcross, Georgia). UV- For measuring absolute solid-state emission quantum yield,
visible spectra were measured at room temperature in CH Cl
a solid sample holder accessory was equipped to the CARY
2
2
and CH CN on a CARY 100 Bio UV-Vis spectrometer. Steady ECLIPSE spectrouorometer.
3
6
Synthesis of [Cu(NHC)(N^N)]PF complexes
Under Ar atmosphere, [Cu(IPr)Cl] (1 equiv.) and N^N ligand (1
equiv.) were combined using 4 ml THF in a 20 ml reaction vial.
The solution was stirred for 1.5 hours and then charged with
KPF6 (10 equiv.) providing a white suspension. The reaction
mixture was kept stirring for 22.5 hours at room temperature,
except for complex 3, aer which time it was ltered through
2 2
a plug of Celite and washed with THF and CH Cl . The ltrate
was placed under vacuum to remove the solvent and the
resultant solid was recrystallized from a mixture of dichloro-
methane and hexane.
Solid state quantum yield measurements
The quantum yield measurements were performed aer cali-
brating the incident light to strike at the center of a custom-
designed sample holder. The holder was made from a pair of
quartz cover glasses, glued together in a squared area (0.5 ꢄ 0.5
cm) aer sample placement. A 5 mg portion of a nely-ground
powder was placed inside the area and distributed evenly using
a at-head spatula to cover the entire surface. For quantum
yield measurements, sodium salicylate was used as the refer-
ence with quantum yield (F) of 0.53. The excitation wavelength
of 340 nm was used to obtain phosphorescence spectra for each
compound ranging from 370 nm to 650 nm. Quantum yields
Fig. 5 Experimental and TD-DFT simulated UV-Vis absorption spectra
of [Cu(IPr)(L4)] in gas-phase and in CH Cl at (a) M06 level and (b)
2 2
wB97XD level.
+
55
were calculated using the formula:
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ꢀ
ꢁ
2
luminescent properties of the complexes are consistent with
their analogs bearing modied IPr and Hdpa ligands.
Furthermore, the character of the electronic states involved
in light absorption and emission were elucidated via DFT and
TD-DFT calculations at three theory levels. The data obtained
using M06, a hybrid functional, and wB97XD, a non-hybrid
IðemÞ
IðemÞ
AðexÞ
AðexÞ
n
Cu
Na
Cu
Na
F
Cu ¼ FNa
ꢄ
ꢄ
ꢄ
n
Na
Cu
where I, A, and n refer to the integrated emission, the absor-
bance at excitation wavelength, and the refractive index,
respectively. The integrated emissions were obtained from the
emission spectra. The absorbance values of the reference and
the sample solutions were used at the same wavelength to
calculate the second term in the formula. For refractive indices,
the n values of CH Cl (n ¼ 1.424) and CH OH (n ¼ 1.331) were
functional, support the dominance of MLCT character in S
transitions and a combination of MLCT and LLCT in S
transitions for the Hdpa-based complexes. The simulated
0
/
S
1
0
/
T
1
2
2
3
UV-Vis absorption spectra indicate a close match with the
experimental data in solution at M06 level. While the wB97XD
level calculation was adopted previously to rationalize the
used as the solvents that dissolve the complexes and the refer-
ence, respectively. The emission measurements were repeated
at least three times and an average of three close peak areas
were used to calculate the integrated emission values. The
compatibility of this method and solid samples was assessed by
comparing the quantum yields of complexes 4, 5, and 11 to
those obtained using integrating sphere equipment. The
difference in the QY values was determined to be ꢃ3%.
+
photophysical properties of the [Cu(NHC)(N^N)] complexes,
our work supports the competency of M06 functional in these
types of calculations as well. The combined experimental and
theoretical ndings offer a deeper understanding of the struc-
+
ture and light-interaction relationship in [Cu(IPr)(N^N)] and
+
[
Cu(SIPr)(N^N)] complexes and
a
more rational-based
approach towards efficient copper-based luminophores.
Electrochemistry
Conflicts of interest
Cyclic voltammetry studies of complexes 1–11 and ferrocene at
n
2
mM concentration were performed using 0.1 M Bu
4
PF
6
/
There are no conicts to declare.
2 2
CH Cl as the supporting electrolyte under nitrogen. The elec-
trochemistry experiment set-up consisted of a three-electrode
cell connected to an external CHI 730C potentiostat run by
Acknowledgements
a personal computer with CHI soware. A glassy carbon elec- We gratefully acknowledge Oklahoma State University start-up
trode (0.5 mm diameter) was employed as the working elec- funding (LT), the National Science Foundation REU program
trode, with Ag/AgNO
3
(0.01 M) as the reference and Pt wire as through the grant CHE-1559874 (BN), the OSU fund (Single
the counter electrode. The working electrode was cleaned Crystal X-ray Diffraction in the OSU core facility center), and the
between experiments using a polishing pad and carefully dried. computing at the OSU High Performance Computing Center
+
All potentials are reported versus [Fe(C
5
H
5
)
2
] /[Fe(C
5
H
5
)
2
] by (HPCC) supported in part through the NSF grant OAC–1531128.
subtracting 0.252 V from the measured value versus Ag/AgNO
3
electrode.
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