Intramolecular non-covalent interactions as a strategy towards controlled photoluminescence in copper(i) complexes

Article abstract of DOI:10.1039/c6tc04989c

In this work, we describe a new strategy for designing photoluminescent Cu(i) complexes. At its core are simple cyclophane inspired N-donor ligands featuring intramolecular interactions between aromatic units within a single molecule. Variation of the steric bulk inflicted a change in intramolecular stacking distances that in turn affected the emission colour of copper(i) complexes tunable in a 0.5 eV range from green to red. As the interactions driving emission are confined to the single molecule, no intermolecular aggregation is required to enable photoluminescence in solution, pristine crystals, or solution-cast polymer films. A crystallographic study provides a link between the spatial proximity of the aromatic rings of the ligands (ranging from 3.349 to 3.731 ?) and the enhancement of emission efficiency, which increases dramatically from 0.02 to 0.78 at 296 K as the ring spacing contracts. Photophysical and theoretical analyses confirm the involvement of intramolecular interactions in the formation of the emissive state and describe the observed phenomena at the molecular level.

Full text of DOI:10.1039/c6tc04989c

View Article Online
View Journal
Journal of
Materials Chemistry C
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: G. Filonenko, R. R.
Fayzullin and J. R. Khusnutdinova, J. Mater. Chem. C, 2017, DOI: 10.1039/C6TC04989C.
This is an Accepted Manuscript, which has been through the
Volume
4 Number 1 7 January 2016 Pages 1–224
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry C
Accepted Manuscripts are published online shortly after
Materials for optical, magnetic and electronic devices
www.rsc.org/MaterialsC
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

Please do not adjust margins
Journal of Materials Chemistry C
Page 1 of 9
View Article Online
DOI: 10.1039/C6TC04989C
Journal Name
ARTICLE
Intramolecular Non-Covalent Interactions as a Strategy Towards
Controlled Photoluminescence in Copper(I) Complexes†
G. A. Filonenko,a R. R. Fayzullinb and J. R. Khusnutdinovaa,*
Received 00th January 20xx,
Accepted 00th January 20xx
In this work we describe a new strategy for designing photoluminescent Cu(I) complexes. At its core are simple cyclophane
inspired N-donor ligands featuring intramolecular interactions between aromatic units within a single molecule. Variation
of the steric bulk inflicted a change of intramolecular stacking distances, that in turn affected the emission colour of
copper(I) complexes tunable in a 0.5 eV range from green to red. As the interactions driving emission are confined to the
single molecule, no intermolecular aggregation is required to enable photoluminescence in solution, pristine crystals, or
solution-cast polymer films. Crystallographic study provides a link between the spatial proximity of the aromatic rings of
the ligands (ranging from 3.349 to 3.731 Å) and the enhancement of emission efficiency, which increases dramatically
from 0.02 to 0.78 at 296 K as the rings spacing contracts. Photophysical and theoretical analyses confirm the involvement
of intramolecular interactions in the formation of the emissive state and describe the observed phenomena on a
molecular level.
DOI: 10.1039/x0xx00000x
www.rsc.org/
recognized as a sustainable alternative to noble metal-based
PL materials.11-14 These compounds include mono- and
binuclear complexes with chelating phosphines,15-18
Introduction
The vast class of photoluminescent (PL) compounds holds
great importance in modern technology. Comprised of purely
organic molecules or transition metal complexes, luminescent
compounds have found broad application in light emitting
devices,1 smart materials for sensing2 and imaging
mononuclear
phenanthroline-19-21
or
diimine-based
complexes22 and trigonal NHC-based and amide-bound
ligands.23-29 Similar to the rest of the TM-based emitters, the
control over emissive properties in Cu(I) compounds is
4
typically achieved by tuning the electronic properties of their
31
applications.3, As the demand for such emitters steadily
ligands30,
or addressing the ligand conformational
grows, new strategies for designing PL materials and
modulating their properties become increasingly important.
Conventional transition metal (TM)-based luminophores
dynamics.32 This implies that the emission of TM-complexes
mainly relies on covalent interactions within one molecule
while the intermolecular and non-covalent interactions
typically remain unutilized.
rely heavily on the use of π-extended ligands required to
create the desired bandgap that facilitates excitation and
emission. This design strategy yielded remarkably efficient PL
complexes with recent examples including Ir,5 Pt,6, 7 Zn,8 and
Au9. Since noble metals currently comprise the most numerous
class of TM-based PL complexes, the search for a cheap yet
efficient and sustainable alternative remains an important
objective.10
Over the last two decades, Cu(I) complexes have been
Figure 1. Comparison between inter- and intramolecular interactions as the driving
factor for photoluminescence. Structure of L1 pyridinophane macrocycle (N4) adopted
from Ref. 33.
One conceptually opposite approach for designing PL
compounds makes use of the intermolecular non-covalent
interactions. Coined in 2001 by Tang, this strategy is referred
to as Aggregation Induced Emission (AIE) and it exploits the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 2 of 9
View Article Online
DOI: 10.1039/C6TC04989C
ARTICLE
Journal Name
intermolecular arrangement in solid or aggregated state to tetrakis[3,5-bis(trifluoromethyl)
35
phenyl]borate)47
This arrangement can, for prepared according to literature procedures. [Cu(MeCN)4]+
were
enable photoluminescence.34,
example, restrict the molecular motions responsible for non- precursors were prepared by dissolving Cu2O in acetonitrile
radiative relaxation or induce photoluminescence in π-π solutions in the presence of aqueous HBF4 or HPF6 followed by
stacked assemblies that emit when aggregated in contrast to two consecutive recrystallizations from cold acetonitrile.48
the non-emissive isolated molecular state.
