White light emission and second harmonic generation from secondary group participation (SGP) in a coordination network

Article abstract of DOI:10.1021/ja2073559

We describe a white emitting coordination network solid that can be conveniently applied as a thin film onto a commercial UV-LED lamp for practical white lighting applications. The solid state material was discovered in an exercise of exploring molecular building blocks equipped with secondary groups for fine-tuning the structures and properties of coordination nets. Specifically, CH₃SCH₂CH₂S- and (S)-CH ₃(OH)CHCH₂S- (2-hydroxylpropyl) were each attached as secondary groups to the 2,5- positions of 1,4-benzenedicarboxylic acid (bdc), and the resultant molecules (L1 and L2, respectively) were crystallized with Pb(II) into the topologically similar 3D nets of PbL1 and PbL2, both consisting of interlinked Pb-carboxyl chains. While the CH₃S- groups in PbL1 are not bonded to the Pb(II) centers, the hydroxy groups in PbL2 participate in coordinating to Pb(II) and thus modify the bonding features around the Pb(II), but only to a slight and subtle degree (e.g., Pb-O distances 2.941-3.116 A). Interestingly, the subtle change in structure significantly impacts the properties, i.e., while the photoluminescence of PbL1 is yellowish green, PbL2 features bright white emission. Also, the homochiral side group in PbL2 imparts significant second harmonic generation, in spite of its seemingly weak association with the main framework (the NLO-phore). In a broad perspective, this work showcases the idea of secondary group participation (SGP) in the construction of coordination networks, an idea that parallels that of hemilabile ligands in organometallics and points to an effective strategy in developing advanced functions in solid state framework materials.

Full text of DOI:10.1021/ja2073559

Article
pubs.acs.org/JACS
White Light Emission and Second Harmonic Generation from
Secondary Group Participation (SGP) in a Coordination Network
Jun He,†,§ Matthias Zeller,‡ Allen D. Hunter,‡ and Zhengtao Xu*,†
†Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China
‡Department of Chemistry, Youngstown State University, One University Plaza, Youngstown, Ohio 44555, United States
S
* Supporting Information
ABSTRACT: We describe a white emitting coordination network solid that
can be conveniently applied as a thin film onto a commercial UV-LED lamp
for practical white lighting applications. The solid state material was discovered
in an exercise of exploring molecular building blocks equipped with secondary
groups for fine-tuning the structures and properties of coordination nets.
Specifically, CH3SCH2CH2S- and (S)-CH3(OH)CHCH2S- (2-hydroxylprop-
yl) were each attached as secondary groups to the 2,5- positions of 1,4-
benzenedicarboxylic acid (bdc), and the resultant molecules (L1 and L2,
respectively) were crystallized with Pb(II) into the topologically similar 3D
nets of PbL1 and PbL2, both consisting of interlinked Pb-carboxyl chains.
While the CH3S- groups in PbL1 are not bonded to the Pb(II) centers, the hydroxy groups in PbL2 participate in coordinating to
Pb(II) and thus modify the bonding features around the Pb(II), but only to a slight and subtle degree (e.g., Pb−O distances
2.941−3.116 Å). Interestingly, the subtle change in structure significantly impacts the properties, i.e., while the
photoluminescence of PbL1 is yellowish green, PbL2 features bright white emission. Also, the homochiral side group in
PbL2 imparts significant second harmonic generation, in spite of its seemingly weak association with the main framework (the
NLO-phore). In a broad perspective, this work showcases the idea of secondary group participation (SGP) in the construction of
coordination networks, an idea that parallels that of hemilabile ligands in organometallics and points to an effective strategy in
developing advanced functions in solid state framework materials.
build up the net, while the latter remain nonbonded, and serve
instead as free-standing functions decorating the host net.
In contrast to the above “orthogonal” approach, here we play
on the idea of utilizing secondary group participation (SGP) in
bonding to the metal centers, so as to fine-tune the
coordination spheres of the metal centers and to impact the
material properties. The goal is to explore secondary groups
that bind to the metal centers, but in a rather weak and subtle
fashion, so as to avoid disrupting the structural integrity of the
scaffold set up by the primary links. In a cross-cutting light, the
idea of SGP closely parallels that of hemilabile ligands in
homogeneous metal-centered catalysis3 − these are hybrid
ligands containing two sets of donors: one binds strongly to the
metal center, whereas the other does so weakly and reversibly.
