Self-assembly of charged bodipy dyes to form cassettes that display intracomplex electronic energy transfer and accrete into liquid crystals

Article abstract of DOI:10.1021/ja3007935

Red- and blue-absorbing boron dipyrromethene dyes, bearing opposite electronic charges, associate in solution to form a 1:2 complex having a stability constant of ca. 10¹⁷ M^-2. The complex can be dismantled by addition of a large exc

Full text of DOI:10.1021/ja3007935

Communication
pubs.acs.org/JACS
Self-Assembly of Charged Bodipy Dyes To Form Cassettes That
Display Intracomplex Electronic Energy Transfer and Accrete into
Liquid Crystals
Jean-Hubert Olivier,† Joaquín Barbera,‡ Effat Bahaidarah,§ Anthony Harriman,§ and Raymond Ziessel*,†
́
†Laboratoire de Chimie Organique et Spectroscopies Avancee
́
s (LCOSA), Ecole Europee
au CNRS, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France
‡Departamento de Química Organ
ica, Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza-CSIC, 50009 Zaragoza,
Spain
́ ̀ ́
nne de Chimie, Polymeres et Materiaux,
LMSPC, UMR 7515 associe
́
́
́
§Molecular Photonics Laboratory, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU,
United Kingdom
S
* Supporting Information
Chart 1. Electrostatically and Covalently Linked Bodipy
Dyes Studied Herein
ABSTRACT: Red- and blue-absorbing boron dipyrrome-
thene dyes, bearing opposite electronic charges, associate
in solution to form a 1:2 complex having a stability
constant of ca. 1017 M−2. The complex can be dismantled
by addition of a large excess of tetra-N-butylammonium
cations. The same complex displays liquid crystalline
properties on heating from rt to above 150 °C, as
characterized by various experimental techniques. Highly
efficient electronic energy transfer from the red to the blue
dye occurs in both the initial complex and the subsequent
mesomorphic state.
onic self-assembly (ISA)1 is a powerful technique that
employs attractive electrostatic forces to organize function-
and the ammonium hexafluorophosphate salt of the distyryl-
Bodipy, Bn,13 in N,N-dimethylformamide (DMF) at 60 °C (see
Supporting Information (SI)). The 2/1 stoichiometry of the
precipitated salt was confirmed by integration of the methyl
singlet (a) at 2.68 ppm for R with the two methylenepropyne
signals (b) at 4.68 ppm for Bn in the H NMR spectrum. The
corresponding covalently linked compound, 4, was isolated as
the neutral analogue via a cross-coupling reaction, promoted by
Pd(O), between the iodophenyl-bearing form of B and the
terminal alkyne of the yellow Bodipy dye (see SI).
Figure 1a presents the absorption and fluorescence spectra of
complex 2. Clearly, the absorption spectrum reflects the
combination of one R (λMAX = 513 nm, εMAX = 75 000
M−1cm−1) and two B dyes (λMAX = 646 nm, εMAX = 240 000
M−1 cm−1). Note that the main absorption of R is shifted from
495 in isolated solution to 513 nm in the complex whereas the
absorption maximum of B remains unchanged. Illumination at
500 nm results in strong fluorescence centered at 670 nm and
for which the emission quantum yield (ΦF) is 0.40 while the
excited-singlet state lifetime (τs) is 4.1 ns. It is known that
isolated R emits strongly at 520 nm but almost no (i.e., ca. 2%
of the total emission) such fluorescence could be detected for
the complex. This is because quantitative (i.e., >98%) energy
I
alized molecular modules into nanometer-sized materials, such
as nanoparticles,2 nanoplatelets,3 nanoblobs,4 nanotubes,5
composite thin films,6 organo-gels,7 or liquid-crystalline
phases.8 In this latter case, the hierarchical superstructure that
forms around oppositely charged dye molecules is favored via
secondary hydrophobic, H-bonding and/or π−π interactions
which help to determine the topology of the final aggregate.9 In
furtherance of this field, we now describe the self-assembly of a
liquid-crystalline material that displays efficient electronic
energy transfer across the emergent mesophase. This
spontaneous assembly of ionic molecules into a functional
liquid-crystalline phase avoids the reliance on the tedious, step-
by-step evolution of covalently linked analogues and equips the
aggregate with additional properties. The complementary
molecular building blocks are formed from two dissimilar
boron dipyrromethene (Bodipy) dyes.10 Thus, the distyryl
Bodipy core, Bn, was selected as an energy acceptor because of
its delocalized planar structure, suitable optical properties,11
and monocationic nature. The more rigid donor, R,12 is based
on a conventional Bodipy dye equipped with two negative
charges at the periphery. Electronic energy transfer (EET)
might be expected from the red dye to the blue counterpart,
provided these molecules can be brought into close proximity.
Complexes 1−3 were formed by way of electrostatic
Received: February 2, 2012
Published: March 28, 2012
interactions between the sodium salt of the sulfo Bodipy, R,
© 2012 American Chemical Society
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Communication
intermolecular association of the complex at high concen-
tration. This latter effect is more prominent in nonpolar
solvents, such as benzene, where the emission maximum for B
moves to ca. 720 nm. It was further found that K1 increases on
cooling the solution in accord with the van’t Hoff expression,
giving the enthalpy change as −5.3 kJ/mol, which is attributed
to an increase in the entropy of the surroundings (see SI).
In the case of 2, it was found that RDA increased progressively
upon addition of tetrabutylammonium perchlorate (TBAP) in
THF (Figure 2). Here, the added cation competes with B for
Figure 1. Absorption, fluorescence, and excitation spectra in THF, at
rt, c = 1 × 10−6 M: (a) complex 2 (λexc = 500 nm, λem = 740 nm), (b)
2/1 mixture of B12 and dye 5 (λexc = 480 nm, λem = 695 nm).
Figure 2. Fluorescence changes at rt, c = 1 × 10−6 M, by successive
addition of TBAP in THF. λexc = 490 nm.
transfer occurs from R to B within the complex, a situation
confirmed by the fact that the excitation spectrum matches the
absorption spectrum over the entire spectral window. Similar
results were found for dyad 4.
association to R and dismantles the original complex;
presumably, a new ion pair is formed by way of electrostatic
binding. In THF, the first B molecule is readily replaced by
TBA+, but this has only a small effect on the fluorescence
spectral profile. Fitting the titration data allows estimation of
the relative binding constants as being ca. 17-fold in favor of B.
Exchange of the more tightly bound B molecule demands a
large excess of TBAP (e.g., >100 mol equiv for complete
replacement) and fully restores emission from the donor. Data
analysis is consistent with the mixed complex having a ca. 104-
fold greater preference for B than TBA+. By taking due account
of the fact that TBAP is likely to form a complex by way of
electrostatic interactions, it follows that complexes such as 2 are
stabilized by ancillary interactions.
The liquid-crystalline properties of 1−3 were examined by
polarizing optical microscopy (POM), differential scanning
calorimetry (DSC), and X-ray diffraction (XRD).14 The results
of these studies are gathered in Table 1. All complexes display
comparable phase behavior with a 3D crystalline phase at rt that
transforms under heating into a hexagonal columnar mesophase
(Colh), which persists on further heating to the point of
thermal decomposition. On cooling the mesophase, in each
case, a mesomorphic glass appears that maintains the structural
characteristics of the hexagonal columnar mesophase (con-
firmed by XRD, see later). Crystallization is not apparent by
either POM or DSC during cooling. For 1 and 2, crystallization
does not take place even on cooling to −40 °C, and therefore
in the second heating and successive scans no transitions are
observed. In contrast, 3 crystallizes on cooling below rt and the
crystal-to-mesophase transition is observed during the second
heating cycle, although at a temperature significantly lower than
seen in the initial cycle (Table 1). The covalently linked dye 4
melts directly to the isotropic liquid, which partially or totally
crystallizes on cooling without passing through a mesomorphic
phase. X-ray patterns recorded at different temperatures
confirm that 4 is crystalline below the transition to the
isotropic liquid at 181 °C. A crystal-to-crystal transition takes
In the absence of anionic charges on R, the situation changes
markedly. For example, mixing B and 5 in a 2:1 stoichiometry
in THF results in an absorption spectrum resembling complex
2 (Figure 1b) but excitation at 480 nm results in strong
fluorescence centered at 511 nm (ΦF = 0.90: τs = 5.3 ns)
together with the very weak emission characteristic of B at 670
nm (ΦF = 0.06). This latter emission is a consequence of the
direct excitation of B since this dye shows slight absorption at
480 nm. Moreover, the absence of EET under these conditions
is indicated by the fact that the excitation spectrum recorded
for emission from B follows the absorption spectrum of B free
from contamination by that of 5. At this stage, we can conclude
that formation of the 2:1 complex requires electrostatic
association of B with R and is manifest by the occurrence of
efficient EET between the partners.
The EET event, being characteristic of the assemblage, is a
useful tool by which to assess the stability of the complex. At a
given concentration, the ratio (RDA) of emission yields for R
and B depends on the nature of the surrounding solvent and
shows a general increase with decreasing static dielectric
constant of the solvent (see SI). It was further noted that RDA
increased on dilution over the range 15 μM to ca. 20 nM, due
to partial dissociation of the complex in line with Le Chateliers’
Principle. Analysis of the experimental data for 2 over this range
shows that the complex dissociates in two distinct steps. The
first molecule of B is lost fairly easily (K1 = 3.5 × 10−7 M), but
the second B molecule is held more tightly (K2 = 7 × 10−11 M).
Interestingly, loss of the first B molecule is accompanied by
only a slight recovery of fluorescence from R, which remains
centered at 520 nm. It is the loss of the second molecule of B
that restores the characteristic emission from the donor. Time-
resolved fluorescence spectroscopy showed that the emission
lifetime of the bis-complex is <30 ps but that of the
monocomplex varies from 70 ps at high concentration to 150
ps at low concentration. This effect is attributed to the weak
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Communication
a
sulfonate anions of R but with their hydrocarbon chains
spreading out toward the periphery.
Table 1. Thermal and X-ray Data for Compounds 1−3
transition temp/°C
(ΔH/kJ·mol)
dmeas (Å) at
25 °C
dcalc
(Å)
Interestingly, an intense, broad fluorescence profile spanning
from 650 to 850 nm is observed for the columnar mesophase of
2 when dispersed on a glass slide. This latter emission is
broadened significantly and red-shifted compared to that from
the corresponding solution phase due to the ordering of the
molecule in the thin film (Figure 3). The shape and intensity of
compd
hk0 a (Å)
1
1st heating: Cr 155
(48.1) Colh ∼215
dec.
49.9
28.7
24.7
19.0
49.8
28.8
24.9
18.8
100
110
200
210
57.5
b
4.4
2
3
1st heating: Cr 77
(107.6) Colh ∼210
dec.
54.2
31.7
27.4
54.5
100
110
200
62.9
63.6
31.45
27.25
b
4.4
1st heating: Cr 85
(260.9) Colh
55.0
31.9
55.1
31.8
100
110
b
1st and 2nd cooling:
Colh 12 (116.1) Cr′
2nd heating: Cr′ 19
(101.8) Colh ∼190
dec.
4.4
a
Temperature at the peak maxima; Cr, Cr′ crystalline phases; Colh:
hexagonal columnar mesophase; Iso: isotropic liquid. X-ray measure-
ments for the cooling process. dmeas and dcalc are the measured and
calculated diffraction spacing; hk0 is the index of the reflections of the
Figure 3. Fluorescence spectrum (λexc = 370 nm) of complex 2 in the
columnar mesophase at 100 °C (red trace). Fluorescence spectrum
(λexc = 370 nm) of complex B12 mixed with dye 5 in a 2 to 1 mol
stoichiometry in the solid state at 100 °C (orange trace).
b
2D lattice of the Colh. Diffuse, broad maximum.
place at 65 °C. The lack of flexibility, the bulky
dimethylaminopropyne groups at the boron center, and the
nonplanar diphenylacetylene spacer in 4 hinder π−π stacking
between neighboring aromatic cores, thereby preventing bulk
phase organization. Such flexibility and the nondirectional
electrostatic interactions confirm the versatility of ISA
processes.
In contrast, variable-temperature X-ray diffraction studies are
consistent with formation of a columnar mesophase of
hexagonal symmetry for complexes 1−3; the lattice constants
a calculated from the registered hk0 X-ray reflections at rt are
given in Table 1. As expected, the magnitude of a depends on
the dimensions of the molecule, increasing progressively with
increasing chain length. No 001 reflections corresponding to
the regular stacking of the molecules within the columnar
mesophase are observed. Therefore, it appears that these
complexes present a disordered, hexagonal columnar mesophase
consistent with the absence of a planar aromatic core inherent
to classical disk-like mesogens.15
So how do these charged compounds pack together in the
mesophase so as to build the resultant columnar structure? The
density, ρ, of the compounds and the number, Z, of molecules
in the unit cell are related by ρ = (M/N)/(V/Z): where M is
the molar mass (g/mol), N is Avogadro’s constant, and V refers
to the unit cell volume (cm3). The latter term can be calculated
from the lattice constant (V = (√3/2)a2c) while the density is
reasonably estimated to be in the range 0.9−1 g cm−3. Within
these various approximations, the mean stacking distance (i.e.,
c) for 1−3 was ∼4−4.5 Å on the basis of Z = 2. This derived
separation is in line with values found for other columnar
mesophases of self-organized nondisk-like molecules9a−c,16 and
is considered to be fully consistent with a scaffold consisting of
stacked pairs of complexes. Such dimers would bear a total
number of 24 peripheral chains able to form a disordered state
surrounding the central columns. In the proposed structure, the
R units must occupy the inner part of the dimer core and the B
units must be located with their ammonium groups close to the
the fluorescence profile depend on the temperature, at least
over the range 20−150 °C. Since there is essentially no
emission from R it is concluded that quantitative EET occurs in
the mesophase. Indeed, mixing complex B12 with 5 in a 2:1
stoichiometry and annealing at 150 °C leads to the appearance
of dual emission at 550 and 750 nm when the film is examined
at 100 °C in the bulk state (Figure 3). This set of experiments
provides strong support for the idea that the complex is not
dismantled at 150 °C and that the mesophase is stable at high
temperature.
In summary, ISA provides an attractive alternative to
covalent bonding in regards to the creation of molecular
entities capable of displaying highly efficacious EET. Moreover,
this approach facilitates the growth of organized mesophases,
under mild conditions, retaining the EET step. A further benefit
of the ISA methodology is that the emergent mesophase has
the potential to function as a superior light-harvesting array for
silicon-based solar cells.
ASSOCIATED CONTENT
■
S
* Supporting Information
General methods, synthetic experimental part, absorption and
fluorescence spectra, NMR spectra, DSC traces, wide-angle and
small-angle X-ray diffraction patterns, stability constant
measurements. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
ziessel@unistra.fr
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the Centre National de la Recherche Scientifique
(CNRS) and the Royal Saudi Government for financial support
of this work. J.B. acknowledges the UE (FEDER funds) and the
Spanish Government (CTQ2009-09030 project).
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