Polymer blends of complexes 4-6 were prepared by mixing
The AIE behaviour is not limited to organic dyes and can be saturated solution of 2-3 mg of the corresponding complex in
found in TM complexes. For example, non-covalent CH2Cl2 with 1 mL of PMMA (poly(methyl methacrylate))
interactions were found to contribute to emission of Zn, Au, solution in CH2Cl2 containing ca. 100 mg of the polymer. The
Pt, Ir, Al and Cu compounds.36-41 However, with very few films were prepared from these blends by drop casting in a
43
exceptions,42,
the non-covalent interactions involved in rectangular PTFE mold followed by drying in vacuo.
modulating photoluminescence are intermolecular and Instrumentation: NMR spectra were measured on JEOL
therefore are highly sensitive to the structure of the bulk ECZ600R 600MHz, JEOL ECZ400S 400 MHz and Bruker Avance
material and thus hard to control and rarely translated to II 400 MHz spectrometers. Full spectra are available from the
solution state.
Supporting Information. Electrospray Ionization Mass
Potentially one can overcome the limitations of the AIE Spectrometry (ESI-MS) measurements were performed on a
approach if the non-covalent interactions driving the emission Thermo Scientific ETD apparatus. Elemental analyses were
are intramolecular and therefore not affected by performed using an Exeter Analytical CE440 instrument.
intermolecular arrangement. Such intramolecular interactions The photoluminescence measurements in degassed
can be found in pyridinophane macrocycles (e.g. N,N’-dialkyl- dichloromethane solutions at varying concentrations were
2,11-diaza[3.3](2,6)-pyridinophane, Figure 1). For example, the performed using a Hitachi F7000 apparatus. Absorbance
N-tert-butyl
substituted
pyridinophane
ligand
is spectra were collected using an Agilent Cary 60 machine.
45
Photoluminescence lifetime measurements were performed
conformationally flexible in transition metal complexes44,
and features tight πꢀπ spacing of 3.275
Å between two using the second harmonics of Spectra-Physics Mai Tai pulsed
pyridine rings in the solid state in the free ligand (Figure 1). If laser and a Hamamatsu Photonics Streak Scope camera. The
the interaction between non-conjugated pyridine rings can be decay data were fitted with a single exponential decay
utilized to enable emission in TM-complexes, the use of function unless specified otherwise. The same laser light
pyridinophane ligands would mark a conceptually new strategy source equipped with a Thorlabs IS236A integrating sphere
for designing emissive TM complexes based on purely non- and an OceanOptics USB4000-ES spectrometer was used for
covalent interactions.
quantum yield measurements in solutions (CH2Cl2, c = 0.5-1
In this work, we demonstrate that such intramolecular µM) and in the solid state. The accuracy of quantum yield data
interactions can be successfully employed to design a new was confirmed using a Hamamatsu Photonics Quantaurus-QY
class of photoluminescent Cu(I) complexes. The colour and system that established the variations in absolute QY to be
quantum yield of the emission could be altered by controlling within 5% for solid and solution samples. Emission spectra
the conformation of the pyridinophane macrocyclic ligand by were recorded using OceanOptics USB4000-ES (solutions,
simply varying N-substituents (R group, Figure 1). Namely, the Figure 6B) or Hamamatsu Photonics Streak Scope camera
variation of the spatial proximity of two non-conjugated (solids, Figure 6A). The IR spectra were recorded using Cary
pyridine rings within the macrocycle allowed for emission in a 630, ATR module. Cyclic voltammetry (CV) studies were
wide range of 524-665 nm with the quantum yield reaching performed with an ALS/CHI Electrochemical Analyzer 660E.
0.74 at room temperature. The photoluminescence of the Electrochemical grade NBu4ClO4 supplied by Sigma-Aldrich was
complexes most emissive in the solid state was also used as the supporting electrolyte. Electrochemical
successfully translated to dichloromethane solutions, thus measurements were performed under nitrogen after the
overturning the prerequisite for emitters driven by the non- sample solutions were purged with nitrogen for 10 minutes. A
covalent interactions to be in the aggregated state.
glassy carbon disk electrode (GCE; d = 1.6 mm) was used as the
working electrode, and a Pt wire as the auxiliary electrode. The
non-aqueous Ag-wire reference electrode assembly was filled
with 0.01 M AgNO3/0.1 M NBu4ClO4/MeCN solution. The
reference electrodes were calibrated against ferrocene. The
ferrocene peak separation in acetonitrile and in CH2Cl2 solution
was 105 and 210 mV at 23 °C, respectively.
Experimental Section
All manipulations unless stated otherwise were performed
using Schlenk or glovebox techniques under dry argon
atmosphere. Anhydrous solvents were dispensed from an
MBRAUN solvent purification system and degassed prior to
use. Anhydrous deuterated solvents were purchased from
Eurisotop and stored over 4Å molecular sieves. All chemicals
unless noted otherwise were purchased from major
commercial suppliers (TCI, Sigma-Aldrich and Nacalai Tesque)
Methods used for DTF calculations and optimized geometries
can be found in section S4 of the Supporting Information.
X-ray diffraction data for single crystal samples was collected
on
monochromated MoKα radiation (0.71073 Å) at -180 °C.
