Restricted conformational flexibility of a triphenylamine derivative on the formation of host-guest complexes with various macrocyclic hosts

Article abstract of DOI:10.1002/chem.201103079

Herein, we report the host-guest-type complex formation between the host molecules cucurbit[7]uril (CB[7]), β-cyclodextrin (β-CD), and dibenzo[24]crown-8 ether (DB24C8) and a newly synthesized triphenylamine (TPA) derivative 1X₃ as the guest co

Full text of DOI:10.1002/chem.201103079

DOI: 10.1002/chem.201103079
Restricted Conformational Flexibility of a Triphenylamine Derivative on the
Formation of Host–Guest Complexes with Various Macrocyclic Hosts
Amal Kumar Mandal, Moorthy Suresh, Priyadip Das, and Amitava Das*[a]
Abstract: Herein, we report the host–
guest-type complex formation between
the host molecules cucurbit[7]uril
(CB[7]), b-cyclodextrin (b-CD), and di-
benzo[24]crown-8 ether (DB24C8) and
tion studies with CB[7] and b-CD in an
aqueous medium. The restricted inter-
nal rotation of the guest molecule on
complex formation with either of these
two host molecules was reflected in the
enhancement of the emission quantum
yield and the average excited-state life-
time for the triphenylamine-based ex-
cited states. Studies with DB24C8 as
the host molecule were performed in
dichloromethane, a medium that maxi-
mizes the noncovalent interaction be-
tween the host and guest fragments.
The Fçrster resonance energy transfer
(FRET) process involving DB24C8 and
a
newly synthesized triphenylamine
(TPA) derivative 1X3 as the guest com-
ponent. The host–guest complex forma-
tion was studied in detail by using
1H NMR, 2D NOESY, UV/Vis fluores-
cence, and time-resolved emission
spectroscopy. The chloride salt of the
TPA derivative was used for recogni-
1ACTHNURGTENUNG(PF6)3, as the donor and acceptor
Keywords: host–guest systems · in-
ternal rotation · noncovalent inter-
actions · pseudorotaxanes · reso-
nance energy transfer
fragments, respectively, was established
by electrochemical, steady-state emis-
sion, and time-correlated single-photon
counting studies.
Introduction
the possibility of probing such conformational or structural
changes on inclusion-complex formation by monitoring asso-
ciated electronic or fluorescence spectral changes. However,
reports on probing such a change in molecular conformation
or relative orientation due to inclusion-complex formation
by monitoring changes in the electronic or luminescence
properties are not very common.[5]
Achieving designer, interwoven complexes, such as pseudor-
otaxanes, rotaxanes, and catenanes, with appropriately sub-
stituted host and guest fragments to realize an assembly that
shows a special property is of current research interest.[1]
Various nonbonding interactions such as p–p,[2] hydrogen-
bonding,[3] hydrophobic, and van der Waals interactions are
generally operational for the generation and stabilization of
these supramolecular architectures.[4] Chemists have ach-
ieved predictable control over these noncovalent interac-
tions and have used these weak forces in their favor to de-
velop a plethora of intricate functional structures. In most
examples, a combination of various nonbonding interactions
is operational to stabilize such intricate supramolecular ar-
chitectures. A change of any or a combination of these inter-
actions may allow host molecules to adopt different relative
conformations/orientations in the host–guest assembly. This
outcome opens up the possibility of achieving desired inter-
component molecular movements as a result of the inclu-
sion-complex formation under the influence of an appropri-
ate external stimulation. Such host or guest molecules, ap-
propriately substituted with a chromogenic fragment, offer
It has been reported that triphenylamine (TPA) and sev-
eral of its derivatives have drawn much attention from re-
searchers with varying interests due to their huge applica-
tion potential in diverse areas, such as, organic light-emitting
diodes (OLED),[6] photoconductors,[7] laser dyes, and mecha-
noluminescent materials.[8] Previous reports of structural[9]
and theoretical studies[10] on TPA suggest that this molecule
preferentially adopts a three-bladed propeller structure with
a planar or nearly planar central N-C-C-C moiety. A sym-
metric top geometry for TPA with at least C3 symmetry is
proposed in the gas phase. Each of the three phenyl groups
in TPA is symmetrically twisted from the plane and makes
the propeller blade. Due to the deviation of the three
phenyl ring from coplanarity, TPA and its derivatives adopt
a 3D structure. Two different opposing forces, namely, the
energy-lowering p-conjugation of the phenyl ring and steric
repulsion between the inner-ring hydrogen atoms, are gener-
ally operational and control this conformation in different
TPA derivatives. The steric repulsion between the inner-ring
hydrogen atoms is highest for the perfectly planar conforma-
tion, which actually makes TPA a conformationally flexible
molecule. This conformational flexibility of TPA derivatives
has a direct relevance to their photoluminescence property
because a fast nonradiative relaxation of the excited state of
these compounds is favored. The lack of rigidity in the twist-
[a] A. K. Mandal, M. Suresh, P. Das, Dr. A. Das
Central Salt and Marine Chemicals Research Institute (CSIR)
Bhavnagar, 364002 Gujarat (India)
Fax : (+91)278-2567562
E-mail: amitava@csmcri.org
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103079.
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FULL PAPER
ed TPA derivatives also adversely affects their application
potential as a host material in OLEDs.[11]
constrictive binding that induces significant steric barriers to
guest association and dissociation.
All these approaches have initiated a surge of interest
among researchers to understand how the flexibility of these
molecules can be controlled to achieve the desired photo-
physical properties. However, efforts in this regard are
mostly restricted to the synthesis of highly rigid framework
TPA derivatives.[12] It is known that macrocyclic hosts
impose a structural confinement on the included chromo-
phoric guest molecules and suppress the nonradiative relax-
ation processes associated with the various molecular move-
ments/vibrations, which in turn improves the luminescence
quantum yield and the lifetime of the photoexcited states.[13]
Among various macrocyclic hosts, cyclodextrin (CD) and
cucurbit[n]uril (CB[n]) are most commonly used for such
studies. However, to the best of our knowledge no such
report with a TPA derivative as a guest molecule has been
reported to unveil the influence of inclusion-complex forma-
tion on the luminescence properties.
