Reversible luminescent colour changes of mononuclear copper(i) complexes based on ligand exchange reactions by N-heteroaromatic vapours

Article abstract of DOI:10.1039/c7dt00532f

[CuCl(PPh₃)₂(4-Mepy)] (PPh₃ = triphenylphosphine, 4-Mepy = 4-methylpyridine), a highly blue-luminescent mononuclear copper(i) complex, shows reversible emission colour change ranging from blue-green to red by vapour exposure to N-heteroaromatics, such as pyridine, pyrimidine, and 2-methylpyrazine. The remarkable colour changes occur due the ligand exchange reaction between the solid and gas. Such ligand exchange reactions would be applicable to various vapours with coordination ability and be useful for the sensing of such volatile organic compounds as well as a new convenient methodology for the preparation of luminescent complexes.

Full text of DOI:10.1039/c7dt00532f

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DOI: 10.1039/C7DT00532F
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Reversible luminescent colour changes of mononuclear copper(I)
complexes based on ligand exchange reactions by N-
heteroaromatic vapours†
Received 00th January 20xx,
Accepted 00th January 20xx
Hiroki Ohara,a Tomohiro Ogawa,a Masaki Yoshida,a Atsushi Kobayashi,a Masako Katoa*
DOI: 10.1039/x0xx00000x
[CuCl(PPh3)2(4-Mepy)] (PPh3 = triphenylphosphine, 4-Mepy = 4-methylpyridine), a highly blue-luminescent mononuclear
copper(I) complex, shows reversible emission colour change ranging from blue-green to red by vapour exposure to N-
heteroaromatics such as pyridine, pyrimidine, and 2-methylpyrazine. The remarkable colour changes occur by the ligand
exchange reaction between solid and gas. Such ligand exchange reactions would be applicable to various vapours with
coordination ability and useful for the sensing of such volatile organic compounds as well as a new convenient
methodology for the preparation of luminescent complexes.
www.rsc.org/
Direct coordination of vapour molecules to the metal sites
of transition metal complexes is another promising strategy for
a more facile way to discriminate vapours of similar molecules.
Introduction
Stimulus-responsive luminescent materials have attracted
much attention because they can be potentially applied in
naked-eye sensors and memory devices.1 In most cases, the
emission colour changes of these materials are based on host-
guest interactions2 or intermolecular interactions such as
Vapours can directly influence the electronic states of the
systems, resulting in large changes in colour depending on the
respective vapour. The method would be applicable to detect
diverse organic vapours containing coordination atoms such as
nitrogen or oxygen. Very recently, we succeeded in the
construction of a vapochromic nickel(II)-quinonoid complex of
this type, [Ni(HLMe)2] (H2LMe = 4-methylamino-6-methyliminio-
3-oxocyclohexa-1,4-dien-1-olate).11 The crystalline sample of
the complex exhibits high-selectivity to methanol vapor
changing colours between purple and orange accompanied by
the change in the spin-state, although the nickel(II) complex
does not emit luminescence.
hydrogen bonds,3
π-π
interactions,4 or donor-acceptor
interactions.5 From the viewpoint of sensors, it is desirable to
show large colour changes in response to external stimuli,
both sensitively and selectively. In addition, multiple-
responses in recognising various stimuli would be useful in
intelligent sensors. Sensor materials that can quickly and easily
detect volatile organic compounds (VOCs) should be important
in environmental monitoring. Platinum(II) and Gold(I)
complexes that exhibit intense luminescence by assembling
have been developed for vapor response systems, i.e.
vapochromic compounds.6,7 Luminescent supramolecular
systems and porous coordination polymers have also been
applied for vapor sensing.8,9 However, the VOC sensors
reproted so far still have limitations on the selective and
multiple detection of vapour molecules because of difficulties
in the fine control of intermolecular interactions, crystal
structures, or pore size of materials. A possible strategy to
overcome this issue is the fabrication of sensor arrays that
detect
a particular vapour was attained by pattern
Scheme 1. Ligand exchange of complex 1 based on solid-vapour reactions.
recognition.10
To demonstrate such
a
sensing system showing
luminescence colour changes, we focused on copper(I)-halide
complexes because they are well-known for being inexpensive
and strongly emissive complexes, and their emission
properties are sensitively affected by the versatile structures
and additional ligands.12 The emission colours of copper(I)-
halide complexes with N-heteroaromatic ligands can be tuned
a.Department of Chemistry, Faculty of Science, Hokkaido University, North-10
West-8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan.
