Synthesis, structure and photophysics of binuclear gold(I) and mercury(II) complexes derived from 2,5-bis(ethynylphenyl)-1,3,4-oxadiazole

Article abstract of DOI:10.1016/j.molstruc.2008.03.003

The organometallic chemistry of the dialkyne ligand 2,5-bis(ethynylphenyl)-1,3,4-oxadiazole with gold(I) and mercury(II) salts has been investigated. Four new group 11 gold(I) and group 12 mercury(II) bis(alkynyl) binuclear complexes functionalized with e

Full text of DOI:10.1016/j.molstruc.2008.03.003

Journal of Molecular Structure 890 (2008) 150–156
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Journal of Molecular Structure
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Synthesis, structure and photophysics of binuclear gold(I) and mercury(II)
complexes derived from 2,5-bis(ethynylphenyl)-1,3,4-oxadiazole q
*
Wai-Yeung Wong , Yan-He Guo
Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The organometallic chemistry of the dialkyne ligand 2,5-bis(ethynylphenyl)-1,3,4-oxadiazole with gold(I)
and mercury(II) salts has been investigated. Four new group 11 gold(I) and group 12 mercury(II) bis(alky-
nyl) binuclear complexes functionalized with electron-deficient oxadiazole (OXD) spacer
[(PR3)AuC „ C(p-C6H4)(OXD)(p-C6H4)C „ CAu(PR3)] (R = Ph, 1; Me, 2) and [RHgC „ C(p-C6H4)(OXD)(p-
C6H4)C „ CHgR] (R = Ph, 3; Me, 4) were prepared in good yields by the base-catalyzed dehydrohalogen-
ation reaction of the corresponding metal chloride precursors with HC „ C(p-C6H4)(OXD)(p-C6H4)
C „ CH at room temperature. We report the optical absorption and photoluminescence properties of
these metallaynes. The structural properties of 1 have been studied by X-ray crystallography, in which
no short-contact aurophilic interaction is observed. The influence of the heavy metal atom on the inter-
system crossing rate from the S1 singlet excited state to the T1 triplet excited state in these metal alkynyl
systems and the spatial extent of the lowest singlet and triplet excitons is systematically characterized.
Our investigations indicate that harvesting of the organic triplet emissions by the heavy-atom effect of
group 11–12 transition metals generally follows the order Au(I) > Hg(II).
Received 21 January 2008
Received in revised form 6 March 2008
Accepted 6 March 2008
Available online 13 March 2008
Keywords:
Gold
Alkynyl
Crystal structures
Mercury
Oxadiazole
Phosphorescence
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
In the past two decades, there has been immense experimental
and theoretical attention focusing on the energy levels of singlet
Advances in the molecular design of modern optoelectronic
materials have made significant contributions toward the develop-
ment of organic electronics. Over the past decades, many endeav-
ors have been made to develop new metal–organic complexes to
be applied in various domains of optoelectronic devices [1] such
as organic light-emitting diodes (OLEDs) [2], photovoltaic cells
[3], field-effect transistors [4], sensors [5], and non-linear optical
systems [6]. In fact, harvesting of triplet excitons using metal-con-
taining complexes is still one of the most promising avenues of
making high-efficiency OLEDs [7]. Strong spin–orbit coupling in-
duced by a heavy-atom effect mixes the singlet and triplet excited
states through efficient intersystem crossing (ISC) so that phospho-
rescence (or triplet emission) can be measured readily using opti-
cal methods. Among these, rigid-rod metallaynes of late transition
metals represent an important class of new materials in which
there is great scope for chemical modification of the conjugated
units [8]. Their interesting photoluminescence and electronic prop-
erties depend on the transition metal, the central organic spacer in
the main chain and the symmetry of the molecules [9].
