Highly fluorescent lyotropic mesophases and organogels based on J-aggregates of core-twisted perylene bisimide dyes

Article abstract of DOI:10.1002/chem.200800915

A new perylene bisimide dye based organogelator PBI1 with a sharp J-type absorption band and favorable emission properties was introduced. Gelation tests performed for PBI1 in 17 different organic solvents show no occurrence of gelation in solvents such a

Full text of DOI:10.1002/chem.200800915

DOI: 10.1002/chem.200800915
Highly Fluorescent Lyotropic Mesophases and Organogels Based
on J-Aggregates of Core-Twisted Perylene Bisimide Dyes
Xue-Qing Li, Xin Zhang, Suhrit Ghosh, and Frank Würthner*[a]
Over the past decade, low molecular weight organic gela-
tors have attracted considerable interest because of their di-
verse applications as supramolecular soft materials.[1] The
formation of organogels[2] is facilitated by highly directional
self-assembly through non-covalent interactions such as p–p
stacking, hydrogen bonding, metal–ion coordination, dipole–
dipole interactions. In the last few years, several functional
dye based building blocks have been reported to form orga-
nogels and the unique features of the 3D networksuper-
structures have been applied for example, for sensors, optoe-
lectronic devices, light harvesting, nucleation of inorganic
materials.[3–8]
These features can be attributed to strong excitonic coupling
as demanded for exciton transport in photonic and photo-
voltaic applications.
The structures of PBI1 and PBI2[11b] are shown in
Scheme 1. PBI1 was synthesized by imidization of tert-
butyl-phenoxy perylene tetracarboxylic acid bisanhydride
with aminoethyltris(dodecyloxy)benzamide in quinoline
using Zn(OAc)2 as catalyst and isolated as a purple powder
G
in 51% yield (details for synthesis and characterization of
PBI1 are given in Supporting Information). The calculated
Organogelators based on numerous electron-rich aromatic
building blocks such as porphyrins, phthalocyanines, oligo
(phenylenevinylenes), oligothiophenes and tetrathiafulva-
lenes have been investigated in the recent past.[7] However,
such examples for electron-poor aromatic systems are still
rare.[8] Perylene bisimides (PBIs) have been extensively
studied as n-type semiconductors in various applications
such as optical recording media, organic photo- and semi-
conductors, and solar cells.[9] Recently, the groups of Shinkai
and Yagai as well as our group have reported the first PBI
based organogelators.[10–12] In these examples, well-defined
fibrous aggregates were observed by atomic force microsco-
py (AFM) and their formation has been attributed to p–p
stacking and intermolecular hydrogen bonding between the
constituent PBI molecules. However, for these gels broad-
ened UV/Vis absorption bands and inferior emission proper-
ties were observed compared to solutions of the monomeric
dyes. In this work, we introduce a new perylene bisimide
based organogelator (PBI1) with an unprecedented sharp J-
type absorption band and favorable emission properties.
Scheme 1. Molecular structures of PBI1 and PBI2.
[a] X.-Q. Li, Dr. X. Zhang, Dr. S. Ghosh, Prof. Dr. F. Würthner
Universität Würzburg, Institut für Organische Chemie and Rçntgen
Research Center for Complex Material Systems, Am Hubland
97074 Würzburg (Germany)
molecular models as well as several crystal structures for re-
lated compounds indicate that PBI2 possesses a planar per-
ylene core, whereas PBI1 exhibits a distorted perylene core
with a twist angle of about 25–308 due to the presence of
four bulky tert-butylphenoxy substituents at the bay posi-
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.200800915.
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COMMUNICATION
tions[13] (see Figure S1 in Supporting Information). We pre-
viously found that PBI2 is an efficient gelator, which can
gel various types of organic solvents at low critical gelation
concentrations (CGC) by virtue of p–p stacking and hydro-
gen-bonding interactions.[11b] In order to examine whether
similar hydrogen-bonding interaction exists among the bay-
substituted PBI1 chromophores, we performed FT-IR spec-
troscopy of PBI1 in apolar solvent methylcyclohexane
(MCH) which revealed the formation of intermolecular hy-
drogen bonds between the benzamide functional groups
(Figure S2 in Supporting Information). Next, gelation tests
were performed for PBI1 in 17 different organic solvents
(Supporting Information). To our surprise, no gelation was
observed in aromatic solvents such as toluene, benzene and
aliphatic solvents such as methylcyclohexane and cyclohex-
ane, but opaque gels were observed in acetone and dioxane,
which are known as hydrogen-bond breaking solvents (Fig-
ure 1a and b). Despite their failure to gel apolar organic sol-
among the columns. As the sample was concentrated (edge
evaporated for 2 h), a grainy optical texture can be ob-
served, which is characteristic of the columns arranged in a
hexagonal order in the hexagonal (M) phase.[14b] Similar tex-
tures can also be observed in cyclohexane and n-hexane that
show both N and M phases in concentrated solutions (see
details in Figure S3, Supporting Information).
