Synthesis, characterization and photophysical properties of PPh 2-C2-(C6H4)n-C 2-PPh2 based bimetallic Au(i) complexes

Article abstract of DOI:10.1039/c1dt11494h

A family of the diphosphines PPh₂C₂(C ₆H₄)_nC₂PPh₂ (n = 0-3), which possess a dialkynyl-arene spacer between the phosphorus atoms, was used for the synthesis of a series of bimetallic gold(i) complexes 1-7. Unlike the corresponding polynuclear Au(i) clusters, which show unique phosphorescence, 1-7 reveal dual emissions consisting of fluorescence and phosphorescence. The results are rationalized, in a semi-quantitative manner, by the trace (1-3) to zero (4-7) contribution of MLCT varying with the number of conjugated phenylene rings. As a result, unlike typical polynuclear Au(i) clusters with 100% triplet state population, the rate constant of the S₁→T₁ intersystem crossing is drastically reduced to 10⁹ s^-1 (4-7)-10¹⁰ s^-1 (1-3), so that the fluorescence radiative decay rate can compete or even dominates. The drastic O₂ quenching of phosphorescence demonstrates the unprotected nature of the emission chromophores in 1-7, as opposed to the well protected, O₂ independent phosphorescence in most multimetallic Au(i) clusters.

Full text of DOI:10.1039/c1dt11494h

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PAPER
Synthesis, characterization and photophysical properties of
PPh2–C2–(C6H4)n–C2–PPh2 based bimetallic Au(I) complexes†
Igor O. Koshevoy,*a,b Chia-Li Lin,c Cheng-Chih Hsieh,c Antti J. Karttunen,a Matti Haukka,a
Tapani A. Pakkanena and Pi-Tai Chou*c
Received 7th August 2011, Accepted 4th October 2011
DOI: 10.1039/c1dt11494h
A family of the diphosphines PPh2C2(C6H4)nC2PPh2 (n = 0–3), which possess a dialkynyl-arene spacer
between the phosphorus atoms, was used for the synthesis of a series of bimetallic gold(I) complexes 1–7.
Unlike the corresponding polynuclear Au(I) clusters, which show unique phosphorescence, 1–7 reveal
dual emissions consisting of fluorescence and phosphorescence. The results are rationalized, in a semi-
quantitative manner, by the trace (1–3) to zero (4–7) contribution of MLCT varying with the number of
conjugated phenylene rings. As a result, unlike typical polynuclear Au(I) clusters with 100% triplet state
population, the rate constant of the S1→T1 intersystem crossing is drastically reduced to 109 s-1 (4–7)–1010
s-1 (1–3), so that the fluorescence radiative decay rate can compete or even dominates. The drastic O2
quenching of phosphorescence demonstrates the unprotected nature of the emission chromophores in 1–
7, as opposed to the well protected, O2 independent phosphorescence in most multimetallic Au(I) clusters.
the gold(I) centers to the phosphorus-containing chromophores
Introduction
results in a large increase in the emission quantum efficiency14
as the presence of the d10 ion diminishes the ability of the
polarizable phosphorus lone pair to participate in nonradiative de-
cay processes (e.g., photoinduced-electron-transfer quenching).15
Additionally, it has been demonstrated that the effect of the
heavy Au(I) ion bound to a phosphine with a polyaromatic
substituent such as phenanthrene leads to the appearance of
pertinent room-temperature phosphorescence originating from an
aromatic ligand, i.e., the phenanthrene moiety.16
The photophysical properties of gold complexes are of funda-
mental importance and have been actively studied for several
decades.1–5 Relevant research has been stimulating significant
progress in preparative gold chemistry as well as attracting consid-
erable theoretical attention.6 Among numerous gold-containing
species, the phosphine-alkynyl complexes are of particular in-
terest due to their ease of preparation and handling, wide
range of accessible modifications of the constituent ligands,
fascinating structural assemblies and intriguing photoluminescent
characteristics.3,7–10 Much of the research effort has been devoted
to the systematic investigation of the emission originating from
gold-perturbed intraligand alkynyl transitions.5,8,11,12 As has been
shown, binding of Au(I) ions to –C CR moieties may induce the
triplet state emission of the organic ligand due to fast intersystem
crossing promoted by spin–orbit coupling of the heavy gold
atom. Surprisingly, the metal-modified emission of the phosphines
has been much less studied, though these ligands possessing
conjugated backbones proved to be useful building blocks in the
construction of both molecular entities and polymeric materials
and demonstrated promising light emissive properties.13 The exam-
ples available in the literature clearly indicate that coordination of
Recently, we have been extensively using bidentate phosphine
ligands with polyaromatic backbones in the synthesis of polynu-
clear luminescent gold(I)-containing alkynyl compounds.17–20 The
resulting complexes exhibit intense room temperature phospho-
rescence arising mainly from the metal–alkynyl cluster cores.
