Hexaazatriphenylene-Based Hydrogen-Bonded Organic Framework with Permanent Porosity and Single-Crystallinity

Article abstract of DOI:10.1002/chem.201701893

Hydrogen-bonded organic frameworks (HOFs) have drawn unprecedented interest because of their high crystallinity as well as facile process for construction, deconstruction, and reassembly arising from reversible bond formation-dissociation. However, struct

Full text of DOI:10.1002/chem.201701893

A Journal of
Accepted Article
Title: Hexaazatriphenylene-Based, Hydrogen-Bonded Organic
Framework with Permanent Porosity and Single-Crystallinity:
Synthesis, Structural, Uptaking and Photophysical
Characterization
Authors: Ichiro Hisaki, Nobuaki Ikenaka, Eduardo Gomez, Boiko
Cohen, Norimitsu Tohnai, and Abderrazzak Douhal
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To be cited as: Chem. Eur. J. 10.1002/chem.201701893
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Hexaazatriphenylene-Based, Hydrogen-Bonded Organic
Framework with Permanent Porosity and Single-Crystallinity:
Synthesis, Structural, Uptaking and Photophysical
Characterization
Ichiro Hisaki,*[a] Nobuaki Ikenaka,[a] Eduardo Gomez,[b] Boiko Cohen,[b] Norimitsu Tohnai,[a] and
Abderrazzak Douhal*[b]
Abstract: Hydrogen-bonded organic frameworks (HOFs) have drawn
unprecedented interests because of their high crystallinity as well as
facile process for construction, deconstruction, and reassembly
arising from reversible bond formation-dissociation. However,
structural fragility and low stability frequently prevent formation of
robust HOFs with permanent porosity. Herein, we report that
hexakis(4-carboxyphenyl)-hexaazatriphenylene (CPHAT) forms
three dimensionally networked H-bonded framework CPHAT-1.
Interestingly, the activated framework CPHAT-1a retains not only
permanent porosity but single-crystallinity, enabling precise structural
characterization and property evaluation on a single crystal. Moreover,
CPHAT-1a retains its framework up to 339 °C or in hot water and in
acidic aqueous solution. These results clearly show that even a simple
H-bonding motif can be applied for construction of robust HOFs, which
creates a pathway to establish a new class of porous organic
frameworks. We also characterize its uptake of gases and I2, in
addition to
a detailed photophysical study (spectroscopy and
dynamics of proton and charge transfers) of its unit in solution, and of
its single crystal under fluorescence microscopy, where we observed
a marked strong anistropy and narrow distribution. The results bring
new findings to advance in HOFs and their possible applications in
science and technology.
Figure 1. Formation of Hexagonally Networked porous frameworks based on
C3-symmetric π-conjugated molecules possessing six carboxyphenyl groups
in periphery. (a) Tp yields layered assembly of 2D-network through discrete
cyclic hydrogen bonding motif so-called PhT as reported previously. (b) HAT
yields 4-fold interpenetrated 3D-network through infinite helical hydrogen
bonding motif.
Introduction
Porous frameworks constructed by organic molecules are
currently one of the central interests in the fields of materials
chemistry and crystal engineering.[1] Particularly, networking
through reversible hydrogen-bonds (H-bonds) frequently provides
frameworks with high crystallinity due to self-repairing of irregular
molecular connection, allowing precise structural analysis and
insight of structure-property relationships. Meanwhile, relatively
weak bonding strength of H-bonds cases fragility of frameworks
and lack of universalistic design principles. To overcome this
problem, various supramolecular synthons[2] have been applied,
resulting in fascinating H-bonded organic frameworks (HOFs)[3]
with permanent porosity.[4]
A dimer of carboxyl groups, which is one of the simplest and
commonest supramolecular synthon,[2,5] is also appropriate to
construct HOFs due to significantly directional nature and facile
synthesis of derivatives.[6] For example, Zentner et al. reported a
porous framework composed of interpenetrated honeycomb
sheets of 1,3,5-tris(4-carboxyphenyl)benzene.[7a] Vaidhyanathan
reported tripodal tricarboxylic acid molecule with flexible sp3
[a]
[b]
Dr. I. Hisaki, N. Ikenaka, Dr. N. Tohnai
Department of Material and Life Science, Graduate School of
Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-
0871 (Japan)
E-mail: hisaki@mls.eng.osaka-u.ac.jp
E. Gomez, Dr. B. Cohen, Prof. Dr. A. Douhal
Departamento de Quimica Fisica, Facultad de Ciencias Ambientales
y Bioquimica, and INAMOL, Universidad de Castilla-La Mancha,
Avenida Carlos III, S/N, 45071 Toledo (Spain)
Abderrazzak.Douhal@uclm.es
Supporting information for this article is given via a link at the end of
the document.
