Physical and photophysical properties of a linear copper(I) complex of a bulky acenapthene-based NHC ligand

Article abstract of DOI:10.1080/00958972.2021.1871901

We report the first example of a charge-neutral linear two-coordinate copper(I) complex bearing a sterically demanding acenaphthoimidazolylidene-based N-heterocyclic carbene (NHC) ligand. The identity and geometry of the complex are confirmed by single-crystal X-ray diffraction (XRD) analysis. The complex is poorly emissive at room temperature, showing either ligand-centered (LC) emission around 340 nm when excited at 300 nm or ligand-to-ligand charge transfer (LLCT) emission at 540 nm when excited at 420 nm; in chloroform, dual emission is observed upon photoexcitation at 300 nm. Nanosecond emission lifetimes were recorded for these processes. This is the first example of emissive linear copper(I) complexes containing this bulky NHC ligand.

Full text of DOI:10.1080/00958972.2021.1871901

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Physical and photophysical properties of a linear
copper(I) complex of a bulky acenapthene-based
NHC ligand
Bryony M. Hockin, Campbell F. R. Mackenzie, David B. Cordes, Alexandra M.
Z. Slawin, Neil Robertson & Eli Zysman-Colman
To cite this article: Bryony M. Hockin, Campbell F. R. Mackenzie, David B. Cordes, Alexandra
M. Z. Slawin, Neil Robertson & Eli Zysman-Colman (2021) Physical and photophysical properties
of a linear copper(I) complex of a bulky acenapthene-based NHC ligand, Journal of Coordination
Chemistry, 74:1-3, 361-379, DOI: 10.1080/00958972.2021.1871901
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JOURNAL OF COORDINATION CHEMISTRY
2021, VOL. 74, NOS. 1–3, 361–379
https://doi.org/10.1080/00958972.2021.1871901
Physical and photophysical properties of a linear copper(I)
complex of a bulky acenapthene-based NHC ligand
Bryony M. Hockina,b , Campbell F. R. Mackenziea , David B. Cordesa
Alexandra M. Z. Slawina , Neil Robertsonb and Eli Zysman-Colmana
,
aOrganic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St
b
Andrews, UK; EaStCHEM School of Chemistry, University of Edinburgh, Edinburgh, UK
ABSTRACT
ARTICLE HISTORY
Received 6 November 2020
Accepted 22 December 2020
We report the first example of a charge-neutral linear two-coordinate
copper(I) complex bearing a sterically demanding acenaphthoimidazo-
lylidene-based N-heterocyclic carbene (NHC) ligand. The identity and
geometry of the complex are confirmed by single-crystal X-ray diffrac-
tion (XRD) analysis. The complex is poorly emissive at room tempera-
ture, showing either ligand-centered (LC) emission around 340 nm
when excited at 300 nm or ligand-to-ligand charge transfer (LLCT) emis-
sion at 540 nm when excited at 420 nm; in chloroform, dual emission is
observed upon photoexcitation at 300 nm. Nanosecond emission life-
times were recorded for these processes. This is the first example of
emissive linear copper(I) complexes containing this bulky NHC ligand.
KEYWORDS
Copper complex; linear
complex; acenaphthoimida-
zolylidene; dual emission;
DFT study
CONTACT Eli Zysman-Colman
School of Chemistry, University of St Andrews, Purdie Building
Robertson neil.robertson@ed.ac.uk
eli.zysman-colman@st-andrews.ac.uk
Organic Semiconductor Centre, EaStCHEM
North Haugh, St Andrews, KY16 9ST, UK; Neil
EaStCHEM School of Chemistry, University of Edinburgh, Joseph Black
Building, David Brewster Rd, Edinburgh, EH9 3FJ, UK
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

362
B. M. HOCKIN ET AL.
1. Introduction
Neutral heteroleptic copper(I) complexes have long been of interest due to their favor-
able optoelectronic properties. This class of complexes has been used as photocata-
lysts [1], in biological imaging [2], and in organic light-emitting diodes (OLEDs) [3].
Most photoactive copper(I) complexes are four-coordinate with a tetrahedral geometry
[4]. A major limitation of four-coordinate copper(I) emitters is the significant non-radia-
tive decay that results from a flattening Jahn–Teller distortion of the complex in the
metal-to-ligand charge transfer (MLCT) excited state, where the metal becomes for-
mally d9 after photoexcitation [5]. This distortion also leads to ligand dissociation and
solvent-induced exciplex formation, both of which adversely affect the emission by
increasing non-radiative decay and decomposition rates of the complex [6]. The use of
large, sterically bulky ligands that inhibit both geometric distortion in the excited state
and exciplex formation, and strong r-donating ligands to reduce ligand dissociation
result in complexes with improved photophysical profiles [7].
Three-coordinate cationic and neutral copper(I) complexes with sterically hindered
phosphine [3c,3d,8] or carbene [9] ligands tend to show high photoluminescence
quantum yields (UPL), emission across the visible spectrum, and display thermally acti-
vated delayed fluorescence (TADF). However, distortion in the excited state and subse-
quent degradation of the complex upon repeated or prolonged irradiation remains a
potential problem as three-coordinate complexes undergo a Y-to-T distortion, which
can lead to exciplex formation and luminescence quenching [10].
Recently, photoactive linear two-coordinate coinage metal complexes have gener-
ated much interest due to their promising photophysical properties. These complexes
usually bear a neutral, bulky r-donating carbene, such as a cyclic alkyl-amino carbene
(CAAC) and an anionic monodentate amide ligand, such as carbazolate and do not
suffer from the same distortion mechanisms as three- and four-coordinate coinage
metal complexes [9c,11]. Several examples of these complexes have been used as
emitters in high-performance solution-processed and vacuum-deposited OLEDs
[11a,11b]. The outstanding photophysical properties of these complexes is a result of
ligand-to-ligand charge transfer (LLCT), with the metal playing mostly a structural role
to reduce the exchange integral between the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) involved in the transition
from the low-lying excited states [12].
Relevant to this study is the neutral heteroleptic complex Cu(IPr)(Cz) [13], that incor-
porates two strongly r-donating ligands: the sterically bulky N-heterocyclic carbene
(NHC), 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IPr), and carba-
zolate, Cz (Figure 1). Cu(IPr)(Cz) shows dual emission upon photoexcitation at kexc
¼
365 nm in the crystalline state, with a high-energy structured fluorescence (kPL from 400
to 500 nm, sPL ¼ 16 ns, 24 ns) and an oxygen-sensitive low-energy phosphorescence (kPL
from 500 to 750 nm, sPL ¼ 55 ms), with a corresponding photoluminescence quantum
yield, UPL, of 33%. A second, more recent study of the related neutral Cu(PyTz)(Cz) com-
plexes (PyTz ¼ 2-methyl-[1,2,3]triazolopyridine) reported LLCT emission between Cz and
PyTz that was assigned as TADF with lifetimes in the crystalline state of 0.6 ls for
Cu(PyTz)(Cz) [14]. Additionally, in THF solution, these complexes form a monomer-dimer
equilibrium and as a result exhibit dual-emission with bands at 437 and 594 nm [14].