Fragments of the main residues of complexes and and
anionic fragments in and were disordered and reported in
a Rigaku XtaLab PRO instrument using graphite
and used without purification. Ligand L1 33
,
2,11-
2,
3
6
46
)
3
6
diaza[3.3](2,6)pyridinophane
(
LNH
and NaBArF (sodium
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 3 of 9
View Article Online
DOI: 10.1039/C6TC04989C
Journal Name
ARTICLE
full in the Supporting information. Complex
2
crystallizes as an complex. Pyridine ligands in the axial plane feature Cu-N
common value for
acetonitrile solvate. Complex
3
contained two independent distances within 1.9-2.1
Å
–
a
molecules in the asymmetric cell. Section S5 of the Supporting pentacoordinate Cu(I) species.49 As expected, amine donors of
Information contains full experimental details regarding data all macrocycles occupy axial sites of the bipyramid and have
collection and structure refinement.
slightly longer Cu-N bonds of ca. 2.4 Å 50
bonding between the amine donors and the metal in both
complexes is manifested in the elongated Cu-N distances in
these complexes compared to their less sterically hindered
counterparts and > 0.044 ).
An intriguing feature of complexes
.
Potentially weaker
1
Results and Discussion
2
3
(
ꢁ
Å
1-3 in the solid state is
Synthesis and structure.
the tight placement of the pyridine rings within the
macrocyclic ligand that may enable the intramolecular
interaction between these aromatic units. Interestingly, the
pyridine units of these cationic complexes are packed tighter
in complexes with greater steric bulk of the substituents at the
amine groups of the macrocycle (Figure 2). Namely, the 3.475
or 3.495 Å distance between centroids of pyridine units in
Pyridinophane macrocycles can be prepared with various
alkyl or functionalized substituents using modifications of
known synthetic protocols.33, 46 We produced a set of ligands
bearing bulky tert-butyl substituents (L1) and less bulky ester
(
L2) and alcohol functionalized (L3) side chains. In acetonitrile
solutions L1
L3 rapidly react with [Cu(MeCN)4]+ precursors to
form bright orange solutions of cationic acetonitrile complexes
(Scheme 1).
-
complexes
1
is ca. 0.16-0.18 Å shorter than that in the less
(3.657 Å). This trend is also
1-3
sterically hindered complex
2
associated with a drastic change of the angle formed between
the pyridine unit planes – the latter is ca. 14% smaller for
complexes
1 compared to complex 2. Taken together, these
features suggest that the macrocyclic ligands with high steric
hindrance at amine donors allow for shorter intramolecular
contacts between pyridine units that can enhance their
interaction. Although the characteristic distance between the
centroids of two pyridine units in complexes 1-3 is longer than
Scheme 1. Synthesis of cationic Cu complexes 1-3
that in the free L1 (Figure 1), only a few known complexes of
N,N-dialkyl-2,11-diaza[3.3](2,6)-pyridinophane complexes have
been reported to have similar Py-Py distances.51-53
Figure 2. Solid state structures of cationic Cu complexes 1-3. Thermal ellipsoids are
shown on 50% probability level, hydrogen atoms, minor disordered fragments, solvent
molecules and anions are omitted for clarity. Ring-ring spacing given for centroids,
substituents at amine groups are omitted in the side view projections. For complex 3
containing two molecules per the asymmetric cell only one molecule is shown. Selected
bond lengths and angles (Å, °): 1PF6: Cu1-N1 2.089(2), Cu1-N2 2.099(2), Cu1-N3
2.444(2), Cu1-N4 2.470(2), Cu1-N5 1.901(2), N1-Cu1-N2 80.38(9); 2: Cu1-N1 2.084(2),
Cu1-N2 2.048(2), Cu1-N3 2.341(2), Cu1-N4 2.400(2), Cu1-N5 1.869(2), N1-Cu1-N2
82.71(8); 3: Cu1a-N1a 2.0770(17), Cu1a-N2a 2.1006(17), Cu1a-N3a 2.3687(17), Cu1a-
N4a 2.3963(17), Cu1a-N5a 1.8840(18), N1a-Cu1a-N2a 82.13(6).
Scheme 2. Synthesis of tetragonal halide complexes 4-6
In attempt to further alter the intramolecular contact
between aromatic units within Cu complexes we aimed at
changing the coordination geometry at the metal center. In
copper complexes this can be done by utilizing soft donor
ligands that promote the formation of tetrahedral complexes
where the pyridinophane ligand would be present in a boat-
chair conformation. Indeed, reactions of copper iodide with
ligands L1 and L2 (Scheme 2) rapidly resulted in the formation
Complexes 1-3 readily crystallize to yield analytically pure
of corresponding iodide complexes
4 and 5 that are readily
solids suitable for single crystal X-ray diffraction studies. Figure
2 shows the solid state structures of these cationic complexes.
In all instances the macrocyclic ligand adopts the boat-boat
conformation and binds in a κ4-mode with pyridine units
occupying two equatorial positions in the complex with
trigonal bipyramidal geometry. We found that the variation of
crystallized as yellow-orange, analytically pure solids.
Furthermore, inspired by recent remarkable examples of
luminescent compounds based on Cu2I2 and Cu4I4 cores30, 31, 54-
56
we attempted the preparation of dinuclear complexes
ligated by L1. Reaction of complex
4 with 0.5 equivalents of
NaBArF as the iodide abstraction agent in dichloromethane
counterion in
1 had little effect of the structure of the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 4 of 9
View Article Online
DOI: 10.1039/C6TC04989C
ARTICLE
Journal Name
-
results in rapid precipitation of sodium iodide and formation of non-coordinated PF6 counterion. At the same time, complex
complex (Scheme 2).
1BF4 features strong bands at 1045 cm-1 and 1030 cm-1 due to
X-ray structure analysis confirms the tetrahedral the presence of the BF4 counteranion, while the region
coordination geometry in complexes
. The macrocyclic between 1650 cm-1 and 1410 cm-1 corresponding to pyridine
ligand in all complexes adopts a boat-chair conformation and ring vibrations remains essentially unchanged as compared to
binds in a κ3-fashion with one amine donor group not bound to 1PF6. In addition, complex
featuring OH groups exhibits free
copper center. Complexes and
have nearly identical Cu1- and hydrogen-bound OH stretching vibrations at 3593 cm-1
N3 bond lengths that implies very similar donor ability of the and a broad band at ~3416 cm-1, respectively.