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nitrogen center yields HOFs exhibiting selective CO2 sorption
behavior.[7b] More recently, Wu and Yuan et al. demonstrated that
a significantly stable HOF with high surface area was nicely
formed with 3,3’,5,5’-tetrakis-(4-carboxyphenyl)-1,1’-biphenyl.[7c]
However, examples of the carboxyl dimer-based HOFs with
permanent porosity and high crystallinity are still limited.
In connection with this, we also reported that C3-symmetric
planar π-conjugated hydrocarbons, such as triphenylene (Tp),
possessing three 4,4’-dicarboxy-o-terphenyl moieties in periphery
form porous hexagonal network (HexNet) structures through the
cyclic H-bonded motif so-called phenylene triangle (PhT), and
that the HexNet sheet stacks without interpenetration to give
flexible layered HOFs with permanent porosity (Figure 1a).[8] In
the present work, we used a heterocyclic π-conjugated system,
hexaazatriphenylene (HAT)[9] with six carboxyphenyl groups, as a
building block and achieved a significantly rigid HOF CPHAT-1
(Figure 1b). HAT has been investigated as electron and/or hole
transport materials in organic semiconducting devices,[9]
mesogens of liquid crystals,[9] ligands of metal complexes,[10]
building blocks of porous materials such as metal organic frame-
works (MOFs)[11] and covalent organic frameworks (COFs)[12] due
to its rigid, planar, π-conjugated skeleton with nitrogen atoms.
However, this is the first example of HAT-based HOF with
permanent porosity. The derivatives with six carboxyphenyl
groups in periphery (CPHAT) yields three-dimensionally
networked rigid pcu-framework with permanent porosity, instead
of 2D layered hxl-framework as in the case of Tp.[8]
Scheme 1 (a) Synthesis and (b) crystallization conditions.
Table 1. Crystallographic parameters of CPHAT-1-(TCB), CPHAT-1a, and
CPHAT-1-(I2)
crystal
System
Space group
a, b / Å
c / Å
CPHAT-1-(TCB)
Trigonal
P-3
CPHAT-1a
Trigonal
R-3
CPHAT-1-(I2)
Trigonal
R-3
One of the most significant nature of the present system is
that the activated framework CPHAT-1a retains not only
permanent porosity, but single-crystallinity after activation.
Furthermore, the framework is thermally stable up to 339 °C and
has resistance against hot water and even aqueous hydrochloric
acid. Such robust framework built by carboxylic acid dimer is
hitherto unknown.
37.2013(7)
7.20124(15)
8630.9(3)
0.0574
37.1764(5)
7.18907(10)
8604.7(2)
0.0500
37.1486(10)
7.1441(4)
8538.1(6)
0.0608
V / Å3
R1
Herein, we present synthesis of CPHAT, preparation of rigid
HOF CPHAT-1, its thermal behaviours, gases and iodine sorption
properties of the activated HOF CPHAT-1a, and detailed
photophysical behaviour in solutions, crystalline bulks, and single
crystals. For the latter, we observed a remarkable fluorescence
anisotropy behaviour reflecting the molecular alignment of the
units within the crystal.
wR2
0.1705
0.1504
0.1866
T / °C
−60
−60
−60
CCDC
1540571
1540572
1540570
low temperature such as at 60 °C, on the other hand, CPHAT
yielded non-porous crystal NP-CPHAT as well as CPHAT-1-
(TCB) concomitantly (Figure S1). In NP-CPHAT, one carboxylic
group of CPHAT forms a H-bond with the nitrogen atom in the
core, preventing formation of a porous networked framework
(Table S1, Figure S2).
Results and Discussion
Synthesis and crystallization.
Hexaaminobenzene (2) was synthesized by hydration of 1,3,5-
triamino-2,4,6-trinitrobenzene (1) according to the literature.[13]
Triple condensation of 2 with dione 3 in acidic condition yielded
hexaazatriphenylene derivative 4 in 83% yield in two steps from
1. Hydrolysis of 4 gave CPHAT in 93%. CPHAT was recrystallized
Crystallography
Crystal CPHAT-1, with the space group of P-3, exhibits a porous
framework based on hexagonally arranged CPHAT molecules
(Table 1 and Figure 2a). CPHAT molecule is one-dimensionally
stacked in an AB pattern with the staggered twisting angle of
60°and intermolecular distances of 3.6 Å (Figure 2b). The
resultant stacking columns are arranged in hexagonal manner,
by slow evaporation from
a
mixed solution of N,N-
dimethylformamide (DMF) and 1,2,4-trichlorobenzene (TCB). At
100 °C, CPHAT yielded a solvated HOF crystal CPHAT-1-(TCB),
in which all carboxylic groups of CPHAT formed H-bonding dimer
to give a networked porous framework (vide infra). At relatively
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Figure 2. Crystal structure of CPHAT-1-(TCB). (a) Packing diagram. (b) 1D Columnar structure composed of π-stacked CPHAT molecules. (c) Visualized surface
of inclusion channels running along the c axis. (d) 1D structure of guest molecules (TCB) aligned in the inclusion channel. (e) Non-planar conformation of CPHAT
drawn in anisotropic displacement ellipsoids with 50% probability. (f) Schematic representations of (top) single and (bottom) 4-fold inter-penetrated H-bonded
frameworks with pcu (primitive cubic) topology (point symbol of the net: [412.63]).
