Spacer-Modulated Differentiation between Self-Assembly and Folding Pathways for Bichromophoric Merocyanine Dyes

Article abstract of DOI:10.1002/chem.201502434

We have synthesized a large series of bis(merocyanine) dyes with varying spacer unit and investigated in detail their self-organization behavior by concentration- as well as solvent-dependent UV/Vis spectroscopy. Our in-depth studies have shown that the s

Full text of DOI:10.1002/chem.201502434

DOI: 10.1002/chem.201502434
Full Paper
&
Self-Assembly
Spacer-Modulated Differentiation Between Self-Assembly and
Folding Pathways for Bichromophoric Merocyanine Dyes
AndrØ Zitzler-Kunkel, Eva Kirchner, David Bialas, Christian Simon, and Frank Würthner*[a]
Abstract: We have synthesized a large series of bis(merocya-
nine) dyes with varying spacer unit and investigated in
detail their self-organization behavior by concentration- as
well as solvent-dependent UV/Vis spectroscopy. Our in-
depth studies have shown that the self-organization of the
present bis(merocyanine) dyes is subtly influenced by the
nature of the spacer unit. The utilization of rigid spacers re-
sults in the formation of self-associated bimolecular com-
plexes with high binding strength, while flexible spacers
drive the respective bichromophoric dyes to intramolecular
folding. Our thorough investigations on the impact of alkyl
spacer chain length on the folding tendency of the present
series of bis(merocyanine) dyes revealed a biphasic behavior,
that is, a steep increase of the folding tendency for the dyes
containing C4 to C7 chains and then a gentle decrease for
dyes with longer alkyl spacer chains as evidenced by free
energy (DG) values for the folding of these dyes. Further-
more, analyses of aggregates’ optical properties based on
exciton theory as well as quantum chemical calculations
suggest a bimolecular aggregate structure for the dye pos-
sessing a rigid spacer and a rotationally twisted pleated
structure for the bis(merocyanine) dyes having spacer units
with less than seven carbon atoms, while the application of
longer alkyl chain linkers (ꢀC7) provides enough flexibility
to orient the chromophores in electrostatically most favored
antiparallel fashion.
Introduction
ing has been well recognized and utilized toward tailored
structures and functions by folding-driven self-assembly.[9]
While this research takes advantage of meanwhile well-estab-
lished peptide sequences that direct particular folding patterns
such as a-helices, b-turns,[10] no such structure-folding property
relationships are available for the design of backbone-directed
aromatic p-stacks. Although the design of tweezers for the
complexation of aromatic molecules has been approached in
great detail[11] and some examples of folding-driven dye aggre-
gation are known,[5] apparently studies devoted to the here ad-
dressed question of “folding versus self-assembly”, that is,
backbone-driven differentiation between intramolecular fold-
ing and intermolecular self-assembly pathways (see the illustra-
tion in Scheme 1, bottom), have only been scarcely attended
for p-conjugated dye molecules.[12] In particular, studies related
to the impact of the chain length and constitution of a linking
tether between two dye molecules are obviously missing.
Our recent research activities on merocyanine dye aggre-
gates suggest that these dyes are particularly suited for such
investigations due to their following outstanding features:
i) This class of dyes show very distinct spectral changes in the
UV-visible spectral range upon aggregation (enabling easy
monitoring),[13] and ii) the binding strength between merocya-
nines is quite high due to pronounced electrostatic support
(dipole–dipole interaction) to the attractive forces evoked by
dispersion forces between aromatic scaffolds that ensure ag-
gregation either by folding or self-assembly.[14] Moreover, in
a recent communication we could demonstrate that an opti-
mized processing procedure taking advantage of a folding-
driven self-assembly sequence of bis(merocyanine) dye 1d (see
Appreciable changes of intrinsic optical[1] and electronic[2]
properties of molecular dyes upon aggregation offer unique
opportunities both for the advancement of supramolecular
chemistry and development of organic materials for electron-
ics, photonics, or photovoltaics.[3] Accordingly, significant ef-
forts have been devoted to control the packing of molecular
dyes in self-assembled materials[4] and a plethora of topologies
(stacks, cycles, polyhedra, micelles, vesicles, layers, liquid crys-
tals, etc.) have meanwhile been realized. In contrast to this tre-
mendous effort on intermolecular self-assembly, far less exam-
ples are known where dye aggregation is directed by intramo-
lecular folding.[5] This is particularly surprising if we consider
the increasingly growing interest in foldamers,[6] and the fact
that the proper folding of biomacromolecules in natural sys-
tems into a particular conformation is prerequisite for the reali-
zation of highly ordered architectures with pivotal functions as
evidenced by, for example, multi-protein complexes of the ri-
bosome, virus capsids and the photosystems in plants and
bacteria.[7] Indeed, within the research field of peptide-based
supramolecular p-conjugated materials,[8] the relevance of fold-
[a] Dr. A. Zitzler-Kunkel, E. Kirchner, D. Bialas, C. Simon, Prof. Dr. F. Würthner
Universität Würzburg
Institut für Organische Chemie & Center for Nanosystems Chemistry
Am Hubland, 97074 Würzburg (Germany)
Fax: (+49)931-31-84756
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.201502434.
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nine in the cyanine limit”) and large dipole moment
of 13 D entailed by its pronounced zwitterionic char-
acter.[16]
Here we report that, to our surprise, all of the pres-
ently investigated bis(merocyanine) dyes with (suffi-
ciently long) flexible tethers preferred the intramolec-
ular folding pathway (Scheme 1, middle), except one
dye bearing a more rigid tether that self-assembled
intermolecularly into
a
bimolecular complex
(Scheme 1, left). The third option (Scheme 1, right),
that is, the formation of supramolecular polymers,[17]
was not observed for the present series of bis(mero-
cyanine) dyes within the concentration range of our
study.
Results
Synthesis
Scheme 1. Resonance structures and schematic illustration of the dimerization of mono-
(ATOP) merocyanine dyes (top) by dipolar interactions and different pathways for the
spacer-modulated self-organization of bichromophoric ATOP merocyanine dyes (bottom)
into well-defined bimolecular complexes (left), extended supramolecular polymers
(right), or the formation of intramolecularly folded structures (middle).
