Synthesis, structure, and luminescent properties of oligothiophene- containing metal-organic frameworks

Article abstract of DOI:10.1039/c2ce25529d

Metal-organic frameworks (MOFs) based on Zn and functionalized bithiophene dicarboxylic acids are reported, [Zn₂(L1)₂(DMA) ₂·3 DMF]_n (1), [Zn(L2)(DMA)₂] _n (2), [Zn(L3)(bpe)_0.5]_n (3), [Zn(L1)(bpe)·2 DMF]_n (4), and [Zn₃(L2) ₃(bpe)₂·4 DMF · H₂O] _n(5) (H₂L1 = 3,3′-diphenyl-2,2′-bithiophene-5, 5′-dicarboxylic acid, H₂L2 = 3,3′-dihexyl-2,2′- bithiophene-5,5′-dicarboxylic acid, H₂L3 = 2,2′- bithiophene-5,5′-dicarboxylic acid, DMA = dimethylamine, DMF = N,N-dimethylformamide, bpe = trans-1,2-bis(4-pyridyl)ethylene). MOFs 1, 2, and 4 consist of 2-D 4-connected sheets while 3 and 5 are 3-D interpenetrated networks with pcu and (4,6)-connected seh topology, respectively. The emission spectra and lifetimes of 1-5 have been determined: 4 and 5 exhibit reduced emission relative to 1 and 2, respectively, attributed to energy transfer quenching by the bpe, whereas 3 does not exhibit a reduction in emission intensity. Solid state lifetime measurements of 1-5 show that the emission decays are similar to those of the constituent ligands.

Full text of DOI:10.1039/c2ce25529d

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PAPER
Synthesis, structure, and luminescent properties of oligothiophene-containing
metal–organic frameworks{
Lyndsey D. Earl, Brian O. Patrick and Michael O. Wolf*
Received 9th April 2012, Accepted 18th June 2012
DOI: 10.1039/c2ce25529d
Metal–organic frameworks (MOFs) based on Zn and functionalized bithiophene dicarboxylic acids
are reported, [Zn2(L1)2(DMA)2?3 DMF]n (1), [Zn(L2)(DMA)2]n (2), [Zn(L3)(bpe)0.5]n (3),
[Zn(L1)(bpe)?2 DMF]n (4), and [Zn3(L2)3(bpe)2?4 DMF ? H2O]n(5) (H2L1 = 3,39-diphenyl-
2,29-bithiophene-5,59-dicarboxylic acid, H2L2 = 3,39-dihexyl-2,29-bithiophene-5,59-dicarboxylic acid,
H2L3 = 2,29-bithiophene-5,59-dicarboxylic acid, DMA = dimethylamine, DMF =
N,N-dimethylformamide, bpe = trans-1,2-bis(4-pyridyl)ethylene). MOFs 1, 2, and 4 consist of 2-D
4-connected sheets while 3 and 5 are 3-D interpenetrated networks with pcu and (4,6)-connected seh
topology, respectively. The emission spectra and lifetimes of 1–5 have been determined: 4 and 5
exhibit reduced emission relative to 1 and 2, respectively, attributed to energy transfer quenching by
the bpe, whereas 3 does not exhibit a reduction in emission intensity. Solid state lifetime
measurements of 1–5 show that the emission decays are similar to those of the constituent ligands.
and understood than in the polydisperse polymeric analogs. It
Introduction
has proven difficult, however, to engineer specific solid state
In photosynthesis, light energy is absorbed and funneled via
interactions in functionalized oligothiophenes, and thus gain
control over the optoelectronic properties of these materials.6,7
energy transfer in the light-harvesting complex, towards the
reaction center to drive electron transfer. The relative orientation
For this reason, it is important to find new ways to control solid-
of components in the light-harvesting complex is essential to
state packing. One approach to this goal is to use MOFs
maximize the efficiency of this process.1 Synthetic molecules
containing thiophene8 and oligothiophene9 dicarboxylates as
ligands to achieve positional and orientational control10 in the
designed to mimic photosynthesis also rely on careful control of
orientation between components, often using covalent links to
solid state. Different coordination modes of the carboxylate
achieve this.2 Metal–organic frameworks (MOFs) provide a
groups can be used to achieve a variety of solid state structures.