allowing formation of 1D inclusion channels along the c axis. The
channel has a triangular shaped cross section with the width
ranging 6.7 to 8.8 Å and ratio of solvent accessible volume per
the unit cell is 31% (Figure 2c). Molecules of TCB are
accommodated in the channel in a parallel fashion with a
host/guest ratio of 1/2 (Figure 2d).
framework with the network topology as same as that of a cubic
framework such as MOF-5 (Figure S3).[16]
Thermal behavior of CPHAT-1-(TCB).
To obtain the information about activation condition, crystalline
bulk of CPHAT-1-(TCB) was subjected to thermogravimetric (TG)
analysis as shown in Figure 3a. The TG curve exhibits gradual
weight loss of 28% up to ca. 300 °C, indicating that TCB
molecules were almost all removed at around this temperature
although the solvent molecules are not easily removed due to a
narrow inclusion channel well-fitted to the solvent molecules.
Further weight loss at temperature higher that ca. 350 °C is
caused by thermal decomposition of CPHAT. An endothermic
behavior in the differential thermal analysis (DTA) curve at 350 °C
to 365 °C also supports the thermal decomposition (Figure S6).
To obtain structural information during desolvation by heating,
PXRD patterns were recorded as heating as-formed crystalline
bulk of CPHAT-1-(TCB) (Figure 3b). The intensity of the
diffraction peaks at 4.7°, 8.2°, 9.5°, and so on increases due to
release of TCB until temperature reached to 200 °C. The peak
positions of the diffraction are the same as those of as-formed
crystalline bulk of CPHAT-1-(TCB), indicating that no structural
change occurred upon desolvation. The diffraction peaks started
to decay from 339 °C and simultaneously new weak peaks
generated at 8.7°, 10.2°, 13.3°, 15.8°, and so on, indicating that
original structure collapsed and changed into another one.
It is noteworthy that CPHAT molecule is not flat but has a
twisted conformation: Root mean square deviation (RMSD) of the
twisted HAT core from the mean square plane is 0.196 Å, which
is larger by 0.106 Å than that in the previously reported 2D
networked system Tp-1.[8] The six peripheral carboxyphenyl
groups in CPHAT alternately direct up and down with an angle of
ca. 30° against the central benzene ring of HAT core (Figure 2e).
Dihedral angle between the peripheral phenylene groups and
HAT core, on the other hand, is much smaller compared with
those in Tp-1 systems[8b] because of lack of sterical hindrance
caused by hydrogen atoms at the ortho-positions: the dihedral
angle in the present CPHAT-1-(TCB) and previous Tp-1 systems
are 25.5–28.0° and 54.7–76.1°,[8b] respectively. These structural
features of CPHAT and Tp are also consistent with those of
optimized structures calculated at the B3LYP/6-31G* level,
although molecular geometries in crystalline states are strongly
affected by packing force (Figure S4).
The observed structural features of CPHAT made it possible
to form a three-dimensionally extended H-bonded framework. In
contrast to a H-bonded motif with a planar cyclic shape in Tp
systems, CPHAT molecules are networked via an infinite three-
fold helical motif with a helical pitch of 14.4 Å (Figure 1b).
Consequently, CPHAT forms a three-dimensionally-networked
framework expressed by pcu (primitive cubic) 46-net of network
topology (point symbol of the net: [412.63]) (Figure 2f).[14,15] The
network is interpenetrated by 4-fold to give rigid porous structure.
It is interesting that C3-symmetric planar molecule yields a
Stability of CPHAT-1a.
On the basis of the above described thermal behaviors,
desolvation of CPHAT-1-(TCB) was accomplished at 200 °C
under vacuum condition for 48 h to yield the activated crystalline
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Figure 4. (a) POM and (b) SEM images of CPHAT-1a crystal-line bulk. (Inset:
magnified SEM image)
Figure 3. Thermal behaviors of CPHAT-1-(TCB) and its activated form
CPHAT-1a. (a) Thermogravimetric (TG) analysis of as-formed CPHAT-1-
(TCB) and activated CPHAT-1a crystalline bulks. Weight loss of 28% at ca.