The latter may therefore pop off to allow the metal center to
take up the substrate and hop back on to stabilize a reactive
intermediate, thus facilitating the catalytic process. This imagery
of a hemilabile donor coming on and off the metal center is
suggestive of a potential solution to the topical issue of
achieving accessible metal sites in coordination networks.2f,4
Namely, SGP could serve to temporarily block a coordination
site on the metal center (while the main framework is being
INTRODUCTION
■
The growing field of coordination networks1 provides great
opportunities for interfacing molecular chemistry with the
development of functional framework materials. Thanks to the
advances in the tectonic/building block approach for network
design and construction, the metric scale and topological
features of the framework can now be reasonably well
correlated with the molecular size and geometry, with a large
body of porous structures achieved as a result. The major
forefront is now shifting toward functionalizing the host
network by means of even closer interaction with molecular
chemistry. A large, attractive exercise would be to implant in
the organized framework matrix the structure and reactivity
motifs established from the solution regime of molecular
chemistry, in order for a close interplay to take root, and to
generate new directions for the science of solid state materials.
The use of secondary groups to modify molecular building
blocks illustrates this megatrend of research.2 By affixing
secondary groups to a prototypal building block, one stands to
couple the rich variety of organic functions to the prospective
host network. A productive approach is to choose secondary
groups that refrain from disruptive metal complexation that
interferes with the primary groups in setting up the host
coordination net. In other words, a clear “orthogonality” or
“division of labor” between the primary and secondary groups is
usually hoped for, i.e., the former bind to the metal centers and
Received: August 5, 2011
Published: December 13, 2011
© 2011 American Chemical Society
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assembled) and later on open up to accommodate the guest
donors.
As a preliminary advance in utilizing SGP for functionalizing
framework materials, we here illustrate how the subtle bonding
effect from SGP could significantly impact the solid state
properties. Reported below is a comparative study on two PbII-
based nets from molecules H2L1 and H2L2 (Scheme 1): PbL1
benzenedicarboxylic acid (H4DMBD) (see also the SI).5
Compound 1 (composition: Pb·L1) was formed by reacting
L1 with Pb(NO3)2 in acetonitrile and water (1:1, v/v) under
solvothermal conditions. X-ray single crystals of 1 (in
centrosymmetric space group C2/c, No. 15) feature chains of
the Pb-carboxyl component integrated into a 3D network by
the aromatic cores of the L1 molecules (Figure 1). The
Scheme 1
and PbL2 (henceforth 1 and 2, respectively). Although 1 and 2
feature similar network topologies (based on the PbII−
carboxylate interactions), the side chains impart a subtle yet
significant difference. In 1, the methylthio unit on the side
chain is not associated with the PbII ion; while in 2, the hydroxy
group of the homochiral side chain is in contact with PbII and
generates, within a single, monolithic structure, the functional
combination of white light photoluminescence and second
harmonic generation (SHG).
EXPERIMENTAL SECTION
The general procedures, as well as the synthetic procedures for L1 and
L2 are included in the Supporting Information (SI).
■
Crystallization of 1. L1 (3.8 mg, 10.0 μmol) and Pb(NO3)2 (5.2
mg, 15.7 μmol) were loaded into a heavy-wall glass tube (soda lime,
10mm OD, 6mm ID), and then a solution of water and acetonitrile (1
mL, 1:1, v/v) was added. The tube was then flame-sealed and heated
at 100 °C in a programmable oven for 48 h, followed by slow cooling
(0.1 °C/min) to room temperature, during which yellow prism single
crystals suitable for single-crystal X-ray diffraction were formed (4.8
mg, 83% yield based on L1). Chemical analysis of the product
C14H16O4PbS4 yielded the following: calcd [C (28.81%), H (2.76%)];
found [C (28.92%), H (2.61%)]. IR (ν/cm−1): 3433 (w), 2970 (w),
2908 (m), 2819 (w), 1523 (s), 1429 (w), 1383 (s), 1320 (w), 1259
(m), 1203 (w), 1091 (w), 986 (w), 904 (w), 845 (s), 788 (s), 691 (w),
621 (m), 567 (w), 512 (w). X-ray powder diffraction of the bulk
sample indicated a pure phase consistent with the single-crystal
structure (Figure S1). Compound 1 is insoluble in common organic
solvents or water.
