Control of H- and J-type π stacking by peripheral alkyl chains and self-sorting phenomena in perylene bisimide homo- and heteroaggregates

Article abstract of DOI:10.1002/chem.200801454

The synthesis, self-assembly, and gelation ability of a series of organogelators based on perylene bisimide (PBI) dyes containing amide groups at imide positions are reported. The synergetic effect of intermolecular hydrogen bonding among the amide functi

Full text of DOI:10.1002/chem.200801454

FULL PAPER
DOI: 10.1002/chem.200801454
Control of H- and J-Type p Stacking by Peripheral Alkyl Chains and Self-
Sorting Phenomena in Perylene BisACTHNUTRGNEUGNimide Homo- and Heteroaggregates
Suhrit Ghosh, Xue-Qing Li, Vladimir Stepanenko, and Frank Wꢀrthner*[a]
Abstract: The synthesis, self-assembly,
and gelation ability of a series of orga-
nogelators based on perylene bisimide
(PBI) dyes containing amide groups at
imide positions are reported. The syn-
ergetic effect of intermolecular hydro-
gen bonding among the amide func-
tionalities and p–p stacking between
the PBI units directs the formation of
the self-assembled structure in solution,
which beyond a certain concentration
results in gelation. Effects of different
peripheral alkyl substituents on the
self-assembly were studied by solvent-
and temperature-dependent UV-visible
and circular dichroism (CD) spectros-
copy. PBI derivatives containing linear
alkyl side chains in the periphery
formed H-type p stacks and red gels,
whereas by introducing branched alkyl
chains the formation of J-type p stacks
and green gels could be achieved. Ster-
ically demanding substituents, in partic-
ular, the 2-ethylhexyl group completely
suppressed the p stacking. Coaggrega-
tion studies with H- and J-aggregating
chromophores revealed the formation
of solely H-type p stacks containing
both precursor molecules at a lower
mole fraction of J-aggregating chromo-
phore. Beyond a critical composition of
the two chromophores, mixed H-aggre-
gate and J-aggregate were formed si-
multaneously, which points to a self-
sorting process. The versatility of the
gelators is strongly dependent on the
length and nature of the peripheral
alkyl substituents. CD spectroscopic
studies revealed a preferential helicity
of the aggregates of PBI building
blocks bearing chiral side chains. Even
for achiral PBI derivatives, the utiliza-
tion of chiral solvents such as (R)- or
(S)-limonene was effective in preferen-
tial population of one-handed helical
fibers. AFM studies revealed the for-
mation of helical fibers from all the
present PBI gelators, irrespective of
the presence of chiral or achiral side
chains. Furthermore, vortex flow was
found to be effective in macroscopic
orientation of the aggregates as evi-
denced from the origin of CD signals
from aggregates of achiral PBI mole-
cules.
Keywords: dyes/pigments · organo-
gelators · self-assembly · systems
chemistry · vortex effect
Introduction
interactions such as hydrogen bonding, metal-ion-to-ligand
coordination, electrostatic interactions, p–p stacking,
dipole–dipole interactions, hydrophobic interactions, and so
forth, have been identified as enabling the construction of
various superstructures from specifically engineered small-
molecule building blocks. Incorporation of building blocks
bearing specific functionality[3] into larger entities has enor-
mous potential for materials science due to the possibility of
bridging the gap between the molecular scale and the mac-
roscopic one in terms of structural order, when precise con-
trol of such a self-assembly process is achieved. Efforts have
been made to create well-ordered supramolecular structures
based on functional dye molecules[4] to expand their utilities
in various organic electronic device applications. It is well
understood that in many of the optical devices the efficient
transport of charge carriers is essential for improved device
performance. For example, in a bulk heterojunction solar
cell, when charge-separated states are generated upon illu-
mination, it is desirable that holes and electrons find the
Supramolecular chemistry has come to the fore as an exten-
sively studied research area in the recent past due to its im-
plications in biology and materials science.[1] Self-organiza-
tion of small molecular entities into well-defined supra-
molecular architectures is involved in various vital biological
functions in living systems.[2] For example, organization of
lipids in the form of bilayers within the cell membrane is
necessary for life. Over the past decade, several noncovalent
[a] Dr. S. Ghosh, X.-Q. Li, V. Stepanenko, Prof. Dr. F. Wꢀrthner
Universitꢁt Wꢀrzburg
Institut fꢀr Organische Chemie and
Rçntgen Research Center for Complex Material Systems
Am Hubland, 97074 Wꢀrzburg (Germany)
Fax : (+49)931-888-4756
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.200801454.
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continuous pathway to the respective electrodes prior to re-
combination. Thus, higher ordering of p- and n-type semi-
conductors based on p-conjugated systems is necessary for
enhanced charge-carrier mobility on the macroscopic
ACHTUNGTRENNUNG
scale.[4i,j]
Organogels[5] are excellent examples of the construction
of higher order self-assembled structures from properly de-
signed small-molecule building blocks. The formation of or-
ganogels is facilitated by the self-assembly of gelator mole-
cules in a specific manner through various noncovalent in-
teractions. In the recent past, several organogelators have
been reported based on p-type organic semiconductors, such
as porphyrins,[6] phthalocyanines,[7] oligophenylenevinyl-
ACHTUNGTRENNUNG
enes,[8a–d] oligophenyleneethynylenes,[8e,f] and oligothio-
phenes.[9] However, such examples for n-type organic semi-
conductors are still rare.[10]
Perylene bisimides (PBIs) have attracted extensive atten-
tion in the last few years due to their excellent n-type semi-
conductivity[11] and intense photoluminescence.[12] Multichro-
mophoric assemblies based on perylene bisimide building
blocks,[13] created by both covalent and noncovalent interac-
tions, have been studied for photonic and electronic purpos-
es. Such chromophore assemblies find various applications,
for example, as light-harvesting systems,[14] photoinduced
electron-transfer systems,[15] organic light-emitting diodes
(OLEDs),[16] organic thin-film transistors,[17] and solar
cells.[18] Various supramolecular approaches have been ex-
ploited to generate self-assembled structures based on this
class of chromophores.[19] We have recently shown that PBI
derivatives equipped with three long alkyl chains at the aro-
matic imide substituents form one-dimensional self-assem-
bled structures in solution by p–p stacking, with dramatic
concentration-dependent variations in photophysical proper-
ties.[20] To strengthen the intermolecular interaction between
the functional PBI units and to increase the length of the
self-assembled domain in solution, we introduced additional
amide functionality at the imide substituents to facilitate hy-
drogen bonding in addition to the p–p stacking (PBI-1).[21]
Indeed, the PBI-1 molecule was found to self-assemble in a
highly dilute solution and to gel a broad range of organic
solvents that have varying dielectric constants. Photophysi-
cal studies in solution revealed the formation of a self-as-
sembled structure with a hypsochromically shifted absorp-
tion band (H-aggregate). The presence of helical structures
was observed in the AFM images of the gel derived from
this achiral gelator.[21] It was understood that in such an or-
ganogel the PBI chromophores were stacked on top of each
other with slight rotational displacement along the long axis
to generate helical aggregates.[22] However, in these systems
both left-handed (M) and right-handed (P) helicity was ob-
served as expected from achiral chromophores. Thus, to ex-
plore the possibility of generating selectively one-handed
helices, we synthesized another structurally similar organo-
gelator containing chiral alkyl side chains (PBI-3). To our
surprise, this PBI chromophore formed an aggregate with
very different spectral properties, that is, a bathochromically
shifted J-band, in contrast to PBI-1, which showed a hypso-
chromically shifted H-type band, and hence the gel of PBI-3
was almost black in color.[23] Such a remarkable difference
in self-assembly behavior between PBI-1 and PBI-3 prompt-
ed us to investigate this system in detail to gain a better in-
sight into the role of the peripheral alkyl side chains on the
self-assembly behavior. When one compares the structures
of PBI-1 and PBI-3, there are three obvious differences:
1) the length of the alkyl side chain is reduced from C12
(PBI-1) to C8 (PBI-3), 2) two methyl groups are introduced
in PBI-3 as branching units in each side chain, whereas all
the alkyl chains are linear in PBI-1, 3) PBI-3 is chiral where-
as PBI-1 is not.
