Self-assembly of Bis(merocyanine) tweezers into discrete bimolecular π-Stacks

Article abstract of DOI:10.1002/chem.200802391

Examples of a novel class of tweezer molecules have been constructed through the tethering of two dipolar merocyanine chromophores by a naphthalenedimethylene or dimethylenediphenylmethane spacer. The electrostatic-interaction-directed self-assembly of th

Full text of DOI:10.1002/chem.200802391

FULL PAPER
DOI: 10.1002/chem.200802391
Self-Assembly of Bis(merocyanine) Tweezers
into Discrete Bimolecular p-Stacks
Andreas Lohr, Matthias Grꢀne, and Frank Wꢀrthner*[a]
Abstract: Examples of a novel class of
ciation of the bis(merocyanine) tweez-
ers is attributed to the strongly dipolar
nature of the merocyanine chromo-
phores and a unique aggregate geome-
try of four p-stacked chromophores
with an alternating arrangement of
their dipole moments. The structural
assignment of the tetrachromophoric
p-stack has been accomplished by
tweezer molecules have been con-
structed through the tethering of two
dipolar merocyanine chromophores by
a naphthalenedimethylene or dimethy-
lenediphenylmethane spacer. The elec-
MALDI-TOF mass spectrometry and
ROESY NMR spectroscopy. Further-
more, molecular modeling studies have
accounted for the relationships be-
tween the dimerization constants and
optical properties of the bimolecular
complexes of the present bis(merocya-
nine) dyes and the structure of the
spacer and the positions at which the
merocyanine chromophores are at-
tached.
trostatic-interaction-directed
self-as-
sembly of these bis(merocyanine)
tweezers affords centrosymmetric bi-
molecular complexes with very high di-
merization constants of up to >109 mꢀ1,
even in the good solvating solvent
chloroform. This pronounced self-asso-
Keywords: pi stacks
aggregation dyes/pigments
molecular tweezers · self-assembly
· dipolar
·
·
Introduction
There has been considerable interest in the self-assembly
of merocyanine dyes,[12–15] a class of chromophores with
promise for nonlinear optical and photorefractive applica-
tions due to their outstanding dipolar and polarizability
properties.[16] Moreover, we have recently shown merocya-
nines to be excellent p-type semiconducting absorber mate-
rials in organic bulk heterojunction solar cells.[17] In our ear-
lier work, we have shown that dipolar merocyanine dyes 1
form centrosymmetric dimer aggregates with high binding
constants of the order of KD =106 mꢀ1 in nonpolar solvents
such as dioxane or CCl4, and that this dimerization arises
primarily from electrostatic interactions between these chro-
mophores that can be traced back to the exceptionally large
ground-state dipole moment mg of about 17 D for 1 (Fig-
ure 1a,b).[12a] This exceptionally large dipole moment is at-
tributed to a major contribution of zwitterionic resonance
structures, as revealed by significant bond length equilibra-
tion within the conjugated path investigated by single-crystal
X-ray diffraction analysis and intense cyanine-type absorp-
tion spectra.[12]
The construction of nanoscale supramolecular architectures
based on directional noncovalent interactions offers tremen-
dous potential in the area of nanotechnology. Thus, an im-
pressive number of discrete, nanoscale assemblies have been
constructed by modern methods of supramolecular chemis-
try employing a variety of weak interactions, including
metal-ion coordination, hydrogen bonding, and p–p interac-
tions.[1,2] However, for the further development of such as-
semblies into molecular-level devices,[3] functional elements
have to be incorporated that enable applications in supra-
molecular catalysis,[4] sensorics,[5] and molecular electron-
ics.[6] The required functionality may be provided by chro-
mophores with unique photophysical and/or electrochemical
properties[7] that enable optical or electrochemical address-
ing and/or readout in devices[8,9] as well as serving as sensi-
tizers[10] or light-harvesting antennae.[11]
[a] Dr. A. Lohr, Dr. M. Grꢀne, Prof. Dr. F. Wꢀrthner
Universitꢁt Wꢀrzburg, Institut fꢀr Organische Chemie
Am Hubland, 97074 Wꢀrzburg (Germany)
Fax : (+49)931-888-4756
As the structures of these dimer aggregates are well-de-
fined, the dipolar interactions of the merocyanine dyes are
highly directional and, therefore, promising for the construc-
tion of more elaborate supramolecular architectures in
which both the binding interaction and functionality are pro-
vided by the chromophores. Indeed, we have recently ob-
E-mail: wuerthner@chemie.uni-wuerzburg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802391.
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complexation of aromatic guests (Figure 1d).[21–23] However,
in most cases the self-association constants are so low that
the guest complexation is not significantly affected. Stronger
self-association occurs mainly in aqueous media owing to
the hydrophobic effect, or if additional noncovalent interac-
tions such as hydrogen bonding are involved. The weak self-
association of common tweezer systems in organic solvents
is reasonable since favorable dispersion interactions from
the van der Waals contact between the p-walls are often
sterically prevented by the spacers and, more importantly,
the electron donor–acceptor (EDA) forces responsible for
strong complexation of electron-deficient guests are very
weak between the mostly electron-rich p-walls of the tweez-
ers (Figure 1d). Accordingly, more favorable EDA interac-
tions can be achieved in the case of heteroaggregates of
electron-rich and electron-deficient building blocks (Fig-
ure 1e), with which the alternate stacking of donor and ac-
ceptor groups allows for multiple EDA interactions.[24] If
such a concept is extended to tweezers of highly dipolar
merocyanine dyes, we can expect the formation of bimolecu-
lar complexes of high thermodynamic stability (Figure 1f).
According to this concept, bis(merocyanine) dyes
equipped with a spacer allowing an interplanar distance be-
tween chromophores of twice the distance observed for a
simple merocyanine dimer of 1 (Figure 1b) should facilitate
self-association into a sandwich-type bimolecular complex of
four merocyanine chromophore units (Figure 1f). Under the
precondition that there are no steric constraints to the for-
mation of a p-stacked arrangement, such dimeric aggregates
should be thermodynamically most favorable as they facili-
tate multiple and cooperative dipolar interactions. The
smallest interplanar distance of the dyes 1 in their crystal
structures was determined to be 3.2–3.5 ꢃ, which ap-
proaches the shortest possible van der Waals distance be-
tween two p-systems.[12b] Thus, an appropriate tether for en-
abling a tight sandwich-like packing of the chromophores
should provide an interplanar distance of the two chromo-
phores of the order of 7 ꢃ. Molecular building blocks that
possess these desired structural specifications are well
known from supramolecular hosts that are capable of com-
plexing extended p-systems, and the special importance of
the host geometry on the complexation properties has been
discussed.[20,25,26] Most recently, Fujita and co-workers have
constructed p-stacks consisting of up to nine discretely
stacked aromatic rings in self-assembled coordination cages
functioning as templates.[27a,b] Matile and co-workers have
recently introduced zipper assemblies with p-stacks com-
posed of more than ten aromatic units.[27c,d] Based on litera-
ture examples and preliminary molecular modeling studies,
we chose 2,7-naphthalenedimethylene and 4,4’-dimethylene-
diphenylmethane spacers to design bis(merocyanine) tweez-
ers for our present study. The former spacer was found to
be the optimal candidate for promoting efficient packing,
while the latter provides a slightly larger distance between
the planes of the two chromophores. Notably, the merocya-
nine chromophore of type 1 offers two positions for the at-
tachment of the spacer, that is, via the pyridine donor group
Figure 1. a) Resonance structures of highly dipolar merocyanine dyes 1
and schematic representations of b) the formation of merocyanine dimer
aggregates by dipole–dipole interactions, c) a p-stack formed from a
tweezer with electron-rich chromophores and an electron-deficient guest
as a host–guest complex (it can also be vice versa), d) a dimer of molecu-
lar tweezers with electron-rich chromophores, e) a heterodimer of an
electron-rich and an electron-deficient building block, and f) the concept
of a bimolecular aggregate of dipolar bis(merocyanine) tweezers.
tained highly defined supramolecular polymers by dipolar
aggregation of ditopic bis(merocyanine) monomers.[13] How-
ever, no directed aggregation of merocyanine dyes into well-
defined discrete aggregates beyond simple dimers has been
reported to date.[18]
Therefore, we have taken inspiration from a class of com-
pounds generally referred to as molecular tweezers or clips
that contain two aromatic chromophores covalently linked
by a single spacer unit. Such molecular hosts are able to
complex aromatic guest molecules between their two p-sur-
faces, which converge on the two divergent p-faces of the
guest, thus forming a sandwich complex held together by
two p-stacking interactions (Figure 1c).[19,20] In organic sol-
vents, a favorable host–guest interaction between the molec-
ular tweezers and the guest molecules only occurs if one
partner is a p-donor and the other is a p-acceptor, or vice
versa. Such host–guest complexes are generally weak, with
association constants in the range 0.1–1000mꢀ1, and are be-
lieved to be held together by electrostatic, polarization, dis-
persion, and charge-transfer forces. In contrast to the vast
number of studies on such tweezer-based host–guest sys-
tems, there have only been a few reports on the self-associa-
tion of such host molecules (tweezers) in solution, which
clearly represents a possible competition process for the
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Chem. Eur. J. 2009, 15, 3691 – 3705

Aggregation of Bis(merocyanine) Tweezers
FULL PAPER
N atom or the imide N atom. On the basis of the abovemen-
tioned considerations, we have synthesized the naphthalene-
dimethylene- and dimethylenediphenylmethane-tethered
bis(merocyanine) dye tweezers 2, 3 and 4, 5 and have car-
ried out detailed investigations of their self-association prop-
erties. Herein, we report that these appropriately designed
tweezers do indeed self-assemble into discrete bimolecular
complexes, which belong to an unprecedented novel class of
strongly bound p-stacks.
prepared from diphenylmethane according to a literature
procedure, was converted into dipyridinium salt 15 in 96%
yield by nucleophilic substitution with 4-picoline.[31] Diamine
17 was obtained from dibromide 14 in two steps according
to a literature method.[32] Amidation of 17 with ethyl cya-
noacetate afforded diamide 18 in 26% yield, which was con-
densed with ethyl acetoacetate to produce bisACHTNUTRGNE(NUG pyridone) 19
in 61% yield. Bis(merocyanine) dyes 4 and 5 were obtained
in yields of 25% and 13%, respectively, by the reaction of
either pyridone 16 with DPFA or 19 with DMF and subse-
quent reaction with salts 15 and 13, respectively.
Mass spectrometry: First evidence of the self-assembly of
the present bis(merocyanine) tweezers into bimolecular
complexes was obtained by mass spectrometry.[33] Samples
for MALDI-TOF mass spectrometry were prepared by
evaporation of the solvent from solutions of 2–5 in chloro-
form containing 2-[(2E)-3-(4-tert-butylphenyl)-2-methyl-
prop-2-enylidene]malononitrile (DCTB) as a matrix. The
spectra recorded in positive-ion mode display peaks that
correspond to the respective singly-charged dimer cations
[M2]+ along with signals for the singly-charged monomer
cations (Figure 2 and Table 1). The measured isotope pat-
terns of the aggregate signals are in good agreement with
the calculated natural abundances, thus confirming the ele-
mental compositions and the dimeric nature of the aggre-
gates in the gas phase. The observation of the respective
dimer signals in the mass spectra corroborates the formation
of discrete bimolecular complexes in solution and partial
fragmentation in the gas phase.
Aggregation studies by UV/Vis spectroscopy: UV/Vis dilu-
tion experiments have been extensively utilized to study dye
aggregation in solution. By this method, electronic interac-
tions between the p-conjugated systems of dyes may be elu-
cidated and the concentration range for aggregate formation
can be easily assessed. For simple processes such as dimeri-
zation or polymerization, binding constants and Gibbs bind-
ing enthalpies may be obtained from such experiments.[2e,34]
In addition, structural information can be obtained from the
splitting of the absorption band due to excitonic coupling of
the transition dipole moments of the chromophores.[35] Thus,
we have performed UV/Vis dilution studies on the bis(mer-
ocyanine) dyes 2–5 as well as on the reference dye 1a. Since
the dipolar interactions between merocyanine dyes are
strongly dependent on solvent polarity, the dilution experi-
ments were performed in chloroform, which is a solvent of
intermediate polarity and was found to be best suited for
studying the aggregation equilibria of the present series of
bis(merocyanine) dyes in the concentration range suitable
for UV/Vis absorption spectroscopy.