However, it has been noted that formation of the simple alkynyl
dimers (RC2Au)PP(AuC2R) (PP = diphosphine) without metal–
metal contacts in some cases also causes a visible increase in the
emission intensity. As the Au(I) complexes with PPh2(C6H4)nPPh2
ligands (n = 1–3) were reported earlier and show rather weak
luminescence,17,21,22 we decided to employ a family of alkynyl-
arene based diphosphines, PPh2C2(C6H4)nC2PPh2 (n = 0–3) for
the preparation as well as detailed photophysical and theoretical
studies of their bimetallic gold(I) alkynyl derivatives.
aUniversity of Eastern Finland, Joensuu, 80101, Finland. E-mail: igor.
koshevoy@uef.fi; Fax: +358 13 2513390; Tel: +358 13 2513325
bSt.-Petersburg State University, St.-Petersburg, Russia
Experimental
cNational Taiwan University, Taipei, 106, Taiwan. E-mail: chop@ntu.edu.tw;
Fax: +886 2 23695208; Tel: +886 2 23630231
General comments
† Electronic supplementary information (ESI) available: X-ray crystal-
lographic data for 6, optimized Cartesian coordinates of 1–3. CCDC
reference number 838558. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt11494h
Au(tht)Cl (tht = tetrahydrothiophene),23 (AuC2R)n (R = Ph, But)24
were obtained according to literature methods. The diphosphines
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PPh2C2(C6H4)nC2PPh2 (n = 0 (L0), 1 (L1), 2 (L2), 3 (L3)) were
prepared as previously reported.19,20,25 Other reagents and solvents
were used as received. The solution 1D 1H, 31P NMR and
1H-1H COSY spectra were recorded on a Bruker Avance 400
spectrometer. Microanalyses were carried out in the analytical
laboratory of the University of Eastern Finland.
(AuC2Ph)2PPh2C2(C6H4)3C2PPh2 (4)
Prepared analogously to 2 using PPh2C2(C6H4)3C2PPh2. Pale
yellow crystalline material (88%). 31P{ H} NMR (CDCl3; d): 16.0
1
1
(s). H NMR (CDCl3; d): 7.89 (dd, 8H, ortho-H (Ph–P), J(HH)
8.1, J(PH) 14.2 Hz), 7.72 (s, -C6H4–C6H4–C6H4- , 4H), 7.69 (s
(coalesced AB system), -C6H4–C6H4–C6H4- , 8H), 7.55–7.47
(m, 16H), 7.28–7.19 (m, 6H). Anal. Calc. for Au2C62H42P2: C,
59.92; H, 3.41. Found: C, 59.79; H, 3.74.
(AuC2CH2O-But)n
Au(tht)Cl (200 mg, 0.624 mmol) was suspended in acetone (10 ml),
followed by HC2CH2O-But (72 mg, 0.0642 mmol) and NEt3
(65 mg, 0.644 mmol). The suspension became a transparent pale-
yellow solution, which was stirred for 20 min in the absence of
light. Addition of water (15 ml) caused precipitation of a flaky
pale solid that was collected by centrifugation, washed with water
(3 ¥ 5 ml) and vacuum dried to give crude (AuC2CH2O-But)n
of sufficient purity, which was used in further synthesis without
additional purification (158 mg, 82%).
(AuC2-But)2PPh2C2(C6H4)3C2PPh2 (5)
(AuC2Bu)n (50 mg, 0.180 mmol) was suspended in CH2Cl2 (10 ml)
and PPh2C2(C6H4)3C2PPh2 (60 mg, 0.93 mmol) was added, and the
yellow suspension turned into a nearly colorless solution within
minutes. It was stirred for 30 min in the absence of light, treated
with activated charcoal, passed through a layer of Al2O3 (neutral,
0.5 ¥ 3 cm), concentrated to ca. 1 ml and diluted with ethanol (5 ml)
to give an opaque solution, in which crystalline needles started to
form. Then more ethanol (5 ml) was added followed by hexanes
(10 ml). The pale crystalline solid was additionally recrystallized
by slow evaporation of a dichloromethane–ethanol solution of 5 at
(AuC2Ph)2PPh2C2C2PPh2 (1)
◦
5 C. Colorless crystalline material (98 mg, 91%). 31P{ H} NMR
(CD2Cl2; d): 16.3 (s). 1H NMR (CD2Cl2; d): 7.95 (dd, 8H, ortho-H
(Ph–P), J(HH) 7.9, J(PH) 14.0 Hz), 7.78 (s, -C6H4–C6H4–C6H4-
1
(AuC2Ph)n (100 mg, 0.336 mmol) was suspended in CH2Cl2 (10 ml)
and PPh2C2C2PPh2 (72 mg, 0.172 mmol) was added, and the yellow
suspension turned into a yellowish solution within minutes. It was
then stirred for 10 min in the absence of light, concentrated to
ca. 3 ml, filtered and precipitated by slow addition of an excess
of hexanes (10 ml). Beige crystalline material was washed with
diethyl ether and vacuum dried (153 mg, 90%). 1 is of limited
stability in solution at room temperature as the nearly colorless
, 4H), 7.75 (s (coalesced AB system), -C6H4–C6H4–C6H4-
8H), 7.59–7.51 (m, 12H), 1.27 (s, t-Bu, 18H). Anal. Calc. for
Au2C58H50P2: C, 57.91; H, 4.19. Found: C, 57.59; H, 4.30.
,
(AuC2CH2O-But)2PPh2C2(C6H4)3C2PPh2 (6)
1
solution of this complex rapidly darkens. 31P{ H} NMR (CDCl3;
Prepared analogously to
2
using (AuC2CH2O-But)n and
d): 18.0 (s). 1H NMR (CDCl3; d): 7.78 (dd, 8H, ortho-H (Ph–P),
J(HH) 7.6, J(PH) 14.1 Hz), 7.57–7.45 (m, 16H), 7.28–7.19 (m,
6H). Anal. Calc. for Au2C44H30P2: C, 52.09; H, 2.98. Found: C,
51.71; H, 3.22.
