Nonlinear-Optical Behaviors of a Chiral Metal-Organic Framework Comprised of an Unusual Multioriented Double-Helix Structure

Article abstract of DOI:10.1021/acs.inorgchem.8b00671

We present here the synthesis of one enantiomeric pair of metal-organic framework materials comprised of a unique multioriented double-helix structure from an achiral spirocenter ligand. Our study clearly shows that the chiral MOF material encompasses concurrently multiple nonlinear-optical functions in the solid state: the noncentrosymmetric structural feature brings the chiral MOF high second-harmonic-generation efficiency; the incorporation of the spirocenter ligand can efficiently produce two-photon-excited photoluminescence with a larger-action cross-sectional value.

Full text of DOI:10.1021/acs.inorgchem.8b00671

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Nonlinear-Optical Behaviors of a Chiral Metal−Organic Framework
Comprised of an Unusual Multioriented Double-Helix Structure
†
,‡
†,‡
†
†
†
†
†
†
Xiaoli Huang, Qiyang Li, Xue Xiao, Shuping Jia, Yue Li, Zhigang Duan, Lei Bai, Ze Yuan,
†
,§
,†
Lin Li, Zhihua Lin,* and Yonggang Zhao*
†
Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation
§
Center for Advanced Materials (SICAM), and College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing
211816, P. R. China
*
S Supporting Information
31
data storage. Recent success in MOF-based NLO materials has
encouraged us to explore the noncentrosymmetric structures
with respect to their multiple optical nonlinearities.
ABSTRACT: We present here the synthesis of one
enantiomeric pair of metal−organic framework materials
comprised of a unique multioriented double-helix structure
from an achiral spirocenter ligand. Our study clearly shows
that the chiral MOF material encompasses concurrently
multiple nonlinear-optical functions in the solid state: the
noncentrosymmetric structural feature brings the chiral
MOF high second-harmonic-generation efficiency; the
incorporation of the spirocenter ligand can efficiently
produce two-photon-excited photoluminescence with a
larger-action cross-sectional value.
In the design of NLO materials, π-conjugated organic
molecules with low dielectric constants, variable spectral
characteristics, and fast NLO response occupy a prominent
position, among which molecules containing spirobifluorene
units are a promising class of conjugated systems. The rigidity of
the spirobifluorene units can increase the effective π-electron
conjugation and suppress nonradiative deactivation pathways,
leading to high photoluminescence (PL) quantum yields.
Moreover, the chemical versatility of (spiro)molecules allows
modulation of the π-conjugation length within a multipolar
structure for enhanced polarization of charge delocalization,
he development of materials with nonlinear-optical (NLO)
which is beneficial for the NLO performance. 3
2−34
T
functions continues to receive researchers’ attention
because of the key role of NLO materials in laser and biomedical
By employing an achiral spirocenter ligand,
4,4′,4′′,4‴-
(9,9′-spirobifluorene-2,2′,7,7′-tetrayl)tetrabenzoic acid
1
−3
35,36
imaging technologies.
Although there has been tremendous
(H L),
we report one enantiomeric pair of 3D MOFs
4
interest in the construction of NLO materials, such as organic
chromophores, polymers, semiconducting materials, and quan-
tum dots, the goal of manipulating the NLO properties of the
materials is still of great challenge. In the last 2 decades, extensive
research on metal−organic frameworks (MOFs) have shown
that they possess numerous advantages over conventional
materials, including, but not limited to, structural tunability,
stability, cost-efficient fabrication, and ease of synthesis on a large
(IAM-1M/1P; IAM = Institute of Advanced Materials)
comprised of unusual double-helix structure in this study.
Associated with the noncentrosymmetric feature of the structure,
chiroptical properties such as circular dichroism (CD), together
with the high SHG activity, are described. Moreover, enhanced
two-photon-excited fluorescence (2PEF) is also observed for the
MOF material compared with its spirobifluorene-based ligand.
Structural analysis revealed that the single crystals in one batch
of IAM-1 contain 1M and its enantiomorph 1P, with chiral space
4
−7
bulk scale.