The synthetic route to the merocyanine dyes 1a–k is
outlined in Scheme 2 and their full chemical struc-
tures are shown in Scheme 3. In the first synthetic
step, amidisation of cyanoacetic acid methyl ester
(NCCH2CO2Me) with the primary amines 2a–k, which
are either commercially available or prepared accord-
Scheme 3) afforded bulk heterojunction solar cells with im-
proved photocurrents, pinpointing the impact of supramolec-
ular order on the functional properties of a material.[15] These
promising preconditions motivated us towards a fundamental
study devoted to self-organization behavior of molecular archi-
tectures in which two merocyanine dyes are tethered by
spacer units of different length and stiffness. Accordingly, we
have synthesized a broad series of bis(merocyanine) dyes and
studied their aggregation behavior in comparison to the
parent amino-thienyl-dioxocyano-pyridine (ATOP) chromophore
(Scheme 1, top), which was introduced more than a decade
ago as a superb merocyanine dye for photorefractive materials
due to its narrow cyanine-type absorption features (“merocya-
ing to literature, under solvent-free conditions at ambient tem-
perature afforded the corresponding amides 3a–k in high
yields. Subsequently, the condensation reaction of cyanoacetic
acid amides 3a–k with ethyl acetoacetate resulted in the corre-
sponding hydroxypyridones 4a–k that were typically used as
obtained (purity >90%) for further reaction. In the final step,
a Knoevenagel condensation reaction of 5-dibutylamino-thio-
phene-2-carbaldehyde with these hydroxypyridones 4a–k in
acetic anhydride afforded the desired merocyanine dyes 1a–k
in 41–93% yields. All unknown target compounds 1c,e–k were
1
characterized by H and 13C NMR, high resolution mass spec-
trometry, UV/Vis spectroscopy, and elemental analysis (for de-
tails see the Experimental Section).
Scheme 2. Synthesis of merocyanine dyes 1a–k. i) NCCH2CO2Me, 258C, 4 h, yields 81–97%; ii) AcCH2CO2Et, piperidine, 1008C, 4–24 h, yields 74–94%; iii) 5-dibu-
tylaminothiophene-2-carbaldehyde, Ac2O, 908C, 1 h, yields 41–93%.
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Scheme 3. Chemical structures of the merocyanine dyes 1a–k investigated in this work.
Concentration-Dependent UV/Vis Studies
The self-organization behavior of mono(merocyanine) refer-
ence dyes 1a,b and bis(merocyanine) dyes 1c–k that bear dif-
ferent spacer units was first explored by concentration-depen-
dent UV/Vis experiments in nonpolar dioxane, since this sol-
vent has turned out to be ideal for the study of self-organiza-
tion of the present series of dyes in the concentration regime
appropriate for UV/Vis spectroscopy. By employing concentra-
tion-dependent UV/Vis spectroscopy, the aggregation mecha-
nism for different dye molecules can be elucidated by fitting
the absorption data to appropriate mathematical models.[14]
Moreover, initial structural information can be derived from the
spectral position and splitting of the aggregate absorption
band by comparison with that of the monomer band, which
Figure 1. Concentration-dependent UV/Vis spectra of a) 1a (c=3”10À5
–
would then allow a classification to H- or J-type[1] excitonic
coupling[18,19] between the interacting transition dipole mo-
ments of adjacent chromophores. The precise magnitude of
the spectral shift is thereby strongly dependent on the transla-
tional and rotational offset between the two interacting chro-
mophores.[18,19] The UV/Vis spectra of the present merocyanine
dyes were measured in dioxane in the concentration range of
around 10À8 to 10À3 m (depending on the solubility properties
of the dyes). As representative examples, the concentration-de-
pendent spectra of reference mono(merocyanine) dye 1a and
those of bis(merocyanine) dyes 1c, 1i and 1k with p-xylylene,
heptyl and dodecyl spacer, respectively, are depicted in
Figure 1. The spectra of the remaining merocyanines are
shown in Supporting Information (Figure S1). As shown in Fig-
ure 1a,b, for the reference dye 1a and bis(merocyanine) 1c,
the variation of concentration resulted in significant spectral
changes. Thus, with decreasing concentration a gradual de-
crease of the H-type aggregate band (denoted as H) with ab-
4”10À3 M), b) 1c (c=9”10À8–1”10À5 M), c) 1i (c=3”10À7–1”10À5 M) and
d) 1k (c=5”10À7–1”10À5 M) in dioxane at 298 K. Arrows indicate the spec-
tral changes upon dilution. The dotted lines in a) and b) correspond to the
calculated ideal monomer and dimer spectra, respectively.
sorption maximum at 505 nm for 1a and 479 nm for 1c, and
a concomitant appearance of a bathochromically shifted in-
tense charge transfer band centered at around 536 nm (denot-
ed as M), which corresponds to the respective monomeric spe-
cies,[16] were observed. The calculated aggregate spectra of 1a
and 1c also visualize next to the intense H-band a second tran-
sition at longer wavelength (J-band), which is suggestive of
a slightly twisted arrangement of the interacting transition
dipole moments that are oriented along the long molecular
axis of the chromophores.[20] Over the whole investigated con-
centration range very clear isosbestic points were observed in
the spectra of both dyes 1a and 1c which indicate a thermody-
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namic equilibrium between the respective monomeric and the
aggregated species. Similar spectroscopic changes were ob-
served for the monomeric reference 1b (see Supporting Infor-
mation, Figure S1a).
made for 1j with C8 spacer (Figure S2f). A similar trend was
also observed for the dye 1k bearing C12 spacer, but the mo-
nomer band at 536 nm was still evident at a THF/MCH ratio of
10:90 (Figure 2c) which might indicate a weaker aggregation
propensity of this dye compared to that of 1j (C8 spacer). The
solvent-dependent spectral changes of 1 f (C5 spacer), 1g (3-
oxopentyl spacer) and 1h (C6 spacer) resemble those of C12-
tethered dye 1k (see Figure S2c,d,e and Figure 2c). The sol-
vent-dependent spectral changes of bis(merocyanine) dyes
1e–k are accompanied by a clear quasi-isosbestic point, re-
spectively, as observed previously for dye 1d with m-xylylene
spacer,[15] revealing a chemical equilibrium between two spe-
cies, that is, the folded and the unfolded conformers of these
molecules.
In striking contrast to the spectra of references 1a,b and
those of bis(merocyanine) 1c, the absorption spectra of the re-
maining bis(merocyanine) dyes 1d–k do not show appreciable
changes upon dilution, even not at a very low concentration
of only about cꢁ1”10À7 m (see Figure 1c,d and Figure S1b–h).
However, over the whole concentration range, for these bis-
(merocyanine) dyes (1d–k) a hypsochromically shifted band
(H-band) of an exciton-coupled merocyanine dimer aggre-
gate[21] was observed, respectively, in coexistence with the
band at around 535 nm for uncoupled chromophores. Thus,
this H-band has to be attributed to the presence of intramolec-
ular folded structures, in which the excitonic coupling occurs
due to the close proximity of two p-stacked merocyanine dyes.
