Supramolecular assembly of isocyanorhodium(i) complexes: An interplay of rhodium(i)···rhodium(i) interactions, hydrophobic-hydrophobic interactions, and host-guest chemistry

Article abstract of DOI:10.1021/jacs.5b03396

A series of tetrakis(isocyano)rhodium(I) complexes with different chain lengths of alkyl substituents has been found to exhibit a strong tendency toward solution state aggregation upon altering the concentration, temperature and solvent composition. Temperature- and solvent-dependent UV-vis absorption studies have been performed, and the data have been analyzed using the aggregation model to elucidate the growth mechanism. The aggregation is found to involve extensive Rh(I)···Rh(I) interactions that are synergistically assisted by hydrophobic-hydrophobic interactions to give a rainbow array of solution aggregate colors. It is noted that the presence of three long alkyl substituents is crucial for the observed cooperativity in the aggregation. Molecular assemblies in the form of nanoplates and nanovesicles have been observed in the hexane-dichloromethane solvent mixtures, arising from the different formation mechanisms based on the alkyl chain length of the complexes. Benzo-15-crown-5 moieties have been incorporated for selective potassium ion binding to induce dimer formation and drastic color changes, rendering the system as potential colorimetric and luminescent cation sensors and as building blocks for ion-controlled supramolecular assembly.

Full text of DOI:10.1021/jacs.5b03396

Article
pubs.acs.org/JACS
Supramolecular Assembly of Isocyanorhodium(I) Complexes: An
Interplay of Rhodium(I)···Rhodium(I) Interactions, Hydrophobic−
Hydrophobic Interactions, and Host−Guest Chemistry
Alan Kwun-Wa Chan, Keith Man-Chung Wong,† and Vivian Wing-Wah Yam*
Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee, Hong Kong) and
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China
S
* Supporting Information
ABSTRACT: A series of tetrakis(isocyano)rhodium(I) com-
plexes with different chain lengths of alkyl substituents has been
found to exhibit a strong tendency toward solution state
aggregation upon altering the concentration, temperature and
solvent composition. Temperature- and solvent-dependent
UV−vis absorption studies have been performed, and the data
have been analyzed using the aggregation model to elucidate the
growth mechanism. The aggregation is found to involve
extensive Rh(I)···Rh(I) interactions that are synergistically
assisted by hydrophobic−hydrophobic interactions to give a
rainbow array of solution aggregate colors. It is noted that the
presence of three long alkyl substituents is crucial for the
observed cooperativity in the aggregation. Molecular assemblies
in the form of nanoplates and nanovesicles have been observed
in the hexane−dichloromethane solvent mixtures, arising from the different formation mechanisms based on the alkyl chain
length of the complexes. Benzo-15-crown-5 moieties have been incorporated for selective potassium ion binding to induce dimer
formation and drastic color changes, rendering the system as potential colorimetric and luminescent cation sensors and as
building blocks for ion-controlled supramolecular assembly.
ligands, [Rh2(bridge)4]2+ were reported with the studies on
INTRODUCTION
■
their structural characterization,2d−h,3a,b as well as photophysical
The d8 square-planar metal complexes have been well-known to
and photochemical properties in association with the presence
exhibit noncovalent metal···metal interactions arising from the
close proximity of the metal centers, imparting them with
intriguing and unique optical properties that have been utilized
for the development of chemosensors, light-emitting devices,
of Rh(I)···Rh(I) interactions.2d,3h Balch and co-workers have
then successfully isolated the monomeric, dimeric, and trimeric
forms of [Rh(CNR)4](X) (X = Cl, BPh4 or SbF6) with their
and molecular wires.1 Square-planar rhodium(I) complexes are
crystallographical characterization established, representing the
first structural characterization of the trimer.4a,b More recently,
one of the important classes of metal complexes that have been
a series of tetrakis(arylisocyanido)rhodium(I) complexes with
extensively explored due to their capability to exhibit Rh(I)···
improved hydrophilicity has been reported by Che and co-
Rh(I) interactions and to display rich photophysical proper-
workers to form self-aggregates in aqueous solution.5g
ties.2−5 However, the rhodium(I) complexes are typically
Although a variety of rhodium(I) complexes have been
unstable toward air and moisture due to the low valence of the
reported to display rich photophysical properties and poly-
metal center and the openness of the structure that facilitate
morphism, those involving the use of Rh(I)···Rh(I) interactions
oxidative addition reaction, making their analysis complicated
and challenging.5b,c,e,f Isocyanide is one of the commonly found
cooperatively with other noncovalent interactions such as
donor−acceptor and hydrophobic−hydrophobic interactions
ligands in stable square-planar rhodium(I) complexes, probably
due to the strong π-accepting ability for the stabilization of the
for stimuli-responsive self-assembly, which are important for
the construction of supramolecular functional nanomaterials,6
electron-rich rhodium(I) metal center, preventing it from
have been relatively under-explored. It is believed that by taking
oxidative addition and giving rise to extraordinarily stable
advantages of the extensive Rh(I)···Rh(I) interactions as well as
materials for applications. Crystallized salts of homoleptic
tetrakis(isocyanido)rhodium(I) complexes have been reported
in the early days by Gray and co-workers to give various colors
modification and structural engineering of the isocyanide
ligands with different geometries, topologies and electronic
of yellow, red, violet, blue, and green solids, arising from the
presence of close Rh(I)···Rh(I) contacts.2a,b A number of
Received: April 1, 2015
dinuclear rhodium(I) complexes with four bridging isocyanido
© XXXX American Chemical Society
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Figure 1. Molecular structures of complexes 1−8 and 1′.
properties, approaches that lead to the construction of
multifunctional materials with diverse shapes, symmetries and
more importantly, the responsiveness and aggregation affinities
toward the external stimuli could be achieved.6 Different chain
lengths of alkyl substituents have been employed to study their
structure−property relationship and more importantly, the
interplay between the hydrophobic effects versus the Rh(I)···
Rh(I) interactions in the molecular aggregation, which may give
rise to the construction of supramolecular architectures with
specific functions. Crown ether components have also been
incorporated into the isocyanide ligands for cation-controlled
supramolecular assembly and as a “proof of concept” model to
study the recognition of different cations of biological interest.7
Concentration-, temperature- and solvent-dependent UV−vis
absorption and microscopic studies have been performed to
investigate their influence on the self-assembly and aggregation
processes. The spectroscopic data have also been analyzed by
the supramolecular aggregation models8 to unravel the self-
assembly mechanisms. It is envisaged that the current system
can act as an efficient spectroscopic probe for the study of
different kinds of intermolecular interactions, which may lead to
unique and interesting photophysical properties, providing
insights into the design of smart and multifunctional materials.
structure. Figure 1 shows the chemical structures of rhodium(I)
complexes 1−8 and 1′.
