Supramolecular Assembly of Complementary Cyanine Salt J-Aggregates

Article abstract of DOI:10.1021/jacs.5b08072

An understanding of structure-property relationships in cyanine dyes is critical for their design and application. Anionic and cationic cyanines can be organized into complementary cyanine salts, offering potential building blocks to modulate their intra/

Full text of DOI:10.1021/jacs.5b08072

Communication
pubs.acs.org/JACS
Supramolecular Assembly of Complementary Cyanine Salt
J‑Aggregates
Zhong’an Li,† Sukrit Mukhopadhyay,‡,⊥ Sei-Hum Jang,† Jean-Luc Bredas,‡,# and Alex K.-Y. Jen*,†
́
†Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, United States
‡School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta,
Georgia 30332, United States
#Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Kingdom
of Saudi Arabia
S
* Supporting Information
ABSTRACT: An understanding of structure−property
relationships in cyanine dyes is critical for their design
and application. Anionic and cationic cyanines can be
organized into complementary cyanine salts, offering
potential building blocks to modulate their intra/
intermolecular interactions in the solid state. Here, we
demonstrate how the structures of these complementary
salts can be tuned to achieve highly ordered J-type
supramolecular aggregate structures of heptamethine dyes
in crystalline solids.
rganizing organic dyes by self-assembly is an efficient
approach to create defined molecular architectures with
O
appealing topology and properties for light-harvesting and other
optoelectronic applications.1 Among various supramolecular
structures, J-aggregates (also called Scheibe aggregates) are
particularly interesting2 because they provide promising optical
properties such as an intense, red-shifted absorption band (J-
band) and a high fluorescence quantum yield in the solid state,
due to extended delocalization of excitons over several molecular
units.3
Figure 1. Molecular structure of cyanines and complementary cyanine
salts studied.
Cyanines are a class of ionic organic compounds with carbon
skeletons containing an odd number of conjugated carbon
atoms, which have been widely studied as photosensitizers for
photography or biological probes.4 Due to their ionic nature and
strong intermolecular interactions, they show a major tendency
to aggregate with each other. Indeed, the first example of a dye J-
aggregate was discovered in a cyanine called pseudoisocyanine
(PIC, Figure 1).2 Nowadays, self-assembly of cyanine dyes, in
particular in the J-aggregate form, is an active area of research, but
most examples reported so far have focused only on cyanines
with relatively short conjugation length (number of carbons in
the conjugation bridge, L ≤ 3).3a,c,5
Cyanines with much longer conjugation length (L ≥ 7), such
as heptamethines, are usually near-infrared (NIR) absorbing and
emitting, making them attractive for various applications such as
in vivo cell imaging,6 photovoltaics,7 and nonlinear optics.8
However, the uncontrolled aggregation of these highly polar-
izable dyes, especially the formation of H-aggregates, often limits
their applicability. For example, H-aggregates can give rise to a
blue-shifted absorption band and strong fluorescence quenching,
which prevents their biomedical applications.9 In addition, the
strong and random ion-pairing/aggregation between these dyes
in the solid state has been shown to induce adverse changes in
their nonlinear optical properties for all-optical switching (AOS)
applications.10 Therefore, it is important to understand the
implications of the varying structural properties in order to
design suitable materials.11
Understanding the role of counterions in the extended
structures of cyanines will provide an effective way to manipulate
the organization of chromophores toward favorable molecular
assemblies.11b,12 Because of their ionic nature, it is possible to
organize anionic and cationic cyanines into complementary salts
for functioning not only as mutually polarizable counterions but
also as useful building blocks to form favorable supramolecular
assemblies in the solid state.13 By rationally tuning the structure
of cyanine dyes as complementary salts, we have recently
demonstrated a new molecular design strategy to achieve J-type
self-assembly of highly polarizable heptamethine dyes in
Received: July 31, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b08072
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Journal of the American Chemical Society
Communication
crystalline solids. It enables us to better understand the structural
properties of these aggregates at the molecular level.
angle (θqui) of 23.8° and a π−π interaction distance of 3.42 Å. It is
also found that there occurs C−H···O bonding between two
neighboring TCF-cyanines (Figure 2C), which may play a role in
forming such a construct. This result suggests that it should be
able to use PIC as a templating counterion to modulate the
spatial arrangements of highly polarizable dyes in the solid state.
