Friedel-crafts bottom-up synthesis of fluorene-based soluble luminescent organic nanogrids

Article abstract of DOI:10.1021/ol500439z

A series of fluorene-based grid molecules (so-called Grid fluorenes) have been synthesized by means of shape-supported cyclization, starting from H-shaped precursors via the alternative Friedel-Crafts reactions of fluorenols and Suzuki cross-coupling reactions with key cyclization yields up to 26%. Fluorenol approaches and nanogrids open a door to soluble one-, two-, or three-dimensional nanoporous polymers as next-generation polymer mechano-semiconductors facing a new era of consciousness.

Full text of DOI:10.1021/ol500439z

Letter
pubs.acs.org/OrgLett
Friedel−Crafts Bottom-up Synthesis of Fluorene-Based Soluble
Luminescent Organic Nanogrids
Long Wang,†,§ Guang-Wei Zhang,†,§ Chang-Jin Ou,† Ling-Hai Xie,*,† Jin-Yi Lin,† Yu-Yu Liu,†
and Wei Huang*,†,‡
†Center for Molecular Systems and Organic Devices (CMSOD), Key Laboratory for Organic Electronics & Information Displays
(KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, 9 Wenyuan Road,
Nanjing 210046, China
‡Jiangsu-Singapore Joint Research Center for Organic/Bio-Electronics and Information Displays and Institute of Advanced Materials,
Nanjing Tech. University, Nanjing 211816, China
S
* Supporting Information
ABSTRACT: A series of fluorene-based grid molecules (so-called Grid
fluorenes) have been synthesized by means of shape-supported cyclization,
starting from H-shaped precursors via the alternative Friedel−Crafts
reactions of fluorenols and Suzuki cross-coupling reactions with key
cyclization yields up to 26%. Fluorenol approaches and nanogrids open a
door to soluble one-, two-, or three-dimensional nanoporous polymers as
next-generation polymer mechano-semiconductors facing a new era of
consciousness.
of them have no solubility, which severely hinders further
ignificant progress has been made in plastic electronics and
Stheir industry owing to molecular design by means of applications in functional organic electronics. It is a challenge to
powerful organic synthesis in the past decades.1 To date,
construct soluble organic optoelectronic frameworks that offer
a new chance to surpass the molecular design of state-of-the-art
three-dimensional organic semiconductors via iterative bottom-
up organic synthesis.12 On the other hand, despite some shape-
persistent 3D organic cages that have been synthesized,13 they
seldom have electroactive backbones, resulting in limited
electronic delocalization which is a prerequisite for the
application of organic electronic devices.14 Furthermore, most
of them have drawbacks with limited expandability and
augmentability15 that severely affect the construction of
complex covalent polymer nanogrids. In this case, it is
significant to explore robust protocols and expandable
molecular π-systems for soluble organic nanoscale grids with
well-defined geometric shapes in the field of organic frame
chemistry that studies the generation of grids and their
networks. Here, we demonstrate a series of fluorene-based
optoelectronic grid molecules with a rectangle shape of that
afford a door toward soluble three-dimensional conjugated
polymers (Figure 1).
various linear small organic molecules, oligomers, π-conjugated
polymers, and π-stacked polymers have been designed and
synthesized intensively for application in organic/polymer light-
emitting devices (O/PLEDs), solar cells, thin-film transistors,
lasers, and memories.2 More recently, an atom-precise bottom-
up approach has been a favored field of study in organic
synthesis to construct various nanocarbons with extraordinary
semiconducting features such as buckybowls,3 nanographene
and nanoribbons,4 and nanorings and nanotubes5 by several
groups.6
Beyond the above-mentioned nanocarbons, three-dimen-
sional metal−organic frameworks (MOFs)7 represent one of
the most elegant geometric architectures which are dramatically
different from dendrimers8 without any regular mesh structures
having been reported continuously by imaginative inorganic
chemists. With MOFs as inspiration, it can be imagined that
framework approaches will create functional organic materials
beyond the application of gas storage and catalysts.9 The active
electron donors or acceptors could be integrated into soluble
organic nanogrids that will impart the distinct charge transfer
and energy transfer behaviors. Under the trend of nanodirected
organic synthesis, covalent engineering and molecular manu-
facturing will hold great promise in creating unprecedented
organic mesh nanomolecules/polymers with intriguing opto-
electronic features that may potentially upgrade organic/plastic
(opto)electronics, molecular electronics, and mechatronics.10
Although covalent-organic frameworks (COFs) with diverse
porous properties11 have been explored by several groups, most
Before bottom-up synthesis, retrosynthetic analysis of
fluorene-based grid molecules with the shape were shown
to be deconstructed in terms of three possible routes, as shown
in Scheme 1. Both routes I (red) and II (orange) involve
synergic cyclization of either a truncated H-core and a clip or
between two bacilliform chains where misplacement probably
occurs easily, leading to cross-linking polymers. We learned that
Received: February 11, 2014
Published: March 10, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
probably are attributed to avoiding polymerization. All the
preliminary results strongly suggest that shape-directed
cyclization via a fluororenol approach is an effective method-
ology to construct complex organic nanogrids.