X-ray Crystal Structures. Single crystal of complex 1′ was
obtained by diffusion of diethyl ether vapor into concentrated
dichloromethane solution of the complex and the structure was
determined by X-ray crystallography. Crystal structure
determination data are summarized in Table S1a, whereas the
selected bond lengths and bond angles are tabulated in Table
S1b in the Supporting Information. Similar to the other
tetrakis(isocyano)rhodium(I) complexes,2−4 the rhodium(I)
metal center is slightly distorted from the ideal square-planar
D4h symmetry with three rhodium metal centers connected by
Rh(I)···Rh(I) interaction. The torsional angles between the
complex cations are about 5−12° when viewed along the Rh−
Rh−Rh axis, in between the ideal staggered and eclipsed
conformation (Figure 2). One of the four isocyanide ligands of
each independent rhodium(I) complex molecule is found to be
twisted from the central rhodium plane at a dihedral angle of
45−47° to minimize the steric repulsion between the
neighboring complex molecules. The C−Rh−C angles (C1−
Rh1−C2 86.9°, C1−Rh1−C3 162.9°; C5−Rh2−C6 86.4°,
C5−Rh2−C7 173.0°; C9−Rh3−C10 86.8°, C9−Rh3−C11
175.9°) are found to deviate from the idealized value of 90° and
180°, respectively, due to the steric demand of the isocyanide
ligand. The trimeric units are aligned in a zigzag form with
alternating Rh···Rh distances (Rh1···Rh2 distance 3.109 Å, and
Rh2···Rh3 distance 3.167 Å) in each trimer unit (Figure 2b).
When viewed along the horizontal axis, the rhodium atoms are
aligned with Rh−Rh−Rh angles of 159° (within the timer) and
120° (between the trimers), while the Rh···Rh distance
between the discrete trimers is found to be 8.07 Å, indicating
that there is absence of Rh(I)···Rh(I) interactions between the
trimers. The interplanar distance between two aryl rings in the
isocyanide ligand in two adjacent complex molecules is found
to be 3.492 Å, indicative of the presence of π−π interactions.
Concentration- and Temperature-Dependent UV−Vis
Absorption Analysis. The UV−vis absorption and emission
properties of the complexes have been investigated and the data
are summarized in Table 1. Generally, the monomeric
rhodium(I) complexes exhibit intense intraligand absorption
bands at 330−350 nm, and a moderately intense absorption at
440−460 nm, assigned as a 4dz2 → 5pz transition with mixing
of a metal-to-ligand charge transfer (MLCT) [dz2(Rh) →
π*(isocyanide)] character, resulting from the overlap of the
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The alkylated nitro-
benzenes are prepared by alkylation of the pyrogallol9a followed
by nitration9b in acetic acid/nitric acid mixture while the crown
ether based nitrobenzenes are prepared from ring-closing
reaction of the pyrocatechol followed by similar nitration
process. The isocyanide ligands are then prepared by the
Hofmann isocyanide synthesis9c from the amino- precursors,
which are first converted from the nitrobenzenes by reduction
in palladium on activated charcoal. Tetrakis(isocyano)rhodium-
(I) complexes are synthesized by the reaction of [Rh(COD)-
Cl]2 (COD = 1,5-Cyclooctadiene) with eight equivalents of the
respective isocyanide ligands in dichloromethane.2b−e The
corresponding hexafluorophosphate salts of the complexes are
prepared by metathesis reaction with ammonium hexafluor-
ophosphate. The identities of the newly synthesized square-
planar rhodium(I) complexes have been confirmed by 1H
NMR, positive FAB-MS and satisfactory elemental analysis.
One of the complexes has also been structurally characterized
by X-ray crystallography with the illustration of the trimeric
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where [Rh] is the total concentration of the complex and A is
the absorbance at 540−570 nm, suggesting that the absorption
in this region should be attributed to the 4dσ*(Rh2) →
5pσ(Rh2)/π*(isocyanide) transition of the dimeric form,
[Rh(CNR)4]22+, due to Rh(I)···Rh(I) interactions, similar to
that reported in the literature.2,3,5g The dimerization constants
are found to be in the order of 104 M−1, which are relatively
higher than that reported for the tetrakis(phenylisocyano)-
rhodium(I) complex.2c,3g These observations suggested that
complexes 1−8 and 1′ show a higher tendency toward self-
aggregation through the combination of Rh(I)···Rh(I), π−π
stacking and hydrophobic−hydrophobic interactions in sol-
ution even at room temperature. It is interesting to note that
the dimerization constants are found to increase with increasing
alkyl chain lengths of the isocyanide ligands from 2.1 × 104 M−1
in complex 2 with ethoxy substituent to 3.2 × 104 M−1 in
complex 6 with hexadecyloxy substituent, attributed to the
gradual increase in the intermolecular hydrophobic−hydro-
phobic interactions in addition to the Rh(I)···Rh(I) and π−π
stacking interactions. Complex 4 serves as a control complex to
illustrate the importance of the three alkyl substituents in
facilitating aggregation as the dimerization constant for 4 is
much lower than the others even though it consists of one long
dodecyloxy substituent. The aggregation behavior of complex
−
1′, which is the PF6 counterpart of complex 1, has also been
investigated by concentration-dependent UV−vis studies. The
spectral change upon increasing the concentration and the
calculated association constant are both similar to that of
complex 1, indicating the limited role of the counterions in
influencing solution state aggregation as revealed by previous
studies.3g For complexes 5−7, absorption shoulders at 642 nm
appeared at high concentrations and are ascribed to 4dσ*(Rh3)
Figure 2. Crystal packing diagrams showing (a) the configuration of
three independent molecules (top view) and (b) the trimeric linearly
stacked structure (side view) with the dotted line indicating the Rh···
Rh interactions within the trimers of cations of complex 1′.
→ 5pσ(Rh3)/π*(isocyanide) transitions of the trimeric [Rh-
(CNR)4]33+ unit. A linear relationship of (A642) versus (A545
)
3/2
in the trimerization plot based on the dimer−trimer
equilibrium3g has been obtained, establishing the formation of
the trimeric species. Table 2 summarizes the dimerization and
trimerization constants for all the complexes.
pz(Rh) orbital with the π* orbital of the isocyanide ligand of
the same symmetry, i.e., 4dz2(Rh) → 5pz(Rh)/π*(isocyanide)
transition (Figure S1a), similar to those reported in the
literature for related systems.2,3 Emissions at 510−540 nm are
observed in the solution state (Figure S1b) and blue-shifted to
around 470 nm in the solid state, while at higher concentrations
in solution state or with lower-energy excitation in the solid
state, emissions at around 700 nm can be observed (Figure
S1c) and are assigned to be originated from the 4dσ*(Rh2) →
5pσ(Rh2)/π*(isocyanide) transition arising from the extensive
Rh(I)···Rh(I) interactions, where dσ* refers to the antibonding
combination of the 4dz2 orbitals.3h Concentration-dependent
UV−vis absorption studies have been performed for the
complexes to visualize their self-assembly behavior (Figure S2).
Solutions of the complexes in dichloromethane at concen-
trations of 10−3 to 10−6 M are prepared. In dilute solutions, the
complexes exhibit absorption bands at 330−460 nm, arising
from the monomer absorptions. Color changes from yellow to
reddish-brown are observed upon increasing the concentration,
ascribed to oligomerization of the complexes via Rh(I)···Rh(I)
interactions.2,3 The absorption tails at 540−570 nm in
complexes 1−8 and 1′ are found to deviate from Beer’s law
upon increasing the concentration while there is a very slight
growth of absorption tail at 680−700 nm (Figure S2). By
applying a monomer−dimer equilibrium,3g,6h,j a dimerization
plot, in which a linear relationship of [Rh]/A1/2 versus A1/2, is
obtained in the concentration range of 2 × 10−3 to 5 × 10−5 M,
Further insights into the mechanism of the oligomerization
processes have been provided by temperature-dependent UV−
vis absorption studies of complexes 1 and 5. Upon lowering the
temperature from 293 to 153 K, 1 with short methoxy
substituent exhibits a gradual increase in the absorption of
dimers at 562 nm with a decrease in the absorption of
monomers at 460 nm (Figure 3) while 5 with long dodecyloxy
substituent shows a significant enhancement in the absorption
of dimers, trimers and oligomers (Figure S3). The plot of the
degree of aggregation as a function of temperature gives a clear
sigmoidal curve, indicative of an isodesmic self-assembly
mechanism during the cooling process for both complexes.