Crystallization of 2. A mixture of L2 (3.5 mg, 10.0 μmol),
Pb(NO3)2 (5.0 mg, 15.1 μmol) and dimethylformamide (DMF)−H2O
(1 mL, 2:1, v/v) was sealed and heated at 85 °C in a programmable
oven for 48 h, followed by slow cooling (0.1 °C/min) to room
temperature. Colorless plate-like single crystals suitable for single-
crystal X-ray diffraction were formed (4.7 mg, 85% yield based on L2).
Chemical analysis of the product C14H16O6PbS2 yielded the following:
calcd [C (30.48%), H (2.92%)]; found [C (30.33%), H (3.03%)]. IR
(ν/cm−1): 3378 (m), 2970 (m), 2960 (m), 2916 (m), 2858 (w), 1551
(s), 1460 (w), 1385 (s), 1265 (m), 1113 (m), 1093 (w), 1030 (w),
939 (w), 853 (m), 846 (m), 788 (w), 749 (w), 650 (m), 590 (w), 505
(w). X-ray powder diffraction of the bulk sample indicated a pure
phase consistent with the single-crystal structure (Figure S2).
Compound 2 is insoluble in common organic solvents or water.
Figure 1. Crystal structure of 1. (A) Local coordination environment
around the Pb2+ ion (only the first S and C atoms are shown for the
side chain. (B) View of the 3D coordination net along the c axis. H
atoms and disordering of side chain atoms are omitted.
asymmetric unit of the unit cell of 1 consists of one-half of a
Pb2+ ion (situated at the special position 0, 1.02, 1/4) and one-
half of the centrosymmetric L1 molecule. Each Pb2+ ion is
coordinated to six carboxyl O atoms (from four L1 molecules;
Pb−O distances: 2.486, 2.551, and 2.686 Å) and two S atoms at
significantly longer distances of 3.159 and 3.403 Å. The
coordination geometry is rather lopsided, with all O and S
donors confined mostly in one hemisphere, indicating
significant 6s2 lone pair effect, which can be rationalized by
mixing the 6s orbital with the 6p states (on the metal) as well as
the 2p states from the ligand O atoms.6 Notice that only the S
atom right next to the carboxyl group is in contact with the
Pb(II) ion, the other one further down the side chain (that of
the −SCH3) is not bonded, and remains free-standing. Of the
two crystallographically inequivalent O atoms, O1 is shared by
two Pb ions at 2.551 and 2.686 Å (while O2 is bonded to one
single Pb ion at 2.486 Å), and each pair of adjacent Pb ions are
bridged by two symmetry-related O1 atoms (Pb−Pb distance:
4.294 Å, see also see Figure 1). The overall feature of rod-
packing in 1 is therefore similar to the reported MOF-70
(PbBDC; BDC: 1,4-benzenedicarboxylate)7 and PbTMBD
[TMBD: tetrakis(methylthio)-1,4-benzenedicarboxylate].4e
RESULTS AND DISCUSSION
■
Syntheses and Structural Studies. Both molecules L1
and L2 can be conveniently prepared from 2,5-dimercapto-1,4-
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Compound 2 was obtained from solvothermally reacting L2
and Pb(NO3)2 in dimethylformamide and water (2:1, v/v) as
colorless, plate-like crystals. This structure also builds on a
similar rod-packing motif with the Pb-carboxylate rods linked
by the aromatic moieties into a 3D net (Figure 2). However,
H-bonding might also modulate the photoluminescence
processes (e.g., the vibrational relaxation and lifetime of the
excitation states), its precise role remains unclear to us at this
stage. The H-bonding to the carboxyl groups also further
polarizes the O−H bonds and draws them near the Pb centers,
which, we suspect, might have served to promote the
coordination to the Pb2+ ions. By comparison, similar factors
are absent in compound 1 to facilitate the bonding of CH3S-
groups to the Pb2+ ion. Moreover, the thioether S is a soft and
relatively weak donor,8 while the Pb2+ ion, as a borderline hard/
soft acid (e.g., chemical hardness ηA values: Hg2+, 7.7; Pb2+, 8.5;
Zn2+, 10.8),9 apparently does not make for stronger affinity for
the thioether S atom, in other words, a softer species, like the
Hg2+ ion, would have been more prone to bond with the
thioether group.