To probe individually the role of these three structural
variations on self-assembly, we have synthesized a series of
PBI organogelators in which the basic building block re-
mains the same but the length as well as the nature of the
alkyl substituents in the benzamide group at the imide posi-
tions are systematically varied. From PBI-1 to PBI-2 the
length of the alkyl chain is reduced from C12 to C8, whereas
in PBI-3 all linear alkyl chains are replaced by chiral C10
chains with branching. In PBI-6 and PBI-7, instead of all
three, only one or two such chiral branched side chains are
introduced. PBI-4 has the similar branching units as in the
case of PBI-3 but the chirality is absent and also the chain
length is much shorter. In PBI-5 the racemic 2-ethylhexyl
group has been introduced to increase the bulkiness without
providing a helical bias.
Herein, we report the synthesis of this series of new PBI
organogelators and give a detailed account of structure-
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Control of H- and J-Type p Stacking
FULL PAPER
property relationship in terms of their self-assembly behav-
ior and gelation capability.
quently, the remaining two OH groups were alkylated to
obtain compound 1 f in 91% yield. For PBI-7, compound 1’
was alkylated in two successive steps; first it was reacted
with the n-octyl bromide by controlling the molar ratio of
the base as well as alkyl bromide to obtain 1’’ in 40% yield
and then the remaining OH group in 1’’ was alkylated with
the respective chiral alkyl bromide to afford 1g in 92%
yield. Once the intermediate 1 was obtained, the subsequent
reaction steps were similar for all the PBI derivatives. The
methyl ester was hydrolyzed by KOH base in 80–90% yield
and then converted to the aminoethylbenzamide derivative
3 via the acid chloride intermediate in about 60–65% yield.
All the PBI derivatives were prepared by coupling the pery-
lene tetracarboxylic acid bisanhydride with the appropriate
aminoethylbenzamide derivative 3 in the presence of Zn-
Results
Synthesis: The synthetic route for various PBI organogela-
tors is outlined in Scheme 1. Commercially available starting
material 3,4,5-trihydroxymethyl benzoate was alkylated with
appropriate alkyl bromides to get trialkoxymethyl benzoate
1. For PBI-1 to PBI-5, the intermediate 1 was produced in a
single reaction step in about 90% yield by using 4.5 equiv of
base and alkyl bromide. For PBI-6, 1 equiv of base and
alkyl bromide was used to selectively alkylate the OH group
in the 4-position of 3,4,5-trihydroxymethyl benzoate. The
monoalkylated product 1’ was obtained in 61% yield. Subse-
AHCTUNGRTEG(NNUN OAc)2 as a catalyst and were isolated as either red or black
Scheme 1. Synthesis of the present series of PBI derivatives.
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11345

F. Wꢀrthner et al.
solids in about 40–55% yield. All the final compounds as
well as the intermediates were characterized by 1H NMR
spectroscopy, HRMS (ESI), and UV-visible spectroscopy.
peak intensity along with a significant blueshift of the ab-
sorption maximum and a loss of the fine structure. Addition-
ally, a new peak appeared at a longer wavelength of 555 nm.
These features suggest the formation of face-to-face p stacks
(H-aggregate) of rotationally displaced PBI chromo-
phores.[20] For PBI-2 and PBI-6, very similar H-type p stack-
ing was observed (see the Supporting Information for de-
tails). In contrast, self-assembled p stacks of PBI-3 and PBI-
4 exhibit a completely different optical signature. Thus, a
new redshifted J-type absorption band appears at 621 nm
with increasing amount of MCH and disappearance of the
characteristic vibronic pattern of the monomeric dye (Fig-
ure 1b). Additionally, a hypsochromically shifted second
broad band shows up at 432 nm. PBI-7 revealed very intri-
guing self-assembly behavior with increasing amount of
MCH; there were two new bands at 550 and 621 nm due to
the formation of H- and J-type structures, respectively (Fig-
ure 1c). Analysis of the aggregate spectrum at 90:10 MCH/
CHCl3 revealed the presence of H- and J-aggregates approx-
imately in a 1:1 ratio. For PBI-5, similar absorption spectra
are measured in pure MCH and in pure CHCl3 (see the Sup-
porting Information for details), thus suggesting the absence
of p stacking in dilute solution.
Aggregation studies: The self-assembly of the present PBI
derivatives was examined by solvent-dependent and varia-
ble-temperature UV-visible spectroscopy. Figure 1a shows
To quantify the propensity for p-stack formation for this
series of dyes, we have estimated the mole fraction of aggre-
gated dyes (aagg) in the respective MCH/CHCl3 solvent mix-
tures from the solvent-dependent UV-visible studies by
using Equation (1).
AmixꢀAmon
aagg
ꢁ
ð1Þ
AaggꢀAmon
In this equation, Amix is the absorbance at 528 nm for the
PBI chromophore at a given solvent mixture, whereas Amon
and Aagg denote the absorbance at 528 nm at the lowest and
highest MCH/CHCl3 ratios, respectively. The aagg values at
various solvent mixtures for all of the PBI derivatives inves-
tigated here are plotted as a function of the solvent compo-
sition in Figure 2. The critical solvent composition in which
the mole fraction of the aggregate is 0.5 (a50) is estimated
from such a plot and the values are reported in Table 1. We
Figure 1. Solvent-dependent (in various MCH/CHCl3 mixtures) UV-visi-
ble absorption spectra of a) PBI-1 (50:50 to 80:20), b) PBI-4 (40:60 to
70:30), and c) PBI-7 (50:50 to 90:10) at a concentration of 1ꢃ10ꢀ5 m at
258C. Arrows indicate the spectral changes upon increasing the amount
of MCH (from 40 to 90%).
the absorption spectra of PBI-1 in various ratios of methyl
cyclohexane (MCH) and chloroform. The latter is a good
solvent for solvation of the p system of PBI dyes, hence the
dyes do not form aggregates at a high CHCl3 content of
50% at the concentration (1ꢃ10ꢀ5 m) applied in these ex-
periments. Thus, for a 50:50 MCH/CHCl3 ratio the major
absorption band shows the well-resolved vibronic structure
ranging from 400 to 550 nm that is characteristic for the S0–
S1 transition of the isolated PBI chromophore. In contrast,
MCH is a bad solvent for the solvation of the p system of
PBI. As a consequence, the dye is not soluble in pure MCH
and aggregation is observed at higher volume ratios of
MCH/CHCl3 as evidenced by distinct spectral changes (Fig-
ure 1a). The most prominent features are a reduction in the
Figure 2. Plot of aagg as a function of solvent composition (MCH/CHCl3)
for various PBI chromophores. The sigmoidal fit for the data points were
obtained by using the Boltzmann function.
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Control of H- and J-Type p Stacking
FULL PAPER
Table 1. The a50 (% of MCH in CHCl3 at which the aagg value is 0.5) and
a50(T) (at 80:20 MCH/CHCl3) values for various PBI chromophores.
values for PBI-1 and PBI-2, both of which form H-aggre-
gate, are identical, suggesting no difference in their aggrega-
tion propensity. In contrast, for PBI-6, in which only one of
the three achiral n-octyl chains of PBI-2 is substituted by a
chiral side chain, the a50 value increases from 64 to 69%
and the a50(T) value decreases from 53 to 478C. Thus, the
presence of a single methyl branch at one of the alkyl side
chains is already able to destabilize the H-type p-stacking
mode. An even lower propensity for the formation of self-
assembled p stacks is revealed by the significantly higher a50
and lower a50(T) values for PBI-7 bearing two methyl
branches at each of the imide substituents. Remarkably
however, for PBI-3 bearing three methyl branches at each
imide substituent, similar a50 and a50(T) values are observed
as for PBI-1 and PBI-2. Clearly, this unexpected strengthen-
ing of p-stack stability is related to the transition to a pure
J-aggregate, which suggests not much difference between
the stability of the H- and J-type p stack. For PBI-4, the
lowest a50 value and, accordingly, the highest p-stack stabili-
ty is found among these PBI dyes, which might be attributed
to an entropic effect due to the presence of much shorter
(C5) alkyl substituents. It is, nevertheless, remarkable that
the structurally closely related dye PBI-5 behaves complete-
ly oppositely, that is, shows the lowest p-stack stability,
which pinpoints the subtle steric effect of peripheral side
chains on self-assembly.