In particular, their optical properties can be
groups of P3 21 and P3 21, respectively. IAM-1 is constructed
systematically tailored by suitable choices of the metal and ligand
constituents, from which the performance of the MOFs can be
1
2
from dinuclear zinc(II) building units with four bridging ligands
Figure 1). A remarkable feature of IAM-1 is their double-helix
8
−11
(
significantly improved to enable viable NLO applications.
arrangements. The spirocenter lignads are interconnected
through the dizinc(II) units to generate the left-handed
double-helical chain in 1M and the right-handed helix in 1P,
with pitches of 16.992(3) Å for 1M and 17.048(7) Å for 1P
To date, intensive studies have been performed on MOF-
based NLO materials, particularly in the second harmonic
1
2−14
generation (SHG).
In principle, noncentrosymmetric
organization is a prerequisite for SHG processes in a bulk
material. To produce noncentrosymmetric MOFs with high
SHG, several strategies have been successfully used, including the
(
1
Figure 2). The most striking feature of the structure is that IAM-
is comprised of three structurally identical double-helical chains
with the same handedness but different orientations along the (1,
, 0), (0, 1, 0) and (1, 1, 0) axes, respectively (Figure 1b). All
three helical chains are parallel to the ab plane and propagate
1
5,16
17,18
use of chiral ligands,1
construction of diamondoid
or
9,20
21,22
0
octupolar structures,
utilizing the structural defects,
23
control of the degree of interpenetration, etc. Compared with
the well-documented SHG-active frameworks, MOFs with
simultaneous two-photon or multiphoton absorption by the
along different directions orthogonal to a 3 screw axis. The
1
double-helical chains with the same orientation are parallel to
each other in the ab plane, while the neighboring double-helical
2
4−27
incorporation of NLO-active organic linkers
encapsulation of suitable dyes in the voids of the MOFs
have only recently been reported and have shown potential
or the
2
8,29
Received: March 16, 2018
30
applications in lasing, three-dimensional (3D) patterning, and
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00671
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Figure 1. View of the spirobifluorene-based ligand and dizinc(II) building unit in IAM-1M (color code: Zn, turquoise; O, red; C, pale gray; N, blue) and
the multioriented double-helical chains along the a axis (a) and c axis (b), in which lime, pale-blue, and lavender represent three helical chains along the
(
1, 0, 0), (0, 1, 0), and (1, 1, 0) axes, respectively. H atoms were omitted for clarity.
Figure 2. Left-handed (M) and right-handed (P) double-helical motifs
composed of achiral ligands in the enantiomorphs IAM-1M and IAM-
1
P, respectively. H atoms were omitted for clarity.
chains with different orientations are alternately arranged and
coupled to each other by sharing the dizinc(II) units, giving rise
to a 3D framework of IAM-1. Notably, because of the same
handedness of the three helical chains, the whole structure is
homochiral. To the best of our knowledge, the previously
reported single- or double-helical MOF structures are all
comprised of helixes in a parallel fashion, whereas the structure
of IAM-1 encompassing concurrently multioriented but
structurally identical double-helical chains has not been reported
before. Moreover, additional independent equivalent frame-
works are present in IAM-1 in order to stabilize the whole
structure, affording a 2-fold interpenetrating framework. It is
worth noting that, because of the interpenetration, a comparably
dense packing of the organic linker in IAM-1 was achieved, with a
Figure 3. (a) CD spectra of IAM-1M and IAM-1P and UV−vis
absorption of a bulk sample of IAM-1 (blue) in N,N-dimethylforma-
mide. (b) Solid-state excitation and fluorescence spectra of H
L and
4
IAM-1.
−3
chromophore linker concentration of 0.86 mol dm . From a
topological point of view, both the dizinc(II) unit and the ligand
can be reduced to the 4-connected node to give a 4,4-connected
enantiomeric form (M) and its opposite form (P) in a ratio of
nearly 50:50.
As is well-known, spirobifluorene derivatives are a class of
efficient blue-light-emitting materials. The one-photon-excited
fluorescence spectra show strong blue emissions for the ligand
2
4
network with qzd topology and the point symbol of {4 8 }.
To verify the occurrence of spontaneous resolution during the
crystal growth, CD spectra were performed. A total of 20
randomly picked individual crystals from one batch were tested.
As anticipated, CD spectra showed the opposite Cotton effect
H L around 428 nm and IAM-1 around 418 nm in the solid state
4
(Figure 3b), with the absolute quantum yields (Φ ) of 33.62%
a,f
and 46.67%, respectively, suggesting excellent PL characteristics
for both.
