Quadruple π stack of two perylene bisimide tweezers: A bimolecular complex with kinetic stability

Article abstract of DOI:10.1002/anie.201302479

Self-assembly: A tweezer-type perylene bisimide (PBI) dyad self-assembles into a defined bimolecular complex composed of a quadruple PBI π stack with remarkable kinetic stability, which is unprecedented for π-stacked dye aggregates (see picture). These persistent supramolecular species are of considerable interest for the elucidation of functional properties of dye aggregates. Copyright

Full text of DOI:10.1002/anie.201302479

Angewandte
Chemie
Communications
Supramolecular Chemistry
C. Shao, M. Stolte,
F. Wꢀrthner*
&&&& — &&&&
Quadruple p Stack of Two Perylene
Bisimide Tweezers: A Bimolecular
Complex with Kinetic Stability
Self-assembly: A tweezer-type perylene
bisimide (PBI) dyad self-assembles into
a defined bimolecular complex composed
of a quadruple PBI p stack with remark-
able kinetic stability, which is unprece-
dented for p-stacked dye aggregates (see
picture). These persistent supramolec-
ular species are of considerable interest
for the elucidation of functional proper-
ties of dye aggregates.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
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DOI: 10.1002/anie.201302479
Supramolecular Chemistry
Quadruple p Stack of Two Perylene Bisimide Tweezers: A Bimolecular
Complex with Kinetic Stability**
Changzhun Shao, Matthias Stolte, and Frank Wꢀrthner*
Self-assembly has become an attractive approach towards
functional materials.[1–5] However, to match with covalent
chemistry, self-assembly processes have to be developed from
reversible thermodynamic regimes to systems of kinetic
stability. Generally, the weak supramolecular interactions
such as hydrogen-bonding[6] and p–p stacking[7,8] rarely pro-
vide sufficient binding strength to lock particular structures in
Figure 1. a) The designed PBI dyad building block and b) the self-
assembly process of monomers into a dimer. Note that the dimer
structure shown here is an idealized cartoon. For a force-field
calculated dimer structure, see the Supporting Information (Fig-
ure S6).
a desired architecture. Accordingly, examples of kinetically
stable supramolecular structures are given either for large but
structurally less defined aggregates[9–11] or discrete self-
assemblies that are attainable by using multiple intermolec-
ular interactions[12–14] or stronger metal ion–ligand interac-
tions,[15–18] the latter represent a borderline case between
covalent and noncovalent chemistry. In our recent work on
perylene bisimide (PBI) dyes,[19,20] we realized that PBI-PBI
interactions are stronger than those of any other p–p stacking
interactions reported for other p scaffolds, for example,
hexabenzocoronenes, phthalocyanines, phenylacetylene mac-
rocycles, and naphthalene bisimides.[21] Thus, we envisioned
that the formation of discrete well-defined and kinetically
persistent PBI aggregates might be feasible for self-assem-
blies of rather modest size.
In the past years, molecular tweezers[22,23] have been used
to form thermodynamically stable assemblies by means of
aromatic p–p interaction or hydrophobic effect.[24–30] How-
ever, to the best of our knowledge, kinetically stable tweezer
assemblies are barely known and no molecular tweezers of
PBI dyes have been reported to date. Herein, we present
a novel tweezer-type PBI dyad with two PBI units attached at
a diphenylacetylene (DPA) backbone which self-assembles
into hitherto unprecedented kinetically stable dimer aggre-
gates (Figure 1).
The target tweezer-type PBI dyad 7 was synthesized
according to the route outlined in Scheme 1 (for details see
the Supporting Information). It contains long “swallow-tail”
alkyl chains attached at the imide terminals of PBI units[31] to
provide sufficient solubility in chlorinated solvents like CHCl3
and CCl4, and a methylene linking group between the PBI
units and DPA backbone to ensure the proper orientation of
Scheme 1. Synthesis of PBI dyad 7: a) BH3·THF, dry THF, 08C, 24 h,
58% for 2 and 45% for 3. b) Imidazole, 1408C, 4 h, 65% for 4 and
61% for 5. c) Trimethylsilylacetylene (TMSA), Pd(PPh3)2Cl2, CuI,
CH2Cl2/NEt3 (v:v=3:1), 408C, 3 h; tetra-n-butylammonium fluoride
(TBAF), THF, RT, 10 minutes, 89% over two steps. d) Pd(PPh3)2Cl2,
CuI, THF/NH(iPr)2 (v:v=5:1), 408C, 18 h, 57%.
[*] Dr. C. Shao, Dr. M. Stolte, Prof. Dr. F. Wꢀrthner
Universitꢁt Wꢀrzburg, Institut fꢀr Organische Chemie and
Center for Nanosystems Chemistry
PBI p systems, while being short enough to preserve rigidity
and spacing constraints of a tweezer architecture.
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: wuerthner@chemie.uni-wuerzburg.de
Because of unavoidable homo-coupling of PBI 6,[32] it was
not possible to isolate the pure PBI dyad 7 by conventional
column chromatography. The material isolated after two
successive chromatographic separations of crude product on
a silica gel column with chloroform as an eluent still contained
[**] Generous financial support by the DFG for the research training
school GRK 1221 “Control of Electronic Properties of Assemblies of
p-Conjugated Molecules” is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302479.
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Figure 2. Semi-preparative HPLC separation of the reaction mixture
containing PBI dyad 7 and homo-coupling byproduct. The stock
solution was concentrated in chloroform and a solvent mixture of
CHCl3/MeOH (v:v=100:1) was used as an eluent. The inset shows
the in situ absorption spectrum of the first fraction.
Figure 3. MALDI-TOF mass spectrum (linear mode) of PBI dyad 7. The
peaks corresponding to monomer and dimer species are marked with
ꢀ
[M]1 and [M]2ꢀ, respectively. 2-[(2E)-3-(4-tert-butylphenyl)-2-methyl-
prop-2-enylidene]malononitrile (DCTB) was used as a matrix.
significant amounts (ca. 15%) of the homo-coupling byprod-
uct of PBI 6. Complete purification of dyad 7 could, however,
be achieved by semi-preparative HPLC (Figure 2) at the
highest flow rate of 9.9 mLminꢀ1 from a concentrated stock
solution of the product mixture in CHCl3 obtained after
column chromatography. The first fraction with a retention
time of 4.5–6 minutes was collected and fully characterized as
pure dyad 7 by 1H and 13C NMR spectroscopy (see Fig-
ures S1–S3 in the Supporting Information), MALDI-TOF
mass spectrometry and elemental analysis.
It was noted that the freshly eluted solution of this fraction
has a red color which gradually turned bright yellow. Such
color changes of the solution are typical for a dissociation of
PBI aggregates into PBI monomers.[33] Analytical HPLC
studies with diode array detection corroborate that this
fraction indeed possesses the characteristic absorption profile
of dyad 7 aggregates (inset of Figure 2, see also Figure S4 in
the Supporting Information). This finding indicates that
under the applied HPLC conditions dyad 7 prevails in
a kinetically trapped aggregated state, whilst the impurities
exist as monomer or in dynamic equilibrium with respective
aggregates. The long tail of the first HPLC fraction can,
therefore, be attributed to the partial disassembly of dyad 7
aggregates while passing through the HPLC column. It is
worth to mention that when a slower flow rate (less than
6 mLminꢀ1) and/or a diluted stock solution was used, the first
fraction did not appear and dyad 7 was thus chromatograph-
ically inseparable from the homo-coupling byproduct.
dyad 7 (10ꢀ4 m in CCl4). As demonstrated in Figure 3, besides
the first peak with an m/z value of 1627 confirming the
molecular mass of dyad 7 molecule, there is an apparent
signal at m/z = 3254 corresponding to the dimer of dyad 7. It is
noteworthy that no further peak at multiples of the m/z =
1627 parent ion value assignable to larger aggregates can be
perceived, implying that dimer complexes are the only
present aggregates of dyad 7 (which complies with the fact
that a single aggregate species could be identified by
analytical HPLC and separated by semi-preparative HPLC).
For the same solution as used for MALDI-TOF studies, that
is, a 10ꢀ4 m solution of dyad 7 in CCl4, dynamic light-scattering
(DLS) measurements were performed. The observed signal
also corroborates the presence of very small assemblies with
a diameter of less than 3 nm, which is in the range of the
measurement limit of the light-scattering instrument (Fig-
ure S5).[39,40] Such a dimension clearly confirms the absence of
large aggregate species of dyad 7 beyond dimers. Moreover,
molecular modeling studies performed with MacroModel
using MM3* force-field calculations support the viability of
a dimer structure (for details see the Supporting Information
and Figure S6).[41]
It is rather uncommon that p–p stacked dye aggregates
can survive the HPLC separation and mass spectrometry
measurements. This reflects that the noncovalently self-
assembled dyad 7 dimer features a high degree of persistence
comparable to the strength of a covalent bond. To quantita-
tively assess the activation parameters for the dissociation of
dyad 7 dimer in solution, we designed kinetic experiments[42]
to monitor the time-dependent absorption changes when
a small amount of concentrated solution containing dyad 7
dimers was dispersed into a thousand times larger volume of
pure chloroform (Figure 4). At a final concentration of 10ꢀ5 m,
dyad 7 molecules have a strong thermodynamic driving force
(DG) to be molecularly dissolved in the good solvating
solvent CHCl3, which is recognized from the, for PBI
monomers characteristic, well-resolved vibronic progressions
between 450 and 550 nm of the final absorption spectra at
The impressive finding that aggregates of PBI dyad 7
survive HPLC separation with eluent of 1 vol% MeOH in
CHCl3 is suggestive of strong and uniform aggregate forma-
tion of dyad 7. On the account of high stability, mass
spectrometry is a promising tool for assessing the aggregate
composition of dyad 7. Among different mass spectrometry
techniques, matrix-assisted laser desorption/ionization
(MALDI) is one of the relatively soft ionization methods
that has been widely applied to monitor noncovalent self-
assemblies and in particular p–p stacked species.[34–38]
MALDI-TOF mass spectra were recorded from a sample of
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tion (4) see the Supporting Information). Fitting the mea-
sured absorption coefficient e(t’) values at 528 nm against the
time t’ provides exactly an exponential function, from which
the rate constant k1 can be acquired (inset of Figure 4). The
measured e(t’) values at different wavelengths fit very well
into Equation (4) with a correlation coefficient better than
0.999 (see Table S1). From the fitting curves, reasonable eM
and C values can be directly obtained. The eM values
correspond well to the absorption of a molecule bearing two
isolated PBI dyes[30] (ffimonomer M), whereas the C values
show the correct positive or negative signs with regard to the
absorption relationship of eM =2 and eM at different wave-
2
lengths. More importantly, all the fittings generate quite
similar k1 values that were averaged to obtain a rate constant
value of 0.16 hꢀ1 for the dissociation process of dyad 7 dimer
and a corresponding half time of t1/2 = 4.31 h.
At different temperatures from 298 to 323 K, the same
kinetic experiments were repeated and thus a series of k1
values could be acquired (Table S2 in Supporting Informa-
tion). Based on these temperature-dependent k1 values the
activation parameters DH° = 81.4 Æ 0.8 kJmolꢀ1 and DS° =
ꢀ55.3 Æ 2.5 Jmolꢀ1 Kꢀ1 can be obtained from the plot of
ln(k1/T) against 1/T by applying the Eyring Equation (5)
(Figure 5).[42]
Figure 4. Recorded absorption spectral changes at time intervals of
15 minutes for 24 h when 3 mL of dyad 7 stock solution at 10ꢀ2 m in
CHCl3 was added into 3 mL of pure CHCl3 solvent at 298 K. Arrows
indicate the absorption changes with time. Inset: A plot of the
absorbance at 521 nm versus time and the best fitting to a simple
mono-exponential equation.
10ꢀ5 m concentration. Accompanied with the pronounced PBI
absorption enhancement, the DPA backbone of dyad 7 shows
a blue-shifted absorption with weaker intensity at around
300 nm. It can be interpreted in terms of DPA deplanarization
synchronous to the PBI disaggregation process. This obser-
vation indicates the freeze of the DPA structure in dimers of
dyad 7 in contrast to its free rotatability in the monomeric
state.[43]
The absorption variations recorded in time intervals of
15 minutes (Figure 4) share several well-defined isosbestic
points at 545, 477, 463, 450, and 266 nm, indicative of the
disassembly process between only two explicit species, that is,
the dimer and monomer of dyad 7. Thus, the dimer dissoci-
ation process can be regarded as a first-order fragmentation
reaction. If we assume that at time t = 0 h, the dimer
concentration cM (0) is equal to a, and after a time interval
2
of t hours, the dimer concentration cM (t) changes to (aꢀx)
2
and the monomer concentration will be accordingly cM(t) =
2x, the following Equations (1) and (2) apply.
Figure 5. Eyring plot for the temperature dependence of the disassem-
bly rate constant k1 of the dyad 7 dimer in CHCl3.
k1
M2
2 M
ð1Þ
ð2Þ
ƒ!
t ¼ 0
t ¼ t
cM ð0Þ ¼ a
cM ðtÞ ¼ aꢀx
cMð0Þ ¼ 0
cMðtÞ ¼ 2x
2
k1
DH°
R
1
T
kB DS°
ð5Þ
ln ¼ ꢀ
T
þ ln
þ
2
h
R
Considering a dead time of t0 = tꢀt’ between the ideal time
t and recorded time t’ and setting a constant value [Eq. (3)]
The Gibbs activation energy of disassembly DG° = 97.9 Æ
0.8 kJmolꢀ1 and disassembly activation energy Ea = 83.8 Æ
0.8 kJmolꢀ1 of dyad 7 dimer in chloroform at room temper-
ature results according to the Equations (6) and (7), respec-
ꢀ
ꢁ
À
Á
e
M2
C ¼
ꢀ eM exp ꢀk1t0
ð3Þ
2
DG° ¼ DH°ꢀTDS°
Ea ¼ DH° þ RT
ð6Þ
ð7Þ
we can obtain Equation (4),
ꢀ
ꢁ
À
Á
e
M2
eðt0Þ ¼ eM
þ
ꢀ eM exp ꢀk1t0 expðꢀk1t0Þ ¼ eM þ C expðꢀk1t0Þ
2
ð4Þ
tively. For comparison, the activation energy Ea of quadruply
hydrogen-bonded 2-ureido-4[1H]-pyrimidinone dimers in
CHCl3 is reported to be 70 Æ 2 kJmolꢀ1.[12] Dyad 7 dimer is
stabilized by triple PBI p–p interaction and thus possesses
where eM and eM are the absorption coefficients of dimer and
2
monomer, respectively (for detailed deduction of Equa-
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Received: March 25, 2013
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