PPh2C2(C6H4)3C2PPh2. Recrystallized by slow evaporation of
a dichloromethane–hexane solution of 6 at 5 ◦C. Colorless
1
1
crystalline material (86%). 31P{ H} NMR (CDCl3; d): 15.8. H
NMR (CDCl3; d): 7.86 (dd, 8H, ortho-H (Ph–P), J(HH) 8.0,
J(PH) 14.2 Hz), 7.72 (s, 4H, -C6H4–C6H4–C6H4- ), 7.69 (s
(coalesced AB system), -C6H4–C6H4–C6H4- , 8H), 7.56–7.46
(m, 12H), 4.26 (s, CH2, 4H), 1.27 (s, t-Bu, 18H). Anal. Calc. for
Au2C60H54O2P2: C, 57.06; H, 4.31. Found: C, 56.65; H, 4.53.
(AuC2Ph)2PPh2C2C6H4C2PPh2 (2)
(AuC2Ph)n (75 mg, 0.252 mmol) was suspended in CH2Cl2 (10 ml)
and PPh2C2C6H4C2PPh2 (64 mg, 0.130 mmol) was added, and the
yellow suspension turned into a yellowish solution within minutes.
It was stirred for 30 min in the absence of light, treated with
activated charcoal, passed through a layer of Al2O3 (neutral, 0.5 ¥
3 cm), concentrated to ca. 5 ml, and precipitated by addition
of an excess of hexanes (15 ml). Colorless crystalline material
was washed with diethyl ether and vacuum dried (125 mg, 91%).
(AuCl)2PPh2C2(C6H4)3C2PPh2 (7)
Au(tht)Cl (100 mg, 0.312 mmol) was dissolved in CH2Cl2 (10 ml)
and PPh2C2(C6H4)3C2PPh2 (101 mg, 0.156 mmol) was added, and
the nearly colorless solution was stirred for 30 min and evaporated.
The pale residue was washed with diethyl ether (2 ¥ 6 cm3) and
recrystallized by slow evaporation of a dichloromethane–hexane
solution of 7 at 5 ◦C. Colorless crystalline material (154 mg, 89%).
1
1
31P{ H} NMR (CDCl3; d): 16.2 (s). H NMR (CDCl3; d): 7.86
(dd, 8H, ortho-H (Ph–P), J(HH) 8.1, J(PH) 14.2 Hz), 7.61 (s,
4H, -C6H4- ), 7.54–7.47 (m, 16H), 7.28–7.19 (m, 6H). Anal.
Calc. for Au2C50H34P2: C, 55.06; H, 3.14. Found: C, 55.02; H,
3.43.
1
1
31P{ H} NMR (CDCl3; d): 3.1 (s). H NMR (CDCl3; d): 7.87
(dd, 8H, ortho-H (Ph–P), J(HH) 7.6, J(PH) 15.0 Hz), 7.73 (s,
4H, -C6H4–C6H4–C6H4- ), 7.71 (s (coalesced AB system),
-
C6H4–C6H4–C6H4- , 8H), 7.50–7.59 (m, 12H). Anal. Calc. for
Au2C46Cl2H32P2: C, 49.71; H, 2.90. Found: C, 49.79; H, 2.96.
(AuC2Ph)2PPh2C2(C6H4)2C2PPh2 (3)
Prepared analogously to 2 using PPh2C2(C6H4)2C2PPh2. Pale
Photophysical measurements. Steady-state absorption and
emission measurements in solution were recorded on a Hitachi
(U-3310) spectrophotometer and an Edinburgh (FS920) fluo-
rometer, respectively. Both the wavelength-dependent excitation
and emission response of the fluorometer have been calibrated.
To determine the photoluminescence quantum yield in solution,
1
yellowish crystalline material (89%). 31P{ H} NMR (CDCl3; d):
1
16.0 (s). H NMR (CDCl3; d): 7.88 (dd, 8H, ortho-H (Ph–P),
J(HH) 8.0, J(PH) 14.2 Hz), 7.66 (AB system C6H4–C6H4-
,
8H), 7.55–7.47 (m, 16H), 7.28–7.19 (m, 6H). Anal. Calc. for
Au2C56H38P2: C, 57.64; H, 3.28. Found: C, 57.30; H, 3.57.
938 | Dalton Trans., 2012, 41, 937–945
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the samples were degassed by three freeze-pump-thaw cycles.
Quinine sulfate monohydrate in sulfuric acid and 7-azaindole in
cyclohexane, with quantum yields of ~0.5726 and ~0.22,27 respec-
tively, served as the standard for measuring the quantum yield.
Nanosecond lifetime studies were performed with an Edinburgh
FL 900 photon-counting system using a hydrogen-filled lamp as
the excitation source. The emission decays were fitted by the sum
of exponential functions with a temporal resolution of ~300 ps via
the deconvolution of instrument response function. Nanosecond-
picosecond lifetime studies were performed with an Edinburgh OB
900-L time-correlated single photon counting system coupled with
a femtosecond Ti-Sapphire oscillator (80 MHz, Spectra-Physics)
coupled with a regenerative amplifier (Spitfire Pro, Spectra-
Physics) that produces 120 fs laser pulses at 800 nm with 1 kHz
repetition rate, and then used to produce the second harmonic
(400 nm). A polarizer was placed in the emission path to ensure
that the polarization of the fluorescence was set at the magic angle
(54.7◦) with respect to that of the pump laser to eliminate the
fluorescence anisotropy. The resolution of the TCSPC system
is limited by the detector response of ~50 ps. The fluorescence
decays were analyzed by the sum of exponential functions with an
iterative convolution method which allows partial removal of the
instrument time broadening and consequently renders a temporal
resolution of ~30 ps.
