Luminescent Tungsten(VI) Complexes: Photophysics and Applicability to Organic Light-Emitting Diodes and Photocatalysis

Article abstract of DOI:10.1002/anie.201608240

The synthesis, excited-state dynamics, and applications of two series of air-stable luminescent tungsten(VI) complexes are described. These tungsten(VI) complexes show phosphorescence in the solid state and in solutions with emission quantum yields up to 22 % in thin film (5 % in mCP) at room temperature. Complex 2 c, containing a 5,7-diphenyl-8-hydroxyquinolinate ligand, displays prompt fluorescence (blue–green) and phosphorescence (red) of comparable intensity, which could be used for ratiometric luminescent sensing. Solution-processed organic light-emitting diodes (OLEDs) based on 1 d showed a stable yellow emission with an external quantum efficiency (EQE) and luminance up to 4.79 % and 1400 cd m^?2respectively. These tungsten(VI) complexes were also applied in light-induced aerobic oxidation reactions.

Full text of DOI:10.1002/anie.201608240

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201608240
German Edition: DOI: 10.1002/ange.201608240
Molecular Photochemistry
Luminescent Tungsten(VI) Complexes: Photophysics and
Applicability to Organic Light-Emitting Diodes and Photocatalysis
Kwan-Ting Yeung, Wai-Pong To, Chenyue Sun, Gang Cheng, Chensheng Ma,
Glenna So Ming Tong, Chen Yang, and Chi-Ming Che*
Abstract: The synthesis, excited-state dynamics, and applica-
tions of two series of air-stable luminescent tungsten(VI)
complexes are described. These tungsten(VI) complexes show
phosphorescence in the solid state and in solutions with
emission quantum yields up to 22% in thin film (5% in mCP)
at room temperature. Complex 2c, containing a 5,7-diphenyl-
8
-hydroxyquinolinate ligand, displays prompt fluorescence
(
blue–green) and phosphorescence (red) of comparable inten-
sity, which could be used for ratiometric luminescent sensing.
Solution-processed organic light-emitting diodes (OLEDs)
based on 1d showed a stable yellow emission with an external
quantum efficiency (EQE) and luminance up to 4.79% and
ꢀ2
1
400 cdm respectively. These tungsten(VI) complexes were
also applied in light-induced aerobic oxidation reactions.
A
lthough tungsten has been used in incandescent light bulbs
for decades, complexes presenting substantial electrolumi-
nescence are not reported. As the spin-orbit coupling
constant (z) of tungsten is comparable to that of iridium
ꢀ
1
and platinum (z [cm ]: 3909 (Ir), 4481 (Pt), and 2433 (W)),
tungsten should display a strong heavy atom effect that
facilitates intersystem crossing from the singlet excited state
[1–4]
Figure 1. a) Chemical structures of 1a–j and 2a–f. b) Perspective views
of 1b (left) and 2e (right) (Hydrogen atoms are omitted for clarity).
to triplet excited state.
Recently, homoleptic arylisocya-
nide tungsten(0) and molybdenum(0) complexes were
reported to display strong phosphorescence with quantum
yields up to 44% and 2.3% respectively, but their air-
[
5]
sensitivity might limit material applications. Herein, two
new classes of air-stable luminescent tungsten(VI) complexes
and their applications in OLEDs and photocatalysis are
described. These tungsten(VI) complexes display dominant
phosphorescence or dual fluorescence–phosphorescence of
comparable intensity, depending on the coordinating ligands.
Figure 1 depicts the structures of 1a–j and 2a–f (syntheses
and characterization data are given in the Supporting
Information, Schemes S1 and S2). Complexes 1i and 1j
incorporate rigid fluorene moieties into the Schiff base ligand
to minimize structural distortion in the emitting excited states.