(
positive or negative) varying from crystal to crystal, indicating
that either 1M or 1P formed under spontaneous resolution
Figure 3a). The CD spectral data reveal that IAM-1 crystallizes
as a racemic mixture of enantiomerically pure crystals of one
In view of the noncentrosymmetric structural feature of IAM-
1, its NLO behavior was studied with the use a confocal laser
scanning microscope equipped with a Ti:sapphire laser (1064
(
B
DOI: 10.1021/acs.inorgchem.8b00671
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
nm output). The SHG efficiency of IAM-1 was measured using
the Kurtz−Perry method at room temperature. Using a 1064
nm laser source through the treated microcrystalline sample
perylene as a standard sample at the same laser wavelength and
37
power. As shown in Figure S15, the ησ value for IAM-1 was
2
found to be in the range of 2.13−6.78 GM with a maximum at
(
particle sizes of 75−150 μm), it was found that IAM-1 displays a
720 nm, which is comparable with the values reported for NLO
24
strong SHG efficiency (Figure 4), which is about 0.56 times that
MOF materials and about 2 times larger than that of the ligand.
This difference between IAM-1 and its ligand can mainly be
attributed to their different fluorescence efficiency in the solid
state, together with the high packing density of the NLO
26
chromophore linker in the framework. Furthermore, the third-
order two-photon upconversion was confirmed by the slope of
PL plotted as a function of the excitation intensity on the log-to-
log scale, which was found to be ≈2.
In conclusion, using an achiral spirocenter ligand, we reported
a chiral 3D MOF material, IAM-1, comprised of an extraordinary
multioriented double-helix structure in this work. The
enantiomeric pairs (1M and 1P) assembled by spontaneous
resolution have been successfully separated. This study shows the
successful integration of multiple NLO behaviors within one
chiral MOF structure: the noncentrosymmetric feature of the
structure brings the chiral MOF high SHG efficiency; the
incorporation of the spirocenter ligand into the framework can
efficiently produce two-photon-excited PL with a larger-action
cross-sectional value compared with the organic linker.
Figure 4. SHG spectra of IAM-1. Inset: Particle size versus SHG
intensity for IAM-1 showing the PM curve.
of LiNdO in the same particle size, that is, an efficiency of 21
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00671.
3
■
times that of KDP and 336 times that of α-quartz. To accurately
investigate the effective SHG of IAM-1, measurements as a
function of the particle size were performed. As shown in Figure
S
*
4, the SHG efficiency of IAM-1 keeps increasing as the particle
size increases, indicating that the material is phase-matchable
1
1
Synthetic routes to the ligand, powder X-ray diffraction,
thermogravimetric analysis, structure analysis, CD, PL,
SHG, and 2PEF spectra and analysis, and crystal data
(
PM). By using the well-known PM material LiNbO as the
3
reference, the effective SHG (d ) of IAM-1 was determined to
eff
−1
be 1.64 pm V , that is, an efficiency of 0.37 times that of LiNbO ,
3
(PDF)
which is concordant with the comparison made on the SHG
intensity.
Accession Codes
A confocal laser scanning microscope was subsequently used
to study the 2PEF of IAM-1 and its ligand at room temperature.
Figure 5 clearly demonstrates bright upconverted PL when IAM-
CCDC 1817958−1817959 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1
EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
E-mail: linzh@njtech.edu.cn (Z.L.).
E-mail: iamygzhao@njtech.edu.cn (Y.Z.).
■
*
*
ORCID
Yonggang Zhao: 0000-0002-0357-382X
Author Contributions
‡
These authors contributed equally.
Notes
The authors declare no competing financial interest.
Figure 5. Two-photon-excited emission spectra of IAM-1 depending on
different excitation wavelengths. The inset shows the two-photon-
excited PL intensity dependence on the laser intensity at 720 nm.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (Grants 21471079 and 21501092)
and the Natural Science Foundation of Jiangsu Province for
Youth (Grant BK20140928).
1
was scanned and irradiated by a tunable femtosecond laser
ranging from 690 to 770 nm, which corresponds to the 2PEF
spectra. PL reached a maximum efficiency when excited at 720
nm, which is roughly located at half of the linear absorption peak
energy. To quantify the 2PEF of the investigated samples, the
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two-photon-action cross section (ησ ) was determined using
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DOI: 10.1021/acs.inorgchem.8b00671
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