Supramolecular construction of fluorescent J-aggregates based on hydrogen-bonded perylene dyes

Article abstract of DOI:10.1002/anie.200701139

(Figure Presented) Luminous nanorods: The self-assembly of core-twisted perylene bisimide fluorophores (see structures) in nonpolar organic solvents is directed by hydrogen-bonding interactions. This supermolecular concept resulted in one-dimensional J-ag

Full text of DOI:10.1002/anie.200701139

Angewandte
Chemie
DOI: 10.1002/anie.200701139
Self-Assembly Processes
Supramolecular Construction of Fluorescent J-Aggregates Based on
Hydrogen-Bonded Perylene Dyes**
Theo E. Kaiser, Hao Wang, Vladimir Stepanenko, and Frank Würthner*
Since their serendipitous discovery in 1936 by Jelley and
Scheibe,[1] cyanine-dye-based J-aggregates (also called
Scheibe aggregates) have evoked much scientific interest.
Such dye aggregates have attracted considerable attention for
various technological applications.[2] However, despite enor-
mous effort made during the past few decades, no examples of
other dye aggregates[3] have been achieved with optical
properties similar to those of cyanine-dye aggregates:
strongly bathochromically shifted absorption and fluores-
cence bands with much narrower bandwidths than those of
the respective monomers, combined with a high fluorescence
quantumyield. Many synthetic dyes, including the remark-
ably photostable and highly fluorescent perylene bisimide
dyes, formpreferentially sandwich-type H-aggregates, which
exhibit unfavorable, strongly quenched fluorescence proper-
ties.[4,5]
through perylene core twisting enforced by bay substituents,[9]
the attachment of trialkoxyphenyl wedges,[10] and head-to-tail
alignment of the dyes through hydrogen-bonding interac-
tions[11] (Figure 1). Traditional dye aggregates of ionic cyanine
dyes are formed only in aqueous systems by van der Waals
In natural light-harvesting pigments, proteins or metal-ion
coordination direct a slipped arrangement of chlorophyll
dyes,[6] which leads to the desired J-type aggregation mode
with pronounced bathochromic shifts of the absorption bands
and high exciton mobility. The latter property is of pivotal
importance for efficient light harvesting.[7] Much research has
been inspired by such examples from nature and devoted to
the self-assembly of structurally related porphyrin dyes.
Supramolecular design has indeed resulted in J-type packing
arrangements of this class of dyes.[8] However, as a result of
the much less favorable optical properties of porphyrin
chromophores, in particular an almost-forbidden lowest-
energy transition with rather weak photoluminescence, the
functional features of porphyrin aggregates compare unfav-
orably with those of their natural chlorophyll counterparts
and cyanine-dye-based synthetic J-aggregates.
Figure 1. Self-assembly of the perylene bisimide dye 1 into J-type
aggregates. a) Molecular structures of 1 and 2 and schematic repre-
sentation of the monomer 1. b) Schematic representation of an
aggregate of 1: Red twisted blocks represent the perylene bisimide
core, gray cones with a blue apex represent the bay substituents, and
green lines represent hydrogen bonds. The dye 1 self-assembles in a
helical fashion as shown in the magnification (substituents have been
omitted and only the left-handed helical structure is shown for
simplicity). c) J-type arrangement of the core perylene bisimide units
in a double stringcable.
Herein, we introduce a highly fluorescent J-aggregate
assembled from the outstanding artificial fluorophore pery-
lene bisimide (also called perylene diimide). The unprece-
dented packing of perylene bisimide dyes in a strongly slipped
arrangement could be tailored by supramolecular design
forces. In contrast, the assembly of J-aggregates of perylene
bisimide dyes is driven by hydrogen bonding and can take
place in an organic environment. This characteristic further
extends the scope of these unique types of aggregates.