Cucurbiturils (CB[n]; n=5–8, 10) are composed of n gly-
coluril units that are coupled in a cyclic manner by 2n meth-
ylene bridges. The pumpkin-shaped CB[n] molecules are
highly symmetrical cage structures with two identical portal
ends flanked with highly polarizable ureido carbonyl groups
and an interior hydrophobic cavity. The presence of the in-
ternal hydrophobic cavity, accessible by two carbonyl open-
ings induces ion–dipole, dipole–dipole, and hydrogen-bond-
ing interactions, thus making this molecule favorable for in-
clusion-complex formation with various guest molecules.[14]
The CB[n] molecules have a common depth (9.1 ꢁ), but
their equatorial widths (5.4 ꢁ for n=7), annular widths
(7.3 ꢁ for n=7), and volumes vary systematically with ring
size. The portals that guard the entry to CB[7] are narrower
than the cavity itself by approximately 2 ꢁ, which results in
b-Cyclodextrin (b-CD) is another commonly used host
molecule, built up from seven glucopyranose subunits that
are connected at the 1,4-position. As a consequence of the
4C1 conformation of the glucopyranose units, all the secon-
dary hydroxy groups are situated on one of the two edges of
the ring, whereas all the primary hydroxy groups are placed
on the other edge. This makes the b-CD ring adopt a conical
cylinder structure. The electrostatic potential at the portals
and within the cavity of CB[7] is significantly more negative
than b-CD. This difference in electrostatic potential has sig-
nificance in their recognition behavior and the relative sta-
bility of the respective host–guest complexes.
A wide range of guests have been shown to form stable
inclusion complexes with cucurbiturils, including cations,[15,21]
neutral species,[16] and anions,[17] thus indicating the versatili-
ty and potential usefulness of a cucurbituril as a host mole-
cule. Whereas CDs can be used to accommodate a variety
of small organic molecules.[18] Apart from these two host
molecules, various crown ether derivatives are another type
of synthetic host for a wide variety of cationic guests. Vari-
ous guests, such as, metal ions, primary alkyl ammonium
ions (RNH3+),[19c,d] secondary dialkyl ammonium ions
(R2NH2+),[19] and imidazolium cations,[20] have received con-
siderable attention among the many guest species that have
been investigated.
We used the three different host molecules CB[7], b-CD,
and DB24C8 to study inclusion-complex formation with a
secondary ammonium salt of a TPA derivative (Figure 1).
These host molecules have comparable cavity volume, but
differ in geometrical features, solubilities, and association
patterns with the ammonium salt-based guest molecules. Re-
stricted tortional motion of the secondary ammonium salt of
Figure 1. Schematic representation of the different macrocyclic hosts and guest molecules used in this study.
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3907

A. Das et al.
the TPA derivative on inclusion-complex formation with dif-
ferent macrocylic host molecules is expected to improve the
emission quantum yield of the TPA core by suppressing the
nonradiative deactivation of the excited states. This behavior
offers an opportunity to probe the formation of such an in-
clusion complex by monitoring the emission response of the
TPA-based fragment. More importantly, studies with three
different host molecules and a common guest molecule
allow a direct comparison of the structural aspects of the
host molecules in influencing the relative stability of the dif-
ferent host–guest complexes thus formed. For each host–
guest complex formation, the respective binding stoichiome-
tries and association constants, along with the relative geo-
metries of the host–guest complexes were studied by using
Figure 2. Partial 1H NMR spectra (500 MHz, D2O, 298 K) of 1Cl3 (3.7ꢂ
10ꢀ3 m) with increasing [CB[7]]. a) In the absence of CB[7] and in the
presence of b) 1.0, c) 2.0, and d) 3.0 equivalents of CB[7].
1
1D and 2D H NMR spectroscopy, mass spectrometry (MS),
steady-state fluorescence spectroscopy, and time-resolved
studies using time-correlated single-photon counting
(TCSPC) technique.
the aromatic protons of 1Cl3 on formation of an inclusion
complex with CB[7] (Figure 2). The 1H NMR signals that
correspond to the aromatic protons of the benzyl unit (Ha,
Hb, and Hc) are broadened with an upfield shift of approxi-
mately Dd=0.83 ppm, whereas the signals for the other aro-
matic protons Hg and Hf of 1Cl3 are shifted downfield (Dd
ꢁ0.06 and 0.28 ppm for Hg and Hf, respectively). The down-
field shifts of signals for the Hg and Hf protons and the sig-
nificant upfield shifts of the signals for the benzyl Ha, Hb,
and Hc protons on inclusion-complex formation are consis-
tent with the fact that CB[7] engulfs the benzyl unit inside
its cavity, whereas the Hg and Hf protons interact with the
carbonyl oxygen atoms of the host portal. These outcomes
suggest the formation of an “external” inclusion complex
with 1Cl3. Presumably, steric hindrance could play a key
role in determining the main binding site for CB[7] and
allow the inclusion of a benzyl unit rather than the phenyl
moiety of the TPA core. Such an “external” complex forma-
tion was has been reported previously for the inclusion com-
plex formed between CB[7] and different derivatives of viol-
ogen as guest species.[21]
Results and Discussion
The secondary ammonium salt of TPA derivatives (1X3; X=
ꢀ
Clꢀ or PF6 ) were synthesized by treating benzylamine with
tris(para-formylphenyl)amine, and the obtained intermedi-
ate was further reduced with NaBH4 in methanol. The re-
spective amine was protonated with the appropriate acid
and isolated as a chloride salt, which was soluble in water
and used for recognition studies with CB[7] and b-CD be-
cause these hosts are only soluble in water. Complexation
studies with DB24C8 were performed in CH2Cl2; therefore,
the hexafluorophosphate salt of the protonated TPA deriva-
tive was used.
Complexation of 1Cl3 with CB[7]: The binding interaction
1
between 1Cl3 and CB[7] was monitored by H NMR spec-
troscopic analysis. Structurally, CB[7] has two distinctly dif-
ferent binding regions: 1) the hydrophobic cavity and 2) the
outer disk formed by the oxygen atoms that stabilize the
positive charge or cations. The guest protons located within
the hydrophobic cavity are expected to be shielded for the
1H NMR spectral signal (Dd<0), whereas guest protons lo-
cated at the more polar outer C=O cavity are deshielded
(Dd>0).