unique opportunity to control energy and electron transfer
Functionalization of the 3-position of oligothiophene derivatives
between different linker groups or a linker and the coordinating
allows for tuning of the structural and electronic properties of the
metal by fixing the position of these in a crystal.3
oligomers,11 and this can also be used to influence the properties of
The optical and electronic properties of poly- and oligothio-
oligothiophene-containing MOFs. In this paper, we explore the
phenes have attracted significant attention, with applications
effect of hexyl and phenyl substituents, and the effect of different
such as chemical and biological sensors, light-emitting and
coordination modes, on the structural and electronic properties of a
photovoltaic devices and electrochromic devices in mind.4 In
series of thiophene-based metal–organic frameworks.
many applications, intermolecular electron and energy transfer
are important. Shorter oligothiophenes (2–6 thiophene rings in
Experimental
length) have the advantage that they can be crystallized, and
characterized by single crystal X-ray diffraction (SCXRD), in
one of several distinctive solid state structures (i.e. herringbone).5
Thus, intra- and intermolecular interactions, which dictate bulk
optical and electronic properties,6 can be better characterized
General methods
Zn(NO3)2?6H2O, trans-1,2-bis(4-pyridyl)ethylene (bpe), and
n-butyllithium were purchased from Sigma-Aldrich. N,N-
Dimethylformamide (DMF) was purchased from Fisher
Scientific. All chemicals were used as received. H2L3 was
prepared by literature procedure.8a THF was distilled from Na/
benzophenone. 1H NMR spectra were collected on a Bruker
AV-300 spectrometer and were referenced to residual solvent.
Emission and excitation spectra were obtained on a Photon
Technology International fluorimeter using a 75 W arc lamp as a
source and were uncorrected for lamp intensity. UV-Vis spectra
Department of Chemistry, University of British Columbia, Vancouver, BC,
V6T 1Z1, Canada. E-mail: mwolf@chem.ubc.ca; Fax: 1-604-822-2847;
Tel: 1-604-822-1702
{ Electronic supplementary information (ESI) available: Crystallographic
and structure refinement data, structural figures of 2, PXRD patterns,
TGA data, emission quenching data. CCDC reference numbers 876807–
876811. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2ce25529d
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were collected using a Cary 5000 spectrometer. A Harrick
Praying Mantis accessory was used to collect UV-Vis spectra of
solid samples. Infrared spectra were obtained on a Thermo
Nicolet 6700 with a Smart Orbit accessory in the range of
4000–400 cm21. CHN elemental analyses were performed using
a EA1108 elemental analyzer. Thermogravimetric analyses
(75.5 MHz, d6-DMSO) d CO2H 162.31, C(3) 142.11, C(2) 135.28,
C(5) 134.44, C(ipso) 134.20, C(4) 133.97, C(ortho, meta) 128.45,
128.11, C(para) 127.64. FT-IR (cm21): 3029 (m, br), 2618 (m,
br), 1670 (s), 1536 (m), 1496 (m), 1425 (s), 1260 (m, br), 1208 (s),
872 (w), 750 (s), 692 (s), 493 (m, br), Elem Calcd for C22H14O4S2:
C, 65.01; H, 3.47; Found: C, 65.28; H, 3.65.
(TGA) were performed using
a Perkin Elmer Pyris 6
Thermogravimentric Analyzer under an air atmosphere at a
rate of 10 uC min21. Powder X-ray diffraction (PXRD) scans
were performed on a Bruker D8 Advance instrument at a scan
3,39-Dihexyl-2,29-bithiophene-5,59-dicarboxylic acid (H2L2).
3,39-Dihexyl-2,29-bithiophene19 (0.985 g, 2.94 mmol) was added
to THF (30 mL). The flask was cooled to 278 uC and
n-butyllithium (4.0 mL, 6.4 mmol, 1.6 M in hexanes) was added
dropwise. The initially colorless solution turned slightly yellow
during the addition. The reaction mixture was warmed to 0 uC
for one hour, then cooled to 278 uC. Addition of excess solid
carbon dioxide pellets resulted in the formation of a light yellow
opaque suspension. The reaction was allowed to warm to room
temperature overnight. Water (25 mL) was then added to quench
the reaction. The aqueous layer was washed with diethyl ether
(3 6 15 mL), and the product was precipitated by addition of
excess 1 M HCl, filtered, washed with 0.1 M HCl and finally
dried in vacuo to afford an off-yellow solid (1.08 g, 87% yield).
rate of 5u min21
.