Upon the synthesis of these giant organic grid nano-
molecules, we characterized their purities and chemical
1
structures by means of GPC, H and 13C NMR spectroscopy,
Figure 1. Schematic of three-dimensional organic nanoframeworks
through the intermediate of organic molecular grids with a shape of
the Chinese symbol .
and MALDI-TOF-MS. They exhibit good solubility in common
solvents such as dichrolomethane, chloroform, and tetrahy-
drofuran (THF) owing to long alkyl side chains. Grid fluorene-
3 with six alkoxy chains shows more solubility than Grid
Scheme 1. Synthetic Routes of the Grid Fluorenes
1
fluorene-2. From H NMR, the signals at 2.6 ppm are ascribed
to the proton of hydroxyl groups in H-tetraOH-2 and -3, and
the chemical shifts of about 3.8 ppm are unambiguously
ascribed to the hydrogen at the first carbon of alkoxy groups
(Figure SI-5). The characteristic peak at 2.6 ppm for hydroxyl
groups disappears, suggesting successful cyclization. With
regard to the 13C NMR spectra of Grid fluorenes, the
disappearance of the peak at 83.4 ppm also suggested the
disappearance of the −OH group, indicating effective trans-
formation of C-9 of fluororenol in H-tetraOH-3 into C-9 of
diarylfluorenes in Grid fluorene-3 (Figure SI-6). The MALDI-
tof mass spectrum technique permits accurate determination of
molecular weights. Grid fluorene-2 exhibited isotopic distribu-
tion with a molecular ion mass of m/z = 2186.5 that is
consistent with a hydrocarbon of the molecular formula
C
154H114O2S6, which completely confirms successful cyclization
(Figure 2). This Grid fluorene-3 gave a molecular ion with a
route III (blue) involves shape-directed cyclization of the H
shape with advantages over two above-mentioned methods.
Furthermore, the regular H shaped configurations have been
designed by the backbone of diaryfluorenes according to our
previous observations.10 Electron-rich 2,2′-bithiophene was
chosen as the crosspiece of due to the high reactivity in
BF3·Et2O-mediated Friedel−Crafts reactions (BFRs) that also
has reversible features similar to dynamic combination
chemistry (DCC) according to our previous work.10,16
Therefore, we decided to adopt route III to synthesize
fluorene-based Grid fluorene-2 and -3 via the BFR and Suzuki
C−C cross-coupling reaction. Specifically, diaryfluorene-based
H-cores were synthesized by the BFR of substituted fluorenol
with bithiophene with yields of 90%. Second, Suzuki cross-
coupling reactions of the tetrabromo-substituted H-core with
the unsubstituted and substituted fluororenol boric acid (4.4
equiv) using the catalyst Pd(PPh3)4 proceeded smoothly to
afford H-tetraOH-2 and H-tetraOH-3 in yields of 40% and
41%, respectively. Finally, the BFR cyclization of H-tetraOHs
with bithiophene produced a yield of 9% for Grid fluorene-2
and 26% for Grid fluorene-3. This result suggests that the long
alkoxy chains can effectively reduce the probability of
intermolecular reactions by increasing the distance of H-
tetraOHs tertiary alcohols. In addition, we found that a low
concentration is favorable for intramolecular closures that
Figure 2. (A) MALDI-tof mass spectra of Grid fluorene-3; (B)
MALDI mass spectra of Grid fluorene-2.
mass of m/z = 2699.0 that is also consistent with the isotopic
distribution expected for a hydrocarbon of the composition
C
186H178O6S6 (Figure 2). The purity was further examined by
GPC analysis with low polydispersity values of ∼1.01. It is
normal that GPC gave a deviating number-average molecular
weight of Mn = ∼1700 g mol−1 for Grid fluorene-2 and ∼2500
g mol−1 for Grid fluorene-3 with regard to the true molecular
weight (Figure SI-10). All results show that the obtained
nanogrids agree well with the designed structures with high
purity.
To evaluate whether fluorene-based Grid fluorenes will be
suitable as nanomolecular building blocks to construct state-of-
the-art three-dimensional polymer semiconductors, we studied
their preliminary thermal, optical, and electrochemical proper-
ties. According to thermogravimetric analysis (TGA), the
decomposition temperatures (Td) are up to 366.5 °C for Grid
fluorene-2 and 402 °C for Grid fluorene-3 (Figure SI-11).
Differential scanning calorimetry (DSC) showed that no
obvious glass transition temperature can be observed below
330 °C, indicating that Grid fluorenes show stable amorphous
states (Figure SI-12).
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Letter
UV/vis absorption spectra exhibited absorption bands at 341
nm for Grid fluorenes in dilute chloroform. For photo-
luminescence spectra, two nanomolecules in dilute solution
show similar blue emission with the peaks at 395−416 nm that
are typical emission behaviors of terfluorenes (Figure SI-13).