Results and Discussion
Synthesis of bis(merocyanine) dyes 2–5: The bis(merocya-
nine) dyes 2–5 were synthesized according to Scheme 1. The
detailed procedures and product characterization data are
given in the Experimental Section. The spacer precursor
naphthalene dicarbaldehyde 6 was prepared in three steps
according to literature procedures starting from commercial-
ly available 2,7-naphthalenediol.[28] Alkylation of 6 with
butyl bromide gave dialdehyde 7 in 43% yield. The bis(pyri-
dinium) salt 9 was synthesized as its tetrafluoroborate by a
sequence of reduction of dialdehyde 7 with NaBH4 to afford
diol 8 (90% yield) followed by tosylation and substitution
with 4-methylpyridine (79% yield). For the synthesis of bis-
ACHTUNGTRENNUNG(pyridone) 12, dialdehyde 7 was first converted into the
Cbz-protected diamine 10 in 70% yield by reductive amina-
tion with benzyl carbamate.[29] After deprotection by catalyt-
ic hydrogenation, the free amine intermediate was reacted
with ethyl cyanoacetate to afford bis(cyanoacetamide) 11 in
66% yield. Condensation of 11 with ethyl acetoacetate gave
Significant spectral changes were observed for dyes 2–5
and 1a upon variation of the concentration in the range
from 1ꢄ10ꢀ7 to 1.4ꢄ10ꢀ3 m using cells of pathlength between
0.01 and 5 cm (Figure 3 and Figure S1). For the reference
dye 1a, the UV/Vis spectra in dilute solution can be ascri-
bed to the monomer, with an intense charge-transfer band
bisACHTUNGTRENNUNG(pyridone) 12 in 72% yield, and the reaction of 12 with
DMF and salt 13[30] afforded the bis(merocyanine) dye 3 in
10% yield. Bis(merocyanine) dye 2 was obtained in 28%
yield by the reaction of pyridone 16[12b] with N,N’-diphenyl-
formamidine (DPFA) and salt 9. Dibromide 14, which was
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3693

F. Wꢀrthner et al.
tion for dye 3 (Figure 3c; for
the spectra of 4 and 5, see Fig-
ure S1 in the Supporting Infor-
mation). However, in 1,2-di-
chloroethane (DCE), a solvent
of higher dielectric permittivity
but similar refractive index,
the dissociation of the aggre-
gates into monomers could be
observed in the applicable con-
centration regime (see Fig-
ure S2).[37]
Over a considerable range of
concentrations,
well-defined
isosbestic points occur, which
clearly indicate the presence of
an equilibrium between two
species, that is, monomer (M)
and dimer (D) species. In this
case,
a simple dimerization
model according to the equa-
tion
cD
1ꢀaM
ð1Þ
KD
¼
¼
cM
2aM2c0
2
can be applied, where KD de-
notes the dimerization con-
stant, cM and cD are the con-
centrations of monomer and
dimer, respectively, c0 is the
total dye concentration, and
aM (=cM/c0) is the fraction of
the monomeric dye in solu-
Scheme 1. Synthesis of bis(merocyanine) dyes 2–5.
occurring in the long-wavelength region (denoted as M).[12]
With increasing concentration, the intensity of the monomer
band is reduced with the concomitant appearance of a hyp-
sochromically shifted band (denoted as D) owing to exciton-
ic coupling of the two chromophores in the formed dimer
unit (Figure 3a). A second excitonic band observed at
longer wavelength (ꢁ560 nm) is indicative of a slightly
twisted arrangement of the two chromophores in the aggre-
gate.[12c,36] The spectral changes observed for the bis(mero-
cyanine) dyes are similar to those observed for 1a, revealing
an intense hypsochromically shifted absorption band and a
less intense absorption band at longer wavelength (a de-
tailed discussion of the bis(merocyanine) dye spectra is pro-
vided later). Thus, they can be attributed to an equilibrium
between two species, that is, the monomers and the bimolec-
ular complexes. Whilst both species are clearly identified in
the concentration-dependent spectra of bis(merocyanine)
dyes 2, 4, and 5, only little dissociation of the bimolecular
complexes of 3 into monomers was observed in CHCl3 down
to a concentration of 10ꢀ7 m, revealing a very strong aggrega-
Figure 2. MALDI-TOF spectra of bis(merocyanine) dyes 2–5. The peaks
corresponding to singly-charged monomer and dimer cations are marked
with [M]+ and [M2]+, respectively.
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Aggregation of Bis(merocyanine) Tweezers
FULL PAPER
Table 1. Monoisotopic mass peaks observed in MALDI-TOF mass spec-
tra of bis(merocyanine) dyes that correspond to bimolecular aggregates.
where eM and eD are the molar absorptivities of a free and a
dimer-bound monomeric unit, respectively. Combining
Equations (2) and (3), we obtain Equation (4):
Dye
Calculated m/z (ion)
Found m/z
2
3
4
5
2277.447 [M2]+
2277.447 [M2]+
2069.279 [M2]+
2069.279 [M2]+
2277.431
2277.430
2069.301
2069.274
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
8KDc0 þ 1ꢀ1
ð4Þ
ꢀ
e ¼
ðeMꢀeDÞ þ eD
4KDc0
Nonlinear regression analysis of the apparent molar absorp-
tivity of the dyes as a function of total monomer concentra-
tion c0 at certain wavelengths based on Equation (4) pro-
vides the dimerization constants and related Gibbs dimeriza-
tion energies,
DDGꢂ ¼ ꢀRTln KD
ð5Þ
in CHCl3 and DCE (for 3). These data are collected in
Table 2. Figure 4 shows the concentration dependence of the
fraction of monomer (aM) calculated from the UV/Vis data
at certain wavelengths as well as the resulting dimerization
isotherms of the nonlinear regression analyses.
Dyes 4 and 2 that are tethered at the N atoms of the pyri-
dine donor groups show dimerization constants of KD =6.2ꢄ
104 mꢀ1 and 4.5ꢄ105 mꢀ1, respectively, while dyes 5 and 3 that
are tethered at the imide N atoms show considerably higher
dimerization constants of KD =2.1ꢄ106 mꢀ1 and >109 mꢀ1, re-
spectively (Figure 4 and Table 2). The last value represents
an approximation because of the abovementioned limited
dissociation of 3 in the applicable concentration regime
down to 10ꢀ7 m, which precludes an exact determination.
These values are about two to six orders of magnitude
higher than the value obtained for the reference dye 1a, and
the corresponding Gibbs energies of dimerization DDG8 are
about two to four times higher than that of 1a. This strong
increase of the binding strength for bis(merocyanine) dyes
2–5 with respect to that of 1a can be attributed to multiple
interactions made possible by the tethering of the chromo-
phores in the bis(merocyanine)s. These data further show
that the position of attachment of the spacer to the chromo-
phores as well as the structure of the spacer have strong in-
fluences on the dimerization constants. Thus, tethering of
the chromophores at the imide N atoms allows for stronger
dimerization of the respective bis(merocyanine) dyes com-
pared to tethering at the N donor atom, and the 2,7-naph-
thalenedimethylene spacer provides higher binding con-
stants than the 4,4’-dimethylenediphenylmethane spacer.
These relationships can be ascribed to varying spatial re-
strictions imposed upon the aggregate structure by the
nature of the spacer and the position of tethering. Unfortu-
nately, determination of the enthalpic and entropic contribu-
tions, DH8 and DS8, to the Gibbs energy of dimerization,
DDG8, by means of a vanꢅt Hoff plot was not successful
owing to the strong temperature dependence of the er of
chloroform.[38]
Figure 3. Concentration-dependent UV/Vis spectra in CHCl3 at 298 K.
The arrows indicate the decrease in the intensity of the monomer band
(denoted as M) and the appearance of a dimer band (denoted as D) with
increasing concentration. The spectra of the monomer (dotted lines) and
the dimer (dashed lines) were calculated from the data at two different
concentrations and the binding constants. a) 1a (c=1.4ꢄ10ꢀ3 to 4.5ꢄ
10ꢀ6 m); b) 2 (c=1ꢄ10ꢀ4 to 7ꢄ10ꢀ7 m); c) 3 (c=1ꢄ10ꢀ4 to 1ꢄ10ꢀ7 m).
tion.[12b] By solving Equation (1), we obtain:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
8KDc0 þ 1ꢀ1
ð2Þ
aM
¼
4KDc0
The apparent molar absorptivity e¯ of the dye in solution
may be expressed as:
It is important to note that the formation of linear oligo-
mers (Scheme 2, left) by dimerization of the chromophores
can be ruled out for all of the bis(merocyanine) dyes studied
here, since analysis of the UV/Vis dilution data based on the
ꢀ
e ¼ aMeM þ ð1ꢀaMÞeD
ð3Þ
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3695

F. Wꢀrthner et al.
larities for the present dimerization process (i=2) in chloro-
form were calculated by assuming that KD =590mꢀ1, which
is the dimerization constant for reference dye 1a
(Table 2).[39] Since the obtained EM values are well above
the concentration regime under consideration in the per-
formed dilution studies, the formation of linear chains can
be excluded for the aggregation of the bis(merocyanine)
dyes studied here.[40]
As noted before, the spectral changes that result from ag-
gregation of dyes can provide information on their aggre-
gate structures. Thus, the absorption spectra of the mono-
mers and the bimolecular complexes of the present series of
bis(merocyanine) dyes were derived from the spectra at two
different concentrations and the dimerization constant ac-
cording to Equation (4) (dashed and dotted spectra in
Figure 3 and Figures S1 and S2), and the respective absorp-
tion maxima are collected in Table 2. The monomer spectra
of the bis(merocyanine) dyes 2 and 4 are in good accord-
ance with that observed for reference dye 1a, showing ab-
sorption coefficients that are about twice as large as those
obtained for mono-chromophoric 1a (Figure 3a,b and Figure
S1a). However, it has to be noted that the monomer
maxima of dyes 2 (lmax =570 nm) and 4 (569 nm) are red-
shifted by about 10 nm with respect to that of 1a (559 nm),
which may be due to a weak intramolecular J-type coupling
between the chromophores.[13a,18] On the other hand, dye 5
shows nearly the same position of the monomer maximum
(lmax =558 nm) as reference 1a, but its absorption coeffi-
cient is less than twice that for mono-chromophoric 1a (Fig-
ure 3a and Figure S1b). Closer inspection reveals that the
absorption band of monomeric 5 is broader (FWHM=
1605 cmꢀ1) than that of 1a (FWHM=1385 cmꢀ1) and has an
additional shoulder at lower wavelength (ꢁ480 nm). The
even more pronounced spectral broadening and the addi-
tional absorption band at lower wavelength of monomeric 3
(Figure S2) compared to 1a suggest a conformational distri-
bution of monomeric 3 that involves a folded species along
with other conformations with more randomly oriented
chromophores. Molecular modeling of monomeric 3 reveals
that such a folded conformation with antiparallel stacked
chromophores can be adopted, thus rationalizing the appear-
ance of the hypsochromically shifted band in terms of H-
type coupling.
Figure 4. Fraction of monomer (aM) calculated from UV/Vis data at a
certain wavelength and results of the nonlinear regression analysis based
on the dimerization model for dyes 3 (555 nm), 5 (555 nm), 2 (570 nm), 4
*
(485 nm), and 1a (505 nm) in CHCl3
(
and solid lines), and 3 (450 nm)
*
in DCE ( and dashed lines).
Scheme 2. Schematic representation of the self-assembly of bis(merocya-
nine) tweezers into cyclic aggregates containing i monomers (right) and
the competing process of linear chain formation (left).
isodesmic model[34] gave only poor fits. Also, the high aggre-
gation constants obtained from the analysis based on the
isodesmic model cannot be explained by the simple dimeri-
zation of two chromophores to form linear oligomers.