Photoluminescence Properties. White light emitting
materials and devices (e.g., light emitting diodes, LED)10 are
of great technological importance for next-generation solid state
lighting and display applications. In this regard, single-
component white light emitters offer advantages for fabrication
and processing, as these circumvent the often complex
procedures of mixing different phosphors to generate the
color compositions for the desired “whiteness” (e.g., a blending
of blue and yellow). Coordination/hybrid networks offer rich
opportunities for developing single-component white light
emitters,11 mainly because the ligand-centered, charge transfer
as well as metal-centered emission processes can potentially all
be modulated in their relative intensities and wavelength
positions, in order for the overall emission to attain the white
light quality. In this vein, main group metals like lead and
bismuth11g,12 are of interest because (1) their ns2np0
configuration and the stereochemically active lone pairs often
give rise to substantial photoluminescent processes involving
the metal centers, contrasting, for example, the Zn2+-based
MOF-5 systems in which the photoluminescence has been
found to be mostly ligand-based;5c,13 (2) as heavy p-block
elements, they tend to display flexible coordination behaviors,
e.g., variable bonding distances, with secondary interactions
furnishing a 7-fold (or higher) coordination configuration. Such
flexible coordination is exemplified in the Pb−S and Pb−O
secondary interactions in 1 and 2, and provides an especially
handy medium to engage the secondary groups (i.e., via SGP)
in the structural and property modifications of the main
framework.
Under UV radiation at RT, the solid sample of 2 exhibits a
bright, white luminescence, whereas the emission from 1
appears yellowish green. For comparison, we first present the
photoluminescence spectra of the corresponding free ligands of
L1 and L2 in THF solutions. As shown in Figure 3, L1 and L2
display similar emission spectra with each consisting of a broad
band that peaks around 450 nm (451 nm for L1, 456 for L2,
with excitation wavelength λex = 370 nm), and the emissions are
blue in color. Such emissions observed of the free ligands in
solutions can be assigned to the typical ligand-centered
transitions (e.g., π* → n, π* → π).
The ligand-centered emission features are carried over into
the room-temperature emission spectra of 1 and 2 (Figure 4),
and they correspond to the high energy (HE) bands with
emission maxima at 468 and 459 nm, respectively. Substantial
differences, however, exist in the low energy (LE) bands of the
emission spectra. Specifically, 1 features a dominant and broad
emission band which peaks at 531 nm (the green region),
resulting in an overall emission color of yellowish green for 1.
Figure 2. Coordination environment of the Pb2+ ions in 2 with atom
labeling (a), and view of the 3D net of 2 along the a axis (b). H atoms
are omitted.
the homochiral 2-hydroxypropyl side chain of L2 imposes a
chiral space group (P21; No. 4), with the asymmetric portion of
the unit cell comprising two Pb2+ centers and two L2 molecules
(Figure 2a, see Figure S3 for more detailed atom labeling). The
hydroxy side chains also impacts the local bonding environ-
ments of the Pb(II) ions. First, the S and O atoms from each
side chain are bonded (chelated) to a common Pb ion
[distances for the 4 side chains: (O6−Pb2, 2.941; S1−Pb2,
3.261 Å), (O12−Pb2, 3.116; S4−Pb2, 3.120 Å), (O5−Pb1,
3.010; S2−Pb1, 3.416 Å), (O11−Pb1, 2.984; S3−Pb1, 3.182
Å)], whereas one of the bridging carboxyl O−Pb bonds on each
Pb ions is substantially elongated (O1−Pb1, 3.376; O3−Pb2,
3.347 Å; the other five O−Pb1 distances: 2.480, 2.579, 2.457,
2.636, 2.863; the other five O−Pb2: 2.497, 2.533, 2.482, 2.735,
2.583 Å). Notice also that the side chain and its ortho-carboxyl
neighbor are bonded to two adjacent, separate Pb ions. In
comparison with the O−Pb distances (2.486, 2.551, 2.686 Å) in
1, the overall carboxyl-Pb interaction in 2 appears to be
significantly weakened, and the adjacent Pb ions in 2 become
farther apart at 4.785 Å (i.e., the −OH serves to push the
carboxyl groups away from the Pb2+). Because the photo-
luminescence process is closely related to the electronic
interaction across the Pb2+ centers and the organic π−systems,
the weakened carboxyl−Pb interaction in 2 might be related to
the distinct white color of the photoluminescence of this
material (as will be discussed again below).
