Highly-phosphorescent tungsten(0) carbonyl pyridyl-imidazole complexes as photosensitisers

Article abstract of DOI:10.1039/c7dt02397a

Photoluminescence data are reported for two W(CO)₄L complexes (L = 2-(1H-imidazol-2-yl)pyridine and 2-(2′-pyridyl)benzimidazole) in room-temperature solutions. The bidentate ligands consist of a pyridine and an imidazole moiety connected by a C-C bond. The complexes have been found to exhibit enhanced phosphorescence from the metal-to-ligand charge transfer (MLCT) excited state with quantum yields in the order of 10^-3, almost two orders of magnitude higher than those reported for W(CO)₄(diimine) complexes. One of the complexes W(CO)₄(2-(2′-pyridyl)benzimidazole) can serve as an efficient visible-light photosensitiser for the isomerisation of aromatic alkenes with efficiency comparable to and even exceeding that of the well-studied ruthenium tris-bipyridine complex, Ru(bpy)Cl₂.

Full text of DOI:10.1039/c7dt02397a

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Highly-phosphorescent tungsten(0) carbonyl
pyridyl-imidazole complexes as photosensitisers†
Cite this: DOI: 10.1039/c7dt02397a
a
Jia Jin Lee,a Chew Pheng Yap,a Tsz Sian Chwee b and Wai Yip Fan
*
Photoluminescence data are reported for two W(CO)4L complexes (L = 2-(1H-imidazol-2-yl)pyridine and
2-(2’-pyridyl)benzimidazole) in room-temperature solutions. The bidentate ligands consist of a pyridine
and an imidazole moiety connected by a C–C bond. The complexes have been found to exhibit
enhanced phosphorescence from the metal-to-ligand charge transfer (MLCT) excited state with quantum
yields in the order of 10−3
, almost two orders of magnitude higher than those reported for
Received 3rd July 2017,
Accepted 31st July 2017
W(CO)4(diimine) complexes. One of the complexes W(CO)4(2-(2’-pyridyl)benzimidazole) can serve as an
efficient visible-light photosensitiser for the isomerisation of aromatic alkenes with efficiency comparable
to and even exceeding that of the well-studied ruthenium tris-bipyridine complex, Ru(bpy)Cl2.
DOI: 10.1039/c7dt02397a
rsc.li/dalton
Introduction
The phosphorescence of transition metal complexes such as
[Ru(bpy)3]Cl2 and Ir(ppy)3 in solutions has been studied in
detail with their quantum yields determined to be in the order
of 10−3 to 0.1.1–10 Studies11–17 have found that a few tungsten(0)
carbonyl complexes, W(CO)4L and W(CO)5L, can also exhibit
phosphorescence in solutions when the lowest-lying excited
state is of MLCT (metal-to-ligand charge transfer) character.
In these tungsten complexes, radiative deactivation from
the excited state is slow, and competing photodissociation
of the ligand also has a relatively low quantum yield.
Notably, Manuta and Lees11 studied the photoluminescence of
W(CO)4(diimine) complexes (diimine = bipyridine (bpy) and its
derivatives such as 4,4′-Me2bpy, phen, 4-Me-phen, 5-Cl-phen)
originating from the MLCT excited state in room-temperature
solutions. The emission maxima lie between 550 and 800 nm
with quantum yields in the order of 10−5 and emission life-
times in the order of 10−7 s.
The bidentate ligands used for coordination to the tungsten
centre have been mostly confined to bpy derivatives. In this
work, we explore two tungsten(0) carbonyl complexes 1 and 2
comprising bidentate ligands that incorporate a pyridine and
an imidazole moiety (Scheme 1). One of the goals is to con-
tinue to improve the phosphorescence quantum yields of
these earth-abundant metal complexes in order to rival the
Scheme 1 Tungsten(0) carbonyl complexes studied.
likes of the more expensive ruthenium(II) and iridium(III) com-
plexes. The luminescence properties of these ligands when co-
ordinated to other metal centres such as Ru(II), Os(II), Pt(II)
or Re(I) have been studied previously.5,6,18–20 Complex 3
(Scheme 1) was also prepared for comparison purposes since
the spectroscopic properties of its W(CO)4(diimine) family of
complexes have been well-studied. We report the luminescence
properties of these tungsten(0) carbonyl complexes and in par-
ticular their potential applications as photosensitisers in
alkene isomerization.