6
4-6
3
4
5
amine ligands of the macrocycles L1 and L2. Consequently, the
Complexes 1-6 in solution.
remote ester group as expected has little effect on the
Having observed the remarkable difference in structural
donating strength of the amine group, but rather influences properties of
ligand conformation via its reduced steric hindrance. Finally, these differences are manifested in solution using NMR
complex
features the shortest distance of 3.349 Å between spectroscopy. Upon complexation with [Cu(MeCN)4]+ the
the centroids of the two pyridine rings among all the geminal protons of pyridylmethylenic groups in L1 L3 lose
4.65 and 3.43
is 28.25° that is significantly smaller than the ppm in CD3CN as two doublets with JHH = 15.2 Hz (See Section
, where the pyridine rings are aligned at S2 of the Supporting Information). The macrocyclic ligand in
complexes and
coordinates exclusively in the κ4-fashion
that is evidenced by the C2v symmetry within the ligand moiety
1-6 in the solid state we further studied how
4
-
1
complexes studied in this work. The angle between two equivalency and appear in H NMR spectra at
δ
2
pyridine rings in
4
angle in complex
43.23°.
5
2
3
1
observed by H NMR at 23 °C. Unlike
2 and 3, complexes 1BF4
and 1PF6 feature broad resonances in 1H NMR at 23 °C that
resolve at -5 °C. Interestingly, a minor isomer with a κ3 binding
mode could be observed in complexes 1BF4 and 1PF6 in a
κ3/κ4 = 28.5/71.5 ratio. This conformation is easily
distinguished from its κ4 counterpart owing to the loss of
symmetry within the macrocyclic ligand. Namely, the two
pyridine backbone protons at meta positions appear as
separate doublets and pyridylmethylenic protons appear as
2
four doublets with different coupling constants JHH of 13 and
15.2 Hz in CD3CN. We attribute this behaviour of L1-ligated
complexes to the increased steric hindrance imposed by tert-
butyl substituents that may hinder formation of the κ4 isomer
and lead to the observed exchange.
Figure 3. Solid state structures of neutral Cu complexes 4, 5 and 6. Thermal ellipsoids
are shown on 50% probability level, hydrogen atoms, minor disordered fragments and
BArF anion in
6 are omitted for clarity. Ring-ring spacing given for centroids,
substituents at amine groups are omitted in the side view projections. Selected bond
lengths and angles (Å, °): 4: Cu1-N1 2.1292(16), Cu1-N2 2.1518(15), Cu1-N3 2.2158(16),
Cu1-I1 2.4907(3), N1-Cu1-N2 78.64(6); 5: Cu1-N1 2.1153(16), Cu1-N2 2.0887(17), Cu1-
N3 2.2235(16), Cu1-I1 2.4711(3), N1-Cu1-N2 81.63(6); 6: Cu1-N1 2.0812(13), Cu1-N2
2.1395(14), Cu1-N3 2.2153(13), Cu1-I1 2.4904(2), N1-Cu1-N2 80.12(5).
1
In agreement with the X-ray diffraction data, the H NMR
analysis of iodide complexes
4-6 confirms their tetrahedral
geometry in CD2Cl2 solution (See Supporting Information). This
can be deduced from a distinct change of the ligand backbone
symmetry in solution when compared to complexes 1-3
described above. Namely, the pyridine backbone protons in
Almost exclusively, the shortest contacts between the
aromatic groups of 1-6 in the solid state are intramolecular.
complex
4 appear as a sharp triplet at δ = 7.25 ppm
For example, the X-ray diffraction data suggests that
corresponding to the proton at para position while meta-H
resonances appear as a broad feature at room temperature. At
-35 °C it resolves into two doublets at δ = 6.86 and 6.65 ppm
complexes of L1 and L3 have no intermolecular Py-Py contacts
in the crystalline state. However, complexes
2 and 5 in solid
state demonstrate short intermolecular distances between
parallel displaced pyridine units in addition to the
intramolecular contacts. The shortest intermolecular Py-Py
3
with identical JHH = 7.7 Hz. Methylene groups on the
macrocycle appear as four doublets consistent with a Cs
symmetric structure. Two separate signals are also observed
for NtBu groups. The H NMR spectrum of
centroid distances of 3.616
Å
(plane shift of 1.572 Å) and
and
1
6 is nearly identical
4.740 Å (plane shift of 3.305 Å) were found in
2
5,
to that of
counterion appearing as singlets at δ = 7.71 and 7.54 ppm.
Interestingly, complexes are dynamic on the NMR
timescale and their resonances are broadened at 23 °C. While
the dynamics in and does not involve changes in the
coordination mode of the ligand, complex in
4 with the exception of resonances of BArF
respectively (see Figure S52 in the Supporting Information).
Being similar to the intramolecular Py-Py distance, these
structural features may potentially impact the properties of
4-6
crystalline
The comparison of infrared spectra of solid samples
complexes and show the presence a C=O stretching band
at 1718 cm-1 and 1722 cm-1, respectively, typical for carbonyl
group of aliphatic esters. IR spectra of cationic complexes 1PF6
, and
exhibit strong band at ~830 cm-1, corresponding to a
2 and 5.