Solvent-Dependent UV/Vis Studies
We have previously shown that, due to the accessibility of
broad solvent permittivity range, tetrahydrofuran (THF; er =
7.58)/methylcyclohexane (MCH; er =2.02) solvent mixtures are
highly suitable to study merocyanine dye aggregation.[15,22] In
nonpolar MCH aggregation of dipolar merocyanines is favored,
while the interaction of dipole moments of dyes with polar
THF stabilizes the monomeric species with respect to the ag-
gregate.[23] Thus, to get more insight into the self-organization
processes of the bis(merocyanine) dyes 1d–k, which do not
display any considerable changes in their absorption spectra
upon dilution in dioxane, we have performed solvent-depen-
dent UV/Vis measurements in THF/MCH mixtures by increasing
the volume fraction of nonpolar MCH from 0 up to 90% under
dilute (c=1”10À6 m) conditions. For comparison, solvent-de-
pendent spectra of reference dye 1a were measured under
the same conditions. Solvent-dependent spectra of bis(mero-
cyanines) 1e, 1i and 1k are shown as examples in Figure 2
and those of reference 1a and remaining bis(merocyanine)
dyes (1d,f,g,h,j) are displayed in Supporting Information (Fig-
ure S2). While the solvent-dependent spectral changes for the
mono(merocyanine) 1a are rather negligible, substantial
changes were observed for the bis(merocyanine) dyes 1d–k.
Among the alkyl chain (C4-C8 and C12) spacer bearing bis-
(merocyanine) dyes, for dye 1e (C4 spacer) less pronounced
spectral changes were observed with a moderate decrease of
the monomer band at 536 nm and emergence of a weak blue-
shifted shoulder at 518 nm upon increasing the volume frac-
tion of MCH (Figure 2a). These observations suggest that the
C4 spacer is too short to effect the stacking of the two mero-
cyanine dyes on top of each other in a common antiparallel ar-
rangement. For other alkyl-spaced bis(merocyanine) dyes con-
siderably stronger spectral changes were observed with vary-
ing solvent composition. For example, in the case of 1i with
C7 spacer the monomer band at 536 nm was vanished with
a concomitant appearance of a hypsochromically shifted in-
tense band centered at 505 nm upon increasing the content of
MCH up to 90% (Figure 2b). Nearly identical observation was
Figure 2. Solvent-dependent UV/Vis spectra of the bis(merocyanine) dyes a)
1e, b) 1i and c) 1k in THF/MCH mixtures (c=1”10À6 m, T=298 K) starting
in pure THF (dotted lines) and successively increasing the volume fraction of
MCH up to 90% (dashed-dotted lines). Arrows indicate the spectral changes
upon increasing the volume fraction of MCH.
DFT Calculations
To rationalize our spectroscopic observations, DFT calculations
(B97D3/def2-SVP) were performed for the monomers of bis-
(merocyanine) dyes 1i, 1d and 1c as well as for the bimolecu-
lar complex of 1c (Figure 3, for further information see Sup-
porting Information). While the chromophores of 1i with flexi-
ble C7 alkyl spacer are planar and exhibit an ideal distance of
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3.4 Š for p-stacking (Figure 3a), one observes a larger distance
in 1d (m-xylylene spacer) of 3.8 Š and a bending of the chro-
mophores, indicating a comparatively less favorable folded
state due to the rigid spacer unit (Figure 3b). The geometry-
optimized structure of monomer 1c shows an even larger dis-
tance of 4.0 Š and a strong bending of the chromophores
since the p-xylylene spacer unit does not allow a perfect align-
ment of the chromophores for geometric reasons and thus dis-
favors the folded state (Figure 3c). Instead, the dimer structure
shown in Figure 3d, which represents a stack of four merocya-
nine chromophores with smaller distances of 3.3 and 3.4 Š,
should be more favored. This bimolecular aggregate structure
is feasible since the p-xylylene spacer allows intercalation of an
additional chromophore between two merocyanine units of
a molecule with a perfect p-stacking distance of ~3.4 Š.[24] By
comparing the DFT energies of the monomer and the dimer of
1c, it becomes apparent that the energy gain for the forma-
tion of the dimer amounts to 0.0530 a.u. (139 kJmolÀ1) per
molecule which can be attributed to the close packing of the
chromophores in the quadruple merocyanine stack. For com-
parison, the intermolecular distances that were found in single
crystals of reference merocyanine 1a are 3.5 and 3.6 Š,[25]
which are even slightly larger than the distances calculated for
the dimer structure of 1c.
assembly process. For a quantitative description of the aggre-
gation process, initially the simplest mathematical models,
these are the dimerization[13,14] and the isodesmic or equal K
model,[14,26,27] were applied to fit the experimentally obtained
data points. As expected based on our previous work on the
aggregation of simple ATOP merocyanine dyes,[13] for the refer-
ence dyes 1a,b the spectral changes could appropriately be
described with the dimerization model (Figure 4), while the
isodesmic model was not applicable to describe their aggrega-
tion process. On the other hand, it is surprising that the con-
centration-dependent spectral changes observed for bis(mero-
cyanine) dye 1c could also be nicely fitted with the dimeriza-
tion model, although our previous work on another class of
bis(merocyanine) dyes bearing p-xylylene spacer unit afforded
supramolecular polymers[28] as depicted in Scheme 1 (on the
right). This illustrates that subtle changes in conformational
preferences and sterical demands have a strong impact on the
formation of preferred self-assembly product.
Figure 4. The fraction of monomeric species aM calculated from the UV/Vis
data at a certain wavelength and results of the nonlinear regression analysis
based on the dimerization model for dyes 1a (~; 537 nm), 1b (&; 537 nm)
and 1c (*; 536 nm) in dioxane.
For the analysis, a monomer-dimer equilibrium of dyes is as-
sumed in solution for which the dimerization constant KD is de-
fined by the law of mass action according to Equation (1).
cD 1 ÀaM
KD ¼
¼
2a2Mc0
ð1Þ
cM2
Figure 3. Geometry-optimized structures in side view of monomers a) 1i,
b) 1d and c) 1c as well as d) the dimer of 1c obtained by DFT calculations
(B97D3/def2-SVP, butyl groups were replaced by methyl groups).