Experimental
Materials
aDepartment of Chemistry, National University of Singapore, 3 Science Drive 3,
117543 Singapore. E-mail: chmfanwy@nus.edu.sg
Tungsten carbonyl (W(CO)6, 99%) was purchased from Strem
Chemicals. 2-(1H-Imidazol-2-yl)pyridine (C5H4NC3N2H3, 97%),
2-(2′-pyridyl)benzimidazole (C5H4NC7H5N2, 97%), 2,2′-bipyri-
dyl ((C5H4N)2, 99%), cis-stilbene (C6H5CHvCHC6H5, 96%),
bInstitute of High Performance Computing, Agency for Science, Technology and
Research (A*STAR), 1 Fusionopolis Way, 138632, Singapore
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7dt02397a
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trans-α-methylstilbene
(C6H5C(CH3)vCHC6H5,
99%), desired solvent, then transferred to a quartz cell of 1 cm path
α-phenylcinnamonitrile (C6H5CHvC(C6H5)CN, 99%) and cin- length and deaerated by nitrogen purging for 10 min. UV-vis
namyl alcohol (C6H5CHvCHCH2OH, 98%) were purchased absorption spectroscopy was carried out using a Shimadzu
from Sigma Aldrich. Cinnamyl bromide (C6H5CHvCHCH2Br, UV-2450 spectrometer. Photoluminescence spectroscopy was
predominantly trans, 95%) was purchased from Alfa conducted using a Horiba Fluoromax-4 spectrophotometer.
Aesar. Tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate Emission quantum yields were determined using [Ru(bpy)3]Cl2
(Ru(C10H8N2)3Cl2·6H2O, 99.95%) was purchased from Fluka as a reference standard. Time-resolved photoemission spec-
Chemicals. Acetone (CH3COCH3, ≥99.8%), toluene (C6H5CH3, troscopy was recorded with a Horiba Fluorolog-3 spectrophoto-
≥99.5%) and ethyl acetate (CH3CO2C2H5, ≥99.5%) were pur- meter, using 374 nm, 240 ps pulses; 438 nm, 260 ps; or
chased from VWR Chemicals. Acetonitrile (CH3CN, ≥99.9%) 483 nm, 205 ps pulses from various laser excitation sources
was purchased from Anhui Fulltime Specialised Solvents & (nanoLEDs).
Reagents. Deuterated Chloroform-D (CDCl3, 98%) was pur-
chased from Cambridge Isotope Laboratories. All solvents and isation of alkenes, an alkene substrate and one of the tungsten
reagents were used without further purification. complexes or Ru(bpy)3Cl2 were stirred in a glass vial filled with
Syntheses of W(CO)4L complexes. W(CO)4L complexes were 10 mL of a suitable solvent at room temperature under visible
prepared in acetone by irradiating tungsten hexacarbonyl, light irradiation using 24 compact fluorescent lamp
For the photosensitization experiments involving isomer-
W
W(CO)6 and 1.5 to 2 equivalents of L for 2 hours. Freeze– (400 nm to 800 nm) for a stipulated duration. Thereafter, 1H
pump–thaw degassing of the solution was carried out three NMR spectroscopy was used to analyse the resultant mixture
times prior to photolysis. After reaction, acetone was removed containing both products and reactants.
by rotary evaporation. The remaining solid was redissolved in
ethyl acetate and purified by column chromatography on silica
gel. The product solution collected was then put through
rotary evaporation to isolate the product.
Results & discussion
Photoluminescence studies
W(CO)4(-(1H-imidazol-2-yl)pyridine) (1): νCO = 2004 (m),
1883 (m), 1870 (sh), 1830 (s) cm−1. ESI-MS (m/z): 441.7 (M−).