JOURNAL OF COORDINATION CHEMISTRY
363
Figure 1. Previous work on 2- and 3-coordinate copper(I) complexes, Cu(IPrBIAN)(py2BMe2) [9b],
Cu(IPr)(Cz) [13], and Cu(PyTz)(Cz) [14] along with synthetic target 3.
A bulkier and more p-accepting analog of IPr is IPrBIAN (1), which has been used
as a ligand in a number of complexes of silver, gold [15], palladium [16], iridium [17],
and ruthenium [18]. The application of these complexes has been in catalysis; palla-
dium complexes of IPrBIAN have been used as mild catalysts for aminocarbonylation
of iodarenes [16a], for Negishi couplings [16b] and Suzuki–Miyaura couplings with ster-
ically hindered substrates [16c], while Hoveyda II-type Ru complexes using IPrBIAN
ligands have been used for ring-closing metathesis [18]. There is only one reported
example of a copper(I) complex using IPrBIAN as a ligand (Figure 1) [9b]. The three-
coordinate Cu(IPrBIAN)(py2BMe2) complex was found to be non-emissive in the solid
state at room temperature and only showed weak, structured ligand-centered (LC)
emission (kPLꢀ610 nm, UPL < 1%, sPL < 10 ns) in a 2-methyltetrahydrofuran glass
at 77 K.
In this work, we report the first example of a neutral linear two-coordinate Cu(I)
complex Cu(IPrBIAN)(Cz) (3) bearing an IPrBIAN NHC ligand. As with the analogous
Cu(IPr)(Cz), it also exhibits dual emission, but in chloroform solution rather than the
solid state [13, 14].
2. Experimental
2.1. Chemical and starting materials
All reagents and solvents were of reagent grade for synthesis, spectroscopic grade for
analysis, and were used as supplied.
2.2. Instrumentation
2.2.1. Purification, NMR spectroscopy, melting point determination, and mass
spectrometry
All reactions were performed using standard Schlenk techniques under an inert (N2)
atmosphere. Flash column chromatography was performed using silica gel (Silica-P
from Silicycle, 60 Å, 40–63 mm). Analytical thin-layer chromatography was performed

364
B. M. HOCKIN ET AL.
on silica plates with aluminum backings (250 mm with indicator F-254). Compounds
were visualized under UV light. Nuclear magnetic resonance (NMR) spectra were
recorded in CDCl3 at room temperature (r.t.) on a Bruker AV400 or AV 500 spectrom-
1
eter at 400 MHz for H NMR and at 101 MHz for 13C NMR, respectively. Chemical shifts
are reported in parts per million (ppm) relative to TMS and calibrated using the
respective residual solvent signals. Melting and decomposition points were determined
by heated solid samples in sealed glass capillaries using a Stuart SMP 30 melting point
apparatus and are uncorrected. High-resolution nano-spray ionization (NSI) mass spec-
tra were collected on a Waters Xevo QTOF instrument by the EPSRC National Mass
Spectrometry Service Centre (NMSSC), Swansea. High-resolution electro-spray ioniza-
tion (ESI) mass spectra were collected on a Bruker MicroTOF 2 instrument by Alan
Walker at the University of Edinburgh.
2.2.2. Cyclic voltammetry and differential pulse voltammetry
Cyclic voltammetry (CV) analysis was performed on a lAUTOLAB Type III potentiostat
at a sweep rate of 100 mV sꢁ1. Differential pulse voltammetry (DPV) was conducted
with an increment potential of 0.004 V and a pulse amplitude of 50 mV, width of
0.05 s, and period of 0.5 s. Samples were prepared as 0.6 mM dichloromethane (DCM)
solutions, which were degassed by sparging with DCM-saturated nitrogen gas for
5 min prior to measurements. All measurements were performed using 0.1 M DCM
solution of tetra-n-butylammonium hexafluorophosphate ([nBu4N]PF6). An Ag/Agþ
electrode was used as the reference electrode while a glassy carbon electrode and a
platinum rod were used as the working electrode and counter electrode, respectively.
The redox potentials are reported relative to a saturated calomel electrode (SCE) with
a ferrocene/ferrocenium (Fc/Fcþ) redox couple as the internal standard (0.46 V vs. SCE
in DCM) [19].
2.2.3. UV–vis absorption spectroscopy
Electronic spectra were recorded using a Jasco V-670 UV/vis/NIR spectrophotometer
controlled with SpectraManager software (compounds 1H.Cl and 2) or a Shimadzu
UV-2600 UV spectrophotometer (3). All samples were prepared in DCM, chloroform or
toluene with varying concentrations in the order of mM. Molar absorptivity determin-
ation was verified by linear least-squares fit of values obtained from at least four inde-
pendent solutions at varying concentrations (for DCM: 0.037–0.18 mM; for chloroform:
0.012–0.054 mM; for toluene: 0.034–0.17 mM).
2.2.4. Steady-state and time-resolved emission spectroscopy
The sample solutions for emission measurements were prepared in toluene, DCM, THF,
or chloroform, with concentrations between 20 and 80 lM, and degassed using three
cycles of freeze-pump-thaw technique using an in-house designed quartz cuvette.
Steady-state emission spectra and excitation spectra were recorded using an
Edinburgh Instruments F980 fluorimeter. Excited-state lifetimes were measured by the
time-correlated single-photon counting (TCSPC) technique using an Edinburgh
Instruments F980 fluorimeter equipped with a pulsed diode laser (exciting at 378 nm);
PL emission was detected at 390 nm (LC emission) or 550 nm (CT emission).