PBI-1
PBI-2
PBI-3
PBI-4
PBI-6
PBI-7
a50
64
54
64
53
60
56
56
–
69
47
77
31
[a]
a50(T) [8C]
[a] PBI-4 was not soluble in 80:20 MCH/CHCl3, thus the experiment
could not be performed.
also probed the self-assembly as a function of temperature
in 80:20 MCH/CHCl3 solvent mixture. For this solvent com-
position, at 308C a mostly self-assembled structure was ob-
served as expected from a previously performed solvent-de-
pendent study. The spectral variations as a function of tem-
perature for PBI-6 are shown in Figure 3. With increasing
Homo- versus heteroaggregation: As PBI-1 and PBI-3 form
H- and J-aggregate, respectively, mixing of these two chro-
mophores should either lead to coaggregation or self-sort-
ing.[24] To elucidate the preferential self-assembly protocol,
UV-visible spectroscopic studies were performed with the
mixture of PBI-1 and PBI-3 in different molar ratios, while
keeping the total chromophore concentration (1ꢃ10ꢀ5 m)
constant. An 80:20 MCH/CHCl3 solvent mixture was used
in this study in which both PBI-1 and PBI-3 are known to
form aggregates of their own. The UV-visible spectra of dif-
ferent mixtures are shown in Figure 4a. The most indicative
band is the J-band at 621 nm that is formed with increasing
molar ratio of PBI-3. Because the H-aggregate does not
have any absorbance at this wavelength, this band is solely
attributed to the J-aggregate. On the other hand, the J-ag-
gregate has a significant absorbance over the whole visible
wavelength range with a minimum at 550 nm. At this wave-
length a maximum is given for the H-aggregates of pure
PBI-1 (compare Figure 1a). Thus, relative absorption inten-
sities at these two wavelengths (550 and 621 nm) can be
used to estimate the amount of J- and H-aggregated PBI
dyes. Towards this goal the spectral contribution of the J-ag-
gregate fraction was subtracted from the original spectrum
to get the spectrum of H-aggregate fraction by using Equa-
tion (3a). Spectral contribution of only J-aggregate in each
mixture was calculated by using Equation (3b).
Figure 3. Temperature-dependent UV-visible absorption spectra of PBI-6
in MCH/CHCl3 mixtures (80:20) at a concentration of 1ꢃ10ꢀ5 m. Arrows
indicate the spectral changes with an increasing temperature from 30 to
608C. The inset shows the mole fraction of aggregate (aagg(T)) as a func-
tion of temperature.
temperature, the band at 550 nm gradually decreases and
eventually disappears at 608C. Further, at elevated tempera-
ture, the broad band becomes structured with pronounced
peaks at 528 and 490 nm. All these observations together
suggest conversion of the aggregated structure to the mono-
meric dye at elevated temperature. The mole fraction of ag-
gregate at each temperature (aagg(T)) was estimated by
using Equation (2):
AðTÞꢀAmon
aaggðTÞ ꢁ
ð2Þ
AaggꢀAmon
in which aagg(T) is the mole fraction of aggregate at temper-
ature T, and Amon, A(T), and Aagg are the absorbance at
528 nm for the monomer, the solution at temperature T, and
the pure aggregate solutions, respectively. The aagg(T) values
were plotted as a function of temperature (see the Support-
ing Information for details) and from such a plot the a50(T)
(temperature at which aagg =0.5) could be estimated for var-
ious PBI derivatives (Table 1). It can be seen that these
ꢀ
ꢁ
A621
A621ðPBI-3Þ
SH ¼ Sorg
ꢀ
ꢂ Sorg
ð3aÞ
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11347

F. Wꢀrthner et al.
SH, SJ, and Sorg correspond to the H-aggregate spectral frac-
tion, J-aggregate spectral fraction, and the original spec-
trum, respectively. A621 and A621ACTHUNTGRNEUNG(PBI-3) are the absorbance
at 621 nm for a particular mixture and for pure PBI-3, re-
spectively. The assumption in this calculation is that the ab-
sorption coefficient at each wavelength remains unchanged
for the J-aggregate even in the mixture. The decoupled spec-
tra for the H- and J-aggregate fractions are shown in
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
cative of mixing among the two chromophores in the aggre-
gate state. When the ratio of PBI-1/PBI-3 is changed from
100:0 to 60:40, there is almost no decrease of the band in-
tensity at 550 nm and only a very modest J-band appears at
621 nm. This suggests that PBI-3 is incorporated into the H-
aggregate of PBI-1 up to a molar content of 40% instead of
sorting out to form its own J-aggregate. As a consequence
of this coaggregation between PBI-1 and PBI-3 the concen-
tration of H-aggregate remains constant for these mixtures.
With further increase of the relative amount of PBI-3, a
nonlinear increase of the absorption intensity at 621 nm is
observed with a concomitant decrease of the absorption in-
tensity at 550 nm. It is noteworthy that the absorbance at
621 nm in all mixtures is less than estimated based on the
amount of PBI-3, whereas that for 550 nm is always en-
larged. This observation suggests that PBI-3 incorporates
more easily into the H-type p stack of PBI-1 than vice
versa. If we assume that all PBI-1 is incorporated in H-ag-
gregates and that the absorbance at 621 nm originates from
pure PBI-3 aggregates, then the relative amount of PBI-3 in
the mixed H-aggregate (aACTHNUTRGENU(GN PBI-3)) in various feed composi-
tions of the two chromophores can be estimated by using
Equation (4):
½PBI-3ꢃꢀ½Jꢃ
ð4Þ
aðPBI-3Þ ¼
ꢂ 100
½PBI-1ꢃ þ ½PBI-3ꢃꢀ½Jꢃ
in which [PBI-1] and [PBI-3] are the feed concentrations of
the two chromophores, respectively, and [J] is the concentra-
tion of J-aggregated dyes. The extinction coefficient (eJ) at
623 nm (21000mꢀ1 cmꢀ1) for the aggregate spectra of pure
PBI-3 was used to calculate the [J] values in each chromo-
phore mixture. aACTHNUTRGNEUNG(PBI-3) values are plotted as a function of
Figure 4. a) UV-visible absorption spectra of a mixture of PBI-1 and
PBI-3 in different ratios in 80:20 MCH/CHCl3 at 258C. The total concen-
tration of chromophores remains constant (1ꢃ10ꢀ5 m) in each mixture.
b) H- and J-aggregated spectral contributions SH (c) and SJ (b) ac-
cording to Equations (3a) and (3b), respectively. c) Change of absorb-
ance at 550 and 621 nm in the spectra represented in b as a function of
the mole fraction of PBI-3. Dashed lines refer to the calculated varia-
tions assuming no mixing among the two chromophores in the aggregate
state. d) Plot of % PBI-3 in the mixed H-aggregate of PBI-1 and PBI-3
as a function of their feed composition.
feed composition of PBI-3 in Figure 4d. It can be seen that
the maximum incorporation of PBI-3 in the mixed stack is
limited to approximately 44%. This indicates that the ratio
of PBI-3/PBI-1 never exceeds 1:1 in the mixed stack.
Gelation study: Gelation ability of these PBI organogelators
was tested at 1 mm concentration in toluene. At this concen-
tration none of them were soluble in toluene at room tem-
perature except PBI-5, but they could be dissolved upon
heating. When the hot solution was allowed to cool down to
room temperature, red gel was observed for PBI-1, PBI-2,
and PBI-6, whereas for PBI-3 and PBI-4 the gel was dark
A621
SJ ¼
ꢂ Sorg
ð3bÞ
A621ðPBI-3Þ
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Control of H- and J-Type p Stacking
FULL PAPER
PBI-4 is quite similar to that of PBI-3 in aromatic solvents.
However, in other tested solvents it did not form a gel;
either it was not soluble even at elevated temperature or
precipitated out upon cooling. PBI-6 exhibits similar gela-
tion capabilities as structurally related PBI-2.