The UV-visible absorption and emission spectra of complex 1
1H NMR. δ (CHCl3): 8.9–9.1 (d, 1H), 8.1–8.3 (m, 2H), 7.66 in toluene at room temperature are illustrated in Fig. 1a. The
(s, 1H), 7.45–7.55 (m, 1H), 7.38 (s, 1H). absorbance band at λabs,max = 460 nm is assigned to a low-
Anal. calc. C, 32.68 H, 1.6 N, 9.53 W, 41.68. Found. lying MLCT transition as the band is found to be highly
C, 32.19 H, 1.99, N, 9.43 W, 42.28. solvent-dependent; with blue-shifts up to 60 nm in more polar
W(CO)4(-(2′-pyridyl)benzimidazole) (2): νCO = 2005 (m), solvents (see Table 1). Similar solvent effects were also pre-
1887 (s), 1870 (sh), 1829 (m) cm−1. ESI-MS (m/z): 491.7 (M−). viously observed for the MLCT of W(CO)4(diimine)11,21,22 and
1H NMR. δ (CHCl3): 9.1–9.2 (d, 1H), 8.45–8.55 (d, 1H), W(CO)5L12 complexes. Correspondingly, excitation at λexc
8.2–8.4 (t, 1H), 7.85–7.95 (m, 1H), 7.75–7.85 (m, 1H), 7.65–7.75 460 nm generates an emission band with λem,max = 590 nm.
(t, 1H), 7.5–7.55 (m, 2H). The emission lifetime (τ) of this band in toluene has also been
Anal. calc. C, 39.13 H, 1.85 N, 8.56 W, 37.43. Found. measured to be 142 ns, which is in the same order of magni-
C, 38.67 H, 2.16 N, 8.57 W, 37.72. tude as the lifetimes reported for phosphorescence of
=
W(CO)4(2,2′-bipyridine) (3): νCO = 2007 (m), 1892 (s), W(CO)4(diimine) complexes11,23 in room-temperature solu-
1877 (sh), 1835 (m) cm−1. ESI-MS (m/z): 450.8 (M−).
1H NMR. δ (CHCl3): 9.0–9.1 (d, 2H), 8.65–8.75 (d, 2H),
8.2–8.3 (m, 2H), 7.6–7.7 (m, 2H).
tions (Fig. 1b).
The observed emission is attributed to phosphorescence
arising from the 3MLCT excited state of complex 1 with the
Anal. calc. C, 37.18 H, 1.77 N, 6.20 W, 40.69. Found. quantum yield (ϕ) estimated to be 8 × 10−3 when calibrated to
C, 37.42 H, 1.82 N, 6.17 W, 40.40.
the well-known standard. Similar results were also obtained
for complex 2, where the phosphorescence quantum yield is
4 × 10−3 (see Table 2 and Fig. 2). In addition, both absorption
Equipment and procedures
Photolysis experiments for the syntheses of W(CO)4L com- and emission bands of 1 and 2 appear to consist of one com-
plexes were conducted using a Legrand broadband lamp ponent in contrast to those of W(CO)4(diimine) complexes
(200–800 nm, 11 W). Fourier Transform Infrared (FT-IR) which show at least two overlap components made up of
spectra were acquired with liquid samples in CaF2 windows the y- and z-polarized transitions respectively.11 The emission
cell of 0.1 mm path length using a Shimadzu IR Prestige-21 spectra of the two complexes were also different in wavelength
spectrometer. Electrospray Ionisation (ESI) was conducted compare to those originating from their respective ligands,
1
using Finnigan MAT LCQ spectrometer. H nuclear magnetic hence verifying that these emissions indeed arise from the
resonance (NMR) spectra were recorded with Bruker AMX 300 complexes and not from the ligands (see ESI†).
NMR spectrometer at room temperature, using CDCl3 cali-
brated to its chemical shift.
Further observations can be made when juxtaposing the
photophysical parameters of 1 and 2 with those of complex 3
In the absorption and photoluminescence studies, each (Table 2).11,24 First, the emission maxima in complexes 1 and 2
sample was prepared by dissolving the complex of interest in a are red-shifted from that in complex 3. The same observation
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Table 2 Photophysical parameters for complexes 1 to 3 in toluene at
298 K (ε = molar absorption coefficient M−1 cm−1
)
MLCT
Complex
λabs,max/nm (ε)
λem,max/nm
τ/ns
ϕa/10−3
1
2
3
463 (6800)
484 (7500)
514 (6200)
590
650
560, 790
142b
215c
348d
8
4
0.05
a Estimated relative to the emission of [Ru(bpy)3]Cl2. b Excited with
438 nm, 260 ps pulses. c Excited with 483 nm, 205 ps pulses.
d Reported lifetime of W(CO)4(4-Me-Phen).10
Fig. 2 Emission spectra of complexes 1 ( , λex = 460 nm), 2 ( , λex
=
Fig. 1 (a) Absorption ( ) and emission ( , λex = 460 nm) spectra, and
(b) emission decay (in log scale) of complex 1 in toluene at 298 K. The
emission curve fits to a biexponential function with two lifetimes, 3.7 ns
and 142 ns of relative amplitudes 0.13. The former lifetime corresponds
to the lifetime of the excitation beam.
