Reversible protonation of amine-functionalized luminescent Au-Cu clusters: Characterization, photophysical and theoretical studies

Article abstract of DOI:10.1039/b922803a

Reaction of the polymeric alkynyl complexes (AuC₂C ₆H₄R)_n (R = 4-NH₂ and 3-NH ₂) with the diphosphine PPh₂C₆H ₄PPh₂ in the presence of Cu⁺ ions gave two novel heterometallic aggregates [{Au₃Cu₂(C ₂C₆H₄R)₆}Au₃(PPh ₂C₆H₄PPh₂)₃](PF ₆)₂ (R = 4-NH₂ (2), 3-NH₂ (3)). The compounds obtained were characterized by NMR spectroscopy and ESI-MS measurements. The solid-state structure of their 4-NMe₂ congener 1 is reported. The complexes 1-3 reversibly react with strong (HSO₃Me and HSO₃CF₃) acids to give the adducts [{Au ₃Cu₂(C₂C₆H₄-R) ₆*(R′SO₃H)₆}Au₃(PPh ₂C₆H₄PPh₂)₃](PF ₆)₂ (R = 4-NMe₂ (4), 4-NH₂ (5), 3-NH₂ (6)) with six acid molecules bound to the amine groups of the alkynyl ligands. Composition and structure of the adducts were established using ESI-MS and multinuclear (³¹P, ¹H and ¹H- ¹H COSY) NMR spectroscopy. It was found that formation of these adducts results in crucial changes of luminescence characteristics of the complexes 1-3 to give substantial (ca. 100 nm) blue shift of the emission maxima and a sharp increase (about an order of magnitude) in luminescence quantum yield for 4-NR₂ substituted derivatives. In the case of 3-substituted complex 3 the effect of adduct formation is much less pronounced and leads to blue-shift of emission maximum for 30 nm accompanied with a small drop in emission quantum yield. Computational studies have been performed to provide additional insight into the structural, electronic and photophysical properties of the starting complexes and their acid adducts. Interpretation of the photophysical effects induced by the adduct formation was suggested. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b922803a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Reversible protonation of amine-functionalized luminescent Au–Cu clusters:
characterization, photophysical and theoretical studies†
Igor O. Koshevoy,*a Antti J. Karttunen,a Sergey P. Tunik,*b Janne Ja¨nis,a Matti Haukka,a Alexei S. Melnikov,c
Pavel Yu. Serdobintsevc and Tapani A. Pakkanen*a
Received 2nd November 2009, Accepted 17th December 2009
First published as an Advance Article on the web 25th January 2010
DOI: 10.1039/b922803a
Reaction of the polymeric alkynyl complexes (AuC2C6H4R)n (R = 4-NH2 and 3-NH2)
with the diphosphine PPh2C6H4PPh2 in the presence of Cu+ ions gave two novel heterometallic
aggregates [{Au3Cu2(C2C6H4R)6}Au3(PPh2C6H4PPh2)3](PF6)2 (R = 4-NH2 (2), 3-NH2 (3)). The
compounds obtained were characterized by NMR spectroscopy and ESI-MS measurements.
The solid-state structure of their 4-NMe2 congener 1 is reported. The complexes 1–3
reversibly react with strong (HSO3Me and HSO3CF3) acids to give the adducts
[{Au3Cu2(C2C6H4-R)6*(R¢SO3H)6}Au3(PPh2C6H4PPh2)3](PF6)2 (R = 4-NMe2 (4), 4-NH2 (5), 3-NH2
(6)) with six acid molecules bound to the amine groups of the alkynyl ligands. Composition and
structure of the adducts were established using ESI-MS and multinuclear (31P, 1H and 1H-1H COSY)
NMR spectroscopy. It was found that formation of these adducts results in crucial changes of
luminescence characteristics of the complexes 1–3 to give substantial (ca. 100 nm) blue shift of the
emission maxima and a sharp increase (about an order of magnitude) in luminescence quantum yield
for 4-NR2 substituted derivatives. In the case of 3-substituted complex 3 the effect of adduct formation
is much less pronounced and leads to blue-shift of emission maximum for 30 nm accompanied with a
small drop in emission quantum yield. Computational studies have been performed to provide
additional insight into the structural, electronic and photophysical properties of the starting complexes
and their acid adducts. Interpretation of the photophysical effects induced by the adduct formation was
suggested.
characteristics of this group of compounds are determined by
the specific features of their molecular and electronic structure.
Introduction
Synthesis, structural characterization and investigation of d10
It is also interesting from the viewpoint of practical applications
coinage metal complexes remain in the focus of many research
that the luminescent properties of these complexes may be easily
groups because of their unique photophysical characteristics and
fine-tuned through variations of the metal core composition and
growing potential of their application in various fields of modern
electron donor ability of the alkynyl ligand substituents. For
technology such as dopant emitters, NLO devices and OLED
example, Au–Ag heterometallic complexes structurally analogous
displays.1 Another point of academic interest in this class of
to the Au–Cu compounds display hypsochromic shift of the
compounds consists of the ability of the d10 coordination centers
to give secondary homo- and heterometallophilic bonds that
results in formation of supramolecular architectures with unusual
chemical and physical properties.2 In our recent papers3-5 we
present a series of Au–Cu and Au–Ag alkynyl-phosphine com-
plexes, which display extremely effective phosphorescence with
nearly negligible oxygen quenching. The unique photophysical
luminescence maxima of about 100 nm,3,6 whereas an increase in
basicity of the alkynyl substituents in the gold-copper compounds
[Au6Cu2(C2C6H4-R)6(PPh2C6H4PPh2)3]2+, R varies from 4-NO2
to 4-NMe2,4 gives a bathochromic shift of nearly the same
magnitude. It is also well known that protonation of the aromatic
and aliphatic amine groups in coordinated ligands may result
in substantial variations of luminescent properties (lem, tobs
,
luminescent quantum yield) of the corresponding complexes.