Herein cM and cD are the concentrations of the monomeric
and dimeric species, respectively, c0 is the total dye concentra-
tion and aM is the fraction of monomeric species in solution.[13]
By solving this equation and taking into account the average
Discussion
ꢀ
absorptivity (e) of a dye in solution, which is the sum of the
contributions of monomeric and dimeric species as shown in
Equation (2):
Evaluation of Self-Assembly Mechanism For Bis(merocya-
nine) Dye 1c Based on Concentration-Dependent UV/Vis
Spectral Data
ꢀ
e ¼eMaM þð1ÀaMÞeD
ð2Þ
The existence of clear isosbestic points in the concentration-
dependent UV/Vis spectra (in dioxane) of the reference mono-
(merocyanines) 1a,b and bis(merocyanine) dye 1c substanti-
ates a chemical equilibrium between monomeric and aggre-
gated species and allows a thermodynamic analysis of the self-
where eM and eD denote the normalized molar absorptivities
of the monomer and dimer, respectively, we obtain Equa-
tion (3):
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pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
aggregate absorption band Dn
in accordance with exciton
¯
À1 þ 8KDc0 þ1
D-M
ð3Þ
ꢀ
e ¼
ðeM ÀeDÞþeD
theory (Figure 5).[18,19] Unfortunately, due to its forbidden
nature, the position of the lowest energy excitonic state
cannot be assigned from our concentration-dependent UV/Vis
spectra (Figure 1) for the tetramer stack of 1c (i.e., bimolecular
assembly of two 1c molecules). Hence, the excitonic coupling
4KDc0
Nonlinear regression analysis of the experimentally deter-
mined extinction coefficients as a function of total dye concen-
tration c0 at certain wavelengths affords the dimerization con-
stants KD and enables the calculation of the associated Gibbs
e can only be estimated from eꢁDn
which is indeed
¯
agg1ÀM
0
almost doubled for the tetrameric stack compared to that of
free dimerization energies (DGD ) according to the Equation (4):
the dimeric stack of 1a or 1b (see Table 1).
0
ð4Þ
DGD ¼ÀRT ln KD
As it is evident from the results summarized in Figure 4 and
Table 1, bis(merocyanine) dye 1c displays significantly higher
(three orders of magnitude) dimerization constant (KD =5.4”
105 mÀ1) compared to those of the reference dyes 1a and 1b
(KD =1.7”102 and 1.2”102 mÀ1). Such a strong increase in the
binding strength for 1c may be rationalized by the presence
of three electrostatically (dipole–dipole interactions) and also
with regard to dispersion interactions favorable contacts be-
tween closely stacked dyes in a bimolecular p-stack composed
of four chromophoric units (see Figure 3d). Accordingly, the di-
merization takes place already at concentrations around
10À5 m, whereas the competing pathway towards supramolec-
ular polymers would only initiate at concentrations of around
0.01m (where the monomeric dyes 1a,b self-assemble into di-
meric units). However, above a certain concentration, that is,
the effective molarity (EM),[29] a transition from cyclic aggre-
gates (i.e., the dimer of 1c) into supramolecular polymers
might occur. As this concentration is >1m (estimated from
EM=KD(1c)/[4”KD(1a)]2), which is far above the concentration
regime of our concentration-dependent UV/Vis measurements,
the formation of linear oligomers by supramolecular polymeri-
zation can be excluded for bis(merocyanine) 1c.
Figure 5. Schematic energy level diagram illustrating allowed electronic tran-
sitions (long arrows) for monomers, H-dimers and H-tetramers according to
the molecular exciton theory. The dipole phase relations for the H-dimer are
also shown. e is the excitonic coupling and f the oscillator strength of the
optical transition, which is for perfectly coplanar transition dipole moments
doubled for the H-band and zero for the J-band according to Kasha
theory.[18,19]
Evaluation of Folding Mechanism for Bis(merocyanine) Dyes
1d–k Based on Solvent-Dependent UV/Vis Data
The concentration-dependent UV/Vis spectra of bis(merocya-
nine) dyes 1d–k in dioxane display a linear increase of absorp-
tion with increasing concentration in accordance with Lam-
bert–Beer’s law for the entire investigated concentration
range. Thus, no changes are observed for the absorption coef-
ficient e for the whole wavelength range over more than
a 100-fold variation in concentration. Although these results
rule out the presence of self-assembly processes, the shape of
the absorption bands indicates different degrees of folding.
Thus, some dyes such as 1i exhibit the most intense band at
508 nm, which is close to the position of the hypsochromically
shifted dimer aggregate band of reference dyes 1a,b, whilst
other dyes such as 1k show the strongest absorption at the
position of the monomer band at 536 nm and only smaller ab-
sorption for the hypsochromically shifted band. Thus, these
bands can be attributed to the presence of two different con-
formational states, that is, folded and unfolded conformations,
whereby the former consists of antiparallel stacked dyes and
the latter prevails without close dye-dye contacts and hence
exhibits negligible excitonic coupling.
Table 1. Dimerization constants KD in dioxane for dyes 1a-c and corre-
0
sponding standard Gibbs free dimerization energies DGD from analysis
based on the dimerization model and absorption maxima of reference
dyes 1a,b and bis(merocyanine) dye 1c.
[a]
0
Dye
KD
[mÀ1
DGD
lM
lagg1
lagg2
Dn
¯
agg1ÀM
]
[kJmolÀ1
]
[nm]
[nm]
[nm]
[cmÀ1
]
1a 1.7”102
1b 1.2”102
1c 5.4”105
À12.4
À11.9
À32.7
537
537
536
505
506
479
547
549
–
1180
1141
2220
[a] Averaged from multilinear regression analysis around the absorption
maximum of the monomer (lM Æ10 nm) or dimer band (lD Æ10 nm), re-
spectively.
A comparison of the aggregate band’s spectral position for
reference dye 1a (505 nm) or 1b (506 nm) with that of bis-
(merocyanine) 1c (479 nm) shows that the latter is much more
blue shifted with respect to the monomer band (~536 nm).
This observation corroborates the suggestion that a bimolecu-
lar p-stack is formed from 1c because the interaction of an in-
creasing number of transition dipole moments in an aggregate
stack should result in a more pronounced spectral shift of the
The equilibrium between unfolded and folded species for
dyes 1d–k could be monitored by solvent-dependent UV/Vis
studies in THF/MCH mixtures (Figure 2 and Figure S2). In the
conducted experiments a clear quasi-isosbestic point, respec-
tively, was observed for all of these bis(merocyanine) dyes, cor-
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cF 1 Àaunfolded
roborating the two-state equilibrium according to Equation (5).
For such situations the thermodynamic driving force for the
folding process may be analyzed according to a model estab-
lished by Moore and Ray[30] to determine the equilibrium con-
stants Keq and the free energy changes DG.