X-Ray crystallography
All crystals were mounted on glass fibers. All measurements were
made on a Bruker X8 APEX II diffractometer with graphite
monochromated Mo-Ka radiation. Data were collected and
integrated using the SAINT12 software package. Data were
corrected for absorption effects using the multiscan technique
(SADABS).13 Structures were solved using direct methods,14 and
all non-hydrogen atoms were refined anisotropically. All non–
N–H hydrogens were placed in calculated positions; all N–H
hydrogens were found in the difference map. Refinements for
1–5 were performed using SHELXL-9715 via the WinGX16
interface. Compound 3 crystallizes with regions of unresolved
electron density in the void volume that could not be properly
modeled. The PLATON/SQUEEZE program was used to
generate a data set free of electron density in those regions.
Compound 5 crystallizes with significant disorder of three of the
n-hexyl groups (C55–C60, C73–C78, and C85–C90). Each group
was modeled in two orientations, and restraints on bond lengths
were employed to maintain reasonable geometries. Additionally,
the material crystallizes with at least four molecules of solvent
DMF and one water molecule in the asymmetric unit. One of
these solvent molecules is disordered about a two-fold rotation
axis in the unit cell. Finally, there are regions of unresolved
electron density that could not be properly modeled. As a result,
the PLATON/SQUEEZE17 program was used to generate a data
set free of electron density in those regions. Crystallographic
details for compounds 1–5 can be found in Table S1.
1
MP 162 uC. H NMR (300 MHz, CD3OD) d 7.68 (s, 2 H); 2.54
(t, J = 7.5 Hz, 4 H); 1.57 (m, 4 H); 1.25 (m, 12 H); 0.86 (t, J =
6.6 Hz, 6 H), 13C NMR (75.5 MHz, CD3OD) CO2H 164.91, C(3)
145.27, C(2) 136.02, C(5) 135.83, C(4) 130.81, CH2 32.76, 31.66,
30.08, 29.88, C_H2–CH3 23.73, CH3 14.56. FT-IR (cm21):
2992 (m), 2853 (m), 2561 (w, br), 1662 (vs), 1525 (s), 1421 (s),
1271 (s), 1194 (s), 863 (m), 750 (m), 714 (m), 497 (m), Elem Calcd
for C22H30O4S2: C, 62.53; H, 7.16. Found: C, 62.60; H, 7.12.
Zn2(L1)2(DMA)2 ? 3 DMF (1). Zn(NO3)2?6 H2O (59.4 mg,
0.200 mmol) and H2L1 (40.6 mg, 0.100 mmol) were dissolved in
DMF (5 mL). The solution was transferred to a 23 mL Teflon-
lined Parr bomb, sealed, and heated at 110 uC for 24 h. The
bomb was cooled at a rate of 3.7 uC h21. Colorless crystals of
Zn2(L1)2(DMA)2 ? 3 DMF were isolated in 64% yield. FT-IR
(cm21): 3258 (w), 3056 (w, br), 1626 (s), 1540 (m), 1422 (s), 1381
(s), 1355 (s), 1213 (m), 1122 (m), 1025 (m), 899 (w), 800 (s),
775 (m), 713 (m), 691 (s), 633 (m), 428 (m). Elem Calcd for
C57H59N5O11S4Zn2: C, 54.81; H, 4.76; N, 5.61. Found: C, 55.36;
H, 4.73; N, 5.30.
Synthesis
Zn(L2)(DMA) (2). Zn(NO3)2?6 H2O (59.4 mg, 0.200 mmol)
and H2L2 (42.2 mg, 0.100 mmol) were dissolved in DMF (6 mL).
The colorless solution was transferred to a 23 mL Teflon-lined
Parr bomb, sealed, and heated at 110 uC for 24 h. The bomb was
3,39-Diphenyl-2,29-bithiophene-5,59-dicarboxylic acid (H2L1).