and triplet states in conjugated polymers and materials [10]. In this
regard, metallaynes offer good prospects for the development. A
concise consideration and a good compromise of the triplet photo-
physics and the corresponding bandgaps in these metalated alky-
nyl complexes are essential for designing and making
technologically useful materials for OLEDs and photovoltaic cells
[11]. Along this line, the trade-off problems for phosphorescence
efficiency/optical bandgap have been addressed and elucidated
[11]. In contrast to the library of work on luminescent metallaynes
with simple electron-rich aromatic and heterocyclic units such as
phenylene, anthryl, fluorenyl and thienyl rings [8,11], there are
no reports of similar systems possessing p-conjugated electron-
deficient oxadiazole group in the main skeleton so far. Aromatic
oxadiazole compounds such as 1,3,4-oxadiazoles have been inves-
tigated as electron-transporting materials within multilayered
OLED devices because the five-membered oxadiazole blocks show
high electron affinity in organic small-molecules and polymers
[12]. Consequently, in keeping with the ongoing research interests
of our group pertaining to the structure–property correlation study
of metallaynes, we report here the first examples of group 11–12
metal acetylide complexes containing the 1,3,4-oxadiazole linker
and study their spectroscopic, structural and photophysical behav-
ior. The nature of the lowest singlet and triplet excited states will
be characterized according to the selection of the terminal late
q
In memory of my mentor, Prof. F. Albert Cotton, for his remarkable contribution
to inorganic and organometallic chemistry throughout his life.
* Corresponding author. Tel.: +852 34117074; fax: +852 34117348.
E-mail address: rwywong@hkbu.edu.hk (W.-Y. Wong).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.03.003

W.-Y. Wong, Y.-H. Guo / Journal of Molecular Structure 890 (2008) 150–156
151
transition metal ions and the results are subsequently compared to
the corresponding biphenylene congeners.
20 min at rt before it was refluxed overnight under N2. The comple-
tion of the reaction was confirmed by IR and TLC. The reaction mix-
ture was filtered and the filtrate evaporated to dryness on a rotary
evaporator. The residue was dissolved in CH2Cl2 and washed suc-
cessively with 10% HCl, water, saturated NaHCO3 and water. The
dichloromethane solution was dried over MgSO4 and evaporated
to dryness under vacuum. Purification was effected by silica col-
umn chromatography using hexane/CH2Cl2 (1:1, v/v) to afford
the title product as a yellow solid in 77% yield (0.32 g). IR (CH2Cl2):
2160 cmÀ1 (mC„C); 1H NMR (CDCl3): d (ppm) 8.07 (d, 4H, J = 8.6 Hz,
phenyl), 7.61 (d, 4H, J = 8.6 Hz, phenyl), 0.28 (s, 18H, SiMe3); 13C
NMR (CDCl3): d (ppm) 164.10, 132.50, 126.66, 126.63, 123.26 (aro-
matic C), 103.90, 97.88 (C „ C), À0.035 (SiMe3); FAB-MS: m/z 415
[M]+. Calcd for C24H26N2OSi2: C, 69.52; H, 6.32; N, 6.76. Found: C,
69.34; H, 6.15; N, 6.58%.
2. Experimental
2.1. General
Solvents were predried and distilled from appropriate drying
agents. All chemicals, unless otherwise stated, were obtained from
commercial sources and used as received. All reactions were car-
ried out under nitrogen atmosphere with the use of standard
Schlenk techniques, but no special precautions were taken to ex-
clude oxygen during work-up. Preparative TLC was performed on
0.7 mm silica plates (Merck Kieselgel 60 GF254) prepared in our lab-
oratory. Infrared spectra were recorded in CH2Cl2 on a Perkin-El-
mer FTIR 550 spectrometer, using CaF2 cells with a 0.5 cm path
length. NMR spectra were measured in appropriate solvents on a
JEOL EX270 or a Varian INOVA 400 MHz FT-NMR spectrometer,
with 1H and 13C NMR chemical shifts quoted relative to tetrameth-
ylsilane and 31P chemical shifts relative to an 85% H3PO4 external
standard. Fast atom bombardment (FAB) mass spectra were re-
corded on a Finnigan MAT SSQ710 mass spectrometer. Absorption
spectra were obtained with a Hewlett–Packard 8453 UV–vis spec-
trometer. For emission spectral measurement, the 325 nm line of a
He–Cd laser was used as an excitation source. The luminescence
spectra were analyzed by a 0.25 m focal length double monochro-
mator with a Peltier cooled photomultiplier tube and processed
with a lock-in-amplifier. For low temperature measurements, sam-
ples were mounted in a closed-cycle cryostat (Oxford CC1104) in
which the temperature can be adjusted from 10 to 330 K. The fluo-
rescence quantum yields (rF) were determined in dichloromethane
solutions at 293 K against the quinine sulfate in 1.0 N H2SO4
(rF = 0.54) [13]. Caution: Organomercurials are toxic, and all exper-
imentation involving these reagents should be carried out in a
well-vented hood.