ð7Þ
ð8Þ
Keq
¼
¼
cU
aunfolded
DG ¼ÀRT ln Keq
where cF and cU represent the concentration of the folded
and unfolded species, respectively. The free energy change be-
tween both conformational states is assumed to linearly
depend on solvent composition in analogy to the solvent de-
naturation of proteins and peptide secondary structures.[31]
This enables an extrapolation of the free energy change in
pure MCH (DG(MCH)) by linear regression analysis of the free
energy values obtained for intermediate solvent compositions
according to Equation (9).
Keq, DG
ƒƒƒ!
ƒƒƒ
unfolded state
folded state
ð5Þ
For the determination of thermodynamic data, the solvent-
dependent absorption coefficients at the center of the aggre-
gate band (lagg in Table 2) are most useful, although other
wavelengths might be utilized as well. Towards a straightfor-
ward procedure, initially two assumptions have to be made.
First, we assume that the extinction values eU obtained in pure
THF correspond to the state where the bis(merocyanine) dyes
are entirely unfolded. Second, as the dyes are not soluble in
pure MCH, we assume that at least for dyes 1d,i,j with the
strongest folding tendency in the most nonpolar solvent mix-
ture (THF/MCH=10:90) the extinction values eF coincide with
the state where these bis(merocyanine) dyes are completely
folded.
DG ¼DGðMCHÞÀm½THFŠ
ð9Þ
Based on this analytical procedure, we obtain the data de-
picted in Figure 6 and the corresponding numerical values are
compiled in Table 2. Detailed thermodynamic data obtained by
this analysis are shown in Supporting Information (Table S1). In
Figure 6a it can be clearly seen that the free energy changes
for the transition region between 20–80 vol% THF follow the
assumed linear dependency on the solvent composition. If we
plot the obtained free energy changes DG(MCH) for pure MCH
against the number of atoms in the tether unit, we obtain the
relationship shown in Figure 6b. There are two interesting fea-
tures to discuss in this plot: The first one is the dependence of
folding on the number of carbon atoms present in the aliphat-
ic tether units (connected by dashed line) and the second one
is the varying spacer units with a chain length of five atoms
that are different in element and/or their conformational
degree of freedom. Regarding the bis(merocyanine) dyes with
different lengths of the alkyl linkers, two regimes can be clearly
distinguished. For C4 to C7 chains a strong linear increase in
the folding tendency is observed. On the other hand, if the
chain length is further increased, a comparatively less intense
decrease of the folding tendency is observed with increasing
chain length. Based on our results discussed above, we can
conclude that within the range of C5 to C12 alkyl chain
lengths, the respective folded species is thermodynamically
more stable in pure MCH. For the shorter linker C4, the ener-
getic benefit from dye-dye interactions is obviously not suffi-
cient to overcome the required distortion of the linker units
and hence this molecule prevails in the open state even in the
least polar solvent MCH. Accordingly, C4 or shorter linkers
would be the choice if supramolecular polymers, as shown in
Scheme 1 (on the right), are targeted. A limited solubility of
these dyes in MCH prohibits such studies as higher concentra-
tions would be required.
Table 2. Extrapolated Gibbs free energy changes in MCH derived from
the analysis of folding process in THF/MCH mixtures, slope of the fitting
line m and absorption maxima and wavelength shifts of folded bis(mero-
cyanine) dyes 1d–k in MCH compared to monomeric dyes.
Dye DG(MCH) [kJmolÀ1
]
m [kJmolÀ1
]
lM [nm] lagg [nm] Dn
[cmÀ1
]
agg-M
¯
1d
1e
1 f
1g
1h
1i
À7.3Æ0.5
1.9Æ0.4
158Æ9
123Æ8
135Æ5
141Æ4
136Æ4
169Æ9
163Æ8
153Æ4
536
536
535
535
536
536
536
536
513
516
513
514
510
505
505
505
836
723
802
764
951
1145
1145
1145
À2.6Æ0.3
À5.2Æ0.2
À2.9Æ0.2
À8.9Æ0.5
À7.6Æ0.4
À3.9Æ0.2
1j
1k
However, for the dyes 1e–h,k with a distinctly lower folding
tendency this assumption obviously does not hold true. There-
fore, we make the simplified but reasonable approximation
that fully folded molecules would show extinction value eF for
their foldamer H-band that are equal to the extinction value eU
of the unfolded species of the respective dye in pure THF (at
the monomer M-band). Based on these considerations, the
mole fraction of the dyes in the unfolded state aunfolded can be
calculated for all solvent compositions using Equation (6).
eðlÞF ÀeðlÞ
ð6Þ
aunfolded
¼
eðlÞF ÀeðlÞU
Herein e(l) denotes the extinction value at a particular wave-
length (l) for an intermediate solvent composition and e(l)F
and e(l)U are the respective values for fully folded and fully un-
folded species that are different at this wavelength. The equi-
librium constant Keq and related Gibbs free energy changes DG
for the conformational transition for any solvent composition
can then be obtained from the following relationships:
If we now analyze the folding tendencies of the dyes 1d,f,g
bearing different spacers that comprise five linking atoms, we
notice a strong dependence on the nature of the spacer unit.
The free energy change in pure MCH markedly increases from
pentyl (1 f; DG(MCH)=À2.6Æ0.3 kJmolÀ1) via 3-oxapentyl
(1g; DG(MCH)=À5.2Æ0.2 kJmolÀ1) to the m-xylylene (1d;
DG(MCH)=À7.3Æ0.5 kJmolÀ1) spacer unit. The crucial part of
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gregate structure, we cannot assume a perfect H-coupling sit-
uation. This is corroborated by a non-negligible absorption for
the lower energy J-transition for self-assembled merocyanine
dimer aggregates (see Figure 1a). Indeed, the intensity of this
band might be utilized to calculate the rotational offset from
the perfect antiparallel arrangement.[19,20] Because this intensity
is, however, prone to significant errors due to the required ex-
trapolation from concentration-dependent studies, we will
here perform an analysis based on the experimentally better
defined exciton coupling energy eꢁDn
values (Table 2) ac-
¯
agg-M
cording to Equation (10).
ꢀ
ꢀ
ꢀ
ꢀ
2
~
mag
ðcos a À3 cos2 qÞ
ð10Þ
e ¼
4pe0rA3B
Herein, mag denotes the transition dipole moment of the
chromophores, e0 the permittivity of vacuum and rAB the dis-
tance between the interacting chromophores (center-to-center
distance). a and q are the rotational and translational offset
angles, respectively, which result from the relative mutual ori-
entation of the transition dipole moments of interacting mole-
cules. According to this equation, the excitonic coupling
strength maximizes for perfect collinear dimer stacks (a=08;
q=908) because only in this case the distance rAB can be as
small as ~3.5 Š. The most collinear arrangement is therefore
given for dimers of dye 1a with an e-value of 1180 cmÀ1
(Table 1). Very close to this value and, indeed, within the exper-
imental error are the values for bis(merocyanine) dyes 1i–k
with C7–C12 alkyl spacers (Table 2). For all other bis(merocya-
nine) dyes the less hypsochromically shifted aggregate band
suggests a more twisted chromophore arrangement in the fol-
damers.
