Strongly luminescent gold(III) complexes with long-lived excited states: High emission quantum yields, energy up-conversion, and nonlinear optical properties

Article abstract of DOI:10.1002/anie.201301149

Photochemistry: A series of emissive gold(III) complexes with fluorene-containing cyclometalating ligands exhibits strong phosphorescence and long-lived excited states with emission quantum yields and lifetimes up to 58 % and 305 μs, respectively. These c

Full text of DOI:10.1002/anie.201301149

Angewandte
Chemie
Communications
Luminescence
Photochemistry: A series of emissive
gold(III) complexes with fluorene-con-
taining cyclometalating ligands exhibits
strong phosphorescence and long-lived
excited states with emission quantum
yields and lifetimes up to 58% and
305 ms, respectively. These complexes
can sensitize energy up-conversion of
9,10-diphenylanthracene (DPA; see pic-
ture) and display rich two-photon
absorption properties (TPA; TTA=trip-
let–triplet annihilation).
W.-P. To, K. T. Chan, G. S. M. Tong, C. Ma,
W.-M. Kwok, X. Guan, K.-H. Low,
C.-M. Che*
&&&& — &&&&
Strongly Luminescent Gold(III)
Complexes with Long-Lived Excited
States: High Emission Quantum Yields,
Energy Up-Conversion, and Nonlinear
Optical Properties
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DOI: 10.1002/anie.201301149
Luminescence
Strongly Luminescent Gold(III) Complexes with Long-Lived Excited
States: High Emission Quantum Yields, Energy Up-Conversion, and
Nonlinear Optical Properties**
Wai-Pong To, Kaai Tung Chan, Glenna So Ming Tong, Chensheng Ma, Wai-Ming Kwok,
Xiangguo Guan, Kam-Hung Low, and Chi-Ming Che*
Long-lived, strongly emissive triplet excited states such as
that of the TbIII complex of N1,N1’,N1’’,N1’’’-(ethane-1,2-diylbi-
s(azanetriyl))tetrakis(ethane-2,1-diyl)tetrakis(2-hydroxy-N3-
methylisophthalamide) and PdII porphyrins[1] can have pro-
found impact in diverse areas including photocatalysis[2] and
optical imaging.[3] Triplet excited states of transition-metal
complexes with lifetimes of over hundreds of microseconds
usually have little metal character as the latter facilitates
radiative decay through a spin–orbit coupling mechanism. In
this context, luminescent AuIII complexes are promising
systems as AuIII ion usually contributes little to the emissive
triplet excited states. As up to now, research on luminescent
gold(III) complexes is in its infancy with the reported
examples hardly showing emission quantum yields over
15% in solutions at room temperature.[4,5] Herein we show
that incorporation of a fluorene moiety adjacent to a C donor
atom of tridentate cyclometalated C^N^C ligands
Scheme 1. Chemical structures of the gold complexes.
(Scheme 1) changes the lowest triplet excited state of the
AuIII complexes from intraligand charge transfer to ligand-
centered p-p* one. Such a subtle structural modification gives
rise to strongly luminescent AuIII complexes with unprece-
dentedly high emission quantum yields of up to 58% and
excited state lifetimes over 200 ms in solutions at room
temperature. These strongly emissive, long-lived excited
states are instrumental to the observation and investigation
of two-photon photophysical and photochemical properties of
luminescent gold(III) systems.