The X-ray crystal structures for 1b, 1 f, 1g, and 2e were
determined (Supporting Information, Table S1). All these
complexes adopt a distorted octahedral geometry with a cis-
dioxo unit (Figure 1; Supporting Information). The WꢀO
[
6–7]
(oxo) distances range from 1.666 to 1.737 ꢀ.
and 1g, the Schiff base ligands adopt a b-cis geometry with
O -W-O angles of 83.07–86.678 (where O = phenolic oxygen
For 1b, 1 f,
[
*] Dr. K.-T. Yeung, Dr. W.-P. To, C. Sun, Dr. G. Cheng, Prof. Dr. C. Ma,
Dr. G. S. M. Tong, C. Yang, Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry, Institute of Molecular
Functional Materials, Department of Chemistry
The University of Hong Kong
p
p
p
atoms). For 2e, the phenolic oxygen atoms of the quinolinate
ligands are trans to each other and the two pyridine nitrogen
atoms are trans to the oxo groups (Figure 1). The N-W-N
angles of 1 f and 1g are 79.06–79.928, which are larger than
that of 1b (71.568), presumably because of the flexibility
offered by the propylene linkage.
Pokfulam Road, Hong Kong SAR (China)
E-mail: cmche@hku.hk
Prof. Dr. C. Ma
The photophysical data are listed in Table 1 and Table S3
School of Chemistry and Chemical engineering, Shenzhen University
Shenzhen 518060 (China)
(
Supporting Information). Figure 2 displays the absorption
and emission spectra for 1a, 1d, 1i, 1j, and 2a–f. Complexes
a–j and 2a–f exhibit strong absorption bands at l < 300 nm
Dr. G. Cheng, Prof. Dr. C.-M. Che
HKU Shenzhen Institute of Research and Innovation
Shenzhen, Guangdong, 518053 (China)
1
3
ꢀ1
ꢀ1
(e = 10.1–89.2 ꢁ 10 m cm ). There are additional strong
bands at 310–350 nm (e = 28.2–29.5 ꢁ 10 m cm ) for 1i and
1j, attributed to pp* transitions of the fluorene moieties.
3
ꢀ1
ꢀ1
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201608240.
1
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Table 1: Photophysical data of tungsten(VI) complexes at room temperature.
Absorption
The lowest energy absorption band
for 1a–j is at 400–427 nm (e = 2.3–
Emission
3
ꢀ1
ꢀ1
3
ꢀ1
3
ꢀ1 [a]
[a]
[a,b]
labs [nm] (e [ꢀ10 mol dm cm ])
lem [nm] (t [ms]) F [%]
7.9 ꢁ 10 m cm ) in dichlorome-
thane while for 2a–f an intense
absorption band is observed for
each complex at 375–396 nm (e =
1
1
1
1
a
b
c
243 (sh, 32.1), 271 (sh, 17.8), 301 (sh, 9.4), 345 (sh, 3.0), 400 (3.6) 595 (14.6)
248 (sh, 20.1), 275 (sh, 14.1), 312 (sh, 6.7), 362 (sh, 2.1), 415 (2.3) 625 (10.0)
0.3
0.2
0.3
2.8
240 (sh, 30.7), 308 (sh, 8.2), 358 (sh, 2.8), 418 (2.9)
243 (35.1), 273 (sh, 17.6), 299 (sh, 12.6), 400 (3.8)
618 (16.4)
600 (96.9)
3
ꢀ1
ꢀ1
d
4.5–13.4 ꢁ 10 m cm ). These low
energy absorption bands with high
e values are insensitive to solvent
polarity (Supporting Information,
Figures S5 and S6) and are assigned
to intraligand (IL) transitions. This
assignment is consistent with the
DFT/TDDFT calculations at the
[
c]
[c]
580
22
1
1
1
1
1
1
2
2
2
2
e
f
g
h
i
245 (22.1), 283 (sh, 11.7), 312 (sh, 8.5), 417 (2.3)
248 (37.6), 297 (sh, 10.1), 413 (3.2)
246 (41.4), 274 (sh, 17.3), 297 (sh, 11.0), 414 (3.7)
244 (23.7), 274 (sh, 12.7), 293 (sh, 8.1), 413 (2.9)
271 (25.0), 312 (sh, 23.6), 330 (sh, 28.6), 349 (29.5), 427 (7.9)
266 (25.1), 327 (29.3), 348 (28.2), 426 (7.7)
263 (47.5), 375 (7.1)
258 (37.5), 385 (4.5)
287 (59.3), 396 (10.5)
284 (62.1), 390 (11.6)
657 (25.7)
622 (77.9)
623 (56.2)
639 (37.2)
598 (62.0)
600 (16.0)
490, 585 (75.9)
515, 600 (10.4)
515, 614 (42.0)
513, 616 (62.0)
0.8
1.6
1.0
1.4
2.1
0.6
0.3
0.1
1.2
0.8
j
a
b
c
d
optimized ground state (S ) geome-
0
tries (see DFT calculations in the
Supporting Information).