300 °C is corresponding to the theoretical value calculated on the basis of the
crystallographically-determined host:guest ratio of 1:2. (b) Changes of powder
X-ray diffraction (PXRD) patterns of as-formed CPHAT-1-(TCB) upon
desolvation by heating. The pattern of CPHAT-1a starts to collapse at ca.
339 °C. Inset: Representative PXRD patterns of as-formed CPHAT-1-(TCB)
(bottom), desolvated CPHAT-1a (middle), and the collapsed form (top).
Figure 5. PXRD patterns of crystalline bulk of CPHAT-1a. (a) Simulated pattern
from the crystallographic data. (b) Experimental pattern before soaking. (c)
Pattern after ex-posing water at 60 °C for 24 h. (d) Pattern after exposing 10%-
HCl aqueous solution at 60 °C for 24 h.
PXRD patterns of crystalline bulk of CPHAT-1a retain the original
profile after soaking in hot water at 60 °C for 24 h and in 10%-HCl
aqueous solution at 60 °C for 24 h, although CPHAT-1a was
immediately dissolved in basic aqueous solution such as KOH
aqueous solution.
material CPHAT-1a. Removal of TCB was confirmed by 1H NMR
spectrum of the resulting material dissolved in a DMSO-d6
solution (Figure S7). TG curve of CPHAT-1a also supported the
desolvation: weight loss of ca. 1% was observed before rapid
weight loss at 355 °C due to decomposition of the compound
(Figure 3a).
Sorption experiments of gases and iodine.
The permanent porosity of CPHAT-1a was evaluated by nitrogen,
hydrogen, and carbon dioxide molecules sorption experiments at
low temperature (77 K, 77 K, and 195 K, respectively) as shown
in Figure 6a. CPHAT-1a exhibits almost no adsorption of N2 and
H2: Uptakes are 7.1 cm3 g−1 (0.32 mmol g−1) and 2.6 cm3 g−1 (0.12
mmolg−1), respectively, at relative pressure (p/p0) of 0.99. N2
adsorption was also conducted at 195 K to confirm a low
temperature effect,[17] resulting in a increase of uptake (21 cm3 g−1,
0.95 mmolg−1). For carbon dioxide, on the other hand, CPHAT-1a
show an uptake of 137 cm3 g−1 (6.13 mmol g−1) at p/p0 of 0.99 with
a typical type-I isotherm. Calculated BET surface area based on
the isotherm of carbon dioxide is 649 m2 g−1. The gas sorption
behavior of CPHAT-1a is different from that of analogous system,
such as Tp-apo, possessing 1D channels composed of H-bonded
PhT motifs previously we reported.[8c] Namely, Tp-apo shows
As shown in Figure 4a, crystalline bulk of CPHAT-1a shows
uniformed birefringence features in polarized optical microscope
(POM) images. Scanning electron microscope (SEM) images also
show that CPHAT-1a retains a clear crystalline habit of a hexagon
rod with diameter up to 10 μm (Figures 4b and S8). These results
strongly indicate that CPHAT-1a retains not only the PXRD profile
but single-crystallinity after activation due to robust three
dimensional networking. Remarkably, we succeeded in
crystallographic analysis on single crystals of CPHAT-1a,
revealing that CPHAT-1a has the same framework as CPHAT-1-
(TCB) (Table 1), thought the channel is empty. CPHAT-1a
belongs to the space group R-3, which is different from that of
CPHAT-1 (P-3) due to the existence of low symmetric guest
molecule (i.e., TCB) in the void space.
Furthermore, we revealed that the framework of CPHAT-1a
resisted both water and acidic conditions. As shown in Figure 5,
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Figure 6. Sorption properties of CPHAT-1a. (a) Gas sorption isotherms. CO2: light blue, N2: black, H2: red. Solid symbol: absorption process, open symbol:
desorption process. (b) Photographs for Iodine sorption and desorption of single crystal of CPHAT-1a. Scale bar: 100 μm. (c) Crystal structure of CPHAT-1-(I2), in
which iodine is drawn by space-fill model. Color code, CPHAT: yellow or blue, I2: purple. (d) Interactions among the carboxyl groups and iodine molecules in
CPHAT-1-(I2). Iodine molecules are disordered into two sites (I and II) with an abundance ratio of 0.61 and 0.39, respectively, although for clarity, only the major
population is shown in (c). The occupancy of total iodine molecules in the channel is 79%. (e) Three-fold helical alignment of iodine molecules in the site I. d and
θ
the intermolecular distance and contact angle between the neighbouring iodine molecules, respectively. Symmetry cords: (A) x, y, y; (B) −1/3−x+y, 1+1/3−x, 1/3+z.
equal amounts of uptake toward both carbon dioxide and nitrogen
molecules, while CPHAT-1a shows selective absorption for the
former. CO2 adsorption selectivity over N2 has been explained by
three aspects: (1) smaller kinetic diameter of CO2,[18] (2) more
attractive and stronger interaction of the gas molecules with pore
surfaces due to a large quadrupole moment (enthalpy factor),[19]
and (3) N2-phobic nature of the framework (entropy factor).[20] In
the present systems, the width of the channels (~6.7Å) is larger
than the kinetic diameters of nitrogen (3.6 Å) and carbon dioxide
molecules (3.3 Å). However, the slightly wider channel of Tp-apo
(by 1.5 Å) compared to that of CPHAT-1a (Figure S9), can have
significant effect on the sorption behavior in connection with
factors (2) and (3).
