Single-crystal-to-single-crystal guest exchange in columnar assembled brominated triphenylamine bis-urea macrocycles

Article abstract of DOI:10.1039/c9cc01725a

Self-assembly of brominated triphenylamine bis-urea macrocycles affords robust porous materials. Urea hydrogen bonds organize these building blocks into 1-dimensional columns, which pack via halogen-aryl interactions. The crystals are stable when emptied,

Full text of DOI:10.1039/c9cc01725a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
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Single-crystal-to-single-crystal guest exchange in columnar
assembled brominated triphenylamine bis-urea macrocycles
a
a
b
b
c
Ammon J. Sindt, Mark D. Smith, Samuel Berens, Sergey Vasenkov, Clifford R. Bowers, and
Linda S. Shimizu *a
jReceived 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Self-assembly of brominated triphenylamine bis-urea macrocycles crystallization are highly sought after since they offer tunability
15
results in the formation of robust porous materials. Urea hydrogen within their host framework. The Shimizu group employs
bonds organize these building blocks into 1-dimensional columns, simple bis-urea building blocks that stack into pillars and
which pack via halogen-aryl interactions. The crystals are stable columns to form nanoporous molecular crystals that can be
129
16
when emptied, show defect free channels by Xe PFG NMR used as containers for photochemical reactions. While
measurements, and exhibit single-crystal-to-single-crystal guest experimental evidence suggests these frameworks remain
exchange.
intact during the process of guest exchange and subsequent
photo-reactions, this is our first demonstration of SC-SC
transformations. We report a porous organic material made
1
Porous materials are advantageous for catalysis, as
nanoreactors, and for the confinement of photo-luminescent
compounds as well as for storage, sensing, and separations
of small molecules. Key to these processes is how the host and
guest influence and interact with each other to afford
synergetic properties. Single-crystal-to-single-crystal (SC-SC)
transformations can follow these molecular processes by
providing atomic details to elucidate the factors that guide the
molecular interactions. SC-SC transformations can be triggered
2
from a triphenylamine (TPA) bis-urea macrocycle
1, which
3
4
5
6
contains two bromo-TPA units (Fig. 1). This macrocycle
crystallizes into columnar structures through urea-urea
interactions with columns packing together with π-π and
halogen-π interactions forming large crystals (35 x 265 μm) that
are robust and suitable for SC transformations. Simple heating
removes the guest affording homogeneous nanochannels.
Immersion of the material into an organic solvent results in a
7
under a number of conditions including: temperature, photo-
irradiation, guest inclusion, pressure,
8
9
10
and mechano-
1
1
responses. Here, we investigate SC-SC guest exchange in
porous organic crystals of triphenylamine bis-urea macrocycles.
Typically, hosts for these studies are assembled from rigid
materials. Examples include metal organic frameworks
1
2
13
(
MOFs), hydrogen bonded organic frameworks (HOFs), and
14
porous organic materials. The latter can be beneficial due to
the structural versatility organic molecules provide and the ease
of forming porous materials simply through crystallization.
However, it can be challenging to predict how molecules will
assemble in the solid-state. Close-packing principles are often
at odds with forming permeable materials. Therefore,
molecular families that reliably form porous materials upon
a.Department of Chemistry and Biochemistry, University of South Carolina,
Columbia, SC 29208. Email: SHIMIZLS@mailbox.sc.edu.
Department of Chemical Engineering, University of Florida, Gainesville, FL 32611.
Department of Chemistry, University of Florida, Gainesville, FL 32611.
Electronic Supplementary Information (ESI) available: Experimental details and
synthesis/characterization of compounds. CCDC 1899526-1899533. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/x0xx00000x
b.
c.
Fig. 1 (A) Self-assembly of macrocycle 1 from the vapour diffusion of DME
into DMSO leads to 1D channels. Subsequent heating activates the
channels for the loading of different guests. (B) SC-SC transformations
observed upon soaking activated host 1 crystals in guest liquids.
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SC-SC transformation to afford a new host:guest complex (Fig.
B). These complexes organize guests near photoactive TPA
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DOI: 10.1039/C9CC01725A
1
units and should consequently enable us to study the effects of
closely oriented guests on the optoelectronic properties.
Macrocycle
1 was synthesized by a strategy similar to its
17
linear counterpart. First, commercial bromotriphenylamine
was converted to the dialdehyde using a Vilsmeier-Haack
reaction followed by hydride reduction resulting in the diol.
Once the alcohols were brominated, two TPA units were
connected with two triazinanone spacers under basic
conditions, resulting in the protected macrocycle. These
macrocycles crystallized in chloroform solutions as colourless
Fig. 2 (A) View along a single column illustrating the 2:1 host:guest ratio
and the three-centred urea hydrogen bonding motif. (B) dnorm surface
showing urea interactions (circled in red). (C) dnorm surface showing
halogen-π interactions (circled in red). (D) Crystal packing showing
select close contacts between columns.
blocks as a 1:8 macrocycle:CHCl
purification (Fig. S9). Subsequent urea deprotection with
diethanolamine under acidic conditions afforded
powder.