Figure 6. a) Plot of the DG values for the folding process of bis(merocyanine)
dyes 1d–k derived from the spectral development at the respective maxi-
mum of the H-aggregate band. The solid lines represent the respective fit-
ting curves from linear regression analysis according to Equation (9). b) Plot
of the free energy change DG(MCH) as the y-intercept from the analysis in
a) as a function of the alkyl chain length of the spacer unit.
the spacer units involves the six bonds between the imide N
atoms at which the two merocyanine chromophore units are
tethered. As only four of these six bonds are freely rotatable
for the m-xylylene spacer (1d), we attribute its highest folding
tendency among these three dyes 1d,f,g to a better preorgani-
zation of the linker with regard to the p-stacked folded struc-
ture. Accordingly, for dye 1d only four rotational degrees of
freedom have to be frozen out in the folded conformer in con-
trast to six for dyes 1 f,g, respectively. For this reason the fold-
ing of 1d should be entropically favored.[32] For 1 f,g, however,
the number of rotable bonds is equal. Here, the 3-oxapentyl
chain in 1g obviously exhibits a higher flexibility of the linker
compared to that of the pentyl chain which prefers a linearly
stretched-out conformation (all-trans).[33] Therefore, the oxygen
atom in the spacer unit facilitates the folding.
A quantification of the rotational displacement angle a be-
tween the two merocyanine chromophores in the foldamers
may be accomplished under the assumptions that i) the trans-
lational displacement angle q is equal to 908 for all foldamers
as well as for the intermolecular dimer aggregates of reference
dyes 1a,b (i.e., perfect co-facial stacking), ii) the transition
dipole moment mag of the involved chromophores and the dis-
tance rAB between those are identical for all investigated dyes
(i.e., close p–p-stacking at van-der-Waals distance), and iii) for
dimer aggregates of the reference dyes 1a,b the rotational dis-
placement angle a is equal to 08 (supported by the crystal
structure[25] for 1a). Based on these assumptions, the quotient
in Equation (10) is a constant and the term 3 cos2q is equal to
zero. Thus, the quantity of the excitonic splitting energy e only
depends on the cosinus of the angle a with e=1180 cmÀ1
”
Structural Insights in Foldamers Based on Exciton Theory
cosa. According to this simple analysis, the rotational displace-
ments within the given foldamers are in the range from 148
(1i–k) to 528 (1e) that are quite reasonable values if we con-
sider the significant number of simplifications made for this
analysis. Obviously, the obtained results corroborate the view
that the higher flexibility imparted by increasing spacer chain
length should lead to dimer stacks with a more collinear ar-
rangement of the transition dipole moments.
Our previous work on self-assembled dimers of dipolar mero-
cyanine dyes such as reference compounds 1a,b and related
merocyanines with various donor and acceptor heterocycles
suggested sandwich-type co-facially stacked centrosymmetric
dimer aggregate structures.[13,25] Because the transition dipole
moments (relevant for exciton coupling) are not perfectly col-
linear with the dipole moments (which direct the antiparallel
arrangement) and, in addition to the dipole–dipole interaction,
dispersion forces have also an influence on the particular ag-
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141.6, 136.2, 127.2, 124.4, 117.2, 112.7, 110.8, 105.2, 91.3, 53.1, 41.5,
28.5, 19.0, 13.1; UV/Vis (THF): lmax (e)=536 nm (251000mÀ1 cmÀ1);
HRMS (ESI, acetonitrile/chloroform, pos. mode): m/z calcd for
C48H56N6O4S2: 844.3799, found: 844.3799 [M]+; elemental analysis
calcd (%) for C48H56N6O4S2: C 68.22, H 6.68, N 9.94, S 7.59; found: C
68.20, H 6.76, N 9.87, S 7.36.
Conclusion
This work apparently constitutes the first investigation on the
effect of various spacer units on the thermodynamics of fold-
ing for bichromophoric p-systems. By tethering two highly di-
polar merocyanine chromophores with various spacers and
performing concentration-dependent UV/Vis studies, we could
distinguish between preferential intermolecular self-assembly
into a quadruple dye stack (only one example) and the more
general folding into intramolecular p-stacks (all other dyes).
Our studies reveal that the final aggregate structure is essen-
tially determined by the nature of the spacer. Thus, a rigid
spacer that preorganizes the chromophores with an interplanar
distance of roughly 7 Š results in the formation of a centrosym-
metric bimolecular complex. This dye assembly comprises a p-
stacked arrangement of four chromophores, excels with high
binding strength (>105 mÀ1) in nonpolar dioxane and spectro-
scopically displays a more than 50 nm blue-shifted absorption
maximum compared to that of the respective monomeric spe-
cies. In contrast, the utilization of flexible linkers leads inevita-
bly to the formation of intramolecular pleated structures,
which are in a chemical equilibrium with disordered, unfolded
species. Based on a solvent-dependent analysis of the free
energy changes between these conformational states, we
found that the folding tendency for different alkyl chain
lengths shows a biphasic behavior, that is, a steep increase of
the folding tendency from C4 to C7 and a subsequent less pro-
nounced decrease for longer alkyl chains. Because the optical
and electronic properties of such bichromophoric aggregate
systems are strongly dependent on the respective aggregate
geometry, our studies might pave the way towards tailored or-
ganic solid-state materials. Folding-driven modulation of opti-
cal and electronic properties of simply tethered bichromo-
phores appears to be a quite attractive research avenue.
General procedure for the synthesis of bis(merocyanine) dyes
1e–k
A mixture of the respective pyridone acceptor 4e–k and 2.2 equiv-
alents of 5-dibutylaminothiophene-2-carbaldehyde in 2–6 mL Ac2O
was stirred at 908C for 60 min. After cooling to room temperature,
the mixture was treated with EtOH/MeOH and the solvent was re-
moved under reduced pressure. The crude products were purified
by column chromatography (silica gel) and subsequent precipita-
tion from CH2Cl2/n-hexane.
N,N’-Tetramethylene-bis(5-dibutylamino-thiophen-2-yl-methyl-
ene-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carboni-
trile) (1e): Compound 1e was synthesized according to the above
general procedure using bis(pyridone) 4e (150 mg, 423 mmol) and
5-dibutylaminothiophene-2-carbaldehyde (225 mg, 931 mmol) yield-
ing a red solid (315 mg, 93%). Column chromatography was per-
formed using dichloromethane/methanol (100:2) as eluent. M.p.