AFM studies: The morphology of the gels was examined by
using atomic force microscopy (AFM). The AFM images of
PBI-2 in toluene are shown in Figure 6. Helical nanofibers[25]
Figure 5. Pictures of gels for various PBI chromophores. In all these
cases, the solution was prepared in toluene by heating the sample and
the picture was taken after leaving the samples at room temperature for
30 min.
green (Figure 5). In all these cases, gelation occurred within
approximately five minutes. No gelation was observed for
PBI-5 and PBI-7. The former was soluble in toluene, ben-
zene, and MCH even at room temperature and remained
dissolved even after a prolonged period. PBI-7 was soluble
at elevated temperature but upon cooling instead of a gel, a
gel-like precipitate was formed. The critical gelation concen-
tration (CGC) values in toluene were very low for all PBI
gelators. For PBI-1 it was 0.1–0.2 wt%, for PBI-2 and PBI-6
it was 0.05–0.08 wt%, and for J-aggregating PBI-4 and PBI-
3 it was also in the range of supergelators[5e] (<0.1 wt%).
To test the versatility of these organogelators, we further
studied gelation property in different organic solvents. The
results are summarized in Table 2. PBI-1 forms a gel in a va-
Figure 6. AFM images of PBI-2: A) height image, B) phase image, C)
cross-section analysis, D) magnified region from image B. Diluted gel
(0.3 mm) in toluene was spin-cast on mica before taking the AFM
images.
Table 2. Gelation study in various solvent systems.[a]
Solvent
PBI-1
PBI-2
PBI-3
PBI-4
PBI-6
toluene
benzene
MCH[b]
hexane
THF
dioxane
Bu2O[c]
acetone
acetonitrile
ethanol
TEA[d]
DMSO
DMF
G
G
G
G
G
G
G
P
P
P
G
P
P
G
G
G
G
P
P
P
P
P
P
P
P
P
G
G
G
G
G
G
G
G
G
G
G
P
G
G
P
G
G
G
G
GP
GP
GP
P
P
P
could be observed without any preferential helicity. Similar
helical fibers could also be observed for the other H-aggre-
gating PBI chromophores (see the Supporting Information
for details), even though none of them possesses any chiral
substituent except PBI-6. It is interesting to note that even
for PBI-6, both left- and right-handed helices were present
in comparable amounts (see the Supporting Information for
details). Single-handed helices were observed only for chiral
J-aggregating gelator PBI-3. For the other J-aggregating dye
PBI-4, both handed helical fibers were observed (Figure 7)
because no chiral substituent is present in this chromophore.
The height and pitch of the fibers obtained for different ge-
lators are presented in Table 3. It can be seen that the gel
fibers in all these cases are almost similar in dimensions
with height ranging from 2.5–4 nm whilst the pitch is quite
diverse, that is, 6–15 nm. AFM morphology of PBI-7, which
formed a mixed aggregate in solution but did not form a
gel, is shown in Figure 8. In this case similar fibers are ob-
served as well. Careful observation revealed two types of
fibers, one of them shows a more ordered structure with
left-handed helicity (indicated by arrow 2) with helical pitch
of 11ꢄ2 nm and the other (indicated by arrow 1) appears
rather disordered. The height ((3.0ꢄ0.2) nm) and width
P
GP
GP
GP
P
P
P
P
P
P
GP
P
P
P
[a] G, GP, and P denote gelation, gel-like precipitation, and precipitation,
respectively. [b] Methylcyclohexane. [c] Dibutyl ether. [d] Triethylamine.
riety of solvents such as aromatic (toluene, benzene), ali-
phatic (MCH, hexane), ether (THF, dioxane), and triethyl
amine. It does not form gel, however, in hydrogen-bond
donor or hydrogen-bond acceptor solvents such as ethanol
or acetone, acetonitrile, DMF, or DMSO. Gelator PBI-2,
which has a shorter alkyl chain (C8) than PBI-1 is effective
in gelating only aliphatic and aromatic solvents. However,
PBI-3 is the most versatile gelator among the present series
and was able to form a gel in every type of tested organic
solvent except in DMF and DMSO. The gelation ability of
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F. Wꢀrthner et al.
((12ꢄ3) nm) of both of these structures are of similar
AHCTUNGTRENNGdUN imensions.
Aggregation study by CD spectroscopy: As PBI-3, PBI-6,
and PBI-7 have chiral side chains, a helical bias is expected
for their self-assembled structures.[26] Figure 9 shows the CD
Figure 9. Solvent-dependent CD spectra of PBI-6 in MCH/CHCl3 mix-
tures from 55:45 to 80:20 at a concentration of 1ꢃ10ꢀ5 m at 258C. Arrows
indicate the spectral changes with increasing amount of MCH.
Figure 7. AFM images of PBI-4: A) magnified phase image from B;
B,C) height images; D) cross section along line a–b in image C; E) mag-
nified height image from C; F) phase image from E. Diluted gel
(0.27 mm) in toluene was spin-cast on mica before taking the images.
spectra of PBI-6 in various solvent compositions. No CD
signal is observed in 55:45 MCH/CHCl3 because the dye
does not form any aggregate in this solvent composition.
However, when the MCH content was increased above 65%
a strong bisignated Cotton effect was observed with positive
and negative maxima at 455 and 500 nm, respectively. An-
other negative band is observed at 547 nm, which corre-
sponds to the shoulder that arises in the UV-visible spectra
upon aggregation. For PBI-3, the major CD signal appeared
at much longer wavelength (645 nm) and accordingly relates
to the J-band observed in the UV-visible spectra (Figure 1b
and ref. [23]). A UV-visible spectroscopic study revealed the
coexistence of H-type and J-type aggregate for PBI-7. Like-
wise, CD spectroscopic studies show spectral changes that
can be attributed to two species (Figure 10). There are
strong negative and positive bands at 500 and 454 nm, re-
spectively, along with a relatively weak negative band at
545 nm. These three bands are quite similar to those ob-
Table 3. AFM data for the PBI gelators.
PBI-1
PBI-2
PBI-3
PBI-4
PBI-6
height [nm]
pitch [nm]
helicity
3.1ꢄ0.3
15ꢄ2.0
P/M
3.9ꢄ0.2
9ꢄ0.4
P/M
2.4ꢄ0.2
6.6ꢄ0.1
P
2.3ꢄ0.2
11ꢄ0.2[a]
P/M
3.4ꢄ0.2
14ꢄ0.5
P/M
[a] Fibers with slightly different pitch are also observed.
Figure 10. Solvent-dependent CD spectra of PBI-7 in MCH/CHCl3 mix-
tures from 55:45 to 90:10 at a concentration of 1ꢃ10ꢀ5 m in a 1 cm cuv-
ette at 258C. Arrows indicate the spectral changes with increasing
amount of MCH.
Figure 8. AFM images of PBI-7: A–C) height images, D) cross-sectional
analysis. Diluted gel (0.3 mm) in toluene was spin-cast on mica before
taking the AFM images.
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Control of H- and J-Type p Stacking
FULL PAPER
served in the case of PBI-6 and thus can be attributed to H-
aggregated species. However, two more bands are seen at
626 and 406 nm that resemble those found for PBI-3 (see
the Supporting Information for details) and are thus evi-
dence for the presence of J-aggregated species. Further-
more, temperature-dependent CD spectroscopy experiments
for PBI-6 solutions confirmed the reversibility of the self-as-
sembly process (see the Supporting Information for details).
At 258C in a 80:20 MCH/CHCl3 mixture, there was a strong
excitonic CD couplet, indicating the presence of chiral ag-
gregate. When the temperature was raised, the intensity of
the CD signal decreased and almost disappeared at 608C,
which revealed conversion of the aggregate structures into
monomers.
phore had been studied before.[21] Variable-temperature UV-
visible spectroscopic studies of PBI-1 in (R)- and (S)-limo-
nene confirmed the formation of H-aggregate at ambient
temperature (see the Supporting Information for details). In
Figure 12, temperature-dependent CD spectra of PBI-1 in
Coaggregation of PBI-1 and PBI-3 was also studied by
using CD spectroscopy (Figure 11). The experiments were
done under identical conditions as the UV-visible spectros-
Figure 12. Temperature-dependent CD spectra of PBI-1 in (S)-limonene
(a) and (R)-limonene (c); concentration: 5ꢃ10ꢀ5 m. Arrows indicate
the spectral changes with decreasing temperature (100 to 308C).