Depending on the nature of the orbitals responsible for emission
and their relation to the amine functionality the protonation of
the coordinated ligands may decrease7 or even completely quench8
luminescence whereas for the other complexes emission quantum
yield can be substantially increased9 upon addition of a proton to
an amine group. The complexes, which display various types of
pH sensitive luminescence, can be used for the design of specific
pH sensors as well as for the construction of pH switchable “on-
off” molecular devices. In this respect we became interested in the
experimental and theoretical investigations of protonation effects
on the photoluminescent properties of the previously reported
aUniversity of Joensuu, Joensuu, 80101, Finland. E-mail: igor.koshevoy@
joensuu.fi, tapani.pakkanen@joensuu.fi; Fax: +358 13 2513390; Tel: +358
13 2513326
bSt.-Petersburg State University, Department of Chemistry, Universitetskii
pr. 26, 198504, St.-Petersburg, Russia. E-mail: stunik@inbox.ru; Fax: +7
812 4283969; Tel: +7 812 4284028
cSt.-Petersburg State University, Department of Physics, St.-Petersburg,
Russia
† Electronic supplementary information (ESI) available: X-ray crystallo-
graphic data in CIF for 1; ESI-MS spectra of 2, 3 and the protonated forms
of 1–3; selected NMR spectra of 2, 3; optimized Cartesian coordinates of
1–6. CCDC reference numbers 753058. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/b922803a
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cluster [Au6Cu2(C2C6H4-4-NMe2)6(PPh2C6H4PPh2)3]2+4 bearing 4-
NMe2 groups and of its novel relatives containing 4-NH2 and 3-
NH2 substituents in the phenylene spacers of the alkynyl ligands.
performed by micropipette addition of a fresh aliquot of MeSO3H
to a sample solution of the corresponding complex. Exhaustive
protonation was usually accomplished with ca. 100-fold excess of
acid.
Experimental
NMR characteristics of protonated forms of the complexes
1,4-Bis(diphenylphosphino)benzene10 and [{Au3Cu2(C2C6H4-
NMe2)6}Au3(PPh2C6H4PPh2)3][PF6]2 (1)4 were synthesized ac-
cording to published procedures. Complexes {Au(1-C2-C6H4-R)}n
[{Au3Cu2(C2C6H4-4-NMe2)6*(MeSO3H)6}
Au3(PPh2C6H4-
1
PPh2)3][PF6]2 (4). 31P{ H} NMR (acetone-d6; d): 44.6 (s, 3P),
1
-144.8 (sept, 1P, PF6). H NMR (acetone-d6; d): {Au(C2C6H4-
11
(R = 4-NH2, 3-NH2) were prepared analogously to (AuC2Ph)n
4-NMe2)2}: 7.01 (d, H-orto, 12H, J(H–H) 7.8 Hz), 7.41 (d,
H-meta, 12H, J(H–H) 7.8 Hz), 3.27 (s, 36H, NMe2) diphosphine:
8.00 (dm(AXX¢), H-ortho, 24H, J(H–H) 7.6, J(P–H) 13.5 Hz),
7.86 (m(A2X2), {P–C6H4–P} 12H, <J(P–H)> 3.3 Hz), 7.68 (t,
H-para, 12H, J(H–H) 7.3 Hz), 7.49 (dd, H-meta, 24H, J(H–H)
7.6, 7.3 Hz).
using commercially available alkynes 1-HC2-C6H4-R. Anhydrous
dichloromethane (Aldrich) was used for the photophysical mea-
surements. Other reagents and solvents were used as received. The
1
1
solution 1D H, 31P NMR and H COSY spectra were recorded
on Bruker Avance 400 and Bruker DPX 300 spectrometers. Mass
spectra were measured on a Bruker APEX-Qe ESI FT-ICR
instrument, in the positive ion mode. Microanalyses were carried
out in the analytical laboratories of the University of Joensuu and
St. Petersburg State University.
[{Au3Cu2(C2C6H4-4-NH2)6*(MeSO3H)6}
Au3(PPh2C6H4-
1
PPh2)3][PF6]2 (5). 31P{ H} NMR (acetone-d6; d): 44.7 (s, 3P),
1
-144.8 (sept, 1P, PF6). H NMR (acetone-d6; d): {Au(C2C6H4-
4-NH2)2}: ca 10.6 (br, HSO3Me*NH2), broaden AB system
of {C6H4-N}, J(H–H) ca 7 Hz, 6.97 (H-ortho, 12H) and 7.04
(H-meta, 12H) diphosphine: 8.01 (dm(AXX¢), H-ortho, 24H,
J(H–H) 7.6, J(P–H) 14 Hz), 7.92 (m(A2X2), {P–C6H4–P} 12H,
<J(P–H)> 3.3 Hz), 7.65 (t, H-para, 12H, J(H–H) 7.4 Hz), 7.48
(dd, H-meta, 24H, J(H–H) 7.6, 7.4 Hz).