Theoretically, however, above a certain overall monomer
concentration known as the effective molarity EM, the for-
mation of linear chains will be favored over that of cyclic ag-
gregates.[39] The effective molarity for self-assembly of a
cyclic aggregate consisting of i monomers (Scheme 2, right)
can be calculated according to:
i
ð6Þ
EM ¼ Ki=ðsKDÞ
where Ki denotes the aggregation constant of the cyclic as-
sembly defined as Ki =caggregate/cmonomeri, s is a symmetry
number (4 in the present cases since both binding sites are
self-complementary), and KD denotes the dimerization con-
stant of the respective monotopic dye.[39a] The effective mo-
Table 2. Dimerization constants KD and corresponding standard Gibbs free dimerization energies DDG8 from analysis based on the dimerization model[a]
and absorption maxima of reference dye 1a and bis(merocyanine) dyes 2–5.
Dye
Solvent
ꢀDDG8 [kJmolꢀ1
]
KD [mꢀ1
]
EM[b] [m]
lM [nm]
lD1 [nm]
lD2 [nm]
Dn˜D1–M [cmꢀ1
]
Dn˜D1–D2 [cmꢀ1
]
1a
4
2
5
3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
DCE
15.8
27.3
32.3
36.1
>50
6.1
590
559
569
570
558
555
548
539
490
488
479
466
460
492
464
559
564
554
553
554
561
520
2520
2917
3333
3538
3721
2077
2999
2520
2760
2830
3376
3688
2500
2321
6.2ꢄ104
4.5ꢄ105
2.1ꢄ106
>109
1.1ꢄ10ꢀ2
8.1ꢄ10ꢀ2
0.38
>180
1a
3
12
DCE
26.3
4.1ꢄ104
7.4ꢄ10ꢀ3
[a] Average from nonlinear regression analysis at four to six different wavelengths. [b] Effective molarity calculated according to Eq. (6) assuming a KD
value of 590mꢀ1
.
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Aggregation of Bis(merocyanine) Tweezers
FULL PAPER
The dimer spectra of bis(merocyanine)s 2–5 show two ab-
sorption bands centered at considerably shorter and compa-
rable wavelengths with respect to the monomer absorption
bands (Table 2). Such a split absorption spectrum has previ-
ously been observed for merocyanine dimers of type 1a and
was attributed to an excitation in the higher- and lower-lying
exciton states of the dimer.[12] However, for the present
dimers of bis(merocyanine)s 2–5, such a simple treatment
according to Kashaꢅs dimer model is not feasible since four
chromophores are kept in close proximity, as suggested by
the high dimerization constants. Thus, four transition dipole
moments have to be considered for excitonic coupling. Nev-
ertheless, some qualitative information can be gleaned from
the spectral changes. The hypsochromic shift of the main
dimer maxima with respect to the monomer maxima in
terms of the wavenumber difference Dn˜D1-M increases from
2520 to 3721 cmꢀ1 in the sequence 1a, 4, 2, 5, and 3
(Table 2). It is noteworthy that the same order is found for
the binding constants for bimolecular association. Thus, the
most tightly bound complex exhibits the most strongly cou-
pled chromophores, which can be related to this having the
shortest distance between the chromophores according to
Kashaꢅs theory.[35] Additionally, the splitting between the
two dimer bands, Dn˜D1–D2, increases from 2520 to 3688 cmꢀ1
in this series. Thus, for the bis(merocyanine) dyes, more
strongly hypsochromically shifted main dimer bands as well
as greater splitting between the dimer maxima are observed
in comparison with those for the reference dye 1a. These
data are in good accordance with a stacked arrangement of
four chromophores in which excitonic coupling to more
than one neighboring dye may occur (Figure 1f). As well as
displaying the most pronounced hypsochromic shift in the
present series of bis(merocyanine) dyes, the dimer band of 3
also has a very narrow bandwidth (FWHM=1200 cmꢀ1), im-
plying a very tightly packed stacking of the chromophores.
Notably, the hypsochromic shift of the absorption band of
this tetrameric stack (with regard to that of the monomer) is
not as pronounced as that observed for bis(merocyanine)
nanorods, with which a much larger number of chromo-
phores are packed in close proximity.[13]
Interestingly, we found the tightly bound bimolecular
complex of 3 to be fluorescent, whereas for the monomeric
tweezers fluorescence is completely quenched (Figure 5).
The latter observation can be attributed to a fast non-radia-
tive deactivation pathway through a bond-twisting mecha-
nism that is a common feature of most cyanine and mero-
cyanine dyes.[41] Thus, the increase of fluorescence intensity
for the dimer of 3 originates from a rigidification of the
chromophores within the tightly packed chromophore
stack.[12c] Similar fluorescence intensity increases have been
reported for merocyanines in frozen solution, in the solid
state, and in an organogel.[15b,42] Clear evidence that the
emission originates from the p-stacked assembly is provided
by the excitation spectrum, which closely resembles the ab-
sorption spectrum of the bimolecular complex. However, it
has to be noted that the fluorescence quantum yield of the
p-stack is still quite low (<10%) in the present case since
Figure 5. UV/Vis absorption (solid line, left scale), fluorescence emission
(dashed line, right scale, lex =462 nm), and excitation (dotted line, right
scale, lem =700 nm) spectra of dye
3 m
dimers in CHCl3 at 1ꢄ10ꢀ6
(298 K).
fluorescence arises from the electronic transition from the
lowest excited state to the ground state, which exhibits only
a small oscillator strength.[12c]
Structural elucidation of dimer aggregates by NMR spec-
troscopy: The structure of the dimer aggregate of bis(mero-
cyanine) 3 was elucidated by NMR spectroscopy as a repre-
1
sentative example. For this purpose, H NMR spectra of 3
were measured in CDCl3 (c=10 mm), in which this dye di-
merizes almost completely (see Figure 4), and in highly
polar [D7]DMF (c=3 mm), in which the dimerization is neg-
1
ligible. The H NMR signals were assigned with the aid of
(1H,1H)-COSY, (1H,13C)-HSQC, and (1H,13C)-HMBC experi-
ments. Table 3 shows the assignments of the significant
proton signals of bis(merocyanine) dye 3 in [D7]DMF and
1
CDCl3. For a complete assignment of the H and 13C NMR
spectra, see Table S1 in the Supporting Information.
For monomeric bis(merocyanine) 3, a simple 1H NMR
spectrum with a single set of signals was obtained in
[D7]DMF (Figure 6a). The respective positions in the two
halves of the dye 3, numbered with and without primes (see
Table 3), are chemically equivalent in the monomeric com-
pound, which is in accordance with rotational freedom
about the single bonds connecting the chromophores to the
naphthalene spacer, thus leading to a C2v-symmetric average
geometry of the bis(merocyanine) dye on the NMR time
scale. Furthermore, the presence of only one doublet for the
protons at positions 1 and 2 (at d=8.52 ppm) as well as for
those at positions 3 and 4 (at d=7.82 ppm) in [D7]DMF
ꢀ
strongly suggests a fast rotation about the C5 C6 bond.
Such rotational motion is known for these merocyanine dyes
and is attributed to their zwitterionic properties, which
[43]
ꢀ
impart strong single-bond character to the C5 C6 bond.
A more complicated spectrum is obtained for the bimo-
lecular complex of 3 in CDCl3 (Figure 6b), which shows
many more signals in comparison with the monomer spec-
trum in [D7]DMF and these signals are also considerably
broader. Assignment of the signals in the spectrum of the
dimer reveals that the chemical equivalence of the positions
in the two halves of the molecule is lost upon dimerization,
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F. Wꢀrthner et al.
Table 3. Assignment of the significant proton signals of bis(merocyanine) 3 in [D7]DMF and CDCl3.
corresponding peaks. Notably,
the methylene protons at posi-
tions 15 and 15’ show two sets
of geminally coupled proton
signals in the spectrum of the
dimer, which indicates restrict-
ed rotation about the methyl-
ene bridge between the spacer
and chromophore units, and a
different magnetic environ-
ment for each methylene
proton in this rigidified dimer
structure. For most of the pro-
tons, an upfield shift (negative
DdH values in Table 3) is ob-
served upon dimerization,
which can be attributed to
shielding of these protons by
the anisotropic contributions
of the aromatic chromophores
and spacers in the dimer struc-
ture. The signals of the protons
of the pyridine donors (at posi-
tions 1–4 and 1’–4’) become
considerably broader, indicat-
ing a slower rotation of the
pyridine donor ring in the
dimer compared to that in the
monomer.
These results are in accord-
ance with a centrosymmetric
dimer structure, as shown in
Figure 7. Within this structure,
the protons on the two halves
of each monomer unit clearly
have different chemical envi-
ronments, which leads to dif-
ferent chemical shifts (i.e., a
splitting of the monomer sig-
nals) of the protons at posi-
tions numbered with and with-
out primes, while the respec-
tive protons on the two mono-
mer units with the same nota-
tion are chemically equivalent
due to the overall centrosym-
metric geometry.
Position
[D7]DMF dH (J[Hz])
CDCl3 dH (J[Hz])
DdH
1,2
1’,2’
3,4
3’,4’
6
6’
7
7’
13
8.52 (d, 7.0, 4H)
7.44 (br, 2H)
7.52 (br, 2H)
ꢀ1.08
ꢀ1.00
ꢀ0.77
ꢀ0.53
ꢀ0.10
0.13
ꢀ0.51
ꢀ0.75
ꢀ0.32
ꢀ0.71
0.00
7.82 (d, 7.0[a], 4H)
7.81 (d, 14.8, 2H)
7.85 (d, 14.9, 2H)
2.47 (s, 6H)
7.05[a] (br, 2H)
7.29 (d, 4.8, 2H)
7.71 (d, 14.8, 1H)
7.94 (d, 14.8, 1H)
7.34 (d, 14.7, 1H)
7.10 (d, 15.2[a], 1H)
2.15 (s, 3H)
13’
15a
15b
15’a
15’b
17
1.76 (s, 3H)
5.22 (s, 4H)
5.22 (d, 16.3, 1H)
5.18 (d, 16.3, 1H)
5.45 (d, 16.0, 1H)
4.97 (d, 16.0, 1H)
6.82 (s, 1H)
ꢀ0.04
0.23
ꢀ0.25
ꢀ0.07
ꢀ0.16
ꢀ0.23
ꢀ0.20
ꢀ0.08
ꢀ0.03
ꢀ0.04
0.06
6.89 (s, 2H)
17’
20
6.73 (s, 1H)
7.04 (s, 1H)
7.27 (s, 2H)
20’
22
22’
25
25’
26,26’
7.07 (s, 1H)
4.09 (br, 2H)
4.14 (br, 2H)
0.98 (t, 7.3, 3H)
1.08 (t, 7.3, 3H)
3.9–4.1 (br, 4H)
4.17 (t, 6.5, 4H)
1.02 (t, 7.4, 6H)
4.39 (t, 7.4, 4H)
ꢀ0.4
[a] Estimated value due to partial overlap or broadening of signals.
In such
a closely packed
dimer, spatial proximities be-
tween the protons of the two
individual molecules in close
vicinity should lead to through-
space couplings that can be
1
Figure 6. Parts of the 600 MHz H NMR spectrum of 3 in a) [D7]DMF (c=3 mm) and b) CDCl3 (c=10 mm) at
293 K. The connecting dotted lines highlight the splitting of selected monomer signals upon dimerization.
since different chemical shifts are observed for the protons
at the positions numbered with and without primes
(Table 3). This splitting of the NMR signals is highlighted
for selected protons in Figure 6 by the lines connecting the
studied by nuclear Overhauser enhancement (NOE) spec-
troscopy. Since the signal intensity of the NOE may become
very low in the intermediate mass range, in the present case
(the masses of the dimers are around 2280 gmolꢀ1) the ro-
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Aggregation of Bis(merocyanine) Tweezers
FULL PAPER
within the same molecule are far beyond the sensitivity of
this technique. Thus, the results of the ROESY experiments
in CDCl3 provide clear evidence for the centrosymmetric
dimer structure as illustrated in Figure 7. The through-space
connectivity of the protons in the 15’b- and 2-positions
found in the ROESY spectrum is in accordance with the
proximity of these protons in the dimer model shown in
Figure 7 and reveals that the positions denoted with primes
are those in the inner halves of the monomer units in the
dimer. This notation is also in agreement with the signifi-
cantly stronger shielding of the protons in the 13’- and 7’-po-
sitions as compared to those in the 13- and 7-positions, re-
spectively. Notably, in the ROESY spectrum of 3 in CDCl3
the signals of the protons denoted with and without primes
are connected to each other by cross-peaks of the same sign
as the diagonal peaks (both negative), which reveals a
supramolecular exchange between the halves of the mole-
cules with and without primes on the time scale of the
mixing times of the ROESY experiments (100 and 300 ms).