Vapour diffusion of 1,2-dimethoxyethane (DME) into a
dimethylsulfoxide (DMSO) solution of
large colourless needles (0.6 x 0.08 x 0.04 mm ) crystallizing in
3
solvate, enabling their
1 as a beige
interactions while the extra phenyl groups in
stacking. The alternative edge-to-face aryl stacking interactions
in and give the channels a curvature highlighted in blue in
1 lead to offset π
1
(~2.5 mg/mL) produced
3
2
3
the space group P2 /c of the monoclinic system. In this
1
Fig. 3B, which oscillates back and forth along the length of the
columns. These oscillations are also pronounced in the offset-
structure, the macrocycle adopts an anti-conformation with
encapsulated, disordered DME solvent in a 2:1 ratio (Fig. 2A).
Both the macrocycle and DME solvent were found on
crystallographic inversion centres with the solvent being
situated across an additional inversion centre leading to its
disorder within the channels. Individual macrocycles assemble
into columnar structures organized by the characteristic three-
aryl stacked structure of host
1. The channels of hosts 2 proved
to be an ideal substrate for monitoring single file diffusion of
20
Xenon. Therefore, we sought to test if the framework of 1 was
stable in the absence of guests to see if it could be used for a
similar application.
To monitor guest removal, thermogravimetric analysis
centred urea hydrogen bond, with N(H)
⋯O distances of
(
TGA) was applied. Host
desorption curve with a weight loss of 5.3% between 0 and 90°
Fig. S18). Higher temperatures (> 90°C) caused degradation of
1·DME crystals displayed a one-step
2
.848(4) and 2.929(4) Å. This creates infinite hydrogen bonded
tubes along the crystallographic b axis with a macrocycle to
macrocycle repeat distance of 4.620(2) Å. π-stacking between
neighbouring TPAs provides additional stabilization within the
columns (Fig. S17).
Individual columns assemble into pseudo-hexagonal rod-
packing arrays, similar to other bis-urea structures. However,
(
the material, which was readily detected by NMR. From the
percent weight loss, we calculated the macrocycle:guest
stoichiometry as 1:0.5. On average a host:guest ratio of 1:0.55
was found over three batches of crystals. The larger crystal size
and heavy bromine atoms in
1 facilitated rapid monitoring of
crystals of
1 are on average ten times larger in width (35 x 265
the empty host framework by SC-XRD. To ascertain if the host
framework would be maintained, one freshly activated crystal
μm versus 3 x 250 μm, Fig. S10) than any other previously
obtained bis-urea macrocycle derivative. Hirshfeld analysis
16
was used to identify key interactions that guide assembly and
subsequent packing of the columns of 1 to facilitate the growth
18
of larger crystals. Fig. 2B and C shows the dnorm surface and
highlights the key urea motif driving individual column
formation as well as close intercolumnar contacts occurring
between the bromine substituents and aryl rings. The halogen-
π interactions illustrated in Fig. 2C displays a Br⋯Caryl distance
of 3.303(3) Å, which is shorter the sum of the vdw radii (3.5 Å)
suggesting that the p-bromophenyl groups significantly
increases intercolumnar interactions in
1 versus the more
19
cylindrical bis-urea macrocycles.
Fig. 3 compares a series of bis-urea macrocycles that
assembled into similar 1-dimensional columns. Host , phenyl
ether ( ), and benzophenone ( ) have similar cavity sizes and
1
2
3
topographies (Table S3). The cavities are roughly elliptical,
displaying cross sectional diameters of ~ 4 Å × 7 Å (Fig. 3A). The
walls of these channels are held together by urea hydrogen
bonds, with further stabilization coming from aryl stacking
Fig. 3 (A) Pore sizes of different bis-urea macrocycles subtracting the vdw
radii. From left to right the urea spacers are 4-bromotriphenylamine,
phenyl ether, and benzophenone. (B) Comparison of their corresponding
1-dimensional columns of 1-3 with their void space highlighted in blue.
interactions. In
2 and 3, these are edge-to-face π-stacking
2
| J. Name., 2012, 00, 1-3
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was examined immediately after TGA completion and a second attenuation for the Xe line at 206 ppm allows us tVoieweAsrttiicmleaOtnelinae
crystal after three days on the lab bench in ambient air. lower limit of 100 ms on the exchange timDeO(Is:e10e.1S0.3I.9)/.CT9hCeCr0e1f7o2r5eA,
Remarkably, the structures, including the columnar framework we can conclude that there are no defects in the channel walls
and packing, were nearly identical to the
that the electron density of the DME was absent (Fig. 1A,
activated host). The largest electron density maxima found small molecules, we treated the activated crystals with a series
1
·DME crystals except that might lead such an exchange.