General procedure for the preparation of heterometallic Au–Cu
complexes
{Au(1-C2-C6H4-R)}n (0.5 mmol) was suspended in CH2Cl2
(15 cm3) and a slight excess of a crystalline diphosphine
(0.28 mmol) was added. The reaction mixture was stirred
for 30 min in the absence of light and then a solution of
Cu(NCMe)4PF6 (0.17 mmol) in CH2Cl2 (5 cm3) was added
dropwise to afford a nearly transparent red solution (2) or orange
suspension (3). The solids were filtered off, the solvent was
removed on a rotovap to give the residues which were repetitiously
recrystallized by gas-phase diffusion of diethyl ether into their
dichloromethane solutions at +5 ◦C (1 was recrystallized by slow
evaporation of its acetone–methanol solution at +5 ◦C).
[{Au3Cu2(C2C6H4-3-NH2)6*(MeSO3H)6}
Au3(PPh2C6H4-
1
PPh2)3][PF6]2 (6). 31P{ H} NMR (acetone-d6; d): 44.9 (s, 3P),
1
-144.8 (sept, 1P, PF6). H NMR (acetone-d6; d): {Au(C2C6H4-3-
NH2)2}: ca 10.6 (br, HSO3Me*NH2), 7.33 (d, 4-H, 6H, J(H–H)
7.8 Hz), 7.12 (dd, 5-H, 6H, J(H–H) 7.8 Hz), 7.04 (d, 6-H,
6H J(H–H) 7.8 Hz), 6.96 (br m, 2-H, 6H) diphosphine: 7.99
(dm(AXX¢), H-ortho, 24H, J(H–H) 7.8, J(P–H) 14 Hz), 7.94
(m(A2¥2), {P–C6H4–P} 12H, <J(P–H)> 3.4 Hz), 7.64 (t, H-para,
12H, J(H–H) 7.5 Hz), 7.46 (dd, H-meta, 24H, J(H–H) 7.8,
7.5 Hz).
[{Au3Cu2(C2C6H4-4-NH2)6}Au3(PPh2C6H4PPh2)3][PF6]2
(2).
Dark red. Yield 63%. ES MS (m/z): [Au6Cu2(C2C6H4NH2)6-
1
(PPh2C6H4PPh2)3]2+ 1671 (calcd 1671). 31P{ H} NMR (acetone-
Photophysical measurements
1
d6; d): 43.0 (s, 3P), -144.8 (sept, 1P, PF6). H NMR (acetone-d6;
An LED (emission maximum at 385 nm) was used to pump
luminescence. The LED was used in continuous and pulse mode
(pulse width 1–20 ms, pulse rise time ~90 ns, repetition rate 100 Hz–
10kHz). A digital oscilloscope Tektronix TDS3014B (Tektronix,
bandwidth 100 MHz), monochromator MUM (LOMO, spectral
resolution 10 nm) and photomultiplier tube (Hamamatsu) were
used for life-time measurements. Emission spectra were recorded
using a HR2000 spectrometer (Ocean Optics). Halogen lamp
LS-1-CAL (Ocean Optics) and deuterium lamp DH2000 (Ocean
Optics) were used to calibrate the absolute spectral response of
the spectral system in the 200–1100 nm range. All solutions were
carefully degassed before life-time measurements. The absolute
emission quantum yield was determined by Vavilov’s method12
using LED (385 nm, continuous mode) pumping and rhodamine
6G in ethanol (Uem = 0.95 0.03) as standard.
d): {Au(C2C6H4-4-NH2)2}: 6.53 (d, H-ortho, 12H, J(H–H)
8.5 Hz), 6.20 (d, H-meta, 12H, J(H–H) 8.5 Hz) diphosphine:
7.94 (dm(AXX¢), H-ortho, 24H, J(H–H) 7.6, J(P–H) 13 Hz),
7.69 (m(A2X2), {P–C6H4–P} 12H, <J(P–H)> 3.3 Hz), 7.63 (t,
H-para, 12H, J(H–H) 7.5 Hz), 7.46 (dd, H-meta, 24H, J(H–H)
7.6, 7.5 Hz). Anal. Calc. for C138H108Au6Cu2F12N6P8: C, 45.60; H,
2.99; N, 2.31. Found: C, 45.99; H, 3.36; N, 2.30%.
[{Au3Cu2(C2C6H4-3-NH2)6}Au3(PPh2C6H4PPh2)3][PF6]2
(3).
Orange. Yield 62%. ES MS (m/z): [Au6Cu2(C2C6H4NH2)6-
1
(PPh2C6H4PPh2)3]2+ 1671 (calcd 1671). 31P{ H} NMR (acetone-
1
d6; d): 43.6 (s, 3P), -144.8 (sept, 1P, PF6). H NMR (acetone-d6;
d): {Au(C2C6H4-3-NH2)2}: 6.67 (dd, 5-H, 6H, J(H–H) 8.0 Hz),
6.50 (dm, 4-H, 6H, J(H–H) 8.0 Hz), ca 6.16 (m, 2-H and 6-H,
12H) diphosphine: 7.97 (dm(AXX¢), H-ortho, 24H, J(H–H) 8,
J(P–H) 14 Hz), 7.70 (m(A2X2), {P–C6H4–P} 12H, <J(P–H)>
3.4 Hz), 7.65 (t, H-para, 12H, J(H–H) 7.5 Hz), 7.48 (dd, H-meta,
24H, J(H–H) 8, 7.5 Hz). Anal. Calc. for C138H108Au6Cu2F12N6P8:
C, 45.60; H, 2.99; N, 2.31. Found: C, 46.01; H, 3.30; N, 2.26%.