(R)-limonene are shown. At elevated temperature, no
Cotton effect was observed, suggesting the absence of any
aggregated structure. However, when the solution was
cooled, a strong Cotton effect was observed with positive
and negative maxima at 458 and 499 nm, respectively, along
with another negative band at 546 nm. The shape of the CD
spectrum is identical to that for chiral chromophore PBI-6
in the presence of excess MCH in CHCl3 (Figure 9). It is
also noteworthy that the De values in Figures 9 and 12 are
similar in magnitude, which suggests a comparable helical
bias imparted by the chiral side chain in PBI-6 and the
chiral solvent for PBI-1.[29] When similar variable-tempera-
ture studies were carried out in (S)-limonene, the CD spec-
tra observed were exact mirror images of those found in the
case of (R)-limonene (Figure 12).
Figure 11. CD spectra of PBI-1 and PBI-3 in different ratios in MCH/
CHCl3 mixtures (80:20) at 258C. Total concentration of the chromo-
phores were kept constant (1ꢃ10ꢀ5 m) in each mixture.
copy experiments. It can be seen that with 100% PBI-1,
there is no CD signal (violet line) because the H-aggregat-
ing chromophore PBI-1 does not possesses any chiral side
chain. However, when 20% of J-aggregating chromophore
PBI-3 was added to 80% of PBI-1, a strong excitonic-type
CD band appeared (blue line) with a positive and negative
maximum at 458 and 499 nm, respectively, along with a
second negative band at 546 nm. With further increase in
the amount of PBI-3 up to 60% in the mixture, there was
little change in the band intensities at 458, 498, and
546 nm[27] but a separate band appeared at 623 nm due to
the formation of pure J-aggregate by PBI-3 (orange line).
At 20:80 PBI-1/PBI-3 a significant decrease of the intensity
of the excitonic CD signal is observed that can be rational-
ized in terms of the lowered concentration of H-aggregate
(pink line). Notably, these results corroborate well to the
UV-visible results shown in Figure 4.
Stirring effect: In the recent past, there has been significant
interest in macroscopic orientation of supramolecular nano-
fibers by external perturbation such as stirring, shaking, and
so forth.[30] Stirring is particularly interesting because its di-
rection as well as the speed can be controlled precisely.
Aida and co-workers have reported remarkable vortex ef-
fects on the CD signal of zinc porphyrin nanofibers,[30a]
which were attributed to the alignment of the nanofibers
macroscopically in the direction of the vortex flow. At the
same time, Meijer and co-workers reported similar behavior
of supramolecular nanostructures generated from oligo(p-
phenylenevinylene) derivatives.[30b] To the best of our knowl-
edge, no such studies have been reported for organogelators
or for perylene bisimide dye aggregates so far. Thus, we
have studied the impact of vortex flow created by a magnet-
ic stirring bar on the CD spectra of H-aggregated achiral
chromophore PBI-1 in toluene. In the absence of stirring no
CD signal was observed. However, upon clockwise stirring a
very strong bisignated CD signal with a positive band at
We also examined the possibility of a preferential popula-
tion of one-handed helices by external chiral bias imparted
by a suitable chiral solvent.[28] The chiral solvent limonene
has been used in this study due to its structural similarity
with toluene, in which the self-assembly of PBI-1 chromo-
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11351

F. Wꢀrthner et al.
458 nm and two negative bands at 498 and 546 nm were ob-
served (Figure 13a). With gradual increase of the stirring
speed, the CD signal intensity increased steadily before it
bled nanofibers by vortex flow, which might be a particular-
ly appealing technique for the orientation of functional ma-
terials based on organogelators.
Discussion
Spectroscopic studies revealed distinct differences among
the present PBI derivatives in terms of their self-assembly
and gelation properties depending on the peripheral alkyl
substituents. It was found that PBI-1, PBI-2, and PBI-6
form H-aggregates, whereas PBI-3 and PBI-4 form J-aggre-
gates, and PBI-7 forms mixtures of H- and J-aggregates. On
the other hand, PBI-5 does not form any aggregate at all in
the investigated concentration range. In our previous work
on PBI-1 and PBI-3, we proposed that the transition from
H- to J-type aggregate may originate from a simple rotation
around the CH2 Namide bond[23] with retention of the syner-
ꢀ
getic effect of p–p stacking and hydrogen-bonding interac-
tion among the amide functionality as shown on the right
side of Figure 14. However, it was not clear how such very
subtle modification in the alkyl side chains could dictate the
preference for one of these two possible p-stacking modes.
Comparison of the self-assembly of PBI-3 and PBI-4 elimi-
nates any specific role of alkyl chain length or chirality, be-
cause the former chromophore is substituted with chiral C10
alkyl groups, whereas the latter is with achiral C5 alkyl
groups, but both of them form J-aggregate. Based on the re-
sults obtained in the present study, we propose that the
bulkiness of the methyl groups in the g position of the pe-
ripheral side chains destabilizes the more densely packed
face-to-face H-type p stacking and favors the J-type p stack-
ing, in which the chromophores are packed with significant
longitudinal displacement (arrow in Figure 14, right-hand
side). Owing to this displacement an increased columnar
cross section is available that provides more space to accom-
modate the alkyl side chains. Thus, PBI-1 and PBI-2 having
linear alkyl substituents prefer to self-assemble as more
compact H-aggregates as does PBI-6 having only one
branched chain among the three alkyl substituents at each
imide position, whereas PBI-3 and PBI-4, both of which
contain three branched alkyl substituents, prefer to self-as-
semble as J-aggregates in which the chromophores are
packed with relatively larger space as shown in Figure 14
(middle). PBI-7 has one linear and two branched alkyl sub-
stituents and in this case neither of the two types of stacking
is largely favored. Therefore, in this case neither H-type nor
J-type aggregate is formed with high preference. Notably,
for this compound aggregation required the highest amount
of MCH in the MCH/CHCl3 mixture and the lowest temper-
ature (Table 1), while no gelation of any tested organic sol-
vents could be observed. This remarkably different behavior
of PBI-7, compared with the very similar compounds PBI-6
and PBI-3, can now be traced back to the disorder in pack-
ing imparted by the coexistence of H- and J-type packing
modes in small domains that do not grow into well-ordered
extended macroscopic fibers (Figure 8). For PBI-5, the pres-
Figure 13. a) Evolution of CD signal upon gradual increase in stirring
(clockwise direction) speed for a solution of PBI-1 (5ꢃ10ꢀ5 m) in toluene
at 308C; inset shows the variation of CD intensity at 446 nm as a function
of stirring speed. b) Evolution of CD signal upon stirring (1300 rpm) in
clockwise (CW) and anticlockwise (ACW) direction for a solution of
PBI-1 (5ꢃ10ꢀ5 m) in toluene at 308C; the inset shows the CD intensity at
446 nm with successive on–off cycles of stirring at 1300 rpm.
was saturated at about 1000 rpm (inset, Figure 13a). If the
CD signal arises in this case from the vortex-induced macro-
scopic orientation of the supramolecular fibers, the direction
of it should be altered when the stirring direction is re-
versed. Indeed, such dynamic behavior is observed upon
changing from clockwise to anticlockwise stirring where a
mirror-image CD spectrum originates (Figure 13b). Further
evidence for the dynamic nature of the macroscopic aggre-
gate orientation is given by the instantaneous on–off switch-
ing of the CD signal upon switching the magnetic stirrer on
and off (inset, Figure 13b). When the stirrer was on, a strong
CD signal could be observed but as soon as it was switched
off, the CD signal intensity decreased to zero. This experi-
ment was repeated for several cycles and identical results
were obtained, which suggests the complete reversibility of
the macroscopic orientation of the PBI aggregates by exter-
nal stirring. A linear dichroism (LD) signal was also ob-
served under vortex flow that does not alter the sign upon
reversal of the stirring direction (see the Supporting Infor-
mation, Figure S12). The remarkable similarity of our results
with those recently published in the literature for two other
dye aggregates[30] corroborates the orientation of self-assem-
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Control of H- and J-Type p Stacking
FULL PAPER
Figure 14. Left: Schematic representation of PBI chromophores with linear (top) and branched (bottom) alkyl substituents. Middle: The transition from
H- (top) to J-type (bottom) p stacking with increasing steric demand of the peripheral alkyl side chains. Right: Packing model for H- (top) and J-type
(bottom) p stacking. In both cases additional rotational offsets are needed to enable both close p–p contact and hydrogen bonding.
ence of larger ethyl groups at the more central b position in-
hibits formation of any p stacking.
