Communications
tions, thus induces chirality, acentricity, and further octupo-
larity of the infinite network.
independent of guest removal and uptake. Moreover, 1a–1 f
exhibit high thermal and solution stability and are totally
transparent in the visible region, and this makes them
attractive candidates for future practical applications.
In conclusion, we have developed a rational synthetic
approach towards chiral octupolar NLO-active solids based
on 3D coordination networks by using chiral and C3-
symmetric tris(pyridylduryl)borane as multidentate ligand.
Current efforts are centered on optimizing the optical
responses of this type of materials by end-functionalizing
the arms of the octupolar units and making organoboron-
based assemblies with unique and practically useful opto-
electronic functions.
Single-crystal X-ray structure determinations revealed
that 1c, 2, and 3 are all isostructural to 1b, while cell-
parameter determinations and powder X-ray diffraction
studies established that 1a, 1d–1 f are also isostructural to
1b. The phase purity of the bulk samples of 1–3 was
established by comparison of their observed and simulated
X-ray powder diffraction patterns (see Figure S7 and S8 in the
Supporting Information). Calculations with the PLATON
program[13] indicated that 1 has about 16% of total volume
occupied by solvent molecules. Thermogravimetric analysis
showed that guest molecules can be removed at 80 and 1308C,
and the frameworks are stable up to about 3808C. Powder
XRD experiments on 1b and 1c indicated that their frame-
works and crystallinity remain intact on complete removal of Experimental Section
Synthesis of 1–3: A mixture of MX2·nH2O (0.005 mmol) and L
guest molecules (see Figure S9 and S10 in the Supporting
Information).
(0.005 mmol) was placed in a small vial containing DMSO (1 mL),
CH3OH (2 mL), and toluene (0. 5 mL). The vial was sealed, heated at
808C for two days, and allowed to cool to room temperature. Crystals
suitable for X-ray diffraction were collected by filtration, washed with
diethyl ether, and dried in air. All products are stable in air and are
insoluble in water and common organic solvents. Crystals of 1a–1 f
are colorless, 2 is blue, and 3 is purple. Yield: 1a, 3.0 mg, 70%; 1b,
3.8 mg, 81%; 1c, 3.8 mg, 72%; 1d, 3.1 mg, 68%; 1e, 3.5 mg, 71%; 1 f,
3.5 mg, 78%; 2, 3.5 mg, 86%; 3, 3.3 mg, 80%. Elemental analysis
calcd (%) for C45H52BBr2CdN3O (1b): C 56.90, H 5.52, N 4.42; found:
C 56.11, H 5.48, N 4.39; IR (KBr): n˜ = 3432 (m), 2986 (w), 1610 (s),
1558 (w), 1420(m), 1394(s), 1300(w), 1260(m), 1218(m), 1068(s),
954(m), 862(m), 820 (m), 802 (m), 652 (m), 614 cmÀ1(w). IR and
analytical data for 1a, 1c–f, 2, and 3 can be found in supporting
information.
Kurtz powder SHG measurements were performed on 1
to confirm its acentricity and to evaluate its potential as a
second-order NLO material.[14] Compound 1a has a powder
SHG intensity of 15 versus a-quartz, which is about 1.5 times
higher than that of technologically useful potassium dihydro-
genphosphate.[15] To investigate the influence of anions on the
SHG response, 1b–1 f were studied. They showed powder
SHG intensities of 24, 35, 11, 20, and 17, respectively, versus
a-quartz (Figure 3). Clearly, the iodide exhibits the strongest,
Received: February 5, 2008
Published online: May 6, 2008
Keywords: boron · chirality · materials science ·
.
metal–organicframeworks · nonlinear optics
Figure 3. Comparison of the SHG intensities of a-quartz (set as unity)
and 1a–1 f.
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and the nitrate the weakest harmonic response. The harmonic
responses of 1 are evidently realized by noncentric arrays of
bridging organoboron skeletons through metal centers. The
anion-dependent NLO activities of 1 probably can be related
to different abilities of the anions to bind cations, which may
modulate charge transfer between metal centers and organo-
boron ligands in coordination networks to generate adequate
electronic asymmetry.[3a] Anion-responsive properties of
assembled materials have recently drawn much attention
but have not yet been well studied in MOFs.[16,17] Further
studies to elucidate the origin of the anion-tunable NLO
properties reported herein are underway.
The SHG measurements on freshly evacuated samples of
1b and 1c revealed intensities of 26 and 35, respectively,
versus a-quartz, both of which compare favorably with the
values of the pristine materials. This indicates that NLO
behaviors of the present frameworks are stable and are
1658; b) O. M. Yaghi, M. OꢀKeeffe, N. W. Ockwig, H. K. Chae,
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 4538 –4541