2.2.3. 2,5-Bis(ethynylphenyl)-1,3,4-oxadiazole L
2,5-Bis(trimethylsilylethynylphenyl)-1,3,4-oxadiazole (0.32 g,
0.77 mmol) and K2CO3 (20.0 mg, 0.15 mmol) were combined in
MeOH/Et2O (40 mL, 1:1, v/v) and the mixture was stirred at rt for
20 h. Infrared spectroscopy showed that the starting material had
been consumed. Upon removal of solvent under reduced pressure,
the crude product was subjected to column chromatography on sil-
ica eluting with hexane/CH2Cl2 (1:1, v/v) to give L as a yellow solid
in 95% yield (0.20 g, 0.74 mmol). IR (CH2Cl2): 3295 (m„CH),
2107 cmÀ1 (mC„C); 1H NMR (CDCl3):
d (ppm) 8.10 (d, 4H,
J = 8.1 Hz, phenyl), 7.65 (d, 4H, J = 8.1 Hz, phenyl), 3.27 (s, 2H,
C „ C); 13C NMR (CDCl3): d (ppm) 164.02, 132.68, 126.70, 125.63,
123.62 (aromatic C), 82.63, 80.19 (C „ C); FAB-MS: m/z 270 [M]+.
Calcd for C18H10N2O: C, 79.99; H, 3.73; N, 10.36. Found: C, 80.08;
H, 3.66; N, 10.16%.
2.3. Complex syntheses
2.3.1. Gold(I) complex 1
Triphenylphosphinegold(I) chloride (99.0 mg, 0.20 mmol) was
2.2. Synthesis of ligands
dissolved in MeOH/CH2Cl2 (6 mL, 5:1, v/v) and
L (27.0 mg,
0.10 mmol) was added. Then NaOH in MeOH (2 mL, 0.2 M) was
added to the mixture, and the mixture was allowed to stir at rt un-
der nitrogen for 2 h. The product was precipitated out and centri-
fuged. The solvent was removed and the product was air-dried to
obtain a yellow solid of 1 (77.0 mg, 65%). IR (CH2Cl2): 2116 cmÀ1
(mC„C); 1H NMR (CDCl3): d (ppm) 8.02 (d, 4H, J = 8.4 Hz, phenyl),
7.64 (d, 4H, J = 8.4 Hz, phenyl), 7.59–7.43 (m, 30H, PPh3); 13C
NMR (CDCl3): d (ppm) 164.20, 134.28, 134.08, 132.75, 131.55,
131.52, 129.87, 129.17, 129.05, 126.45 (aromatic C), 128.50,
121.69 (C „ C); 31P {1H} NMR (CDCl3): d (ppm) 43.13; FAB-MS:
m/z 1187 [M]+. Calcd for C54H38N2OP2Au2: C, 54.65; H, 3.23; N,
2.36. Found: C, 54.49; H, 3.28; N, 2.28%.
2.2.1. 2,5-Bis(4-bromophenyl)hydrazine
This compound was prepared according to a slightly modified
literature method [14]. 4-Bromobenzoic chloride (2.40 g,
11.0 mmol) and hydrazine (0.25 g, 7.80 mmol) were dissolved in
chloroform (50 mL). Triethylamine (2.20 g, 21.8 mmol) was then
added at 0 °C and the solution was stirred for 30 min. After warm-
ing to room temperature (rt), the solution was allowed to react
overnight. A white precipitate then came out and was collected,
washed several times sequentially with 10% NaHCO3 and water
to obtain bis(4-bromophenyl)hydrazine (1.40 g, 64%) for the next
step. This intermediate (1.40 g, 3.53 mmol) was added to POCl3
(30 mL) and the solution was refluxed for 8 h under N2. The solu-
tion was then reduced to 5 mL in vacuo. Upon cooling, the solution
was poured into ice/water, and a white solid precipitated out. The
white precipitate was filtered off, washed with water and cold eth-
anol. The pure white needles were obtained by recrystallization
from ethanol to give 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole
(1.30 g, 97%). 1H NMR (CDCl3): d (ppm) 8.00 (d, 4H, J = 8.4 Hz, phe-
nyl), 7.68 (d, 4H, J = 8.4 Hz, phenyl). FAB-MS: m/z 380 [M]+. Calcd
for C14H8N2Br2O: C, 44.25; H, 2.12; N, 7.37. Found: C, 44.20; H,
2.01; N, 7.42%.