Unlike the cooperative self-assembly that will start with an
unfavorable nucleation step with equilibrium constant Kn
followed by a more favorable elongation step once the critical
nucleus M* is formed from the nucleation step,8b the isodesmic
assembly can be described as the equal-K model (Ki = Ki+1) as
the rate of each monomer addition step is the same and
governed by a single equilibrium constant Ke with a gradual
increase in the aggregated species.8b To allow a further
understanding of the aggregation process, the absorption data
have been fitted with the temperature-dependent supra-
molecular aggregation model,8b confirming the isodesmic
nature of the process. The enthalpy change (ΔH) and the
melting temperature (Tm) for dimerization have also been
C
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Table 1. Photophysical Data for the Rhodium(I) Complexes
complex
medium (T/K)
electronic absorption λmax/nm (ε/dm3 mol−1 cm−1
)
emission λem/nm (τo/μs)
ΦPL × 103
a
1
CH2Cl2 (298)
333 (21,990), 400 (4,880), 457 (760)
529 (<0.1)
471 (<0.1)
471 (<0.1)
744
0.1
b
solid (298)
b
solid (77)
c
glass (77)
2
3
4
5
6
7
8
CH2Cl2 (298)
338 (30,340), 405 (4,580), 462 (670)
330 (24,540), 402 (4,240), 441 (2,200)
332 (20,220), 397 (2,950), 453 (890)
332 (35,690), 399 (5,350), 464 (940)
335 (24,120), 396 (3,020), 460 (490)
353 (39,500), 401 (4,000), 460 (1,020)
529 (<0.1)
469 (<0.1)
472 (<0.1)
731
0.4
0.1
0.2
0.8
0.5
0.3
0.2
b
solid (298)
b
solid (77)
c
glass (77)
CH2Cl2 (298)
520 (<0.1)
470 (<0.1)
471 (<0.1)
642
b
solid (298)
b
solid (77)
c
glass (77)
CH2Cl2 (298)
503 (<0.1)
471 (<0.1)
471 (<0.1)
544
b
solid (298)
b
solid (77)
c
glass (77)
CH2Cl2 (298)
515 (<0.1)
469 (<0.1)
472 (<0.1)
627
b
solid (298)
b
solid (77)
c
glass (77)
CH2Cl2 (298)
525 (<0.1)
470 (<0.1)
472 (<0.1)
618
b
solid (298)
b
solid (77)
c
glass (77)
CH2Cl2 (298)
538 (<0.1)
470 (<0.1)
472 (<0.1)
637
b
solid (298)
b
solid (77)
c
glass (77)
CH2Cl2 (298)
345 (34,660), 399 (4,030), 459 (1,020)
535 (<0.1)
470 (<0.1)
472 (<0.1)
b
solid (298)
b
solid (77)
c
glass (77)
634
a
b
c
Measured at concentration = 1 × 10−5 to 1 × 10−4 M. Solid-state emission was recorded after grinding. In ethanol−methanol (4:1 v/v) glass.
interactions. More importantly, complex 5 (ΔH = −41.88 kJ
mol−1) results in a much more negative enthalpy value than 1
(ΔH = −25.02 kJ mol−1) during the aggregation process,
stemming from the additional hydrophobic−hydrophobic
interactions contributed from three long dodecyloxy compo-
nents which can synergistically assist the stacking of rhodium(I)
complexes in the process.5a Interestingly, the changes in
association energies obtained from the concentration-depend-
ent and temperature-dependent aggregation studies are found
to be in similar order of magnitude compared to other alkyl-
substituted systems.5h,i A change in the Gibbs free energy of
association (ΔG) from n-butyl (−12.41 kJ mol−1) to n-octyl
(−16.59 kJ mol−1) has been reported in the concentration-
dependent aggregation studies of alkyl-substituted imidazolium
chlorides,5h while the enthalpy of association (ΔH) has been
shown to become more negative from n-butyl (−16.3 kJ mol−1)
to n-decyl (−23.9 kJ mol−1) in the temperature-dependent
aggregation studies of a series of alkyl-substituted quinacrido-
nes.5i The more negative association energies have been
attributed to the additional attractive hydrophobic−hydro-
phobic interactions.5h,i Thus, the difference in association
energies between different alkyl-substituted rhodium(I) com-
plexes may therefore provide some insights into the estimation
of the extent of hydrophobic interactions between the alkyl
chains.
Table 2. Association Constants (log K) of the Rhodium(I)
Complexes for Concentration-Dependent Studies
a
d
e
ΔGd
/
ΔGt /
b
c
complex
solvent
log Kd log Kt
kJ mol−1
kJ mol−1
f
1
1′
2
3
4
5
6
7
dichloromethane
dichloromethane
dichloromethane
dichloromethane
dichloromethane
dichloromethane
dichloromethane
dichloromethane
4.02
3.95
4.32
4.47
2.37
4.55
4.51
3.62
−
−
−
−
−22.93
−22.54
−24.64
−25.50
−13.52
−25.96
−25.73
−20.65
−
−
−
−
f
f
f
2.92
2.82
2.51
3.18
−16.66
−16.09
−14.32
−18.14
a
Association constants are determined in the concentration range of 2
b
× 10−3 to 5 × 10−5 M. Dimerization constant determined from
c
dimerization plot of the monomer−dimer equilibrium. Trimerization
constant determined from trimerization plot of the dimer−trimer
d
e
f
equilibrium. ΔGd = −RT ln Kd. ΔGt = −RT ln Kt. Trimeric
absorption band cannot be observed.
determined from the isodesmic model, consistent with the
values obtained from the corresponding van’t Hoff analysis.
Table 3 summarizes the thermodynamic data obtained from the
temperature-dependent aggregation process. The negative
enthalpy change has been attributed to the molecular
aggregation arising from the intermolecular Rh(I)···Rh(I),
π−π stacking interactions and hydrophobic−hydrophobic
D
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Figure 4. UV−vis absorption spectra of complex 5 with different
solvent compositions. The insets showing the corresponding color
changes upon changing the solvent composition from left to right
correspondingly (dilute CH2Cl2, concentrated CH2Cl2, CH2Cl2 mixed
with ether, CH2Cl2 mixed with acetone, CH2Cl2 mixed with methanol
and chloroform).
formation of aggregates. 1 and 5 have been spectroscopically
studied in the hexane−dichloromethane solvent mixtures to
investigate the effects of alkyl chain lengths and solvent
polarities toward aggregation. Upon increasing the hexane
content (nonpolar solvent) in the hexane−dichloromethane
mixture of 1, the color of the solution changed gradually from
pale yellow (100% dichloromethane) via orange-red (50%
hexane) to purple (80% hexane), while more dramatic color
changes from yellow through purple to purplish-blue (100%
hexane) was observed for 5 with the long dodecyloxy
substituent. From the UV−vis absorption spectrum of 1
(Figure 5a), the absorptions of dimers and trimers appear at
556 and 642 nm respectively upon increasing the hexane
content. On the other hand, the UV−vis absorption studies of 5
(Figure 6a) first illustrate the sequential appearance of the
absorptions of dimers (547 nm), trimers (671 nm) and then
higher-ordered oligomers (789, 908 nm) upon increasing the
hexane content. The observations suggested that both 1 and 5
would undergo solvent-induced aggregation by the addition of
a nonpolar solvent, but to different extents toward the process.
