Guest-responsive fluorescence of inclusion crystals with π-stacked supramolecular beads

Article abstract of DOI:10.1002/anie.201106849

Luminescent jewels: Unusually shaped fluorescent supramolecular clusters assemble into one-dimensional π-stacked supramolecular beads to eventually crystallize with a wide range of solvent molecules (see picture). The included solvent molecules modulate the fluorescence colors of the inclusion crystals from blue to orange-yellow as is known for the colors of gemstones.

Full text of DOI:10.1002/anie.201106849

Angewandte
Chemie
DOI: 10.1002/anie.201106849
Luminescent Inclusion Crystals
Guest-Responsive Fluorescence of Inclusion Crystals with p-Stacked
Supramolecular Beads**
Tomoaki Hinoue, Mikiji Miyata, Ichiro Hisaki, and Norimitsu Tohnai*
Lattice inclusion host compounds entrap guest molecules of
various sizes into their flexible cavities formed by the host
[1]
framework. The inclusion phenomena have been exten-
sively investigated because of the potential application of
[
2]
these compounds in many fields such as separation,
[
3]
[4]
[5]
storage, catalysis, and chemical sensing. Considerable
efforts have demonstrated that bulky and rigid molecules,
or awkwardly shaped molecules, tend to include guest
[
6]
molecules because they do not pack easily without guests.
For example, crystals of anthracene derivatives bearing
diphenylphosphanyl groups at the 9- and 10-positions
include toluene molecules and show on/off fluorescence
[
5a]
switching by absorption and desorption of the guest.
Inclusion of various guests into fluorescent hosts will
afford a multitude of fluorescent colors as if doping
strongly affects the colors of jewels, as seen in sapphires
and rubies. However, chemical affinity of their inclusion
spaces often limits the appropriate guest molecules (i.e.
polar or nonpolar molecules). Therefore, it still remains
challenging to design new lattice host compounds that
efficiently incorporate and respond to a wide range of
guests.
Previously, we readily prepared bulky and rigid super-
molecules from simple molecules: triphenylmethylamine
Figure 1. a) Supramolecular clusters with cubic H-bonding network com-
prising TPMA and a variety of monosulfonic acids such as benzenesul-
[
7]
(
TPMA) and a variety of sulfonic acids. Four TPMA
molecules and four monosulfonic acid molecules formed fonic acid. The shortest S–S distance is 5.3–5.7 ꢀ. b) Novel supramolec-
cubic hydrogen-bonding (H-bonding) networks completely ular cluster with chairlike H-bonding network comprising TPMA and 1,8-
ADS. The intramolecular S–S distance is 5.2–5.3 ꢀ. This complex did not
form cubic networks. c) The novel supramolecular cluster in crystals.
d) Space-filling representation of the cluster. e) Schematic representation
of the cluster. Yellow and gray represent anthracene and triphenylmethyl
covered with their substituents (Figure 1a). Herein, in
order to create fluorescent materials responsive to various
guests, we have designed a new host supermolecule that
possesses two aromatic fluorophores at the periphery. The
moieties, and blue, red, orange, and white represent N, O, S, and H
fluorescent host forms one-dimensional (1D) p-stacked atoms, respectively.
[
*] T. Hinoue, Prof. Dr. M. Miyata, Dr. I. Hisaki, Dr. N. Tohnai
Department of Material and Life Science
Graduate School of Engineering
assemblies based on p–p stacking between the fluorophores.
That is to say, the fluorophores serve as supramolecular glue
[
8]
as well as emitter. The stacking geometry changes through
crystallization with guest molecules, resulting in guest-
2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
E-mail: tohnai@mls.eng.osaka-u.ac.jp
[
5c,9]
responsive fluorescence.
In that respect, we employed
Dr. N. Tohnai
Japan Science and Technology Agency (JST), PRESTO
the supramolecular complex comprising TPAM and anthrace-
1,8-disulfonic acid (1,8-ADS). This disulfonic acid has two
sulfonate groups at a sulfur–sulfur (S–S) distance of approx-
imately 5.3 ꢀ, which corresponds to the typical nearest S–S
distance in the cluster with a cubic H-bonding network (5.3–
4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
[
**] This work was financially supported by the Research fellowships of
the Japan Society for the Promotion of Science (JSPS) for Young
Scientists, the Global COE (center of excellence) Program “Global
Education and Research Center for Bio-Environmental Chemistry”
of Osaka University, and Grants-in-Aid for Scientific Research on
Innovative Areas “Coordination Programming” (area 2107, no.
5
.7 ꢀ). Therefore, they should have formed cubic networks
with two anthracene fluorophores at the peripheries of the
resulting cluster. Surprisingly, however, this supramolecular
complex formed not cubic H-bonding networks but topolog-
ically different, chairlike H-bonding networks (Figure 1b).