[
c]
[c]
508, 646
2.5
1.2
0.4
All of these complexes display
photoluminescence at room tem-
perature (Table 1; Supporting Infor-
mation, Figure S3) with emission
quantum yields (F) in the range of
0.1–2.8% in dichloromethane solu-
tions and up to 22% in thin film at
room temperature. Complexes 1a–
2
2
e
f
283 (89.2), 382 (13.4)
262 (43.5), 283 (41.7), 292 (42.0), 383 (9.0)
501, 602 (25.4)
520, 598 (12.7)
[a] Dichloromethane solutions. [b] Emission quantum yields (F) were measured with [Ru(bpy) ][PF6]2
3
(
bpy=2,2’-bipyridine; F=0.062) in degassed acetonitrile as a standard reference. [c] Absolute
emission quantum yields of thin-film samples were measured with a Hamamatsu Quantaurus-QY
Absolute PL quantum yields spectrometer C11347. 5 wt% in mCP thin film.
1
j display orange to red emission at
5
95–657 nm with a shoulder at 500–560 nm in dichlorome-
thane solution (Figure 2; Supporting Information, Figures S7
and S8). The lifetimes of the 595–657 nm emission are in the
microsecond regime (10–97 ms) and the radiative decay rate
2
ꢀ1
constants (k ) lie in the range of 1.8–3.8 ꢁ 10 s . The
r
excitation spectra for 1a–j agree well with the absorption
spectra of the corresponding complexes. Varying the solvent
polarity does not induce any significant shift in emission lmax
(
Supporting Information, Figure S10). The orange–red emis-
3
sion is assigned to a IL excited state. In the solid state, the
emission blue-shifts to 540–652 nm at 298 K (Supporting
Information, Table S3).
For 2a–f, dual fluorescence–phosphorescence are
observed with emission lmax values at 490–520 nm (high
energy (HE); t < 0.1 ms) and at 585–616 nm (low energy
(LE); t = 10–76 ms; Figure 2b). The HE emission band is
insensitive to oxygen whereas the LE emission band is
strongly quenched under aerated condition (see 2e in the
Supporting Information, Figure S12; see 2c in Figure 6). This
leads to a change in emission color from orange to green. The
1
HE and LE emission bands are assigned to IL fluorescence
3
and IL phosphorescence of the quinolinolate ligands, respec-
tively. Varying the solvent polarity results in only little change
in the wavelength of the HE and LE bands of 2e (within
ꢀ1
4
00 cm ; Supporting Information, Figure S11). The HE
emission is more intense than the LE emission when there
is no aryl substituent on the quinolinolate (Qin) ligand (2a
and 2b) but the trend is reversed for 2c–2 f when the Qin
ligand has aryl substituents on the 5- and 7-position.