Both the absorption data of 1 and 5 have been analyzed by
the equilibrium model on solvent-dependent dynamics
developed by Meijer and co-workers.8c A sigmoidal curve was
observed for the plot of the degree of aggregation at 556 and
642 nm versus solvent composition in 1. The plot is well fitted
with the equilibrium model (Figure S4), indicating that the
solvent-induced assembly process of 1 follows an isodesmic
mechanism. However, a nonsigmoidal curve is obtained for 5
upon plotting the degree of aggregation at 789 or 908 nm
versus solvent composition (Figure S5), and a cooperative
mechanism was confirmed by the aggregation equilibrium
model. The cooperativity of 5 is suggested to be attributed to
the three long dodecyloxy substituents, which can assemble
with each other in high hexane content to assist the packing of
the molecules, synergistically facilitating the intermolecular
Rh(I)···Rh(I) and π−π stacking interactions.5a,11,12 Together
with the enhanced solubility, the rhodium(I) metal centers in
the aggregates of 5 would therefore be expected to be brought
into a closer proximity and more importantly, have a larger
propensity to assemble with each other and give rise to higher-
ordered oligomers with more drastic color changes in high
hexane content. Complex 5 has also been found to exhibit
cooperativity and drastic color changes in the UV−vis
absorption studies in other solvent mixtures, including
Figure 3. (a) UV−vis spectra of 1 in 4 × 10−4 M from 293 to 153 K in
dichloromethane. Insets: The corresponding sigmodal plot at 562 nm
absorption against temperature fitted with the isodesmic aggregation
model, with insets showing the enthalpy data and melting temperature.
(b) Van’t Hoff plot of elongation equilibrium constant Ke in
dichloromethane in various temperatures with insets showing the
thermodynamic data obtained.
Table 3. Thermodynamic Parameters for the Temperature-
Induced Aggregation Process of 1 and 5
ΔH/
ΔS/
TΔS/
ΔG/
melting
complex kJ mol−1 J mol−1 K−1 kJ mol−1 kJ mol−1 temperature/K
a
b
a
b
a
a
a
a
b
1
5
−24.56
−25.02
−46.03
−41.88
−63.27
−18.85
−42.57
−5.71
−3.47
201
229
a
a
b
−142.84
a
Determined by van’t Hoff plot from UV−vis absorption studies at
b
different temperatures. Determined from curve fitting by applying a
global nonlinear least-squares procedure using the temperature-
dependent equilibrium aggregation model from UV−vis absorption
at different temperatures.
Mixed Solvent Aggregation-Induced Assembly Stud-
ies. Solvent-induced aggregations have been reported by our
group in the square-planar alkynylplatinum(II) terpyridine
system6h,i,10 to exhibit drastic changes in absorption colors and
luminescent properties. In this work, the strategy of using
mixed solvent system is extended to the rhodium(I) system to
induce self-assembly properties, given the high propensity of
rhodium(I) complexes in exhibiting Rh(I)···Rh(I) interactions.
With an interplay of the extensive Rh(I)···Rh(I) interactions as
well as hydrophobic−hydrophobic interactions, the square-
planar rhodium(I) system is expected to exhibit diverse and
enhanced solvent-induced aggregation behaviors.
Upon dissolution of the rhodium(I) complexes in different
solvent mixtures such as hexane−dichloromethane or acetone−
dichloromethane, drastic color changes from pale yellow to a
rainbow array of colors ranging from deep purple to deep blue
and green were observed (Figure 4), attributed to the
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acetone−dichloromethane and methanol−dichloromethane
mixtures (Figure S6). It is observed that there was an increase
in degrees of aggregation and propagation from dimeric to
oligomeric aggregates by increasing the nonsolvent content,
giving rise to the different colors in the solvent mixtures (Figure
4), probably facilitated by the lower solubility of 5 in acetone
and methanol.6h,i,10
The assembly process of the control complex, 4, is found to
follow an isodesmic mechanism in the hexane−dichloro-
methane solvent mixture even though it contains the long
dodecyloxy substituents. The Gibbs free energy (ΔG) of the
assembly processes of the complexes in hexane−dichloro-
methane solvent mixture are also determined, with 4 (−38.5 kJ
mol−1) being less negative than 1 (−40.27 kJ mol−1) and than 5
(−42.08 kJ mol−1), similar to the trends obtained from the
temperature-dependent and concentration-dependent aggrega-
tion processes, which could be ascribed to the progressional
enhancement of Rh(I)···Rh(I) and hydrophobic−hydrophobic
interactions on going from 4 to 1 to 5. Table 4 summarizes the
Table 4. Thermodynamic Parameters for the Solvent-
a
Induced Aggregation Process of 1, 4 and 5
complex
ΔG/kJ mol−1
m/kJ mol−1
σ
b
b
b
b
b
c
1
4
5
−40.27
−38.55
−42.08
−36.82
64.47
1.00
1.00
0.57
0.76
0.17
b
b
70.50
b
b
120.00
c
c
50.00
d
d
d
−48.63
116.00
a
Determined from curve fitting by applying a global nonlinear least-
squares procedure using the equilibrium model from UV−vis
Figure 5. (a) UV−vis spectra of complex 1 upon increasing the hexane
content from 0 to 80% in dichloromethane. (b) Transmission (TEM)
and (c) scanning (SEM) electron micrographs showing the nanoplate
formation.
b
absorption studies at different solvent mixtures. In hexane−
c
dichloromethane solvent mixture. In acetone−dichloromethane
d
solvent mixture. In methanol−dichloromethane solvent mixture.
thermodynamic data obtained in the solvent-induced aggrega-
tion processes. The ready tunability of the current system can
therefore be possibly utilized for the control and manipulation
of supramolecular assembly11 and stimuli-responsiveness12 via
simple structure−property modification and variation of
external stimuli, rendering the present system a versatile
building block and motif for the construction of functional
molecular materials.
Morphological Studies. Transmission electron micros-
copy (TEM) and scanning electron microscopy (SEM) are
used to probe the presence of supramolecular self-assemblies in
the solvent-induced aggregation process. For complex 1 with
three short methoxy substituents, nanoplates are formed in 60%
hexane−dichloromethane solution with lengths and diameters
of 1−2 μm and 150−200 nm, respectively. The heights of the
nanoplates have been determined to be about 20 nm by atomic
force microscopy (AFM) (Figure 7c). In the case of complex 2
with the three ethoxy substituents, molecular vesicles formed by
small granular assemblies are observed upon increasing the
hexane content (Figure S7a). Complex 4 with only one long
alkyl chain on each ligand gives rise to the irregular cubes or
lamellas with dimensions of about 500 nm, which are similarly
observed in the square-planar palladium(II) system with the
same ligand and are believed to adopt a similar assembly
mechanism.13 With three dodecyloxy substituents on each
ligand, complex 5 similarly exhibits molecular vesicles in 70%
hexane−dichloromethane solution (Figure 6b). More interest-
ingly, the vesicles will contract toward the center when the
Figure 6. (a) UV−vis spectra of complex 5 upon increasing the hexane
content from 0 to 100% in dichloromethane. The inset shows the
corresponding color changes upon changing the solvent composition
from all dichloromethane (left) to all hexane (right). (b) TEM (left)
and SEM (right) images showing the vesicle formation upon
aggregation.