269–2708C; 1H NMR (400 MHz, CD2Cl2): d=7.58 (m, 4H), 6.41 (d,
J=5.2 Hz, 2H), 3.97 (t, J=5.4 Hz, 4H), 3.55 (t, J=7.7 Hz, 8H), 2.46
(s, 6H), 1.78–1.68 (m, 8H), 1.65 (t, J=5.4 Hz, 4H), 1.47–1.37 (m,
8H), 0.99 (t, J=7.3 Hz, 12H); 13C NMR (101 MHz, CDCl3): d=175.8,
163.4, 162.4, 157.9, 151.9, 142.1, 124.5, 117.4, 110.3, 107.5, 95.3,
39.7, 29.8, 29.5, 25.5, 20.2, 18.9, 13.9; UV/Vis (CH2Cl2): lmax (e)=
540 nm (264000mÀ1 cmÀ1); HRMS (ESI, acetonitrile/chloroform, pos.
mode): m/z calcd for C44H56N6O4S2 796.3799, found: 796.3802 [M]+.
N,N’-Pentamethylene-bis(5-dibutylamino-thiophen-2-yl-methyl-
ene-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carboni-
trile) (1 f): Compound 1 f was synthesized according to the above
general procedure using bis(pyridone) 4 f (200 mg, 543 mmol) and
5-dibutylaminothiophene-2-carbaldehyde (273 mg, 1.14 mmol)
yielding a red solid (280 mg, 64%). Column chromatography was
performed using dichloromethane/methanol (100:2) as eluent. M.p.
1
171–1728C; H NMR (400 MHz, CD2Cl2): d=7.56 (d, J=5.2 Hz, 2H),
7.54 (s, 2H), 6.40 (d, J=5.2 Hz, 2H), 3.94 (t, J=7.4 Hz, 4H), 3.53 (t,
J=7.6 Hz, 8H), 2.44 (s, 6H), 1.81–1.61 (m, 12H), 1.47–1.36 (m, 10H),
0.98 (t, J=7.4 Hz, 12H); 13C NMR (101 MHz, CD2Cl2): d=176.3,
163.6, 162.5, 158.3, 152.5, 142.1, 124.7, 117.8, 111.2, 107.3, 94.7, 54.2
(overlapped with solvent signal), 39.9, 29.6, 28.1, 25.1, 20.4, 18.9,
13.9; UV/Vis (CH2Cl2): lmax (e)=539 nm (277000mÀ1 cmÀ1); HRMS
(ESI, acetonitrile/chloroform, pos. mode): m/z calcd for
C45H58N6O4S2: 810.3956, found: 810.3963 [M]+; elemental analysis
calcd (%) for C45H58N6O4S2: C 66.63, H 7.21, N 10.36, S 7.91; found:
C 66.39, H 7.34, N 10.29, S 7.87.
Experimental Section
For materials and methods and for the synthesis of precursor
amides 3g,i–k and hydroxypyridones 4c,e–k see the Supporting
Information.
Synthesis of target compounds 1c,e–k
N,N’-(para-Phenylenedimethylene)-bis(5-dibutylamino-thiophen-
2-yl-methylene-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-
carbonitrile) (1c):
A
mixture of bis(pyridone) 4c (200 mg,
N,N’-[2,2’-Oxybis(dimethylene)]-bis(5-dibutylamino-thiophen-2-
yl-methylene-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-
carbonitrile) (1g): Compound 1g was synthesized according to
the above general procedure using bis(pyridone) 4g (300 mg,
810 mmol) and 5-dibutylaminothiophene-2-carbaldehyde (427 mg,
1.78 mmol) yielding a red solid (532 mg, 81%). Column chromatog-
raphy was performed using chloroform/methanol (100:7) as eluent.
M.p. 215–2168C; 1H NMR (400 MHz, CDCl3): d=7.50 (d, J=5.1 Hz,
2H), 7.47 (s, 2H), 6.35 (d, J=5.1 Hz, 2H), 4.21 (t, J=6.2 Hz, 4H),
3.76 (t, J=6.3 Hz, 4H), 3.54 (t, J=7.8 Hz, 8H), 2.41 (s, 6H), 1.78–
1.68 (m, 8H), 1.47–1.37 (m, 8H), 0.99 (t, J=7.4 Hz, 12H); 13C NMR
(101 MHz, CDCl3): d=176.0, 163.5, 162.3, 158.1, 152.1, 141.9, 124.7,
117.4, 110.7, 107.2, 94.8, 67.6, 53.9, 38.9, 29.5, 20.2, 18.8, 13.9; UV/
Vis (CH2Cl2): lmax (e)=538 nm (248000mÀ1 cmÀ1); HRMS (ESI, aceto-
nitrile/chloroform, pos. mode): m/z calcd for C44H56N6O5S2:
497 mmol) and 5-(dibutylamino)thiophene-2-carbaldehyde (262 mg,
1.09 mmol) in Ac2O (2 mL) was stirred at 908C for 120 min. The re-
action mixture was treated with iPrOH and n-hexane, filtered and
washed successively with n-hexane, Et2O and toluene to give the
crude product (336 mg, 80%) in a purity of ~80% (corresponding
to a yield of ~64% of 1c). Owing to the very poor solubility of 1c
only a small amount of the crude product was purified by column
chromatography (silica gel, CHCl3/MeOH 100:5 v/v) and subse-
quent precipitation from CH2Cl2/n-hexane for the characterization
and UV/Vis spectroscopy. M.p. 305–3068C; 1H NMR (400 MHz,
[D6]DMSO): d=8.02 (d, J=5.2 Hz, 2H), 7.87 (s, 2H), 7.16 (s, 4H),
6.85 (d, J=5.3 Hz, 2H), 4.99 (s, 4H), 3.60–3.53 (m, 8H), 2.46 (s, 6H),
1.68–1.57 (m, 8H), 1.37–1.27 (m, 8H), 0.91 (t, J=7.4 Hz, 12H);
13C NMR (151 MHz, [D6]DMSO): d=176.4, 162.2, 161.3, 158.4, 153.8,
Chem. Eur. J. 2015, 21, 14851 – 14861
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812.3748, found: 812.3750 [M]+; elemental analysis calcd (%) for
C44H56N6O5S2: C 65.00, H 6.94, N 10.34, S 7.89; found: C 64.74, H
7.30, N 10.04, S 7.60.
performed using dichloromethane/methanol (100:2) as eluent. M.p.