JOURNAL OF COORDINATION CHEMISTRY
365
Table 1. Selected crystallographic data for 1H.Cl, 2, and 3.
Compound
1H.Clꢂ1.5CH2Cl2
2,038,660
C38.5H44Cl4N2
676.60
2
3
CCDC
2,038,661
C37H40ClCuN2
611.74
2,038,662
C49H48CuN3
742.48
Formula
Mw (g molꢁ1
)
Crystal description
Yellow prism
0.20 ꢃ 0.07 ꢃ 0.03
Pnma
Yellow prism
0.12 ꢃ 0.04 ꢃ 0.03
C2/c
Yellow plate
0.27 ꢃ 0.12 ꢃ 0.02
P2/c
Crystal size [mm3]
Space group
a [Å]
b [Å]
24.722 (3)
17.017 (2)
18.298 (2)
90.00
7697.9 (15)
8
32.5243 (9)
27.2274 (8)
32.8590 (9)
90.440 (2)
29097.5 (14)
32
19.9728 (5)
8.6927 (2)
23.2178 (6)
90.744 (3)
4030.68 (17)
4
c [Å]
b [ꢄ]
Vol [Å]3
Z
q (calc) [g cmꢁ3
]
1.168
0.334
1.117
0.698
1.223
0.579
l [mmꢁ1
]
F(000)
2856
10304
1568
Reflections collected
Independent reflections (Rint
Parameters, restraints
GoF on F2
91939
7311 (0.0911)
454, 28
193229
50753
9302 (0.0466)
591, 48
)
34205 (0.1243)
1515, 96
0.978
1.027
1.059
R1 [I > 2r(I)]
0.0686
0.0756
0.0532
wR2 (all data)
0.2153
0.71, ꢁ0.45
0.2257
0.96, ꢁ0.62
0.1314
0.42, ꢁ0.47
Largest diff. peak/hole [e/Å3]
2.3. X-ray crystallography
X-ray diffraction (XRD) data for all compounds were collected at 173 K using a Rigaku FR-
X Ultrahigh Brilliance Microfocus RA generator/confocal optics with an XtaLAB P200 dif-
fractometer [Mo Ka radiation (k ¼ 0.71075 Å)]. Intensity data were collected using x steps
accumulating area detector images spanning at least a hemisphere of reciprocal space.
Data for all compounds analyzed were collected using CrystalClear [20] and processed
(including correction for Lorentz, polarization, and absorption) using either CrystalClear
or CrysAlisPro [21]. Structures were solved by direct (SIR2004) [22] or dual-space (SHELXT)
[23] methods and refined by full-matrix least-squares against F2 (SHELXL-2018/3) [24].
Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined
using a riding model. Compound 3 was twinned with a twin law of ꢁ1 0 0 0 ꢁ 1 0 0 0 1
and a refined twin fraction of 0.15. Both 1H.Cl and 2 showed void space containing
poorly ordered solvent (1H.Cl: 215 Å3, 2: 4291 Å3) and the SQUEEZE [25] routine imple-
mented in PLATON [26] was used to remove the contribution to the diffraction pattern of
the unordered electron density in the void spaces. All calculations except SQUEEZE were
performed using the CrystalStructure [27] interface. Selected crystallographic data are
presented in Table 1. Deposition numbers 2,038,660–2,038,662 contain the supplemen-
tary crystallographic data for this article. These data are provided free of charge by the
joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe
Access Structures service (www.ccdc.cam.ac.uk/structures).
2.4. Synthesis of complexes
Bis(2,6-(diisopropyl)phenyl)acenaphthylene-1,2-diimine: To a two-neck round bottom flask
equipped with a reflux condenser under nitrogen was added acenapthenequinone
(1.35 g, 7.40 mmol, 1 equiv.) in acetonitrile (65 mL) and the solution was refluxed for
45 min. Acetic acid (12 mL) was then added and stirring continued until the total