The phosphorescence lifetime (>ms) was then detected by an
optical spectrum analyzer, consisting of an intensified charge-
coupled device (ICCD) with detector control (PI-MAX, Princeton
Instruments) in conjunction with a monochromator (SP2300i,
Acton). The gate channel of the ICCD was open at 50 ns
and synchronized with the firing time of the excitation pulse.
Typically, the second harmonic (400 nm, fwhm <20 ns) of a
Nd:YAG pumped Ti:sapphire laser (LT-2211, LOTIS TII) was
used as the excitation pulse. For the higher energy excitation at e.g.
340 nm, a traveling-wave optical parametric amplifier of white-
light continuum (TOPAS-C, Spectra Physics) was driven by the
fundamental of the above mentioned regenerative amplifier. The
output of TOPAS-C produced NIR signal beam (1150–1560 nm,
1 kHz, 80 mJ) was selected as 1360 nm and then used to produce
the fourth harmonic (340 nm) as an excitation source.
The femto-picosecond transient absorption measurements were
performed according to previous reports.28 However, the system
has been modified to improve the sensitivity and stability. In
brief, a Spitfire Pro system (Spectra-Physics) was used as the
laser source. This system comprises a seed laser, a pump laser,
a stretcher, a Ti:sapphire regenerative amplifier and a compressor.
The output of the system consists of pulses of 800 nm, 1 W,
120 fs (fwhm) at a repetition rate of 1 kHz. The pump–probe
spectroscopic setup is mainly based on an ExciPro spectrometer
(CDP System Corp). The Spitfire Pro output is first split (50%) into
two beams, in which the pump is converted to designated excitation
wavelengths by coupling it into a third-harmonic generator (for
266 nm excitation). The probe beam is first passed through a
computer-controlled delay line, and then focused on a 1 mm thick
sapphire plate to generate a white light continuum (350–1000 nm).
The pump beam is then passed through a computer controlled
optical chopper, and focused (3 mm) onto the sample cell. The
sample cell is a 1 mm optical path quartz cylindrical cell placed
in a variable speed rotating holder. The white light probe beam
is collimated and focused into the sample cell, superimposed to
the pump beam at an angle of about 5 degrees. To minimize the
temporal chirp in the spectrum, parabolic mirrors are used for
focus and collimation. After passing through the sample cell,
the white continuum is coupled into a 100 mm optical fiber
connected to a diode array. Typically, time-resolved absorption
spectra were acquired averaging over 200 excitation pulses at any
delay time. With 120 fs pump and probe pulses, the effective time
resolution of the ultrafast spectrometer that is, the risetime of an
“instantaneous” signal, is about 220 fs.
X ray structure determination. The crystal of 6 was immersed
in cryo-oil, mounted in a Nylon loop, and measured at a
temperature of 100 K. The X-ray diffraction data was collected
on a Bruker Kappa ApexII diffractometer using Mo-Ka radiation
(l = 0.71073 A). The APEX229 program package was used for cell
˚
refinements and data reduction. The structure was solved by direct
methods using the SIR9730 program with the WinGX31 graphical
user interface. The absorption effects were corrected by semi-
empirical methods based on equivalent reflections (SADABS).32
Structural refinements were carried out using SHELXL-97.33 The
Au molecule is located around the center of symmetry, and the
asymmetric unit contains half of the Au molecule. Both the hexane
and dichloromethane of crystallization are disordered over two
sites with occupancy ratios of 0.51/0.49 and 0.57/0.43. The C–C
and C–Cl distances in these molecules were restrained. Similarly,
the heavy atoms in these solvent molecules were restrained so that
their Uij components approximate to isotropic behavior. The O–
tBu units were also slightly disordered but no disorder model was
used in the final refinement. The hydrogen atoms were positioned
geometrically and constrained to ride on their parent atoms,
˚
with C–H = 0.95–0.99 A and Uiso = 1.2–1.5·Ueq (parent atom).
The crystallographic details are summarized in the corresponding
footnote.‡
Computational details. The studied structures were inves-
tigated with the Second-Order Approximate Coupled-Cluster
method (CC2), which is an approximation of the Coupled Cluster
Singles and Doubles (CCSD) method.34 All atoms were de-
scribed by a triple-valence-zeta quality basis set with polarization
functions (def2-TZVP for Au, def-TZVP for other elements).35
Scalar relativistic effects were taken into account by employing
a 60-electron relativistic effective core potential for gold.36 The
resolution-of-the-identity technique was used to speed up the
calculations.37 The studied complexes can rotate freely around
the P–C2–(C6H4)n–C2–P axis and we utilized C2h point group
symmetry to speed up calculations (utilizing C2v point group
symmetry yielded the same results). The geometries of the ground
and excited states were fully optimized at the RI-CC2/TZVP level
of theory using the TURBOMOLE program package (versions 6.2
and 6.3).38 Additional electronic structure analyses were carried
out using the AOMix program package.39
‡ Crystal data for 6: C130H130Au4Cl8O4P4, M = 2951.69, yellow needle, 0.29
3
¯
¥ 0.11 ¥ 0.09 mm , triclinic, space group P1, a = 11.6702(9), b = 13.6745(11),
◦
˚
c = 21.1659(17) A, a = 97.135(3), b = 103.103(2), g = 93.052(2) , V =
3
-3
˚
3252.7(4) A , Z = 1, Dc = 1.507 g cm , F000 = 1454, Mo-Ka radiation, l =
0.71073 A, T = 100(2)K, 2qmax = 52.0◦, 32 989 reflections collected, 12 098
˚
unique (Rint = 0.0324). Final GooF = 1.074, R1 = 0.0569, wR2 = 0.1331, R
indices based on 12 098 reflections with I > 2s(I) (refinement on F2), 708
parameters, 212 restraints; wR2 = 0.1558 (all data).