2.3.2. Gold(I) complex 2
Similar to 1, complex 2 was prepared from Au(PMe3)Cl instead
of Au(PPh3)Cl and collected as a yellow solid in 87% yield (28.0 mg
from 25.0 mg of Au(PMe3)Cl). IR (CH2Cl2): 2104 cmÀ1 (mC„C); 1H
NMR (CDCl3): d (ppm) 7.98 (d, 4H, J = 8.4 Hz, phenyl), 7.57 (d, 4H,
J = 8.4 Hz, phenyl), 1.54 (s, 18H, Au(PMe3)); 13C NMR (CDCl3): d
(ppm) 164.09, 132.62, 132.19, 128.43, 126.32 (aromatic C),
128.08, 121.53 (C „ C), 15.91 (Au(PMe3)); 31P {1H} NMR (CDCl3):
d (ppm) 2.08; FAB-MS: m/z 814 [M]+. Calcd for C24H26N2OP2Au2:
C, 35.40; H, 3.22; N, 3.44. Found: C, 35.23; H, 3.19; N, 3.27%.
2.2.2. 2,5-Bis(trimethylsilylethynylphenyl)-1,3,4-oxadiazole
To 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole (0.38 g, 1.00 mmol)
in triethylamine (50 mL) were added CuI (25.0 mg), Pd(OAc)2
(25.0 mg) and PPh3 (75.0 mg) under N2. After the mixture was stir-
red at rt for 15 ꢀ 20 min, trimethylsilylacetylene (0.28 mL,
2.00 mmol) was added. The mixture was stirred for a further 15–
2.3.3. Mercury(II) complex 3
Phenylmercury(II) chloride (51.0 mg, 0.16 mmol) was dissolved
in MeOH/CH2Cl2 (6 mL, 5:1, v/v) and L (87.8 mg, 0.07 mmol) was
added. Then 0.20 M basic NaOH in MeOH (2 mL) was subsequently
added to give a pale-yellow suspension. Then, the mixture was

152
W.-Y. Wong, Y.-H. Guo / Journal of Molecular Structure 890 (2008) 150–156
allowed to stir at rt under nitrogen atmosphere for 2 h. The crude
product was precipitated out and centrifuged. The solvent was dec-
anted and a yellow solid was washed with MeOH (2Â 20 mL) and
air-dried to afford a pure sample of 3 (32.0 mg, 69%). IR (CH2Cl2):
2134 cmÀ1 (mC„C); 1H NMR (CDCl3): d 8.10 (d, 4H, J = 8.1 Hz, phe-
nyl), 7.65 (d, 4H, J = 8.1 Hz, phenyl), 7.43–7.26 (m, 10H, HgPh);
FAB-MS: m/z 824 [M]+. Calcd for C30H18N2OHg2: C, 43.75; H,
2.20; N, 3.40. Found: C, 43.80; H, 2.10; N, 3.44%.
characterized by a single sharp m(C „ C) absorption band at ca.
2104–2116 cmÀ1 (for Au complexes) and 2130–2134 cmÀ1 (for
Hg complexes). The IR spectrum of each compound shows no char-
acteristic „CAH stretching band in the range 3200–3300 cmÀ1
,
thus confirming that 2,5-bis(ethynylphenyl)-1,3,4-oxadiazole is
capped by metal groups via r bonds. The room temperature 31P
NMR spectrum of 1 and 2 each displays a sharp singlet at d 43.13
and 2.08, respectively, which indicates a symmetrical arrangement
of PAuC „ C groups in solution. 1H NMR resonances arising from
the protons of the organic moieties were observed. The low solubil-
ity of 3 in chlorinated solvents led to some difficulties for its com-
plete 13C NMR characterization. The formulas of the metal
complexes were successfully established by the presence of in-
tense molecular ion peaks in their respective positive FAB mass
spectra.