F
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Figure 7. Schematic diagrams illustrating (a) the formation of the nanoplates from 1 and (b) the nanovesicles from 5. (c) Atomic force micrograph
(AFM) of 1 indicating the thickness of the nanoplates.
hexane content is further increased to 90%, resulting in denser
and closer-packed vesicles (Figure S7b). Such observations are
found to be consistent with the results from dynamic light
scattering (DLS) experiments, in which a decrease in particle
size distribution was observed upon increasing the hexane
content (Figure S7c). Energy-dispersive X-ray spectroscopy
(EDX) has also been performed to establish the amorphous
nature of both the nanoplates and nanovesicles (Figure S8−
S9), as well as the hole at the center of the nanovesicles.
Complex molecules of 1 are found to assemble with each other
in a linear fashion via extensive Rh(I)···Rh(I) interactions,
similar to that of other rhodium(I) complexes,5a,b as revealed by
the X-ray crystal structure. The resulting linear assembly would
further stack closely on each other to end up with nanoplates
(Figure 7a). For the complexes bearing the three long alkyl
substituents, the assembly would further assemble in a
multidimensional manner cooperatively by an interplay of the
hydrophobic−hydrophobic interactions exerted by the longi-
tudinal alkyl substituents and the Rh(I)···Rh(I) interactions
(Figure 7b and Figure S7a). However, such kind of assemblies
will not have a high packing density as a result of the floppiness
of the alkyl chains, giving rise to a molecular vesicle instead of
G
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rod-shaped structures upon further heaping together. By further
increasing the hexane content, the long hydrophobic alkyl
chains would be more extended and well dispersed toward each
other to further enhance the hydrophobic−hydrophobic
interactions, such that the nanoassembly of 5 can heap with
others in close proximity to end up with smaller but more
compact vesicles, which are also in accordance with the
dramatic UV−vis absorption changes at high hexane content.
Cation-Induced Supramolecular Assembly. In view of
the rich and tunable aggregation properties associated with
hydrophobic−hydrophobic interactions and Rh(I)···Rh(I)
interactions exhibited by the isocyanorhodium(I) complexes,
further effort has been initiated to explore the utilization of the
switching on and off of Rh(I)···Rh(I) interactions through
cation-induced supramolecular assembly. Benzo-15-crown-5
moieties, which have been previously demonstrated to
encapsulate the sodium ion within the crown ether or having
two of them binding to potassium ion selectively in a sandwich
mode,14 have been incorporated into the molecular structure of
complex 7 to turn on the Rh(I)···Rh(I) interactions through
cation binding. The complexation of complex 7 toward alkali-
metal and alkaline-earth-metal ions have also been studied to
rationalize the mechanism of recognition (Figure 8−9). The
also consistently illustrate the significant enhancement of the
emission of dimers at 690 nm and also the drop of emission of
monomers upon the addition of potassium ion. The 1:1
complexation mode (or 2:2) is further confirmed by the
method of continuous variation from the Job’s plot and also by
ESI mass spectrometry (Figure S10). The binding constants
(log K) have been determined to be 4.29 and 4.12 by UV−vis
and emission titration studies respectively, similar to the values
determined from the dibenzo-15-crown-5 gold(I) complex
systems.14c,16b−d The detection limit was determined to be 1.33
× 10−5 M.
The rhodium(I) complexes are originally aggregated in
acetonitrile solution in the oligomeric form due to the extensive
Rh(I)···Rh(I) interactions to give the greenish-yellow color.
However, upon the addition of KClO4, two potassium ions are
encapsulated by two rhodium(I) complex cations in a sandwich
fashion to transform the oligomers to dimers, associated with a
sharp color change to deep blue (Figure 9a). The studies of 7
with Ba(ClO4)2 show similar observations with the decrease of
the absorption of oligomers and significant enhancement of
absorption and emission of dimers (Figure S11), indicating a
binding mechanism similar to that of potassium.15a The log K
value with barium ion is found to be 3.24, which is smaller than
that of potassium. The more positively charged barium ions,
though with similar ionic radius (1.35 Å) compared to
potassium ion (1.38 Å), may give rise to a less stable adduct
due to electrostatic repulsion. It is also noted that the
encapsulated potassium ions could be extracted by the addition
of 18-crown-6, which has a stronger binding affinity than 15-
crown-5 toward the corresponding cations,15b leading to revival
of the original absorption and emission spectra, making the
cation-induced aggregation process reversible.
In comparison, the binding toward lithium, sodium and
magnesium ions, which are smaller in size but better fitted into
benzo-15-crown-5, result in solution color changes of 7 from
greenish-yellow to yellow. The corresponding UV−vis titration
studies indicate that the absorptions of oligomers decrease with
an increase in the absorption of monomers, while the emission
titration studies only show very minor changes upon the
addition of the these ions. The encapsulation of these cations
by benzo-15-crown-5 may lead to the electrostatic repulsion
between the rhodium(I) complex molecules and hence
deaggregation (Figure S12). Attempts to obtain the binding
constants are unsuccessful because no satisfactory fit to the
absorption/emission data could be obtained. Both 1:1 and 1:2
binding adducts have been observed in the ESI mass spectra. It
is likely that the binding process may not be as simple as we
thought and may probably involve both 1:1 and 1:2
complexations toward the cations. Complex 8 with the
benzo-18-crown-6 moieties, in which the cavity can encapsulate
the potassium ion selectively16a and also bind to the larger
cesium ion in a sandwich mode,16b,c has been chosen to verify
the cation-induced assembly. Deaggregation and dimerization
of the complex molecules are observed in both UV−vis and
emission studies upon the addition of KClO4 and CsClO4
respectively, confirming the proposed processes (Figure S13).
On the other hand, similar UV−vis and emission spectral
changes have been observed upon the addition of KClO4 into 7
Figure 8. (a) UV−vis spectral changes of complex 7 upon the addition
of different alkaline and alkaline earth metal cations. The inset
illustrates the corresponding color changes of complex 7 upon
addition of the cations, from left to right: Complex 7 in acetonitrile,
NaClO4, Mg(ClO4)2, Ba(ClO4)2, KClO4 and 18-crown-6 after
addition of cations. (b) Responses of 7 upon addition of different
metal ions with the emission intensity monitored at 690 nm upon
excitation at the isosbestic wavelength.
addition of potassium ion to a solution of 7 in acetonitrile (0.1
M nBu4NPF6) will result in drastic color change from greenish-
yellow to deep blue. The UV−vis titration studies indicate that
there is a decrease in the absorption of oligomers at 867 and
761 nm with the simultaneous growth of the absorption of
dimers at 583 nm (Figure 9). The emission titration studies
−
with PF6 as the counteranion. The Job’s plot again indicates
the 1:1 complexation mode and the binding constants (log K)
have been determined to be 4.49 and 4.33 by UV−vis and
emission titration studies respectively, indicating that the
counteranion may not have a direct effect toward the process
H
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Figure 9. (a) Schematic diagram showing the rhodium(I) dimer induced by the binding of potassium ion. (b) UV−vis absorption and emission
n
spectral changes upon addition of KClO4 into 7 in acetonitrile with 0.1 M Bu4NPF6. The insets show the Job’s plot and the plot of emission
■
intensity ( ) and its theoretical fit (), respectively.