4
6
5
5
2
dichloromethane solution exists as a mixture of a major
tetrahedral complex and a minor trigonal bipyramidal complex
1
in H NMR
,
in a 61/39 ratio at -35 °C. Resonances of complex
5
2
3
coalesce at ca. 10 °C, suggesting a higher exchange rate in
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 5 of 9
View Article Online
DOI: 10.1039/C6TC04989C
Journal Name
CD2Cl2 in compared to complex
ARTICLE
5
4
with the coalescence
temperature of 25 °C (See Figures S27 and S32 in the
Supporting Information).
Figure 5. UV-Vis absorbance spectra for complexes 1-6 in dichloromethane and
acetonitrile solutions in 300-600 nm region.
Upon irradiation with 400 nm light, complexes
1-6 emit
visible light in the solid state. Emission colour varied from red
to green depending on the complex used. While cationic
complexes
1BF4), 637 (
emit light at 585 (
1
-
3
emit red light at 619 (identical for 1PF6 and
) and 665 ( ) nm, the neutral iodide complexes
), 564 ( ) and 524 ( ) nm (Figure 6 A). The
2
3
4
5
6
photoluminescence lifetimes determined by time-resolved
photoluminescence measurements were in range of 2-30 µs
(see Table 1).
Figure 4. Cyclic voltammograms of solutions of complexes 1PF6 (1 mM) and 2 (1.2 mM)
in 0.1M N Bu4ClO4/CH3CN at 23 °C and -35 °C. Scan rate 100 mVs-1; 1.6 mm glassy
carbon disk electrode.
Interestingly, complex
5 had a distinctly different
photoluminescence decay profile in the crystalline state (See
Figure S41 in the Supporting Information). Unlike its
Conformational equilibria in acetonitrile solutions of
can also be readily observed with cyclic voltammetry when
temperature is varied. At 23 °C, both 1PF6 and show an
1 and
counterparts, complex
components in the crystalline state with lifetimes of τ = 0.97
and 6.59 µs. Such behaviour in crystalline can occur due to
5 was found to have two decay
2
2
5
oxidation wave in MeCN solution with large separation
between anodic and corresponding reverse cathodic waves
(Figure 4). Similar E1/2 = -222.5 mV and -229.5 mV vs. Fc/Fc+
measured at 23 °C for complexes 1PF6 (Figure 4, left, red line)
intermolecular interactions, i.e. formation of excimers, that
was consistent with our X-ray diffraction studies showing
relatively short intermolecular Py-Py distances (Figure S52 in
the Supporting Information) (vide supra). We found that the
emission of the long lived component was significantly red-
shifted compared to the short lived component (Figure S44 in
the Supporting information). To assign the decay components
and
2 (Figure 4, right, red line) respectively, implies that the
electronic properties of the remote ester functional group do
not significantly affect the electronics at the metal center. In
line with NMR data that suggests the existence of two
conformations in 1PF6, at -35 °C this complex features two
oxidation waves, at 59 and 640 mV vs Fc/Fc+, assigned to the
to particular state of complex
amorphous blended into a PMMA film where the formation
of excimers would be less favourable. We found that
amorphous emits at λmax = 610 nm that is ca. 45 nm red-
shifted from the crystalline . We, therefore, conclude that a
long lived red-shifted component is produced by monomeric
rather than its excimers. Interestingly, complex – the cationic
counterpart of , features a simple monoexponential decay
5 we analysed the emission of
5
species with
κ
4- and
κ
3-bound ligands respectively (Figure 4).50
, shown to exist as a single conformer in
5
By contrast, complex
2
solution by 1H NMR, correspondingly revealed
oxidation event even at -35 °C (Figure 4).
Photophysical studies.
a
single
5
5
2
5
Complexes 1-6 absorb light in near UV region in the solid
suggesting that the presence of intermolecular aggregation
does not necessarily contribute to emission.
state. Microspectrophotometry analysis of crystalline samples
revealed the broad absorption bands in the 300-500 nm range
with peak maxima of ca. 395-420 nm (see Figure S37 in the
Supporting Information). When dissolved in CH3CN or CH2Cl2,
complexes
1
-
6
feature similar intense absorption bands in the
π*
deep UV region at 250-300 nm associated with ligand π
-
transitions and less intense broad metal-to-ligand charge-
transfer bands at 395-400 nm (see Figure 5 and Figure S38 in
the Supporting Information). The free ligands L1-L3 shows
π* transitions at 260-270 nm, having negligible
similar π
-
absorption above 300 nm (see Figure S37 in Supporting
Information).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 6 of 9
View Article Online
DOI: 10.1039/C6TC04989C
ARTICLE
Journal Name
Figure 6. Normalized emission spectra of complexes 1-6 in solid state (a) and in CH2Cl2
solution (b) at T = 23 °C; excitation at 400 nm. Photographs of crystalline samples are
taken with 365 nm light excitation.
The absolute luminescence quantum yield of copper
complexes varied significantly from 0.02 to 0.78. Interestingly,
the quantum yield and the spatial arrangement of the pyridine
rings within the ligand were correlated: complexes with tight
intramolecular distances between two pyridine rings were
more efficient emitters. Namely, cationic pentacoordinate
complexes 1BF4 and 1PF6 that have tight pyridine-pyridine
distances have a quantum yield that is at least 10-fold higher
than analogous complexes
observed in iodide complexes
2
and
3
. The same trend is
4
,
6
and 5
where the latter has
the longest pyridine-pyridine centroid distance and the lowest
corresponding quantum yield.