THF (35 mL) was added to a round bottom flask with a side-
arm charged with 3,39-diphenyl-2,29-bithiophene18 (1.12 g,
3.51 mmol). The flask was cooled to 278 uC and n-butyllithium
(4.6 mL, 7.3 mmol, 1.6 M in hexanes) was added dropwise. The
initially colorless solution turned slightly yellow-green during the
addition. The reaction was first warmed to 0 uC for one hour,
then cooled back to 278 uC. Excess solid carbon dioxide pellets
were added, resulting in the formation of an off-white opaque
suspension. The flask was allowed to warm to room temperature
overnight. Water (30 mL) was then added to quench the
reaction. The aqueous layer was washed with diethyl ether
(3 6 15 mL), and the product was precipitated by addition of
excess 1 M HCl, filtered, washed with 0.1 M HCl, methanol, and
diethyl ether, and finally dried in vacuo to afford a white solid
(1.30 g, 91% yield). MP . 300 uC. 1H NMR (300 MHz, d6-
DMSO) d 7.74 (s, 2 H), 7.24 (m, 6 H), 7.10 (m, 4 H). 13C NMR
cooled at
a . Colorless crystals of
rate of 3.7 uC h21
Zn(L2)(DMA) were isolated in 52% yield. FT-IR (cm21): 3282
(w), 2925 (m), 2852 (m), 1623 (s), 1538 (m), 1426 (s), 1394 (s),
1362 (s), 1199 (w), 1024 (w), 904 (w), 799 (w), 773 (s), 714 (m),
454 (m, br). Elem Calcd for C24H34NO4S2Zn: C, 52.69; H, 6.45;
N, 2.56. Found: C, 52.92; H, 6.85; N, 2.32.
Zn(L3)(bpe)0.5 (3). Zn(NO3)2?6 H2O (29.7 mg, 0.100 mmol),
H2L3 (12.7 mg, 0.0500 mmol), and bpe (9.0 mg, 0.050 mmol)
were dissolved in DMF (5 mL). The solution was transferred to a
23 mL Teflon-lined Parr bomb, sealed, and heated at 110 uC for
24 h. The bomb was cooled at a rate of 3.7 uC h21. Yellow
crystals of Zn(L3)(bpe)0.5 were isolated in 72% yield. FT-IR
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(cm21): 1614 (m), 1519 (m), 1431 (m), 1379 (s, br), 1075 (w), 1033
(m), 953 (w), 820 (w), 798 (w), 766 (s), 652 (w), 550 (m), 446 (m,
br). Elem Calcd for C16H9NO4S2Zn: C, 47.01; H, 2.22; N, 3.43.
Found C, 46.68; H, 2.59; N. 3.37.
Zn(L1)(bpe)?2 DMF (4). Zn(NO3)2?6 H2O (59.4 mg, 0.200 mmol),
H2L1 (40.6 mg, 0.100 mmol), and bpe (18.2 mg, 0.100 mmol) were
dissolved in DMF (6 mL). The light yellow solution was left at
room temperature open to atmosphere and light yellow crystals of
Zn(L1)(bpe) ?2 DMF formed after two days and were isolated in
42% yield. FT-IR (cm21): 1611 (s), 1537 (m), 1497 (m), 1425 (s),
1365 (s), 1350 (s), 1210 (m), 1028 (m), 834 (m), 754 (s), 691 (s),
570 (s), 518 (w), 406 (w). Calcd C40H36N4O6S2Zn, C, 60.19.; H, 4.54;
N, 7.01; Found C, 60.34; H, 4.07; N, 6.53.