The perylene bisimide dye 1 (Figure 1a) was synthesized
from1,6,7,12-tetrachloroperylene-3,4:9,10-tetracarboxylic
acid bisanhydride (3) as outlined in Scheme 1. The imidiza-
tion of 3 with racemic a-methylbenzylamine afforded the
perylene bisimide 4, and nucleophilic substitution of the four
chlorine atoms in 4 by 4-methoxyphenol gave compound 5.
Cleavage of the methyl ether groups in the bay substituents
and the N-methylbenzyl groups in the imide positions with
BBr3 in anhydrous dichloromethane provided the depro-
tected perylene bisimide intermediate. Subsequent esterifi-
cation of the phenol groups with 3,4,5-tridodecyloxybenzoic
acid afforded the target dye 1[12] in 63% yield. The reference
compound 2 (structure shown in Figure 1a) was synthesized
similarly by using n-butylamine for the imidization of 3.
[*] T. E. Kaiser, Dr. H. Wang, V. Stepanenko, Prof. Dr. F. Würthner
Universität Würzburg
Institut für Organische Chemie and
Röntgen Research Center for Complex Material Systems
Am Hubland, 97074 Würzburg(Germany)
Fax: (+49)931-888-4756
E-mail: wuerthner@chemie.uni-wuerzburg.de
[**] We gratefully acknowledge the Alexander von Humboldt Foundation
(postdoctoral fellowship for H.W.) and the Fonds der Chemischen
Industrie for financial support of this research, and C. Z. Shao for
the synthesis of the reference compound 2.
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Scheme 1. Reagents and conditions: a) (ꢁ)-d,l-a-methylbenzylamine,
propionic acid, reflux, 20 h, 45%; b) 4-methoxyphenol, K2CO3, NMP, Ar
atmosphere, 1208C, 69 h, 49%; c) BBr3, CH2Cl2, 08C!RT, 5 h;
d) 3,4,5-tridodecyloxybenzoic acid, DCC, DMAP, DPTS, molecular
sieves (4 Š), DMF, CH2Cl2, 0!508C, 91 h, 63%. DCC=N,N’-dicyclo-
hexylcarbodiimide, DMAP=4-dimethylaminopyridine, DMF=N,N-
dimethylformamide, DPTS=4-(dimethylamino)pyridinium 4-toluene-
sulfonate, NMP=1-methyl-2-pyrrolidinone.
The UV/Vis absorption and fluorescence emission spectra
of the monomeric dye 1 in CH2Cl2 (Figure 2a) exhibit all the
typical spectroscopic features of tetraphenoxy-substituted
perylene bisimide chromophores.[9] The absorption maximum
of the strongly allowed S0–S1 transition appears at 570 nm
(absorption coefficient: e = 41600mꢀ1 cmꢀ1), and the fluores-
cence emission spectrum with a maximum at lmax = 602 nmis
a mirror image of the S0–S1 absorption band. Thus, these
spectra reveal a Stokes shift of 32 nm. Owing to a pronounced
vibronic progression and conformational disorder imparted
by the core-twisted chromophore (Figure 1a), the S0–S1
absorption and emission bands are both rather broad with
full-width-at-half-maximum (fwhm) values of 2393 cmꢀ1
(absorption) and 1660 cmꢀ1 (emission).
In striking contrast to those of the monomer, the
absorption and emission spectra of aggregated dye 1 in the
nonpolar solvent methylcyclohexane (MCH) exhibit strongly
bathochromically shifted and unusually narrow bands of
greater intensity (Figure 2a). The maximum of the aggregate
absorption band is shifted to lmax = 642 nm, the fwhm value is
reduced to 885 cmꢀ1, and the dipole moment of the S0–S1
transition is increased from 6.7 D (monomer) to 8.3 D
(aggregate).[13] The fluorescence spectrumof the aggregate
has a mirror-image relationship to the absorption spectrum. It
Figure 2. a) UV/Vis (solid lines) and fluorescence spectra (dashed
lines) of 1 in CH2Cl2 (10ꢀ5 m, thin line) and MCH (10ꢀ5 m, bold line).