366
B. M. HOCKIN ET AL.
dissolution of acenapthenequinone. 2,6-diisopropylaniline (2.84 g, 16 mmol, 2 equiv.)
was added dropwise. The solution was heated at reflux for 5 h and then cooled to
room temperature. The precipitate was filtered and washed with hexane. After air-dry-
ing, the product was obtained as an orange powder. Mass: 2.977 g. Yield: 80%. Mp:
316–320 ꢄC. 1H NMR (400 MHz, CDCl3) d (ppm): 7.87 (d, J ¼ 8.24 Hz, 2H), 7.35 (t,
J ¼ 7.92 Hz, 2H), 7.29–7.22 (m, 6H), 6.63 (d, J ¼ 7.20 Hz, 2H), 3.03 (h, J ¼ 6.96 Hz, 4H),
1.23 (d, J ¼ 6.84 Hz, 12H), 0.97 (d, J ¼ 6.96 Hz, 12H). 13C NMR (101 MHz, CDCl3) d
(ppm): 160.80, 147.28, 147.00, 135.26, 132.00, 129.30, 129.17, 128.72, 128.10, 127.93,
127.71, 124.81, 124.13, 123.29, 123.23, 123.18, 122.05, 28.43, 28.17, 23.25, 22.95, 22.68.
HR-MS (NSI): Calculated for [M þ H]þ(C36H41N2): 501.3270 m/z Found [M þ H]þ:
501.3270 m/z. Characterization matches that previously reported [28].
7,9-bis(2,6-diisopropylphenyl)-7H-acenaphtho[1,2-d]imidazol-9-ium chloride, 1H.Cl: To a
nitrogen-flushed thick-walled reaction vessel was added bis(2,6-(diisopropyl)phenyl)a-
cenaphthylene-1,2-diimine (0.500 g, 1.0 mmol,
1
equiv.) under nitrogen.
Methoxymethyl chloride (MOMCl, 1.5 mL, 20 mmol, 20 equiv.) was added and the reac-
tion was heated at 100 ꢄC with stirring for 16 h. After cooling to room temperature,
10 mL of diethyl ether was added and a yellow precipitate formed. The precipitate
was filtered and washed with diethyl ether to give the product, which was dried under
vacuum. Residual MOMCl waste was quenched with aqueous ammonia. The product
1
was isolated as a yellow powder. Mass: 0.410 g. Yield: 70%. Mp: 349–352 ꢄC. H NMR
(400 MHz, CDCl3) d (ppm): 12.10 (s, 1H), 8.02 (d, J ¼ 8.04 Hz, 2H), 7.69 (t, J ¼ 7.88 Hz,
2H), 7.60 (dd, J ¼ 8.30, 7.12 Hz, 2H), 7.49 (d, J ¼ 7.84 Hz, 4H), 7.23–7.21 (m, 2H), 2.73 (h,
J ¼ 6.84 Hz, 4H), 1.43 (d, J ¼ 6.84 Hz, 12H), 1.18 (d, J ¼ 6.84 Hz, 12H). 13C NMR
(101 MHz, CDCl3) d (ppm): 135.89, 129.32, 128.33, 124.75, 123.93, 123.81, 69.31, 29.09,
23.89, 23.60. HR-MS (NSI): Calculated for [M]þ (C37H41N2): 513.3270 m/z Found [M]þ:
513.3262 m/z. Characterization matches that previously reported [28].
(7,9-bis(2,6-diisopropylphenyl)-7H-acenaphtho[1,2-d]imidazol-9-ium)copper(I) chloride,
2: To a nitrogen flushed 100 mL RBF were added 1H.Cl (400 mg, 0.720 mmol, 1 equiv.),
CuCl (85.0 mg, 0.863 mmol, 1.2 equiv.), NaOtBu (0.069 g, 0.72 mmol, 1 equiv.) and dry
THF (60 mL). The cloudy orange solution was stirred overnight under nitrogen during
which it turned colorless. The solution was exposed to air, filtered through a celite
plug and the solvent removed under reduced pressure. To the residue was added
toluene (30 mL) and the solution was filtered again, and the filtrate collected. The solv-
ent was evaporated to give the product as a yellow solid. Mass: 0.425 g. Yield: 80%.
1
Mp: 270–276 ꢄC. H NMR (400 MHz, CDCl3) d (ppm): 7.80 (dd, J ¼ 8.40, 0.64 Hz, 2H),
7.59 (t, J ¼ 7.80 Hz, 2H), 7.45–7.40 (m, 6H), 7.18–7.16 (m, 2H), 2.84 (h, J ¼ 6.88 Hz, 4H),
1.35 (d, J ¼ 6.88 Hz, 12H), 1.15 (d, J ¼ 6.84 Hz, 12H). 13C NMR (101 MHz, CDCl3) d
(ppm): 145.70, 130.95, 128.46, 127.95, 124.66, 121.14, 29.07, 25.02, 23.89. HRMS (ESI):
Calculated [M]þ (C37H40N2CuCl): 610.2176 m/z (M þ ion not observed). Calculated
[M þ 2H-CuCl]þ (C37H42N2): 514.3343 Found [M þ 2H-CuCl]þ: 514.3334 m/z.
Characterization matches that previously reported [9b].
Copper(I) (7,9-bis(2,6-diisopropylphenyl)-7H-9l4-acenaphtho[1,2-d]imidazol-8-yl)(9H-car-
bazol-9-yl), [Cu(IPrBIAN)(cz)], 3: To a solution of 2 (180 mg, 0.290 mmol, 1 equiv.) in
DCM (25mL) was added carbazole (49.2 mg, 0.290mmol, 1 equiv.), NaOH (1 mL, 0.5 M
solution), and Bu4NCl (8.0 mg, 0.03 mmol, 0.1 equiv.). The solution was stirred for 48h