E-mail: mkato@sci.hokudai.ac.jp; Fax: +81-11-706-3447; Tel: +81-11-706-3817
†Electronic Supplementary Information (ESI) available: PXRD and luminescence
spectral changes on vapour exposure, 1H NMR spectra, DFT calculation details, and
X-ray structural data for
3 and 4. CCDC 1519416 and 1519417, respectively. See
DOI: 10.1039/x0xx00000x
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Journal Name
widely from red to blue by varying the ligands.13,14 These
properties suggest the sensing ability of copper(I)-halide
complexes to N-heteroaromatic compounds.
based on the single crystal of 4 (Fig. S4). Anal. Calcd foVriew Article Online
C41H36N2P2ClCu: C, 68.61; H, 5.06; N, 3.90. FoDuOndI::1C0.,160839.6/C6;7DHT,050.15342;F
N 3.81.
Preparation of single crystals of 4. To [CuCl(PPh3)3]·CH3CN (101.2
mg, 0.114 mmol) was added 2-Mepyz (1 mL). Yellow crystals were
obtained by slow diffusion of diethyl ether vapour into the solution
overnight. Yield. 52.0 mg, 63%. 1H NMR (270 MHz, CD3CN, Fig.
S3(b)) δ ppm 2.50 (s, 3H, CH3), 7.30–7.42 (m, 30H, PPh3), 8.36–8.46
(m, 3H, pyz-H3,5,6). Anal. Calcd for C41H36N2P2ClCu: C, 68.61; H,
5.06; N, 3.90. Found: C, 68.60; H, 4.94; N3.85.
Herein, we report a new vapour-responsive system based
on
luminescent
mononuclear
copper(I)
complexes
N-
[CuCl(PPh3)2(L)] (PPh3
=
triphenylphosphine,
L
=
heteroaromatic compounds) (Scheme 1). The complexes
exhibited remarkable luminescence colour changes in the solid
state in response to the vapours of various N-heteroaromatic
compounds. We found that the phenomenon occurred due to
the ligand exchange of vapour molecules derived from N-
heteroaromatic compounds such as pyridine and pyrimidine.
Measurements.
Luminescence Spectroscopy. The luminescence spectrum of each
sample was measured using a JASCO FR-6600 spectrofluorometer at
room temperature. The luminescence quantum yield was recorded
on a Hamamatsu Photonics C9920-02 absolute photoluminescence
quantum yield measurement system equipped with an integrating
sphere apparatus and 150-W CW Xenon light source. Hamamatsu
Photonics A10095-03 non-luminescent quartz sample holder was
used for absolute emission quantum yield measurements of the
solid samples in air. The accuracy of the instrument was confirmed
by the measurement of the quantum yield of anthracene in
degassed ethanol solution (Φ = 0.27).17 Emission life time
Experimental
Materials
All commercially available starting materials were used as received
and solvents were used without any purification. Unless otherwise
stated, all manipulations were conducted in air. Complexes
[CuCl(PPh3)2(4-Mepy)] (1) (4-Mepy = 4-methylpyridine),14
[CuCl(PPh3)2(py)] (2),15 and [CuCl(PPh3)3]·CH3CN16 were prepared
according to the literature procedure. The complex 2 was also
prepared by a vapour exchange method described below.