The hydroxy (−OH) groups also form significant hydrogen
bonds with the carboxylate O atoms O12−O3, 3.110; O11−
O10, 2.889; O6−O4, 2.793; O5−O2, 2.697 Å. Even though the
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the HE emission band increases with the excitation wavelengths
changing from 300 to 360 nm, and decreases from 360 to 420
nm; but no apparent changes occur on the LE band.
Nevertheless, the overall emissions of 2 remain well within
the white domain of the chromaticity diagram throughout the
excitation wavelength range from 300 to 400 nm (Figure 5).
The remarkable persistence of the white light feature observed
Figure 3. Room temperature emission spectra of L1 and L2 in THF
(5 × 10−5 M, λex = 370 nm).
Figure 5. CIE-1931 chromaticity diagram and the positions (marked
by the crosses) for emissions of 1 (λex = 365 nm) and 2 [irradiated by
300 (a), 350 (b), and 400 nm (c)].
of 2 is rare, and provides advantages for lighting applications.
Measurements on the powder sample of 2, using the
integration sphere setup,14 indicates a fluorescence quantum
efficiency on the order of 2−3%. Notice though that the
integration sphere method tends to underestimate the actual
value because of reabsorption of the emitted light by the
sample;14c,15 the underestimation could have been especially
significant for 2, which exhibits strong overlap between the
emission and absorption, e.g., between 450 and 550 nm (see
more in the SI, e.g., Figure S4). Nevertheless, the quantum
efficiency found here is relatively low (even though the
photoluminescence appears quite bright to the eye, e.g., see
Figure 6), being comparable to a white emitting nanocrystal
sample of CdSe.16
The colors of the emissions can be better quantified with the
CIE (Commission Internationale de l′Eclairage) chromaticity
Figure 4. Room temperature emission spectra of (a) 1 in the solid
state (λex = 365 nm) and (b) 2 in the solid state under the excitation of
300, 340, 360, 400, and 420 nm.
By comparison, the LE emission band of 2 becomes weaker
than its HE band, with a “fat” tail spreading well into the red
region beyond 600 nm. The major upshot is a decrease of the
green contribution in the emission spectra of 2, leading to a
distinct white emission.
As also shown in Figure 4, the emissions of 1 and 2 respond
differently to the variation of excitation wavelengths. For
excitation wavelengths between 320 and 420 nm, the emission
features of 1 stay mostly unchanged. As for 2, the intensity of
Figure 6. Photographs of LEDs: (A) a 3 mm reference UV LED (not
turned on, available from Le Group Fox, Inc.); (B) the same UV LED
turned onthe emission has a blue tinge; (C) the same LED coated
with a thin layer of compound 2 (not turned on); (D) the coated LED
turned on and illuminating bright white light.
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diagram, which serves to specify how the human eye perceives
light with a given spectrum.17 For this, the spectrum is
deconvoluted into three functions related to the primary color
components of red, green and blue. The (three) coefficients for
the functions are then normalized (this leaves out the
brightness factor) to give the two independent variables x
and y. The coordinates (x, y) thus derived specify the
chromaticity (color) of the spectrum. As seen in Figure 5,
the chromaticity coordinates for 1 (spectrum of Figure 4a) are
(0.31, 0.42), which is located in the yellowish green gamut. By
comparison, the coordinates for the emissions of 2, when
excited by 300, 350, and 400 nm light are (0.27, 0.30; a), (0.25,
0.29; b), and (0.24 0.28; c), respectively, all falling within the
white gamut of the CIE-1931 color space chromaticity diagram
(Figure 5).