The structures of the gold(III) complexes 1a–6a are
shown in Scheme 1. Details for their synthesis and character-
ization are given in the Supporting Information. X-ray crystal
structures of 2a and 6a have been determined.[6] As shown in
Figure 1, the Au atoms of both 2a and 6a adopt a distorted
square-planar geometry with C–Au–C angles of 162.3(5)–
162.5(1)8. The Au–C (C^N^C ligand) and Au–N distances are
2.058(3)–2.082(12) ꢀ and 1.958(10) –2.005(2) ꢀ, respectively,
both of which are similar to those of other cyclometalated
gold(III) complexes. Interestingly, the Au–C(acetylide) dis-
tance of 2a is 1.915(11) ꢀ, which is notably shorter than
related reported Au–C(acetylide) distances.[4a,b,5a]
[*] Dr. W.-P. To, K. T. Chan, Dr. G. S. M. Tong, Dr. C. Ma, Dr. X. Guan,
Dr. K.-H. Low, Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry
Institute of Molecular Functional Materials and
Department of Chemistry, The University of Hong Kong
Pokfulam Road, Hong Kong (China)
E-mail: cmche@hku.hk
Prof. Dr. C.-M. Che
In dichloromethane solutions, complexes 1a–5a show
intense absorption bands at 250–350 nm (e = 2–5 ꢁ
104 dm3 molꢀ1 cmꢀ1) and moderately intense absorption
bands at 375–440 nm (e = 1–2 ꢁ 104 dm3 molꢀ1 cmꢀ1); 6a exhib-
its intense absorption bands at 250–320 nm (e = 2 ꢁ 104–1.2 ꢁ
HKU Shenzhen Institute of Research and Innovation
Shenzhen 518053 (China)
Dr. W.-M. Kwok
Department of Applied Biology & Chemical Technology
The Hong Kong Polytechnic University
Hung Hom, Kowloon, Hong Kong (China)
105 dm3 molꢀ1 cmꢀ1)
and
370–410 nm
(e = 1–3 ꢁ
104 dm3 molꢀ1 cmꢀ1). These absorption bands are assigned to
intraligand transitions localized on the C^N^C ligand.
All of these gold(III) complexes having a C(fluorene)-
donor atom display vibronic structured emissions with peak
maxima at 538–546 nm and vibrational spacings of about
1300 cmꢀ1 in CH2Cl2 at room temperature; these emissions
are assigned to metal-perturbed intraligand transitions of the
C^N^C ligand (Figure 2). Their solution emission quantum
[**] This work was supported by the Area of Excellence Scheme (grant
number AoE/P-03/08), the National Key Basic Research Program of
China (grant number 2013CB834802) and the CAS-Croucher
Foundation Funding Scheme for Joint Laboratories, and in part
using the HKU Information Technology Services research comput-
ing facilities supported by the Hong Kong UGC Special Equipment
Grant (SEG HKU09).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301149.
2
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fluorescence with the fluorescence lifetime
determined to be about 0.58 ps for 4b (lmax
ꢁ 465 nm), about 0.52 ps for 6a (lmax
ꢁ 425 nm), and about 0.15 ps for 4bb (lmax
ꢁ 430 nm). The sub-picosecond fluorescence
decay is indicative of the presence of
extremely fast nonradiative deactivation of
the S1 state which we attribute to intersystem
crossing (ISC) from the singlet to triplet
manifold with nearly unitary quantum effi-
ciency (Fisc ꢁ 1.0).[4c] With this, the phosphor-
escence quantum yields of the complexes
investigated are determined solely by the
nature and dynamic competition between
the radiative (kr) versus the nonradiative
(knr) decay of the emitting triplet excited
states. The values of kr and knr which were
derived from the measured Fem and t0 are
Figure 1. ORTEP representations of 2a (left) and 6a (right) with ellipsoids set at 35%
probability. Hydrogen atoms and solvent molecules are omitted for clarity.
listed in Table 1. It is clear that the core cause for the
remarkably high Fem and the exceptionally long t0 of the AuIII
Table 1: Emission data of the AuIII complexes.