To achieve one-dimensionally aligned supramolecular
structure of iodine molecules,[21] we attempted to introduce iodine
into the channel of CPHAT-1a. Crystals of CPHAT-1a were
exposed to iodine vapor at ambient condition, resulting in
penetration of iodine into the crystals. As shown in Figure 6b, an
edge of a single crystal of CPHAT-1a was immediately colored
with dark red, and the colored region was extended and reached
to the other edge within 10 min, giving iodine included crystal
CPHAT-1-(I2). The rate of iodine penetration depends on sizes,
shapes, and conditions (e.g. existence of cracks) of the selected
single crystals. After removing the iodine source, the colored
crystals were then gradually discolored from the edge that colored
first in the absorption process in relatively longer period of time
compared with the absorption process (e.g. 60 min for the present
system, Figure 6b).
To identify an iodine-included structure and interaction
among iodine molecules and CPHAT molecular units in the
crystal, CPHAT-1-(I2) was subjected to single-crystal X-ray
diffraction analysis at −60 °C. As a result, we revealed that
CPHAT-1-(I2) has the same framework and the nearly identical
cell parameters with CPHAT-1a (Table 1), and moreover, that
iodine molecules are accommodated in the channels (Figure 6c).
The iodine in the channel is disordered into two positions (sites I
and II) with an abundance ratio of 0.61 and 0.39 (Figure 6d). The
occupancy of total iodine molecules was estimated by
crystallographic analysis to be 79%, indicating strong affinity
between iodine and surface of the channel. Indeed, the iodine
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molecule form a halogen-bond[22] with the oxygen atom of the
carboxy group on the channel surface [distance of O···I: 3.08 Å,
angles of I−I···O and C−O···I: 150.4° and 144.5°, respectively]. In
the channel, iodine molecules are aligned in a three-fold helical
fashion (Figure 6e). Each of iodine molecules contacts with the
neighboring ones with a nearly perpendicular manner [e.g. I2(A)
and I2(B)]: the intermolecular distance (d) and contact angle
between the neighboring iodine molecules (θ) are 3.60 Å and
82.8°, respectively, which are typical values for the perpendicular
halogen-bond in a crystalline state.[23]
To get information on the photodynamics of CPHAT and 4 in
DMF solutions, we performed picosecond time-resolved studies,
exciting at 370 nm and observing at different wavelengths.
Figures 7a and S11a show representative emission decays of
CPHAT and 4 in DMF solutions, respectively. Also, Table S2
gives the time constants (τi), contributions (ci) and preexponential
factors (ai normalised to 100) obtained from multiexponential
global fits of the decays. For 4, we got two decaying components,
whose times are 160±30 ps and 2.41±0.20 ns. The contribution of
the shorter decay is larger than that of the longer one. Based on
solvent effect on emission spectra of molecular systems that
present the same core (HAT) with different surrounding structures,
an ICT process has been suggested to occur.[26] Our time-
resolved results point out to an ICT process within 4 in DMF,
whose time is < 15 ps (the time resolution of our picosecond
setup). Thus, we assign the shortest component, 160±30 ps, to
the lifetime of the initially excited species, and which
photoproduces a charge transfer structure, whose emission
lifetime is 2.41±0.20 ns (Figure S11c). The fluorescence decays
of CPHAT in DMF exhibit a complex multiexponential behaviour,
with time constants of 70±20 ps, 0.23±0.05, 1.0±0.1 and
4.90±0.20 ns (Table S2). The shortest component is decaying at
the bluest part of the spectrum, and becomes a rising one from
525 nm. The intermediate times, 0.23±0.05 and 1.0±0.1 ns, are
decaying in all the spectral range. Finally, the longest component
(4.90±0.20 ns) does not appear at the blue part of the spectrum
and decays from 500 nm. To explain this complex and rich
behaviour, we suggest the involvement of two processes at the
S1 state, and explained it by following (Figure 7c). Upon electronic
excitation of CPHAT, a fast ICT takes places (<15 ps), as it occurs
in 4. The initially excited CPHAT has a fluorescence lifetime of
230±50 ps, while the photoproduced ICT species decays in 1 ns.