b) Temperature-dependent UV/Vis spectra of 1 in MCH (1.5”10ꢀ5 m)
at 20–908C; arrows indicate the spectroscopic changes with increasing
&
temperature. Inset: Changes in absorption at 642 nm ( ) and 553 nm
~
( ) with increasingtemperature; lines were calculated accordingto a
sigmoidal fit. c) Temperature-dependent fluorescence spectra of 1 in
MCH (6”10ꢀ7 m, lex =476 nm) at 15–508C; arrows indicate the
spectroscopic changes with increasing temperature.
the hydrogen-bonding imide units in 1. For direct comparison,
the N-butyl-substituted reference dye 2, whose imide units
can not undergo hydrogen bonding, was investigated by UV/
Vis spectroscopy. In contrast to the results with 1, only a slight
bathochromic shift of the absorption band was observed for 2
upon aggregation in MCH at roomtemperature (see the
Supporting Information). This small bathochromic shift can
be attributed to the formation of a dimeric aggregate species
on the basis of our previous studies.[14]
The unusual spectroscopic properties of the J-aggregate of
1 are encoded in its molecular structure: a twisted p-
conjugated core and two self-complementary arrays of
hydrogen-bond donors/acceptors (Figure 1a). Upon self-
assembly, these dyes form p–p-stacked dimeric units (con-
sisting of one red and one orange building block in Figure 1c)
that interconnect through hydrogen bonds to provide a
closely packed one-dimensional supramolecular polymer[15]
(Figure 1b) which exhibits all the typical features of a J-
aggregate. The most important of these features is a
fluorescent excitonic state, which results fromthe slipped
arrangement of the building blocks.
has a maximum at lmax = 654 nmwith a very small Stokes shift
ꢀ1
of 12 nmand an equally small fwhmvalue of 878 cm
.
Temperature-dependent UV/Vis spectroscopy revealed that
the formation of aggregates of dye 1 is fully reversible, and a
well-defined isosbestic point was observed at 575 nm(Fig-
ure 2b). These spectra of the aggregate of dye 1 are distinct
frompreviously observed spectra of aggregates of perylene
bisimide dyes,[4,5,14] apparently as a result of the presence of
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Experimental evidence for the supramolecular model
shown in Figure 1b derives fromvarious spectroscopic and
microscopic investigations. The p–p-stacking interaction
between the perylene cores in a slipped arrangement is
evident fromthe UV/Vis absorption data, which can be
interpreted according to exciton theory.[2c,16] Concentration-
and temperature-dependent FTIR and 1H NMR spectroscopy
model (Figure 1; we carried out AMBER force-field calcu-
lations on the molecular model of the aggregate structure (see
the Supporting Information)).
We obtained a fluorescence quantumyield of 0.93 ꢁ 0.01
for monomeric 1 in CH2Cl2 and a value of 0.96 ꢁ 0.03 for its J-
aggregate in MCH by applying the conventional method for
the determination of the relative fluorescence quantum
yield.[18] The unquenched emission of this aggregate is also
evident fromthe temperature-dependent fluorescence spec-
tra obtained upon excitation at lex = 476 nm, at the isosbestic
point (Figure 2c). These results were further substantiated by
the fluorescence quantumyield of 0.85 ꢁ 0.02 measured for a
concentrated solution of 1 in MCH with an integrating sphere
(by using a system for the measurement of the absolute
fluorescence quantumyield). [18]
ꢀ
confirmed the formation of N H···O hydrogen bonds
between the imide hydrogen atoms and the carbonyl oxygen
atoms. The free-NH stretching vibrations in the FTIR spectra
of monomeric 1 in CH2Cl2 occur at n˜(NH) = 3368 cmꢀ1. This
band shifts upon hydrogen bonding to 3172 cmꢀ1 in MCH and
3178 cmꢀ1 in the solid state. The concomitant appearance of a
ꢀ1
=
new stretching frequency at n˜(C O) = 1676 cm assigned to
hydrogen-bonded carbonyl groups was observed along with a
=
decrease in the intensity of the bands at n˜(C O) = 1734 and
Several kinds of J-type aggregates, including J-aggregates
based on cyanines,[1,2] chlorins, and porphyrins,[6–8] have been
reported and investigated extensively in the past. However,
most previously reported J-aggregates exhibit only weak
fluorescence, which makes them less suitable for many
applications, in particular for demanding sensory and pho-
tonics applications.[19] To the best of our knowledge, no J-
aggregate with fluorescence quantumyield of near unity has
been reported previously.[20]
The electronic properties of the J-aggregates of dye 1 were
explored by polarization (Figure 4) and time-resolved fluo-
rescence measurements (see the Supporting Information). A
strong increase in the fluorescence anisotropy value from
0.032 (monomers at 808C in MCH) to 0.158 (aggregates at
208C in MCH) was observed for the whole S0–S1 band. This
result indicates the formation of an extended aggregate and
collinearity between the dipole moment of the S0–S1 tran-
sition and the long axis of the aggregate.