kr [103 sꢀ1 [c]
]
knr [103 sꢀ1 [d]
]
[b]
Complex lmax [nm]
(t0 [ms])[a]
Fem
1a
2a
3a
4a
4b
4c
4d
5a
6a
540, 580 (182)
0.43
0.44
0.51
0.55
0.58
0.40
0.13
0.45
2.4
2.1
2.5
2.4
2.4
1.3
1.1
1.8
0.35
3.1
2.7
2.4
1.9
1.7
2.0
7.3
2.3
6.6
540, 579 (207)
540, 578 (201)
539, 579 (233)
538, 579 (242)
546, 583 (305)
545, 583 (120)
540, 579 (244)
Figure 2. a) UV/Vis absorption spectra of 4b, 4c, and 6a in dichloro-
methane. b) Emission spectra of 4b, 4c, and 6a in degassed dichloro-
methane solutions (1ꢀ10ꢀ5 moldmꢀ3) at room temperature. The
asterisk indicates a second-order transmission of the 350 nm excita-
tion.
404, 521, 560 (143) 0.05
yields are in most cases over 0.40 and up to 0.58, which is
unprecedented in the reported luminescent gold(III) com-
plexes. Besides the commonly encountered acetylide and N-
heterocyclic carbene ligands, cyanide ligand is also effective
to switch on the intraligand state (3IL) emission of cyclo-
metalated AuIII complexes and in this work, an emission
quantum yield of 0.40 for 4c is observed. The solvent effect on
the emission of 4b has been examined. Both the emission
quantum yield and lifetime of 4b in THF (F = 0.58, t =
259 ms) are similar to those values in CH2Cl2, but these
values mildly decrease when the solvent is changed to CH3CN
(F = 0.42, t = 204 ms) and MeOH (F = 0.37, t = 193 ms). The
self-quenching rate constants for the emissions of 1a–5a and
4b–4d (5.0 ꢁ 106–8.7 ꢁ 107 moldmꢀ3 sꢀ1) are significantly
lower than that of 6a (1.7 ꢁ 109 moldmꢀ3 sꢀ1) presumably the
dialkylfluorene moiety on the C^N^C ligand strongly dis-
favors intermolecular close contact of the gold complexes.
The simultaneous high emission quantum yields (Fem up
to 58%) and long emission lifetimes (t0 up to 305 ms) of these
AuIII complexes are striking. Indeed, the emission quantum
yields of 1a–5a and 4b–4c are more than 100-folds higher
than the related Au(C^N^C) complexes reported in liter-
ature.[5a] Femtosecond time-resolved emission measurements
(350 nm excitation) with dichloromethane solutions of 4b, 6a,
and a closely related AuIII complex 4bb revealed (see the
Supporting Information) a very rapid decay of the S1
[a] Phosphorescence lifetime. [b] Phosphorescence quantum yield mea-
sured at 1ꢀ10ꢀ5 moldmꢀ3 at room temperature using [Ru(bpy)3][PF6]2 as
standard. [c] Radiative decay rate constant estimated by kr =F/t.
[d] Nonradiative decay rate constant estimated by knr =(1ꢀF)/t.
complexes (1a–4a, 4b, 4c, and 5a) is their low kr ( ꢁ 1.3–2.5 ꢁ
103 sꢀ1) combined with a similarly low or even lower knr
( ꢁ 1.7–3.1 ꢁ 103 sꢀ1), a trait which is unique when compared
to the luminescent AuIII complexes in literature. The some-
what lower/shorter Fem/t0 displayed by 4d and 6a is due to
modestly reduced kr ( ꢁ 0.4–1.1 ꢁ 103 sꢀ1) and a slight increase
in knr ( ꢁ 6.6–7.3 ꢁ 103 sꢀ1). On the other hand, the markedly
lower/shorter Fem/t0 of the reported parental 4bb (in this
work: Fem = 0.0004; t0 = 0.017 ms; kr = 2.35 ꢁ 104 sꢀ1; knr =
5.9 ꢁ 107 sꢀ1) is obviously a result of its much faster knr,
though its greater kr also contributes. The drastic difference
between 4b and 4bb in terms of kr and especially knr implies
that the triplet states of these two apparently similar
complexes feature distinctively different electronic and/or
structural characters. This is substantiated by DFT and
TDDFT calculations revealing that the emitting triplet of
4b is the C^N^C centered pp* state whereas that of 4bb is
the ligand-to-ligand charge-transfer (LLCT) state in parent-
age. Thus in contrast to 4b with its triplet and S0 states having
similar structures and thus slow knr, 4bb possesses a low-lying
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and structurally highly distorted triplet state which serves to
promote effective nonradiative decay (to S0) thereby leading
to very large knr.