To gain insight into the molecular interaction and the ge-
ometry of the host–guest complex, we performed 2D
1H NMR spectroscopic studies. The NOESY spectrum of
1Cl3 in presence of three molar equivalents of CB[7] were
recorded (Figure 3a) and shows cross-peaks between the
protons (H1, H2, H3) of CB[7] and the aromatic protons of
1Cl3. These cross-peaks corroborate the formation of an in-
clusion complex between CB[7] and 1Cl3 in aqueous solu-
tion, whereas the binding interactions seem to be dominated
by hydrophobic forces, with one of the two aromatic rings
being included in the CB[7] cavity. It is anticipated that the
host–guest complex would experience increased steric
crowding when CB[7] engulfs the phenyl ring of the TPA
core rather than the external benzyl moiety of 1Cl3 inside its
cavity. Thus, the experimental data tend to imply that CB[7]
preferred the formation of an “external” inclusion complex.
The changes in the chemical shift values (Dd in ppm) for
1
Figure 2 shows the H NMR spectra of 1Cl3 recorded in
D2O in the absence (Figure 2a) and presence of CB[7] (Fig-
ure 2b–d) and reveals that the chemical shift of all the pro-
tons of 1Cl3 are shifted on host–guest inclusion-complex for-
mation relative to free 1Cl3. A closer look at the Figure 2 re-
veals that the signals for two sets of aliphatic protons of 1Cl3
shift in opposite directions; that is, an upfield shift of ap-
proximately Dd=0.4 ppm is observed for Hd, whereas a
downfield shift of approximately Dd=0.19 ppm is evident
for He. These chemical shifts indicate that Hd of 1Cl3 is in-
cluded inside the shielding region of the CB[7] cavity and
He of 1Cl3 is located in the deshielding region, outside of
the carbonyl portal. Appreciable shifts are also observed for
1
He of 1Cl3 in the H NMR spectrum with varying concentra-
tions of CB[7] in D2O were used for a Job plot analysis (Fig-
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Complexation of a Triphenylamine Guest with Macrocyclic Hosts
FULL PAPER
Figure 4. a) Scatchard plot for the complexation of 1Cl3 with CB[7] (D2O,
258C, [1Cl3]0 =3.7ꢂ10ꢀ3 m; y=ꢀ407.59x2 +433.88x+11.824, r2 =0.98),
where p is the fraction of 1Cl3 units bound. Error bars in p= ꢂ0.02;
error bar in p/[CB[7]]= ꢂ0.87. b) Job plot that reveals a 1:3 binding stoi-
chiometry between 1Cl3 and CB[7] in D2O by using changes in the chem-
ical shift d data for He ([1Cl3]0 +[CB[7]]0 =6.8ꢂ10ꢀ3 m).
on the p/[CB[7]] axis gave the value of K1 =(1.2ꢂ0.5)ꢂ
101 mꢀ1. The initial slope of the first six data points for low
values of p (Figure 4a) gave the value of 2K2ꢀK1 to give
K2 =(1.5ꢂ2.6)ꢂ102 mꢀ1. The final slope, calculated from last
nine data points for high values of p, gave the value of
[22b]
ꢀ3K3
to give K3 =(1.0ꢂ1.9)ꢂ102 mꢀ1 and Kav =(K1 +K2 +
K3)/3=(9.0ꢂ1.2)ꢂ101 mꢀ1 (see the Supporting Information
for further details).[22d] On the basis of on the above infor-
mation, a structural representation of the inclusion complex
1Cl3·3CB[7] is shown in Figure 3b.
Figure 3. a) Partial 2D NOESY NMR spectrum (500 MHz, D2O, 298 K)
of a solution of 1Cl3 (3.7ꢂ10ꢀ3 m) and CB[7] (11.2ꢂ10ꢀ3 m). b) Mode of
binding interaction between 1Cl3 and CB[7] is shown.
ESI mass-spectrometric studies were performed to con-
firm the host–guest complex formation and establish the
binding stoichiometry. The mass spectrum was dominated
by signals that correspond to the inclusion complexes with a
1:3 binding stoichiometry (Table 1). The calculated and ob-
served masses for 1Cl3·3CB[7] are summarized in Table 1.
ure 4b), which signified a 1:3 binding stoichiometry for the
inclusion-complex formation between 1Cl3 and CB[7]. A
Scatchard plot (Figure 4a) was drawn to study the relation-
ship between the three binding sites of 1Cl3 during complex
formation with CB[7]. The non-
linear nature of this plot with a
maximum value reveals that
positive cooperativity drives the
complex-formation process.[22]
Analysis of the Scatchard
plot enabled us to find out the
stoichiometric binding constants
K1, K2, and K3 and the apparent
average association constant
Kav. From the plot of p/[CB[7]]
versus p (p=extent of complex-
Table 1. The MALDI-TOF and ESI mass spectrometric data for the inclusion complex 1Cl3·3G (G=CB[7], b-
CD, or DB24C8).
Host–guest system
Molecular formula
m/z
calcd
observed
1Cl3·3CB[7]
1Cl3·3b-CD
G
1369.59
1342.16
2409.52
650.52
1369.02, 1369.32, 1369.65[a]
1341.45, 1341.75, 1342.13[a]
2409.55[b]
{1Cl3+3b-CDꢀ3Cl+H2O}3+/3
1
(PF6)3·3DB24C8
650.16, 650.48, 650.81[c]
[a] ESI mass spectrometric data were recorded in the positive-ion mode with water as the solvent.
[b] MALDI-TOF mass spectrometric data were recorded in the positive-ion mode with CH2Cl2/CH3CN (4:1,
v/v) as the solvent. [c] ESI mass spectroscometric data were recorded in the positive-ion mode with CH2Cl2/
ation; Figure 4a), the intercept CH3CN (4:1, v/v) as the solvent.
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A. Das et al.
Complexation of 1Cl3 with b-CD: Four protons H1, H2, H4,
and H6 are located on the external surface of the molecule
in b-CD, whereas other two protons H3 and H5 are in the in-
terior of the cavity at the wide and narrow rims, respective-
ly. Thus, the signals for H3 and H5 are expected to be mostly
influenced on inclusion-complex formation and are most
sensitive to the inclusion of the guest molecule in the b-CD
cavity. This outcome was indeed evident when the complex
formation between 1Cl3 and b-CD was monitored by
1H NMR spectroscopic analysis in D2O with varying concen-
trations of b-CD. Figure 5 reveals that on the addition of in-
ring of 1Cl3 occupies a more shielded environment of the
hydrophobic b-CD cavity in the inclusion complex. Figure 5
also reveals that on the formation of an inclusion complex
with b-CD both Hd and He of 1Cl3 are shifted downfield.