2.3.4. Mercury(II) complex 4
A procedure similar to that used for 3 was employed to obtain
the title compound in 76% yield. IR (CH2Cl2):2130 cmÀ1 (mC„C);
1H NMR (CDCl3): d 8.11–8.04 (m, 4H, phenyl), 7.66–7.58 (m, 4H,
2
phenyl), 0.74 (s, 6H, JHg–H = 148.5 Hz, HgMe); 13C NMR (CDCl3):
d 163.98, 146.76, 132.49, 126.57, 123.60 (aromatic C), 122.67,
104.06 (C „ C), 7.12 (HgMe); FAB-MS: m/z 700 [M]+. Calcd for
C20H14N2OHg2: C, 34.34; H, 2.02; N, 4.00. Found: C, 34.40; H,
2.08; N, 3.89%.
3.3. Crystal structure analyses
The three-dimensional molecular structure of 1 was analyzed
by X-ray crystallography and a perspective view of 1 is shown in
Fig. 1. Pertinent bond distances and angles are given in Table 1.
The crystal structure consists of discrete binuclear molecules in
which two terminal organometallic Au(PPh3)+ groups are linked
by the 2,5-bis(ethynylphenyl)-1,3,4-oxadiazole moiety to afford
the rigid molecular structure. The dihedral angles made between
the central five-membered oxadiazole plane with the two neigh-
boring inner C6H4 rings are ca. 7.2° and 7.9°. The coordination
geometry is linear about the gold centers. The C „ C bond
lengths in the ethynyl bridge of 1.198(11)–1.200(11) Å are fairly
typical of metal-alkynyl r-bonding [11c,17,18]. The bond angles
for the metalAC „ C units are close to linearity and conform
to the rigid-rod nature of the compound. No apparent short
intermolecular contacts or p-stacking interactions are observed
in 1. In contrast to many Au(I) compounds, where there fre-
quently exists short AuÁ Á ÁAu contacts, aurophilicity is absent in
the crystal lattice of 1 [19].
2.4. X-ray crystallography
Colorless crystals of 1 suitable for X-ray diffraction experiment
were grown by slow evaporation of its solution in a CHCl3/hexane
mixture at rt. Geometric and intensity data were collected at 293 K
using graphite-monochromated Mo-Ka radiation (k = 0.71073 Å)
on a Bruker Axs Smart 1000 CCD area detector diffractometer.
The collected frames were processed with the software SAINT
[15a], and an absorption correction was applied (SADABS) [15b]
to the collected reflections. The structure was solved by the direct
methods (SHELXTL) [16] in conjunction with standard difference
Fourier techniques and subsequently refined by full-matrix least-
squares analyses on F2. All non-hydrogen atoms were assigned
with anisotropic displacement parameters. Crystal data for
1ÁCHCl3: C55H39N2Cl3OP2Au2, M = 1306.10, monoclinic, space group
P21/c, a = 6.9195(9), b = 24.577(3), c = 29.218(4) Å, b = 91.891(2)°,
U = 4966(1) Å3, Z = 4, T = 293 K, l(Mo-Ka) = 6.169 mmÀ1, 24,638
reflections measured, 8696 unique, Rint = 0.0783, final R1 = 0.0545,
wR2 = 0.1184 for 4534 [I > 2r(I)] observed reflections.
3.4. Electronic absorption and photoluminescence spectra
The photophysical data of the new compounds 1–4 measured in
CH2Cl2 are shown in Table 2. All of the metal alkynyls display sim-
ilar structured absorption bands in the near UV region (Figs. 2–5).