(Figure S14). By taking the advantages of such cation-induced
aggregation-deaggregation processes with the modulation of the
Rh(I)···Rh(I) interactions, metal cations with different ionic
size could be differentiated by their specific spectral response to
enable the system to serve as potential building blocks for the
construction of supramolecular assemblies and colorimetric and
luminescent cation sensors.
color changes and enhancement of NIR emission arising from
the dimer formation. The studies have provided fundamental
understanding on noncovalent metal···metal interactions and
their versatile self-assembly governed by external stimuli. These
findings and discoveries may lead to the design of smart and
multifunctional systems with unique optical properties.
EXPERIMENTAL SECTION
■
Materials and Reagents. 5-Isocyano-1,2,3-trimethoxybenzene,17
3,4,5-triethoxy-aniline,18 3,4,5-tributoxyaniline,19 4-isocyano-1-
(dodecyloxy)benzene,13 3,4,5-tris-(dodecyloxy)aniline,9a 3,4,5-tris-
(hexadecyloxy)aniline,9a 15-isocyanobenzo-15-crown-520 and 18-iso-
cyanobenzo-18-crown-616b are prepared according to reported
procedures. [Rh(COD)Cl]2 was purchased from Strem Chemicals,
and all the tetrakis(isocyano)rhodium(I) complexes were prepared
according to modification of the literature procedure.3g All solvents for
synthesis were of analytical grade and were used as received.
CONCLUSION
■
In summary, we have demonstrated the induced self-assembly
behavior of a series of tetrakis(isocyano)rhodium(I) complexes
by variation of concentration, solvent composition, temperature
and addition of cations as a result of extensive Rh(I)···Rh(I)
interactions. More importantly, it is found that the molecular
structure actually plays a crucial role toward the supramolecular
assembly process. By the modification of the number of alkyl
substituents and the corresponding chain length, the
aggregation behaviors of the complex molecules could be
perturbed. Complex 5 with three long dodecyloxy substituents
in each isocyanide ligand is observed to exhibit rainbow arrays
of solution colors in the different solvent mixtures as a result of
molecular aggregation via the Rh(I)···Rh(I) interactions
synergistically with the hydrophobic−hydrophobic interactions.
Cooperative aggregation mechanism has also been identified for
the corresponding complex upon analyzing the spectroscopic
data and fitted to the aggregation model, while the complexes
with short or less alkyl substituents just exhibit ordinary
isodesmic aggregation mechanism. The morphologies of the
complexes have also been visualized by TEM and SEM,
indicating that the complex molecules would aggregate to form
nanoplates and nanovesicles depending on the substitution
pattern of the alkyl substituent in hexane−dichloromethane
solvent mixture. The rhodium(I) complex with the benzo-15-
crown-5 moieties has also been demonstrated to selectively
encapsulate potassium ion in a sandwich mode with drastic
Synthesis. 5-Isocyano-1,2,3-triethoxybenzene. 3,4,5-Triethoxya-
niline (3.65 g, 16 mmol) was added to a mixture of dichloromethane
(15 mL), chloroform (10 mL), sodium hydroxide (2.60 g, 65 mmol),
water (10 mL) and benzyl triethylammonium chloride (0.23 g, 1.0
mmol), and then the reaction mixture was heated to 50 °C overnight.
After cooling to room temperature, the mixture was extracted with
dichloromethane. The organic layer was dried over anhydrous MgSO4,
and filtered. The filtrate was evaporated to dryness and the residue was
purified by column chromatography on silica gel (230−400 mesh)
using n-hexane−dichloromethane (1:1 v/v) as the eluent to give the
product as a yellow oil (2.25 g, 9.57 mmol, 60%). 1H NMR (400 MHz,
CDCl3, 298 K, relative to Me4Si)/ppm: δ 1.29 (t, J = 6.0 Hz, 3 H,
−CH3), 1.43 (t, J = 6.0 Hz, 6 H, −CH3), 4.05 (m, 6 H, −OCH2−),
6.57 (s, 2 H, −C6H2−). HRMS (Positive EI): m/z found (calcd for
C13H17NO3), 235.1202 (235.1284).
5-Isocyano-1,2,3-tributoxybenzene. The compound was prepared
according to the procedure described for 5-isocyano-1,2,3-triethox-
ybenzene, except 3,4,5-tributoxyaniline (1.41 g, 4.56 mmol) was used
in place of 3,4,5-triethoxyaniline to give the product as a yellow oil
(1.01 g, 3.16 mmol, 71%). 1H NMR (400 MHz, CDCl3, 298 K, relative
to Me4Si)/ppm: δ 0.96 (t, J = 6.0 Hz, 9 H, −CH3), 1.49 (t, J = 6.0 Hz,
I
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6 H, −CH2−), 1.74 (m, 2 H, −CH2−), 1.79 (m, 4 H, −CH2−), 3.95
(t, J = 6.0 Hz, 6 H, −OCH2−), 6.57 (s, 2H, −C6H2−). HRMS
(Positive EI): m/z found (calcd for C19H29NO3), 319.2160
(319.2147).
C79H126ClN4O13Rh (3·H2O), found (calcd): C, 64.01 (64.19); H, 8.40
(8.59); N, 3.93 (3.79).
Complex 4. The compound was prepared according to the
procedure described for complex 1, except 4-isocyano-1-(dodecyloxy)-
benzene (0.20 g, 0.70 mmol) was used in place of 5-isocyano-1,2,3-
trimethoxybenzene to give complex 4 as a brown solid (0.11 g, 0.09
mmol, 98%). 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si)/
ppm: δ 0.88 (t, J = 6.0 Hz, 12 H, −CH3), 1.26 (m, 72 H, −CH2−),
1.44 (m, 8 H, −CH2−), 1.78 (m, 8 H, −CH2−), 3.97 (t, J = 6.0 Hz, 8
H, −OCH2−), 6.91 (d, J = 8.0 Hz, 8 H, −C6H4−), 7.43 (d, J = 8.0 Hz,
8 H, −C6H4−). IR (KBr disk): v = 2170 cm−1 (v(CN)). MS
(positive FAB): m/z 1252 [M − Cl]+, 964 [M − isocyanide − Cl]+.
Elemental analyses calcd for C79.5H125Cl2N4O4Rh (4·0.5CH2Cl2),
found (calcd): C, 69.53 (69.46); H, 9.10 (9.17); N, 4.13 (4.08).
Complex 5. The compound was prepared according to the
procedure described for complex 1, except 5-isocyano-1,2,3-tris-
(dodecyloxy)benzene (0.20 g, 0.30 mmol) was used in place of 5-
isocyano-1,2,3-trimethoxybenzene to give complex 5 as a deep blue
5-Isocyano-1,2,3-tris(dodecyloxy)benzene. The compound was
prepared according to the procedure described for 5-isocyano-1,2,3-
triethoxybenzene, except 3,4,5-tris(dodecyloxy)aniline (1.52 g, 2.35
mmol) was used in place of 3,4,5-triethoxyaniline to give the product
1
as a yellow solid (1.22 g, 1.86 mmol, 79%). H NMR (400 MHz,
CDCl3, 298 K, relative to Me4Si)/ppm: δ 0.88 (t, J = 6.0 Hz, 9 H,
−CH3), 1.33 (m, 48 H, −CH2−), 1.43 (m, 6 H, −CH2−), 1.72 (m, 2
H, −CH2−), 1.80 (m, 4 H, −CH2−), 3.93 (t, J = 6.0 Hz, 6 H,
−OCH2−), 6.56 (s, 2H, −C6H2−). HRMS (Positive EI): m/z found
(calcd for C43H77NO3), 655.5893 (655.5903).