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Results and discussion
Synthesis and characterization
Reactions of the polymeric gold(I) phenylacetylide with a family
of diphosphine ligands PPh2–C2–(C6H4)n–C2–PPh2 (n = 0 (L0),
1 (L1), 2 (L2), 3 (L3)) result in clean and high yield formation
of the corresponding bimetallic complexes (PhC2Au)PPh2–C2–
(C6H4)n–C2–PPh2(AuC2Ph) (n = 0 (1), 1 (2), 2 (3), 3 (4)), depicted
in Scheme 1. Analogously, treatment of the (AuC2But)n and
(AuC2CH2OBut)n acetylides with PPh2C2(C6H4)3C2PPh2 diphos-
phine gave the dimeric compounds 5 and 6, which are similar
to 4. This depolymerization methodology is a general and well
documented synthetic approach, leading to a variety of gold
alkynyl complexes.9,22,24 In the obtained species the phosphine
ligands bridge two AuI centers, which evidently demonstrates
two-coordinate linear geometry, typical for the alkynyl-phosphine
compounds.8,9,11,22 The reaction of the labile Au(tht)Cl precur-
sor with L3 ligand gave a chloro-gold(I) diphosphine complex
(ClAu)PPh2–C2–(C6H4)n–C2–PPh2(AuCl) inhigh yield(Scheme 1).
Fig. 1 Structural plot of the complex (AuC2CH2OBut)2PPh2C2(C6H4)3-
C2PPh2 (6). Selected interatomic distances (A) are: Au(1)–C(47) 1.996(11),
Au(1)–P(1) 2.276(2), Au(1)–Au(2) 3.1287(5), Au(2)–C(54) 1.992(14),
Au(2)–P(2) 2.262(3).
˚
patterns with those of the recently reported AuI–AgI compounds
based on these diphosphines.20 The amount of the signals observed
together with their multiplicity and relative intensities completely
match the proposed structures and indicate that these compounds
exist in their molecular forms in solution.
X-ray diffraction analysis of complex 6 revealed a dimeric
structure, in which two molecules are linked together by aurophilic
interactions (Fig. 1). A similar structural motif was observed
in analogous gold complexes based on the 1,4-bis(diphenyl-
phosphino)benzene
or
1,2-bis(diphenylphosphino)ethane
ligands.2,21 Solution NMR spectroscopic data (see Experimental)
correspond to the symmetrical monomolecular form of 6,
indicating dissociation of the Au–Au bonds in fluid medium.21
Photophysical properties
Fig. 2 and 3 show absorption and emission spectra of complexes 1–
7 and ligand L3 (see Scheme 1 for the structures) in CH2Cl2 at room
temperature. Solid-state emission spectra at room temperature
are shown in Fig. S1, ESI.† Note that the separation into
two figures is mainly to avoid spectral congestion. Based on
the theoretically determined photophysical characteristics (vide
infra), the lowest lying absorption in the singlet manifold for
1–3 is ascribed to a combination of the intraligand p→p* of
both p-conjugated C C–Ar and PPh2C C(C6H4)nC CPPh2
Scheme 1 Synthesis of the complexes 1–7: CH2Cl2, 298 K, 30 min, yields
86–91% (P = PPh2).
The solid-state structure of complex 6 has been determined by
an X-ray diffraction study (Fig. 1). The rest of the complexes
(1–5, 7) were characterized by NMR spectroscopic studies and
elemental analyses. The 31P NMR spectra of 1–6 display singlet
resonances in the region 15.8–18 ppm, while a high field signal
at 3.1 ppm is observed for 7. The detected low field shift
in comparison to the free phosphine ligands unambiguously
indicates the coordination of the P donor atoms to the gold(I)
ions.
Fig. 2 UV/vis absorption and normalized emission spectra of 1–4 in
degassed CH2Cl2 at room temperature, lexcit = 300 nm for 1–3, lexcit = 330
nm for 4.
Assignment of the proton NMR spectra of complexes 1–7
has been confirmed by the comparative analysis of their spectral
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orbital energy is virtually insensitive to the p-elongation. As a
result, for the titled complexes 1–7, the % contribution of the
metal dp orbital to the HOMO is expected to be decreased upon
increasing n from 0 to 3. Up to n = 3, the dp energy is substantially
lower than that of the p orbital, giving null contribution of the
dp→p* transition, i.e., no MLCT contribution to the lowest lying
transition.
The viewpoint of the lowest lying transition having a small
%MLCT character in 1–3 (<5%) and ~0% MLCT character
in 4–7 is firmly supported by the observed emission properties.