These findings signify hydrogen-bond formation between
ꢀ
these protons and the hydroxy functionality of the CH2OH
groups of the narrower rim of b-CD. A significant upfield
shift for the protons of b-CD, positioned inside the cavity
(i.e., H3 and H5) and the protons located near the narrow
rim (i.e., H6) of b-CD are also evident on inclusion-complex
formation (Figure 5). The chemical shifts for the protons H2
and H4 positioned outside the cavity are insignificant on for-
mation of the inclusion complex. All these data tend to con-
firm the formation of the “host–guest” inclusion complex
1Cl3·3b-CD.
The information regarding the side of inclusion of the
guest 1Cl3 into the b-CD cavity, that is, whether through the
narrower or wider rim of b-CD, could be derived from the
relative change in the chemical shift values for the H3 and
H5 protons of the host molecule b-CD. It is reported that in
general DdH >DdH for the inclusion of the guest molecule
3
5
through the wider rim side of b-CD and vice versa.[23] In the
present study, the observed chemical shift of H3 (DdH
=
3
0.50 ppm upfield shift) is more prominent than that for H5
(DdH =0.46 ppm upfield shift), thus confirming the penetra-
5
tion of 1Cl3 through the wider rim side of b-CD (Figure 6).
The 2D NOESY spectrum revealed cross-peaks between the
aromatic protons and the H5 and H6 protons of b-CD (see
the Supporting Information).
To study the involvement of the three binding sites of
1Cl3 with b-CD, 1H NMR spectroscopic studies were per-
formed with a series of solutions for which the initial con-
centration of 1Cl3 was kept constant at 8.4 mm, while the
concentration of b-CD was systematically varied. On the
1
Figure 5. Partial H NMR spectra (500 MHz, D2O, 298 K) of a solution of
1Cl3 (8.4ꢂ10ꢀ3 m) with increasing [b-CD]. a) In the absence of b-CD and
in the presence of b) 1.0, c) 2.0, and d) 3.0 equivalents of b-CD. The spec-
trum portrays a system of slow exchange, such that the two observable
sets of signals that correspond to the complexed and uncomplexed (de-
noted by star) species.
creasing amounts of b-CD, the
resonance of the aromatic pro-
tons of 1Cl3 in the 1H NMR
spectrum were shifted upfield,
whereas the signals for the aro-
matic protons of uncomplexed
1Cl3 remain unaffected. These
signals indicate that the ex-
change between the free and
bound guest with b-CD is slow
1
on the H NMR timescale. The
upfield shifts seen in Figure 5
are most prominent (Dd
ꢁ0.68 ppm) for the three pro-
tons of the benzyl ring Ha, Hb,
and Hc relative to the other two
aromatic protons Hg and Hf
(Ddꢁ0.10 and 0.07 ppm, re-
spectively) of the phenyl ring of
the TPA core in 1Cl3. This be-
Figure 6. Schematic diagram to reveal the mode of penetration of 1Cl3 in the b-CD cavity on the inclusion-
havior suggests that the benzyl complex formation.
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Complexation of a Triphenylamine Guest with Macrocyclic Hosts
FULL PAPER
Figure 8. Partial 1H NMR spectra (500 MHz, D2O, 298 K) of 1
ACHTUNGTRENNUNG
varying [DB24C8] (CD2Cl2/CD3CN (4:1, v/v), 258C, [1ACHTUNGTRENNUNG
10ꢀ3 m). a) In the absence of DB24C8 and in the presence of b) 1.0, c) 2.0,
and d) 3.0 equivalents of DB24C8. The spectrum portrays a system of
slow exchange, such that the two observable sets of signal correspond to
the complexed (denoted by star) and uncomplexed DB24C8.
Figure 7. a) Scatchard plot for the complexation of 1Cl3 with b-CD,
ꢀ
where p is the fraction of NCH2 units of 1Cl3 that exist as the included
complex with b-CD as the host component (D2O, 25 8C, [1Cl3]0 =8.4ꢂ
10ꢀ3 m; error bars in p= ꢂ0.01; error bar in p/
[b-CD]= ꢂ2.88). b) Com-
ꢀ
ꢀ
plexation of 1
(PF6
)
3
ꢀ
units of 1
host (CD2Cl2/CD3CN (4:1, v/v), 258C, [1
in p= ꢂ0.01; error bar in p/[DB24C8]= ꢂ2.37).
ACHTUNGTRENNUNG(PF6 ) bound as the included complex with DB24C8 as the
3
tra for respective protons of 1
in the free and complexed forms (Figure 8). The signal for
Hd and He of the uncomplexed benzyl moiety of 1(PF6)3
ACHTUGNTRENUN(GN PF6)3 and DB24C8 that exist
AHCTUNGTRENNUNG
(PF6ꢀ)3]=7.6ꢂ10ꢀ3 m; error bars
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
appear at 4.21 and 4.14 ppm, which were shifted downfield
on the formation of an inclusion complex and appeared at
4.63 ppm. An increase in the intensity of this signal at
4.63 ppm was observed with an increase in the concentration
of DB24C8. The percentage of ammonium ion-based thread
basis of these data, the extent of complexation p was deter-
mined[22d] and a Scatchard plot[22] was constructed (Fig-
ure 7a). The linear nature of this plot demonstrates that the
complex formation between 1Cl3 and b-CD is statistical,
that is, the three binding sites behave independently. The
average association constant[22] Kav =(3.4ꢂ12.8)ꢂ102 mꢀ1
was evaluated from the intercept and the slope of the
plot.[24] As the complexation process was slow on the
1H NMR timescale, the stoichiometry of the complex was
1ACTHUNGRTNEUNG(PF6)3 that was present as the complexed form was evaluat-
ed by comparison of the integration of these two sets of sig-
nals, which varied from 42.1 to 96.3% when [DB24C8]=
6.2–56.1 mm. A similar downfield shift was reported for the
methylene protons in the 1H NMR spectra of complexes
formed between DB24C8 and substituted dibenzylammoni-
um cations.[26] Interestingly, a gradual upfield shift for the
1
determined by simple integration of the H NMR signals for
the relevant protons of the free and complexed 1Cl3 and
was found to be 1:3. ESI-MS measurements provided evi-
dence for the 1:3 binding stoichiometry and the formation
of the host–guest complex 1Cl3·3b-CD (Table 1).
signals for the Hd and He protons of uncomplexed 1ACHTUNGTRENNUNG(PF6)3
was also observed on an increase in the concentration of
1
DB24C8 during the H NMR spectroscopic titration process.