480 nm) and 3 ( , multiplied by factor of 50, λex = 515 nm) in toluene at
298 K.
spin–orbit coupling (SOC), which is enhanced by the heavy
tungsten atom. We posit that the rate of ISC could be much
faster in 1 and 2 due to higher contributions of W in the
MLCT states. In addition, SOC is expected to be more effective
when orbitals involved in the electronic transition (i.e., d orbi-
tals on W and π orbitals on the ligand) are overlapping and
spatially close. Compared to complex 2, the ligand π orbitals in
1 are relatively more localized and stronger SOC effects could
result in a higher rate of ISC and a subsequently higher
quantum yield of emission. The explanation is also consistent
with the observation of only very weak phosphorescence from
3 as its bipyridine ligands are expected to be most delocalised
amongst the three complexes.
Table 1 MLCT absorption maxima of complexes 1 to 3 in various sol-
vents at 298 K (ε = dielectric constant)
Solvent
Acetonitrile
(ε = 37.5)
Acetone
(ε = 20.7)
Dichloromethane
(ε = 8.93)
Toluene
(ε = 2.38)
Complex
1
2
3
405
450
452
426
462
465
446
478
481
463
484
514
has previously been made between [Ru(bpy)3]2+ and
[Ru(bpy)2L]2+ where L is a pyridyl-imidazole derivative.5,18
Photosensitisation
Second and more significantly, the quantum yields of 1 and 2 We are interested in finding potential applications for the syn-
are two orders of magnitude higher than that of complex 3. thesised tungsten complexes. There has been much research
Fig. 2 compares the intensity of the emission spectra of com- carried out on photoredox catalysis using iridium and ruthe-
plexes 1, 2 and 3, where that of complex 3 has to be multiplied nium pyridine complexes.25–28 However non-redox photopro-
by a factor of 50 to be visible on the same scale.
cesses such as alkene isomerization via excited triplet state
The rate of intersystem crossing (ISC) between the singlet energy transfer are less investigated. Fabry and co-workers29
and triplet manifold within each complex also plays an impor- have used phosphorescent metal complexes such as [Ru(bpy)3]Cl2
tant role. The non-radiative transition is largely driven by as visible-light photosensitisers to isomerise alkenes into
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Table 3 Isomerisation reactions of trans- and cis-stilbene with visible light irradiation
Entry
Catalysta
Substrateb
Solvent
Duration/h
Conversionc/%
TONd
1
2
3
4
5
6
7
8
[Ru]
[Ru]
1
1
2
2
2
2
2
2
3
3
trans-
cis-
trans-
cis-
trans-
cis-
trans-
cis-
trans-
cis-
trans-
cis-
Acetonitrile
Acetonitrile
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
18
18
18
18
18
18
48
48
18
18
18
18
94
4
11
9
49
31
74
26
78
21
0
6.3
<1
<1
<1
3.3
2.1
4.9
1.7
5.2
1.4
—
9
Acetonitrile
Acetonitrile
Toluene
10
11
12
Toluene
0
—
a 15 mol% catalyst loading. b 0.1 mmol substrate. c Conversion is calculated by comparing the integrations of peaks corresponding to alkenyl H.
d Turnover number (TON) is calculated by dividing the conversion by the mol% catalyst loading.
their corresponding geometric isomers. With their enhanced
To expand the substrate scope, complex 2 was used to
phosphorescence, we reason that complexes 1 and 2 could be photosensitise the isomerisation of other trans-alkenes
developed into novel and cheaper visible-light photosensitisers (Scheme 2 and Table 4). Interestingly the isomerisation of
for such isomerisation reactions.
three of the trans-alkenes is more efficient when complex 2 is
The efficiency of the tungsten complexes as photosensiti- used instead of [Ru(bpy)3]Cl2. On the other hand, trans-cinnamyl
sers is compared with the commonly-used [Ru(bpy)3]Cl2. Using
stilbenes as the test case, we first verify that [Ru(bpy)3]Cl2 is
able to photosensitise trans- to cis-stilbene isomerization.