Whereas the observed pronounced bathochromic shift
and the band narrowing for the aggregate of dye 1 provide a
clear indication of a strong coupling between the aggregated
chromophores, the decrease in the fluorescence lifetime from
6.8 ns (monomer in MCH) to 2.6 ns (J-aggregate in MCH)
allows the elucidation of exciton delocalization. According to
exciton theory, such enhanced radiative decay (“superra-
diance”) is related to the presence of a coherent excited
domain of a number, N, of monomers.[2c] In the absence of
nonradiative quenching processes (as confirmed by the high
1700 cmꢀ1 as a result of stretching vibrations of non-hydro-
gen-bonded carbonyl groups (see the Supporting Informa-
tion). Furthermore, 1H NMR spectroscopy of 1 as solutions in
CDCl3 and [D8]toluene revealed pronounced downfield shifts
for the imide hydrogen atoms with increasing concentration,
whereas the signals in the spectra of 1 in [D14]MCH are rather
broad even under dilute conditions as a result of the
polymeric nature of the aggregates (see the Supporting
Information).
ꢀ
Once the existence of local N H···O hydrogen bonding
and p–p-stacking contacts between self-assembled dyes 1 had
been confirmed by spectroscopic methods, atomic force
microscopy (AFM) was used to elucidate the size and shape
of these aggregates. AFM images on silicon wafers (Figure 3;
see also the Supporting Information) reveal a network of
defined fibers whose size (height: 2.0 ꢁ 0.2 nm; width: 8.4 ꢁ
2.6 nm) is in agreement with the expected dimensions[17] of a
double string cable of the type suggested in Figure 1b.
According to this model, we expect quite a rigid inner core
(diameterꢂ 3.0 nm) composed of helically twisted perylene
bisimides (red) with four appended aromatic substituents
(blue) and surrounded by a flexible periphery of twelve
C12H25 aliphatic chains (gray). The aliphatic chains are
wrapped around the helical core as proposed in Figure 1b.
Remarkably, at the highest possible resolution of our AFM
imaging (Figure 3b; see also the Supporting Information), a
helical pitch of p = 13.0 ꢁ 3.4 nmbecomes apparent for these
nanosized fibers. This value is in proper agreement with our
Figure 3. a,b) Tapping-mode AFM images of self-assemblies of 1 on a
silicon wafer spin coated (2000 rpm) from a 9”10ꢀ6 m solution in
MCH. Scale bars: 150 nm; z scale: 6 nm. Inset in (b): Height profile
alongthe thin line.
Figure 4. Fluorescence anisotropy, r, of the perylene bisimide 1
(10ꢀ6 m) in MCH at 258C ( ) and 808C ( ). The corresponding
nonpolarized absorption spectra of 1 (258C: dashed line; 808C: solid
line) have been added for comparison.