Protonation of 1–3 in solution (CD3COCD3 for NMR ex-
periments and CH2Cl2 for photophysical measurements) was
Computational details
The structures of the studied complexes were optimized using
the BP86 density functional method.13 Standard GGA density
functionals such as BP86 cannot accurately describe the energetics
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of weak dispersive metal-metal interactions between closed-shell
metal atoms. However, the structural chemistry of the supramolec-
ular complexes studied here is dominated by the electrostatic
interactions between the Cu(I) ions, the [PhC2AuC2Ph] rods,
and the gold-diphosphine “belt”.4,6 The copper and gold atoms
were described with a triple-valence-zeta quality basis set with
polarization functions (def2-TZVP),14 employing a 60-electron
relativistic effective core potential for gold.15 A split-valence
basis set with polarization functions on non-hydrogen atoms
was used for all the other atoms (def2-SV(P)).16 The multipole-
accelerated resolution-of-the-identity technique was used to speed
up the calculations.17 Solvent effects were taken into account by
using the COSMO solvation model18 (solvent: CH2Cl2, dielectric
constant e = 9.1 at 20 ◦C). Due to the symmetric nature of
the studied systems, the reported computational results were
obtained using the following point group symmetries to facilitate
comparisons with the experiments: D3h for 1 and 2; C3h for
3–6. However, unconstrained optimizations were also performed
to check that the symmetry constraints did not introduce any
inconsistencies. All electronic structure calculations were carried
out with TURBOMOLE program package (version 5.10).19
Fig. 1 Schematic structure of the dications [Au6Cu2(C2C6H4-R)6(PP)3]2+
(1–3), (R = 4-NMe2, 4-NH2, 3-NH2).
Results and discussion
3 display singlet signals (43.0 ppm and 43.6 ppm) in accord
with the D3h symmetry of both molecules with the coordinated
phosphorus atoms positioned in the corners of the [Au3(PP)3]
Synthesis and characterization
In our recent publications we reported a synthetic strategy
to prepare a family of the heterometallic AuI–CuI clusters of
the general formula [Au6Cu2(C2C6H4-R)6(PPh2C6H4PPh2)3][PF6]2
(R = NO2, H, OMe, NMe2.4 Using this approach two novel
substituted derivatives (R = 4-NH2 and 3-NH2) were obtained
in a moderately good yield as shown in Scheme 1.
1
triangular “belt”. The 1D and COSY H NMR spectra of both
compounds (see Experimental and Fig. S2–S3†) show a typical set
of resonances in the low-field part of the spectra corresponding
to the protons of the diphosphine ligands. It is worth noting that
the position of these resonances and observed coupling patterns
are nearly identical to those found earlier4,6 for the other “rods-
in-belt” structures based on the PPh2C6H4PPh2 ligand, which is
a consequence of a very weak effect4 of the alkynyl substituents
on the phosphine proton shielding. The high field part of the
proton spectra of 2 and 3 displays the signals of protons of the
alkynyl phenylene spacers, which appear as AB system in 2 (4-NH2
substituent) and as a more complicated set of resonances for a less
symmetric phenylene moiety containing the NH2 substituent in
the 3-position. According to the relative intensities of the signals
1
observed in the H NMR spectra of 2 and 3 three diphosphine
ligands of the “belt” and all six alkynyl ligands of the central
{Cu2(Au(C2C6H4-R)2)3} core are equivalent, which is a clear
indication of the D3h symmetry of the molecules under study as
shown in Scheme 1.
Scheme 1
Clusters 2 and 3 didn’t give single crystals suitable for
X-ray diffraction analysis and their composition and structure
were established on the basis of analytical data, ESI-MS, NMR
spectroscopic study and comparison with the corresponding
parameters of the congeners mentioned above. Both compounds
appeared as doubly charged [Au6Cu2(C2C6H4-NH2)6(PP)3]2+
(PP = PPh2C6H4PPh2) molecular ions in the ESI-MS spectra, see
Fig. S1 (ESI†), with the isotopic patterns of these signals matching
completely the calculated ones. This observation is indicative
of preparation of the “rods-in-belt” supramolecular aggregate
closely analogous to the complexes synthesized earlier4 using other
substituted alkynyl ligands (Fig. 1).
It has been found that the clusters 1–3 containing amine
nitrogen in the peripheral part of the molecule can be protonated
using an excess of the strong non-coordinating sulfonic acids
(MeSO3H and CF3SO3H). Exhaustive protonation of all six amine
groups is completed under ca. 100-fold excess of the acids, the
methylsulfonic acid affords more stable protonated products.
Investigation of the acetone solution of 1 in the presence of an
excess of CH3SO3H using ESI-MS, Fig. 2, revealed a weak signal
of the non-protonated starting cluster M2+, observed at m/z 1755,
together with the following signals of protonated forms: [M +
MeSO3H + H]3+ (m/z 1202), [M + MeSO3H + 2H]4+ (m/z 902),
[M + 2MeSO3H + 2H]4+ (m/z 926).