Figure 7. Structural model of the centrosymmetric dimer of 3 based on
NMR studies and molecular modeling. The arrows indicate spatial prox-
imities between the green highlighted protons as evidenced by ROESY
cross-coupling peaks (see Figure 8). The alkyl chains are replaced by
methyl groups for simplicity.
Molecular modeling: We performed molecular modeling
studies on the present series of bis(merocyanine) dyes with
the aim of obtaining a qualitative explanation for the de-
pendence of the dimerization constants and absorption
properties on the molecular structure of the bis(merocya-
nine) monomers revealed by the UV/Vis studies (see
above). However, the structural characterization of supra-
molecular assemblies by computational methods is still very
limited because an accurate description of the intermolecu-
lar forces involved requires higher-level calculations that are
limited to small systems. Simple force-field calculations are
in general not applicable since they cannot account for the
non-uniform charge distribution in the delocalized p-system
responsible for the strong dipolar character of the merocya-
nine chromophore 1. As has recently been shown, the dime-
rization of simple merocyanine chromophores 1 can only be
accurately described by more demanding DFT-D methods,
whilst even semi-empirical or standard DFT methods fail
due to missing dispersion corrections.[44]
Keeping in mind the noted deficiency of this method, the
packing of the chromophores in the dimer structures of bis-
(merocyanine) monomers 2–5 was assessed by geometry op-
timization through applying simple force-field calculations
(Hyperchem, MM+).[45] A molecular dynamics simulation
was used to equilibrate each supramolecule, and this was
followed by energy minimization of the resulting structures
to full convergence. As shown in Figure 9, the geometry-op-
timized dimer models of all of the bis(merocyanine) com-
pounds in the present series adopt a centrosymmetric struc-
ture, which has been experimentally confirmed for dye 3 by
the 2D NMR experiments. The chromophores are arranged
in a stacked fashion and interlock like two tweezers from
opposite directions. In such a dimer structure, the electron-
rich and electron-deficient parts of the chromophores are
positioned on top of each other to maximize the electrostat-
ic interactions. These multiple dipolar interactions account
for the strong increase of the dimerization constant of the
Figure 8. Selected areas of the 600 MHz ROESY NMR spectrum of dye
3 in CDCl3 (c=10 mm) at 274 K and 100 ms mixing time. Positive and
negative signals are represented by solid and dashed contours, respective-
ly.
tating-frame
Overhauser
enhancement
spectroscopy
(ROESY) technique was applied. The through-space con-
nections were thus determined for dye 3 by ROESY NMR
in CDCl3 at a concentration of 10 mm (Figure 8). All of the
observed cross-peaks can be unambiguously assigned to in-
termolecular through-space couplings because none of them
were found in [D7]DMF.
For the pairs of protons that show cross-peaks in the
ROESY spectrum (Figure 8), the intermolecular distances
in a centrosymmetric dimer according to Figure 7 are in the
range 2.5–2.9 ꢃ (based on MM+ optimized dimers; see
below), which is well within the range of ROESY experi-
ments. The intramolecular distances between these protons
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3699

F. Wꢀrthner et al.
Figure 9. MM+ geometry-optimized models of dimer aggregates of bis(merocyanine) dyes 2–5 and the structures of these dyes. The spacer units are
shown in green and all alkyl chains have been replaced by methyl substituents.
bis(merocyanine) dyes with respect to simple dimers of ref-
erence dye 1a.
spacer show a considerably larger distance between the
methylene C atoms of 9.1 ꢃ (Figure 9). Thus, the molecular
geometry of the monomeric units is strongly distorted in
these cases to allow the approach of the chromophore
planes to a close van der Waals contact. From these structur-
al differences between the monomers, incorporating either
the 2,7-naphthalenedimethylene spacer or the 4,4’-dimethy-
lenediphenylmethane spacer, it is apparent that the latter is
less suited to supporting a tightly packed chromophore
stack. In addition, higher entropic losses have to be over-
come in the dimerization of 4,4’-dimethylenediphenylme-
thane-tethered dyes in comparison with the 2,7-naphthalene-
dimethylene-tethered dyes, since a larger number of rotors
have to be frozen out in the former case for the formation
More detailed inspection reveals that the 2,7-naphthalene-
dimethylene-tethered bis(merocyanine)s 2 and 3 have a very
tight packing of the two inner chromophores in the bimolec-
ular complex, with an ideal van der Waals distance to both
neighboring chromophores in the range 3.5–3.6 ꢃ
(Figure 9). The intramolecular distance between the methyl-
ene C atoms of the spacers is 7.4 ꢃ (arrows), which is nearly
twice the distance between the chromophore planes, and
suggests that the monomers do not have to assume a distort-
ed geometry to form these tightly packed dimer structures.
On the other hand, the dimers of bis(merocyanine) dyes 4
and 5 incorporating the 4,4’-dimethylenediphenylmethane
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Aggregation of Bis(merocyanine) Tweezers
FULL PAPER
of the rigid dimers.[46] These considerations account for the
higher dimerization constants of the naphthalene-tethered
dyes 2 and 3 compared to those of the diphenylmethane-
tethered dyes 4 and 5, respectively (see Table 2).
chloroform. These self-association constants are indeed sev-
eral orders of magnitude higher than those reported for tra-
ditional tweezer systems.[21] Thus, a new design principle for
ultra-strong bimolecular association has been introduced
based on the antiparallel stacking of highly dipolar chromo-
phores (Figure 1f). This new design principle should be of
interest for the development of many other well-defined p-
stack architectures that warrant in-depth studies of their op-
tical and electrochemical properties. These studies will be of
relevance to the optimization of energy levels and the
charge- and exciton-transport properties of bulk heterojunc-
tion solar cell materials.[17] In our opinion, physical data for
well-defined dye stacks should ideally bridge the gap be-
tween the properties of individual dyes (e.g. half-wave po-
tentials in cyclic voltammetry) and those observed in bulk
materials (e.g. HOMO and LUMO levels as obtained from
photoelectron spectroscopy). Our current efforts are direct-
ed along these lines.
Higher dimerization constants were also found for the
bis(merocyanine) dyes tethered via the imide N atoms (3
and 5) compared to those tethered via the pyridine donor N
atoms (2 and 4). A closer look at the dimer models of the
bis(merocyanine) dyes 2 and 4 reveals that the terminal car-
bonyl groups of the chromophores are in close proximity to
the aromatic spacers, which might result in repulsive forces
between the carbonyl oxygen with negative partial charge
and the p-cloud of the spacers (Figure 9). Such repulsive
forces are avoided in the case of the bis(merocyanine) dyes
3 and 5 tethered at the imide N atoms since these dyes can
slip against each other until the electrostatic interaction is
optimized (Figure 9). Such a slipped arrangement might
even allow aromatic edge-to-face interactions between the
pyridine donor group with partial positive charge and the ar-
omatic spacers.[47] Edge-to-face interactions can be quite
strong (of the order of several kJmolꢀ1) and may contribute
substantially to the stronger dimerization constants observed
in the case of the bis(merocyanine) dyes tethered via the
imide N atoms compared to those tethered via the pyridine
donor group N atoms.
The dimer structures obtained from molecular modeling
can also account for the UV/Vis absorption properties of
the bis(merocyanine) dimers. The strong hypsochromic
shifts, Dn˜D1–M, of the main dimer absorption bands with re-
spect to the monomer bands as well as the stronger splitting
of the dimer bands, Dn˜D1–D2, compared to that of the refer-
ence merocyanine 1a result from the tightly packed bimo-
lecular complexes of the bis(merocyanine) dyes, which allow
excitonic coupling between four chromophores in a card-
pack arrangement with almost parallel transition dipole mo-
ments.[36] Moreover, the increases of Dn˜D1–M and Dn˜D1–D2 in
the sequence 4, 2, 5, and 3, which corresponds to the se-
quence of increasing dimerization constants (see Table 2),
can be attributed to increasingly tight packing of the chro-
mophores since exciton coupling theory predicts an inverse
third power relationship between the distance of the dipole
centers and the coupling energy of the transition dipoles J,
which determines the energetic levels of the excitonic
states.[48]
Experimental Section
General: Solvents and reagents were obtained from commercial suppliers
and were purified according to standard procedures.[49] Naphthalene di-
carbaldehyde 6,[28] pyridinium salt 13,[30] dibromide 14,[31] hydroxypyri-
done 16,[12b] and diamine 17[32] were prepared according to literature pro-
cedures. Column chromatography was performed on silica gel (Merck
Silica 60, particle size 0.04–0.063 mm) and TLC was conducted on silica
gel plates (60 F254, Merck). Melting points were determined on a Linkam
TP 94 heating stage and are uncorrected. NMR experiments were con-
ducted on a Bruker Avance 400 and a Bruker DMX 600 spectrometer
with TMS or the residual solvent signal as internal standard. High-resolu-
tion ESI-TOF and EI mass spectrometry was carried out on a microTOF
focus instrument (Bruker Daltronik GmbH) in positive mode and a Fin-
nigan MAT 90 instrument, respectively. MALDI-TOF MS measurements
were performed on a Bruker Autoflex II. Elemental analyses were ob-
tained with a CHNS 932 analyzer (Leco Instruments GmbH).
UV/Vis absorption: UV/Vis spectra were measured on a Perkin Elmer
Lambda 950 spectrometer in conventional quartz cells of 0.001–5.0 cm
pathlength to cover a suitable concentration range. The spectral band-
width and the scan rate were 2 nm and 140 nmminꢀ1, respectively. Stock
solutions of each compound were accurately prepared, and dilutions of
these stock solutions were used for absorption measurements over a con-
centration range by taking into account the solubility and absorbance of
the respective compound. The temperature was controlled with a PTP-1
peltier element (Perkin Elmer). The solvents for UV/Vis absorption were
of spectroscopic grade and were used as received.
2D NMR spectroscopy: Measurements were performed at 274 K and
293 K in standard 5 mm NMR tubes on a Bruker DMX 600 spectrometer
equipped with a 5 mm 13C/1H cryoprobe. Prior to two-dimensional NMR
measurements, the samples were degassed by bubbling dry argon through
them for at least 30 min. A set of standard 1H, 13C, DEPT, (1H,1H)-
COSY, (1H,13C)-HSQC, and (1H,13C)-HMBC spectra was recorded for
Conclusion
1
compound 3 in CDCl3 and in [D7]DMF using the H and 13C solvent sig-
Molecular tweezers obtained by tethering of two identical,
highly dipolar merocyanine chromophores to appropriate
spacer units self-assemble into centrosymmetric dimers con-
taining a p-stacked arrangement of four chromophores.
Owing to the alternating orientation of the dipole moments
of the four chromophores in these bimolecular complexes
and ideal interplanar distances for p–p stacking of about
3.5 ꢃ, very high dimerization constants of up to >10ꢀ9 mꢀ1
have been realized, even in a good solvating solvent such as
nals as internal standards (d=7.26 and 77.00 ppm for CDCl3 and d=2.75
and 29.76 ppm for the high-field methyl signal of [D7]DMF). Two-dimen-
sional ROESY spectra of 3 in CDCl3 were acquired with a standard
pulse sequence over a sweep width of 7184 Hz using 8192 data points in
the t2 dimension and 512 increments in the t1 dimension for different
mixing times of tm =300 ms (at 274 and 293 K) and 100 ms (at 274 K). A
total of 32 scans was collected for each t1 increment with an acquisition
time of 0.57 s followed by an additional relaxation delay of 2.5 s. The
spectra were acquired at different temperatures and mixing times to dif-
ferentiate between ROE connections and chemical exchange peaks. A
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F. Wꢀrthner et al.