The emission properties of 1d and 2d in thin films have
been examined. The emission maximum of 1d in 1,3-
bis(carbazol-9-yl)benzene (mCP) thin film (5 wt%) is at
580 nm and is insensitive to complex concentration; the
Figure 2. UV/Vis absorption and emission spectra of a) 1a, 1d, 1i, 1j,
and b) 2a–f at room temperature, measured in degassed dichloro-
methane solutions.
2
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emission quantum yield is 22% at room temperature
Supporting Information, Figure S13). For 2d-doped mCP
thin film, two emission maxima at approximately 504–508 nm
and the HE band for 2d in their respective steady-state
(
luminescence spectra. The PF decay time constants (t ),
f
attributed to the ISC of the S excited state to the triplet
1
(
HE) and approximately 642–650 nm (LE) are observed.
DFT/TDDFT calculations were performed to shed light
manifold (T ), are 64, 52, and 42 ps for 1a, 1d, and 1i,
n
respectively, and 110, 102, and 93 ps for 2b, 2d, and 2e,
respectively; the Schiff base complexes generally have a faster
ISC efficiency than the quinolinolate complexes.
Figure 5 and Figures S15–18 (Supporting Information)
display the frontier molecular orbital (MO) surfaces of 1a,
on the effect of auxiliary ligand on the intersystem crossing
ISC) efficiency. For facile ISC, the presence of a close-lying
triplet excited state (T ) relative to the singlet excited state
(
n
(
usually the first singlet excited state, S ), with a small
1
j DE(S ꢀT ) j , is crucial. Figure 3 depicts an energy level
1d, 2b, and 2e. For 1a and 1d, the S excited state is mainly
derived from a HOMO!LUMO transition and the closest T3
1
n
1
Figure 3. Energy level diagrams for 1a, 1d, 2b, and 2e. The singlet
excited states (blue solid line) and triplet excited states (purple dashed
lines) are of the same parentage. A negative sign in DEST means that
the S excited state is below the T excited state.
1
3
diagram for the relative energies of the S and the closest
triplet excited state for 1a, 1d, 2b, and 2e at their respective
Figure 5. a) HOMO (right) and H-3 (left) for 1d. b) LUMO (left) and
L+1 (right) for 2b.
1
optimized S geometries. The triplet excited states (which
0
happened to be the T3 excited state for all complexes
excited state has a significant contribution from the H-2
(H-3)!LUMO transition for 1a (1d). The singlet counter-
part of the T excited state is S for 1a and S for 1d. Owing to
investigated) lying closest to the S excited state of 1a and
1
ꢀ
1
1
d are 790 and 573 cm
below the S1 excited state,
3
5
6
respectively; on the other hand, the T excited state is 1770
the longer bridging alkyl chain in 1d, the angle between the
two phenolic moieties of 1a and 1d are approximately 688 and
828, respectively. As the two phenolic moieties are in a nearly
orthogonal conformation in 1d, both HOMO and H-3 are
mainly localized on one of the phenolic moieties (> 90%;
Figure 5a). On the other hand, as the angle is smaller in 1a,
there is electron density on both phenolic moieties in the H-2
of 1a (74 and 14%; Supporting Information, Figure S15), thus
destabilizing the H-2 of 1a relative to H-3 of 1d. In effect, the
energy gap between the HOMO and H-2 of 1a is smaller than
the corresponding gap between the HOMO and H-3 of 1d.
Moreover, because of the localized nature of the relevant
orbitals, the S –T energy gap of 1d and the S –T energy gap
3
ꢀ
1
and 1612 cm above the S excited state for 2b and 2e,
1
respectively. Thus, ISC efficiency correlates well with the
singlet–triplet energy gap (j DE(S ꢀT ) j).