JOURNAL OF COORDINATION CHEMISTRY
367
during which a color change from yellow to orange was observed. Fifty milliliter of water
was added, and the solution stirred for 1 h. The aqueous layer was extracted three times
with DCM (50mL) and the solvent removed under reduced pressure to give an orange
solid. This was recrystallized from DCM/hexane, filtered and washed with hexane to give
the product as orange needles (Mass: 0.126 g). Yield: 58%. Mp: >300 ꢄC. 1H NMR
(400MHz, CDCl3) d (ppm): 8.08 (d, J ¼ 7.08Hz, 2H), 7.92 (d, J ¼ 7.60Hz, 2H), 7.84
(d, J ¼ 8.20Hz, 2H), 7.81–7.74 (m, 3H), 7.59–7.50 (m, 6H), 7.49–7.39 (m, 6H), 7.18
(d, J ¼ 7.0 Hz, 2H), 7.01–6.98 (m, 3H), 6.88 (t, J ¼ 7.20 Hz, 2H), 6.37 (d, J ¼ 8.10 Hz, 2H),
2.98 (h, J ¼ 6.90Hz, 4H), 1.31 (d, J ¼ 6.90 Hz, 12H), 1.18 (d, J ¼ 6.90 Hz, 12H). 13C NMR
(101MHz, CDCl3) d (ppm): 146.31, 145.71, 130.95, 128.47, 128.03, 125.95, 124.66,
123.23, 121.23, 120.45, 119.55, 115.10, 114.46, 110.72, 29.06, 25.00, 23.88, 1.17. HRMS
(ESI): Calculated [M]þ (C49H48N3Cu): 741.3150 m/z Found [M]þ: 741.3198 m/z.
2.5. Density functional theory (DFT) studies
Density functional theory (DFT) calculations were performed using the Gaussian 09 (Rev.
D.01; Gaussian Inc., Wallingford, CT) [29] suite of software. All calculations were per-
formed using the B3LYP functional [30] and the 6–31 G(d,p) basis set [31] for non-metal
atoms and the SBKJC VDZ ECP basis set [32] for Cu atoms, using the conductor-like polar-
izable continuum model (CPCM) [33] for DCM solution. Singlet ground state geometry
optimizations were carried out using crystallographic structures as starting points; vibra-
tional frequency calculations were performed to ensure that the optimized geometries
represented the local minima. Geometry optimization of the triplet state was performed
using the optimized singlet structure as the starting point. Excited-state energies were
calculated using the TD-DFT formalism at the same level of theory [34].
Calculated structures and Kohn-Sham orbitals were visualized with Gaussview ver-
sion 5.0 (Semichem Inc., Shawnee, KS) [35]. Chemissian version 4.60 (Lenoid
Skripnikov, St. Petersburg, Russia) [36] was used to calculate and visualize the frac-
tional contributions of various groups to each molecular orbital. GaussSum version 2.2
(Noel O’Boyle, Dublin, Ireland) [37] was used to calculate and visualize UV–vis spectra
and oscillator strength.
3. Results and discussion
3.1. Synthesis and characterization of the precursors and copper complexes
IPrBIAN synthesis proceeded without the requirement for the commonly used ZnCl2 tem-
plating agent [38]. The IPrBIAN imidazolium salt 1H.Cl was synthesized from IPrBIAN in
good yields using a solvent-free procedure adapted from the literature; addition of
diethyl ether afforded the product as a yellow powder [39]. The complexation of this pro-
ligand 1H.Cl to copper(I) chloride proceeded using sodium tert-butoxide at room tem-
perature to afford the desired [Cu(IPrBIAN)Cl] complex 2 in good yield (Figure 2)
Substitution of the chlorido ligand with carbazolate proceeded in reasonable yield
under basic conditions and in the presence of tetrabutylammonium chloride as a
phase transfer catalyst to afford [Cu(IPrBIAN)(Cz)], 3, as an orange powder [40]. Pro-

368
B. M. HOCKIN ET AL.
a
Figure 2. Synthesis of NHC ligands and 1–3. Reagents and conditions: MeCN, AcOH, 80 ꢄC, 5 h;
c
bMOMCl (40 mmol), 100 ꢄC, 16 h; CuCl (1 equiv.), NaOtBu (2 equiv.), THF, r.t., 24 h. dCarbazole,
NaOH (0.5 M, 2 mL), Bu4NCl, DCM, 48 h, r.t.
1
ligand 1H.Cl and complexes 2 and 3 were characterized by H and 13C NMR spectros-
copy, melting point determination, HR ESI-MS, and single-crystal XRD analysis.
3.2. Solid-state structures
Yellow crystals of 1H.Cl and 2 were obtained by slow diffusion of diethyl ether into a
solution of 1H.Cl in MeCN; or pentane into a solution of 2 in DCM. Orange crystals of
3 were obtained by slow diffusion of pentane into a solution of 3 in DCM. Crystal
structures of 1H.Cl and 2 are shown in Figure 3 while the crystal structure of 3 is
shown in Figure 4. The structure of 2 shows very slight distortion from linearity at the
copper, with a bond angle of 177.50(2)ꢄ.
Complex 3 shows a similar, slight distortion from linearity with a C1-Cu1-N52 bond
angle of 176.39(10)ꢄ, and the C1-Cu bond lengths do not vary significantly between 2
and 3, at 1.851(6) and 1.864(2) Å, respectively. The related complex Cu(IPr)(Cz) has a
slightly longer Cu–C bond distance of 1.877(2) Å [13], suggesting that the acenaph-
thene backbone present on 2 and 3 slightly increases the carbene r-donation and
strengthens the Cu–Ccarbene bond. In 3 the IPrBIAN and Cz ligands are nearly co-planar,
with an angle between the IPrBIAN and Cz planes of 7.65ꢄ. Literature examples of lin-
ear Cu(NHC)(Cz) complexes exhibit a range of angles between the NHC and Cz ligands,
from orthogonal in Cu(IPr)(Cz) (IPr-Cz plane angle is 89.9ꢄ) [13], to almost co-planar in
Cu(PyTz)(Cz) (PyTz-Cz plane angle is 5.9ꢄ) [14], with an intermediate value of 33.9ꢄ
observed for Cu(IMes)(Cz) [13].

JOURNAL OF COORDINATION CHEMISTRY
369
Figure 3. Molecular structure of (a) one independent cation of 1H.Cl and (b) one independent
molecule of 2. Solvent and hydrogen atoms have been omitted for clarity. Thermal ellipsoids are
drawn at the 50% probability level.
Figure 4. Views of the structure of 3, Cu(IPrBIAN)(Cz). (a) Thermal ellipsoid plot, ellipsoids are
drawn at the 50 % probability level. (b) Side view of a wireframe representation of the structure.
Hydrogens and the minor component of the disorder have been omitted for clarity.
A key difference between the predicted and the observed structures is the uneven
copper-NHC bond angles in the crystal structure; the angle N2–C1–Cu1 [124.7(2)ꢄ] dif-
fers noticeably from N5–C1–Cu1 [130.1(2)ꢄ], resulting in a slightly skewed orientation
of the NHC relative to the predicted structure (where the two angles are the same).
i
This skewing of the NHC in the crystal structure such that the Pr groups on one side
of the complex show greater proximity to the Cz than the other is likely due to crystal
packing. Apart from this discrepancy in NHC orientation, the observed bond lengths
and angles match well with those predicted by DFT (Table 2).