Conclusion
Further indirect support for this hypothesis based on
steric constraint is gathered from the coaggregation studies
of PBI-1 and PBI-3. It was found from UV-visible spectros-
copy as well as from CD spectroscopic studies that, when
purely J-aggregating chromophore PBI-3 was mixed with
purely H-aggregating PBI-1, at lower PBI-3/PBI-1 ratio,
only H-aggregate was formed containing both kinds of chro-
mophores. The maximum incorporation of PBI-3 in such a
mixed H-type p stack was calculated to be 44%. This ratio
implies that PBI-3 is incorporated in the mixed H-type stack
until it can be located in-between two PBI-1 units (at the
most 50% PBI-3 can be incorporated in the mixed stack to
make this arrangement possible), so that the unfavorable
steric constraint among two adjacent face-to-face stacked
PBI-3 units does not arise. The ratio of 1:1 would imply a
perfectly alternating arrangement of the two different chro-
mophores in the mixed H-type stack, which is, however, not
entropically favored. At a ratio of 0.44:0.56, there is still
some disorder in the mixed-stacked system leading to the
lowest Gibbs energy. In contrast, self-sorting of PBI-1 and
PBI-3 dyes prevails at high PBI-3 content due to the ener-
getically disfavored packing of PBI-1 dyes in a J-type pack-
ing mode.
We have investigated the influence of peripheral alkyl side
chains on the self-assembly and gelation properties of a
series of structurally related PBI chromophores. It was
found that, depending on the nature of the alkyl substitu-
ents, the p-stacking mode of the chromophores could be al-
tered from commonly seen H type to the rather uncommon
J type.[31,32] Our studies revealed that steric effects in the pe-
ripheral side chains dictate the mode of self-assembly. PBI
chromophores substituted with linear alkyl chain (least
steric demand) formed sandwich-type H-aggregates whereas
PBI chromophores bearing branched alkyl groups (higher
steric demand) formed slipped J-aggregates,[32] or no aggre-
gates at all. For mixtures of H- and J-aggregating chromo-
phores PBI-1 and PBI-3 coaggregation in H-type p stacks
was observed in the presence of larger amounts of the H-ag-
gregating chromophore PBI-1 up to a 1:1 ratio. Whilst such
coaggregation suggests the intimate alternate packing of the
two chromophores at a 1:1 ratio, self-sorting of PBI-1 and
PBI-3 dyes prevails at high PBI-3 content due to the ener-
getically disfavored packing of PBI-1 within J-aggregates.
The results of this work may be important for the emerging
field of systems chemistry.[24,33] They offer the relevant
guidelines for a rational design of PBI J-aggregates. Having
understood the self-assembly properties at the supramolec-
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11353

F. Wꢀrthner et al.
matography (silica gel, chloroform) to get the pure product as a light
yellow oil (40%). 1H NMR (400 MHz, CDCl3, TMS, 300 K): d=7.28 (s,
1H), 7.16 (s, 1H), 5.82 (s, 1H), 4.20 (t, 2H), 4.01 (m, 6H), 3.87 (s, 3H),
1.84–1.14 (m, 22H), 0.93–0.85 ppm (m, 21H); UV/Vis (CH2Cl2): lmax
(e)=305 (2100), 268 nm (9200mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for
C26H45O5 [M+H]+: 437.3261; found: 437.3269.
ular level, we are currently engaged in exploiting these ma-
terials in bulk heterojunction solar cells.[34]
Experimental Section
3,4,5-TrisACTHNUTRGNE(NUG octyloxy)methyl benzoate (1b): 3,4,5-Trihydroxymethyl ben-
zoate (5.0 g, 0.027 mol) was dissolved in dry DMF (50 mL) and to this so-
lution K2CO3 (16.76 g, 0.121 mol) and n-octyl bromide (17.19 g,
0.089 mol) were added, and the reaction mixture was stirred at 708C for
48 h, then cooled to room temperature and poured into ice-cold water
(300 mL). A light brown oil was separated from the mixture and was
then extracted with diethyl ether (3ꢃ50 mL), and the combined organic
layer was washed with water and brine and dried over Na2SO4. The sol-
vent was evaporated and the crude product (80%) was found to be ade-
quately pure from TLC analyses and 1H NMR spectroscopy. Thus, the
product was used for the next step as such. 1H NMR (400 MHz, CDCl3,
TMS, 300 K): d=6.93 (s, 2H), 3.96–3.92 (m, 6H), 3.88 (s, 3H), 1.71 (m,
6H), 1.33–1.25 (m, 30H), 0.96 ppm (m, 9H); UV/Vis (CH2Cl2): lmax (e)=
301 (4520), 274 nm (9200mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for
C32H56NaO5 [M+Na]+: 543.4020; found: 543.4020.
Materials and methods: All solvents and reagents were purchased from
commercial sources and purified by using standard methods.[35] The sol-
vents for spectroscopic studies were of spectroscopic grade and used as
received. 1H NMR spectra were recorded on a 400 MHz Bruker spec-
trometer and all the spectra were calibrated against tetramethylsilane
(TMS). UV-visible spectra were measured by using
a Perkin-Elmer
Lambda 40P spectrometer equipped with a Peltier system as temperature
controller. The CD spectra were recorded with a Jasco J-810 spectropo-
larimeter. AFM measurements were carried out under ambient condi-
tions by using a MultiMode Nanoscope IV system operating in tapping
mode in air. Silicon cantilevers (OMCL-AC160TS) with a resonance fre-
quency of ꢁ300 kHz were used.
UV-visible and CD spectroscopic studies: Stock solutions (concentration
1ꢃ10ꢀ4 m) of PBI dyes were made in CHCl3. A 0.2 mL aliquot of the
stock solution was transferred to six different volumetric flasks, each of
2 mL volume. The final volume was adjusted by adding different amounts
of CHCl3 and methyl cyclohexane (MCH) in different flasks to get vari-
ous solutions of the same concentration (1ꢃ10ꢀ5 m) but in different sol-
vent compositions. The solutions were allowed to equilibrate for 3 h prior
to the spectroscopic measurements. For the coaggregation studies, stock
solutions of PBI-1 and PBI-3 in CHCl3 were mixed in various ratios and
then MCH was added. For variable-temperature UV-visible and CD
spectroscopic experiments, a 20 min interval was given before each mea-
surement after the desired temperature was reached.
Compounds 1d–1g were synthesized according to the above-described
procedure for compound 1b.
3,4,5-(Tris-3-methylbutyloxy)methyl benzoate (1d): Light green oil
(72%); 1H NMR (400 MHz, CDCl3, TMS, 300 K): d=7.26 (s, along with
residual solvent peak), 4.05–4.01 (m, 6H), 3.89 (s, 3H), 0.97–0.93 ppm
(m, 18H); UV/Vis (CH2Cl2): lmax (e)=301 (4590), 274 nm
(9660mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for C23H38NaO5 [M+Na]+:
417.2611; found: 417.2586.
3,4,5-(Tris-2-ethylhexyloxy)methyl benzoate (1e): Light green oil (15%);
1H NMR (400 MHz, CDCl3, TMS, 300 K): d=7.24 (s, 2H), 3.90–3.81 (m,
11H), 1.54–0.88 ppm (m, 43H); UV/Vis (CH2Cl2): lmax (e)=301 (4470),
274 nm (9180mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for C32H57O5 [M+H]+:
521.4200; found: 521.4201.