Much to our satisfaction, the impact of intramolecular
interactions in complexes
4 and 6 is sufficient to enable
Table 1. Summary of absorption and emission characteristics of complexes 1-6 in the crystalline state
CIE color coordinates (x,y)g
0.5938,0.4043
d
Py-Py, Åb
Complex
λemi, nm
619
Φa
τ, µs
30.05
24.79
18.20
2.63
krad, 10-3s-1
11.64
12.10
1.77
3.4950(16)
3.4752(12)
3.6568(13)
1PF6
0.35
0.30
0.03
0.02
0.78
0.08
0.53
0.5889,0.4093
1BF4
619
0.6036,0.3940
2
3
4
5
6
637
0.6486,0.3507
3.6211(12) and 3.5806(12)e
665
9.46
0.5142,0.4756
3.3494(10)
585
15.79
2.31c
13.90
49.40
34.63d
38.13
0.4388,0.5375
3.7315(12)
564
0.2798,0.5906
3.4647(11) and 3.595(7)/ 3.611(8)f
524
a absolute quantum yield, all measurements performed with excitation at 400 nm; b solid state distance between centroids built on pyridine units; c weighed average
value derived from the two component fitting, see section S2 in the Supporting Information for decay profiles; d calculated using weight averaged lifetime value;
e values for the symmetrically independent molecules A and B, respectively; f values for the first unit and both disordered components of the second unit of binuclear
complex 6, respectively; g CIE 1931, chromaticity diagram containing solid and solution state data can be found on Figure S46 of the supporting information .
photoluminescence even in solution. Complex
4 emits yellow- decrease to the prevalence nonradiative relaxation pathways
orange light (CIE (0.527,0.4512)) at 600 nm in dichloromethane expected to take effect in solution. Namely, chemical exchange
with absolute quantum yield of 0.28 (Figure 6 B). Its halide- associated with the solution dynamics of the pyridinophane
bridged counterpart
at 545 nm with a quantum yield of 0.15. Interestingly, the nonradiative relaxation.
emission integral intensity and absorbance of and is
6 emits green light (CIE (0.3719,0.5375)) macrocycle may be one of the major contributors to the
4
6
directly proportional to their concentration (See Figure S43
Supporting Information) confirming their nature as molecular
On the origin of the emission.
Our structural and photophysical data suggests that
emitters that are unaffected by intermolecular forces. intramolecular Py-Py interactions have a crucial effect on the
Furthermore, the solution-like emission characteristics were emissive properties of pyridinophane complexes. In this
observed for PMMA films containing complexes
marginally affected by a change of the aggregation state, films transitions involved in photoluminescence. Having complex
containing emit at 580 nm with a quantum yield of 0.29 and as the representative example we analyzed its electronic
films fabricated with complex
4 and 6.Very respect, it is particularly important to reveal the electronic
4
4
6
emit with
Φ
= 0.26 at 548 nm structure using DFT calculations to identify the photophysical
events leading to emission.
(See Figure S45 in the Supporting Information).
The emission lifetime of complexes
4
and
6
in
Frontier orbital analysis shows that the HOMO orbital in 4
dichloromethane is considerably lower than that in the solid in the ground state is confined mainly to Cu and I centers (57 %
state. Time resolved photoluminescence spectroscopy and 23 %, Figure 7 A) with lower energy HOMO-1 orbital
measurements estimate the lifetimes to be 4.3 µs for complex having similar character. The spatially separated pyridine rings
4
and 0.9 µs for complex
6
in CH2Cl2. We attribute the lifetime are directly involved in the formation of the excited S1 state of
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 7 of 9
View Article Online
DOI: 10.1039/C6TC04989C
Journal Name
complex
ARTICLE
4
since they form the LUMO orbital set that is overlap between the Py units (Figure 6 A). Similar to the LUMO
confined to both pyridine rings (44 % and 48 % for Py units in of S0, the higher energy SOMO of T1 is almost exclusively
LUMO, Figure 6 A). Interestingly, parts of LUMO confined to confined to pyridine units (52 and 41 % for each Py unit). This,
different Py rings have
a positive overlap, therefore, in turn, suggests stronger bonding between the Py units in T1
population of LUMO should enhance the interaction between compared to the ground state.
the Py units.
In summary, pyridine units in complex 4 not only interact
but directly influence its emissive properties in two ways.
Firstly, they provide electron accepting LUMO orbitals in the
ground state (S0) confined exclusively to the pyridine units.
Secondly, the interaction between pyridine units strengthens
upon excitation and directs the formation of stable geometries
of excited S1 and T1 states – the crucial step that precedes
emission.
Conclusion
In conclusion, we have developed a new class of transition
metal-based compounds that rely on intramolecular non-
covalent interactions to enable photoluminescence. In
contrast to the majority of emissive TM complexes that are
tuned via modification of the donor/acceptor properties of
their ligand, the compounds reported in this work are tuned by
the variation of steric properties of macrocyclic ligands that
impacts the intramolecular non-covalent interactions within
the ligand molecule.
Figure 7. DFT computed (A) frontier HOMO/LUMO orbitals of complex 4 in the ground
state S0 and the highest populated SOMO orbital of T1 state in CH2Cl2 at isosurface value
0.03 and (B) optimized structures of S0, S1 and T1 states. tBu substituents omitted for
clarity, the values indicated correspond to the distances between centroids built on
pyridine units.
As a result, simple pyridinophane macrocycles act as
potent and tunable ligands that yield copper(I) complexes
emitting in the range of 524-665 nm with a quantum yield up
to 78% at 23 °C. Due to the molecular nature of emission,
TD-DFT analysis predicts the S0-S1 transition to have mixed
metal-to-ligand and halide-to-ligand charge-transfer (M+X)LCT
character associated with calculated absorption at 385 nm. In
agreement with theoretical prediction the lowest energy
absorption band was experimentally observed at 400 nm, a
common value for MLCT/XLCT bands in Cu complexes.17 The
nature and localization of the LUMO orbitals that are
populated upon excitation is the major factor governing the
emission. When the LUMO orbital set is populated during
excitation one would expect the strong attractive forces to
emerge that, in turn, should alter the geometry of the excited
state. Indeed, the TD-DFT analysis indicates the contraction of
complexes
4 and 6 are capable of solution state emission with
quantum yields of 28 and 15 % correspondingly. Our
experimental data suggests a strong link between the extent of
intramolecular aromatic interaction and the quantum yield of
photoluminescence and DFT calculations explain how these
interactions are involved in the formation of emissive states.