264–2658C; 1H NMR (400 MHz, CD2Cl2): d=7.61 (m, 4H), 6.41 (d,
J=5.2 Hz, 2H), 3.92 (t, J=7.5 Hz, 4H), 3.54 (t, J=7.7 Hz, 8H), 2.48
(s, 6H), 1.78–1.68 (m, 8H), 1.64–1.54 (m, 4H), 1.47–1.33 (m, 16H),
0.99 (t, J=7.3 Hz, 12H); 13C NMR (101 MHz, CD2Cl2): d=176.1,
163.7, 162.5, 158.3, 152.5, 142.4, 124.6, 117.8, 111.0, 107.5, 95.3, 53.8
(overlapped with solvent signal), 40.0, 30.0, 29.8, 29.6, 28.3, 27.5,
N,N’-Hexamethylene-bis(5-dibutylamino-thiophen-2-yl-methyl-
ene-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carboni-
trile) (1h): Compound 1h was synthesized according to the above
general procedure using bis(pyridone) 4h (100 mg, 262 mmol) and
5-dibutylaminothiophene-2-carbaldehyde (138 mg, 575 mmol) yield-
ing a red solid (182 mg, 84%). Column chromatography was per-
formed using dichloromethane/methanol (100:2) as eluent. M.p.
269–2708C; 1H NMR (400 MHz, CD2Cl2): d=7.58 (m, 4H), 6.40 (d,
J=5.1 Hz, 2H), 3.93 (t, J=7.5 Hz, 4H), 3.53 (t, J=7.7 Hz, 8H), 2.46
(s, 6H), 1.77–1.68 (m, 8H), 1.66–1.56 (m, 4H), 1.47–1.36 (m, 12H),
0.99 (t, J=7.4 Hz, 12H); 13C NMR (101 MHz, CD2Cl2): d=176.2,
163.7, 162.5, 158.3, 152.4, 142.3, 124.6, 117.8, 111.0, 107.4, 95.1, 53.6
(overlapped with solvent signal), 40.0, 29.6, 28.3, 27.3, 20.5, 18.9,
13.9; UV/Vis (CH2Cl2): lmax (e)=539 nm (279000mÀ1 cmÀ1); HRMS
(ESI, acetonitrile/chloroform, pos. mode): m/z calcd for
C46H60N6O4S2: 824.4112, found: 824.4118 [M]+; elemental analysis
calcd (%) for C46H60N6O4S2: C 66.96, H 7.33, N 10.19, S 7.77; found:
C 67.28, H 7.40, N 10.27, S 7.77.
20.5,
19.0,
13.9;
UV/Vis
(CH2Cl2):
lmax
(e)=540 nm
(289000 LmolÀ1 cmÀ1); HRMS (ESI, acetonitrile/chloroform, pos.
mode): m/z calcd for C52H72N6O4S2: 908.5051, found: 908.5048 [M]+;
elemental analysis calcd (%) for C52H72N6O4S2: C 68.69, H 7.98, N
9.24, S 7.05; found: C 68.79, H 8.14, N 9.27, S 6.84.
Keywords: dipolar interaction · dyes/pigments · excitonic
coupling · foldamers · self-assembly
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N,N’-Heptamethylene-bis(5-dibutylamino-thiophen-2-yl-methyl-
ene-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carboni-
trile) (1i): Compound 1i was synthesized according to the above
general procedure using bis(pyridone) 4i (150 mg, 378 mmol) and
5-dibutylaminothiophene-2-carbaldehyde (200 mg, 832 mmol) yield-
ing a red solid (205 mg, 65%). Column chromatography was per-
formed using dichloromethane/methanol (100:2) as eluent. M.p.
149–1508C; 1H NMR (400 MHz, CD2Cl2): d=7.58 (m, 4H), 6.40 (d,
J=5.2 Hz, 2H), 3.92 (t, J=7.5 Hz, 4H), 3.54 (t, J=7.7 Hz, 8H), 2.46
(s, 6H), 1.77–1.68 (m, 8H), 1.65–1.56 (m, 4H), 1.47–1.33 (m, 14H),
0.99 (t, J=7.3 Hz, 12H); 13C NMR (101 MHz, CD2Cl2): d=176.2,
163.7, 162.5, 158.2, 152.4, 142.2, 124.6, 117.8, 111.0, 107.5, 95.0, 53.8
(overlapped with solvent signal), 40.0, 29.6, 29.5, 28.2, 27.4, 20.5,
18.9, 13.9; UV/Vis (CH2Cl2): lmax (e)=539 nm (272000mÀ1 cmÀ1);
HRMS (ESI, acetonitrile/chloroform, pos. mode): m/z calcd for
C47H62N6O4S2: 838.4269, found: 838.4266 [M]+; elemental analysis
calcd (%) for C47H62N6O4S2: C 67.27, H 7.45, N 10.01, S 7.64; found:
C 66.98, H 7.56, N 9.93, S 7.40.
N,N’-Octamethylene-bis(5-dibutylamino-thiophen-2-yl-methyl-
ene-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carboni-
trile) (1j): Compound 1j was synthesized according to the above
general procedure using bis(pyridone) 4j (100 mg, 244 mmol) and
5-(dibutylamino)-thiophene-2-carbaldehyde (130 mg, 536 mmol)
yielding a red solid (157 mg, 76%). Column chromatography was
performed using dichloromethane/methanol (100:2) as eluent. M.p.
224–2258C; 1H NMR (400 MHz, CD2Cl2): d=7.58 (m, 4H), 6.40 (d,
J=5.2 Hz, 2H), 3.92 (t, J=7.5 Hz, 4H), 3.54 (t, J=7.7 Hz, 8H), 2.47
(s, 6H), 1.78–1.68 (m, 8H), 1.64–1.54 (m, 4H), 1.47–1.33 (m, 16H),
0.99 (t, J=7.3 Hz, 12H); 13C NMR (101 MHz, CD2Cl2): d=176.1,
163.7, 162.5, 158.2, 152.4, 142.3, 124.6, 117.8, 111.0, 107.5, 95.1, 53.8
(overlapped with solvent signal), 40.0, 29.8, 29.6, 28.2, 27.5, 20.5,
18.9, 13.9; UV/Vis (CH2Cl2): lmax (e)=539 nm (277000mÀ1 cmÀ1);
HRMS (ESI, acetonitrile/chloroform, pos. mode): m/z calcd for
C48H64N6O4S2: 852.4425, found: 852.4428 [M]+; elemental analysis
calcd (%) for C48H64N6O4S2: C 67.57, H 7.56, N 9.85, S 7.52; found: C
67.51, H 7.59, N 9.78, S 7.30.
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Published online on September 8, 2015
Chem. Eur. J. 2015, 21, 14851 – 14861
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim