Alternated stacks of nonpolar oligo(p-phenyleneethynylene)-BODIPY systems

Article abstract of DOI:10.1002/chem.201203279

Supramolecular chemistry: The self-assembly of an oligo(p- phenyleneethynylene)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (OPE-BODIPY) system into alternated 1D π stacks in solution exhibited liquid-crystalline properties at room temperature. This system represents a new approach towards a controlled dye organization exclusively driven by π-π interactions (see figure). Copyright

Full text of DOI:10.1002/chem.201203279

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DOI: 10.1002/chem.201203279
Alternated Stacks of Nonpolar Oligo(p-phenyleneethynylene)-BODIPY**
Systems
Alexander Florian, Marꢀa Josꢁ Mayoral, Vladimir Stepanenko, and
Gustavo Fernꢂndez*[a]
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)[1]
dyes have become increasingly relevant in the last two dec-
ACHTUNGTRENNUNGades due to their remarkable optical properties, that is, high
molar-absorption coefficients and fluorescence-quantum
yields, excellent (photo)chemical stability, and ease of func-
tionalization.[2] Typical applications encompass fields as di-
verse as optical chemosensors,[3] laser dyes,[4] fluorescent
probes for cell imaging,[5] organic light-emitting diodes,[6]
photovoltaic devices,[7] or photodynamic therapy agents.[8] In
contrast and despite the tendency of BODIPY dyes to self-
assemble into H- or J-type dimer aggregates through stack-
ing of the dipyrromethene core,[9] their supramolecular as-
pects have thus far been somewhat limited. Some examples
of BODIPY-based liquid-crystalline materials,[10,12c–d] or
H-[11] or J-type[12] aggregates driven by hydrogen bon-
ding,[12c–d] electrostatic interactions,[12a] the hydrophobic ef-
fect,[11b,12a] or the addition of ions[11a,12b] have recently been
reported. However, little effort has been devoted to the in-
vestigation of BODIPY aggregates based on p–p aromatic
interactions, despite the relevance of these interactions in
numerous natural supramolecular assemblies or in the nano-
meter-scale dye organization in optoelectronic devices.[13]
Herein, we describe the self-assembly behavior of oligo(p-
phenyleneethynylene) (OPE)-BODIPY systems into highly-
organized alternated p stacks in cyclohexane exclusively
driven by p–p aromatic interactions. OPEs are known for
their ability to self-assemble into a wide variety of supramo-
lecular structures.[14] Derivative 1 (Scheme 1) was synthe-
sized by Sonogashira cross-coupling reaction of 2,6-diiodo-
BODIPY (2)[15] and ethynylphenyl-based derivative 3 con-
taining long solubilizing dodecyl chains (for synthetic details,
see the Supporting Information) and fully characterized by
NMR, IR, MALDI-TOF, high-resolution ESI, UV/Vis, fluo-
rescence, and elemental analyses (see the Supporting Infor-
mation).
Scheme 1. Chemical structure and cartoon representation of the OPE–
BODIPY derivative investigated in the present work.
The UV/Vis spectra of 1 showed important contributions
from both OPE and BODIPY subunits, that is, a transition
with a vibronic pattern centered at about l=340 nm attrib-
uted to the p–p* transition of the OPE units[16] and an in-
tense sharp band at about l=577 nm and a small second
transition at about l=400 nm that can be assigned to the
S0!S1 and S0!S2 transitions of the BODIPY core, respec-
tively (Figures 1 and S1 in the Supporting Information).[17]
Comparison of the UV/Vis spectrum of 1 with those of ref-
erence compounds 2 and 3 in dichloromethane showed that
[a] A. Florian, Dr. M. J. Mayoral, Dr. V. Stepanenko, Dr. G. Fernꢀndez
Institut fꢁr Organische Chemie and
Center for Nanosystems Chemistry
Universitꢂt Wꢁrzburg
Am Hubland, 97074 Wꢁrzburg (Germany)
Fax : (+49)931-31-84756
E-mail: gustavo.fernandez@uni-wuerzburg.de
Figure 1. UV/Vis spectra of 1 in CH2Cl2 (1ꢃ10ꢀ5 m) and cyclohexane (1ꢃ
10ꢀ5 and 1ꢃ10ꢀ2 m) and emission spectra of 1 in cyclohexane (1ꢃ10ꢀ5 and
1ꢃ10ꢀ2 m) at 298 K. The UV/Vis spectra of 2 and 3 in CH2Cl2 at 298 K
have been represented for comparison. Black c=1 in CH2Cl2; d=1
in cyclohexane; light c=1 in cyclohexane 1ꢃ10ꢀ2 m; b=2 in
CH2Cl2; g=3 in CH2Cl2.
[**] BODIPY=4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203279.
Chem. Eur. J. 2012, 18, 14957 – 14961
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
14957

the absorption maxima of the BODIPY and OPE units in 1
experienced a significant bathochromic shift from l=521 to
571 and from 312 to 340 nm, respectively, which is indicative
of electronic communication between both OPE and
BODIPY fragments (Figure 1).
sponding to the aromatic protons (Hd) of the OPE frag-
ments splits into a multiplet upon increasing concentration
(Figure 2). Both phenomena indicate the formation of p
stacks, in which chemical equivalence is lost upon aggregate
formation. The fact that an analogous signal splitting is
absent in CD2Cl2 substantiates that aggregation exclusively
occurs in cyclohexane (Figure S2 in the Supporting Informa-
The UV/Vis spectral changes of 1 in dilute (10ꢀ5 m) solu-
tion at room temperature are nearly solvent independent.
The S0!S1 transition corresponding to the BODIPY frag-
ment experiences negligible shifts when moving from mod-
erately polar THF, chloroform, or dichloromethane (l=
570 nm) to nonpolar CCl4, cyclohexane, or methylcyclohex-
ane (l=578 nm; Figures 1 and S1 in the Supporting Infor-
mation). In contrast, the vibronic structure of the transition
between l=300–350 nm became more pronounced in non-
polar solvents, which suggests a greater bias towards the pla-
narization of the alkyne/aryl groups belonging to the OPE
fragments.[18] The higher degree of preorganization of 1 in
nonpolar solvents anticipates an efficient p stacking when a
sufficiently high concentration has been reached. In fact,
UV/Vis studies in cyclohexane at 298 K revealed a progres-
sive broadening of the transitions at l=340 and 577 nm
when the concentration exceeds 5ꢃ10ꢀ4 m up to 1ꢃ10ꢀ2 m,
characteristic of aggregate formation (Figure 1). At the
highest concentration, a blueshifted shoulder (l=554 nm)
belonging to the S0!S1 BODIPY transition becomes notice-
able, which suggests only a weak intermolecular excitonic
coupling between BODIPY chromophores. The absence of
more hypso- or bathocromically shifted transitions indicates
that no H- or J-type excitonic coupling took place.
tion).
Variable-temperature
1H NMR
studies
in
[D12]cyclohexane (1 mm) are reminiscent to concentration-
dependent experiments (Figure S3 in the Supporting Infor-
mation). A detailed inspection of the chemical shifts against
temperature revealed an isodesmic supramolecular polymer-
ization, in which the addition of each monomeric unit to the
growing aggregate is energetically equivalent.[19] At room
temperature, the binding constant (Ka) associated to the
self-assembly process of 1 in cyclohexane was calculated to
be 1ꢃ104 mꢀ1 (Figure S4 and Table S2 in the Supporting In-
formation). CONTIN analysis of the autocorrelation func-
tion obtained from dynamic light scattering (DLS) experi-
ments (cyclohexane, 2 mm, 298 K) revealed that the aggre-
gates possess an angle-dependent size ranging from about
300 nm to 3 micrometers (Figure S5 in the Supporting Infor-
mation), which suggests the formation of relatively large 1D
aggregates.
The arrangement of the monomeric units within the asso-
ciates was elucidated by rotating-frame Overhauser effect
spectroscopy (ROESY) NMR experiments. The appearance
of several cross-peaks in [D12]cyclohexane at 4 mm can be
unambiguously assigned to intermolecular through-space
couplings, because these are absent in CD2Cl2, a solvent in
which no aggregation takes place (Figures 3a, S2, and S6 in
the Supporting Information). Interestingly, coupling signals
between the aromatic protons (Hd and He) of the OPE frag-
ments and the methyl groups (Hb and Hc) of the BODIPY
unit are noticeable, as well as between the aromatic protons
Hd and He (Figures 3a and S6 in the Supporting Informa-
tion). This coupling pattern along with the absence of dis-
tinct red- or blueshifted transitions in UV/Vis studies rules
out the formation of H- or J-type p stacks, in which the
BODIPY units are assembled on top of each other and dem-
onstrates that the BODIPY and OPE fragments are alter-
nately stacked within the aggregates, as illustrated in Fig-
ure 3b–d (see also Figure S7 in the Supporting Information).
DFT calculations supported this hypothesis. Geometry-
optimized (B3LYP/6-31G**) monomeric building blocks
were brought into van der Waals contact, and the resulting
dimer aggregate was energetically minimized. In this ar-
rangement, the distance between the BODIPY centers in
the p stack is 7.1 ꢅ, and the slip angles that result from the
rotational (a) and translational (q) offset of two parallel-ar-
ranged molecules are 7 and 398, respectively (Figures 3c,d,
and S7 in the Supporting Information). The long distance
between the centers of two adjacent BODIPY molecules in
the p stack (7.1 ꢅ) implies a weak intermolecular coupling
between BODIPY chromophores and, therefore, a negligi-
ble exciton splitting of the excited states in the stack. Ac-
cordingly, a quantitative analysis of the spectral shift of the
Concentration-dependent 1H NMR experiments between
0.05 and 10 mm ([D12]cyclohexane, 400 MHz, 298 K) con-
firmed the existence of a self-assembly process. If the con-
centration increases, a shielding of most of the resonances
takes place (Figure 2). Simultaneously, the singlet corre-
Figure 2. Partial 1H NMR spectra of
298 K) at different concentrations.
1 ([D12]cyclohexane, 400 MHz,
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Chem. Eur. J. 2012, 18, 14957 – 14961