Moreover, as a result of ICT where a large electronic redistribution
has occurred, the ICT structure of CPHAT becomes more acidic
as it has a carboxylic group, and undergoes an intermolecular
excited-state proton-transfer (PT) reaction to the surrounding
solvent (DMF), which has a high basicity through its H-bonding
ability (β=0.69).[27] Our analysis of the fluorescence decays at
different wavelengths of observation shows that this time is 70±20
ps. Previous study of comparable HOF systems showed the
formation of this kind of H-bonds in DMF, and which play a key
role in the crystals formation, morphology and stability.[8] Finally,
the resulting formed anion species of CPHAT has an emission
lifetime of 4.90±0.20 ns (Figure 7c).
Steady-state photobehavior
We carried out UV-vis absorption and emission measurements of
CPHAT and its methyl ester derivate 4 in DMF where both
chromophores are relatively well soluble (Figure S10). The UV-
vis absorption spectra present similar shape with a maximum at
368 and 359 nm for CPHAT and 4 respectively. Previous studies
of the parent molecule, HAT, have shown that the UV-vis
absorption spectrum in chloroform presents a maximum at 260
nm corresponding to the S0→S3 transitions, along with a broad
unresolved band around 300 nm, assigned to the S0→S2
transition.[24] HAT structure presents a D3h symmetry, making
forbidden the S0→S1 transition. A theoretical study suggests that
the forbidden transition should be located around 350 nm. Our
data show that the absorption spectra of both CPHAT and 4 in
DMF are characterized by strong absorptions at 368 and 359 nm,
respectively. We assign them to S0→S1 transition, reflecting a
change in the symmetry of this chromophore when compared with
the parent HAT. This change is clearly associated with the
presence of the benzoic acid substituents around the core, in
agreement with our theoretical calculations (Figure S5). As
mentioned above, CPHAT molecule has a twisted conformation
and the RMSD of the twisted HAT core from the mean square
plane is 0.196 Å. Additionally, the absorption spectra of CPHAT
and 4 show a red-shift compared to that of HAT, and which can
be ascribed to a larger π-conjugation, owing to the phenyls and
carboxylic groups. Other systems have shown similar red shift in
the absorption spectra as a result of increase in the electron
density.[25]
The emission spectra of CPHAT and 4 in DMF have a
maximum intensity at 444 and 419 nm, respectively. The
abnormal Stockes shift values (4650 and 4000 cm-1 for CPHAT
and 4, respectively) are results of an intramolecular charge
transfer (ICT) reaction at the S1 state. The full-width at half-
maximum (FWHM) of the emission band of CPHAT (3650 cm-1)
is significant larger than that of 4 (3000 cm-1), and reflects the
formation of another excited species in CPHAT. Indeed, its
emission band shows a long tail at the red side of the spectrum
(Figure S10). The fluorescence quantum yields (φ) of CPHAT and
4 (0.04 and 0.05, respectively) are low due to an efficient crossing
from (π-π*) to no fluorescent (n-π*) states, caused by the
presence of nitrogen atoms. Next part will give us information on
the excited state relaxation.
To get more information on the spectral photodynamics of
CPHAT and 4 in DMF solutions, we recorded the time-resolved-
emission spectra (TRES), following excitation at 370 nm (Figures
7b and S11b, respectively). The TRES of 4 at early times (<1 ns)
are characterized by an emission band centred on 425 nm that
decreases in intensity as time increases. This behaviour is
concomitant with the appearance at longer times of a new band
with a maximum of emission intensity at 475 nm, and which is
reminiscent to the presence of a photoproduced charge transfer
species. Note that this band is not clear in the CW emission
spectrum, as its intensity is weaker than the one at 425 nm. Figure
7b shows the TRES of CPHAT in DMF. At t = 0 ps, the early band
which appears at 435 nm is assigned to the emission of the
Time-resolved photobehavior
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It is natural to think that CPHAT-1-(TCB) and CPHAT-1a crystals
may show photoinduced charge-carrier transport ability owing to
the 1D stacked rigid structure of CPHAT. Therefore, their
crystalline bulks were subjected to flash-photolysis time-resolved
microwave (FP-TRMC) measurements.[28] Surprising, neither
crystalline bulks of these crystals show charge transport ability.
The observed insulation property is presumably brought from the
fact that CPHAT moieties are stacked with 60° rotation, which is
known as the unfavourable arrangements for efficient hopping of
charge carrier species.[29] CPHAT-1-(TCB) and CPHAT-1a
crystal bulks show fluorescence bands with emission maxima at
460 nm, and the quantum efficiencies are 0.041 and 0.016,
respectively. These low values reflect efficient crossing from (π,
π*) state to the less emissive (n, π*) one, as happening in
solutions. It should be noted that other no-radiative decay
channels might be accessible in solid state, as a result of stacking
molecular interactions.[30] Finally, crystals of CPHAT trapping
iodine molecules, CPHAT-1(I2), have a complete emission
quenching due to a charge transfer between the chromophore of
the crystals and iodine molecules, in agreement with the X-ray
diffraction results (Figure 6d). The following paragraph will
provide us information on the fluorescence at a single crystal level.