Finally, our results introduce a new strategy for designing
photoluminescent compounds that employs subtle
conformational modifications as the main tuning motif. Being
intrinsically sensitive to the local steric environment, the new
emissive complexes hold a good potential to be utilized in
sensing and imaging applications.
the Py-Py spacing by
excited state S1 (Figure 6 B).
As the emission lifetimes observed for complex
relatively long (15.79 s in the solid state and 4.3
Δ = 0.25 Å upon the formation of the first
4
are
µ
µ
s in
Acknowledgements
solution), the possible contribution of the triplet state to
overall emission is likely. In agreement with this suggestion,
DFT analysis estimates the emission maxima for the triplet
state at 614 nm while the S1 state would emit at 557 nm.
Interestingly, the relaxation of the T1 state geometry that
precedes the emission is also associated with the
GAF is sincerely thankful to Dr. Guanying Li, Mr. Christopher
Eugene Petoukhoff, Dr. Kieran Deasy and Dr. Emilio Juarez
Perez of OIST for enthusiastic discussions during the
preparation of this work. JK thanks Dr. Hiroyasu Sato (Rigaku)
for discussions regarding the X-ray data analysis. The authors
acknowledge the Okinawa Institute of Science and Technology
Graduate University for full funding of the project.
rearrangement of the stacking pyridine units within
to S1, the T1 state has a significantly shorter Py-Py contacts
compared to the ground state ( = 0.23 ). As expected,
4. Similar
Δ
Å
formation of T1 state strengthens the interaction between the
Py units in macrocyclic ligand as evidenced by the frontier
orbital analysis. Namely, the population of LUMO level in S0
results in formation of the new SOMO of T1 featuring a positive
Notes and references
1
R. C. Evans, P. Douglas and C. J. Winscom, Coord. Chem. Rev.,
2006, 250, 2093-2126.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 8 of 9
View Article Online
DOI: 10.1039/C6TC04989C
ARTICLE
Journal Name
2
3
4
5
J. Wu, W. Liu, J. Ge, H. Zhang and P. Wang, Chem. Soc. Rev., 32 C. T. Cunningham, K. L. H. Cunningham, J. F. Michalec and D.
2011, 40, 3483-3495. R. McMillin, Inorg. Chem., 1999, 38, 4388-4392.
K. Li, W. Qin, D. Ding, N. Tomczak, J. Geng, R. Liu, J. Liu, X. 33 C.-M. Che, Z.-Y. Li, K.-Y. Wong, C.-K. Poon, T. C. W. Mak and
Zhang, H. Liu, B. Liu and B. Z. Tang, Sci. Rep., 2013, , 1150. S.-M. Peng, Polyhedron, 1994, 13, 771-776.
Z. Wang, J. Nie, W. Qin, Q. Hu and B. Z. Tang, Nat. Commun., 34 J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam and B. Z.
2016, Tang, Chem. Rev., 2015, 115, 11718-11940.
S. Lee, S.-O. Kim, H. Shin, H.-J. Yun, K. Yang, S.-K. Kwon, J.-J. 35 Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun., 2009,
3
7.
Kim and Y.-H. Kim, J. Am. Chem. Soc., 2013, 135, 14321-
14328.
F. Julia, D. Bautista, J. M. Fernandez-Hernandez and P.
4332-4353.
36 X. Zhang, Z. Chi, Y. Zhang, S. Liu and J. Xu, J. Mater. Chem. C,
2013, 1, 3376-3390.
6
7
8
Gonzalez-Herrero, Chem. Sci., 2014,
X. Yan, T. R. Cook, P. Wang, F. Huang and P. J. Stang, Nat.
Chem., 2015, , 342-348.
R. Sakamoto, T. Iwashima, J. F. Kögel, S. Kusaka, M. Tsuchiya,
Y. Kitagawa and H. Nishihara, J. Am. Chem. Soc., 2016, 138
5666-5677.
T. Seki, Y. Takamatsu and H. Ito, J. Am. Chem. Soc., 2016, 40 K. Huang, H. Wu, M. Shi, F. Li, T. Yi and C. Huang, Chem.
138, 6252-6260. Commun., 2009, 1243-1245.
10 D. Volz, M. Wallesch, C. Flechon, M. Danz, A. Verma, J. M. 41 Y. You, H. S. Huh, K. S. Kim, S. W. Lee, D. Kim and S. Y. Park,
5
, 1875-1880.
37 T. U. Connell, J. M. White, T. A. Smith and P. S. Donnelly,
Inorg. Chem., 2016, 55, 2776-2790.
38 Q. Zhao, L. Li, F. Li, M. Yu, Z. Liu, T. Yi and C. Huang, Chem.
Commun., 2008, 685-687.
7
,
39 G.-G. Shan, H.-B. Li, H.-Z. Sun, D.-X. Zhu, H.-T. Cao and Z.-M.
Su, J. Mater. Chem. C, 2013, 1, 1440-1449.
9
Navarro, D. M. Zink, S. Brase and T. Baumann, Green Chem.,
2015, 17, 1988-2011.
Chem. Commun., 2008, 3998-4000.