~
*
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Communications
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fluorescence quantum yields of the monomer 1 (0.93) and
aggregate of 1 (0.96)), the size of the coherent domain could
be estimated for 1 at roomtemperature to be about three dye
molecules from Equation (1):
N tagg ¼ Fagg tmon
ð1Þ
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tives, VCH, Weinheim, 1995.
[12] The perylene bisimide 1 was characterized by NMR spectros-
copy, mass spectrometry, and elemental analysis: m.p. 246–
2478C; 1H NMR (CDCl3, 400.13 MHz, 300 K, TMS): d = 8.31 (s,
2H, NH), 8.17 (s, 4H, Hpery), 7.33 (s, 8H, Haryl), 7.15–7.10 (m, 8H,
in which Fagg denotes the fluorescence quantumyield of the
aggregate and tagg and tmon the fluorescence lifetimes of the
aggregate and monomer, respectively.[21] On the basis of
previously reported studies on cyanine- and chlorin-dye
aggregates,[7,22] we would expect an increase in the size of
the coherent domain upon a decrease in temperature.
Previously reported J-aggregates were either discovered
by serendipity[1] or developed on the basis of concepts found
in nature.[6–8] Herein we have shown that the formation of a J-
aggregate can be encoded in the molecular building block
according to supramolecular design principles.[11] The out-
standing fluorescence properties of the chosen fluorophore
provided for the first time J-aggregates that fluoresce with
quantumyields of near unity. As the perylene bisimide
chromophore also has other favorable properties, such as high
photostability and semiconductivity, we anticipate that these
perylene bisimide J-aggregates will be integrated into many
devices for fluorescence sensing, photonics, and organic
photovoltaics.
Haryl), 6.97–6.92 (m, 8H, Haryl), 4.02–3.93 (m, 24H, OCH2), 1.85–
1.62 (m, 24H, CH2), 1.55–1.35 (m, 24H, CH2), 1.35–1.10 (m,
192H, CH2), 0.84–0.78 ppm(m, 36H, CH 3); MS (MALDI-TOF,
positive mode, DCTB): m/z calcd for C220H332N2O28: 3450.46
[M+2H]+; found: 3450.40; UV/Vis (CH2Cl2): lmax (e) = 570
(41600), 533 (26800), 444 nm(19000 mꢀ1 cmꢀ1); fluorescence
(CH2Cl2, lex = 535 nm): lmax = 602 nm, Ffl = 0.93 ꢁ 0.01; elemen-
tal analysis: calcd (%) for C220H330N2O28·H2O (3466.46): C 76.17,
H 9.65, N 0.81; found: C 76.04, H 9.55, N 0.89.
Received: March 15, 2007
Revised: April 13, 2007
[13] Transition dipole moments were calculated according to a
reported method: W. Liptay, R. Wortmann, H. Schaffrin, O.
Burkhard, W. Reitinger, N. Detzer, Chem. Phys. 1988, 120, 429 –
438. The molar extinction coefficient and the transition dipole
moment for the aggregate are given as the values per aggregate-
bound monomeric unit.
[14] Z. Chen, U. Baumeister, C. Tschierske, F. Würthner, Chem. Eur.
J. 2007, 13, 450 – 465.
[15] R. P. Sijbesma, F. H. Beijer, L. Brunsveld, B. J. B. Folmer,
J. H. K. K. Hirschberg, R. F. M. Lange, J. K. L. Lowe, E. W.
Meijer, Science 1997, 278, 1601 – 1604.
Published online: June 20, 2007
Keywords: dyes/pigments · J-aggregates · nanostructures ·
.
self-assembly · supramolecular chemistry
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[17] For a discussion, see the Supporting Information.
[18] For further details, see the Supporting Information.
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[20] Previously reported fluorescence quantumyields for cyanine-
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Dnfwhm,mon/Dnfwhm,agg
= N (G. Busse, B. Frederichs, N. K. Petrov,
S. Techert, Phys. Chem. Chem. Phys. 2004, 6, 3309 – 3314).
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