2D ROESY spectrum of 3 in [D7]DMF was recorded at 293 K with
300 ms mixing time and similar experimental parameters.
vent was removed under reduced pressure and the oily residue was sub-
jected to column chromatography (silica gel, CH2Cl2/EtOAc 95:5). Re-
peated recrystallization from EtOH afforded fine white needles (3.49 g,
5.83 mmol, 70%). M.p. 103–1048C; TLC (CH2Cl2/EtOAc 95:5): Rf =0.34;
1H NMR (400 MHz, CDCl3, 258C): d=7.59 (br, 2H; naph-H), 7.3 (m,
10H; Ph-H), 6.98 (s, 2H; naph-H), 5.27 (br, 2H; NH), 5.11 (s, 4H;
OCH2Ph), 4.48 (d, J=5.9 Hz, 4H; CH2NH), 4.07 (t, J=6.4 Hz, 4H;
OCH2CH2), 1.84 (m, 4H; OCH2CH2), 1.53 (m, 4H; CH2CH3), 1.00 ppm
(t, J=7.4 Hz, 6H; CH3); HRMS (ESI, pos. mode, MeCN/CHCl3 1:1):
m/z: calcd for C36H42N2O6Na [M+Na]+: 621.29351; found: 621.29399; ele-
mental analysis calcd (%) for C36H42N2O6: C 72.22, H 7.07, N 4.68;
found: C 72.14, H 7.03, N 4.71.
3,6-Dibutoxynaphthalene-2,7-dicarbaldehyde (7): Dihydroxynaphthalene
6 (6.0 g, 27.8 mmol) and K2CO3 (19 g, 139 mmol) were suspended in
DMF (100 mL) and the mixture was heated under an argon atmosphere
to 608C. n-Butyl bromide (7.62 g, 55.6 mmol) was added over a period of
5 min and the mixture was heated for an additional 17 h. The hot mixture
was poured into iced water (200 mL) under vigorous stirring and the sus-
pension was stirred for a further 10 min. The precipitate was collected by
filtration, washed with water, and chromatographed on a short column
(silica gel, CH2Cl2). The crude product was recrystallized twice from ace-
tone, which furnished pale-yellow needles (3.88 g, 11.8 mmol, 43%). M.p.
131–1338C; TLC (CH2Cl2): Rf =0.21; 1H NMR (400 MHz, CDCl3, 258C):
d=10.53 (s, 2H; CHO), 8.37 (s, 2H; naph-H), 7.04 (s, 2H; naph-H), 4.18
(t, J=6.4 Hz, 4H; OCH2), 1.91 (m, 4H; OCH2CH2), 1.57 (m, 4H;
CH2CH3), 1.03 ppm (t, J=7.5 Hz, 6H; CH3); HRMS (ESI, pos. mode,
MeCN/CHCl3 1:1): m/z: calcd for C20H24O4Na [M+Na]+: 351.15668;
found: 351.15691; elemental analysis calcd (%) for C20H24O4: C 73.15, H
7.37; found: C 72.95, H 7.30.
N,N’-[(3,6-Dibutoxynaphthalene-2,7-diyl)bis(methylene)]bis(2-cyanoace-
tamide) (11): In an argon-flooded flask, the Cbz-protected diamine 10
(3.0 g, 5.01 mmol) was dissolved in a mixture of EtOAc (120 mL) and
MeOH (60 mL) and the solution was degassed by applying a vacuum and
flooding with argon. Pd/C (10%, 100 mg) was then added to the solution
and H2 gas was supplied by means of a balloon. The mixture was vigo-
rously stirred under gentle heating at 408C for 2 h and then filtered
through Celite. The solvent was removed under reduced pressure, the re-
sulting colorless oil (1.64 g) was dissolved in MeOH (20 mL), and this so-
lution was degassed as described above. Ethyl cyanoacetate (1.17 mL,
11.0 mmol) was added and the resulting mixture was stirred at RT for
1 h. Additional MeOH (20 mL) was then added and the mixture was re-
fluxed for 90 h. After cooling to RT, the precipitate obtained was collect-
ed by filtration and washed with MeOH. After drying in vacuo, a white
powder was obtained. For elemental analysis, the product was recrystal-
lized from MeCN/AcOH 1:1 (1.54 g, 3.31 mmol, 66%). M.p. 248–2498C;
TLC (CH2Cl2/MeOH 95:5): Rf =0.30; 1H NMR (400 MHz, [D6]DMSO,
258C): d=8.53 (t, 3J=5.6 Hz, 2H; NH), 7.56 (s, 2H; naph-H), 7.23 (s,
2H; naph-H), 4.36 (d, J=5.6 Hz, 4H; CH2NH), 4.08 (t, J=6.4 Hz, 4H;
OCH2CH2), 3.73 (s, 4H; CH2CN), 1.79 (m, 4H; OCH2CH2), 1.50 (m, 4H;
CH2CH3), 0.97 ppm (t, J=7.4 Hz, 6H; CH3); HRMS (ESI, pos. mode,
MeOH): m/z: calcd for C26H32N4O4Na [M+Na]+: 487.23158; found:
487.23181; elemental analysis calcd (%) for C26H32N4O4: C 67.22, H 6.94,
N 12.06; found: C 66.89, H 6.88, N 11.72.
3,6-(Dibutoxynaphthalene-2,7-diyl)dimethanol (8): Dialdehyde 7 (1.0 g,
3.05 mmol) was suspended in a mixture of THF (2 mL) and EtOH
(13 mL). NaBH4 (248 mg, 6.53 mmol) was added and the suspension was
stirred at room temperature (RT) for 2.5 h. Hydrochloric acid (10%,
7 mL) was then carefully added and the mixture was extracted four times
with CH2Cl2. The combined organic phases were washed with brine and
water, dried over Na2SO4, and the solvent was removed in vacuo.
Column chromatography (silica gel, CH2Cl2/MeOH 9:1) and crystalliza-
tion from methyl tert-butyl ether/n-hexane afforded a pale-pink solid
(910 mg, 2.74 mmol, 90%). M.p. 93–948C; TLC (CH2Cl2/MeOH 9:1):
Rf =0.42; 1H NMR (400 MHz, CDCl3, 258C): d=7.60 (s, 2H; naph-H),
7.02 (s, 2H; naph-H), 4.79 (s, 4H; CH2OH), 4.12 (t, J=6.4 Hz, 4H;
OCH2CH2), 2.25 (br, 2H; OH), 1.86 (m, 4H; OCH2CH2), 1.55 (m, 4H;
CH2CH3), 1.01 ppm (t, J=7.4 Hz, 6H; CH3); HRMS (ESI, pos. mode,
MeCN/CHCl3 1:1): m/z: calcd for C20H28O4Na [M+Na]+: 355.18798;
found: 355.18842; elemental analysis calcd (%) for C20H28O4: C 72.26, H
8.49; found: C 72.27, H 8.48.
1,1’-[(3,6-Dibutoxynaphthalene-2,7-diyl)bis(methylene)]bis(6-hydroxy-4-
methyl-2-oxo-1,2-dihydropyridine-3-carbonitrile) (12): Cyanoacetamide
11 (2.0 g, 4.31 mmol) was suspended in piperidine (7 mL) and ethyl ace-
toacetate (3.25 mL, 25.7 mmol) was added to the suspension. The mixture
was heated to 1008C for 3.5 h and then the solvent was distilled off under
reduced pressure. After the addition of concentrated HCl (10 mL), the
mixture was sonicated until a homogeneous suspension was formed. The
mixture was then diluted with water to three times the original volume.
The product was collected by filtration (G3 frit) and washed several
times with water and Et2O to give the product (1.85 g, 3.10 mol, 72%) in
sufficient purity (>85%) for further reaction. Owing to the very poor
solubility of 12 in all tested solvents other than dimethyl sulfoxide, only
small amounts were purified by column chromatography for characteriza-
tion purposes (silica gel, CH2Cl2/MeOH 7:3): TLC (CH2Cl2/MeOH 7:3):
Rf =0.45; 1H NMR (400 MHz, [D6]DMSO, 258C): d=7.24 (s, 2H; naph-
H), 6.91 (s, 2H; naph-H), 5.58 (s, 2H; pyridone-H), 5.10 (s, 4H; CH2N),
4.11 (t, J=6.3 Hz, 4H; OCH2CH2), 2.24 (s, 6H; pyridone-CH3), 1.79 (m,
4H; OCH2CH2), 1.51 (m, 4H; CH2CH3), 0.98 ppm (t, J=7.4 Hz, 6H;
CH3); HRMS (ESI, pos. mode, MeCN): m/z: calcd for C34H36N4O6Na
[M+Na]+: 619.25271; found: 619.25274.
1,1’-[(3,6-Dibutoxynaphthalene-2,7-diyl)bis(methylene)]bis(4-methylpyri-
dinium) ditetrafluoroborate (9): Diol 8 (500 mg, 1.50 mmol) was dis-
solved in 4-methylpyridine (3.5 mL, 35 mmol). Tosyl chloride (572 mg,
3.00 mmol) was added and the solution was stirred at RT for 2 h. Addi-
tional tosyl chloride (143 mg, 0.75 mmol) was then added and the mixture
was stirred for a further 2 h. Acetonitrile (7 mL) was added and the solu-
tion was refluxed for 2 h. The solvent was then removed under reduced
pressure and the residue was dissolved in MeOH (2 mL). In a second
flask, a saturated aqueous NaBF4 solution (22.5 mL) was diluted with
water (7.5 mL) and the methanolic solution was dropped into it under
gentle stirring. The resulting mixture was briefly heated to reflux, cooled
slowly, and kept overnight at room temperature. The precipitate formed
was collected by filtration and recrystallized from iPrOH/EtOAc. An ad-
ditional recrystallization from iPrOH afforded pale-brown needles
(780 mg, 1.18 mmol, 79%). M.p. 117–1208C; 1H NMR (400 MHz,
CD3OD, 258C): d=8.81 (d, J=6.7 Hz, 4H; py-H), 8.11 (s, 2H; naph-H),
7.91 (d, J=6.3 Hz, 4H; py-H), 7.31 (s, 2H; naph-H), 5.84 (s, 4H;
-CH2N), 4.09 (t, J=6.6 Hz, 4H; OCH2), 2.67 (s, 6H; py-CH3), 1.75 (m,
4H; OCH2CH2), 1.37 (m, 4H; CH2CH3), 0.97 ppm (t, J=7.4 Hz, 6H;
CH3); HRMS (ESI, pos. mode, MeOH): m/z: calcd for C32H40BN2O2F4
[MꢀBF4]+: 570.31546; found: 570.31469; elemental analysis calcd (%)
for C32H40B2F8N2O2·2H2O: C 55.36, H 6.39, N 4.03; found: C 55.36, H
6.24, N 3.97.