The gradual upfield shift of the uncomplexed benzylic
protons Hd and He with an increasing concentration of the
crown ether as the host component has also been observed
by many research groups. Gibson et al. concluded for analo-
Complexation of 1
host–guest complex between 1AHCNUTGTRENNUNG
ACHTUNGTRENNUNG
polar solvent was established by 1H NMR spectroscopic
analysis. Earlier reports reveal that pseudorotaxanes exist in
equilibrium with the individual components in a completely
dissociated form.[25] Various nonbonding interactions present
in the thus-formed pseudorotaxane are responsible for the
stability of the included complex (i.e., the pseudorotaxane)
over its precursors (i.e., the macrocyclic host and thread as
the guest). In the present study, a slow kinetic exchange (on
ꢀ
gous systems that the buildup of the concentration of PF6
ions around the free quaternary ammonium ion during the
inclusion-complex formation was not responsible for this
shift.[22d] Stoddart and co-workers reported that a similar up-
field shift is the consequence of the shielding effect from a
p-stacked catechol ring.[27] Figure 8 reveals that in the pres-
1
ꢀ
ent study the H NMR signals for the OCH2 groups of the
1
the H NMR timescale at 500 MHz) as a result of the sur-
crown ether moieties are shifted upfield on complexation,
which is an observation that is consistent with the previous
reports in which similar upfield shifts were registered for
DB24C8 and its derivatives upon complexation with diben-
mountable steric barrier for the slippage of the DB24C8
ring over the relatively bulky benzyl unit was present. This
behavior accounts for observed signals in the H NMR spec-
1
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3911

A. Das et al.
zylammonium ions.[28] Upfield shifts are evident for all the
aromatic protons of 1(PF6)3 with increasing concentration of
DB24C8 (Figure 8). The aromatic protons of DB24C8 are
also shifted upfield on complexation, which is used to esti-
mate the extent of complexation.
(ca. 6%) was observed when b-CD was used. No shift in the
absorption maxima was observed in either case (see the
Supporting Information). However, both features are indica-
tive of binding of the dye to the macrocycle. The absorption
AHCTUNGTRENNUNG
spectrum of 1ACHTNUGTRNEG(UN PF6)3 was recorded in presence of three
To study the relationship between the three binding sites
equivalents of DB24C8 in dichloromethane, thus giving two
maxima at l=281 and 317 nm. The band at l=281 nm was
assigned for a DB24C8-based transition, which for free
DB24C8 appeared at l=274 nm. The band at l=317 nm
showed hypochromism (ca. 6%) when recorded in the pres-
ence of DB24C8, though no shift in the absorption maxima
at l=317 nm was observed.
The solvatochromic behavior of 1X3 was examined in dif-
ferent solvents with varying polarities (see the Supporting
Information). A redshift of Dl=20 nm in the emission band
maxima was observed in cyclohexane (lmax, em =364 nm for
lext =309 nm) and water (lmax, em =384 nm for lext =309 nm).
In contrast, no shift in the absorption band was noticed with
the change in media polarity. Interestingly, the emission
spectra were broader with a solvent of higher polarity. The
observed redshift and broader emission band could be at-
tributed to an internal charge-transfer excited state, which is
also very likely to be twisted in geometry due to the flexible
nature of the molecule. Such behavior is not uncommon for
numerous fluorescent molecules with donor–acceptor com-
ponents, including TPA derivatives.[30]
of
1ACHTUNGTRENNUNG(PF6)3 during the complex-formation process with
1
DB24C8, H NMR spectroscopic studies were performed on
a series of dichloromethane/acetonitrile (4:1 v/v) solutions,
for which the initial concentration of 1ACTHNUTGRNEG(UN PF6)3 was kept at
7.68 mm and the concentration of DB24C8 was varied sys-
tematically. The p value was evaluated for these data and a
Scatchard plot[22] was prepared (Figure 7b). The linear
nature of this plot demonstrated that the complexation be-
tween 1ACHTUNGTRENNUNG(PF6)3 and DB24C8 was statistical. From the inter-
cept and the slope of the plot, Kav =(1.9ꢂ6.7)ꢂ102 mꢀ1 was
evaluated.[29] The value of the average binding constant,
evaluated following the same method adopted by Gibson
et al.,[22d] for the corresponding tritopic ammonium guest
1,3,5-tris
G
benzene
trishexafluorophosphate and DB24C8 was Kav =1.5ꢂ102 mꢀ1
in CD3CN. The lower polarity of the solvent medium
(CD2Cl2/CD3CN=4:1, v/v) in the present study could have
contributed to a slightly higher value for Kav than that re-
ported in CD3CN.[22d] As the complexation process is slow
1
on the H NMR timescale, the stoichiometry of the complex
was determined to be 1:3 by simple integration of the signals
for the relevant protons in the free and complexed states. A
binding stoichiometry of 1:3 was also confirmed from mass-
spectrometric analysis (ESI-MS and MALDI-TOF; Table 1).
These results also support the nonvanishing dipole
moment of TPA in solution and are consistent with a molec-
ular geometry that is slight nonplanar at the central nitrogen
atom. The solvatochromic behavior was also supported by a
decrease in the fluorescence quantum yield with an increase
in solvent polarity (F=0.0099 and 0.0119 in water and
CH2Cl2, respectively). The lower quantum yield in water is
presumably due to the lowered energy of the increased sta-
bilization of the highly polar intramolecular charge-transfer
(ICT) states.[31] The emission band maxima for 1Cl3 in water
appeared at lmax =388 nm for excitation at l=309 nm and a
significant change in the lumi-
UV/Vis, fluorescence, and TCSPC studies: The UV/Vis and
ꢀ
steady-state emission spectra for 1X3 (X=Clꢀ, PF6 ) were
recorded in either water or dichloromethane (the relevant
data are summarized in Table 2). The absorption spectrum
of 1Cl3 in water showed an intense absorption band (e=
2.8ꢂ104 Lmolꢀ1 cmꢀ1) with an absorption maxima at lmax
=
nescence intensity was observed
in the presence of CB[7] and b-
Table 2. Spectral parameters for 1X3.