Synthesis of [CuCl(PPh3)2(py)] (2) (py = pyridine) by ligand
vapour exposure. Complex 1 (50 mg, 0.070 mmol) was exposed to
py vapour overnight. Complex 2 was obtained quantitatively as
white powder. The powder X-ray diffraction (PXRD) pattern of the
white powder was consistent with the simulated pattern based on
the single crystal X-ray analysis of 2 (see the “vapochromic
behaviour” paragraph in Results and Discussion). Anal. Calcd for
C40H34N2P2ClCu: C, 70.08; H, 5.02; N, 1.99. Found: C, 70.04; H, 5.06;
N 2.04.
Synthesis of [CuCl(PPh3)2(pym)] (3) (pym = pyrimidine) by ligand
vapour exposure. Complex 1 (50 mg, 0.070 mmol) was exposed to
pym vapour overnight. Complex 3 was obtained quantitatively as
pale yellow powder. The PXRD pattern of the powder sample dried
in the vacuum was consistent with the simulated pattern based on
the single crystal X-ray analysis of 3, although a different pattern
was obtained for the sample before drying (Fig. S1). 1H NMR
spectrum indicated that the crystal form before drying included an
excess amount of pym as crystal solvent, and the dried form was
consistent with the crystalline sample (Fig. S2) Anal. Calcd for
C40H34N2P2ClCu: C, 68.28; H, 4.87; N, 3.98. Found: C, 68.58; H, 4.96;
N 3.85.
measurements were conducted by using a Hamamatsu Photonics,
C4334 system equipped with a streak camera as a photo detector
and a nitrogen laser for the 337 nm excitation.
Table 1. Crystal Parameters and Refinement Data for complexes 3 and 4.
3
4
Formula
C40H34ICuN2P2
703.67
C41H36ClCuN2P2
717.70
Formula weight
Crystal system
Space group
a (Å)
Triclinic
P-1
monoclinic
P21/c
11.456(1)
13.009(2)
13.566(2)
9.712(1)
37.383(5)
10.869(2)
b (Å)
c (Å)
α
β
γ
(deg)
(deg)
(deg)
78.890(7)
68.086(6)
66.559(6)
1718.6(5)
200(1)
2
90
115.945(4)
90
V (Å3)
3548.3(9)
173(1)
4
T (K)
Z
no. of reflections collected
no. of unique reflections
GOF on F2
13922
7770
36563
8006
Preparation of single crystals of 3. To complex 1 (31.0 mg, 43.3
mmol) was added pym (1 mL). Pale yellow crystals were obtained
by slow diffusion of diethyl ether vapour into the solution for one
week. Yield. 5.5 mg, 18%. 1H NMR (270 MHz, CD3CN, Fig. S3(a)) δ
ppm 7.29–7.44 (m, 31 H, phenyl group in PPh3 + pym-H5), 8.74 (d, J=
4.9 Hz, 2H, pym-H3,6), 9.15 (s, 1H, pym-H2). Anal. Calcd for
C40H34N2P2ClCu: C, 68.28; H, 4.87; N, 3.98. Found: C, 68.41; H, 4.73;
N3.92.
1.034
1.058
1.343
0.0676
0.0525
0.1145
Dcalc (g cm-3)
1.360
Rint
0.0246
0.0343
0.0895
R1 (I > 2σ
(I))a
b
wR2
aR1 = Σ||Fo| − |Fc||/Σ|Fo|. b wR2 = [Σw(Fo2 − Fc2)/Σw(Fo)2]1/2, w = [σc2(Fo2) + (xP)2
yP]−1, P = (Fo2 − 2Fc2)/3.
+
Synthesis of [CuCl(PPh3)2(2-Mepyz)] (4) (2-Mepyz = 2-
methylpyrazine) by ligand vapour exposure. Complex 1 (50 mg,
0.070 mmol) was exposed to 2-Mepyz vapour for one week.
Complex 4 was obtained quantitatively as yellow powder. The PXRD
pattern of the powder was consistent with the simulated pattern
Other measurements. The 1H NMR spectrum of each sample was
measured using a JEOL EX-270 NMR spectrometer at room
temperature. Elemental analysis was conducted at the analysis
centre of Hokkaido University.