Zn3(L2)3(bpe)2?4 DMF?H2O (5). Zn(NO3)2?6 H2O (29.7 mg,
0.100 mmol), H2L2 (21.0 mg, 0.0500 mmol), and bpe (9.0 mg,
0.050 mmol) were dissolved in DMF (4 mL). The light yellow
solution was left at room temperature open to atmosphere. Light
yellow crystals of Zn3(L2)3(bpe)2?4 DMF?H2O formed after three
days and were isolated in 27% yield. FT-IR (cm21): 2925 (m), 2854
(m), 1597 (s), 1532 (m), 1421 (s), 1390 (s), 1351 (s), 1195 (w), 1101
(w), 1030 (w), 957 (w), 827 (w), 799 (w), 772 (m), 717 (w), 551 (m),
420 (w, br). Elem Calc for C102H134N8O17S6Zn3: C, 57.44; H, 6.33;
N, 5.25; Found C, 58.49; H, 5.59; N, 3.04.20
Results and discussion
Crystal structures
The coordination modes of functionalized bithiophene linkers
discussed in this work are shown in Chart 1. Zn(NO3)2?6H2O
and H2L1 were reacted under solvothermal conditions to form
colorless crystals of Zn2(L1)2(DMA)2?3 DMF (1) (DMA =
dimethylamine). DMA is a known hydrolysis product of DMF21
and has been observed in other metal–organic frameworks.22
Compound 1 crystallizes in the monoclinic space group P21/n
(Fig. 1) and has a Schla¨fli symbol of (44.62). The unit cell
contains two crystallographically non-equivalent zinc centers,
two L122 ligands, two DMA molecules, and three non-
coordinated DMF molecules, one of which is disordered. In
the structure the zinc centers form a binuclear ‘‘paddlewheel’’
cluster where each zinc has a square pyramidal geometry, four
carboxylate groups from two non-equivalent L122 (average Zn–
Fig. 1 Solid state structure of 1. Non-coordinating solvent and
hydrogens have been removed for clarity. a) Metal coordination of 1.
b) 2-D sheets along the bc plane.
(k1–k1)–(k1–k1)–m4 fashion and are easily distinguishable by their
differing thienyl-thienyl S–C–C–S torsion angles. At one site, the
S3–C–C–S4 torsion angle is 56u, the phenyl groups of L122 are
nearly parallel to one another and have a centroid-centroid
˚
distance of 4.22 A: this excludes justifying the observed geometry
due to cofacial p–p interactions. At the other site, the S1–C–C–
S2 torsion angle is 70u and the phenyl groups are nearly
orthogonal to each other. Molecular mechanics modeling of
3,39-diphenyl-2,29-bithiophene suggests that an anti conforma-
tion with a thienyl-thienyl torsion angle of 153u is the energetic
minimum.18 A syn conformation with a torsion angle of 42u was
found to be an energy minimum for 2,29-bithiophene23 but this
conformation was precluded for 3,39-diphenyl-2,29-bithiophene
due to the steric bulk of the phenyl groups.18 Fig. 1b shows that
˚
O distance = 2.05 A) coordinate in a bisbidentate fashion
(Fig. 1a) at the equatorial positions, and a DMA caps the zinc at
˚
the axial position (average Zn–N distance = 2.03 A). The two
distinct crystallographic environments for L122 coordinate in a
Chart 1 Coordination modes of oligothiophene linkers observed in this work. a) (k1–k1)–(k1–k1)–m4; b) (k1)–(k1)–m2; c) (k1–k1)–(k2)–m3; d) (k1–k1)–
(k1)–m3.
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the L122 ligands link the zinc centers to form 2-D grids that
stack upon one another. Non-coordinated DMF fills the void
space between the 2-D sheets. It should be noted there is no
p-type interaction between the phenyl rings in the sheets of 1.
The reaction of H2L2 with Zn(NO3)2?6H2O under solvother-
mal conditions results in the formation of Zn(L2)(DMA) (2).
Compound 2 crystallizes in the space group P21/n and is
structurally similar to 1 (Figure S1). The metal-carboxylate
simplified framework has a pcu topology where the binuclear
zinc paddlewheel acts as the six-connected node. There is one
L322 environment that adopts a bis-bidentate coordination
mode (Chart 1a). The rings of the bithienyl ligand have a S1–C–
C–S2 torsion angle of 145.6u, and the pyridyl rings of bpe are
˚
coplanar. The Zn–O bond distances range from 2.01–2.09 A, and
˚
the Zn–N distance is 2.02 A.
Addition of 4,49-bipyridine (bpy) or bpe to Zn(NO3)2?6 H2O
and H2L1 under solvothermal conditions consistently gives
phase-pure 1 without incorporation of the N-heterocyclic ligand
into the MOF. However, reaction of Zn(NO3)2?6 H2O, H2L1,
and bpe (2 : 1 : 1) in DMF at room temperature resulted in the
formation of colorless crystals of Zn(L1)(bpe) ? 2 DMF (4) after
a few days. Compound 4 crystallizes in the triclinic space group
˚
Zn–O distance ranges from 2.04–2.05 A, and the metal-DMA
22
˚
Zn-N distance is 2.06 A. The crystallographic site of L2
coordinates in a (k1–k1)–(k1–k1)–m4 fashion (Chart 1a) and has a
S1–C–C–S2 torsion angle of 68u. The hexyl chains of 2 reside in
the void that non-coordinating solvent occupied in 1.