To investigate the aggregation properties of PBI1 in more
detail, we carried out photophysical studies in solution.
Firstly, optical properties of PBI1 in the monomeric and ag-
gregated states were investigated by temperature-dependent
UV/Vis and fluorescent spectroscopy in different solvents
(Figure 2). The UV/Vis spectra of PBI1 in CH2Cl2 (or in
MCH at higher temperature) exhibit typical spectroscopic
features that are expected for monomeric tetraaryloxy-sub-
stituted PBI chromophores.[15] The absorption maximum at
583 nm indicates a strongly allowed S0!S1 transition. The
fluorescent emission spectrum with a peakmaximum at
621 nm is mirror image of the absorption band. The absorp-
tion and emission bands are rather broad with full-width-at-
half-maximum (fwhm) values of 2311 and 1780 cmꢀ1, respec-
tively. The broadness of these bands results from the distor-
tion of the chromophore and various conformational states
of the four aryloxy groups.
Unlike those in CH2Cl2, the absorption spectra of PBI1 in
MCH exhibit a sharp intense band that is bathochromically
shifted by ca. 13 nm from the monomer band (Figure 2a).
The fluorescent spectrum has a mirror image relationship to
Figure 1. Photographs of PBI1 in different solvents under a UV lamp
(366 nm): a) in MCH at c=10 mm (2.9 wt%); b) in acetone at c=1.5 mm
(0.4 wt%) and in dioxane at c=4 mm (1.1 wt%); c) OPM image of a vis-
cous solution of PBI1 in MCH (8 mm, 2.3 wt%); d) a zoomed region
from c).
vents, the signature of aggregate formation was evident in
these aliphatic solvents (e.g., methylcyclohexane, cyclohex-
ane, and n-hexane) from the increased viscosity of the solu-
tions at higher concentrations (c>10 mm). Notably, no pre-
cipitation tookplace at these rather high concentrations but
textures that resemble those found for chromonic liquid
crystals[14] were observed under the optical polarizing micro-
scope (OPM). The formation of a nematic (N) phase in a
concentrated MCH solution was indicated by a schlieren
texture (Figure 1c and d). In this phase, the mesogens typi-
cally stackto form columns, but there is no positional order
Figure 2. a) UV/Vis (left) and fluorescence (right) spectra of PBI1 in
CH2Cl2 (dashed lines, 1.1”10ꢀ5 m) and MCH (solid lines, 1.1”10ꢀ4 m) at
208C; b) temperature-dependent absorption spectra of PBI1 (1.1”
10ꢀ4 m, in MCH) from 108C to 908C. The arrows indicate spectral
changes with increasing temperature.
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F. Würthner et al.
the absorption spectrum with a Stokes shift of only 11 nm.
The fwhm values of absorption and emission spectra are sig-
nificantly reduced to 850 and 786 cmꢀ1, respectively. Tem-
perature-dependent UV/Vis experiments in MCH testify the
reversibility of the self-assembly process. When the temper-
ature is raised from 10 to 908C, the sharp band at 598 nm
gradually disappears and the monomeric species is recov-
ered as evidenced by re-appearance of the broad spectrum
(Figure 2b). These spectral features clearly indicate the re-
versible formation of J-aggregates of PBI1 in apolar solvent
which is in contrast to the H-aggregating PBI2 and other
PBI-based organogelators.[10,11] The very intense J-band of
PBI1 with relatively small bathochromic shift suggests the
presence of tightly aggregated chromophores at a slip angle
close to the magic angle of 54.78 that is unprecedented for
this class of chromophores.[16,17] These unusual spectral and
packing features are encoded in the unique molecular struc-
ture of PBI1: a twisted p-conjugated core and additional
amide groups that can assist intermolecular H-bonds.
TEM (Figure 3a,b) as expected for lyotropic mesophases.[14]
These aggregates appear much more fluid-like as compared
to the entangled fibers observed for gels of PBI2[11b] (Fig-
ure 3e,f). They are able to cause an increase in viscosity of
the medium (Figure 1a) but are unable to gel the solvent.[19]
Interestingly, TEM images from a suspension of PBI1 gel in
acetone revealed distinctly different aggregate morphologies
as shown in Figure 3c and d. The willow-leaf-like aggregated
units appear to be more flexible and they can gradually en-
tangle to each other at the ends to give star-shaped struc-
tures. Those “stars” further intercross to form the 3D net-
workstructures, which are essential for gelation. Similar
spectroscopic and microscopic features of PBI1 could also
be observed in dioxane (Figures S4 and S6).
Similar optical properties of PBI1 can also be found in
acetone and dioxane, in which organogels were formed.