Single Crystal Fluorescence Microscopy
To get precise information of solid state fluorescence, we
conducted fluorescence confocal microscopy measurements on
single crystals of CPHAT-1-(TCB) and CPHAT-1a. The emission
spectra of CPHAT-1-(TCB) and CPHAT-1a have emission
maxima at 458 nm and 450 nm, respectively (Figure 8a).
Additionally, CPHAT-1-(TCB) has a broad shoulder at around 550
nm, which is not observed in that of CPHAT-1a. The emission
decays, recorded over the entire emission spectrum of CPHAT-
1a, give lifetimes of τ1 = 0.20 ± 0.05 ns (92%) and τ2 = 0.8± 0.1
ns (8%), while those of CPHAT-1-(TCB) are τ1 = 0.30 ± 0.05 ns
(85%), τ2 = 1.40 ± 0.15 ns (12%), and τ3 = 4.5 ± 0.2 ns (3%)
(Figure 8b). However, when the decays were collected at blue
(420 - 470 nm) and green (515 – 565 nm) part of the spectrum,
the fits give: τ1 = 0.30 ± 0.05 ns (90%) and τ2 = 1.1± 0.1 ns (10%);
and τ1 = 0.90 ± 0.1 ns (85%) and τ2 = 4.0± 0.1 ns (15%),
respectively. The lifetimes at the blue emission are comparable to
those of CPHAT-1a, suggesting that the emitters from both
crystals in this spectral region are very similar in nature. On the
other hand, the appearance of the red-edge shoulder in the
emission spectrum of CPHAT-1-(TCB), and the presence of the
new, longer lifetime in the green spectral region indicate the action
of specific interactions (H-bonds) between the carboxylic groups
of the crystal building unit and those of TCB, in agreement with
the results obtained for the CPHAT chromophore in DMF
solutions. Within the experimental error, we observed no
significant dependence of either the emission spectra
position/shape or the emission lifetime values on the crystal
orientation and the targeted position in the crystal. 4 under the
microscope shows an emission band with a maximum at 455 nm,
and a fluorescence lifetime of 0.31±0.05 (97%) and 1.80±0.20
(3%), similar to the obtained for CPHAT-1a. As 4 exhibits an
amorphous structure, its fluorescence anisotropy is ~0 (Figure
Figure 7. (a) Emission decays of CPHAT in DMF upon excitation at 390 nm,
and observation at the indicated wavelengths. The solid lines are from the
best-fit using a multiexponential function. (b) Normalized time-resolved
emission-spectra of CPHAT in DMF upon excitation at 370 nm, and gating at
the indicated delay times. The peak at 420 nm is from the solvent raman
emission. (c) Proposed photodynamic scheme for CPHAT in DMF solutions,
where τICT and τPT are the times for intramolecular charge transfer and
intermolecular proton transfer reactions.
initially excited CPHAT population. The peak at 420 nm arises
from the Raman signal of the solvent. As the gating time increases,
the spectrum shifts to longer wavelengths, and becomes wider.
The spectral shift from 0 to 6 ns gating time is large; about 3070
cm-1. This confirms the photoproduction of another structure (ICT
one) from the initially excited species. At gating times longer than
~750 ps, the spectrum exhibits a new band whose maximum is
centred at 532 nm, and a red shift of 4200 cm-1. This time-
dependent spectral photobehavior indicates
a subsequent
process within the ICT structure. Thus, while the earliest spectral
shift reflects the ICT process within the initially excited species,
the latest one is due to a subsequent CPHAT anion formation as
explained above (Figure 7c).
Solid-state Photobehaviour
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Figure 8. (a) Emission spectra and (b) decays of CPHAT-1-(TCB) (blue) and CPHAT-1a (red) single crystals. The excitation wavelength was 390 nm and the
decays were measured over the whole spectral range using a 430 nm long-pass filter (HQ430LP, Chroma). Dependence of the emission anisotropy on the
orientation of single crystals of (c) CPHAT-1-(TCB) and (d) CPHAT-1a with respect to the plane of observation. i) Histogram of the emission anisotropy for the
horizontal crystal orientation; ii) histogram of the emission anisotropy for the vertical crystal orientation. (Insets) anisotropy distribution image for the corresponding
crystal orientation. The anisotropy was calculated with respect to the excitation polarization (vertical). The limits for the anisotropy are -0.4 to 1.0, where -0.4
corresponds to perpendicular orientation and 1.0 to parallel.