In this study, particular attention is paid to the observation
of dual emission for all titled compounds. Evidently, as shown
in Fig. 2, complexes 1–3 exhibit very distinct dual emission
consisting of fluorescence and phosphorescence in the short and
long wavelength regions, respectively. The classification of each
is based on the associated radiative decay rate constant (kr), for
which the fluorescence and phosphorescence are in the range of
107–109 s-1 and ꢀ105 s-1, respectively (see Table 1).
Fig. 3 UV/vis absorption and normalized emission spectra of 5–7 in
degassed CH2Cl2 and L3 in aerated CH2Cl2 at room temperature, lexcit
330 nm for 5–7, lexcit = 300 nm for L3. Inset: the enlargement of spectrum
of 5–7 in the region of 530–700 nm. Note that no phosphorescence can be
resolved from emission of L3.
=
Moreover, as shown by quantum yield data (Table 1), the
longer wavelength emission band is drastically quenched by the
presence of O2 (in aerated solution). Thus, its assignment to the
phosphorescence seems to be unambiguous, which, in theory,
is quenched by O2 via a well established mechanism associated
with collisionally induced energy transfer.41 Extending to n = 3,
complex 4 exhibits an intense fluorescence band with a maximium
at ~375 nm, accompanied by a rather weak phosphorescence
maximized at ~550 nm in degassed CH2Cl2 solution. Complexes 5
and 6, which are close relatives of 4 having the phenyl groups
in R positions replaced by t-butyl, 2-methoxy-2-methylpropyl
(see Scheme 1), respectively, and 7 possessing chloride ligands
instead of the alkynyl ones, also exhibit similar dual emission
peaks with fluorescence being dominant. In fact, for complexes
5–7, spectral enlargement is necessary in order to resolve the
obscure phosphorescence in degassed solution (see inset of Fig.
3). In contrast to the polymetallic Au(I) containing clusters,20,42
phosphorescence emission of the titled bimetallic complexes
1–7 is drastically quenched by triplet molecular oxygen, an
inherent feature of many luminescent coordination transition
metal complexes.43 This is apparently due to the unprotected
Au(I)-alkyne chromophores in the bimetallic complexes, which
are exposed to the surrounding solvents in which O2 molecules are
dissolved.
(n = 0–2) ligands and their cross excitation, i.e. the interligand
p → p* transitions of the alkynyl and phenyl moieties, mixed
with a minor percentage of Au(I)-to-alkyne metal-to-ligand charge
transfer (MLCT) transition.
As for 4–7, the lowest lying transition is dominated by intrali-
gand p→p* of the PPh2C C(C6H4)3C CPPh2 chromophore,
whereas the contribution of MLCT is virtually zero (vide infra).
The pure p→p* allowed transition is manifested in the fact
that complexes 5–7 exhibit similar absorption peaks at 330 nm,
despite their possessing different R-terminated alkynyl groups
and chloride (see Scheme 1), with e as high as ≥105 cm-1 M-1.
This assignment is consistent with the previous reports on the
relevant alkynyl analogues, for which the dominant absorption
band in the spectral region of 250–320 nm is ascribed to
the characteristic band of the alkynyl-phosphine moieties.21,22,40
Further confirmation is given by the absorption spectrum of ligand
L3 (PPh2C C(C6H4)3C CPPh2), which exhibits nearly identical
spectral features with respect to those of complexes 5–7 (Fig. 3).
The above trend of transition character can be rationalized by
the increase of p-conjugation from n = 0 to n = 3 of the ligands
PPh2C C(C6H4)nC CPPh2, such that its corresponding HOMO
(LUMO) is increased (decreased). Conversely, the metal (Au(I)) dp
Table 1 Photophysical properties of the Au(I) complexes 1–7 and ligand (L3) in CH2Cl2
lab /nm (10-4 e/cm-1 M-1)
l
em/nm
Ua
Ub
t
obs/nsc
t
obs/msd kr/s-1,e
1
2
3
4
5
6
7
286(2.0), 302sh(1.4)
394, 455
370, 482, 517
0.0003 [fl], 0
0.001 [fl], 0
0.0003 [fl], 0.0017
0.001 [fl], 0.006
0.077 [fl], 0.023
0.4 [fl], 0.02
0.99 [fl], <0.01
0.93 [fl], 0.01
0.99 [fl], <0.01
0.002[fl]
0.013
0.020
0.050
0.45
0.66
0.55
0.88
73
296
1.30 ¥ 107 [fl], 1.93 ¥ 103
5.00 ¥ 107 [fl], 82.2
1.57 ¥ 109 [fl], 77.7
2.68 ¥ 109 [fl], 29
1.52 ¥ 109 [fl]
272sh(1.8), 283(2.1), 294(2.4), 312(2.2)
271(3.9), 285(4.7), 299sh(4.8), 315 (5.6)
271(4.1), 285(4.8), 325 (7.0)
325(12.6)
325(11.8)
277sh(3.6), 327(5.9)
352, 370, 528, 570 0.077 [fl], 0
375, 394, 549, 591 0.4 [fl], 0
375, 392, 550, 593 0.99 [fl], 0
375, 393, 547, 595 0.93 [fl], 0
375, 394 550, 593
372, 386
680
1175
1322
2300
1.70 ¥ 109 [fl], 7.6
1.72 ¥ 109 [fl],
0.99 [fl], 0
0.002 [fl]
0.58
0.87
L3 328(5.7)
2.3 ¥ 106 [fl]
a Measured in aerated CH2Cl2. b Measured in degassed CH2Cl2. c tobs denotes the lifetime of the S1 state, which is resolved by pico-femtosecond transient
absorption for 1–3 and by fluorescence time-correlated single photon counting measurement for 4–7. d tobs denotes the lifetime of the phosphorescence
band. e kr are deduced from data obtained in the fluorescence and phosphorescence band. Information regarding the fluorescence band is marked with
[fl]. 7-Azaindole in cyclohexane (lexcit = 300 nm for 1–3 and L3) and quinine sulfate monohydrate in sulfuric acid (lexcit = 330 nm for 4–7) were used as
standards for the quantum yield (U) measurements, respectively.