370
B. M. HOCKIN ET AL.
Table 2. Comparison of observed bond distances and angles for 3 with values calculated for 3.
Bond lengths (Å)
Bond angles (ꢄ)
Obs.
Obs.
(X-ray)
Calcd. (DFT)
(X-ray)
Calcd. (DFT)
Cu1–C1
Cu1–N52
N52–C53
N52–C64
C1–N2
C1–N5
N2–C3
N5–C4
C3–C4
N2–C16
N5–C28
1.864 (2)
1.8540 (19)
1.376 (3)
1.375 (3)
1.365 (3)
1.370 (3)
1.371 (3)
1.387 (3)
1.353 (3)
1.441 (3)
1.549 (16)
1.864
1.855
1.384
1.384
1.370
1.370
1.381
1.381
1.380
1.438
1.438
C1–Cu1–N52
N2–C1–Cu1
N5–C1–Cu1
C64–N52–Cu1
C53–N52–Cu1
C1–N5–C28
C1–N2–C16
NHC–Cza
Cu–Czb
176.39 (10)
124.74 (16)
130.12 (16)
126.34 (16)
126.83 (18)
121.9 (8)
123.68 (18)
7.65
179.99
127.23
127.23
126.69
126.69
122.36
122.36
4.35
6.8 (5)
–
–
0
–
–
–
–
aThe angle between the NHC plane and the Cz plane. bCu1–N52–C53–C54 Torsion angle.
3.3. DFT and TD-DFT calculations
DFT and time-dependent DFT (TD-DFT) calculations of 2 and 3 were undertaken to
provide insight into ground and excited-state properties. The optimized ground state
structure of 2 matches well with the observed crystal structure. The HOMO is distrib-
uted mostly on the IPrBIAN ligand (87%) and the copper (10%), with the LUMO local-
ized solely on the IPrBIAN (100%); the corresponding HOMO-LUMO gap is 3.12 eV.
According to TD-DFT most of the transitions possessing a strong oscillator strength (f
> 0.02) originate from LC transitions of the IPrBIAN and a small contribution from cop-
per to IPrBIAN MLCT (Table 3). The first three transitions (S1, S2, and S3) correspond to
the lowest energy absorption band and are assigned as HOMO!LUMO, H-1!LUMO,
and H-2!LUMO, respectively. All of these transitions have major IPrBIAN-centered
intra-ligand CT character; the S2 transition also has some minor Cu (dp) ! IPrBIAN
ꢅ
(p ) MLCT character.
Two optimized structures were obtained for 3. The lowest-energy conformer has
the IPr(BIAN) and Cz ligands almost co-planar (angle between the mean-planes of the
two ligands is 4.35ꢄ), similar to the geometry observed in the X-ray structure (angle
between mean-planes of 7.65ꢄ). The second, higher energy, conformer (12 kJ molꢁ1
destabilized) has the two ligands orthogonal to each other (angle of rotation between
the two ligands is 89.66ꢄ). These results are in contrast to calculations conducted on
other linear (NHC)-Cu-Cz complexes in which the lowest energy calculated structures
dispose the NHC and Cz ligands orthogonal to each other [13]. From TD-DFT calcula-
tions, Li et al. concluded that the nonplanar geometry between the carbene and the
Cz weakens the charge transfer character and that the emission becomes almost
entirely LC in nature, whereas in the case of 3, the weak emission at 550 nm can be
assigned to originating from a LLCT state. A rigid potential energy scan (PES) of the
angles between the IPrBIAN and Cz ligands in 3 (Figure 5(a)) revealed an energy bar-
rier to interconversion between the two conformers of approximately 23 kJ molꢁ1
.
Given the energy difference between the two conformers and the rotational barriers,
it is expected that the compound will exist only in the lower energy co-planar con-
formation. Excited state calculations (TD-DFT and triplet structure optimization) were
thus performed using the lower energy co-planar conformation of 3.

JOURNAL OF COORDINATION CHEMISTRY
371
Figure 5. a) Rigid potential angle energy scan of 3; the optimized geometry of the orthogonal
structure was used as the starting point, with the angle between the IPrBIAN and Cz ligands
rotated through 180ꢄ in 10ꢄ steps. Representation of the two optimized ground state structures
observed; b) higher energy orthogonal conformation; c) lower energy co-planar conformation.
Figure 6. Summary of DFT calculations of 3. a) Energy level diagram calculated using DFT using
ꢅꢅ
B3LYP/SBKJC-VDZ ECP for the Cu(I) center and 6–31 G
CPCM(CH2Cl2) and a 0.5 eV threshold of degeneracy (isocontour of 0.03), showing HOMO-2, HOMO,
for the atoms C, H, and N with
LUMO, and LUMO þ 1 orbitals of 3. b) Triplet spin density of 3 (isovalue of 0.02).
In the ground state, the HOMO of 3 is distributed mostly on the carbazolate ligand
(94%) with a minor contribution from the copper center (5%), while the LUMO is local-
ized on the naphthalene moiety of the IPrBIAN (100%) (Figure 6(a)). The calculated
HOMO-LUMO gap is 2.46 eV. The relative LUMO energies of 2 and 3 are similar (ꢁ2.44
and ꢁ2.23 eV, respectively), with the substitution of the chlorido ligand for the more
strongly r-donating carbazolate ligand in 3 resulting in a destabilization of the LUMO.
According to TD-DFT, and in agreement with literature studies of the related com-
plex Cu(IPr)(Cz) [13], most of the transitions with a strong oscillator strength (f > 0.02)