This topological difference led to a rather favorable supra-
22108517) from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT) (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106847.
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Figure 2. Schematic representation of the hierarchical construction of p-stacked inclusion assemblies with guest-dependent fluorescent properties
from the ellipsoidal supramolecular cluster comprising TPMA and 1,8-ADS. Purple cube, red octahedron, and green disc represent guest (dopant)
molecules.
molecular cluster, which was unusually shaped and exposed
the anthracene fluorophores more. Consequently, the novel
supramolecular clusters were robustly formed to construct p-
stacked 1D assemblies that incorporate many kinds of
nonpolar and even polar guest molecules. Their fluorescence
colors widely changed depending on the included (doped)
guests, thus behaving like “organic” jewels, suggesting their
potential as chemo- and biosensors.
ellipsoids (site B in Figure 2). Toluene and 1,4-dioxane were
incorporated into both sites. The precise geometry of p–p
stacking and relative positions between the beads slightly
changed so as to pack efficiently with the guests (Figure 2 and
Figure 3, and Table 1). Nevertheless, assembly manners of the
clusters and the beads are basically similar in any crystals.
The versatile inclusion ability of these clusters seems to be
attributed to the presence of accessible oxygen atoms of the
sulfonate groups as well as the shape of the resulting
supramolecular beads (Figure 1d). The ellipsoidal clusters
exposed their sulfonate groups on the edge of the chair,
though the cubic H-bonding networks in our previous cluster
were efficiently covered by their substituents. Without
interrupting formation of the H-bonding core, these acces-
sible oxygen atoms captured alkyl halides, 1,4-dioxane, and
toluene in site A by CH–O interactions with CH at a position
or of phenyl groups of the guests. In other words, the oxygen
atoms serve not only as structural elements in forming the H-
bonding cores but also function as receptors for molecules
having weak acidic protons. On the other hand, other guests
were incorporated into site B without CH–O interactions. In
these inclusion crystals, the guests adequately filled the gaps
between the beads. The accessible oxygen atoms caught the
neighboring beads instead of the guests. This observation
suggests that both the receptor oxygen atoms and the
awkward shape of the supramolecular beads contribute to
incorporating various guest molecules. Interestingly, despite
their awkward shape, they are also able to crystallize without
guest molecules from methanol and 2-pentanol; in the guest-
free crystal, two non-equivalent beads coexisted in the same
lattice to complement each awkward shape as if beads
themselves are hosts and guests.
Recrystallization of the supramolecular complex of
TPMA and 1,8-ADS from mixtures of methanol and a variety
of organic solvents resulted in many kinds of crystal habits,
depending on the solvents. X-ray crystallographic studies
clearly demonstrated that these crystals included not only
[10]
nonpolar solvents but also polar solvents.
In all of the
crystal structures, two disulfonic acid molecules (four sulfo-
nate groups) and four TPMA molecules robustly formed a
discrete chairlike H-bonding network. Parallel orientation of
the sulfonate groups in the 1,8-ADS molecule probably did
not afford a stable cubic network. Alcohols such as methanol
and 2-pentanol, which are often involved in H-bonding
networks, were not incorporated but did not break such
discrete networks upon crystallization. This observation
indicates the high stability of this chairlike H-bonding net-
work. The network was surrounded by anthracene backbones
and triphenylmethyl groups, which led to ellipsoidal supra-
molecular clusters (major and minor axis are 23 ꢀ and 18 ꢀ,
respectively; Figure 1c–e). The clusters exposed two of the
p planes of the anthracene moieties on the opposite sides of
the ellipsoid. These clusters were hierarchically assembled
into inclusion crystals (Figure 2). Each cluster was linked by
face-to-face p–p interactions between the anthracene moi-
eties to give beadlike p-stacked motifs. These supramolecular
beads were bundled by CH–p interactions between triphe-
nylmethyl groups, leaving two kinds of inclusion spaces
depending on the guest molecules. 1,2-Dichloroethane, 1-
bromobutane, and 1,3-diiodopropane were incorporated into
the gaps between both sides of the ellipsoids (site A in
Figure 2), whereas acetonitrile, ethyl acetate, and acetone
were entrapped into the gaps between top and bottom of the
The guest molecules are located above or beside the
anthracene moieties either within the sum of the van der
Waals radii or within 0.2 ꢀ over this value (Figures 3 and S2).