The NMR spectroscopic data give further evidence in favor
of this structural hypothesis. The 31P NMR spectra of 2 and
These observations indicate that the starting supramolec-
ular “rods-in-belt” aggregate is not destroyed in the
2678 | Dalton Trans., 2010, 39, 2676–2683
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both [Au6Cu2(C2–C6H4-3-NH2)6*(H+)6(PP)3]8+ and [Au6Cu2(C2–
C6H4-3-NH2)6*(MeSO3H)6(PP)3]2+ stoichiometries for 6.
The 1H NMR experiments on the protonation of 1–3 were
carried out in deuterated acetone because of considerably higher
solubility of the protonated clusters in this solvent compared to
CD2Cl2. This allows precipitation of the products to be avoided
and keeps the solutions under study homogeneous. Stepwise ad-
dition of the acid to the starting solution of 1 (Fig. 3) showed very
minor changes in the low field signals corresponding to the diphos-
phine belt. In contrast, the signals of the NMe2 moiety and those
of the alkynyl-phenylene spacers immediately start to broaden,
which points to a dynamic exchange between partially protonated
forms of the cluster. As expected all these signals experience low
field drift, the phenylene spacer resonance corresponding to the
protons adjacent to amine group has been eventually shifted for
ca. 1.2 ppm. The resulting spectrum of 4 (Fig. 3, complex to acid
ratio 1 : 90) displays a spectroscopic pattern, which looks very
similar to that of the starting cluster. It contains the singlet of
methyl groups (protonated NMe2 fragment), the AB system of the
protons of phenylene spacers and nearly unchanged resonances
of the tri(gold-diphosphine) belt in the low field part of the
spectrum. The acidic protons associated with the amine groups
do not appear in the spectrum due to fast exchange with free acid
molecules. Relative intensities of the signals and their multiplicity
fit completely the D3h symmetry group of the molecule formed
upon protonation as shown in Scheme 1. It is hardly possible
to exclude the acid counterion dissociation but this or any other
dissociative process is obviously very fast in the NMR time scale,
which keeps observed spectroscopic patterns compatible with the
D3h symmetry of the adduct.
Fig. 2 ESI-MS spectrum of 1 in the presence of MeSO3H. Inset–spectrum
of 1 under acid-free conditions.
protonation reaction, which is evidently related to the amine
groups of the alkynyl ligands. Emergence of the signals containing
nondissociative acid molecules evidently stems from the unfavor-
able electrostatic effect of proton addition to the amine groups,
which would eventually bring six more positive charges to the
relatively small fragments of the molecule in the case of exhaustive
protonation. Compensation of the proton positive charges by
the acid counterion helps to relieve the electrostatic repulsion
and stabilizes the final product. It is also worth noting that
instability of the purely protonated adducts formed upon reaction
with stronger CF3SO3H acid (Fig. S4†) can be also considered
as an additional argument in favor of the acid counterion
stabilizing effect. ESI-MS experiments did not show the signals
corresponding to addition of five and six protons (acid molecules)
to the amine substituents very probably due to instability of these
aggregates under the conditions of the ESI-MS measurements, in
particular, due to extremely high dilution of the mixture that shifts
the equilibrium towards acid dissociation products. Nevertheless,
partial protonation of 1 (addition of less than six acid molecules)
in the presence of acid excess is not compatible with the NMR
spectroscopic data, vide infra, which clearly point to retention
of the D3h symmetry for the protonated product. Thus, the
[Au6Cu2(C2–C6H4-NMe2*HO3SMe)6(PP)3]2+composition can be
most probably ascribed to the final adduct formed upon exhaustive
protonation of 1.
[Au6Cu2(C2C6H4NMe2)6(PP)3]2++ MeSO3H ꢀ
[Au6Cu2(C2C6H4NMe2*HSO3Me)6(PP)3]2+
(1)
The ESI-MS pattern obtained for the reaction mixture con-
taining the protonated form of 2 displays an analogous set of
signals (Fig. S5†) that is indicative of essentially similar adduct
composition formed in this case. The ESI-MS spectrum of 3
in the presence of MeSO3H excess (Fig. S6†) displays a weak
signal of nonprotonated M2+ ion together with two signals of
mono- and doubly protonated ions, (M + H)3+ and (M +
2H)4+, respectively. No signals corresponding to coordinated acid
molecules or multiple protonated adducts were found in the
spectra. This observation can be explained by the meta amine
group location on the phenylene spacer in 3. Such stereochemistry
evidently results in orientation of the NH2 substituents outside the
cluster core and probably makes the addition of the acid molecules
sterically hindered because of the van der Waals contacts with the
diphosphine belt. However, the NMR spectroscopic data obtained
for the protonated adduct of 3 still require symmetrical and
exhaustive protonation of this complex that is compatible with
Fig. 3 1H NMR spectra of 1 in the aromatic region upon stepwise
addition of MeSO3H, concentration of 1 is ca. 5 ¥ 10-3 M, 298 K,
acetone-d6.