AHCTUNGTRENGN(UN 5Z,5’Z)-5,5’-{(3,6-Dibutoxynaphthalene-2,7-diyl)bis[methylenepyridin-1-
yl-4-ylidene-(1Z)-ethane-2,1-diylidene]}bis(1-dodecyl-4-methyl-2,6-dioxo-
1,2,5,6-tetrahydropyridine-3-carbonitrile) (2): A mixture of dodecylpyri-
done 16 (275 mg, 0.836 mmol) and N,N’-diphenylformamidine (170 mg,
0.866 mmol) in Ac2O (1.5 mL) was stirred at room temperature for
15 min. The mixture was then heated to 908C for an additional 30 min to
complete the reaction. After cooling the reaction mixture to room tem-
perature, dipyridinium salt 9 (217 mg, 0.33 mmol) and KOAc (97 mg,
1 mmol) were added, and the resulting mixture was heated at 1008C for
14 h. The solution was concentrated in vacuo and the crude product was
purified by column chromatography (silica gel, CH2Cl2/MeOH 95:5)
(105 mg, 92 mmol, 28%). M.p. 2978C; TLC (CH2Cl2/MeOH 93:7): Rf =
Dibenzyl
mate (10): Dialdehyde
[(3,6-dibutoxynaphthalene-2,7-diyl)bis(methylene)]biscarba-
(2.75 g, 8.37 mmol) and benzyl carbamate
7
(2.79 g, 18.5 mmol) were dissolved in a mixture of acetonitrile (90 mL)
and toluene (90 mL). Under stirring, Et3SiH (2.96 mL, 18.6 mmol) and
then TFA (0.94 mL) were added and stirring was continued at RT for
48 h. Additional portions of Et3SiH (2.96 mL, 18.6 mmol) and TFA
(0.94 mL) were added and the solution was stirred for a further 24 h. The
reaction mixture was then washed with saturated aqueous NaHCO3 solu-
tion (2ꢄ90 mL) and brine (90 mL). After drying over Na2SO4, the sol-
3702
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3691 – 3705

Aggregation of Bis(merocyanine) Tweezers
FULL PAPER
0.27; 1H NMR (400 MHz, CD2Cl2/CD3OD 4:1, 258C): d=8.02 (d, J=
7.0 Hz, 4H; py-H), 7.87 (s, 2H; naph-H), 7.70 (s, 4H; methine-H), 7.47
(d, J=7.1 Hz, 4H; py-H), 7.14 (s, 2H; naph-H), 5.43 (s, 4H; CH2N), 4.09
(t, J=6.6 Hz, 4H; CH2O), 3.90 (t, J=7.6 Hz, 4H; CH2N), 2.41 (s, 6H;
pyridone-CH3), 1.79 (m, 4H; OCH2CH2), 1.55 (m, 4H; NCH2CH2), 1.44
(m, 4H; CH2), 1.4–1.2 (m, 36H; CH2), 0.98 (t, J=7.4 Hz, 6H; CH3),
0.85 ppm (t, J=6.8 Hz, 6H; CH3); HRMS (ESI, pos. mode, MeOH):
m/z: calcd for C72H94N6O6Na [M+Na]+: 1161.71271; found: 1161.71164;
UV/Vis (CH2Cl2, 258C, 10ꢀ5 m): lmax (e)=560 nm (226900mꢀ1 cmꢀ1).
tated by adding ethyl acetate (200 mL). The solid was collected by filtra-
tion, washed three times with ethyl acetate, and dried in vacuo to afford
bis(hydroxypyridone) 19 as a light-brown powder in sufficient purity for
further reaction (2.49 g, 5.05 mmol, 61%). M.p. >3008C (decomp);
1H NMR (400 MHz, [D6]DMSO, 258C): d=7.14 (s, 8H; Ph-H), 5.60 (s,
2H; pyridone-H), 5.05 (s, 4H; CH2N), 3.85 (s, 2H; PhCH2Ph), 2.21 ppm
(s, 6H; pyridone-CH3); 13C NMR (100 MHz, [D6]DMSO, 258C): d=
161.0, 160.7, 158.8, 140.6, 134.8, 129.0, 128.0, 117.9, 92.7, 88.8, 43.7, 40.7,
21.0 ppm.
AHCTUNGTERG(NNUN 5Z,5’Z)-5,5’-{Methylenebis[4,1-phenylenemethylenepyridin-1-yl-4-yli-
ACHTUNGTRENNUNG
dene-(1Z)-ethane-2,1-diylidene]}bis(1-dodecyl-4-methyl-2,6-dioxo-1,2,5,6-
tetrahydropyridine-3-carbonitrile) (4): A mixture of hydroxypyridone 16
(353.5 mg, 1.11 mmol) and N,N’-diphenylformamidine (217.8 mg,
1.11 mmol) in acetic anhydride (1.85 mL) was stirred for 30 min at RT
and then heated to 1108C for a further 45 min. After cooling to RT, bis-
(pyridinium) salt 15 (200 mg, 370 mmol) and KOAc (107 mg, 1.11 mmol)
were added and the resulting mixture was heated at 1008C for 17 h. The
solvent was then removed under reduced pressure and the residue was
subjected to column chromatography (silica gel, CH2Cl2/MeOH 95:5).
The product was dissolved in a small amount of CH2Cl2/MeOH 9:1 and
reprecipitated by adding an excess of MeOH to furnish a dark powder
(95 mg, 92 mmol, 25%). M.p. 232–2338C; TLC (CH2Cl2/MeOH 9:1): Rf =
0.54; 1H NMR (400 MHz, [D6]DMSO, 258C): d=8.49 (d, J=7.1 Hz, 4H;
py-H), 7.80–7.68 (m, 8H; py-H, methine-H), 7.36 (d, J=8.2 Hz, 4H; Ph-
H), 7.28 (d, J=8.2 Hz, 4H; Ph-H), 5.45 (s, 4H; NCH2Ph), 3.93 (s, 2H;
PhCH2Ph), 3.79 (t, J=7.4 Hz, 4H; NCH2CH2), 2.39 (s, 6H; pyridone-
CH3), 1.44 (m, 4H; NCH2CH2), 1.22 (m, 36H; CH2), 0.83 ppm (t, J=
6.8 Hz, 6H; CH3); HRMS (ESI, pos. mode, MeCN/CHCl3 1:1): m/z:
calcd for C67H83N6O4 [M+H]+: 1035.64703; found: 1035.64030.
A
ACHTUNGTRENNUNG
(597 mg, 1 mmol) and dimethylformamide (220 mg, 3 mmol) in Ac2O
(0.8 mL) was heated at 908C for 45 min. After cooling to RT, 1-dodecyl-
4-methylpyridinium bromide (13) (685 mg, 2 mmol) and Et3N (0.2 g,
2 mmol) were added and the mixture was heated at 1008C for 14 h.
Thereafter, the solution was concentrated in vacuo and the crude product
was purified by column chromatography (silica gel, CH2Cl2/MeOH
96.5:3.5) (110 mg, 97 mmol, 10%). M.p. 2708C (decomp); TLC (CH2Cl2/
MeOH 93:7): Rf =0.58; 1H NMR (600 MHz, [D7]DMF, 208C): d=8.52
(d, J=7.0 Hz, 4H; py-H), 7.82 (m, 8H; methine-H, py-H), 7.27 (s, 2H;
naph-H), 6.89 (s, 2H; naph-H), 5.22 (s, 4H; naph-CH2N), 4.39 (t, J=
7.4 Hz, 4H; CH2N), 4.17 (t, J=6.5 Hz, 4H; CH2O), 2.47 (s, 6H; pyri-
done-CH3), 1.93 (m, 4H; CH2), 1.87 (m, 4H; CH2), 1.59 (m, 4H; CH2),
1.4–1.2 (m, 36H; CH2), 1.02 (t, J=7.4 Hz, 6H; CH3), 0.88 ppm (t, J=
7.0 Hz, 6H; CH3); 13C NMR (150 MHz, [D6]DMF, 208C): d=163.30,
163.03, 156.89, 156.05, 155.44, 142.46, 138.94, 133.86, 125.81, 123.29,
122.80, 121.12, 119.89, 114.68, 105.44, 105.17, 86.15, 67.86, 59.08, 38.69,
32.10, 31.60, 31.21, 29.72, 29.22, 26.26, 22.81, 19.62, 18.39, 13.97,
13.85 ppm; HRMS (ESI, pos. mode, MeCN): m/z: calcd for
C72H94N6O6Na [M+Na]+: 1161.71271; found: 1161.71166; UV/Vis (DMF,
258C, 5ꢄ10ꢀ5 m): lmax (e)=510 nm (126000mꢀ1 cmꢀ1).
AHCTUNGTERG(NNUN 5Z,5’Z)-1,1’-[Methylenebis(4,1-phenylenemethylene)]bis{5-[2-(1-dode-
cylpyridin-4(1H)-ylidene)ethylidene]-4-methyl-2,6-dioxo-1,2,5,6-tetrahy-
dropyridine-3-carbonitrile} (5): A mixture of bis(hydroxypyridone) 19
(100 mg, 203 mmol), DMF (44 mg, 602 mmol), and acetic anhydride
(0.1 mL) was heated at 908C for 90 min. After cooling to RT, 1-dodecyl-
4-methylpyridinium bromide (13) (139 mg, 406 mmol) and triethylamine
(0.56 mL, 406 mmol) were added and the mixture was heated at 908C for
an additional 3 h. After removal of the solvent under reduced pressure,
MeOH was added to the residue. The precipitated solid was collected by
filtration and washed with diethyl ether. The crude product was subjected
to column chromatography (silica gel, CH2Cl2/MeOH 95:5). The product
was redissolved in a small amount of CH2Cl2/MeOH 9:1 and reprecipitat-
ed by adding an excess of MeOH. After filtration and drying, bis(mero-
cyanine) dye 5 was obtained as a dark-red powder (28 mg, 27 mmol,
13%). M.p. 291–2928C; TLC (CH2Cl2/MeOH 9:1): Rf =0.31; 1H NMR
(400 MHz, [D6]DMSO, 258C): d=8.46 (d, J=7.2 Hz, 4H; py-H), 7.83 (d,
J=7.2 Hz, 4H; py-H), 7.74 (m, 4H; methine-H), 7.15 (d, J=8.3 Hz, 4H;
Ph-H), 7.08 (d, J=8.2 Hz, 4H; Ph-H), 4.98 (s, 4H; NCH2Ph), 4.28 (t, J=
7.04 Hz, 4H; NCH2CH2), 3.82 (s, 2H; PhCH2Ph), 2.42 (s, 6H; pyridone-
CH3), 1.82 (m, 4H; CH2CH2), 1.23 (m, 36H; CH2), 0.85 ppm (t, J=
7.0 Hz, 6H; CH3); HRMS (ESI, pos. mode, CH2Cl2): m/z: calcd for
C67H83N6O4 [M+H]+: 1035.64703; found: 1035.64607.
1,1’-[Methylenebis(4,1-phenylenemethylene)]bis(4-methylpyridinium) di-
bromide (15): A mixture of dibromide 14 (800 mg, 2.26 mmol) and 4-pi-
coline (0.92 mL, 9.2 mmol) in acetonitrile (25 mL) was heated under
reflux for 24 h. Removal of the solvent and the excess 4-picoline under
reduced pressure yielded dipyridinium salt 15 as a white powder (1.17 g,
2.16 mmol, 96%). M.p. 2208C (decomp); 1H NMR (400 MHz, D2O,
258C): d=8.63 (d, J=6.7 Hz, 4H; py-H), 7.81 (d, J=6.7 Hz, 4H; py-H),
7.32 (d, J=8.1 Hz, 4H; Ph-H), 7.26 (d, J=8.1 Hz, 4H; Ph-H), 5.66 (s,
4H; CH2N), 3.93 (s, 2H; PhCH2Ph), 2.63 ppm (s, 6H; py-CH3); 13C NMR
(100 MHz, D2O, 25 8C): d=163.9, 146.3, 146.2, 134.4, 133.1, 132.5, 132.1,
66.7, 43.8, 24.7 ppm; HRMS (ESI, pos. mode, MeOH): m/z: calcd for
C27H27N2 [MꢀHꢀ2Br]+: 379.21688; found: 379.21688.
N,N’-[Methylenebis(4,1-phenylenemethylene)]bis(2-cyanoacetamide)
(18): A mixture of diamine 17 (8.3 g, 36.7 mmol) and ethyl cyanoacetate
(380 mL) was heated at 1008C for 62 h. After cooling to RT, ethyl acetate
(500 mL) was added and the mixture was left to stand overnight. The
precipitated solid was collected by filtration, washed with ethyl acetate,
and dried in vacuo. Recrystallization from acetonitrile yielded bis(cya-
noacetamide) 18 as a brownish powder (3.50 g, 9.71 mmol, 26%). M.p.
