A hydrogen-bonded organic framework based on redox-active tri(dithiolylidene)cyclohexanetrione

Article abstract of DOI:10.1039/d0cc07776c

Redox-active hexakis(4-carboxyphenyl) tri(dithiolylidene)cyclohexanetrione (CPDC) was synthesized. The CPDC-based porous framework, constructedviaanomalistic helical hydrogen-bonding, exhibites permanent porosity and photoconductivity.

Full text of DOI:10.1039/d0cc07776c

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Hydrogen-bonded organic framework based on redox-active
tri(dithiolylidene)cyclohexanetrione
Kilingaru I. Shivakumar,a Shin-ichiro Noro,b Yuna Yamaguchi,c Yusuke Ishigaki,d Akinori Saeki,e
Received 00th January 20xx,
Accepted 00th January 20xx
Kiyonori Takahashi,a Takayoshi Nakamura,*a and Ichiro Hisaki*c
DOI: 10.1039/x0xx00000x
Redox-active
cyclohexanetrione (CPDC) was synthesized. The CPDC-based dithiolylidene-based
porous framework, constructed via anomalistic helical hydrogen- tri(dithiolylidene)cyclohexanetrione (DC).14 To date, C2-
hexakis(4-carboxyphenyl)
tri(dithiolylidene)-
In this study, we became interested in redox-active
C3-symmetric core,
bonding, exhibited permanent porosity and photoconductivity.
symmetric X-shaped tetrathiafulvalene (TTF)-tetrabenzoic acid
based HOF has been reported to furnish a simple 2D topology
of rhombic networked (RhombNet) sheets, which stacked
without interpenetration.15 Recently, Xiao’s group used the
same molecule to obtain stable, H-bonded porous cocrystals
that showed photocurrent response.16 TTF based MOFs and
COFs are also well-studied.17 However, derivatives of DC are
severely understudied, despite its first synthesis in 1980,14 and
certainly have not been used for the construction of porous
frameworks, in spite of its fascinating properties such as
amphoteric redox, solvatofluorochromism and charge
conduction upon suitable substitution.18, 19
Hydrogen-bonded organic frameworks (HOFs),1-7 endowed with
reversible intermolecular H-bonds and consequent high
crystallinity, are new entrants to the family of crystalline porous
materials.8-10 Their unique features such as facile processability,
self-healing, and regeneration make HOFs an attractive
alternative to metal-organic frameworks (MOFs) and covalent
organic frameworks (COFs).1-7 These characteristics combined
with their applications in selective sorption, catalysis,
photoluminescence, and proton conduction underpin the
growing interest in HOFs in recent years.1-7
A C3- or C6-symmetric, shape-persistent, hexatopic carboxylic
acid derivative is one of the useful building blocks to construct
stable HOFs with permanent porosity because they can form
multiple intermolecular hydrogen bonds in well-defined and
predictable manners.3 We have earlier reported that C3-
symmetric planar π-conjugated hydrocarbons, such as
triphenylene derivative Tp, forms a HOF with hexagonal
network structures facilitated by a cyclic H-bonded motif,11, 12
while hexaazatriphenylene derivative CPHAT forms a HOF with
interpenetrated cpu-networks via a helical H-bonding motif13
(Fig. 1a).
a. Research Institute for Electronic Science, Hokkaido University, Sapporo, Hokkaido
001-0020, Japan.
b. Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Hokkaido
060-0810, Japan
c. Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama,
Toyonaka, Osaka 560-8531, Japan.
d. Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
e. Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka,
565‐0871 Japan
† Electronic Supplementary Information (ESI) available: Synthesis, photophyics,
electrochemistry, crystallography details and thermal analysis. CCDC-2038905 See
DOI: 10.1039/x0xx00000x
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Fig. 1 Hydrogen bonded motifs observed in HOFs of hexatopic building block
molecules. (a) Cyclic and helical motif formed by Tp and CPHAT, respectively. (b)
CPDC forming an anomalistic helical motif and not cyclic.
behaviour of 4 as the cyclic voltammetry was lesVsiesweAnrtsicilteivOenlinine
distinguishing peaks (Fig. S9). The voltammogram shows multi-
DOI: 10.1039/D0CC07776C
stage amphoteric redox behaviour with three prominent
oxidation and three reduction peaks. The three sets of redox
peaks plausibly suggest that all the three dithiole rings undergo
sequential oxidation and reduction. The HOMO and LUMO
energies were determined from the onset oxidation and
reduction potentials and found to be −5.82 and -3.57 eV,
respectively, affording an electrochemical band gap of 2.25 eV.
Theoretical calculations carried out at B3LYP/6-31G** level on
CPDC and a methyl ester of CPDC revealed stabilization of
Herein, we report the first 3D-networked HOF of
carboxyphenyl-substituted DC derivative, CPDC. Interestingly,
the HOF CPDC-1 is constructed through anomalistic helical H-
bonded motif, instead of planar or simple helical motifs
reported in the literature. This unusual H-bonding motif
probably originates from slightly larger bite angle of CPDC (Fig.
1c), which can contribute to establish a rule of thumb for
designing HOF. The synthesis of CPDC, preparation and
crystallography of HOF CPDC-1 along with its thermal, gas
sorption and photoconductive behaviours are described. Also,
the photophysical and electrochemical properties of an ester
derivative of CPDC are studied.
HOMO
by
electron-withdrawing
carboxyl/carboxylate
substituents (Figs. S11 and S12). The CPDC and its methyl ester
possess the theoretical HOMO, LUMO and band gap energies of
ca. 5.6-5.7, 2.2-2.3, and 3.35 eV, respectively (Table 1).
CPDC was synthesized as shown in Scheme 1. The
dibenzonitrile derivative 1, obtained according to the literature
procedure20 from commercially available 1,3-dithiole-2-thione
in two steps with overall yield of 37%, was hydrolysed in an
acidic medium to obtain dibenzoic acid 2 in 54% yield. Fisher
esterification of 2 resulted in dibenzoate ester 3 in 93% yield.
The aromatic electrophilic substitution of phloroglucinol by 3,
mediated by triethylamine in presence of silver nitrate,19
afforded the C3-symmetric hexabenzoate 4 in 86% yield. The
target compound CPDC was obtained by subjecting
hexabenzoate 4 to saponification in 99% yield.
Table 1 Energy levels of molecular orbitals in CPDC and its esters.
a
b
c
EHOMO
(eV) a
EHOMO
(eV) b
ELUMO
(eV) a
ELUMO
Eg
Eg
Eg
Compound
(eV) b (eV) (eV) (eV)
CPDC
ester
CPDC
-5.82
n.d.
-5.57
-5.69
-3.57
n.d.
-2.21 2.25 3.35 2.54
-2.33
n.d. 3.36 n.d.
Determined from the aelectrochemical experiment on 4, btheoretical method
applied to methyl ester of CPDC, and cabsorption spectrum of 4. Eg denotes band
gap between HOMO and LUMO, and n.d. - not determined.
Slow evaporation of the solution of CPDC in a mixed solvent
system consisting dimethylacetamide and methyl benzoate
(MB) at 80 °C furnished orange needle-like solvated HOF
crystals denoted as CPDC-1, in which all the six carboxylic acid
groups participate in the formation of an H-bonded dimer. The
CPDC-1 crystallized in the space group of C2/c and exhibits a
porous framework constituted by hexagonally arranged CPDC
molecules (Fig. 2a and Table S4). The ratio of total potential
solvent area calculated from the PLATON software is 43% (cell
volume: 7485.9 Å, void volume: 3219.4 Å). CPDC molecules are
organized in an AB pattern with slipped π-stacking and stacked
in a head-to-tail fashion with an intermolecular distance of 3.48
Å (Fig. 2b and 2c), while the four peripheral phenylene rings of
CPDC are involved in parallel-displaced π-stacking, two each
with layers above and below, by a shortest distance of 3.29 Å
(Fig. S13a). The CPDC core is not strictly flat; nevertheless, has
high planarity: the mean planes of dithiole rings deviate by 2.8–
3.1° from mean plane of cyclohexanetrione and the root mean
square deviation (RMSD) of the central DC core is 0.048 Å.
Intramolecular S‧‧‧O distances are 2.49–2.68 Å and are smaller
than the sum of van der Waals (vdW) radii of the two atoms (Fig
S13b). The two sulphur atoms in the dithioylidene rings form
intermolecular S‧‧‧S contacts at the distances of 3.77 and 4.00
Å. The former, although greater than sum of vdW radii of two
sulphur atoms of 3.60 Å, is comparable to the intermolecular
S‧‧‧S contacts of 3.82 Å observed in conductive charge-transfer
Scheme 1 Synthesis of CPDC.
The absorption spectrum of ester derivative 4 exhibits three
prominent peaks at 246, 370, and 470 nm with the last showing
maximum intensity (Fig. S8). The optical band gap obtained
from the absorption edge of 488 nm was determined to be 2.54
eV. In order to obtain corresponding emission, the peak at 370
nm was chosen for excitation. The hexabenzoate 4 exhibited
fluorescence with the intensity maximum at 491 nm with
relative quantum yield (F) of 0.76%. Differential pulse
voltammetry (DPV) was performed to determine the redox
salt
tetrathiafulvalene-tetracyanoquinodimethane
TCNQ).21
(TTF-
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DOI: 10.1039/D0CC07776C
Fig. 2 Crystal structure of CPDC-1. (a) Packing diagram, where molecules with diametrically opposed orientation are classified by colours (blue and yellow). (b)
Relative orientation of two stacked molecules. (c) Interlayer slipped head-to-tail stacking of CPDC molecules viewed along the b-axis. (d) Partially H-bonded sheet
structure of CPDC, where two different partial helical turns (e, f) formed through intralayer H-bonds (intra-HB) connect each other through inter-layer H-bonds
(inter-HB) to give anomalous helical motif (g).
Interestingly, four carboxy groups of CPDC form intra-layer from room temperature to 668 K (Fig. S16). The initial patterns
H-bonds to give a rhombic structural unit, while the other two obtained upon heating the bulk crystals are weak until 330 K
form inter-layer ones (Fig. 2d). There are two kinds of open-end owing to the severe disorder of solvent molecules in the voids,
triangular motif with the different diameters. The motif with a which is often observed in the case of HOFs.12 The intensity of
smaller diameter has a pair of phenylene groups appended to the peaks gradually increased while losing the first solvent
the same dithiole ring as a vertex (Fig. 2e), while the one with a molecule and hits a plateau at 455 K that continues up to 580 K
larger diameter possesses a pair of phenylene groups appended - the temperature range at which CPDC-1 loses the two
to the adjacent dithioles as a vertex (Fig. 2f). These two types of remaining MB molecules to undergo complete desolvation.
motif are connected through interlayer H-bonds to form 3D Subsequently, the intensity dips slightly before peaking at 645
infinite anomalistic helix along the c axis with pitch of 8.6 Å (Fig. K. Beyond 645 K the intensity starts to descend, indicating the
2g). Alternating diameters of helical channel results in shish- gradual collapse of the H-bonded framework.
kebab-shaped 1D inclusion channel (Fig. S14). The resultant
Activation of CPDC-1 was performed by heating the crystals
whole framework has pcu network topology with [412.63] point at 473 K for 24 hours under vacuum conditions. Complete
symbol.
removal of the solvent from pores of the crystals and retention
1
The observed H-bonding manner is in contrast to the cyclic of H-bonded framework were evident from the H NMR and
and infinite helical motifs observed in Tp-1 and CPHAT-1, PXRD patterns, respectively (Figs. S17 and S18). The permanent
respectively (Fig. 1). A closer look at the bite () angles made by porosity of CPDC obtained upon activation of CPDC-1 was
the phenylene groups with the π-conjugated central cores evaluated by gas adsorption experiments at various
reveals a trend in the type of H-bonding motifs formed. The temperatures: CO2 (195 K), N2 (77 K), O2 (195 and 77 K), propane
smaller bite angles of 63.2–66.8° in Tp-1 justify the formation of (298 K) and propene (298 K) (Fig. S19). The desolvated CPDC-1
the discrete cyclic motif akin to an equilateral triangle. On the exhibited type-I sorption isotherms for CO2, propane, and
other hand, a marginal increase to 69.9° enables the H-bonding propene, while type-II isotherms for N2 and O2. Oxygen uptake
motif in CPHAT-1 to form acyclic infinite helical moiety. A at 195 K, 107. 5 kPa was 15.0 cm3 (0.67 mmol)/g and at 77 K,
further increase in the bite angles to 70.7–72.9°, although trivial, 20.0 kPa was 10.8 cm3 (0.48 mmol)/g, while nitrogen at
allows CPDC to form the complicated helix.