Essentially similar spectroscopic changes were observed upon
addition of MeSO3H to the acetone-d6 solutions of 2 and 3
(Fig. S7† and Experimental). The amine moieties in both 2 and
3 are easily protonated to give the adducts 5 and 6, respectively,
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which retain the structure of the starting clusters with six acid
molecules symmetrically added to the NH2 substituents. In the
proton spectra of 5 and 6 one can observe a typical set of
resonances corresponding to the tri(gold-diphosphine) belt, which
remains nearly unchanged upon protonation together with the
signals of the phenylene spacers being shifted downfield compared
to the starting clusters. The acidic proton bound to the amine
group [–NH2*HSO3Me] appears in the spectrum as a broadened
singlet at ca 10.6 ppm upon addition of a large excess of the acid.
Thus, the most probable structure of 5 and 6 resembles that shown
in Scheme 1 and Fig. 1 with the acid molecules (protons) bound
to the nitrogen atoms of the corresponding amine groups.
It has been found earlier that cluster 1 demonstrates decent
luminescence in dichloromethane solution20 which was ascribed to
the phosphorescence from the triplet excited state associated with
electron transitions between the orbitals located mainly within
the heterometallic cluster core. It was also shown that among
their close relatives containing various para-substituents (NO2,
H, OMe) in the phenylene spacers of the alkynyl ligands the
amine derivative shows the lowest quantum yield and longest
emission wavelength, the latter being ascribed4 to the strong donor
properties of the amine moiety, which increase the HOMO energy
to a higher extent compared to the shift of the LUMO. These
orbitals give the main contribution to the LSOMO-HSOMO pair
of the triplet emitting state for the complexes under study that
decreases the MO gap responsible for the emission observed. This
is indicative of fairly strong electronic communication between the
heterometallic cluster core and para-substituents of the aromatic
ring through the phenylene spacer.
In general luminescence of the complexes studied shows a
substantial Stokes shift (210–320 nm) and an emission lifetime
in the microsecond domain that is characteristic of its origination
from the triplet state manifold. However, in contrast to similarity in
absorption properties the phosphorescence characteristics of 1 and
2 differ substantially to those revealed for 3. Emission wavelengths
of 1 and 2 in dichloromethane solution are close each other (715
and 708 nm) whereas that of 3 (608 nm) is blue-shifted for about
one hundred nm. Analogously, lifetimes of emitting excited states
for 1 and 2 (0.81 and 0.32 ms) are considerably shorter compared
to 3.30 ms in the case of 3. And finally, the emission Q. Y. for
3 is an order of magnitude higher compared to those found for
1 and 2.
These observations illustrate clearly a well known strong
resonance conjugation between the para-substituents of the aro-
matic ring and a much weaker inductive influence of the meta-
substituents that determines a lower donor effect of the 3-NH2
group compared to those of NMe2 and NH2 in para-position.
Because of this the electron density transfer onto MOs responsible
for the triplet state emission is substantially higher in 1 and 2,
which results in a bathochromic shift of the emission maxima
(see above) and in a dramatic decrease in the phosphorescence
Q. Y. The effects observed are completely in line with the
experimental data4 obtained for a series of closely analogous
“rods-in-belt’ complexes containing para-substituents (NO2, H
and OMe) of various electron donor strengths in the alkynyl
ligands, the higher donicity causing a red shift of emission
maximum and decrease in emission Q. Y.
The most interesting feature of the complexes under study is a
variation of their photophysical properties upon protonation of
the amine groups. Luminescence and absorption responses of 1 in
the CH2Cl2 solution during titration with MeSO3H are shown
in Fig. 4 and Fig. S8†, respectively. The absorption spectrum
variation is rather small, whereas the luminescence characteristics
experience dramatic changes both in emission wavelength and
intensity.
Compounds 2 and 3 display closely analogous UV-vis spec-
troscopic characteristics summarized in Table 1. Similar to 1
these complexes show strong absorption bands at ca 260 nm,
featureless-shoulder absorption between 290 and 315 nm and
clear-cut long wavelength bands at 395, 397 and 401 nm for 1, 2,
and 3, respectively. According to the electronic structure analysis,
see computational section and ref. 4, the high energy absorption
≡
can be assigned to the intraligand p→p* (C CAr) transitions
The acid addition first results in nearly complete emission
quenching in the area corresponding to incomplete protonation
of the starting cluster and then appears at substantially shorter
wavelength demonstrating a sharp growth in Q. Y. The completely
protonated complex 4 displays a slightly lower lifetime but the
radiative decay rate constant and Q. Y. are approximately an order
of magnitude higher compared to the initial values. A closely
analogous behavior was observed upon protonation of 2 with
in the alkynyl moiety, featureless absorption at about 300 nm
to the transitions from s(Au–P) orbitals to antibonding orbitals
of alkynyl or phosphine ligands, whereas low energy absorption
bands with a tail extended to 550 nm have primarily MLCT
character, from the orbitals of central cluster fragment to the
orbitals of the phosphine ligands and alkynyl p* orbitals, see
below.