Host–guest system
e [Lmolꢀ1 cmꢀ1
]
labs [nm]
lem [nm]
F
CD (Figure 9). In the case of
CB[7], an enhancement of
430% was observed (F=
0.0283) for the highest concen-
tration of CB[7] used (CB[7]/
1Cl3 =8:1; [1Cl3]=9.2ꢂ10ꢀ6 m;
Figure 9a).
However, the changes were
different for b-CD from that
observed for CB[7] (Figure 9b):
1Cl3
2.8ꢂ104[a]
3.7ꢂ104[b]
309[a]
317[b]
388[a]
0.0099[a]
0.0119[b]
0.0580[c]
0.0283[a]
0.0182[a]
0.0386[b]
1
N
369,[b] 482,[b] 370[c]
1Cl3·3CB[7]
1Cl3·3b-CD
2.9ꢂ104[a]
2.6ꢂ104[a]
3.5ꢂ104[b]
309[a]
307[a]
317[b]
384[a]
380[a]
1
N
368,[b] 500[b]
[a] Measurements were performed in water. [b] Measurements were performed in dichloromethane. [c] Meas-
urements were performed in glycerol.
309 nm; whereas, lmax =317 nm and e=3.7ꢂ104 Lmolꢀ1 cmꢀ1
in dichloromethane. The observed absorption band was at-
tributed to the triphenylamine-based p–p* transition. Ab-
sorption spectra for 1Cl3 were also recorded in water in the
presence of three equivalents of CB[7] or b-CD. Interesting-
ly, the addition of CB[7] induced a hyperchromism (ca. 7%)
of the absorption band for 1Cl3, whereas a hypochromism of
an increase of 260% in luminescence intensity (F=0.0182)
and a blueshift of Dl=15 nm for the band maxima were ob-
served at the highest concentration of b-CD used (i.e., b-
CD/1Cl3 =15:1). Molecule 1Cl3 showed weak emission in
the free state, and the respective quantum yield was F=
0.0099. This difference in fluorescent enhancement of 1Cl3
in the presence of CB[7] and b-CD is essentially due to the
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Complexation of a Triphenylamine Guest with Macrocyclic Hosts
FULL PAPER
at l=388 nm for 1Cl3, showed an average lifetime of
0.80 ns. The decay profiles were also recorded in the pres-
ence of three molar equivalents of CB[7] and b-CD
(Table 3) to give average lifetimes of 1.05 and 1.45 ns, re-
Table 3. Fluorescence lifetime t data obtained by using a nano-LED as
an excitation source (l=280 nm) and monitoring the wavelength lem in
water and dichloromethane at 258C.
Host–guest system
lem [nm]
t [ns]
c2
1Cl3
388[a]
388[a]
388[a]
300[b]
380[b]
300[b]
0.80
1.05
1.45
0.57
0.91
0.92
1.06
1.12
1.17
0.91
1Cl3·3CB[7]
1Cl3·3b-CD
DB24C8
1
1
U
2.98
0.42 (61.3%), 2.23 (38.7%)
[a] Measurements were performed in water. [b] Measurements were per-
formed in dichloromethane. c2 is a numerical value that reflects the over-
all goodness of fit.
spectively. The increase in the average lifetime of 1Cl3 in the
presence of CB[7] and b-CD also supports the restriction of
internal rotation in the excited state on inclusion-complex
formation. This behavior along with the decrease in the
local polarity is expected to decrease the deactivation pro-
cesses through a nonradiative pathway and to subsequently
enhance the luminescence quantum yield.
Figure 9. Fluorescence spectra of 1Cl3 in water upon the addition of in-
creasing concentrations of a) CB[7] and b) b-CD (1Cl3 =9.2ꢂ10ꢀ6 m,
lexc =309 nm).
fundamental differences in the topology of the upper and
lower rims of the two classes of macrocyclic host and the
difference in their binding abilities. To develop a better un-
derstanding about the origin of this effect, additional meas-
urements were made in glycerol, a medium known for its
high viscosity so that restricted molecular movements could
be induced. The quantum yield recorded for 1X3 in glycerol
was F=0.058, which is significantly higher than those deter-
mined in the presence of the macrocyclic hosts CB[7] and b-
CD (Table 2). This outcome suggests that the origin of this
binding-induced fluorescence enhancement is attributed
mainly to the restricted internal rotation of the molecule on
the formation of the inclusion complex. It is widely accepted
that internal rotation generally provides additional channels
for nonradiative de-excitation of the excited states.[32] Addi-
tionally, recent reports also revealed significant fluorescence
enhancements for TPA-based dye molecules as a result of
restricted rotational freedom upon binding to DNA and
RNA matrices.[33] In the present study, an appreciable steric
hindrance was induced in 1Cl3 to inhibit the rotational free-
dom of the TPA-based core on the formation of inclusion
complexes with these macrocyclic hosts. Additionally, the re-
duction of local polarity experienced by the TPA derivative
within the macrocyclic host cavity, relative to that in aque-
ous solution, might have also contributed to the observed
fluorescence enhancement.
The electronic spectra for 1ACHTNUGRTNEUNG(PF6)3 and its inclusion com-
plex with DB24C8 in dichloromethane were different from
those for 1Cl3·3CB[7] and 1Cl3·3b-CD (see the inset of Fig-
ure 10a and the Supporting Information). For the 1:3 com-
plex between DB24C8 with 1
the absorption spectrum of the [4]pseudorotaxane complex
was different from its individual components 1(PF6)3 and
DB24C8. The redshifted absorption band for the [4]pseudo-
rotaxane complex appeared at l=282 nm relative to the ab-
ACHTUNTGRENNGU(PF6)3 (i.e., 1ACHTUGNTRENN(UGN PF6)3·3DB24C8),
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
sorption band maxima for DB24C8 at l=275 nm. This no-
ticeable shift in the absorption band maxima is indicative of
an interaction in the ground state for the [4]pseudorotaxane.