These are consistent with the value of the terfluorene
derivatives that are probably ascribed to the π−π* electronic
transition of backbones according to our previous reports.10
Their absorption spectra in spin coated thin films exhibited the
peaks in 349 nm. Grid fluorenes 2 and 3 in thin films show the
emission bands at ∼431 nm (Figure 3). These results suggest
HOMO and LUMO are localized at different molecular
segments. The electron density distribution at HOMOs are
localized at 2,2′-bithiophene units, whereas the LUMOs are
predominately determined by one of the linear arms of the
terfluorene chain. The electrostatic potential analysis shows that
the p orbitals of π-conjugated systems that are oriented
perpendicular to the molecular plane make the electrostatic
potential more negative in the upper and underside of the
molecule.
In summary, we proposed a grid-directed bottom-up
synthesis that will create unprecedented soluble organic
nanoobjects such as one-dimensional nanoladders and two-
dimensional nanomeshes. Toward this target, a series of
fluorene-based grid molecules with a configuration of the
Chinese symbol
have been designed rationally and
synthesized via a Friedel−Crafts protocol of fluorenols starting
from H shapes. H shapes are favorable for effective closure to
give Grid fluorene-3 with yields up to 26%. Luminescent Grid
fluorenes will be potential nanoporous organic semiconductors.
Organic grid approaches will be one effective strategy for
covalent nanobuilding blocks in organic nanoscience and three-
dimensional organic/polymer semiconductors in plastic elec-
tronics, offering a new starting point toward organic robotics
and a new era of molecular machine consciousness. Soluble
giant organic nanogrids and detailed structure−property
relationships, as well as their organic nanomaterials, are
currently being studied in our laboratory.
Figure 3. (a) Absorption (solid symbol) and emission (hollow
symbol) spectra in thin films; (b) Cyclic voltammograms; (c)
Optimized geometry of Grid fluorene-1; (d) HOMO of Grid
fluorene-1; (e) LUMO of Grid fluorene-1; (f) Electrostatic potential
surface of Grid fluorene-1. The color code ranges from 0.01 eV (red)
to 0.01 eV kcal mol−1 (blue).
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR, MALDI-tof-MS, and detailed description of experiments.
These materials are available free of charge via the Internet at
http://pubs.acs.org.
that the rigid three-dimensional oligofluorenes have stable
morphological stability probably owing to the resulting
macrocycles with high steric hindrance, suppressing interchain
packing and undesired chain aggregation.17
AUTHOR INFORMATION
■
Corresponding Authors
Cyclic voltammogram (CV) measurements were carried out
to investigate the oxidation and reduction behaviors of Grid
fluorenes (Figure 3). The oxidation potentials were recorded at
1.05 V for Grid fluorene-2 and 1.18 V for Grid fluorene-3,
respectively. The highest occupied molecular orbital (HOMO)
energy levels were estimated to be −5.75 eV for Grid fluorene-2
and −5.88 eV for Grid fluorene-3. The reduction onset
potentials were measured to be −2.43 V for Grid fluorene-2
and −2.42 V for onset Grid fluorene-3, from which the LUMO
energy levels of Grid fluorene-2 and Grid fluorene-3 were
calculated to be −2.37 and −2.3 eV, respectively. As a result,
the electrochemical band gap of 3.38 eV for Grid fluorene-2
and 3.58 eV for Grid fluorene-3 does not closely match the
optical band gap of 3.06 eV for Grid fluorene-2 and 3.08 eV for
Grid fluorene-3.
To gain insight into the fundamental electronic structure
features, Grid fluorenes without any alkyl substitution have
been analyzed by density functional theory (DFT) at the
B97D/TZVP level. Geometry optimization, frontier molecular
orbitals, and electrostatic potential (ESP) have been
summarized in Figure 3. For fully optimized geometry
conformations, the target Grid fluorene has the length ×
width size of ∼2.2 nm ×1.1 nm. The rectangle Grid fluorenes
have two approximately square subgrid structures with a mesh
area of 0.9 × 0.7 nm2 indicating potential as organic
nanoporous materials. From visualizing the molecular orbitals,
*E-mail: iamlhxie@njupt.edu.cn.
*E-mail: iamwhuang@njupt.edu.cn.
Author Contributions
§These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The project was supported by the National Natural Science
Funds for Excellent Young Scholar (21322402), National
Natural Science Foundation of China (U1301243, 21274064,
21144004, 61177029, 20974046, 51273092, 61106058), The
Program for New Century Excellent Talents in University
(NCET-11-0992), Doctoral Fund of Ministry of Education of
China (20133223110007), Natural Science Foundation of
Jiangsu Province (BK2011761, BK2008053, SJ209003), and
Funding of Jiangsu Innovation Program for Graduate
Education (CXZZ11_0413, CXLX13_450). The project was
funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions, PAPD.
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