1
3
Figure 4 and Figure S14 (Supporting Information) respec-
tively depict the evolutions of femtosecond time-resolved
fluorescence (fs-TRF) and the corresponding decays of the
emission intensities for 1i and 2d. Photoexcitation at 350 nm
in dichloromethane solution results in broad and structureless
prompt fluorescence (PF) with an emission peak at lf
ꢁ
530 nm (1i, Figure 4a) and approximately 515 nm (2d,
Figure S14), which coincides with that of the shoulder for 1i
6
3
5
3
of 1a are comparatively large. As a consequence, the S –T
1
3
energy gap for both 1a and 1d is small and the T excited state
3
is below the S excited state. Furthermore, as the relevant
1
MOs are more localized for 1d, the T excited state is closer in
3
energy to the S excited state for 1d than for 1a (Figure 3).
1
Hence, 1d has a faster ISC rate, which is consistent with the
experimental findings.
The S excited states for 2b and 2e are also derived from
1
a HOMO!LUMO transition but the triplet excited state
Figure 4. a) Temporal evolution and b) experimental (*) and fitted
lines) intensity decay profiles of fs-TRF for 1i in dichloromethane
lying closest to the S1 excited state, T , is derived from
(
3
upon photoexcitation at lexc =350 nm.
a HOMO!L + 1 transition and the singlet counterpart of T3
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is S . As shown in Figure 5b and Figures S17 and S18
cient energy transfer from the PYD2 host to the tungsten
emitter. EQE-luminance characteristics of these devices
(Figure 7b) showed a maximum EQE of 4.79% for the
device with 4 wt% 1d at a luminance of approximately
40 cdm . High luminance up to 1400 cdm was realized for
the device with 2 wt% 1d. To our knowledge, this is the first
realization of a molecular tungsten OLED device.
4
(
Supporting Information), both the HOMO and L + 1 orbitals
are delocalized over the two Qin ligands, resulting in a small
ꢀ
1
DE(S ꢀT ) energy gap (< 2000 cm ). On the other hand,
4
3
ꢀ
1
ꢀ2
ꢀ2
DE(S ꢀS ) is more than 3500 cm ; hence, the T excited state
4
1
3
ꢀ
1
is more than 1600 cm above the S excited state. (Figure 3).
1
For 2e with aryl substituents at the Qin ligand, the frontier
orbitals are more delocalized, leading to more mixing among
different transitions and a larger DE(S ꢀT ) energy gap
The photocatalytic activity of 1i and 2c using light-
induced cyanation of tertiary amines as examples, has been
examined (Scheme 1; Supporting Information, Tables S5 and
4
3
(
Supporting Information). Thus, j DE(S ꢀT ) j is smaller and
1
3
ꢀ
the ISC rate of 2e is faster than 2b.
S6). Their excited-state reduction potentials E(W*/W ) are
Application studies of these tungsten(VI) complexes are
described below. Complex 2c acted as a ratiometric sensor for
[
8]
dissolved oxygen. As shown in Figure 6, the phosphores-
cence of 2c decreased upon diffusion of air into the solution
while the fluorescence remained unchanged. The emission
color gradually changed from orange to green.
Scheme 1. Reaction 1) Oxidative cyanation of tertiary amines photo-
catalyzed by 1i and 2c. Reaction 2) Oxidative hydroxylation of aryl
boronic acids photocatalyzed by 2c.
+
/0
estimated to be 0.59 and 0.40 V versus Cp Fe , respectively.
2
Both 1i and 2c (2 mol%) were found to catalyze the
formation of a-aminonitriles in good to excellent yields
(77–88% with 1i; 78–95% with 2c) under l > 370 nm light
irradiation. Singlet oxygen sensitized by the triplet-excited
[
9]
tungsten(VI) complex is proposed to be the active oxidant.
Figure 6. a) Emission spectral change of 2c in dichloromethane upon
diffusion of air into the solution. The arrow indicates the change in
emission intensity. b) Photo showing the emission of a dichlorome-
Besides, aerobic oxidative hydroxylation of aryl boronic acids
to aryl alcohols using 2c as photocatalyst has also been
demonstrated. Irradiation (l > 370 nm) of a mixture of aryl
ꢀ5
[10]
thane solution of 2c (5ꢀ10 m) under air and nitrogen, respectively
(lexc =365 nm).
boronic acids, 2c (1 mol%), and diisopropylethylamine in
DMF, afforded the respective aryl alcohols with yields of 57–
82% in three hours. These examples reveal the potentially
rich photochemistry of luminescent tungsten(VI) complexes.