To this end, PIC was mixed with TCF-heptamethine C3 to form
a complementary salt, CC2 (Figure 1 and Scheme S2). However,
we could not obtain a single crystal for structure analysis. This is
likely due to the incompatibility in molecular size between PIC
and C3, making it difficult to form an extended structure.
Therefore, quinolinium-heptamethines C4 and C5 (Figure 1
and Scheme S2) were further designed and synthesized as
compatible counterions for C2 and C3, respectively. Here, the
phenyl substitution at the center of the bridge in C3 and C5 is
introduced to suppress potential H-type aggregation. Indeed, a
comparison of thin-film absorption spectra between C2 and C3
(Figure 3A), or C4 and C5 (Figure 3B), clearly suggests that H-
In this study, we have used 2-(3-cyano-4,5,5-trimethyl-5H-
furan-2-ylidene)malononitrile (TCF) and N-ethyl-2-methyl-
quinolinium as end groups for anionic and cationic cyanines,
respectively. The TCF end group has high electronic
delocalization and was developed initially as an efficient electron
acceptor for dipolar chromophores with strong intramolecular
charge transfer properties.14 TCF-heptamethines, such as C2
(Figure 1), absorb in the NIR (∼900 nm) with a large extinction
coefficient (>3 × 105 M−1 cm−1),15 which enables them to have
many possible applications.7b,16 For example, C2 with aromatic
counterions has been demonstrated to have large third-order
nonlinearities for AOS applications.16c However, the crystal
structure of C2 has shown unfavorable supramolecular assembly
with an unexpected anti-conformation in which dimethyl groups
in TCF are on the different sides of polymethine backbone and
break symmetry in the solid state.15 This deleteriously affects its
optical properties. However, quinolinium-cyanines are known to
exhibit diverse aggregations dependent on their conjugated
backbone, due to strong π−π interactions between their end
groups.3c The PIC compound with L = 1 is known to form J-
aggregates, while the pinacyanol compound (PIN) with L = 3 is
reported to form H-aggregates. In this context, the cationic PIC
molecules are anticipated to maintain their J-aggregation
characteristics in the solid state, which can function as a template
to organize complementary anionic TCF-cyanines into an
extended structure.
To validate this idea, we first mixed PIC with the TCF-cyanine
C1 (L = 3) to form a new complementary salt CC1 (Figure 1 and
Scheme S1). A single crystal of CC1 was successfully obtained.
Its refined structure (monoclinic, P21/n space group) is
presented in Figure 2 (See SI for details). As shown in Figure
Figure 3. Neat thin-film absorption spectra for cyanine dyes: (A) TCF-
cyanines C2 and C3; (B) quinolinium-cyanines C4 and C5; (C)
complementary cyanine salts CC2−CC4. Note: The absorption
maxima in diluted solutions of dyes are listed in Table S1: 900 nm for
C2; 880 nm for C3; 861 nm for C4; 858 nm for C5; 882 nm for CC1;
868 nm for CC2; and 870 nm for CC4.
type aggregation is mitigated when the chloride atom in the
bridge is replaced with a phenyl ring. Moreover, it is worth to
note that phenyl substitution also leads to a blue-shifted
absorption band when compared to chloride substitution, due
to the inductive effect of the substituents (Figure S2). However,
the absorption peaks from quinolinium-cyanines (<0.01 eV)
showed a smaller blue shift than those of TCF-cyanines (∼0.03
eV). Results from time-dependent density functional theory
(TD-DFT) calculations suggest that this difference in blue shift is
attributed to the extent of delocalization of the frontier molecular
orbitals of the respective cyanines near their end groups (Table
S5, see SI for details).