372
B. M. HOCKIN ET AL.
Figure 7. a) Absorption spectra of solutions of 1H.Cl (blue line), 2 (red line) and 3 (black line) in
dichloromethane. b) Experimental absorption spectra (solid lines) of solutions of 3 in DCM. Spectra
predicted by TD-DFT in DCM shown as dashed lines; predicted oscillator strength for each transi-
tion shown as grey lines.
originate from LC transition of the Cz. However, unlike previous examples of two-
coordinate copper(I) complexes [11c], the oscillator strength of the S1 transition is very
small (f ¼ 0.0015). The calculated S1 and T1 energies for 3 are 2.122 and 2.027 eV,
respectively, resulting in a singlet-triplet energy gap, DEST of 95 meV. The small DEST,
reflective of the CT character of the S1 and T1 states, implies that this compound
should exhibit TADF [41]. Indeed, the S1 state can best be described as a LLCT
between Cz and IPrBIAN (HOMO!LUMO, 99%). The first singlet excited state with a
significant oscillator strength is S4 (f ¼ 0.1923), which is also LLCT in nature
(HOMO!LUMO þ 1, 94%). The T1 state is predominantly LLCT between Cz and
IPrBIAN (HOMO!LUMO, 77%), although there is some contribution from an LC transi-
tion localized on the IPrBIAN ligand (HOMO-2!LUMO, 22%).
The geometry of 3 at the lowest energy excited state (T1) was then optimized; its
structure is broadly similar to the ground state structure, except for slightly shorter bond
lengths to the copper center (Cu1–C1 and Cu1–N52 bond distances are 1.846 Å and
1.839 Å, respectively) and a reduction in the already small angle between the mean-
planes of the IPrBIAN and Cz ligands to 2.52ꢄ. The spin density of the T1 state of 3 (Figure
6(b)) is distributed across both the IPrBIAN and Cz ligands, corroborating the LLCT nature
of the triplet state determined by TD-DFT and consistent with the experimental results
(vide infra). The calculated vertical emission from this triplet state is 822 nm.
3.4. UV–vis absorption properties
The UV–vis absorption spectra of 1H.Cl, 2, and 3 were recorded in DCM solution at
room temperature (Figure 7(a)) and the data are summarized in Table 4. Overlays of
experimental and calculated absorption spectra alongside predicted oscillator
strengths for the associated vertical transitions of 3 are shown in Figure 7(b). Selected
transitions from TDDFT calculations are summarized in Table 3.
The three compounds 1H.Cl, 2, and 3 all show a strong absorption band at
233–235 nm with e ¼ 23–27 ꢃ 103
M
ꢁ1 cmꢁ1 and another between 279 and 292 nm
with e ¼ 18–24 ꢃ 103
M
ꢁ1 cmꢁ1, both of which are characteristic of locally-excited

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373
Table 4. Absorption data of 1H.Cl, 2, and 3 in DCM at room temperature.
Compound
kabs (nm) (e ꢃ 10ꢁ3/Mꢁ1cmꢁ1
233 (27.4), 282 (20.1), 320 (11.3)
)
1H.Cl
2
3
233 (23.1), 292 (18.4), 388 (1.10), 426 (1.23)
235 (27.5), 279 (24.4), 310 (13.9), 326 (14.6), 335 (13.2), 341 (12.6), 375 (1.90), 420 (0.54)
transitions on the NHC, as predicted by TD-DFT (Table 3). Carbazole has a characteris-
tic absorption spectrum with peaks at 266 and 290 nm [42]; however, these are largely
obscured in the absorption spectrum of 3, with only a peak attributed to a LC transi-
tion on carbazole observed as a shoulder at 310 nm. The transitions observed at 326
and 375 nm can both be assigned to predominantly 1LLCT transition from Cz to
IPrBIAN (HOMO-5!LUMO and HOMO!LUMO þ 1, respectively).
Both 2 and 3 show a broad, weak CT transition at 426 nm (2) and 420 nm (3). For
2, this CT absorption band encompasses the first three transitions (S1, S2, and S3); all
ꢅ
have both Cu (dp) ! IPrBIAN (p ) MLCT and IPrBIAN LC character (Table 3). For 3, this
LC absorption band is assigned to the S3 transition, which is HOMO-2!LUMO with a
predicted absorption maximum of 425 nm. This transition is predominantly a singlet
1
1LLCT transition from Cz to IPrBIAN with a minor contribution from a singlet MLCT
ꢅ
component involving Cu (dp) ! IPrBIAN (p ) (Table 3).
3.5. Electrochemical properties of 3
The redox behavior of 3 in degassed DCM was assessed by CV and DPV, respectively,
at room temperature (Figure 8). Complex 3 exhibited an irreversible oxidation at
0.60 V versus SCE (Figure 8(a)) based on the peak value from the DPV scan. Based on
the DFT calculations, the oxidation wave is assigned to oxidation of the carbazolate;
pa
the Eox for carbazole is significantly anodically shifted at ca. 1.16 V versus SCE [43].
Indeed, this irreversible oxidation behavior is typical of such copper(I) carbazolate-con-
taining complexes, with many literature examples also showing irreversible oxidation
waves [11c]. For example, Cu(MAC )(Cz) complexes (MAC ¼ N,N0-bis(diisopropyl-
ꢅ
ꢅ
phenyl)-5,5-dimethyl-4-keto-tetrahydropyrimidin-2-ylidene) exhibit an irreversible oxi-
pa
dation wave at Eox ¼ 0.56 V (vs. SCE in MeCN), with the oxidation primarily centered
on the carbazolate ligand [11c]. This oxidation potential is very similar to 3 and so
similar electronic coupling is present. Both 1H.Cl and 2 exhibit an irreversible reduc-
pc
tion wave at Ered of ꢁ1.3 V versus SCE (Figure 8(b)). This reduction wave is also pre-
pc
sent in the CV and DPV of 3, shifted anodically to Ered of ꢁ1.2 V versus SCE,
indicating a slightly more facile reduction of the NHC in this complex.
3.6. Emission properties of 3
Complex 3 exhibits interesting emission properties in solution. When 3 is photoexcited
into the carbazolate LC band at 300 nm, structured LC emission from the carbazolate
ligand was observed with kPL between 338 and 359 nm (Figure 9(a) and Table 5). The
LC emission is solvent insensitive and for measurements in degassed toluene, THF,
and DCM solutions, this was the only emission observed. In degassed chloroform,
however, an additional emission band was observed with kPL of 539 nm. Based on