Gelation tests: The measured amount of PBI gelator and appropriate sol-
vent were put together in a screw-capped sample vial and it was heated
until all the solute was dissolved and then allowed to cool down to room
temperature. After leaving the sample for 1 h at ambient temperature,
the formation of gel was tested by the “stable-to-inversion of a vial”
method.[5d]
4-[(S)-3,7-Dimethyloctyloxy]-3,5-dioctyloxymethyl benzoate (1 f): Yield:
91%; 1H NMR (400 MHz, CDCl3, TMS, 300 K): d=7.24 (s, 2H), 4.00–
3.98 (m, 6H), 3.88 (s, 3H) 1.82–0.85 ppm (m, 49H); UV/Vis (CH2Cl2):
lmax (e)=301 (4620), 274 nm (9350mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for
C34H61O5 [M+H]+: 549.4513; found: 549.4522.
AFM studies: For AFM studies, the gel in toluene was diluted and then
was spin-coated onto mica under 2000 rpm rotating speed.
Synthesis and characterization: Synthesis and characterization of PBI-
1[21] and PBI-3[23] were described previously. All the other PBI organoge-
lators investigated here were synthesized by means of a similar proce-
dure.
3,4-[Bis-(S)-3,7-dimethyloctyloxy]-5-(octyloxy)methyl benzoate (1g):
Light yellow oil (92%); 1H NMR (400 MHz, CDCl3, TMS, 300 K): d=
7.26 (s, 2H), 4.05–4.00 (m, 6H), 3.99 (s, 3H), 1.84–1.13 (m, 32H), 0.94–
0.85 ppm (m, 21H); UV/Vis (CH2Cl2): lmax (e)=301 (4600), 274 nm
(9310mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for C36H65O5 [M+H]+:
577.4826; found: 577.4818.
4-[(S)-3,7-Dimethyloctyloxy]-3,5-dihydroxymethyl benzoate (1’): A mix-
ture of 3,4,5-trihydroxymethyl benzoate (3.23 g, 17.50 mmol), (S)-3,7-di-
methyloctyl bromide (3.62 g, 17.50 mmol), and K2CO3 (2.42 g,
17.50 mmol) in dry DMF (30 mL) was stirred at 808C for 24 h under an
inert atmosphere. The reaction mixture was cooled to room temperature
and poured into ice-cold water (200 mL) and was extracted with diethyl
ether (3ꢃ50 mL). The combined organic layer was dried over Na2SO4
and the solvent was evaporated to get the crude product as a brown oil,
which was purified by using column chromatography (silica gel, chloro-
form) to get the pure product as a light brown oil (61%). 1H NMR
(400 MHz, CDCl3, TMS, 300 K): d=7.20 (s, 2H), 5.52 (s, 2H), 4.16–4.12
(m, 2H), 3.87 (s, 3H), 1.86–1.13 (m, 10H), 0.94–0.85 ppm (m, 9H); UV/
Vis (CH2Cl2): lmax (e)=306 (1300), 295 (1800), 260 nm (7900mꢀ1 cmꢀ1);
HRMS (ESI): m/z calcd for C18H27O5 [MꢀH]ꢀ: 323.1864; found:
323.1864.
3,4,5-TrisACTHNUTRGNE(UNG octyloxy)benzoic acid (2b): KOH (5.6 g) was dissolved in water
(40 mL) and was added to a solution of 1b in ethanol (9.0 g, 40 mL), and
the emulsion was stirred at 1008C for 5 h. After 1 h, the emulsion
became a clear solution, indicating the progress of the reaction. After
5 h, the heating was stopped and the reaction mixture was poured into a
solution of concentrated HCl (15 mL) in ice-cold water (350 mL). A
white precipitate came out, which was filtered and washed with distilled
water and dried under vacuum. The crude product was obtained as a
white powder (90%), which was taken to the next step without further
purification. M.p. 66–688C; 1H NMR (400 MHz, CDCl3, TMS, 300 K):
d=10.6 (broad peak, 1H), 7.03 (s, 2H), 3.99–3.93 (m, 6H), 1.72 (m, 6H),
1.33–1.25 (m, 30H), 0.96 ppm (t, J=7.08 Hz, 9H); UV/Vis (CH2Cl2): lmax
(e)=303 (4470), 276 nm (8470mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for
C31H54NaO5 [M+Na]+: 529.3863; found: 529.3863.
3-Octyloxy-4-[(S)-3,7-dimethyloctyloxy]5-hydroxymethyl benzoate (1’):
Compound 1’ (1.56 g, 4.81 mmol), n-octyl bromide (0.929 g, 4.81 mmol),
and K2CO3 (0.663 g, 4.81 mmol) were taken in a flask along with dry
DMF (20 mL) and the reaction mixture was stirred at 808C for 24 h
under an inert atmosphere. The heating was stopped, the reaction mix-
ture was cooled to room temperature, then poured into ice-cold water
(120 mL), and extracted with diethyl ether (3ꢃ30 mL). The combined or-
ganic layer was dried over Na2SO4 and the solvent was evaporated to get
the crude product as a brown oil. It was purified by using column chro-
Compounds 2d–2g were synthesized according to the above-described
procedure for compound 2b.
3,4,5-(Tris-3-methylbutyloxy)benzoic acid (2d): White powder (87%);
m.p. 758C; 1H NMR (400 MHz, CDCl3, TMS, 300 K): d=7.33 (s, 2H),
4.06 (m, 6H), 1.89–1.61 (m, 9H). 0.97–0.96 ppm (m, 18H); UV/Vis
(CH2Cl2): lmax (e)=303 (5100), 276 nm (9700mꢀ1 cmꢀ1); HRMS (ESI):
m/z calcd for C22H36NaO5 [M+Na]+: 403.2455; found: 403.2455.
11354
www.chemeurj.org
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 11343 – 11357

Control of H- and J-Type p Stacking
FULL PAPER
3,4,5-(Tris-2-ethylhexyloxy)benzoic acid (2e): Light yellow waxy material
(87%); 1H NMR (400 MHz, CDCl3, TMS, 300 K): d=7.25 (s, 2H), 3.81
(m, 6H), 1.67–0.86 ppm (m, 43H); UV/Vis (CH2Cl2): lmax (e)=303
(5100), 276 nm (9700mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for C31H53O5
[MꢀH]ꢀ: 505.3898; found: 505.3899.
imidazole (10.5 g) were taken together in a flask and were stirred at
1308C for 24 h under an argon atmosphere. The reaction mixture was
cooled to room temperature and poured into methanol (100 mL). The
red precipitate was filtered and the solid was washed several times with
methanol. The crude product was purified by using column chromatogra-
phy with silica gel as the stationary phase and 5% MeOH in CHCl3 as
the eluent. The pure product was obtained as a bright red powder
(35%). M.p. 258–2608C; 1H NMR (400 MHz, CDCl3, TMS, 300 K): d=
8.50 (d, J=7.6 Hz, 4H), 8.36 (d, J=8.0 Hz, 4H), 7.08 (s, 2H), 7.00 (s,
4H), 4.53 (m, 4H), 3.98 (m, 12H), 3.88 (m, 4H), 1.80–1.15 (m, 72H),
0.88 ppm (m, 18H); UV/Vis (CHCl3): lmax (e)=528 (79100), 491 (48300),
460 nm (17900mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for C90H125N4O12
[M+H]+: 1453.9288; found: 1453.9285.
4-[(S)-3,7-Dimethyloctyloxy]-3,5-diACTHNUGTRNEUNG(octyloxy)benzoic acid (2 f): White
1
waxy material (90%); H NMR (400 MHz, CDCl3, TMS, 300 K): d=7.19
(s, 2H), 4.00–3.97–3.88 (m, 6H), 1.82–0.85 ppm (m, 49H); UV/Vis
(CH2Cl2): lmax (e)=303 (4900), 276 nm (8700mꢀ1 cmꢀ1); HRMS (ESI):
m/z calcd for C33H57O5 [MꢀH]ꢀ: 533.4211; found: 533.4215.
3,4-[Bis-(S)-3,7-dimethyloctyloxy]-5-octyloxybenzoic acid (2g): White
1
powder (91%); m.p. 60–628C; H NMR (400 MHz, CDCl3, TMS, 300 K):
d=7.31 (s, 2H), 4.09–4.00 (m, 6H), 1.85–1.14 (m, 32H), 0.95–0.85 ppm
(m, 21H); UV/Vis (CH2Cl2): lmax (e)=303 (4900), 276 nm
(8700mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for C35H61O5 [MꢀH]ꢀ: 561.4525;
found: 561.4527.