The single-crystal structure (orthorhombic, Pbca space group)
of C5 has been refined also (see SI for details). The unit cell is
shown in Figure 4a, where continuous parallel double-layers of
cationic cyanines were separated by a single sheet of borate
anions. The symmetric molecules exhibit a quasi-planar
conjugated skeleton (Figure S5), and the phenyl substituents
are significantly twisted relative to the conjugated backbone
(∼70°) as a result of steric hindrance between the ortho-
hydrogen atoms of the phenyl ring and the backbone. As noted
previously, the quinolinium-cyanines exhibit diverse aggregation
behaviors dependent on their conjugation length. Here, as shown
in Figure 4B, even with the long heptamethine chains, strong
π−π interactions between the end groups still lead to the
formation of a highly ordered herringbone-type molecular
Figure 2. Crystal structure of CC1: (A) molecular packing of the
quinolinium (marked in blue) and TCF (marked in red), cyanines
viewed along the a axis; (B) packing mode of the quinolinium-cyanines
with a slippage angle (θqui = 23.8°) and a π−π interaction distance (dπ−π
= 3.42 Å); (C) packing mode of the TCF-cyanines through the O1−
C9H contact with a distance of 2.56 Å (the C−H···O bond angle is
159.3°). For clarity, most of the hydrogen atoms are omitted. The
oxygen atoms are marked in red, nitrogen atoms in blue, carbon atoms in
gray, and hydrogen atoms in white.
2A, both the PIC and C1 in CC1 show a highly ordered
molecular packing structure along the (110) plane; in particular,
the PIC molecules form extended J-aggregates as we expected,
which potentially dictates the spatial arrangement of the anionic
TCF-cyanines to be J-type as well. Figure 2B depicts a typical
π−π stacking mode among PIC molecules with a small slippage
B
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Journal of the American Chemical Society
Communication
Figure 4. Crystal structure of C5: (A) unit cell of the quinolinium-
cyanines (marked in blue) and borate (marked in red) viewed along the
a-axis; (B) extended packing mode of the quinolinium-cyanines with a
herringbone angle (θH) of 52.0° and a π−π interaction distance (dπ−π
=
3.30 Å) viewed along the b-axis. For clarity, the hydrogen atoms are
omitted. The nitrogen atoms are marked in blue and carbon atoms in
gray.
packing structure along the (020) plane. This can be attributed to
the twisted phenyl ring at the center of the bridge serving as an
effective “spacer” to mitigate the deleteriously random
intermolecular interactions, as demonstrated in the film
absorption spectra. Thus, the above observations indicate that
C5 has the potential to be used as a templating counterion to
direct the spatial arrangement of an anionic TCF-heptamethine.
By mixing quinolinium and TCF-heptamethines through a
simple ion exchange, the complementary salts CC3 and CC4
have been prepared (Figure 1 and Scheme S2). The close λmax
and εmax (Table S1) between the quinolinium and TCF-
heptamethines causes some overlap of their solution absorption
bands; therefore, only one slightly broader combined transition
peak can be observed (Figure S3). Their neat thin-film
absorption spectra are shown in Figure 3C, all of which show a
red-shifted (>0.06 eV) and broadened absorption band as
compared to those obtained from the solution spectra, which
suggest a strong ion-pairing and/or aggregation in the solid
state.10 However, CC4 exhibits a much sharper absorption band
than CC3 (0.64 eV vs 0.78 eV), indicating somewhat narrower
distribution of conformational variation of aggregates for the
CC4 as a result of the spacing effect from the twisted phenyl ring
at the center of the bridge.
Figure 5. Crystal structure of CC4: (A) molecular packing of the
quinolinium- (marked in blue) and TCF- (marked in red) cyanines
viewed along the a axis; (B) unit cell viewed along the c axis illustrating
the slippage angle (θqui = 21.3°) and the π−π interaction distance (dπ−π
(1)
TCF
= 3.43 Å); (C) packing mode of the TCF-cyanines (θ
= 26.4°)
through the N5−H14 contact with a distance of 2.71 Å (the C−H···N
(2)
TCF
bond angle is 157°); (D) packing of the TCF-cyanines (θ = 24.1°)
through the N2−H18 contact with a distance of 2.59 Å (C−H···N bond
angle of 169°). For clarity, most of the hydrogen atoms and the solvent
molecules are omitted. The oxygen atoms are marked in red, nitrogen
atoms in blue, carbon atoms in gray, and hydrogen atoms in white.
TCF-cyanines, which also form J-aggregate-like stacks along the
(011) plane (Figure 5A). As shown in Figure 5C,D, nonclassical
C−H···N bonding, in two different configurations, between the
phenyl rings on the bridge and the TCF end groups, plays a
significant role in reinforcing the stability of such structures.