Table 1 The photophysical properties of complexes 1–6 in CH2Cl2, 298 K
R
Complex
l
abs /nm (10-3e/cm-1 M-1)
262 (165), 291 (137), 315sh (129), 395 (78)
l
em/nmb Quantum yield (Q. Y.)c
t
obs/ms
10-5 kr/s-1
10-6 knr/s-1
4-NMe2
4-NH2
3-NH2
1
715
575
0.06 0.01
0.48 0.05
0.018 0.003
0.6 0.1
0.41 0.05
0.26 0.04
0.81 0.08
0.54 0.05
0.32 0.03
0.52 0.05
3.3 0.3
0.74 0.05 1.2 0.1
8.9 0.9 1.0 0.2
0.56 0.05 3.1 0.3
12.1 1.0
1.24 0.1
4 (protonated)a 262 (168), 325sh (60), 408 (51)
2
262 (79.6), 290 sh (117), 312sh (93) 397 (63) 708
5 (protonated) 262 (175), 332sh (50.0), 408 (48.0)
263sh (159), 313sh (59), 401 (52)
6 (protonated) 264 (164), 332sh (76), 407 (54),
580
608
575
0.71 0.15
0.18 0.03
0.7 0.2
3
1.0 0.2 (1.0)d 2.6 0.3e
3.3 0.5 (0.2)d
a Protonation performed with 100-fold excess of MeSO3H. b lex = 385 nm. c Rhodamine 6G was used as a standard for the quantum yield measurements.
d Relative contribution of the corresponding exponent into double-exponential decay. e Relative to main component of the decay.
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Table 2 Selected interatomic distances and electronic properties of the
heterometallic AuI–CuI clusters 1–6 obtained at the BP86 level of theory
1 (X-ray)
1
2
3
4
5
6
˚
Interatomic distances/A
Au(central)–Au(belt)
Au(central)–Au(central)
C(Ph)–N
2.82–2.84 2.90 2.91 2.93 2.96 2.97 2.93
3.20–3.51 3.46 3.44 3.43 3.39 3.35 3.42
1.37–1.42 1.38 1.38 1.39 1.48 1.46 1.47
N–H+
—
—
—
—
—
—
1.14 1.19 1.09
1.44 1.36 1.63
O(MeSO3-) ◊ ◊ ◊ H+
HOMO–LUMO gap (eV)
1.27 1.35 1.64 2.01 1.99 1.79
Fig. 4 Changes of emission spectra of 1, C = 2.3–2.7 ¥ 10-4 M, in degassed
CH2Cl2 upon addition of MeSO3H, room temperature, lex = 385 nm.
fragment, the metal–metal distances within the heterometallic core
decrease in comparison to the non-protonated clusters 1 and 2.
This effect, illustrated by the changes in the Au–Au distances, is
absent in the case of the clusters 3 and 6, where the electron donor
effect of the meta-NH2 group is lower. The N–H+ distances are
shortest in the meta-substituted cluster 6. In clusters 4 and 5 the
acid molecules are clearly connected to the amine groups only
through one O ◊ ◊ ◊ H+ contact, while in cluster 6 the acid molecules
extremely high growth in emission efficiency to give rise to a Q. Y.
value of 0.6 and kr = 12.1 ¥ 105 s-1 for the protonated adduct
5 (cf. 0.018 and 0.56 ¥ 105 s-1 for 2). It is worth noting that
the protonation reaction is reversible and its equilibrium can be
easily shifted backwards upon addition of NEt3 to restore initial
emission properties that can be considered as a “turn-on-turn-off”
tool.
also show a second fairly short O ◊ ◊ ◊ H+ distance of 1.81 A
˚
Very different variations in luminescence characteristics were
found in the case of meta-substituted complex 3, Table 1. Upon ex-
haustive protonation this compound displays a moderate (33 nm)
blue shift in emission maximum and, in contrast to complexes
1 and 2, relatively small reduction in Q. Y. The effects observed
are evidently related to the alteration of the amine group donor
properties accounted for by addition of a proton to the nitrogen
atom (formation of the adduct with the acid molecule). In the
case of the para-substituted alkynyl ligands the effect of donicity
reduction is easily transferred to the orbitals of the heterometallic
core to increase the energy gap between MOs responsible for
triplet state emission as described earlier4 and to give 140 and
128 nm blue shifts in emission wavelengths. Protonation of the
meta-substituents in 3 quite expectedly causes much lower changes
in electron density on the cluster core orbitals that provokes a
relatively small hypsochromic shift of emission maximum.
(Figure S10†).
We investigated the frontier orbital characteristics of both
S0 and T1 states of the clusters 1–6 to shed light on their
photophysical behavior. The relaxed geometry of the lowest energy
triplet state T1 was generally found to be very close to the ground
state structure for all clusters, the general structural motif of the
clusters remaining unperturbed. Accordingly, for the studied T1
states, the highest singly occupied orbital (HSOMO), occupied
by the excited electron, is closely related to the LUMO of the S0
state, and the lowest singly occupied orbital (LSOMO) is closely
related to the HOMO of the S0 state. Hence, it can also be expected
that the theoretical HOMO–LUMO gaps reproduce the observed
qualitative trends for the emission characteristics. The calculated
HOMO–LUMO gaps of the non-protonated clusters decrease in
the order 3 > 2 > 1 (Table 2), which is also the experimentally
observed ordering of the emission wavelengths for these clusters.
Furthermore, in agreement with the experimentally observed blue
shift of the emission maxima for the protonated clusters, the energy
gaps of the clusters 4–6 are larger than the energy gaps of the non-
protonated clusters, particularly for the para-substituted ones.
As can be expected from the structural characteristics of the
para-substituted clusters 1 and 2, their HOMOs are very similar to
each other (Figures S10 and S11†). The HOMOs are delocalized
within the heterometallic central fragment and the contribution
of the NMe2 and NH2 groups is significant. The LUMOs of 1
and 2 are mainly centered on the “belt” fragment, with some
small contributions from the alkynyl groups of the Au–Cu central
cluster. In the case of the meta-substituted cluster 3, the HOMOs
are also delocalized within the heterometallic core (Figure S12†),
but due to the lower electron-donating effect of the meta-NH2
group the HOMOs lie lower in energy in comparison to 1 and 2,
resulting in a larger HOMO–LUMO gap. This effect influences the
LUMOs of 3, as well the alkynyl groups of the central fragment
contributing more to the LUMO in comparison to 1 and 2.
Investigation of the frontier orbital characteristics of the lowest
energy triplet state does not change the general picture, but the
HSOMOs of the clusters 1 and 2 do possess some additional
Computational results
The structural and electronic properties of the AuI–CuI clusters
were further elucidated by means of density functional calculations
(for computational details, see the Experimental section). The
availability of an X-ray crystal structure of 1 (Figure S9†) allows
for the comparison between the experimental and theoretical
geometries, which are in good agreement. The properties of the
structurally optimized clusters 1–6 are summarized in Table 2 and
Figure S10.†
In the case of the protonated clusters 4–6, complete protonation
is sterically feasible without any significant changes to the general
structural motif of the clusters. This process is energetically
feasible, as well. For example, in the case of cluster 1, the
reaction energies for the protonation, calculated as DEprot
=
E[1*(MeSO3H)n] - [E(1) + n E(MeSO3H)], are -98, -142, -201,
-256, -287, and -338 kJ mol-1 for n = 1–6.
The different donor properties of the para- and meta-
substituents give rise to some evident structural trends. In clusters
4 and 5, where the protonation strongly reduces the ability of the
NMe2 and NH2 groups to contribute electron density to the central
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contributions from the central fragment in addition to the orbitals
of the “belt” fragment.
restores initial emission properties that can be considered as a
“turn-on-turn-off” tool. Protonation of the 3-NH2 substituted
complex 3 demonstrated much smaller blue-shift of emission
wavelength (ca. 30 nm) and ca. 40% drop in quantum yield. The
computational studies performed proved the energetic feasibility
of the complete protonation of the clusters 1–3 and revealed
the structural trends accompanying this process. Analysis of the
frontier orbital characteristics of the complexes 1–6 suggested a
tentative explanation of the photophysical changes observed.
Protonation has significant effects on the frontier orbital char-
acteristics of the clusters, as the protonated NMe2 and NH2 groups
contribute much less electron density to the central fragment.
Because of the significantly reduced substituent effects, the orbitals
of the central fragment centered on the heterometallic core and
the alkynyl groups are shifted down in energy and the composition
of the HOMOs is fairly similar for clusters 4–6. Analogously,
for clusters 4–6, the LUMO is centered primarily on the alkynyl
groups of the central fragment, and the LUMO+1 is centered on
the “belt” fragment. The lowest energy triplet states consistently
possess the same central fragment based characteristics, with
LSOMO related to HOMO and HSOMO related to LUMO.
One of the interesting photophysical aspects of the studied
clusters is the greatly improved quantum yield of clusters 1 and 2
upon complete protonation. In the protonated clusters 4 and 5, the
frontier orbital characteristics suggest the triplet emission occurs
within the core of the central fragment, in which case spin–orbit
coupling due to metal orbitals would enable efficient radiative
decay of the excited triplet state. On the contrary, in the case of
the non-protonated complexes 1 and 2, the excited triplet state
strongly involves the p* orbitals of the “belt” fragment, hindering
efficient spin–orbit coupling with the metal orbitals of the central
fragment thus promoting radiationless decay.
Another intriguing photophysical feature of the present Au–Cu
clusters 1 and 2 is the nearly complete disappearance of emission
resulting from the partial protonatation of the amine groups. We
elucidated the photophysical effects of the partial protonation by
inspecting the frontier orbitals of the cluster 1 with the number of
MeSO3H molecules varying from zero to six. Partial protonation
has distinct effects on the frontier orbital characteristics, as
illustrated for a four-fold protonated cluster 1*(MeSO3H)4 in
Figure S13.† As the protonation of the amine groups reduces their
electron-donating effects, the orbitals of the protonated C2C6H4-
4-NMe2 ligands are shifted down in energy and the HOMOs
become localized on the non-protonated C2C6H4-4-NMe2 ligands.
In contrast, the dominant contributions to the LUMO come
from the protonated ligands, the non-protonated ligands having a
smaller contribution. The strongly reduced delocalization of the
central fragment based frontier orbitals makes the excited triplet
state much less susceptible to spin–orbit coupling effects of the
metal orbitals, thus annihilating pathways for effective radiative
decay.
Acknowledgements
Financial support from Academy of Finland (I. O. K.), Russian
Foundation for Basic Research (grants 07-03-00908-a and 09-03-
12309) and European Union: European Regional Development
Fund (A. J. K., grant 70026/08) is acknowledged.
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Conclusion
We have reported the structurally similar luminescent AuI–CuI
alkynyl-diphosphine clusters 1–3 bearing NMe2 and NH2 groups
and their reactions with strong sulfonic acids (HSO3Me and
HSO3CF3), which result in protonation of the amine func-
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2682 | Dalton Trans., 2010, 39, 2676–2683
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