A R T I C L E S
Stylianou et al.
Scheme 1. H4TBAPy Ligand
the network topology on the fluorescence of conformationally
flexible stilbene-based MOFs,18 whose emission can be irrevers-
ibly modified upon guest removal, and shown that guests can
control fluorescence wavelengths. More weakly emitting 4,4′,4′′-
nitrilotris(benzoic acid)-based materials can have their guest-
sensitive emissions tuned by the presence or absence of π-π
interactions in desolvated and solvated materials.19 Such studies
now present the opportunity to exploit framework-forming
ligands based on cores specifically selected for their desirable
optical properties, in particular strong and long-lived emission,
that are functionalized to generate large accessible pore volumes.
This is an important step in order to design fluorescent materials
with the required lifetimes and quantum yields for viable sensing
devices.20
In(III) as the inorganic component to form coordination-based
reticular structures with spatially separated pyrene-derived
TBAPy fluorophore ligands. Indium has previously been used
to prepare topologically diverse MOFs from small carboxylate-
based ligands,25 which have been investigated for hydrogen
storage,6d ion exchange,26 and catalysis.27 It was anticipated that
the combination of fluorophore separation in the solid state and
the [Kr]4d10 configuration of In(III) would limit destructive
nonradiative pathways to yield highly emissive permanently
porous framework structures.
In this paper we report a new tetracarboxylate ligand, TBAPy,
based on the optically active and strongly fluorescent pyrene
core as illustrated in Scheme 1, which is functionalized in the
1-, 3-, 6-, and 8-positions with benzoate fragments. (E)-Stilbene
and pyrene are attractive molecules, due to their use as optical
brighteners, dyes, or dye precursors. We have chosen the pyrene
core as a linker, as its lifetime and quantum yield (τ ) 113
ns,21 φ ) 0.6522 in EtOH at 293 K) are much higher than those
of (E)-stilbene (τ ) 1.7 ns,23 φ ) 0.02 in acetonitrile24), and
The reaction of In3+ and TBAPy in acidic solution gives a
three-dimensional noninterpenetrated porous and strongly fluo-
rescent framework. The specific geometry derived from the
ligand and metal ion connectivity affords significant permanent
porosity and modulates the ligand-ligand interactions respon-
sible for the optical response. The fluorescent behavior of the
framework reversibly responds to the precise nature of the guests
(of a range of shapes and sizes permitted by the window and
pore sizes) through the position, intensity, and lifetime of the
emission. Moreover, the quantum yield and microsecond lifetime
of the framework are comparable to those of Eu(III) cryptate
based commercial sensors.28
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Experimental Section
General Information. Reagents and solvents were purchased
from Sigma-Aldrich and used as received without further purifica-
tion. Elemental analyses (C, H, and N) were obtained using a
Thermo EA1112 Flash CHNS-O Analyzer. X-ray powder diffrac-
tion data were collected in transmission geometry using a STOE
Stadi-P diffractometer with Cu KR radiation at 298 K, except in
the case of toluene-loaded 1′, for which the data were collected at
station I11 of the Diamond Synchrotron, U.K., at a wavelength of
0.82633 Å.
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Synthesis of 1,3,6,8-Tetrakis(benzoic acid)pyrene (TBAPy)
Ligand. Pyrene (5 g, 24 mmol) in 100 mL of nitrobenzene was
reacted with excess bromine (5.6 mL, 109 mmol) for 14 h at 120
°C to give 1,3,6,8-tetrabromopyrene. Yield: 98% based on pyrene.
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1
The product 1,3,6,8-tetrabromopyrene was judged by solution H
NMR to be of sufficient purity to be used in the following reaction
without further purification.
A mixture of (4-(methoxycarbonyl)phenyl)boronic acid (5 g, 32.9
mmol, 6 equiv), 1,3,6,8-tetrabromopyrene (2.85 g, 5.5 mmol),
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4120 J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010