A new MOF containing non-functionalized bithienyl dicar-
boxylate groups and the N-heterocyclic linker bpe was also
synthesized. Reaction of Zn(NO3)2?6 H2O, H2L3 and bpe
(2 : 1 : 1) in DMF under solvothermal conditions afforded
yellow crystals of Zn(L3)(bpe)0.5 (3). Compound 3 is a three-
fold interpenetrated 3-D framework belonging to the space
group C2/c and is similar in structure to a previously reported
MOF containing L322 and 4,49-bipyridine.9a,b Fig. 2 shows the
coordination center and extended structure of 3. The structure
consists of binuclear paddlewheel clusters coordinated to four
carboxylate groups of four L322 and two bpe ligands in the
axial positions that contribute to the six-connected node. The
¯
P1 (Fig. 3). In this structure, each zinc atom has a tetrahedral
geometry and is coordinated to two monodentate L122 and two
bpe ligands. L122 adopts a (k1)–(k1)–m2 coordination (Chart 1b).
˚
˚
Zn–O contact lengths of 1.96 A and 2.69 A clearly indicate one
of the carboxylate oxygen atoms is involved in bonding. The bpe
is part of a 2-D grid (Fig. 3b), rather than replacing DMA in the
structure of 1 and acting as a linker to connect 2-D sheets of
L122 groups. The S1–C–C–S2 torsion angle of the bithienyl unit
˚
is 59.9u, and the phenyl-centroid contact distance is 4.62 A. This
distance is longer than that observed for 1. The zinc-linked bpe
chains shown in Fig. 3b have a Zn–Zn–Zn angle of 131.3u,
whereas zinc atoms along the chain of L122 are colinear
(Fig. 3c). Zn–Zn–Zn angles at the bpe-L122 intersection are
87.5u and 92.5u, nearly right angles. Fig. 3d shows three identical
and independent wave-like nets that form a complete 2-D sheet
in the ab plane. The net trilayers are separated by the bithienyl
phenyl rings and non-coordinated DMF. The solid state
structure suggests there is no p interaction of the phenyl rings
within the net trilayer.
Similarly, addition of bpe to Zn(NO3)2?6 H2O and H2L2
under solvothermal conditions consistently gives phase-pure 2
without incorporation of bpe into the structure. Reaction
of Zn(NO3)2?6 H2O, H2L2 and bpe (2 : 1 : 1) in DMF at
room temperature, however, afforded yellow crystals of
Zn3(L2)3(bpe)2?4 DMF?H2O (5) after one week. Elemental
analysis of 5 corresponds to the solvent-free material (calculated
for solvent-free Zn3(L2)3(bpe)2: C, 59.32; H, 5.75; N, 3.07).
Compound 5 is a two-fold interpenetrated three-dimensional
framework belonging to the space group C2/c. The framework
has (4,6)-connected seh topology and is assigned the Schla¨fli
symbol of (3.42.52.6)(32.42.52.64.74.8). Fig. 4a shows a simplified
unit cell of 5. There are two distinct zinc environments in 5: one
environment consists of binuclear ‘‘paddlewheel’’ clusters
coordinated to four carboxylate groups of four L222 and two
bpe groups in the axial positions that contribute to the six-
connected node, while the other environment is a mononuclear
four-coordinate center coordinated to two L222 and two bpe
ligands. There are three L222 environments that have coordina-
tion modes (a), (c), and (d), respectively (Chart 1). The
corresponding S–C–C–S torsion angles of L222 are 117u (S1–
C–C–S2), 2118u (S5–C–C–S6), and 66u (S3–C–C–S4). The
pyridyl rings are coplanar for one bpe group, but the other
bpe ligand has pyridyl rings that are twisted by approximately
20u (N1–C36–C37–N2). Ribbons consisting of two L222 ligands,
two bpe ligands, two binuclear centers, and two connected nodes
Fig. 2 Solid state structure of 3. a) Metal coordination of 3. b) 3-D
framework along the bc plane. Interpenetration has been removed for
clarity.