5-Isocyano-1,2,3-tris(hexadecyloxy)benzene. The compound was
prepared according to the procedure described for 5-isocyano-1,2,3-
triethoxybenzene, except 3,4,5-tris(hexadecyloxy)aniline (1.42 g, 1.74
mmol) was used in place of 3,4,5-triethoxyaniline to give the product
1
1
solid (0.08 g, 0.03 mmol, 77%). H NMR (400 MHz, CDCl3, 298 K,
as a yellow solid (1.02 g, 1.24 mmol, 71%). H NMR (400 MHz,
relative to Me4Si)/ppm: δ 0.88 (t, J = 6.0 Hz, 12 H, −CH3), 1.26 (m,
192 H, −CH2−), 1.40 (m, 24 H, −CH2−), 1.78 (m, 24 H, −CH2−),
3.94 (t, J = 6.0 Hz, 24 H, −OCH2−), 6.71 (s, 8 H, −C6H2−). IR (KBr
disk): v = 2160 cm−1 (v(CN)). MS (positive FAB): m/z 2727 [M −
Cl]+. Elemental analyses calcd for C172H310ClN4O13Rh (5·H2O),
found (calcd): C, 74.21 (74.29); H, 11.11 (11.24); N, 2.09 (2.01).
Complex 6. The compound was prepared according to the
procedure described for complex 1, except 5-isocyano-1,2,3-tris-
(hexadecyloxy)benzene (0.20 g, 0.24 mmol) was used in place of 5-
isocyano-1,2,3-trimethoxybenzene to give complex 6 as a deep green
CDCl3, 298 K, relative to Me4Si)/ppm: δ 0.88 (t, J = 6.0 Hz, 9 H,
−CH3), 1.26 (m, 72 H, −CH2−), 1.44 (m, 6 H, −CH2−), 1.78 (m, 2
H, −CH2−), 3.94 (t, J = 6.0 Hz, 6 H, −OCH2−), 6.56 (s, 2H,
−C6H2−). HRMS (Positive EI): m/z found (calcd for C54H103NO3),
813.7940 (813.7937).
Complex 1. 5-Isocyano-1,2,3-trimethoxybenzene (0.32 g, 1.65
mmol) in dichloromethane (15 mL) was added to [Rh(COD)Cl]2
(0.10 g, 0.20 mmol, COD = cyclooctadiene) in dichloromethane (15
mL). The mixture was allowed to stir for 30 min at room temperature.
The solution was concentrated by evaporation under reduced pressure,
and the residue was washed with diethyl ether (20 mL) to give 1 as a
blue solid (0.30 g, 0.33 mmol, 82%). 1H NMR (400 MHz, CDCl3, 298
K, relative to Me4Si)/ppm: δ 3.85 (s, 12 H, −OCH3), 3.88 (s, 24 H,
−OCH3), 6.86 (s, 8 H, −C6H2−). IR (KBr disk): v = 2160 cm−1
(v(CN)). MS (positive FAB): m/z 875 [M − Cl]+, 682 [M −
isocyanide − Cl]+. Elemental analyses calcd for C40H46ClN4O13Rh (1·
H2O), found (calcd): C, 51.54 (51.71); H, 5.07 (4.99); N, 5.95 (6.03).
Complex 1′. The saturated methanol solution of ammonium
hexafluorophosphate was dropwisely added to a concentrated
methanol solution 1 (0.05 g, 0.05 mmol) to give 1′ as green solid
quantitatively. Subsequent recrystallization by the layering of diethyl
ether onto a concentrated solution of 1′ gives the product as a
greenish-yellow microcrystal. 1H NMR (400 MHz, CDCl3, 298 K,
relative to Me4Si)/ppm: δ 3.85 (s, 12 H, −OCH3), 3.87 (s, 24 H,
−OCH3), 6.84 (s, 8 H, −C6H2−). 31P NMR (376 MHz, CDCl3,
relative to H3PO4): δ 143.98 (heptet, JP−F = 1764 Hz). IR (KBr disk):
v = 2160 cm−1 (v(CN)), 840 (v(P−F)). MS (positive FAB): m/z
875 [M − PF6]+. Elemental analyses calcd for C44H54Cl2F6N4O12PRh
(1′·CH2Cl2), found (calcd): C, 45.61 (45.97); H, 4.58 (4.73); N, 5.17
(4.87).
1
solid (0.08 g, 0.02 mmol, 78%). H NMR (400 MHz, CDCl3, 298 K,
relative to Me4Si)/ppm: δ 0.88 (t, J = 6.0 Hz, 12 H, −CH3), 1.26 (m,
288 H, −CH2−), 1.44 (m, 24 H, −CH2−), 1.76 (m, 24 H, −CH2−),
3.92 (t, J = 6.0 Hz, 24 H, −OCH2−), 6.73 (s, 8 H, −C6H2−). IR (KBr
disk): v = 2160 cm−1 (v(CN)). MS (positive FAB): m/z 3398 [M −
Cl]+. Elemental analyses calcd for C223H412ClN4O13Rh (6), found
(calcd): C, 76.67 (76.96); H, 11.80 (11.93); N, 1.79 (1.61).
Complex 7. The compound was prepared according to the
procedure described for complex 1, except 15-isocyanobenzo-15-
crown-5 (0.10 g, 0.34 mmol) was used in place of 5-isocyano-1,2,3-
trimethoxybenzene to give complex 7 as a brown solid (0.05 g, 0.04
mmol, 90%). 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si)/
ppm: δ 3.69 (m, 32 H, −OCH2CH2OCH2CH2O−), 3.91 (t, J = 6.0
Hz, 16 H, −CH2O−), 4.15 (t, J = 6.0 Hz, 16 H, −OCH2−), 6.86 (d, J
= 6.0 Hz, 4 H, −C6H3−), 7.06 (m, 8 H, −C6H3−). IR (KBr disk): v =
2160 cm−1 (v(CN)). MS (positive FAB): m/z 1276 [M − Cl]+, 982
[M
−
isocyanide − Cl]+. Elemental analyses calcd for
C63H86ClN4O20Rh (7), found (calcd): C, 55.45 (55.73); H, 6.35
(6.38); N, 4.18 (4.13).
Complex 8. The compound was prepared according to the
procedure described for complex 1, except 18-isocyanobenzo-18-
crown-6 (0.10 g, 0.30 mmol) was used in place of 5-isocyano-1,2,3-
trimethoxybenzene to give complex 8 as a brown solid (0.04 g, 0.03
mmol, 74%). 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si)/
ppm: δ 3.71 (m, 48 H, −OCH2CH2OCH2CH2CH2CH2O−), 3.92 (t, J
= 6.0 Hz, 16 H, −CH2O−), 4.15 (t, J = 6.0 Hz, 16 H, −OCH2−), 6.80
(m, 4 H, −C6H3−), 6.92 (m, 4 H, −C6H3−) 7.02 (dd, J = 8.0 Hz, 2.0
Hz, 4H, −C6H3−). IR (KBr disk): v = 2160 cm−1 (v(CN)). MS
(positive FAB): m/z 1452 [M − Cl]+, 1114 [M − isocyanide − Cl]+.
Elemental analyses calcd for C69H94Cl3N4O24Rh (8·CH2Cl2), found
(calcd): C, 52.22 (52.69); H, 6.18 (6.02); N, 3.67 (3.56).