In summary, we have realized the feasibility of developing
air-stable luminescent tungsten(VI) complexes with room
temperature emission quantum yields reaching 22%. Chang-
ing the supporting ligand allows modulation of ISC efficiency
and dual fluorescence–phosphorescence with comparable
intensities has been realized. It is conceivable that air-stable
phosphorescent tungsten complexes, such as the tungsten(VI)
species described herein, could become as competitive as
other luminescent complexes of iridium(III) and platinum-
Solution-processed OLEDs were fabricated to examine
the electroluminescence (EL) properties of the tungsten(VI)
complexes using 1d as an example (details in the Supporting
Information). 2,6-dicarbazolo-1,5-pyridine (PYD2) was used
as the host. As depicted in Figure 7a, EL spectra of the 1d
device displayed a broad structureless emission with a max-
imum at 577 nm and CIE chromaticity coordinates of (0.46,
0
.50). Such emission was similar to the PL of 1d in mCP thin
film (Supporting Information, Figure S13), suggesting effi-
[
11]
(II). The absence of a deactivating d-d ligand field excited
state in tungsten(VI) may provide an advantage over other
metal complexes when materials with high-energy phosphor-
escence are desired.
Acknowledgements
This work was supported by the Theme-Based Research
Scheme (Project No. T23-713/11), the Innovation and
Technology Commission of the HKSAR Government
(ITS/140/16), Guangdong Special Project of the Introduction
Figure 7. a) Normalized EL spectra (2 wt% (c), 4 wt% (c),
6
wt% (c)) and b) EQE-luminance characteristic of OLEDs based
on 1d with 2 wt% (&
), 4 wt% (*), and 6 wt% (~) dopant concen-
trations.
4
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of Innovative R&D Teams and GuangDong Aglaia
Optoelectronic Materials Co., Ltd., and Basic Research
Blakemore, A. A. Rachford, P. J. LaBeaume, J. W. Thackeray,
J. F. Cameron, J. R. Winkler, H. B. Gray, J. Am. Chem. Soc. 2013,
35, 10614; c) L. A. Bꢂldt, X. Guo, A. Prescimone, O. S. Wenger,
Angew. Chem. Int. Ed. 2016, 55, 11247; Angew. Chem. 2016, 128,
1413.
1
Program
of
Shenzhen
(JCYJ20160229123546997,
JCYJ20160530184056496). This work was also conducted in
part using the research computing facilities and/or advisory
services offered by Information Technology Services, the
University of Hong Kong. KTY acknowledges the receipt of
a postgraduate studentship from The University of Hong
Kong.
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Conflict of interest
[
The authors declare no conflict of interest.
Keywords: dual fluorescence–phosphorescence ·
organic light-emitting devices · phosphorescence ·
photocatalysis · tungsten
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[
Manuscript received: August 23, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Molecular Photochemistry
Luminescent tungsten complexes were
realized by coordination of polydentate
ligands to a tungsten metal center. These
metal complexes exhibit interesting pho-
tophysical and photocatalytic properties
with intense phosphorescence at room
temperature and dual fluorescence–
phosphorescence covering the entire
spectrum. They are capable phosphores-
cent dopants for solution-processable
organic light-emitting diodes.
K.-T. Yeung, W.-P. To, C. Sun, G. Cheng,
C. Ma, G. S. M. Tong, C. Yang,
C.-M. Che*
&&&& — &&&&
Luminescent Tungsten(VI) Complexes:
Photophysics and Applicability to
Organic Light-Emitting Diodes and
Photocatalysis
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!