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Fig. 3 Solid state structure of 4. Non-coordinating solvent and hydrogens have been removed for clarity. a) Metal coordination of 4. b) 2-D grid along
the ab plane. c) Simplified 2-D grid highlights the zigzag connectivity of bpe. d) Net trilayers that forms a complete 2-D sheet.
Fig. 4 Solid state structure of 5. Hexyl chains, non-coordinating solvent, and hydrogens removed for clarity. a) Metal coordination of 5. b) Simplified
view along the c axis, interpenetration removed. Dashed arrows (red and green) show the planes of the ribbon, and solid lines (blue) show the plane of
22
connecting L222. c) ‘‘Ribbon’’ of L2 and bpe. d) View along c axis including solvent, disorder, hexyl chains, and interpenetration.
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(Fig. 4c) represent the primary connectivity modes in 5. There
are two planes oriented 50u from each other that the ribbons
occupy, which intersect and share nodes at the mononuclear zinc
center. In addition, the ribbons of the two planes are connected
via a L222 linkage of two binuclear units. A simplified view
down the c axis of 5 highlights these features (Fig. 4b). Fig. 4d
shows that the L222 hexyl chains are disordered and fill much of
the free space of the framework. This type of self-assembled
aggregation has been observed in other metal–organic frame-
works.24 It is possible that the occupancy of the otherwise void
space by the hexyl chains plays a key role in the formation of this
somewhat unusual solid state structure.
Table 1 Photophysical data for ligands and 1–5
Abs. lmax (nm) Em. lmax (nm) tA (ns) (A)a tB (ns) (B)a
H2L1
H2L2
H2L3
bpe
1
2
3
4
5
390
400
395
470
485
490
1.1
1.2
1.3
2.5
0.5 (80%)
1.2 (100%)
2.5 (100%)
2.5 (100%)
0.5 (67%)
—
—
—
—
295b, 430
380
395
400, 525
470
435
2.2 (20%)
—
—
—
380
380
390, 450
A
500
470
435, 460, 530
2.7 (33%)
a
f(t) = Ae2t/t + Be2t/t , where tA and tB are the lifetimes, A and B
B
b
are normalized intensity. Solution state absorption.
Powder X-ray diffraction patterns and thermogravimetric analysis
Photophysical properties
The phase purity of bulk samples of 1–5 was determined using
powder X-ray diffraction (PXRD) and comparing the experi-
mental patterns to those predicted from the single crystal data.
Figures S2–S6 show the experimental and simulated powder
patterns for 1–5, respectively. The PXRD patterns of the bulk
material matches the pattern calculated from the single crystal
data for all of the MOF compounds. There is a broad peak
present in 5 that is not predicted in the simulated pattern and
grows in with exposure to atmosphere. This feature may be
attributed to the loss of non-coordinating solvent.25
To investigate the excited state electronic behavior of 1–5, the
photoluminescence spectra of the MOFs and ligands were
obtained (Fig. 5) and are summarized in Table 1. The
photoluminescence spectra of the ligands H2L1, H2L2, H2L3,
and bpe in the solid state (Fig. 5a) show single broad emission
bands. In solution, the H2L1 and H2L3 emission maxima are
identical to those observed in the solid state, whereas the solution
state emission of H2L2 blue-shifts to 430 nm (Figure S10). The
emission spectrum for bpe shows a maximum at 525 nm in the
solid state (Fig. 5a). These species all have short excited state
emission lifetimes (Table 1), consistent with singlet state emission
from p–p* states.26
Thermogravimetric analysis (TGA) was performed on 1–5 to
analyze the composition and evaluate the thermal stability of
these compounds (Figure S7). Table S2 summarizes the results of
the analyses. Loss of both coordinating and non-coordinating
solvent is observed. Major decomposition occurs at y350 uC
and is attributed to the loss of the organic linkers. The remaining
weight after this decomposition corresponds to the formation of
ZnO.