DFT/TDDFT calculations performed on 4b and 4bb
showed that, for both complexes, their HOMOs are mainly
localized on the phenylacetylide ligand, and in each case the
H-1 (second highest occupied molecular orbital) is a p-
(C^N^C) orbital and the LUMO is predominantly a p*-
(C^N^C) orbital in character (see the Supporting Informa-
tion for the MOs). When one of the phenyl rings of the
C^N^C cyclometalated ligand is replaced by a fluorenyl ring,
the separations between HOMO and H-1 orbitals at the
ground state geometry decrease from 0.64 eV (4bb, benzene)
to 0.20 eV (4b, fluorene). This is attributed to the more
conjugated fluorene leading to destabilization of the p-
(C^N^C) orbital. Thus the lowest singlet excited state, S1,
of 4bb, comes from a HOMO!LUMO transition and is
1
assigned to LLCT. On the other hand, the S1 state of 4b is
made up mainly of a H-1!LUMO transition and is best
1
described as a pp* excited state localized on the C^N^C
cyclometalated ligand.
TDDFT calculations (M062X/6-311G*(SDD)) based on
triplet geometries of 4b and 4bb gave emission wavelengths
of 580 nm (4b) and 454 nm (4bb-1), respectively, which match
with the experimental data (538 nm, 4b; 473 nm, 4bb). The
3
lowest triplet excited state, T1, of 4b is a pp* excited state
localized on the C^N^C cyclometalated ligand with an
optimized geometry similar to that of S0 (Figure 3). The
highly similar geometries of the T1 and S0 states means that
the nonradiative decay rate constant (knr) of T1 to S0 has to be
small. For 4bb in the ground state (S0), as the phenylacetylide
ligand can be coplanar or perpendicular to the C^N^C ligand,
two forms of 4bb were optimized as 4bb-1 (coplanar) and
4bb-2 (perpendicular), respectively.[5a] 4bb-1 is only 0.1 kcal
molꢀ1 more stable than 4bb-2. Starting from S0 of 4bb-1,
optimization of the triplet state gave a coplanar structure with
Figure 3. Comparison of the changes of key geometrical parameters of
triplet state (T1) with respect to the corresponding ground state (S0;
ground states in regular fonts, triplet states in italic fonts). 4b: C3–
C4=1.222/1.222, C4–C5=1.425/1.426, N–Au–C3=179.5/179.6, Au–
C1–N–C2=0.1/0.1, C1-Au-C5-C6=50.7/52.9 and 4bb-2: C3–
C4=1.221/1.232, C4–C5=1.426/1.416, N–Au–C3=180.0/179.6, Au–
C1–N–C2=0.0/20.6, C1-Au-C5-C6=ꢀ90.0/ꢀ99.5.
(Figure 4). Excitation of either 4b or DPA alone under the
same conditions did not invoke any delayed fluorescence.
Quenching experiments revealed that the rate constant of
quenching of 4b* by DPA is 1.87 ꢁ 109 dm3 molꢀ1 sꢀ1 revealing
an efficient energy transfer from 4b* to DPA. The delayed
fluorescence intensity of DPA showed a quadratic depend-
ence (y = x2) on the excitation laser power density (Wcmꢀ2),
confirming that the delayed fluorescence is generated by
a two-photon process (see the Supporting Information).[7] To
achieve up-conversion by means of TTA, we employed laser
light at 476 nm to selectively excite the sensitizer and
measured the fluorescence of DPA generated. As shown in
Figure 4b, excitation of a mixture of sensitizer and DPA by
a laser at 476 nm generated the fluorescence of DPA with
complete quenching of the phosphorescence of the sensitiz-
ers. The up-conversion quantum yields were estimated to be
3.5–9.8% (Table 2).