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For complexes 1–4, in which the only structural difference lies
in the number (n) of the phenylene rings in the spacer of the
bridging diphosphine, the peak wavelengths for both fluorescence
and phosphorescence are progressively red shifted with an increase
in n from 0 to 3. The results are in accordance with computational
results, concluding that the LUMO is mainly located at the
diphosphine ligands PPh2–C2–(C6H4)n–C2–PPh2 (n = 0–3) for both
singlet and triplet manifolds. Thus, upon increasing the number
of phenylene rings from n = 1 to n = 3, the p-conjugation,
in part, is expected to be increased, resulting in a decrease
of the lowest lying transition gap.20,21 For 4–7, similar spectral
features (peak wavelength and profile) for both fluorescence and
phosphorescence are in accordance with computational studies.
For n = 3, the HOMO and LUMO are predominantly p and p* in
character, respectively, and are located over the PPh2–C2–(C6H4)n–
C2–PPh2 ligand moiety.
transient absorption spectroscopy (see Experimental section for
detail). Using complex 3 as an example, the results shown in
Fig. 4 clearly indicate an instant rise of the absorption peak at ~585
nm after 266 nm (~120 fs pulse width) excitation in CH2Cl2. The
585 nm transient absorption then gradually decreases following a
single exponential decay (see Fig. 5) with a time constant of 50
ps (monitored at 617 nm). The decay of the 585 nm absorbance
is simultaneously accompanied by the rise of an absorption band
with a maximum at ~510 nm.
Despite the fact that most Au(I) homometallic phosphine
compounds demonstrate solely phosphorescence emission,4,10,22,44
complexes 1–7, especially 4–7, exhibit high fluorescence quantum
yields. The dominant fluorescence suggests a relatively slow S1 →
Tm (m ≥ 1) intersystem crossing rate so that the S1 → S0 fluorescence
radiative decay is able to compete and even dominate. To simplify
the approach, we assume that the intersystem crossing takes place
solely via the S1 → T1 pathway. The corresponding rate constant,
kisc, is then expressed as
Fig. 4 Femtosecond transient absorption spectra of 3 in CH2Cl2 mea-
sured following excitation at 266 nm.
2
<yT HSO yS1
>
1
kisc
∝
(1)
(DES −T )2
1
1
where Hso is the Hamiltonian for spin–orbit coupling (SOC), and
DES1 is the energy difference between S1 and T1 states. Since the
–T
1
metal dp orbital is directly coupled into the spin–orbit coupling
matrix, the rate of S1 → T1 intersystem crossing should be greatly
influenced as well as enhanced by those low lying transitions
incorporating a metal dp orbital,45 the result of which can be
defined as an internal heavy atom effect. Conversely, for the lack of
MLCT and hence no dp contribution, the coupling matrix should
be rather small. In this case the Au(I) atom simply executes an
external heavy atom effect, which is qualitatively proportional
to Z8/r6 (Z: the atomic number, r: distance between the metal
atom and the center of the chromophore for the transition), if
Au(I) is treated as a hydrogen-like atom via an oversimplified
approach. Standing on this basis, the above spectral properties
can be rationalized by the rather small %MLCT or even zero
MLCT contribution to the lowest lying transition (vide infra) for
the titled complexes. Since the metal dp orbital in e.g. 4–7 is not
directly involved in the transition, Au(I) only executes the external
heavy atom effect rather than the internal heavy atom effect,46
giving rise to a small kisc, and hence the dominant fluorescence.
In general, similar regularities have been observed in solid-state
emission spectra of the complexes 2–7 (Fig. S1, ESI,† the emission
of 1 was too weak to be resolved). Compounds 2, 3 and 4 exhibit
dual emission (fluorescence and phosphorescence) while dominant
fluorescence was recorded for 5–7. This indicates that there is
no drastic difference in the intersystem crossing in solid, and
potential Au–Au intermolecular interactions (in fact determined
for complex 6, Fig. 1) should have a negligible effect on the
emission parameters. To gain further insight into the relaxation
dynamics, we also carried out the pico-femtosecond UV/Vis
Fig. 5 Relaxation dynamics of transient absorption for 3 in CH2Cl2
excited at 266 nm (~120 fs), monitored at 617 nm (black line) and 500
nm (blue line). The red line is the fit of the experimental data with an
exponential function.