Compounds 1–5 exhibit emission profiles that are attributed
to the constituent ligands (Fig. 5b–f). Emission of 1 matches the
H2L1 emission profile well, while H2L2-attributed emission of 2
shifts from lmax 485 nm to 430 nm upon metal coordination in
compound 2: its emission is similar to the solution state profile of
Fig. 5 Photoluminescence spectra for ligands and compounds 1–5. Excitation spectra are shown as dashed lines, and emission spectra are shown as
solid lines. Intensities are normalized. a) Excitation and emission spectra of ligands in the solid state. H2L1, lex: 380 nm , lem 470 nm (black), H2L2, lex:
385 nm, lem 480 nm (red), H2L3, lex: 390 nm, lem 490 nm (blue), and bpe, lex: 460 nm, lem 530 nm (magenta) b) Compound 1: lex 380 nm, lem 470 nm
c) Compound 2: lex 395 nm, lem 435 nm d) Compound 3: lex 380 nm, lem 500 nm e) Compound 4: lex 380 nm (blue), lex 440 nm (red), lem 470 nm
(blue), lem 530 nm (red) f) Compound 5: lex 390 nm (blue), lex 450 nm (red), lem 435 nm (blue), lem 525 nm (red).
5806 | CrystEngComm, 2012, 14, 5801–5808
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H2L2. The emission of 3 is attributed to H2L3. Any possible
contribution from bpe is overshadowed by the intense, broad
band emission attributed to H2L3. The emission profile of
compound 4 is dominated by the H2L1 emission. Some
contribution from the bpe is observed in 4 when the material is
excited with lower energy light (Fig. 5e). In 5, features of both
organic linkers are readily discernable in the emission spectrum.
Exciting at 390 nm results in emission dominated by H2L2
features at 430–460 nm and a bpe shoulder at 525 nm, whereas
exciting at 460 nm produces emission with bpe character.
The emission intensities of compounds 1–3 are qualitatively
comparable to the emission intensities of their respective bithienyl
ligands in the solid state. However, the emission intensities for 4
and 5 are significantly weaker than for 1 and 2. This qualitative
Conclusions
A series of new bithienyl-containing metal–organic frameworks
have been synthesized and characterized. The structure of these
materials can be tuned by changing the functionality at the
3-position of the thienyl unit. The thienyl torsion angle of L122
in metal–organic frameworks deviates from the calculated
solution-state geometry. However, the near co-facial interaction
of the phenyl moieties is not close enough to indicate significant
p–p interaction. Addition of aliphatic functional groups drive
out non-coordinating solvent as seen in 2. In the three-
dimensional framework 5, the presence of hexyl chains can lead
to the formation of hydrophobic pockets. Addition of an
N-heterocyclic ligand results in materials where the ligand is
incorporated into a 2-D sheet (4) or the dimensionality of the
materials is increased (3, 5). Compound 3 has a simplified pcu
topology whereas 5 has a seh topology as well as multiple metal-
center environments. Compounds 1–5 retain similar emission
properties to the constituent ligands. The emission can be
quenched via an incomplete energy transfer with the addition of
an appropriate co-ligand as seen in 4 and 5. This behavior would
be beneficial for emission-based applications of these materials
such as in chemical sensors or light-emitting devices.
reduction in emission intensity is not observed when comparing 3
9a,b
and the previously reported Zn(L3)(DMF)3
polymer.
coordination
One possible explanation for these results is that the bpe linker
is quenching the L122 and L222-based emission. However,
previously synthesized bpe-containing coordination polymers do
not exhibit fluorescence quenching.27 The solid state UV-Vis
spectrum of bpe shows a broad feature between 390 nm and
480 nm with lmax at 430 nm (Figure S8). To probe if bpe is
responsible for the reduced emission intensity of the bithienyl
linkers, the emission of mixtures of bpe and bithienyl ligands
were investigated in the solid state and in solution (Figures S9–
S12). The presence of bpe did not change the emission intensity
of either bithienyl linker in solution or in the solid state. In
solution, this can be attributed to the absorption maximum of
bpe occurring at 295 nm compared to 430 nm in the solid state.
This suggests that if incomplete energy transfer is the quenching
mechanism, the linkers need to be within the structured
framework to achieve the proximity needed for resonance energy
transfer. Also, the observation that emission intensity is not
reduced in 3 suggests that bpe itself is not quenching bithienyl-
based emission.
Acknowledgements
We thank the Natural Sciences and Engineering Council
(NSERC) of Canada for funding this research.
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