The lowest energy transitions are predominantly intraligand in
3. Results and discussion
3.1. Synthesis
1
nature consisting of both acetylenic and aromatic (pp*) character,
All of the metal alkynyl complexes were obtained in good yields
by the general reaction routes as described in Scheme 1. The organic
precursor L was chosen as a versatile synthon in the present study to
form a series of group 11–12 metal acetylide complexes by adapta-
tion of the base-catalyzed dehydrohalogenation procedures re-
ported in the literature [17,18]. We have prepared the d10 digold(I)
diacetylide complexes 1 and 2 by reaction of Au(PPh3)Cl or Au(P-
Me3)Cl with L under ambient conditions in a mole ratio of 2:1 in
the presence of NaOH in MeOH [17]. Likewise, mercuration of L with
two molar equivalents of PhHgCl or MeHgCl using methanolic NaOH
as a base at rt gave yellow solids identified as 3 and 4, respectively,
which are the isoelectronic and isolobal analogues of the gold(I)
counterparts [18]. The yields of these transformations are high in
each case (ca. 65–87%). All of the new complexes are air-stable pale
yellow solids and can be stored without demanding any special pre-
cautions. They generally exhibit good solubility in chlorocarbons
such as CH2Cl2 and CHCl3, but are insoluble in hydrocarbons.
possibly mixed with some admixture of metal orbitals. The 0–0
absorption peak is assigned as the S0 ? S1 transition from the high-
est occupied molecular orbital (HOMO) to the lowest unoccupied
molecular orbital (LUMO), which are mainly delocalized p and p*
orbitals [17,18]. As compared to the band at kabs = 307 nm in
CH2Cl2 for L (Fig. 6), we find that the position of the lowest energy
absorption band for 1–4 is red-shifted after the inclusion of metal
fragment. This reveals that p-conjugation is preserved through the
metal site by mixing of the frontier orbitals of metal and the ligand.
In energy terms, the experimentally determined HOMO–LUMO en-
ergy gaps (Eg) as measured from the onset absorption wavelength
are also tabulated in Table 2. According to the type of the metal
groups, the optical energy gaps of the polyynes follow the experi-
mental order 3 > 1 and 4 > 2.
At 293 K, all of the complexes in CH2Cl2 solutions emit
1
purple-blue (pp*) fluorescence (S1 ? S0) peak near 400 nm that
is characterized by the small Stokes shift between the bands in
the absorption and the emission spectra. At 77 K, each of them
shows dual emission peaks of different intensities with the high-
er-energy peak due to fluorescence and the lower-energy one
due to phosphorescence. The triplet emission peak occurs at ca.
512, 513, 508 and 517 nm for 1–4, respectively, in frozen CH2Cl2
3.2. Spectroscopic properties
The IR, NMR and MS data of our compounds shown in Section 2
agree with their chemical structures. The solution IR spectra are

W.-Y. Wong, Y.-H. Guo / Journal of Molecular Structure 890 (2008) 150–156
153
O
N
O
O
Et3N / CHCl3
POCl3
Br
Cl
N
+ NH2NH2.H2O
Br
Br
NHNH
overnight
0~r.t.
N2 / reflux
Me3Si
N
N
CuI, Pd(OAc)2, PPh3
NEt3
Br
SiMe3
Br
Me3Si
O
O
N
N
K2CO3 / MeOH
O
L
Au(PR3)Cl (2 equiv.)
NaOH / MeOH
HgRCl (2 equiv.)
NaOH / MeOH
N
N
N
N
(R3P)Au
Au(PR3)
RHg
HgR
O
O
R = Ph 3
R = Me 4
R = Ph 1
R = Me 2
Scheme 1. Synthetic routes to binuclear gold(I) and mercury(II) complexes 1–4.
Fig. 1. A perspective drawing of 1 with the thermal ellipsoids drawn at the 25% probability level. The labels on the phenyl ring carbon atoms are omitted for clarity.
Table 1
Selected bond lengths (Å) and angles (°) for 1
cited state that is more sensitive to thermally activated non-radia-
tive decay mechanisms [20]. As the temperature is lowered from
293 to 77 K, there is a notable red shift in the PL maximum for 3
and 4 which is presumably a manifestation of the solid-state aggre-
gation effect in the frozen phase at 77 K [21]. However, this solid-
state effect is not significant for the Au(I) complexes 1 and 2.