Changes in the luminescence spectrum for 1ACHTNUGTRNEG(UN PF6)3 record-
ed in the absence and presence of three molar equivalents
of DB24C8 were even more significant. DB24C8 in di-
chloromethane showed an emission band with maxima at
lmax =310 nm on excitation at l=280 nm (where DB24C8
absorbs predominantly); however, the emission intensity at
l=310 nm for [4]pseudorotaxane was completely quenched
when compared with free DB24C8. Subsequently, an in-
crease in emission intensity at l=369 nm, which corre-
sponds to a 1
compared with that for a comparable concentration of free
(PF6)3. These steady-state emission results suggest a possi-
ACHTUGNERTN(NUNG PF6)3-based emission, was observed when
1ACHTUNGTRENNUNG
ble energy-transfer (ET) process that involves the low-lying
excited state of DB24C8 as the donor state and the even
lower excited state of 1ACHTNUGTRNEUGN(PF6)3 as the acceptor state. To check
the thermodynamic feasibility of such an ET process, it is es-
sential to have some idea about the relative energies for the
LUMO for the chromophores, namely, DB24C8 and 1ACHTUNGTRENNUNG(PF6)3,
Luminescence-decay profiles for the excited state were
measured for 1Cl3 in the presence and absence of CB[7] and
b-CD by using time-correlated single-photon counting with
a nano-sized light-emitting diode (nano-LED) as an excita-
tion source at l=280 nm. Luminescence decay, monitored
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3913

A. Das et al.
Figure 11. Relative energies for the HOMO and LUMO levels for
DB24C8 and 1ACHTNUGRTNEUNG(PF6)3, which were evaluated from the respective ground-
state redox potentials and E0–0 values.
To quantify the ET efficiency, we compared the fluores-
cence quantum yields of the donor in the presence and ab-
sence of the acceptor using Equation (1), in which Fdonor
and Fdonorꢀacceptor correspond to the fluorescence quantum ef-
ficiency of the donor in the absence and presence of the ac-
ceptor, respectively.
Figure 10. a) Fluorescence spectra of 1
creasing [DB24C8] (lexc =310 nm, [1
(PF6)3]=7.3ꢂ10ꢀ6 m; inset: absorp-
tion spectra for (PF6)3 (blue line) DB24C8 (green line), and 1-
(PF6)3·3DB24C8 (red line)). b) Emission spectra for DB24C8 at lexc
280 nm (purple line=DB24C8 (2.2ꢂ10ꢀ5 m), red line=3.0 equiv
DB24C8+1(PF6)3, blue line=1
(PF6)3 (7.3ꢂ10ꢀ6 m); inset: emission and
absorption spectral overlap for DB24C8 and 1(PF6)3, respectively).
ACHTUGNTERN(NUNG PF6)3 in dichloromethane with in-
AHCTUNGTRENNUNG
1ACHTUNGTRENNUNG
G
=
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ꢀdonorꢀacceptor
ð1Þ
ꢀdonor
The ET quantum yield for 1ACHTUNTGERN(NUNG PF6)3·3DB24C8 complex was
FET ¼ 1 ꢀ
in the [4]pseudorotaxane complex. Cyclic voltammetry (CV)
experiments were carried out in dichloromethane/acetoni-
trile and revealed two oxidation processes: the first at E=
calculated to be 71.2%. The ET rate was evaluated by using
Equation (2) and was found to be 4.3ꢂ109 sꢀ1. These data
enable us to calculate the Fçrster distance of R0 =43.1 ꢁ by
using a standard equation reported previously [Eq. (2)].[35]
ACHTUNGTRENNUNG
+
1ACHTUNGTRENNUNG
1
tdonor
1
energy levels for the respective components were calculated
by using the value of the optical-band gap (E0–0) obtained
from the intersecting wavelength of the normalized emission
and excitation spectra, which were E=4.21 and 3.58 eV for
KET
¼
ð
Þ
ð2Þ
ð1=FETÞ ꢀ 1
To obtain more information about such ET processes,
DB24C8 and 1
G
fluorescence lifetime measurements were carried out for 1-
These values enabled us to evaluate the energies for the
LUMO for these two chromophores, which were E=ꢀ2.72
and ꢀ2.39 V for LUMO1 for DB24C8 and LUMO2 for 1-
ACHUTNGRENU(NG PF6)3·3DB24C8, DB24C8, and 1ACHTUNGTRENN(UNG PF6)3 in dichloromethane
with a nano-LED as an excitation source at l=280 nm. The
decay profile of DB24C8 was best fit to a single-exponential
process with t=0.57 ns for lmon =300 nm. However, a simi-
ACHTUNGTRENNUNG(PF6)3, respectively (Figure 11). Thus, the energy gap (i.e.,
LUMO1ꢀLUMO2) of DE=ꢀ0.33 V supports the thermody-
lar study for a 1:3 mixture of 1ACHTNUTRGNE(GUN PF6)3 and DB24C8 (lmon =
namic feasibility of an ET process. More importantly, a
300 nm) revealed that the lifetime of the DB24C8-based
decay component decreased from 0.57 to 0.41 ns, thus sug-
gesting an efficiency of only 28% for the ET process [see
Eq. (1) and Table 3].
closer look at the emission spectra for 1ACTHNUTGRNEG(UN PF6)3 and the ab-
sorption spectra for DB24C8 reveals a significant spectral
overlap (see the inset of Figure 10a), thus further corrobo-
rating the possibility of the Fçrster resonance energy-trans-
fer (FRET) mechanism. Additionally, the normalized ab-
sorption spectrum for the [4]pseudorotaxane complex was
identical with the normalized excitation spectrum for 1-
The apparent disagreement in the ET efficiency, calculat-
ed on the basis of the quantum yields (71.2%) and time-re-
solved spectra (28%) suggests the possibility of other
quenching mechanisms being operational in addition to the
1
(PF6)3 (lems =369 nm), thus further confirming an efficient
resonance ET process. The results of the H NMR spectro-
ET process.[34]
scopic studies described earlier suggest incomplete complex
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Complexation of a Triphenylamine Guest with Macrocyclic Hosts
FULL PAPER
(India). All the solvents were dried and distilled prior to use following
standard procedures.