Temperature-dependent UV/Vis spectra of PBI1 in these
hydrogen-bond breaking solvents (Figure S4) clearly show J-
type aggregating behavior, that is, upon aggregation the ab-
sorption maximum shifts bathochromically. However, in
these solvents aggregation tookplace only at considerably
higher concentrations and lower temperatures (in acetone:
c>1.0 mm, T<308C and in dioxane: c>2.5 mm, T<208C;
see Supporting Information) when compared with that in
apolar solvent such as MCH. The relatively lower propensity
to aggregate formation of PBI1 in acetone and dioxane is
conceivable since the solvent molecules in these cases are
capable of interacting with the amide functionality of the
dye molecules through hydrogen bonding,[18] and thus can
weaken the inter-chromophoric interactions. However, at
significantly higher concentration and lower temperature,
dye aggregation was evidenced from distinct spectral
changes (bathochromic shift of ca. 39 nm in acetone) that
are indicative of a packing motif with more displaced dye
molecules according to exciton coupling theory[17] (see calcu-
lations in the Supporting Information). Further evidence of
solvent participation in hydrogen bonding with the amide
functionality of PBI1 was gathered from 1H NMR studies
(Figure S5). The amide protons of PBI1 were found to be
significantly downfield shifted in acetone compared to that
in chloroform or benzene as expected for hydrogen-bonding
interactions with the solvent molecules.
Figure 3. TEM images of PBI1 in MCH (1.0”10ꢀ4 m, a and b) and a sus-
pension of a gel in acetone (ca. 1.0”10ꢀ3 m, c and d); e) and f) show the
long aggregated fibers of PBI2 from a suspension of a gel in dioxane (ca.
1.0”10ꢀ3 m). The scale bar in a)–d) corresponds to 500 nm and in e) and
f) corresponds to 1000 nm.
Whilst the solution spectroscopic studies revealed the ag-
gregate formation in various solvents, transmission electron
microscopy (TEM) studies provided direct evidences for the
formation of extended networks. Figure 3 shows typical
TEM images of aggregated structures of PBI1 and PBI2 in
different solvents. As reported previously, PBI2 can form
long well-defined fibers and further entangle to network
structures in many apolar solvents.[11b] In contrast, for PBI1
less regular structures are observed. In MCH loosely con-
nected bundles of aggregate structures are visualized by
In contrast to the morphology of core-twisted PBI1, the
core-planar PBI2 gives very long (several microns) 1D fiber
in dioxane. This observation strongly indicates that the dif-
ferences in gelation behavior of these dyes are originated
from the structural change of the perylene cores. Possibly,
strong p–p interaction among planar PBI2 chromophores
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Highly Fluorescent Mesophases and Organogels
COMMUNICATION
rules out the possibility of solvent insertion irrespective of
the nature of the solvent, while the core-twisted PBI1
allows solvent participation in H-bonding because of the rel-
atively weaker inter-chromophoric interactions.
Acknowledgements
We are grateful to the Fonds der Chemischen Industrie for financial sup-
port. S.G. and X.Z. thankthe Alexander von Humboldt Foundation for
postdoctoral fellowships.
It is noteworthy that the fluorescence of PBI2 is rather
weakdue to a photoinduced electron transfer process from
the electron-rich trialkoxyphenyl group to the electron poor
PBI core.[20] Thus, the fluorescence quantum yield is 0.03 for
the monomers in dichloromethane and less than 0.01 for the
aggregates in toluene. In contrast, very exciting fluorescence
properties for PBI1 are given under various conditions that
are attributed to a more electron-rich PBI core and a more
favorable dye packing. The fluorescence quantum yields for
monomeric PBI1 were found to be 0.78ꢁ0.02, 0.76ꢁ0.01,
and 1.00ꢁ0.01 in dichloromethane, acetone, and dioxane,
respectively. For the J-aggregate in MCH, the quantum
yield was found to be 0.82ꢁ0.01. Such unquenched emission
upon dye aggregation in solution is indeed a rare phenom-
enon.[16] Even in the gel phases, remarkable fluorescence
quantum yields were determined: 0.20ꢁ0.01 in acetone and
0.39ꢁ0.01 in dioxane where the concentrations were 2.0 mm
and 5.0 mm, respectively (see details in Supporting Informa-
tion).
In summary, we have introduced highly fluorescent PBI J-
aggregate based lyotropic mesophases and gels of organic
solvents. UV/Vis studies revealed more pronounced aggre-
gation in apolar solvents like MCH due to intermolecular
p–p stacking and hydrogen bonding interactions. On the
other hand, in polar solvents like acetone and dioxane, par-
ticipation of the solvent molecules in hydrogen bonding sig-
nificantly reduced the aggregation propensity but enforced
the gel formation at higher concentrations (Figure 4). The
Keywords: hydrogen bonds
mesophase · organogel · perylene bisimide dye
· liquid crystal · lyotropic
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Received: May 13, 2008
Published online: July 30, 2008
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