Based on the absorption and photoluminescence data, we can
obtain experimental values of the lower-lying excitations (Table
3) and construct an energy scheme as shown in Fig. 8 for 1–4 that
provides clearly the spatial extent of the singlet and triplet exci-
tons. The energy values are absolute values with respect to the S0
ground state. Values of DE(So À T1) (energy gap between So and
T1) were compiled to be 2.40–2.44 eV. The measured DE(S1 À T1)
values are 0.78, 0.79, 0.71 and 0.69 eV for 1–4, respectively, and
they correspond well with the S1 À T1 energy gap of 0.7 0. l eV
for similar p-conjugated Pt(II), Au(I) and Hg(II) acetylides
[9a,17b,18b]. We attribute such a constant DE(S1 À T1) value to
the exchange energy and possibly some additional constant contri-
bution due to the admixture of the metal orbitals [22]. From the
S1 energy levels obtained by absorption studies, it is clear that
the S1 states are notably lower for the d10 Au(I) than their Hg(II)
congeners, i.e. the order of p-delocalization through the metal
Au(1)AP(1)
Au(1)AC(19)
C(19)AC(20)
2.277(2)
1.989(9)
1.198(11)
Au(2)AP(2)
Au(2)AC(36)
C(35)AC(36)
2.264(2)
1.998(9)
1.200(11)
175.2(2)
173.8(7)
P(1)AAu(1)AC(19)
Au(1)AC(19)AC(20)
177.1(2)
174.0(8)
P(2)AAu(2)AC(36)
Au(2)AC(36)AC(35)
(Figs. 2–5). The large Stokes shifts of these lower-lying emission
peaks from the dipole-allowed absorptions (1.15–1.49 eV, see Figs.
2–5), plus the long emission lifetimes (sP) in the microsecond re-
gime are indicative of their triplet parentage, and they are thus as-
3
signed to the (pp*) excited states of the diethynylheteroarylene
core (i.e. T1 ? S0 emission). Such assignment can be further sup-
ported by the observed temperature dependence of the emission
data for 1 (see Fig. 7), in accordance with earlier work on metal diy-
nes and polyynes [11]. From 290 to 11 K, the singlet emission peak
intensity increases only by a factor of 3.0 but the intensity of the
lower-lying triplet emission drastically increases by a factor of
>100 which is accompanied by a well-resolved vibronic structure,
and such an increase in intensity indicates a long-lived triplet ex-

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W.-Y. Wong, Y.-H. Guo / Journal of Molecular Structure 890 (2008) 150–156
Table 2
Photophysical data for the gold(I) and mercury(II) alkynyl complexes
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
kabs (nm)a
Eg (eV)b
kem (nm)
CH2Cl2
CH2Cl2 (290 K)c
Frozen CH2Cl2 (77 K)d
1
276 sh (2.5)
299 sh (4.5)
332 (8.6)
3.35
366 sh
387 (0.28, 1.1)
405 sh
388 (S1)
404 sh
412 sh
340 (8.6)
360 sh (4.4)
512 (13.4) (T1)
542 sh
557 sh
2
3
4
L
297 sh (3.1)
331 (6.3)
347 (4.7)
3.31
3.44
3.48
3.59
365 sh
385 (0.34, 1.2)
403 sh
386 sh (S1)
513 (15.9) (T1)
531
558 sh
577 sh
358 sh (3.6)
300
400
500
600
700
288 sh (2.1)
320 (4.8)
345 sh (1.9)
369 (0.28, 1.3)
369 (0.47, 1.2)
380 sh
Wavelength (nm)
394 (S1)
508 (13.6) (T1)
533 sh
Fig. 3. Optical absorption spectrum at 293 K (Á Á Á) and PL spectra at both 293 (—)
and 77 K (---) of 2 in CH2Cl2.
552 sh
280 sh (1.9)
319 (4.8)
340 sh (2.2)
318 sh
401 (S1)
419 sh
517 (11.4) (T1)
561 sh
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
264 sh (1.8)
307 (5.1)
338 sh
357 (0.64, 1.4)
372
a
b
c
Extinction coefficients (104 MÀ1 cmÀ1) are shown in parentheses.