Alternated Stacks of Nonpolar BODIPY Systems
COMMUNICATION
Figure 4. a) SEM image of 1 onto silicon wafer. b) Optical and c) fluores-
cence micrograph obtained upon drop-casting a solution of 1 from cyclo-
hexane (4 mm, 298 K) on glass substrate. d) Cartoon representation of
the molecular arrangement of 1 in the needle-like aggregates.
chains, as illustrated in Figure 4d. This feature is further
ꢀ
ꢀ
substantiated by the appearance of CH2 stretching vibra-
tions at n˜ =2919 (anti) and 2850 cmꢀ1 (sym) in the IR spec-
trum (Figure S10 in the Supporting Information).[21]
Interestingly, the molecular organization in the bulk state
proved to be significantly different. POM investigations re-
vealed that OPE–BODIPY derivative 1 smears over the
glass substrate by applying a slight pressure (Figure S11a in
the Supporting Information), which suggests a liquid-crystal-
line behavior at room temperature. The typical pseudo fan-
shaped textures observed when cooling the sample from the
isotropic liquid (ca. 938C) to room temperature indicated
the formation of a columnar mesophase (Figure 5b; S11b–c,
and S12 in the Supporting Information), which was con-
firmed by small-angle X-ray scattering (SAXS) experiments
(see the Supporting Information). The diffractogram of 1
showed seven sharp peaks in the low-angle region that can
be indexed in a 2D-noncentered rectangular lattice (Fig-
Figure 3. a) ROESY NMR spectrum of 1 ([D12]cyclohexane, 600 MHz,
4 mm, 298 K). The red and green circles highlight the relevant coupling
signals. b) Cartoon representation of the molecular arrangement of 1 in
the associates. Blue arrows represent the intermolecular coupling signals
observed in ROESY studies. c), d) Ground-state geometry of 1 opti-
mized by using DFT calculations (B3LYP/6-31G*). The alkyl chains have
been removed for simplicity.
AHCTUNGTREGuNNUN res S13, S14, and Table S3 in the Supporting Informa-
aggregate absorption band cannot be estimated by Kashaꢆs
exciton theory.[20]
tion).[22] In the wide-angle region, a broad halo at about
4.7 ꢅ due to the liquid-like order of the aliphatic chains con-
firms the liquid-crystalline nature of the phase. The sharp
peak at hp–p of about 3.2 ꢅ corresponds to short range p–p
stacking interactions and clearly indicates that the aromatic
cores are not tilted with respect to the columnar axis. The
number of molecules within a columnar disc (Z) showed
that only one molecule of 1 is necessary to form a columnar
discotic slice. These self-assemble on top of each other into
columns that are subsequently packed in a rectangular fash-
ion in the liquid-crystal state (see Figure 5c and the Sup-
porting Information).[22]
Scanning electron microscopy (SEM), polarized optical
microscopy (POM), fluorescence microscopy, and atomic
force microscopy (AFM) studies on silicon wafer, glass sub-
strate, or highly ordered pyrolytic graphite (HOPG), respec-
tively, provided final evidence supporting the formation of
1D associates. The images obtained from concentrated (4ꢃ
10ꢀ3 m) solutions of 1 in cyclohexane showed the presence of
fluorescent needle-like associates that are several tens of mi-
crometers long and between 90 and 240 nm wide (Fig-
ACHTUNGTRENNUNGures 4a–c; S8, and S9 in the Supporting Information). These
dimensions suggest the bundling of individual alternated p
stacks through interdigitation of the peripheral dodecyl
The optical properties in the bulk state are in accordance
with the columnar arrangement observed by SAXS. The flu-
Chem. Eur. J. 2012, 18, 14957 – 14961
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14959

G. Fernꢀndez et al.
Acknowledgements
We sincerely acknowledge Prof. Frank Wꢁrthner for many helpful discus-
sions and his generous support, Ana Reviejo for graphic design, and the
Alexander von Humboldt Stiftung and Bundesministerium fꢁr Bildung
und Forschung for financial support in the framework of the Sofja Kova-
levskaja program.
Keywords: conjugated systems
· BODIPY dyes · pi
interactions · self-assembly · supramolecular chemistry
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on top of each other giving rise to columnar rectangular
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new approach towards a controlled dye organization with
potential application in optoelectronics. The investigation of
related donor–acceptor systems based on BODIPY dyes will
be the next step to achieve this goal.
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Received: September 13, 2012
Published online: October 23, 2012
Chem. Eur. J. 2012, 18, 14957 – 14961
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14961