215–2188C; 1H NMR (400 MHz, [D6]DMSO, 258C): d=8.64 (br, 2H;
NH), 7.17 (s, 8H; Ph-H), 4.23 (d, J=5.7 Hz, 4H; CH2NH), 3.89 (s, 2H;
PhCH2Ph), 3.65 ppm (s, 4H; CH2CN); 13C NMR (100 MHz, [D6]DMSO,
258C): d=162.0, 140.1, 136.0, 128.5, 127.5, 116.1, 42.3, 40.3, 25.2 ppm; MS
(EI, 70 eV): m/z: 360 [M]+, 276; elemental analysis calcd (%) for
C21H20N4O2: C 69.98, H 5.59, N 15.55; found: C 70.01, H 5.78, N 15.39.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft
(grant project: Wu 317/1–5) and the Fonds der Chemischen Industrie. We
thank Dr. M. Bꢀchner for carrying out the MS measurements.
1,1’-[Methylenebis(4,1-phenylenemethylene)]bis(6-hydroxy-4-methyl-2-
oxo-1,2-dihydropyridine-3-carbonitrile) (19): Metallic sodium (2.5 g,
110 mmol) was added to ethanol (45 mL) and the mixture was heated to
reflux until all of the sodium had reacted to form sodium ethanolate. Cy-
anoacetamide 18 (3.0 g, 8.32 mmol) and ethyl acetoacetate (24 mL,
190 mmol) were added and the resulting mixture was heated under reflux
for 20 h. After the addition of water (90 mL) and further heating for
20 min, hydrochloric acid (10%, 190 mL) was carefully added to the hot
solution. During cooling of the solution, a brown solid precipitated,
which was collected by filtration and washed with water. After drying in
vacuo, the crude product was dissolved in DMSO (50 mL) and reprecipi-
[1] J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives,
VCH, Weinheim, 1995.
[2] a) L. J. Prins, D. N. Reinhoudt, P. Timmerman, Angew. Chem. 2001,
113, 2446–2492; Angew. Chem. Int. Ed. 2001, 40, 2382–2426; b) B. J.
Holliday, C. A. Mirkin, Angew. Chem. 2001, 113, 2076–2097;
Angew. Chem. Int. Ed. 2001, 40, 2022–2043; c) F. Hof, S. L. Craig,
C. Nuckolls, J. Rebek, Jr., Angew. Chem. 2002, 114, 1556–1578;
Chem. Eur. J. 2009, 15, 3691 – 3705
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3703

F. Wꢀrthner et al.
Angew. Chem. Int. Ed. 2002, 41, 1488–1508; d) M. Schmittel, V. Kal-
sani, Top. Curr. Chem. 2005, 245, 1–53; e) Z. Chen, A. Lohr, C. R.
Saha-Mçller, F. Wꢀrthner, Chem. Soc. Rev. 2009, 38, 564–584.
[3] a) V. Balzani, A. Credi, M. Venturi, Molecular Devices and Ma-
chines, VCH, Weinheim, 2003; b) Molecular Switches (Ed.: B. L.
Feringa), Wiley-VCH, Weinheim, 2001.
[4] a) M. L. Merlau, M. del Pilar Mejia, S. T. Nguyen, J. T. Hupp,
Angew. Chem. 2001, 113, 4369–4372; Angew. Chem. Int. Ed. 2001,
40, 4239–4242; b) V. F. Slagt, P. W. N. M. van Leeuven, J. N. H.
Reek, Angew. Chem. 2003, 115, 5777–5781; Angew. Chem. Int. Ed.
2003, 42, 5619–5623; c) M. S. Masar III, N. C. Gianneschi, C. G. Oli-
veri, C. L. Stern, S. T. Nguyen, C. A. Mirkin, J. Am. Chem. Soc.
2007, 129, 10149–10158.
[5] a) A. T. Wright, E. V. Anslyn, Chem. Soc. Rev. 2006, 35, 14–28;
b) S. W. Thomas III, G. D. Joly, T. M. Swager, Chem. Rev. 2007, 107,
1339–1386.
[6] a) A. P. H. J. Schenning, E. W. Meijer, Chem. Commun. 2005, 3245–
3258; b) R. L. Carroll, C. B. Gorman, Angew. Chem. 2002, 114,
4556–4579; Angew. Chem. Int. Ed. 2002, 41, 4378–4400.
[15] a) S. Yagai, M. Higashi, T. Karatsu, A. Kitamura, Chem. Mater.
2005, 17, 4392–4398; b) S. Yagai, M. Higashi, T. Karatsu, A. Kita-
mura, Chem. Commun. 2006, 1500–1502.
[16] a) Y. V. Pereverzev, O. V. Prezhdo, L. R. Dalton, ChemPhysChem
2004, 5, 1821–1830; b) J. J. Wolff, R. Wortmann, Adv. Phys. Org.
Chem. 1999, 32, 121–217; c) T. Verbiest, S. Houbrechts, M. Kaura-
nen, K. Clays, A. Persoons, J. Mater. Chem. 1997, 7, 2175–2189;
d) F. Wꢀrthner, R. Wortmann, K. Meerholz, ChemPhysChem 2002,
3, 17–31.
[17] N. M. Kronenberg, M. Deppisch, F. Wꢀrthner, H. W. A. Lademann,
K. Deing, K. Meerholz, Chem. Commun. 2008, 6489–6491.
[18] For the optical properties and formation of intramolecular aggre-
gates of tethered merocyanine or squaraine dyes, see: a) L. Lu, R. J.
Lachicotte, T. L. Penner, J. Perlstein, D. G. Whitten, J. Am. Chem.
Soc. 1999, 121, 8146–8156; b) T. Katoh, Y. Inagaki, R. Okazaki, J.
Am. Chem. Soc. 1998, 120, 3623–3628; c) S. Zeena, G. K. Thomas, J.
Am. Chem. Soc. 2000, 122, 7859–7865; d) E. N. Ushakov, S. P.
Gromov, O. A. Fedorova, Y. V. Pershina, M. V. Alfimov, F. Barigel-
letti, L. Flamigni, V. Balzani, J. Phys. Chem. A 1999, 103, 11188–
11193; e) E. Arunkumar, P. Chithra, A. Ajayaghosh, J. Am. Chem.
Soc. 2004, 126, 6590–6598.
[7] P. Gregory, High Technology Applications of Organic Colorants,
Plenum Press, New York, 1991.
[19] For reviews on molecular tweezers and clips, see: a) F.-G. Klꢁrner,
B. Kahlert, Acc. Chem. Res. 2003, 36, 919–932; b) M. Harmata, Acc.
Chem. Res. 2004, 37, 862–873.
[8] a) S. Yagai, T. Karatsu, A. Kitamura, Chem. Eur. J. 2005, 11, 4054–
4063; b) S.-S. Sun, J. A. Anspach, A. J. Lees, Inorg. Chem. 2002, 41,
1862–1869; c) J.-P. Collin, C. Dietrich-Buchecker, P. Gavina, M. C.
Jimenez-Molero, J.-P. Sauvage, Acc. Chem. Res. 2001, 34, 477–487;
d) D. P. Funeriu, J.-M. Lehn, K. M. Fromm, D. Fenske, Chem. Eur. J.
2000, 6, 2103–2111; e) V. Balzani, A. Credi, F. M. Raymo, J. F. Stod-
dart, Angew. Chem. 2000, 112, 3484–3530; Angew. Chem. Int. Ed.
2000, 39, 3348–3391.
[9] a) S. J. Lee, C. R. Luman, F. N. Castellano, W. Lin, Chem. Commun.
2003, 2124–2125; b) M. Ruben, J.-M. Lehn, G. Vaughan, Chem.
Commun. 2003, 1338–1339; c) A. Tsuda, S. Sakamoto, K. Yamagu-
chi, T. Aida, J. Am. Chem. Soc. 2003, 125, 15722–15723; d) G. A.
Mines, B. C. Tzeng, K. J. Stevenson, J. Li, J. T. Hupp, Angew. Chem.
2002, 114, 162–165; Angew. Chem. Int. Ed. 2002, 41, 154–157;
e) R. K. Castellano, S. L. Craig, C. Nuckolls, J. Rebek, Jr., J. Am.
Chem. Soc. 2001, 123, 7876–7882.
[20] a) C.-W. Chen, H. W. Whitlock, Jr., J. Am. Chem. Soc. 1978, 100,
4921–4922; b) C. S. Wilcox, L. M. Greer, V. Lynch, J. Am. Chem.
Soc. 1987, 109, 1865–1867; c) S. C. Zimmerman, M. Mrksich, M.
Baloga, J. Am. Chem. Soc. 1989, 111, 8528–8530; d) M. Harmata,
C. L. Barnes, S. R. Karra, S. Elahmad, J. Am. Chem. Soc. 1994, 116,
8392–8393; e) F.-G. Klꢁrner, J. Panitzky, D. Blꢁser, R. Boese, Tetra-
hedron 2001, 57, 3673–3687; f) B. Branchi, V. Balzani, P. Ceroni,
M. C. Kuchenbrandt, F.-G. Klꢁrner, D. Blꢁser, R. Boese, J. Org.
Chem. 2008, 73, 5839–5851.
[21] For self-association of molecular tweezers in organic solvents, see:
a) J. N. H. Reek, J. A. A. W. Elemans, R. de Gelder, P. T. Beurskens,
A. E. Rowan, R. J. M. Nolte, Tetrahedron 2003, 59, 175–185; b) A.
Wu, A. Chakraborty, J. C. Fettinger, R. A. Flowers II, L. Isaacs,
Angew. Chem. 2002, 114, 4200–4203; Angew. Chem. Int. Ed. 2002,
41, 4028–4031; c) A. Wu, P. Mukhopadhyay, A. Chakraborty, J. C.
Fettinger, L. Isaacs, J. Am. Chem. Soc. 2004, 126, 10035–10043;
d) J. N. H. Reek, A. E. Rowan, M. J. Crossley, R. J. M. Nolte, J. Org.
Chem. 1999, 64, 6653–6663; e) T. Haino, T. Fujii, Y. Fukazawa, J.
Org. Chem. 2006, 71, 2572–2580.
[22] For self-association of molecular tweezers in water, see:
a) J. A. A. W. Elemans, A. E. Rowan, R. J. M. Nolte, J. Am. Chem.
Soc. 2002, 124, 1532–1540; b) L. Isaacs, D. Witt, J. Lagona, Org.
Lett. 2001, 3, 3221–3224; c) J. N. H. Reek, A. Kros, R. J. M. Nolte,
Chem. Commun. 1996, 245–247; d) F.-G. Klꢁrner, B. Kahlert, A.
Nellesen, J. Zienau, C. Ochsenfeld, T. Schrader, J. Am. Chem. Soc.
2006, 128, 4831–4841; e) L. Isaacs, D. Witt, Angew. Chem. 2002, 114,
1985–1987; Angew. Chem. Int. Ed. 2002, 41, 1905–1907;
f) J. A. A. W. Elemans, R. de Gelder, A. E. Rowan, R. J. M. Nolte,
Chem. Commun. 1998, 1553–1554.
[10] a) M. Bonchio, T. Carofiglio, M. Carraro, R. Fornasier, U. Tonellato,
Org. Lett. 2002, 4, 4635–4637; b) R. Cibulka, R. Vasold, B. Kçnig,
Chem. Eur. J. 2004, 10, 6223–6231; c) A. Bauer, F. Westkꢁmper, S.
Grimme, T. Bach, Nature 2005, 436, 1139–1140.
[11] a) V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni, M. Ven-
turi, Acc. Chem. Res. 1998, 31, 26–34; b) Y. Kuramochi, A. Satake,
Y. Kobuke, J. Am. Chem. Soc. 2004, 126, 8668–8669; c) F. C. De
Schryver, T. Vosch, M. Cotlet, M. van der Auweraer, K. Mꢀllen, J.
Hofkens, Acc. Chem. Res. 2005, 38, 514–522; d) C.-C. You, C. Hip-
pius, M. Grꢀne, F. Wꢀrthner, Chem. Eur. J. 2006, 12, 7510–7519.
[12] a) F. Wꢀrthner, S. Yao, Angew. Chem. 2000, 112, 2054–2057; Angew.