The LE bands centering around 531 and 515 nm in the
emission spectra of 1 and 2 can be attributed to ligand-to-metal
charge transfer (LMCT) between the aromatic π systems and
the p orbitals of the Pb2+ centers, whereas the tailing features
beyond 600 nm are suggestive of metal-centered transitions
involving the s and p orbitals of s2-metal cluster com-
pounds.12f−l The white light emission in 2 therefore stems
primarily from its LMCT being significantly reduced relative to
the ligand-centered transitions; whereas for 1, the LMCT
appears to be more efficient and its emission band becomes
stronger than that of the intraligand transitions. In this light, the
weakened LMCT in 2 relates well to its weaker Pb2+−carboxyl
interactions that are caused by the perturbation of the −OH
groups (see above discussion of the crystal structures). By
comparison, in 1, the Pb2+−carboxyl interactions are stronger,
which might have led to more effective electronic coupling
between the aromatic π systems and the metal center. In other
words, the intraligand excitation in 1 is therefore more
effectively diverted and channeled to the metal center to
generate the dominant LMCT feature.
Emission spectra taken at the lower temperature of 77 K are
also informative (Figure S5). Specifically, the intraligand
transition bands (around 450 nm) are largely diminished
both for 1 and 2, suggesting that the intraligand excitation is
more effectively channeled to the metal center at low
temperature (presumably because less thermal motion exists
to perturb such channeling). While the residual intraligand
emission of 1 at 77 K becomes hardly observable, that of 2 is
more significant. The relative intensity of the intraligand bands
observed here is in line with the rt data in Figure 4 and also
points to more efficient LMCT in 1. In addition, like at rt, 1
continues to show a LMCT band at the longer wavelength
(peaking at 540 nm), compared with the 520 nm for 2. The
longer wavelengths of the LMCT in 1 (both at rt and 77 K)
appear to be correlated with its stronger Pb-carboxyl interaction
(as compared with the case of 2), which is also consistent with
an earlier observation that stronger Pb−O linkages in similar
Pb-carboxylate systems are associated with larger red shifts of
the related emission bands.12e
The photoluminescent processes in coordination networks
are, however, complex, and the emission characteristics (e.g.,
position and intensity of the peak) can be greatly influenced by
the structures and conformations (e.g., rigidity) of the
molecules. The recently reported Bi(III) and Pb(II) pyridine-
2,5-benzenedicarboxylate networks, for example, exhibit MLCT
bands that, unlike the current systems, are instead blue-shifted
relative to the ligand emissions.11g Further electronic structure
calculations are needed for elucidating the structure−property
correlations in these and related photoluminescent networks.
Along this direction, using side chains as secondary functions
offers unique opportunities for systematically modifying and
fine-tuning the prototypal framework structure, and for
probing, within the matrices of structurally well-defined
crystalline frameworks, how structural changes impact photo-
luminescent properties.
Fabrication of White Light LED. By a simple and efficient
dip coating procedure, a thin film of compound 2 can be
applied onto a commercially available UV-LED lamp (similar to
that of Ki and Li11b), and thus accomplish the conversion into a
new LED that emits bright white light (Figure 6). Specifically,
an as-made crystalline sample of 2 (about 10 mg) was loaded
into an agate mortar and manually ground with the pestle for
20 min to afford a fine powder. Ethanol (1.0 mL, ABS) was
then added to the mortar. Further grinding (for about 1 min)
was applied until a significant portion of the alcohol has
evaporated and a homogeneous slurry was achieved. A
commercial 360 nm UV-LED lamp (Le Group Fox, Inc.) was
then dipped into the slurry, held therein for several seconds,
and then taken out for the coating to dry in air. This dip coating
process can be repeated to further ensure an even and
continuous coating of 2 on the lens/housing of the LED. The
performance of the white light LED thus fabricated is stable for
months in air and the white color quality remains consistent
over a wide temperature range (e.g., −20 °C to rt). This simple
experiment serves to demonstrate the use of compound 2 in a
practical lighting application.