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X-ray crystallography
The vapochromic luminescence was investigated through
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powder X-ray diffraction (PXRD) measurDeOmI:e1n0t.1s0.3F9i/gC.7D2Ts0h05o3w2Fs
Single-crystal X-ray analysis. All single-crystal X-ray diffraction
measurements were conducted using a Rigaku Mercury CCD
diffractometer with graphite monochromated Mo Kα radiation (λ =
0.71069 Å) and a rotating anode generator. Each single-crystal was
mounted on a MicroMount Using paraffin oil. The crystal was then
cooled using an N2-flow type temperature controller. Diffraction
data were collected and processed using CrystalClear.18 Structures
were solved by the direct method using SIR-200419 for 3 and 4.
Structural refinements were conducted by the full-matrix least-
squares method using SHELXL-97.20 Non-hydrogen atoms were
refined anisotropically, and hydrogen atoms were refined using the
riding model. All calculations were conducted using the Crystal
Structure crystallographic software package.21 Crystallographic data
obtained for each complex are summarized in Table 1 and
deposited to the Cambridge data centre (CCDC-1519416, 1519417).
Powder X-ray diffraction measurements. Powder X-ray
the PXRD pattern changes in the case that complex
exposed to py vapour. The PXRD pattern after py vapour
exposure (Fig. 2(b)) was completely different from that of
1 was
1
(Fig. 2(a)). The pattern was almost consistent with the
simulation pattern calculated from the single-crystal X-ray
diffraction (SXRD) data of [CuCl(PPh3)2(py)] (
indicating the formation of complex by ligand exchange
2
) Fig. 2(c)),14
2
based on vapour exposure. By exposing the sample after
exchange to 4-Mepy vapour again, the PXRD pattern for
complex
1 was recovered (Fig. 2(d)). Similar reversible PXRD
pattern changes by ligand exchange were observed for pym
and 2-Mepyz vapours (Figs. S1 and S4). We confirmed the
purity of the complexes [CuCl(PPh3)2(pym)]
(3
)
and
[CuCl(PPh3)2(2-Mepyz)] ( ) produced by the vapor exchange on
4
the basis of the elemental analysis (see experimental section).
diffraction was conducted using a Bruker D8 Advance
diffractometer equipped with a graphite monochromator using Cu
Kα radiation and one-dimensional LinxEye detector.
Theoretical Calculations
Density functional theory (DFT) calculations were performed with
B3LYP functional22 and the LANL2DZ basis23 set using Gaussian 03
program.24 Molecular structures determined by X-ray
crystallographic analysis were used for calculations. Molecular
orbitals are shown using Avogadro 1.10. 25
Results and discussion
Vapochromic behaviour. Crystal samples of [CuCl(PPh3)2(4-
Mepy)] (4-Mepy = 4-methylpyridine) (1), which were prepared
as reported previously,14 exhibited strong blue emission, as
shown in Fig. 1 (a, c, e). Interestingly, upon exposure to the
vapours of pyridine (py), pyrimidine (pym), or 2-
methylpyradine (2-Mepyz), the emission colour changed from
blue to blue-green, yellow, or red, respectively (Fig. 1 (b, d, f)).
The NMR spectra and elemental analyses of the exposed
samples after drying in a vacuum clearly indicated that the
replacement of the 4-Mepy ligand by the other L occurred
completely. Therefore, the vapor exposure can be used as a
convenient preparation method of the complexes as described
in the experimental section.
Fig. 2. Reversible PXRD pattern changes of complex 1; (a) PXRD pattern of complex 1,
and (b) that after py vapour exposure. (c) The simulation patterns of complex 2, and (d)
PXRD pattern after exposing the sample (b) to 4-Mepy vapour.