3
its emission originating from the LLCT excited state. The
geometrical difference between T1 and S0 states of 4bb-1 is
small, which means that the nonradiative decay rate constant
(knr) of T1 to S0 in 4bb-1 is slow. However in the case of 4bb-2
which is just 0.1 kcalmolꢀ1 higher than 4bb-1, the optimized
T1 of 4bb-2 was found to be 5.3 kcalmolꢀ1 more stable than
the above optimized T1 of 4bb-1 having a coplanar structure.
Population of electron density into the ds* orbital of Au–
C^N^C in its T1 results in a highly distorted geometry with
a dihedral angle of 20.68 between Au–C1 and N–C2, and with
the bond lengths of Au–C(C^N^C) and Au–N(C^N^C)
substantially increased to 2.194 and 2.215 ꢀ, respectively (see
Figure 3). Such dramatic excited-state structural distortion
provides a fast channel for nonradiative decay, accounting for
the very low emission quantum yield (4 ꢁ 10ꢀ4) of 4bb.
In view of their high emission quantum yields and long
excited-state lifetimes, we have examined the capacity of the
present AuIII complexes to sensitize triplet–triplet annihila-
tion (TTA).[7,8] Excitation (355 nm, Nd:YAG laser) of a mix-
ture of 4b (2 ꢁ 10ꢀ5 moldmꢀ3) and 9,10-diphenylanthracene
(DPA, 5 ꢁ 10ꢀ5 moldmꢀ3) in deoxygenated dichloromethane
resulted in the observation of both prompt and delayed
fluorescence of DPA and residual phosphorescence of 4b
In the literature, the main strategies used to construct
molecules with large two-photon absorption (TPA) cross-
sections are to increase the conjugation of the chromophore
and to incorporate donor and acceptor moieties so as to
create a large dipole moment.[9] Without undertaking the
aforementioned structural modification, some of the present
AuIII complexes could give the green 538–540 nm emission
(clearly visible to the naked eye, Figure 5) by excitation with
4
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a focused 756 nm laser beam, the spectra of which
are the same as their corresponding emission
spectra obtained by excitation with a xenon lamp
at 400–430 nm. As all of these complexes do not
show any linear absorption at 756 nm, the emission
generated by excitation at 756 nm can be attributed
to TPA-induced emission. The two-photon absorp-
tion cross sections (s2) were estimated to be 3.4–
9.4 GM (Table 2) by the literature method with
fluorescein as reference.[10]
As shown in Figure 5, the emission intensity of
4b displays a quadratic dependence (y = x2) on the
laser power density, confirming that two light
photons at 756 nm are required for one emitted
photon. As 4b could sensitize TTA and displays
TPA properties, 4b was used to generate the
fluorescence of DPA by a TTA process.[11] When
a deoxygenated solution of 4b and DPA in dichloro-
methane was brought to the focus point of the
Figure 4. a) Time-resolved emission spectra of a mixture of 4b (2ꢀ10ꢀ5 moldmꢀ3
and DPA (5ꢀ10ꢀ5 moldmꢀ3) in degassed dichloromethane recorded from 1 to
21 ms after laser pulse excitation (355 nm). The red arrows indicate the change in
)
emission intensity. b) Emission spectra of mixtures of DPA (5ꢀ10ꢀ4 moldmꢀ3) and
different sensitizers (2ꢀ10ꢀ4 moldmꢀ3) in degassed dichloromethane using
476 nm laser light for excitation. Data from 471 to 481 nm were not recorded
because of the strong scattering of the laser.
Table 2: Up-conversion and two-photon absorption data for selected
complexes.
756 nm laser beam, a blue line with an emission spectrum
identical to the fluorescence spectrum of DPA has been
recorded (Figure 6). Control experiment revealed that this
blue line emission can be observed only when 4b is present.