The 510 nm transient exhibits dual rise time, which is fitted
to be the system response limit (<250 fs) and 48 ps (monitored
at 500 nm). The latter component, within experimental/fitting
error, is well correlated with the decay of the 585 nm transient
absorbance. The former system-response-limited rise component
can be rationalized by the overlap between rise and decay of the
510 nm and 585 nm bands, respectively. It is thus reasonable for
us to assign the 585 nm and 510 nm absorption bands originating
from the S1 → Sn and T1 → Tm transient absorptions, respectively,
which have a precursor-successor type of relationship. Assuming
942 | Dalton Trans., 2012, 41, 937–945
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Table 2 Computational photophysical results for the clusters 1–7 and the ligand L3 (CC2/TZVP level of theory)
lab S0 → S1 (nm)
lab S0 → S2 (nm)
lem S1 → S0 (nm)
lem T1 → S0 (nm)
Theor.a
Exp.
302
312
315
325
325
325
327
328
Theor.
266 (0.15)
274 (1.59)
293 (0.79)
295 (0.45)
—
Exp.
286
294
299
285
—
—
277
—
Theor.
387
397
363
378
377
376
386
378
Exp.
394
370
370
394
392
393
394
386
Theor.
392
518
550
564
565
565
569
—
Exp.
455
517
570
591
593
595
593
—
1
310 (1.04)
306 (1.01)
311 (2.42)
325 (3.35)
325 (3.19)
324 (3.29)
333 (3.16)
325 (3.03)
2
3
4
5
6
—
7
247 (0.03)
—
L3
a Wavelengths in nm, oscillator strengths given in parentheses.
that the rate of S1 → T1 intersystem crossing dominates the decay
of the S1 state, the rate constant of the intersystem crossing is
thus determined to be ~(50 ps)-1 equivalent to 2.0 ¥ 1010 s-1.
A similar procedure is applied for complexes 1 and 2 and the
resulting time constant of intersystem crossing is indicated by
the inverse of the observed fluorescence lifetimes (Table 1), which
are deduced to be 7.7 ¥ 1010 s-1 and 5.0 ¥ 1010 s-1, respectively.
As for 4–7, the rate of intersystem crossing is apparently much
slower, so that a time-correlated single photon counting technique
was applied to resolve the lifetime of fluorescence (Table 1).
Note that the rate of intersystem crossing for 4–7 should be
f
f
slower than 1/t obs, where t
is the observed lifetime of the
obs
fluorescence. As a result, the data shown in Table 1 clearly
f
indicates that the rate of intersystem crossing (1/t obs) for 4–7
is ~1–2 ¥ 109 s-1, which is apparently smaller than that of 1–3 by
more than one order of magnitude, consistent with the decrease
of %MLCT in the lowest lying transition state in a qualitative
manner.
Computational studies
We investigated the structural and photophysical characteristics
of the Au(I) complexes 1–7 and the phosphine ligand L3 by
using ab initio quantum chemical methods (CC2/TZVP level
of theory). First, we optimized the structures at the S0 ground
state. Comparison of the interatomic distances in the optimized S0
ground state and the X-ray structure of the complex 6 showed the
theoretical and experimental structures to be in good agreement:
Fig. 6 Transition densities for the S0 → S1 and S0 → S2 singlet excitations
of complexes 1–4 (CC2/TZVP level of theory, density isovalue 0.002 a.u.).
During the transition, the electron density increases in the blue areas and
decreases in the red areas.
˚
˚
˚
˚
R(Au–C): 1.96 A vs. 1.992–1.996 A; R(Au–P): 2.26 A vs. 2.262–
p → p* character of the S0 → S1 excitation is also confirmed
by investigation of the photophysical characteristics of the pure
phosphine ligand L3 (see ESI† for details).
˚
˚
2.276 A; R(C6H4–C6H4): 1.48 A vs. 1.476–1.487 A. Next, we
determined the singlet excitation spectra of the structures at the
optimized S0 geometry. The calculated excitation features are listed
in Table 2. The theoretical S0 → S1 and S0 → S2 excitation
wavelengths are in good agreement with the experimental ones,
in particular for the S0 → S1 transitions. The transition densities
for the S0 → S1 and S0 → S2 singlet excitations shown in Fig. 6
shed light on the absorption properties of the complexes 1–4. The
transition densities for the complexes 5–7 and the ligand L3 are
shown in the ESI.† As the number of the phenylene rings in the
PPh2–C2–(C6H4)n–C2–PPh2 ligand increases, the contribution of
the Au atoms and the alkynyl ligands to the S0 → S1 excitation
decreases, and for the complex 4, the excitation is practically a
pure p → p* excitation within the phosphine ligand. The singlet
excitations of complexes 5–7 are very similar to complex 4, as can
also be expected from the identical excitation wavelengths. The
To determine the emission energies for both fluorescence and
phosphorescence, we first optimized the structures of the S1, S2
and T1 excited states. The emission energies for the complexes
were then determined at the optimized excited state geometries.
The calculated wavelengths of the emission features are listed in
Table 2. The predicted emission wavelengths are not as accurate
as the predicted excitation energies, but their trends are still in
agreement with the experimental values. The transition densities
for the S1 → S0 and T1 → S0 emissions of the complexes 1–4 are
shown in Fig. 7. The T1 → S0 emission characteristics of complexes
5–7 are very similar to those of complex 4 (see ESI† for details).
For complex 4, both the lowest energy singlet and triplet emissions
are of p* → p type within the phosphine ligand, while for 2 and
3, the Au atoms and alkynyl ligands also contribute to the singlet
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Acknowledgements
We thank the University of Eastern Finland (Finnish–Russian
collaborative grant and Spearhead project), Academy of Finland
(grant 138560/2010, A.J.K.) and St.-Petersburg State University
(grant 12.37.132.2011) for financial support.
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