Crystal structures of complexes. Fig. 3 shows the crystal
structures of complexes
3
and 4. The teterahedral
coordination stuctures including chloride, two triphenyl
phosphine and a N-heteroaromatic ligand are similar to those
for complexes
geometry.14,15 The coordination of 2-Mepyz at the 4-position
of nitrogen (N1) for would be reasonable considering the
1 and 2 showing the typical mononuclear Cu(I)
4
stric effect of the methyl group (Fig. 3(b)). For both complexes,
no special intermolecular interactions can be seen in the
crystal, although there are trivial short contacts of Cl···H for
3
and N···H for in the crystal structures, respectively (Fig. S5).
4
Therefore, the luminescence would mainly reflect the
electronic properties of the molecular characters.
Fig. 1. Photographs of crystals of complex 1 under UV light before (a, c, e) and after
vapour exposure of (b) py for 1 day, (d) pym for 2 days, and (f) 2-Mepyz for 3 days,
respectively, under ambient vapour pressure at room temperature.
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Table 2. Luminescence properties of crystals 1–4 at 298 K.
View Article Online
DOI: 10.1039/C7DT00532F
Complex
1 g
2 h
3
4
λmaxa (nm) 468
498
526
627
τ (µs (An)) b 5.8 (0.43)
3.3 (0.13)
12.5 (0.89)
2.3 (0.35)
4.3 (0.69)
3.3 (0.88)
5.5 (0.13)
10.7 (0.64)
Φ c
0.99
0.98
0.40
0.09
τav d (µs)
9.4
12
3.8
3.7
(a)
(b)
−
1
kr e (s )
1.1 × 105
1.1 × 103
8.0 × 104
1.6 × 103
1.0 × 105
1.5 × 105
2.4 × 104
2.5 × 105
Fig. 3. Molecular structures of (a) 3 and (b) 4. Hydrogen atoms are omitted for clarity.
Thermal ellipsoids are displayed at the 50% probability level. Coordination Bond
distances: For 3, Cu1–Cl1 = 2.3510(6), Cu1–P1 = 2.2531(5), Cu1–P2 = 2.2741(6), Cu1–N1
= 2.128(1), For 4, Cu1–Cl1 = 2.3174(8), Cu1–P1 = 2.2764(8), Cu1–P2 = 2.2626(9), Cu1–
N1 = 2.136(4).
−
1
knr f (s )
a
b
Emission maximum. Emission lifetimes. Emission decays were
c
analysed with two components: I = A1exp(−t/τ1) + A2exp(−t/τ2).
d
Photoluminescence quantum yield of the crystal sample.
Average emission lifetime, which was estimated based on the
following equation for the two exponential decay components:
2
2
Luminescence properties. Fig. 4 shows the emission spectra of τav = (A1τ1 + A2τ2 ) / (A1τ1+ A2τ2). e Radiative rate constant, which
was estimated by Φ/τav. f Non-radiative rate constant, which was
complexes
1
–
4
as crystals. The emission maxima (
λ
,
max) of the
, and 4,
estimated by kr(1 − Φ)/Φ. g Ref.14. h Ref.26.
complexes are 468, 498, 526, and 627 nm for
1
,
2
3
respectively. The large emission spectral shifts depending on
the N-heteroaromatic ligands (L) suggest the involvement of
the π* orbital of L in the emission state. Therefore, this Cu(I)
complex system can be a good detector for N-heteroaromatic
14
molecules. Time-dependent density functional theory (TD-DFT)
4
12
10
8
3
calculations for complexes
3 and 4 indicated that the electron
density in the highest occupied molecular orbital (HOMO) was
distributed over the Cu and halogen atoms, while that in the
lowest unoccupied molecular orbital (LUMO) was localized on
L (Fig. S6, Tables S1 and S2), suggesting that the lowest excited
2
states for both
3 and 4 can be assigned to the metal-to-ligand
1
charge transfer (MLCT) states mixed with halide-to-ligand
charge transfer (XLCT) states ((M+X)LCT states), as in the case
and 2 14,26
of the complexes 1 .
6
15
16
17
18
19
20
21
22
Wavenumber/ 1000 cm-1
Fig. 5. Plots of ln(knr) vs. the wavenumber for [CuX(PPh3)2(L)] (L = N-heteroaromatics).