Chem. Int. Ed. 2000, 39, 1978–1981; b) F. Wꢀrthner, S. Yao, T. De-
baerdemaker, R. Wortmann, J. Am. Chem. Soc. 2002, 124, 9431–
9447; c) U. Rçsch, S. Yao, R. Wortmann, F. Wꢀrthner, Angew.
Chem. 2006, 118, 7184–7188; Angew. Chem. Int. Ed. 2006, 45, 7026–
7030; d) J. Seibt, A. Lohr, F. Wꢀrthner, V. Engel, Phys. Chem.
Chem. Phys. 2007, 9, 6214–6218.
[13] a) S. Yao, U. Beginn, T. Gress, M. Lysetska, F. Wꢀrthner, J. Am.
Chem. Soc. 2004, 126, 8336–8348; b) A. Lohr, T. Gress, M. Dep-
pisch, M. Knoll, F. Wꢀrthner, Synthesis 2007, 3073–3082; c) A.
Lohr, M. Lysetska, F. Wꢀrthner, Angew. Chem. 2005, 117, 5199–
5202; Angew. Chem. Int. Ed. 2005, 44, 5071–5074.
[14] a) F. Wꢀrthner, S. Yao, B. Heise, C. Tschierske, Chem. Commun.
2001, 2260–2261; b) F. Wꢀrthner, S. Yao, J. Org. Chem. 2003, 68,
8943–8949; c) F. Wꢀrthner, J. Schmidt, M. Stolte, R. Wortmann,
Angew. Chem. 2006, 118, 3926–3930; Angew. Chem. Int. Ed. 2006,
45, 3842–3846; d) J. Schmidt, R. Schmidt, F. Wꢀrthner, J. Org.
Chem. 2008, 73, 6355–6362; e) I. Bolz, C. Moon, V. Enkelmann, G.
Brunklaus, S. Spange, J. Org. Chem. 2008, 73, 4783–4793; f) I. Bolz,
S. Spange, Chem. Eur. J. 2008, 14, 9338–9346.
[23] For self-assembly of molecular clips into lamellar films, see: a) S. J.
Holder, J. A. A. W. Elemans, J. J. J. M. Donners, M. J. Boerakker, R.
de Gelder, J. Barberꢆ, A. E. Rowan, R. J. M. Nolte, J. Org. Chem.
2001, 66, 391–399.
[24] For self-assembly of heteroaggregates see, for example: a) G. J. Ga-
briel, B. L. Iverson, J. Am. Chem. Soc. 2002, 124, 15174–15175;
b) Q.-Z. Zhou, X.-K. Jiang, X.-B. Shao, G.-J. Chen, M.-X. Jia, Z.-T.
Li, Org. Lett. 2003, 5, 1955–1958; c) H. M. Colquhoun, Z. Zhu, D. J.
Williams, Org. Lett. 2003, 5, 4353–4356; d) Q.-Z. Zhou, M.-X. Jia,
X.-B. Shao, L.-Z. Wu, X.-K. Jiang, Z.-T. Li, G.-J. Chen, Tetrahedron
2005, 61, 7117–7124; e) S. Ghosh, S. Ramakrishnan, Angew. Chem.
2005, 117, 5577–5583; Angew. Chem. Int. Ed. 2005, 44, 5441–5447;
f) A. R. Pease, J. O. Jeppesen, J. F. Stoddart, Y. Luo, C. P. Collier,
J. R. Heath, Acc. Chem. Res. 2001, 34, 433–444; g) P. Talukdar, G.
Bollot, J. Mareda, N. Sakai, S. Matile, J. Am. Chem. Soc. 2005, 127,
6528–6529; h) P. Talukdar, G. Bollot, J. Mareda, N. Sakai, S. Matile,
3704
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3691 – 3705

Aggregation of Bis(merocyanine) Tweezers
FULL PAPER
Chem. Eur. J. 2005, 11, 6525–6532; i) H. Tanaka, S. Litvinchuk, D.-
H. Tran, G. Bollot, J. Mareda, N. Sakai, S. Matile, J. Am. Chem. Soc.
2006, 128, 16000–16001; j) S. Bhosale, A. L. Sisson, P. Talukdar, A.
Fꢀrstenberg, N. Banerji, E. Vauthey, G. Bollot, J. Mareda, C. Rçger,
F. Wꢀrthner, N. Sakai, S. Matile, Science 2006, 313, 84–86.
[25] a) M. Bꢀhner, W. Geuder, W.-G. Gries, S. Hꢀnig, M. Koch, T. Poll,
Angew. Chem. 1988, 100, 1611–1614; Angew. Chem. Int. Ed. Engl.
1988, 27, 1553–1556; b) B. Odell, M. V. Reddington, A. M. Z.
Slawin, N. Spencer, J. F. Stoddart, D. J. Williams, Angew. Chem.
1988, 100, 1605–1608; Angew. Chem. Int. Ed. Engl. 1988, 27, 1547–
1550.
spectively. Data from CRC Handbook of Chemistry and Physics
(Ed.: D. R. Lide), CRC Press, Boca Raton, 1995.
[38] For the determination of DH8 and DS8, a linear relationship be-
tween lnKD and Tꢀ1 is required. In the present cases, no such linear
relationship was observed, which may be attributed to the strong de-
pendence of er of chloroform on temperature, varying from 5.02 at
283 K to 4.29 at 323 K. For comparison, these values correspond to
the er values of BuOAc and Et2O, respectively, at 293 K. Thus, DH
shows a temperature dependence in the present case, where bimo-
lecular association is driven by electrostatic attraction (see ref. [12]).
Interestingly, the strong dependence of er on temperature is also
manifested in the absorption maxima lmax of this strongly solvato-
chromic class of merocyanine dyes (see Figure S3).
[39] a) G. Ercolani, Chem. Commun. 2001, 1416–1417; b) G. Ercolani, J.
Phys. Chem. B 1998, 102, 5699–5703; c) X. Chi, A. J. Guerin, R. A.
Haycock, C. A. Hunter, L. D. Sarson, J. Chem. Soc. Chem.
Commun. 1995, 2563–2565.
[40] Since the condition EMsKꢃ185n also holds in the present cases, an
almost complete aggregation to the dimers can be obtained at a cer-
tain initial monomer concentration; see ref. [39] for details.
[41] a) H. Ephardt, P. Fromherz, J. Phys. Chem. 1989, 93, 7717–7725;
b) R. F. Khairutdinov, N. Serpone, J. Phys. Chem. B 1997, 101,
2602–2610; c) V. Karunakaran, J. L. P. Lustres, L. Zhao, N. Ernsting,
O. Seitz, J. Am. Chem. Soc. 2006, 128, 2954–2962.
[42] a) F. Wꢀrthner, R. Sens, K.-H. Etzbach, G. Seybold, Angew. Chem.
1999, 111, 1753–1757; Angew. Chem. Int. Ed. 1999, 38, 1649–1652.
[43] The crystal structures of merocyanine chromophores of this type
reveal a significant bond length equilibration within the conjugated
path (refs. [12a,b]). Thus, a bond length of 1.435 ꢃ was obtained for
[26] a) B. J. Whitlock, H. W. Whitlock, J. Am. Chem. Soc. 1994, 116,
2301–2311; b) R. M. Izatt, J. S. Bradshaw, K. Pawlak, R. L. Bruen-
ing, B. J. Tarbet, Chem. Rev. 1992, 92, 1261–1354.
[27] a) Y. Yamauchi, M. Yoshizawa, M. Fujita, J. Am. Chem. Soc. 2008,
130, 5832–5833; b) M. Yoshizawa, J. Nakagawa, K. Kumazawa, M.
Nagao, M. Kawano, T. Ozeki, M. Fujita, Angew. Chem. 2005, 117,
1844–1847; Angew. Chem. Int. Ed. 2005, 44, 1810–1813; c) A. L.
Sisson, N. Sakai, N. Banerji, A. Fꢀrstenberg, E. Vauthey, S. Matile,
Angew. Chem. 2008, 120, 3787–3789; Angew. Chem. Int. Ed. 2008,
47, 3727–3729; d) N. Sakai, A. L. Sisson, T. Bꢀrgi, S. Matile, J. Am.
Chem. Soc. 2007, 129, 15758–15759. This group has also accom-
plished unique self-assembled ion channels and pores containing ex-
tended p-stacks of aromatic compounds: see refs. [24g–j].
[28] a) E. Bamberger, Justus Liebigs Ann. Chem. 1890, 257, 1–55; b) T. J.
Katz, L. Liu, N. D. Willmore, J. M. Fox, A. L. Rheingold, S. Shi, C.
Nuckolls, B. H. Rickman, J. Am. Chem. Soc. 1997, 119, 10054–
10063; c) G. Zagotto, M. Palumbo, E. Uriarte, L. Bonsignore, G.
Delogu, G. Podda, Il Farmaco 1998, 53, 675–679.
[29] For reductive aminations using triethylsilane as reductant, see: D.
Dubꢇ, A. A. Scholte, Tetrahedron Lett. 1999, 40, 2295–2298.
[30] a) B. D. Coleman, R. M. Fuoss, J. Am. Chem. Soc. 1955, 77, 5472–
5476; b) J. O. Morley, R. M. Morley, R. Docherty, M. H. Charlton, J.
Am. Chem. Soc. 1997, 119, 10192–10202.
[31] J. Blacker, J. Jazwinski, J.-M. Lehn, Helv. Chim. Acta 1987, 70, 1–12.
[32] J. Campos Rosa, D. Galanakis, A. Piergentili, K. Bhandari, C. R.
Ganellin, P. M. Dunn, D. H. Jenkinson, J. Med. Chem. 2000, 43,
420–431.
ꢀ
the C5 C6 bond, which is very close to that for a standard single
bond (1.46 ꢃ).
[44] S. Grimme, J. Antony, T. Schwabe, C. Mꢀck-Lichtenfeld, Org.
Biomol. Chem. 2007, 5, 741–758.
[45] Hyperchem, Hypercube Inc., 1115 NW 4th Street, Gainesville, FL
32601, USA.
[46] a) J. Rebek, Jr., B. Askew, M. Killoran, D. Nemeth, F.-T. Lin, J. Am.
Chem. Soc. 1987, 109, 2426–2431; b) S. C. Zimmerman, M. Mrksich,
M. Baloga, J. Am. Chem. Soc. 1989, 111, 8528–8530.
[47] a) M. J. Cloninger, H. W. Whitlock, J. Org. Chem. 1998, 63, 6153–
6159; b) H. Adams, K. D. M. Harris, G. A. Hembury, C. A. Hunter,
D. Livingstone, J. F. McCabe, Chem. Commun. 1996, 2531–2532;
c) F. J. Carver, C. A. Hunter, P. S. Jones, D. J. Livingstone, J. F.
McCabe, E. M. Seward, P. Tiger, S. E. Spey, Chem. Eur. J. 2001, 7,
4854–4862.
[33] For reviews on mass spectrometry in supramolecular chemistry, see:
a) C. A. Schalley, Int. J. Mass Spectrom. 2000, 194, 11–39; b) B. Bay-
tekin, H. T. Baytekin, C. A. Schalley, Org. Biomol. Chem. 2006, 4,
2825–2841.
[34] a) K. A. Connors, Binding Constants, Wiley, New York, 1987;
b) R. B. Martin, Chem. Rev. 1996, 96, 3043–3064.
[48] a) A. S. Davydov, Theory of Molecular Excitons, Plenum Press, New
York, 1971; b) H. Fidder, J. Knoester, D. A. Wiersma, J. Chem.
Phys. 1991, 95, 7880–7890.
[35] M. Kasha, H. R. Rawls, M. Ashraf El-Bayoumi, Pure Appl. Chem.
1965, 11, 371–392.
[36] The transition dipole moment of the merocyanine chromophore is
oriented along the long molecular axis.
[49] D. D. Perrin, W. L. F. Armarego, Purification of Laboratory Chemi-
cals, Pergamon Press, Oxford, 1980.
[37] The electric permittivities er of CHCl3 and DCE are 4.81 and 10.42,
respectively, and their refractive indices n2D0 are 1.444 and 1.446, re-
Received: November 17, 2008
Published online: February 19, 2009
Chem. Eur. J. 2009, 15, 3691 – 3705
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3705