All other PBI gelators were made by following the above procedure.
N,N’-DiACTHUNRTGNEUNG[3,4,5-(tris-4-methylbutyloxy)benzoylaminoethyl]perylene-
3,4:9,10-tetracarboxylic acid bisimide (PBI-4): Yield: 49%; m.p. 285–
2878C; 1H NMR (400 MHz, CDCl3, TMS, 300 K): d=8.66 (d, J=8.0 Hz,
4H), 8.58 (d, J=8.04 Hz, 4H), 7.25 (along with residual solvent peak),
6.98 (s, 4H), 4.56 (m, 4H), 4.06–3.96 (m, 16H), 1.72–1.60 (m, 16H), 0.97–
0.92 ppm (m, 36H); UV/Vis (CHCl3): lmax (e)=528 (80200), 491 (51100),
460 nm (18100mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for C72H88N4NaO12
[M+Na]+: 1223.6291; found: 1223.6290.
3,4,5-TrisACHTUNGTRENNUNG(octyloxy)benzoylaminoethylamide (3b): Compound 2b (2.88 g,
5.69 mmol) was dissolved in dry dichloromethane (30 mL). Thionyl chlo-
ride (15 mL) was added along with 3 drops of dry DMF. The reaction
mixture was stirred at room temperature for 12 h. The stirring was stop-
ped and the solvents were evaporated under vacuum. The crude product
was obtained as a light yellow solid and it was taken to the next step
without further purification. It was redissolved in dry dichloromethane
(30 mL) and the solution was added dropwise to an ice-cold flask con-
taining ethylene diamine (30 mL). The reaction mixture was stirred in
the same ice bath for another 3–4 h and then at room temperature for
12 h. Then the reaction mixture was diluted with dichloromethane
(60 mL) and washed with water, aqueous NaHCO3, and brine. The organ-
ic layer was dried over Na2SO4 and the solvent was evaporated to get the
crude product, which was then added to ethanol (60 mL) and kept in the
refrigerator for 2 h. The precipitate was filtered under vacuum and dried
to get the crude product as a light yellow waxy material (65%). It was
taken to the next step without further purification. 1H NMR (400 MHz,
CDCl3, TMS, 300 K): d=8.00 (broad peak, 1H), 6.91 (s, 2H), 3.94 (m,
6H), 3.46 (m, 2H), 2.95 (m, 2H), 1.71 (m, 6H), 1.33–1.25 (m, 30H),
0.96 ppm (t, 9H); UV/Vis (CH2Cl2): lmax (e)=295 (3120), 264 nm
(9540mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for C33H61N2O4 [M+H]+:
549.4625; found: 549.4626.
N,N’-DiACHTNUTRGNEUNG[3,4,5-(tris-2-ethylhexyloxy)benzoylaminoethyl]perylene-3,4:9,10-
tetracarboxylic acid bisimide (PBI-5): Yield: 54%; m.p. 215–2178C;
1H NMR (400 MHz, CDCl3, TMS, 300 K): d=8.56 (d, J=7.6 Hz, 4H),
8.43 (d, J=8.1 Hz, 4H), 7.05 (t, 2H), 7.00 (s, 4H), 4.55 (m, 4H), 3.89–
3.82 (m, 16H), 1.76–1.28 (m, 54H), 0.90 ppm (m, 36H); UV/Vis (CHCl3):
lmax (e)=528 (81300), 491 (51400), 460 nm (19100mꢀ1 cmꢀ1); HRMS
(ESI): m/z calcd for C90H124N4NaO12 [M+Na]+: 1475.9108; found:
1475.9108.
N,N’-Di{4-[(S)-3,7-dimethyloctyloxy-3,5-dioctyloxy]benzoylaminoethyl}-
perylene-3,4:9,10-tetracarboxylic acid bisimide (PBI-6): Yield: 48%; m.p.
251–2548C; 1H NMR (400 MHz, CDCl3, TMS, 300 K): d=8.57 (d, 4H,
J=7.7 Hz), 8.47 (d, 4H, J=8.0 Hz), 7.02 (t, 2H), 6.98 (s, 4H), 4.55 (m,
4H), 3.01–3.87 (m, 16H), 1.82–0.84 ppm (m, 60H); UV/Vis (CHCl3): lmax
(e)=528 (80400), 491 (50000), 460 nm (18000mꢀ1 cmꢀ1); HRMS (ESI):
m/z calcd for C94H133N4O12 [M+H]+: 1509.9914; found: 1509.9909.
N,N’-DiACHTNUTRGNEUNG{3,4-bis-[(S)-3,7-dimethyloctyloxy-5-octyloxy]benzoylaminoethyl}-
perylene-3,4:9,10-tetracarboxylic acid bisimide (PBI-7): Yield: 50%; m.p.
218–2208C; 1H NMR (400 MHz, CDCl3, TMS, 300 K): d=8.56 (d, J=
7.7 Hz, 4H), 8.44 (d, J=8.0 Hz, 4H), 7.00 (t, 2H), 6.94 (s, 4H), 4.49 (m,
4H), 3.01–3.82 (m, 16H), 1.81–0.83 ppm (m, 68H); UV/Vis (CHCl3): lmax
(e)=528 (81700), 491 (51200), 460 nm (18500mꢀ1 cmꢀ1); HRMS (ESI):
m/z calcd for C98H140N4NaO12 [M+H]+: 1566.0148; found: 1566.0344.
Compounds 3d–3g were synthesized according to the above-described
procedure for compound 3b.
3,4,5-(Tris-3-methylbutyloxy)benzoylaminoethylamide (3d): Light yellow
solid (64%); m.p. 968C; 1H NMR (400 MHz, CDCl3, TMS, 300 K): d=
6.98 (s, 2H), 6.59 (broad peak, 1H), 4.05–3.98 (m, 6H), 3.48 (m, 2H),
2.93 (m, 2H), 1.71–1.61 (m, 9H), 0.96–0.94 ppm (m, 18H); UV/Vis
(CH2Cl2): lmax (e)=295 (3110), 264 nm (9530mꢀ1 cmꢀ1); HRMS (ESI):
m/z calcd for C24H43N2O4 [M+H]+: 423.3217; found: 423.3217.
3,4,5-(Tris-2-ethylhexyloxy)benzoylaminoethylamide (3e): Light yellow
waxy material (65%); H NMR (400 MHz, CDCl3, TMS, 300 K): d=6.97
Acknowledgements
1
(s, 2H), 6.55 (broad, 1H), 4.02–3.99 (m, 6H), 3.51–3.40 (m, 2H), 2.94 (m,
2H), 1.54–1.30 (m, 27H), 0.91–0.88 ppm (m, 18H); UV/Vis (CH2Cl2):
lmax (e)=295 (3150), 264 nm (9530mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for
C33H61N2O4 [M+H]+: 549.4625; found 549.4625.
Financial support by DFG for the research training school GRK 1221
“Control of electronic properties of aggregated p-conjugated molecules”
is gratefully acknowledged. S.G. thanks the Alexander von Humboldt
Foundation for a postdoctoral fellowship.
4-[(S)-3,7-Dimethyloctyloxy]-3,5-diACTHNUGRTNEUNG(octyloxy)benzoylaminoethylamide
(3 f): Light yellow waxy material (62%); 1H NMR (400 MHz, CDCl3,
TMS, 300 K): d=6.98 (s, 2H), 6.54 (broad, 1H), 4.02–3.99 (m, 6H), 3.50–
3.46 (m, 2H), 2.95 (t, 2H), 1.82–0.85 ppm (m, 49H); UV/Vis (CH2Cl2):
lmax (e)=295 (3330), 264 nm (9670mꢀ1 cmꢀ1); HRMS (ESI): m/z calcd for
C35H64N2O4 [M+H]+: 577.4938; found: 577.4938.
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Chem. Eur. J. 2008, 14, 11343 – 11357
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 11343 – 11357

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Received: July 18, 2008
Published online: November 13, 2008
Chem. Eur. J. 2008, 14, 11343 – 11357
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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