Complex 2. The compound was prepared according to the
procedure described for complex 1, except 5-isocyano-1,2,3-triethox-
ybenzene (0.20 g, 0.85 mmol) was used in place of 5-isocyano-1,2,3-
trimethoxybenzene to give complex 2 as a brown solid (0.10 g, 0.09
mmol, 87%). 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si)/
ppm: δ 1.34 (t, J = 6.0 Hz, 12 H, −CH3), 1.43 (t, J = 6.0 Hz, 24 H,
−CH3), 4.06 (q, J = 6.0 Hz, 24 H, −OCH2−), 6.78 (s, 8 H, −C6H2−).
IR (KBr disk): v = 2160 cm−1 (v(CN)). MS (positive FAB): m/z
1042 [M]+. Elemental analyses calcd for C55H78ClN4O13Rh (2·H2O),
found (calcd): C, 57.71 (57.87); H, 6.38 (6.59); N, 5.16 (4.91).
Complex 3. The compound was prepared according to the
procedure described for complex 1, except 5-isocyano-1,2,3-tributox-
ybenzene (0.20 g, 0.63 mmol) was used in place of 5-isocyano-1,2,3-
trimethoxybenzene to give complex 3 as an oily green solid (0.10 g,
Physical Measurements and Instrumentation. 1H NMR
spectra were recorded on a Bruker DPX-300 (300 MHz) or Bruker
DPX-400 (400 MHz) Fourier transform NMR spectrometer with
chemical shifts recorded relative to tetramethylsilane (Me4Si). Positive
FAB mass spectra were recorded on a Thermo Scientific DFS High
Resolution Magnetic Sector Mass Spectrometer. IR spectra were
obtained as KBr disks on a Bio-Rad FTS-7 FTIR spectrometer (4000−
400 cm−1). Elemental analyses of the newly synthesized complexes
were performed on a Flash EA 1112 elemental analyzer at the Institute
of Chemistry, Chinese Academy of Sciences. The electronic
1
0.07 mmol, 90%). H NMR (400 MHz, CDCl3, 298 K, relative to
Me4Si)/ppm: δ 0.96 (t, J = 6.0 Hz, 36 H, −CH3), 1.48 (m, 24 H,
−CH2−), 1.70 (m, 24 H, −CH2−), 3.96 (m, 24 H, −OCH2−), 6.75
(s, 8 H, −C6H2−). IR (KBr disk): v = 2160 cm−1 (v(CN)). MS
(positive FAB): m/z 1394 [M − Cl]+, 1060 [M − isocyanide − Cl]+,
727 [M − 2isocyanide − Cl − CH3]+. Elemental analyses calcd for
J
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Article
absorption spectra were obtained by using a Hewlett−Packard 8452 A
diode array spectrophotometer and Lambda 750 UV−vis−NIR
Spectrophotometer. The concentrations of solution samples for
electronic absorption measurements were typically in the range of 1
× 10−6 to 2 × 10−4 mol dm−3. Steady state excitation and emission
spectra were recorded at room temperature and at 77 K on a Spex
Fluorolog-3 model FL3-211 fluorescence spectrofluorometer equipped
with an R2658P PMT detector. Solid-state photophysical studies were
carried out with solid samples contained in a quartz tube inside a
quartz-walled Dewar flask. Measurements of the ethanol−methanol
(4:1, v/v) glass or solid-state sample at 77 K were similarly conducted
with liquid nitrogen filled into the optical Dewar flask. The
concentrations of the complexes in ethanol−methanol (4:1, v/v) for
glass emission measurements were usually in the range of 10−6 mol
dm−3. All solutions for photophysical studies were degassed on a high-
vacuum line in a two-compartment cell consisting of a 10 mL Pyrex
bulb and a 1 cm path length quartz cuvette and sealed from the
atmosphere by a Bibby Rotaflo HP6 Teflon stopper. The solutions
were rigorously degassed with at least four successive freeze−pump−
thaw cycles. Emission lifetime measurements were performed by using
a conventional laser system. The concentrations of the complexes in
solution for lifetime measurements were usually about 2 × 10−5 mol
dm−3. The excitation source used was a 355 nm output (third
harmonic) of a Spectra-Physics Quanta-Ray Q-switched GCR-150−10
pulsed Nd:YAG laser. Luminescence decay signals were detected by a
Hamamatsu R928 PMT and recorded on a Tektronix Model TDS-
620A (500 MHz, 2GS/s) digital oscilloscope and analyzed by using a
program for exponential fits. Luminescence quantum yields were
measured by the optical dilute method reported by Demas and
Crosby.21a A degassed aqueous solution of [Ru(bpy)3]Cl2 (Φ = 0.042,
excitation wavelength at 436 nm) was used as the reference.21b TEM
experiments were performed on a Philips CM100 transmission
electron microscope. They were conducted at the Electron Microscope
Unit of The University of Hong Kong. The samples were prepared by
dropping a few drops of solutions onto a carbon-coated copper grid.
Slow evaporation of solvents in air for 10 min was allowed before
placing the samples into the instrument. The AFM images were
obtained using a Digital Instruments Nanoscope III AFM with a scan
rate of 1.0 μm s−1. The sample was prepared by dropping a few drops
of solution onto a silicon wafer which was then dried under a vacuum
before the measurement. The detection limit was determined from the
3σ/m method, where σ is the standard deviation of the blank host
emission, which was determined from a series of ten independent
measurements of the emission intensity, and m is the calibration
sensitivity, which corresponds to the slope of the linear part of the
titration curve.22
AUTHOR INFORMATION
Corresponding Author
■
*wwyam@hku.hk
Present Address
†Department of Chemistry, South University of Science and
Technology of China, No. 1088, Tangchang Boulevard,
Nanshan District, Shenzhen, Guangdong, PR China.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
V.W.-W.Y. acknowledges support from The University of Hong
Kong and the URC Strategic Research Theme on New
Materials. This work has been supported by the University
Grants Committee Areas of Excellence Scheme (AoE/P-03/
08) and a General Research Fund (GRF) grant from the
Research Grants Council of Hong Kong Special Adminstrative
Region, P.R. China (HKU 7060/12P). A.K.-W.C. acknowl-
edges the receipt of a Hung Hing Ying Scholarship and a Hong
Kong Ph.D. Fellowship administered by the University of Hong
Kong and the Research Grant Council, respectively. Dr. Vonika
Ka-Man Au is gratefully acknowledged for her helpful
discussions. Miss Bella Hoi-Shan Chan from the Chinese
University of Hong Kong is gratefully acknowledged for
mounting the crystals for structural determination and solving
the X-ray crystal structure.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Crystal and structure determination data (CCDC 1052139);
photophysical data; concentration-dependent UV−vis absorp-
tion spectra of 1, 2, 5, 6 and their corresponding dimerization
plot for monomer−dimer equilibrium; temperature-dependent
UV−vis absorption data of 1 and 5, and curve-fitting by the
temperature-dependent supramolecular aggregation model and
van’t Hoff analysis; mixed solvent UV−vis absorption data of
the complexes and curve-fitting with the equilibrium model on
solvent-dependent dynamics; TEM data of 2; DLS data of 5 in
70 and 90% hexane−dichloromethane solution; energy-
dispersive X-ray spectroscopy (EDX) data and selected area
electron diffraction (SAED) pattern of 1 and 5; UV−vis and
emission titration studies of 7 with Ba(ClO4)2 and NaClO4;
positive ESI-mass spectra of 1:1 host−guest mixture of 7 and
potassium ions; UV−vis and emission titration studies of 8 with
KClO4 and CsClO4. The Supporting Information is available
free of charge on the ACS Publications website at DOI:
10.1021/jacs.5b03396.
K
DOI: 10.1021/jacs.5b03396
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