Therefore, the included guest molecules must have an
influence on the fluorescence properties. In fact, these crystals
displayed a wide range of emission colors from blue to
orange-yellow depending on the included guests under UV
1
56
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Angewandte
Chemie
Figure 3. Crystal structures of the supramolecular clusters a) without guests, c) with 1,2-dichloroethane, e) ethyl acetate, g) toluene, and i) 1,3-
diiodopropane. The clusters stack perpendicularly to form supramolecular beads. The p-stacking geometries within the beads depend on the
guests (b, d, f, h, and j). Two types of stacking geometry coexist in the guest-free crystal. Guest molecules are represented by green sticks in (a),
(
c), (e), (g), and (i), and those near the fluorophores are represented by transparent sticks in (b), (d), (f), (h), and (j). Hydrogen atoms and minor
components of disordered 1,2-dichloroethane and toluene molecules are omitted for clarity.
Table 1: Structural and spectral properties of the crystals comprising 1,8-
ADS and TPMA with various guests.
Guests
lem
[
lex
[nm]
dp–p
[ꢀ]
Overlap
[%]
[
a]
[b]
[c]
[d]
nm]
no guest
473
409
418
3.25, 3.34
3.44
43, 40
41
452
5
5
5
37
37
44
420
428
419
3.40
3.32
3.30
51
38
41
5
5
46
54
421
421
3.32
3.40
39
62
5
5
64
70
420
402
3.32
3.33
54
51
[a] Maximum wavelength of emission spectra of the inclusion crystals
excited at 340 nm. [b] Maximum wavelength of excitation spectra
monitored at their respective emission bands. [c] Interplane distances
between the anthracene planes of dimer pairs. In guest-free crystals, two
non-equivalent dimer pairs coexist in the same lattice. [d] The ratio of the
area of overlapped moieties of anthracene aromatic rings in the crystal
structures calculated as the overlapped area divided by the whole area of
the anthracene ring in Figure 3b,d,f,h, and j, and Figure S1b, d, f, and h.
Figure 4. a) Photographs of guest-free and inclusion crystals of supra-
molecular complexes of 1,8-ADS and TPMA with various guest molecules
under UV irradiation (l=365 nm). b) Normalized fluorescence spectra of
the guest-free and inclusion crystals excited at 340 nm.
irradiation (Figure 4a). Figure 4b shows the fluorescence
spectra of the inclusion crystals excited at 340 nm. The
maximum wavelength of the fluorescence emission spectra
allowed the formation of ground-state dimers to emit red-
shifted fluorescence.
(
lem) widely shifted from 452 nm to 570 nm. Guest-free
Inclusion of guest molecules into the lattice caused larger
red-shifts of lem by 60–100 nm compared to the guest-free
crystal but only 1,2-dichloroethane did blue-shift the emis-
sion. In both cases, the maximum wavelength of the excitation
crystals showed a broad emission profile around 470 nm.
This result is in contrast to the dilute solution of anthracene
and 1,8-ADS, the fluorescence spectra of which have vibra-
tional structures with lem of approximately 400 nm. Such large
red-shifts and broadenings of fluorescence spectra are char-
acteristics for excimer or dimer fluorescence. The p-stacked
geometry of anthracene dimers in these crystals probably
spectra (l ) are red-shifted from guest-free crystals by 10-
ex
20 nm. These phenomena imply that the red-shifts of lem and
lex in the inclusion crystals are caused by large stabilization of
excited states or destabilization of ground states by lattice
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.
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Communications
guests. On the other hand, the blue-shift of lem is attributed to
its unique geometry of p-stacked anthracene dimers. That is,
an inclusion crystal with 1,2-dichloroethane showed a parallel
slipped stacked geometry (Figure 3d), whereas the other
inclusion crystals displayed common offset (centre-to-edge)
stacked geometry (Figure 3b,f,h,j). Such obvious differences
in stacking geometry of fluorophores has a significant
influence on the formation of their excited complexes. It
was reported that anthracenophane as a model of anthracene
dimers shows a blue-shift in fluorescence with slipping the
Keywords: hydrogen bonds · inclusion crystals · luminescence ·
sensors · supramolecular cluster
.
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Experimental Section
[
10] CCDC 848993, 848994, 848995, 848996, 848997, 848998, 848999,
849000, 849001 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif..
Single-crystal X-ray diffraction data were collected on a Rigaku
RAXIS-RAPID imaging-plate diffractometer with graphite-mono-
chromated CuKa radiation (l = 1.54187 ꢀ). Solid-state fluorescence
spectroscopy was carried out with a FP-6500 spectrofluorometer
(JASCO). The crystals that were used for the spectroscopy were
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ground to crystalline powder. See the Supporting Information for
more experimental details.
Tanaka, J. Am. Chem. Soc. 1976, 98, 5910.
Received: September 27, 2011
Published online: November 14, 2011
1
58
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Angew. Chem. Int. Ed. 2012, 51, 155 –158