374
B. M. HOCKIN ET AL.

JOURNAL OF COORDINATION CHEMISTRY
375
Figure 8. a) Cyclic voltammogram and DPV voltammogram of 3 in degassed DCM solution show-
ing the first oxidation of 3. b) Cyclic voltammogram and DPV voltammogram of 1H.Cl, 2, and 3 in
degassed DCM solution showing the first reduction of 1H.Cl, 2, and 3, respectively. Scan rate for
–1
CV: 100 mV s in cathodic direction.
Figure 9. a) Normalized emission spectra of 3 in degassed solvents, kexc ¼ 300 nm. b) Normalized
excitation (solid lines, kem ¼ 550 nm) and emission (dashed lines, kexc ¼ 420 nm) spectra of 3 in
degassed solvents. Photos of emission from solutions of 3 when excited by UV torch at 365 nm in
aerated solution of c) toluene, d) DCM, e) THF, and f) chloroform.
Table 5. Solution state emission data of 3 at 298 K.
kexc ¼ 300 nm
kexc ¼ 420 nm
kPL/nm
Solvent
kPL/nm
Toluene
DCM
THF
339, 353
338, 353
344, 359
355, 539
583
568
542
539
Chloroform
DFT calculations, we assign this emission to originate from an LLCT state involving a
Cz to IPrBIAN transition. Upon photoexcitation into the LLCT absorption band at
420 nm, emission from the LLCT state is observed in all solvents (kPL between 539 and
580 nm). In chloroform solution the energy of the LLCT emission remains unchanged

376
B. M. HOCKIN ET AL.
regardless of the excitation wavelength. The difference in emission behavior for 3 in
different solvents is seen visually where the chloroform solution appears yellow/green,
while the toluene, THF and DCM solutions present as violet to blue (Figure 9(c–f)).
Excitation spectra were collected for all solutions, monitoring the LLCT emission (kem
¼ 550 nm). There exists a strong correlation between the absorption and excitation
spectra, with strong excitation below 340 nm and a weaker low energy excitation
band centered around 420 nm, indicating no photodegradation of the complex.
There is no delayed component to the emission, with all emission lifetimes for
degassed chloroform solution in the ns range for both the LC emission [kem
¼
390 nm, s ¼ 1.64 ns (60%) and 4.8 ns (40%)] and LLCT emission [kem ¼ 550 nm, s
¼ 1.4 ns (75%) and 7.4 ns (23%)]. The photoluminescence quantum yield for the very
weak emission could not be accurately determined.
Dual emission has previously been observed for two-coordinate copper(I) carbazo-
late complexes. Cu(IPr)(Cz) shows dual emission in the crystalline powder state [13];
however, only carbazolate-based LC emission in THF solution (10 mM) was observed,
with a maximum at 342 and a shoulder at 378 nm. This is similar in both energy and
structure to the LC emission that was observed for 3 in THF solutions (20–80 mM) with
a peak at 344 and shoulder at 359 nm. Recently, dual emission was reported for
Cu(PyTz)(Cz) in THF solution, ascribed to a carbazolate-based LC state (kPL ¼ 437 nm)
and an LLCT state (kPL ¼ 594 nm) [14]. It was proposed that the dual emission for
Cu(PyTz)(Cz) was due to a mixture of conformers in solution, with LC emission origi-
nating from complexes with orthogonally disposed ligands and intermolecular LLCT
resulting from through space interaction between ligands in p-stacked dimers of co-
planar molecules. The use of a monomer-dimer equilibrium in solution to explain the
dual emission was supported by DFT calculations on the copper complex and a series
of concentration, temperature, and excitation energy studies on an analogous gold
complex, Au(PyTz)(Cz). The origin of the solvent-specific dual emission of 3 in chloro-
form is presently unknown.
4. Conclusion
Two Cu(I)-NHC complexes, 2 and the novel two-coordinate 3, both bearing the
unusual NHC-BIAN ligand 1 were synthesized and characterized by various analytical
techniques. Ligand 1 coordinates to Cu(I) as a neutral ligand to give 2; further com-
plexation with carbazole gave 3. Both 2 and 3 show mixed LC transitions lower than
300 nm and weak LLCT transitions further into the visible region. Complex 3 exhibited
interesting dual emission properties in solution; this is seen as separate emission
depending on excitation source for most solvents, and is especially strong in chloro-
form solution, an interesting phenomenon that warrants further study. Although no
delayed component was observed for the emission at room temperature, further
investigation of this interesting emission is ongoing.
Disclosure statement
No potential conflict of interest was reported by the authors.

JOURNAL OF COORDINATION CHEMISTRY
377
Funding
CFRM and EZ-C wish to thank the Engineering and Physical Sciences Research Council (EP/
M02105X/1 and EP/R035164/1). We would like to thank the Engineering and Physical Sciences
Research Council and CRITICAT Centre for Doctoral Training for financial support (Ph.D. student-
ship to B. H.; EP/L016419/1).CFRM and EZ-C wish to thank the Engineering and Physical Sciences
Research Council (EP/M02105X/1 and EP/R035164/1) and CRITICAT Centre for Doctoral Training
for financial support (Ph.D. studentship to B. H.; EP/L016419/1).
ORCID
Bryony M. Hockin
http://orcid.org/0000-0002-4687-3806
Campbell F. R. Mackenzie
http://orcid.org/0000-0001-7465-8989
David B. Cordes
http://orcid.org/0000-0002-5366-9168
Alexandra M. Z. Slawin
http://orcid.org/0000-0002-9527-6418
Neil Robertson
Eli Zysman-Colman
http://orcid.org/0000-0002-9230-6124
http://orcid.org/0000-0001-7183-6022
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