Red circle: X = Cl−, Blue triangle: X = Br−, Black square: X = I−, The numbers for the
complexes 1–4 are shown in the graph. Other data are taken from the previous
reports.26
The photophysical properties of complexes
are summarized in Table 2. Although the luminescence
quantum yields of and are quite high, those of and 4
1–4 at 298 K
1
2
3
decreased with the red-shift of the emission wavelength. The
trend can be explained roughly by the energy gap law as
shown in Fig. 5.26,27 For the complex
3, however, the slight
deviation from the trend suggests additional factors for the
nonradiative deactivation of the luminescent state. It would be
related to the flexiblity of the crystal structure. In fact,
relatively large thermal ellipsoids for some of phenyl groups
were found in the X-ray structure of
formation of a different crystal structure from that of
saturated vapour atmosphere of pym and the transformation
to the crystal structure of by drying also suggest the
3
(Fig. 3(a)). The
3
in the
Fig. 4. Luminescence spectra of 1–4 crystals at 298 K: λex = 350 nm for 1, 2 and 3, and
ex = 400 nm for 4.
3
λ
flexibility of the crystal structure of
section).
3 (see experimental
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To investigate the ligand exchange behaviour, we
measured the time-course of emission spectra under vapour
exposure. Fig. 6 shows luminescent spectral changes of in
the presence of 2-Mepyz vapour. With vapour exposure, the
emission intensity of gradually decreased and a new band
appeared at a longer wavelength, and finally the spectral
change was completed exhibiting the band corresponding to
Acknowledgements
View Article Online
DOI: 10.1039/C7DT00532F
This work was partly supported by JSPS KAKENHI Grant Number
JP15H00858 in Scientific Research on Innovative Areas “Artificial
Photosynthesis (AnApple)”, JP26410063 in Scientific Research
(C), and JP26410063 (15K17827) in Scientific Research, Young
Scientists (B).
1
1
that for complex
occurred in the solid state followed by structural
transformation from to . Similar spectral changes of
complex were observed in the cases of py and pym vapour
4. This suggests the ligand exchange reaction
Notes and references
1
4
1
1
(a) Y. Sagara and T. Kato, Nat. Chem. 2009, 1, 605; (b) Z. Chi,
exposure, although the reaction times were much faster (Fig.
S7). Py, pym, and 2-Mepyz took 2, 11 and 22 h, respectively, to
complete the exchange. Such different reaction times can be
explained based on the basicity and vapour pressure of L. The
pKa of the conjugate acids and vapour pressure values (in
parentheses, mmHg) at 298 K were reported as follows:28 6.0
(6), 5.2 (19), 1.3 (18), and 1.5 (9) for 4-Mepy, py, pym and 2-
Mepyz, respectively. The low basicity (i.e. low pKa of the
conjugate acid) and low vapour pressure for 2-Mepyz would
result in the slowest exchange among the three.
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Fig. 6. Luminescence spectral changes of complex 1 under exposure to 2-Mepyz vapour
(λex = 350 nm). Inset is an enlarged drawing.
Conclusions
9
In conclusion, we found a new vapour response system of
luminescent mononuclear Cu(I) complexes based on the ligand
exchange reaction of N-heteroaromatic compounds. By vapour
exposure to complex 1, mononuclear complexes 2, 3, and 4
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were easily and purely obtained by the replacement of 4-Mepy
with py, pym, and 2-Mepyz, respectively, changing the
luminescence colours drastically and reversibly from blue to
red. Such ligand exchange reactions would be applicable to
various vapours with coordination ability. The wide emission
wavelength shifts in exchange reactions are able to recognize
N-heteroaromatic vapours for sensing applications. The
response time could be improved by the fabrication of thin
layers or nano sheets. Therefore, the ligand exchange
reactions based on vapour exposure would be useful for easy
sensing of VOCs and for a new convenient methodology for
the preparation of luminescent complexes. Further studies
,
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