42 M. Osawa, Chem. Commun., 2014, 50, 1801-1803.
11 N. Armaroli, G. Accorsi, F. o. Cardinali and A. Listorti, in 43 R.-H. Wei, J.-F. Chen, J.-Q. Feng, J.-S. Hu and D.-K. Cao, RSC
Photochemistry and Photophysics of Coordination Adv., 2015, , 14359-14365.
Compounds I, Springer Berlin Heidelberg, Berlin, Heidelberg, 44 J. R. Khusnutdinova, N. P. Rath and L. M. Mirica, Inorg.
5
2007, 280, 69-115.
12 P. C. Ford, E. Cariati and J. Bourassa, Chem. Rev., 1999, 99
Chem., 2014, 53, 13112-13129.
,
45 S. P. Meneghetti, P. J. Lutz and J. Kress, Organometallics,
2001, 20, 5050-5055.
13 A. l. Lavie-Cambot, M. Cantuel, Y. Leydet, G. Jonusauskas, D. 46 F. Bottino, M. Di Grazia, P. Finocchiaro, F. R. Fronczek, A.
3625-3648.
M. Bassani and N. D. McClenaghan, Coord. Chem. Rev., 2008,
252, 2572-2584.
Mamo and S. Pappalardo, J. Org. Chem., 1988, 53, 3521-
3529.
14 O. Horváth, Coord. Chem. Rev., 1994, 135, 303-324.
15 T. Hofbeck, U. Monkowius and H. Yersin, J. Am. Chem. Soc.,
2015, 137, 399-404.
47 N. A. Yakelis and R. G. Bergman, Organometallics, 2005, 24,
3579-3581.
48 G. J. Kubas, B. Monzyk and A. L. Crumblis, in Inorganic
Syntheses, John Wiley & Sons, Inc., 2007, 68-70.
49 S. Holler, M. Tüchler, F. Belaj, L. F. Veiros, K. Kirchner and N.
C. Mösch-Zanetti, Inorg. Chem., 2016, 55, 4980-4991.
50 J. R. Khusnutdinova, J. Luo, N. P. Rath and L. M. Mirica, Inorg.
Chem., 2013, 52, 3920-3932.
16 S. B. Harkins and J. C. Peters, J. Am. Chem. Soc., 2005, 127
2030-2031.
,
17 A. Tsuboyama, K. Kuge, M. Furugori, S. Okada, M. Hoshino
and K. Ueno, Inorg. Chem., 2007, 46, 1992-2001.
18 M. Nishikawa, Y. Wakita, T. Nishi, T. Miura and T.
Tsubomura, Dalton. Trans., 2015, 44, 9170-9181.
19 D. R. McMillin and K. M. McNett, Chem. Rev., 1998, 98, 1201-
1220.
51 W.-T. Lee, S. B. Muñoz, D. A. Dickie and J. M. Smith, Angew.
Chem. Int. Ed., 2014, 53, 9856-9859.
52 W. O. Koch and J. T. Kaiser, Chem. Commun., 1997, 2237-
2238.
21 D. G. Cuttell, S.-M. Kuang, P. E. Fanwick, D. R. McMillin and 53 M. Jagoda, S. Warzeska, H. Pritzkow, H. Wadepohl, P. Imhof,
20 G. Blasse and D. R. McMillin, Chem. Phys. Lett., 1980, 70, 1-3.
R. A. Walton, J. Am. Chem. Soc., 2002, 124, 6-7. J. C. Smith and R. Krämer, J. Am. Chem. Soc., 2005, 127,
22 C. Bizzarri, C. Strabler, J. Prock, B. Trettenbrein, M. 15061-15070.
Ruggenthaler, C.-H. Yang, F. Polo, A. Iordache, P. Braggeller 54 K. Tsuge, Chem. Lett., 2013, 42, 204-208.
and L. D. Cola, Inorg. Chem., 2014, 53, 10944-10951.
23 M. Nishikawa, T. Sano, M. Washimi, K. Takao and T.
Tsubomura, Dalton. Trans., 2016, 45, 12127-12136.
55 S. Perruchas, X. F. Le Goff, S. Maron, I. Maurin, F. Guillen, A.
Garcia, T. Gacoin and J.-P. Boilot, J. Am. Chem. Soc., 2010,
132, 10967-10969.
24 V. A. Krylova, P. I. Djurovich, M. T. Whited and M. E. 56 M. Vitale and P. C. Ford, Coord. Chem. Rev., 2001, 219-221
3-16.
,
Thompson, Chem. Commun., 2010, 46, 6696-6698.
25 V. A. Krylova, P. I. Djurovich, J. W. Aronson, R. Haiges, M. T.
Whited and M. E. Thompson, Organometallics, 2012, 31
7983-7993.
,
26 V. A. Krylova, P. I. Djurovich, B. L. Conley, R. Haiges, M. T.
Whited, T. J. Williams and M. E. Thompson, Chem. Commun.,
2014, 50, 7176-7179.
27 R. Marion, F. Sguerra, F. Di Meo, E. Sauvageot, J.-F. o. Lohier,
R. Daniellou, J.-L. Renaud, M. Linares, M. Hamel and S.
Gaillard, Inorg. Chem., 2014, 53, 9181-9191.
28 A. J. M. Miller, J. L. Dempsey and J. C. Peters, Inorg. Chem.,
2007, 46, 7244-7246.
29 K. J. Lotito and J. C. Peters, Chem. Commun., 2010, 46, 3690-
3692.
30 Y. Okano, H. Ohara, A. Kobayashi, M. Yoshida and M. Kato,
Inorg. Chem., 2016, 55, 5227-5236.
31 H. Araki, K. Tsuge, Y. Sasaki, S. Ishizaka and N. Kitamura,
Inorg. Chem., 2005, 44, 9667-9675.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 9
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC04989C
2081x1040mm (72 x 72 DPI)