The average bond-length alternation (BLA) is an important
parameter characterizing the geometries and electronic structure
of cyanines.8 Here, the BLA of C5 in the crystal structure is only
0.012 (Table S9), while those of the complementary TCF-anion
and quinolinium-cation in the crystal of CC4 are found to be
0.015 and 0.020 Å, respectively (Table S10). These values are
slightly larger than those obtained from density functional theory
(DFT) calculations (Table S2); however, they remain relatively
small when compared to those reported in the literature.12c,15
This can be attributed to the favorable spatial arrangement of
cyanines within the fairly symmetric counterionic environment.
In CC4, the highly polarizable and delocalized cyanine pairs with
mutually compatible size and complementary charges clearly
facilitate the symmetrical distribution of electrons between
heptamethines, as shown in Figure S8.
In conclusion, through rational molecular design, comple-
mentary salts of quinolinium- and TCF-cyanines have been
prepared to modulate the molecular packing of heptamethine
dyes in the solid state while maintaining their highly polarizable
and symmetric nature. We demonstrated that well-known J-
aggregating cationic cyanines such as PIC can be used as
structure-directing counter cations for anionic cyanines. Our
results revealed that the phenyl substitution at the center of
conjugation backbone not only suppresses the potential H-type
aggregation efficiently but also plays an important role in
reinforcing the stability of J-type supramolecular aggregate
structures. We also reported the first crystal structure of a
The single-crystal structure (triclinic, P1 space group) of the
̅
complementary cyanine salt CC4 has been determined and is
presented in Figure 5 (see SI for details). To the best of our
knowledge, this is the first crystal structure reported for a
complementary cyanine salt with a highly polarizable heptame-
thine backbone. Interestingly, Figure 5A shows that the
oppositely charged cyanines in the salt form a cooperative self-
assembly and individually pack in alternating J-type fashion,
similar to the packing observed in CC1. This is consistent with
our initial concept of using complementary cyanines as
counterions to control the spatial arrangements and interactions
of highly polarizable dyes in the solid state.
The unit cell is shown in Figure 5B, and both cationic and
anionic cyanines in the salt maintain overall symmetric
geometries, without significant twisting of the cyanine cores
(Figure S6). Unlike in the crystal structure of C5, here, the strong
π−π interactions between the end groups direct adjacent
quinolinium cyanines to pack in a dimeric J-type structure with
a small θqui of 21.3° and a π−π interaction distance of 3.43 Å
(Figure 5B). Since the quinolinium-cyanines also act as
counterions, the electrostatic interactions between the cationic
and anionic stack dictate the spatial arrangement of the anionic
C
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thine backbone, which shows that both cationic and anionic
cyanines in the complementary salt form a cooperative self-
assembly, resulting in highly ordered J-aggregate-like molecular
packing. This study provides a molecular design strategy useful to
modulate the aggregation and ion-pairing properties of highly
polarizable cyanines with long conjugation length. Such rational
design can prove highly beneficial for application of cyanine dyes
in biomedicine, optics, and other technologies.
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ASSOCIATED CONTENT
■
S
* Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b08072.
Synthetic details and characterization data, linear optical
properties, computational results, X-ray crystal structure,
and 1H NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*ajen@u.washington.edu
Present Address
⊥The Dow Chemical Company, Midland, 48674, United States.
(12) (a) Gieseking, R. L.; Mukhopadhyay, S.; Shiring, S. B.; Risko, C.;
Bred
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as, J.-L. J. Phys. Chem. C 2014, 118, 23575. (b) Mukhopadhyay, S.;
as, J.-L. Chem. Sci. 2012, 3, 3103. (c) Bouit,
Notes
Risko, C.; Marder, S. R.; Bred
P.-A.; Aronica, C.; Toupet, L.; Guennic, B. L.; Andraud, C.; Maury, O. J.
Am. Chem. Soc. 2010, 132, 4328.
́
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was funded by the AFOSR MURI (FA9550-10-0558).
A.K.-Y.J. thanks the Boeing-Johnson Foundation for its support
and J.L.B. acknowledges generous support from King Abdullah
University of Science and Technology. The authors would like to
thank Prof. Kaminsky (UW) for X-ray structure refinement.
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