Quadratic Nonlinear Optical Properties. Progress in the
crystal engineering of coordination networks provides helpful
insight into the control of acentric assembly of polar and
multipolar organic molecules, so as to impact the diverse
properties of second harmonic generation, piezoelectrics,
pyroelectrics and ferroelectrics.18 Considerable efforts have
been devoted to addressing the intriguing issue of assembling
nonchiral molecules into acentric networks. For example, Ward
and co-workers devised guanidinium sulfonate (GS) sheets to
orient the pillar groups in polar order.19 Another notable
approach builds on the interplay of dipolar linkers and
interpenetration, e.g., an odd-fold interpenetration of diamond-
oid nets is perforce acentric.18c Along this line, we have
explored star-shaped molecules as building blocks that
effectively preclude interpenetration, so as to preserve the
acentricity of 3D nets.20 Subsequent developments include the
construction of octupolar networks21 and the use of backfolded
molecules to induce acentric features in solid state networks.22
A commonality of these ideas is to focus on the backbone of
the molecule, and to design its shape and function so as to
induce acentricity on the main grid of the framework. Quite
often, one aims for frameworks featuring connectivities
(topologies) that are inherently acentric (e.g., the diamondoid
or gyroid net), and is thus limited to frameworks as such. By
comparison, by attaching chiral side groups to the molecular
backbone, one can hope to relay the chiral character to the
main grid, and thus work to include networks of acentric
topologies in their prototypal forms. However, a key question
remains: could the acentric information from the side groups be
effectively imparted to the main grid, so as to achieve, e.g.,
significant quadratic NLO properties? For this, close inter-
actions with the main frame as effected by SGP is especially
pertinent. The structure of 2, with the homochiral 2-
hydroxypropyl side chain in well-defined contacts with the
Pb(II) centers of the main framework, provides an opportunity
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to probe the impact of the side chain chirality upon the main
framework.
AUTHOR INFORMATION
Corresponding Author
*zhengtao@cityu.edu.hk
■
The homochiral side chains impose the polar space group
P21 on the crystal structure of 2. However, the structure
exhibits a pseudoinversion center (located at 0.242, 0.249, and
0.490) with an 81% fit for the space group P21/c (P21/a in the
used setting). Specifically, the main grid and most of the side
chain atoms fit the pseudoinversion symmetry; the only
nonfitting atoms are the terminal O and C atoms on the side
chain (i.e., O11, O12, C25, and C28). With the framework
consisting of the more polarizable Pb-organic π structures as
the major NLO chromophore, it would be remarkable if the
very subtle structural influence from the side chains, so subtle
as to keep the formal inversion symmetry of the main frame
atoms, shall prove to be impactful on the NLO properties.
Measurement of the second harmonic generation (SHG) by
the Kurtz powder method18a,23 found that compound 2 is
nonphase-matchable and exhibits a significant SHG efficiency
about 40 times larger than α-SiO2 (for 45−63 μm particle size,
see also Figure S6). Such preliminary results suggest effective
impact from the chiral side chain, in spite of the
pseudoinversion symmetry of the main framework. Because
the solid of 2 also features significant emission intensity around
532 nm, a contribution from photoluminescence to SHG
efficiencies can not, however, be completely ruled out at this
stage. Further experimental studies based on the systematic and
flexible modification of the secondary groups shall unveil more
cases of SGP-induced acentric properties in coordination
networks, in order to test the generality of this approach in
preparing acentric materials, and to provide a basis for
establishing the structure−property correlation thereof.
Present Address
§Faculty of Chemical Engineering and Light Industry,
Guangdong University of Technology, Guangzhou 510006,
Guangdong, China.
ACKNOWLEDGMENTS
■
We thank Drs. P. Shiv Halasyamani and Jeongho Yeon at the
University of Houston for conducting the SHG measurement
and analysis for compound 2. This work is supported by the
Research Grants Council of the Hong Kong Special
Administrative Region [Project 9041437 (CityU 103009) and
Project CUHK2/CRF/08] and City University of Hong Kong
(Project No. 7002590). The single crystal diffractometer was
funded by NSF Grant 0087210, by the Ohio Board of Regents
Grant CAP-491, and by YSU. We thank Mr. Chen Yang and
Mr. Kai Li from Prof. Chi-Ming Che’s group at HKU for
assistance with the quantum yield measurement and Dr.
Zhengqing Guo for photographing the LEDs.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Additional experimental procedures, full crystallographic data in
CIF format, X-ray powder diffraction patterns, low-temperature
emission spectra and diffusion reflectance spectra for 1 and 2,
quantum yield measurement procedures, second harmonic
generation (SHG) curve, and TGA plot of 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Ed. 2008, 47, 7263. (i) Ma, S.; Sun, D.; Simmons, J. M.; Collier, C. D.;
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