Indeed, irradiating trans-stilbene in the presence of a catalytic
amount of the ruthenium complex in acetonitrile resulted in
94% conversion to cis-stilbene (Table 3, entry 1). On the other
hand, only 4% conversion of cis-stilbene to trans-stilbene was
achieved under the same conditions (Table 3, entry 2), which
agrees with the previous report that the trans-to-cis stilbene
isomerisation is largely one-directional in the presence of
[Ru(bpy)3]Cl2.
Under the same conditions but in toluene, trans-to-cis stil-
bene conversion of 11% and cis-to-trans conversion of 9%
(Table 3, entries 3 and 4) were achieved using 1 as the photo-
sensitiser. As increasing the reaction time also did not
improve the conversion rate, complex 1 has limited use as an
efficient photosensitiser.
However, complex 2 performs better with much higher
Scheme 2 Substrate scope of the alkene isomerisation reactions.
trans-to-cis and cis-to-trans conversion rates respectively
(Table 3, entries 5 and 6). To ascertain whether the isomerisa-
tion has reached its photostationary state, the reaction time
was increased to 48 hours. About 74% of initial trans-isomer
Table 4 Isomerisation reactions of other trans-alkenes with visible light
was converted to the cis-isomer (Table 3, entry 7), while 26% of
irradiation for 18 hours
the cis-isomer was converted to the trans-isomer (Table 3, entry
Conversionb/%, TONc for the catalystd
8), leading to a trans- to cis-isomer ratio of 1 : 3 in toluene. The
isomerisation also proceeds in acetonitrile with a comparable
ratio of 1 : 4 (Table 3, entries 9 and 10). It is worth noting that
the isomerization is proceeding in a catalytic manner although
with small turnover numbers.
As further controls, we note that conversion in either direc-
tion was not observed when complex 3 was used (Table 3,
entries 11 and 12). In addition, the W(CO)6 precursor and
the ligands themselves also do not function as visible-light
photosensitisers.
Substratea
2 in toluene
[Ru] in acetonitrile
trans-α-Methylstilbene
α-Phenylcinnamonitrile
Cinnamyl bromide
Cinnamyl alcohol
65, 4.3
32, 2.1
0
23, 1.5
0
22, 1.5
0
19, 1.3
a 15 mol% catalyst loading. b Conversion is calculated by comparing
the integrations of peaks corresponding to alkenyl H. c TON is calcu-
lated by dividing the conversion by the mol% catalyst loading.
d 0.1 mmol substrate.
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bromide can only be converted to its cis-isomer by the ruthe-
nium complex.
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The isomerisation of alkenes is believed to be initiated by
the transfer of energy from the triplet state of the photosensiti-
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ser donor.29 For example, the MLCT state of [Ru(bpy)3]Cl2 is
generated during irradiation and lies at 2.12 eV (ref. 1) which
is very close to the triplet state of trans-stilbene (2.1 eV) but
lower than that of cis-stilbene (2.6 eV).30 Thus, upon collisional
energy transfer in the solution phase, trans-stilbene can act as
the more efficient triplet energy acceptor and undergoes iso-
merisation. However, if the energy difference is the only cri-
terion, one expects α-phenylcinnamonitrile, which has lower
lying excited singlet and triplet states due to the electron-with-
drawing resonance effect of the –CN group, to undergo isomer-
isation.31 In contrast cinnamyl bromide and cinnamyl alcohol
would remain resistant to isomerisation by virtue of their
higher-lying triplet states.
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Conclusions
We have shown that W(CO)4L complexes where L is a bidentate
ligand carrying pyridine-imidazole moieties exhibit enhanced
phosphorescence from the MLCT excited state, with quantum
yields up to two orders of magnitude higher than those pre-
viously reported for W(CO)4(diimine) complexes. Interestingly
complex 2 can also function as an efficient photosensitiser
for the geometric isomerisation of aromatic alkenes.
Investigations of the reaction mechanism are underway to
provide more insights into the process.
23 P. Fleischauer and P. Fleischaucr, Chem. Reu., 1970, 70,
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This work is supported by a National University of Singapore
research grant 143-000-641-112.
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