101.1kPa was 14.6 cm3 (0.65 mmol)/g. The CO2 uptake of 67.4
Thermogravimetric analysis of as-formed CPDC-1 revealed cm3 (3.01 mmol)/g at 102.3 kPa was highest among the gases
complete desolvation at 585 K, corresponding to a weight loss used for the experiment, attributed to the smaller kinetic
of 26% (calculated for three MB molecules 26.3%) in two steps diameter22 and large quadrupolar moment of CO2 that leads to
(Fig. S15). The first step from 345 to 455 K accounts for the increased electrostatic affinity with the aromatic and carbonyl
weight loss of one MB molecule, while the second step from 455 pore surfaces.23 These results indicate that the HOF has micro
to 585 K corresponds to losing of two remaining MB molecules. pore channels with very small pore width. A comparison of the
Further heating leads to rapid weight loss beyond 660 K, which simulated PXRD pattern obtained from the crystal structure of
indicates the onset of decomposition. In order to obtain CPDC-1 with the PXRD pattern of activated CPDC-1 reveal
information regarding the structural changes of CPDC-1 upon marginal shift of the low angle 020 and 110 peaks towards
heating, variable temperature powder X-ray diffraction (VT- higher 2 values in the latter, indicating slight shrinkage of the
PXRD) patterns were recorded on as-formed crystalline bulks framework upon activation (Fig. S18). The Brunauer–Emmett–
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Teller (BET) surface area, SA(BET), calculated based on CO2 authors thank Profs. Y. Sagara and N. Tamaoki at RIES, Hokkaido
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isotherm was 249 m2 g-1. Furthermore, we carried out propane University for purification of compoundsDbOyI:p1r0e.1p0a3r9a/Dti0vCeCH0P77L7C6.C
and propene sorption experiments at room temperature to find
out whether the microporous nature of the void can selectively
uptake propene over propane. Unfortunately, we could not
Conflicts of interest
observe significant difference in sorption volumes of the two,
with 21.7 cm3 (0.97 mmol)/g of propene and 18.9 cm3 (0.84
mmol)/g of propane being adsorbed at 100.4 kPa.
There are no conflicts to declare.
The crystalline bulk of activated HOF CPDC-1 exhibited a
photocurrent of Σμ = 2.8 × 10−5 cm2V−1s−1 upon exciting at 355
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nm as revealed from flash-photolysis time-resolved microwave
25
conductivity (FP-TRMC) measurement (Fig. 3).24,
The
observed conductivity in the present DC based HOF is
comparable with that obtained in TTF based MOF26 reported by
Dincă’s group and could be attributed to the molecular overlap
of dithiolylidene rings and close intermolecular S‧‧‧S contacts.
The microcrystalline nature of the sample used for the
measurement precluded us from determining the quantum
efficiency of the charge carrier generation () of CPDC-1.
Consequently, the charge carrier mobility (Σμ) remains
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Fig. 3 Transient microwave photoconductivity obtained for the activated CPDC-1 crystals
as revealed from the flash photolysis-TRMC measurement. The excitation wavelength
used was 355 nm.
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To conclude, by effectively employing the C3-symmetric
tri(dithiolylidene)cyclohexanetrione (DC)-based HOF CPDC-1,
we have demonstrated the pivotal role of bite angle ( ) on the
type of H-bonded motif and network formed. Thanks to five-
membered dithiolylidene rings, the larger bite angle in CPDC
precludes the formation of the frustrated triangular cyclic motif,
instead provides an acyclic anomalistic helix with alternating
diameters. As a consequence of the H-bonded helical structure,
the CPDC-1 forms a robust 3D, non-interpenetrated network
that supports the framework till 645 K. The highly π-delocalised
redox core of CPDC promotes intermolecular π-stacking and S‧
‧‧S interactions, supporting charge conduction in HOF. Thus,
this study offers a promising perspective to tailor the complex
topologies and properties of HOFs by bringing subtle
geometrical
changes
in
the
building
blocks.
This work was supported by KAKENHI (JP18H01966 and
JP19H04557) and the Dynamic Alliance for Open Innovation
Bridging Human, Environment and Materials from MEXT Japan.
I.H. thanks Izumi Science and Technology Foundation. K.I.S.
thanks RIES, Hokkaido University for financial support. The
25. A. Saeki, Polym. J., 2020, 52, 1307-1321.
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26. T. C. Narayan, T. Miyakai, S. Seki and M. Dincă, J. Am. Chem.
Soc., 2012, 134, 12932-12935.
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