Estimated from the onset wavelength of the optical absorption.
Fluorescence quantum yields (%) and lifetimes (ns) shown in parentheses
(rF, sF) are measured in CH2Cl2 relative to 1.0 M quinine sulfate in H2SO4 (rF = 0.54).
d
Phosphorescence lifetimes sP (ls) at 77 K for the peak maxima are shown in
parentheses. sh, shoulders or weak bands.
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
Wavelength (nm)
Fig. 4. Optical absorption spectrum at 293 K (Á Á Á) and PL spectra at both 293 (—)
and 77 K (---) of 3 in CH2Cl2.
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength (nm)
Fig. 2. Optical absorption spectrum at 293 K (Á Á Á) and PL spectra at both 293 (—)
and 77 K (---) of 1 in CH2Cl2.
chromophore is Au(I) > Hg(II) [17d]. We observe that the S1 energy
level and the bandgap for 1 and 4 are higher relative to the biphen-
ylene-spaced gold(I) and mercury(II) analogues [(PPh3)AuC „ C(p-
C6H4)(p-C6H4)C „ CAu(PPh3)] (kabs = 326 nm, Eg = 3.24 eV) and
[MeHgC „ C(p-C6H4)(p-C6H4)C „ CHgMe] (kabs = 309 nm, Eg =
3.31 eV), respectively [23]. This may be attributed to the partial
conjugation interruption of the conjugated chain in the presence
of an electron-accepting oxadiazole ring which results in an angu-
lar geometry.
300
400
500
600
Wavelength (nm)
Fig. 5. Optical absorption spectrum at 293 K (Á Á Á) and PL spectra at both 293 (—)
and 77 K (---) of 4 in CH2Cl2.
tallic materials. The group 11–12 transition elements in these oxa-
diazole-phenylene derivatives can exert heavy-atom effects to
trigger the phosphorescence emission at low temperatures. Future
work in this direction should elucidate a direct evaluation of the
role of a metal center on the properties of conjugated compounds
4. Concluding remarks
This report describes the use of 2,5-bis(ethynylphenyl)-1,3,4-
oxadiazole in the production of a new series of luminescent bime-

W.-Y. Wong, Y.-H. Guo / Journal of Molecular Structure 890 (2008) 150–156
155
1.0
0.8
0.6
0.4
0.2
0.0
1.0
4.0
S1 Abs. 3.89
S1 Abs. 3.87
S1 Abs 3.63
S1 Abs 3.57
0.8
0.6
0.4
0.2
0.0
3.5
3.0
2.5
S1 PL 3.36
S1 PL 3.15
S1 PL 3.36
S1 PL 3.09
S1 PL 3.22
S1 PL 3.21
S1 PL 3.20
S1 PL 3.20
T1 PL 2.44
T1 PL 2.42
T1 PL 2.42
T1 PL 2.40
S0
S0
S0
S0
2.0
0
300
400
500
600
2
3
4
1
Wavelength (nm)
Fig. 8. Electronic energy level diagram of 1–4 determined from absorption and PL
data. For the S1 PL levels, the dashed lines represent those obtained from the PL data
at 293 K in CH2Cl2 whereas the solid lines those from the PL data at 77 K in frozen
CH2Cl2. The S0 levels are arbitrarily shown to be of equal energy.
Fig. 6. Optical absorption (Á Á Á) and photoluminescence (PL) spectra (—) of 2,5-bi-
s(ethynylphenyl)-1,3,4-oxadiazole L in CH2Cl2 at 293 K.
bridge Crystallographic Centre (Deposition No. CCDC-675080).
Copies of this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(Fax: + 44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk or
www: http://www.ccdc.cam.ac.uk).
Acknowledgments
Financial support from a CERG Grant from the Hong Kong Re-
search Grants Council of the Hong Kong SAR, PR China (Project
No. HKBU 2024/04P) and a Faculty Research Grant from the Hong
Kong Baptist University (FRG/06-07/II-63) is gratefully acknowl-
edged. We also thank Prof. K.-W. Cheah for the access of the facil-
ities for variable temperature photoluminescence measurements.
Fig. 7. Temperature dependence of the PL spectra of 1.
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