formation between DB24C8 and 1
rotaxane formation. Presumably there is a spectral overlap
of the emission of the uncomplexed DB24C8 with the ab-
sorption spectra of 1(PF6)3, and the light emitted by
DB24C8 is in turn absorbed by 1(PF6)3, thus causing the
trivial sensitization effect between the free components in
addition to the FRET. Therefore, quenching of the DB24C8
emission due to this trivial sensitization effect might have
added up to overall quenching of the donor chromophore
(i.e., DB24C8), which in turn showed the greater quenching
and thus the higher ET efficiency. The absence of any rise-
time when the fluorescence decay was monitored at the ac-
ceptor emission suggests a very fast ET, which could not be
identified by the resolution of our spectrophotometer.[36]
ACHTUGNTRENUN(GN PF6)3 for the [4]pseudo-
ACHTUNGTRENNUNG
1
The H NMR spectra were recorded on a Bruker 500 MHz FT NMR Ad-
vance-DPX 500 spectrometer at room temperature (258C). The chemical
shifts d are reported in parts per million (ppm) and relative to trimethyl-
silane (TMS) as an internal standard. The coupling constants J are given
in Hertz. ESI mass-spectrometric measurements were carried out on a
Waters Q-TOF Micro instrument. The UV/Vis spectra were obtained by
using a Cary 500 Scan UV/Vis–NIR spectrometer. Steady-state emission
spectra at room temperature were obtained on a Fluorolog (Horiba Jovin
Yvon) luminescence spectrofluorimeter. Time-resolved emission studies
were carried out using the TCSPC technique (Edinburgh Instruments
F900). The redox values were measured by cyclic volotametry (CV) with
a bipotentiostat (AFCBPI, PINE Instrument Co). A conventional three-
electrode cell assembly was used with a platinum-disk electrode as the
working electrode, a platinum wire as a counterelectrode, and an Ag/
AgCl electrode as a reference electrode. All the measurements were car-
ried out in dichloromethane/acetonitrile containing 0.1m tetrabutylam-
monium hexafluoroborate [(n-C4H9)4NBF4] as a supporting electrolyte.
The scan rate used was 0.5 Vsꢀ1, ferrocene (Fc) was used as an internal
standard for the CV studies, and the Fc/Fc+ couple appeared at E=
0.395 V. The methodologies adopted for the calculation of errors in solu-
tion preparation and instrumental analysis are provided in the Supporting
Information.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Conclusion
In summary, the complexation behavior of a newly synthe-
sized TPA derivative with three different host molecules,
namely, b-CD, CB[7], and DB24C8, has been demonstrated.
MALDI-TOF mass-spectrometric and 1H NMR and 2D
NOESY spectroscopic experiments revealed a 1:3 binding
stoichiometry, the extent of complexation, and the topology
of the inclusion processes. 1H NMR spectroscopic studies
also revealed a cooperative binding process for CB[7],
whereas statistical binding prevailed for hosts b-CD and
DB24C8. Enhanced fluorescence and an increase in the
average lifetime of 1X3 on inclusion-complex formation with
b-CD or CB[7] can be attributed to the restricted internal
rotation of the TPA core. This finding was confirmed by
steady-state and time-resolved emission studies. The choice
of DB24C8 as a host component offered us the unique op-
portunity to study the binding-induced FRET process that
Synthetic procedure of 1X3: Benzylamine (0.5 g, 5.2 mmol) was added to
tris(para-formylphenyl)amine (0.5 g, 1.5 mmol) dissolved in dry acetoni-
trile/methanol (30 mL, 1:1, v/v), and the reaction mixture was stirred vig-
orously at room temperature. After 24 h, methanol (20 mL) was added to
the reaction mixture, which was cooled to 08C. NaBH4 (0.5 g) was added
portionwise to the stirred cooled reaction mixture, which was stirred for
another 1 h at 08C. The reaction mixture was allowed to reach the room
temperature and stirred for a further 2 h. The solvent was removed
under reduced pressure, the residue was extracted three times with
CHCl3 and water, and the organic layers were combined and dried over
anhydrous sodium sulfate. The solvent was removed under reduced pres-
sure to give the crude product, which was purified on a silica gel with
CH3OH/CHCl3 (4:96, v/v) as the eluent. The desired compound was iso-
lated as a sticky brown solid. A solution of HCl (concentrated HCl
(0.5 mL) dissolved in acetone (2 mL)) was added dropwise to the sticky
solid dissolved in acetone (20 mL) and stirred for 2 h. A brown solid was
formed, which was isolated by filtration and air dried (0.6 g, 65%). Salt
1ACHTUNGTRENNUNG
involves DB24C8 as a donor molecule and 1ACHTNUGTRNEG(UN PF6)3 as an ac-
ceptor. This outcome was established by using ground-state
redox-potential and steady-state luminescence data. A trivi-
al ET and a FRET process are proposed to be the reason
for the quenching of the DB24C8-based luminescence for
AHCTUNGTRENNUNG
298 K): d=7.47–7.45 (15H, m), 7.37 (6H, d, J=8.5 Hz), 7.06 (6H, d, J=
8.5 Hz), 4.21 (6H, s), 4.14 ppm (6H, s); 13C NMR (500 MHz, CD3CN,
298 K): d=147.6, 131.2, 129.7, 129.2, 128.6, 124.9, 123.8, 116.9, 50.6 ppm;
HRMS (ESI): m/z calcd for [C42H46N4]4+: 606.3722; found: 606.3697
(error= ꢂ4.2 ppm).
studies with 1ACHTUNGTRENNUNG(PF6)3 and DB24C8. Thus, the present studies
have revealed that fluorescence spectroscopy can also be
used as a tool to probe the structural confinement imposed
on TPA derivatives by macrocyclic hosts in addition to de-
tailed NMR spectroscopic studies. More importantly, the re-
sults of the steady-state and time-resolved fluorescence stud-
ies also helped to understand the change in environment of
the TPA core from the bulk of the solvent medium to a hy-
drophobic environment during complexation with b-CD and
CB[7].
Acknowledgements
A.D. thanks DST (India) and CSIR (India) for supporting this research.
A.K.M., M.S., and P.D. acknowledge CSIR for their research fellowships.
The authors are thankful to Prof. H.W. Gibson of Virginia Polytechnic
Institute and University (USA) for his suggestion and help in the inter-
pretation of certain parts of the NMR data and error analysis. We also
thank Dr. S. Basa of CSMCRI for his help in evaluating cumulative
errors. The authors are thankful to the reviewers for their suggestions,
which were very helpful in improving this manuscript.
Experimental Section
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vents were of reagent grade and were procured from S.D. Fine Chemicals
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3916
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Chem. Eur. J. 2012, 18, 3906 – 3917

Complexation of a Triphenylamine Guest with Macrocyclic Hosts
FULL PAPER
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Received: October 1, 2011
Published online: February 17, 2012
Chem. Eur. J. 2012, 18, 3906 – 3917
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3917