S13). Remarkably, the single crystals of CPHAT-1-(TCB) and
CPHAT-1a show highly anisotropic emission behaviours (Figure
8c,d). Both CPHAT-1-(TCB) and CPHAT-1a present similar
values of the anisotropy that depend on the orientation of the
crystals. The anisotropy for those oriented perpendicularly to the
plane of observation has a value of 0.65, while those with parallel
orientation give a value of -0.32. This behaviour (strong and in-
sign different anisotropies) suggests an ordered crystalline
structure for both CPHAT-1-(TCB) and CPHAT-1a with
preferential orientation of the molecular dipole moments
perpendicular to the long crystal axis. The emission
polarization/anisotropy behaviour of the ordered parent molecules
in the single crystals imply that they form a π-stack with the π-π
stacking direction parallel to the length axis of the crystal. This
observation is reminiscent to the crystal morphology and structure
shown in Figure 2. Similar behaviour has been reported for other
single crystalline molecular structures formed either by π-π
stacking or J-aggregates interactions between the constituents
[30c,31] It is remarkable that the anisotropy distribution histogram for
CPHAT-1-(TCB) monocrystal is significantly broader (full width at
half maximum, FWHM ~ 0.12) than the CPHAT-1a narrow one
(FWHM ~ 0.05) (Figure 8a,b). This indicates that the presence of
TCB molecules in the crystal structure of CPHAT-1a disrupts the
π-π packing of the parent HAT molecules and leads to a more
heterogeneous distribution of the molecular interactions between
the chromophore building units within the studied CPHAT-1-
(TCB) single crystals. We reflected this difference in the colour of
the imaged crystals under different orientations (Figure 8c). On
the contrary, the CPHAT-1a crystal exhibits a very homogenous
distribution, as a result of similar interactions of the unit forming
the crystal (Figure 8d). Thus, the observed difference in the
FWHMs of the anisotropy distributions can act as a “ruler” of the
molecular orientation within the crystal, which should be relevant
to the defect formation, of great relevance for MOFs, COFs, and
HOFs stability and use.[32]
Conclusion
We revealed that a new hexaazatriphenylene derivative with six
carboxyphenyl groups, CPHAT, forms three dimensionally
networked framework with pcu topology, which subsequently
interpenetrated by 4-fold to yield porous HOF CPHAT-1. The
activated CPHAT-1a retains not only permanent porosity but
single-crystallinity. Furthermore, CPHAT-1a keeps its framework
up to 340 °C, or in hot water, and in acidic aqueous solutions.
Such structural stability of CPHAT-1a enables precise structural
characterization of the framework accommodating guest
molecules based on single crystalline X-ray diffraction analysis
and evaluation of their structure dependent properties. These
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(g) J. Lü, C. Perez-Krap, M. Suyetin, N. H. Alsmail, Y. Yan, S. Yang, W.
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Xiang, B. Chen. J. Am. Chem. Soc. 2015, 137, 9963-9970.
results clearly show that even a simple H-bonding motif can be
applied for construction of robust HOFs.
Surprisingly, CPHAT in DMF solutions exhibits a strong
allowed S0→S1 transition, while it is prohibited in its parent
molecule (the core), which does not display this absorption band.
The change in the symmetry of CPHAT is due to the introduction
of the benzoic groups in the core. These groups act as a glue to
build up single crystals (of micrometers in length) having a clean
rod shape. CPHAT molecule shows broad emission band as a
result of an ultrafast ICT process followed by an intermolecular
proton transfer reaction (70 ps) at the excited state with DMF
molecules to generate the anion of CPHAT. Fluorescence
microscopy study at single crystal level reveals an ordered
crystalline structure for both CPHAT-1-(TCB) and CPHAT-1a with
preferential orientation of the molecular dipole moments.
Interestingly, the interaction of CPHAT-1a with TCB molecules in
the formed crystal leads to a heterogeneous distribution of the
molecular interactions as a result of H-bonds formation with the
co-solvent upon crystal formation.
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This work is supported by a Grant-in-Aid for Scientific Research
(C) (JPT15K04591) and for Scientific Research on Innovative
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the FPU fellowship. We thank Prof. Shu Seki at Kyoto University
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Significantly robust hydrogen-bonded
I. Hisaki,* N. Ikenaka, E. Gomez, B.
Cohen, N. Tohnai, A. Douhal*
organic
framework
retaining
permanent porosity and single-
crystallinity was established with
Page No. – Page No.
hexaazatriphenylene
(CPHAT), enabling precise structural
characterization
evaluations.
derivative
Hexaazatriphenylene-Based,
Hydrogen-Bonded Organic
Framework with Permanent Porosity
and Single-Crystallinity: Synthesis,
Structural, Uptaking and
and
property
Photophysical Characterization
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