Importantly, a remarkably high anti-Stokes shift of 1.36 eV
(756!413 nm) can be achieved by combining TPA and TTA.
The emission intensity of DPA caused by TPA-induced TTA
was reported to display a quartic (y = x4) incident light power
Complex
FUC [%][a]
s2 [GM][b]
Power dependence[c]
2a
3a
4a
4b
4c
4d
4.9
3.5
5.9
9.8
7.9
5.7
7.4
7.8
3.4
9.4
3.5
4.4
1.89
1.87
2.09
1.87
–
–
[a] Up-conversion quantum yield in dichloromethane using 4-(dicyano-
methylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran as standard
(F=0.10 in dichloromethane).[8d] [b] Two-photon absorption cross-
section determined using fluorescein as standard. [c] Slope of the
double-log plot of the emission intensity induced by two-photon
absorption against the excitation power density.
Figure 6. Emission of a mixture of 4b (2ꢀ10ꢀ4 moldmꢀ3) and DPA
(5ꢀ10ꢀ4 moldmꢀ3) in dichloromethane induced by two-photon absorp-
tion excited with 756 nm laser light. Left: Photos showing the emission
of a mixture of 4b (2ꢀ10ꢀ4 moldmꢀ3) and DPA (5ꢀ10ꢀ4 moldmꢀ3) in
degassed dichloromethane excited with a 756 nm laser. Right: Emis-
sion spectra using different excitation power densities.
dependence.[11] In this study, the power dependence obtained
from a double-log plot of the emission intensity against the
excitation power density was found to be 3.31 (i.e. y = x3.31),
suggesting that it is at least a three-photon process.
While applications of TPA in fluorescence microscopy[12]
and photodynamic therapy[13] are well-documented, research
regarding the photochemistry/photocatalysis stemming from
TPA is utterly unexplored.[14] The importance of TPA-induced
photochemistry lies in the use of light radiation that is
normally not absorbed by transition-metal complexes (sensi-
tizers) (l > 700 nm). In our initial trials, we found that:
1) Three turnovers of imines were furnished by irradiating
a mixture of 4b and dibenzylamine with laser light at 756 nm
under aerobic conditions for 2 h. 2) Formation of the methyl
viologen radical cation (MV+C) was observed by irradiating
Figure 5. Emission of 4b (2ꢀ10ꢀ4 moldmꢀ3) in dichloromethane
excited with 756 nm laser light. a and b) Photos showing the emission
of solution. c) Emission spectra using different excitation power
densities. d) Plot of emission intensity against excitation power density
(W cmꢀ2). e) Double-log plot of emission intensity against excitation
power density.
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a mixture of 4b, MV2+, and triethanolamine (TEOA) in
degassed acetonitrile with 800 nm laser light (Scheme 2).
Control experiments showed that in both cases no photo-
chemistry could be observed in the absence of 4b. With these
preliminary results, we conceive that solar energy, which is
concentrated in the near-infrared spectral region, could be
harnessed for the photochemical reactions of phosphorescent
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[6] CCDC 919888 (6a) and 919889 (2a) contain the supplementary
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Scheme 2. Left: Photochemical reactions catalyzed by 4b using a near-
infrared laser as light source. Right: Photos showing the color of the
solution of reaction 2 in 1) the absence and 2) presence of 4b after
irradiation by 800 nm laser light for 30 minutes (MV2+ =methyl viol-
ogen dication).
gold(III) complexes that do not absorb strongly in the visible
to near-infrared spectral region.
In conclusion, a series of metal–organic gold(III) com-
plexes which display unprecedentedly high emission quantum
yields and long-lived excited states have been synthesized. We
have also described, for the first time, the TTA and TPA
properties of metal–organic gold(III) complexes and the use
of TPA for photoinduced oxidation and electron-transfer
reactions.
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113, 9266 – 9269.
Received: February 8, 2013
Revised: April 22, 2013
Published online: && &&, &&&&
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