To investigate the ability of this host to absorb and store
-
3
inside the channels within either data set was 0.21 e / Å , i.e. of halogenated benzenes. Host
1·DME crystals were
essentially background noise (see S.I.). These results consistently activated for SC-SC exchange by heating at 90˚C for
demonstrate the stability of this assembled material under ~ 2.5 h until no further weight loss was detected via TGA (Fig.
ambient conditions in the absence of guests.
S18). Freshly activated crystals (5 mg) were then immersed in a
To further characterize the pore space architecture of host liquid guest (1 mL) for 1 day followed by examination with SC-
, freshly evacuated crystals were pressurized to 9.5 bar (at 298 XRD. SC-SC transformations were observed giving five host
1
K) with isotopically enriched Xe gas and sequentially examined
1
·guest structures that displayed 1:0.5 host guest stoichiometry
129
129
by Xe NMR. Xe NMR has previously been used to study 1D including ·C ·C F, ·C Cl, ·C Br, and ·C I. All the
1
6
6
H ,
1
H
6 5
1
6
H
5
1
6
H
5
6 5
1 H
channels since the 129Xe NMR chemical shift tensor is highly inclusion crystals were found to be isoskeletal with one another,
sensitive to the pore-space structure and shows dependence on the original DME solvate, and the activated host. Fig. 1B
de-shielding due to Xe-Xe interactions, especially at higher Xe highlights the similarity between these host:guest complexes.
21
loadings in single-file nanotubular pores, where cross-sectional
In all cases, the guests showed a moderate amount of
dimensions are comparable to the vdw diameter of the Xe atom disorder within the columns. Fortunately, the halogen-
2
2,23
129
(0.44 nm).
Fig. 4A and B show the NMR spectra for Xe·
1
substituted guests permitted more reliable determination
129
at 295 and 243 K referenced to gas phase Xe at 0 ppm. A well- despite this disorder due to the larger X-ray scattering factors
defined axially symmetric chemical shift anisotropy (CSA) (especially for Cl, Br and I). The guests aligned in a planar tape-
powder pattern with δiso = 217 ppm emerges upon cooling the like manner within the channels with guests being
sample to 243 K. This Xe CSA tensor is consistent with a high crystallographically modelled on two independent sites, one
Xe loading in channels with the dimensions of host
129
23
1
.
The having the halogen-benzene bond more perpendicular to the
symmetric peak centered near 310 ppm is attributed to highly macrocycle, shown for iodobenzene in Fig. 5A (red structure)
confined Xe atoms residing in pores with (dynamically with the other in a slightly tilted orientation (Fig. 5A, orange).
averaged) cubic symmetry in host 1, tentatively identified as the Both of these sites were located near inversion centres (Fig. 5A,
inter-columnar pores (Fig. S20). The ratio of the areas of the blue and green) giving a total of four possible sites for the guest
adsorbed Xe peaks are close to 3:1 at both temperatures. In the location, with each having similar occupancies. This disorder
spectrum recorded at 295 K, the small peak that appears near was quite similar across all structures. The alignment of the
2
60 ppm (indicated by the arrow in Fig. 4A) suggests that Xe is guests in all of these structures may arise from C-H
in chemical exhange between the two types of pore spaces. and/or C-H π interactions; however, these details are
Xe PFG NMR experiments were performed at 298 K to obscured by the crystallographic disorder.
investigate the diffusion of Xe atoms adsorbed inside the 1-D In summary, a brominated TPA bis-urea macrocycle
channels. Unfortunately, short T NMR relaxation times (see assembled to form robust crystals with accessible columnar
S.I.) prevented us from using sufficiently large gradient pulse channels suitable for SC-SC guest exchange. The host is stable
⋯halogen
⋯
129
2
129
durations and amplitudes to measure intra-channel diffusion. when emptied and exhibits confined Xe NMR signals when
However, these diffusion studies allowed us to qualitatively pressurized under Xenon.
1
29
Xe PFG NMR measurements
examine the exchange of Xe atoms in and out of the channels suggest these channels are homogeneous and defect free.
on the time scale of the diffusion observation (5 - 100 ms). It Most intriguingly, assembly of this macrocycle orients the
was found that a complete diffusion attenuation of the gas- individual TPAs close in space to potential guests and enforces
phase line could be achieved with an expected gas-phase close contacts between the two units. Since TPAs are known to
diffusivity of 6.7x10-7 m /s at 298 K. However, there was no undergo chemical or electrochemical oxidation to generate
noticeable diffusion attenuation of the line corresponding to Xe radical cations, these crystalline materials offer potential
atoms adsorbed in the channels (Fig. 4C). This was seen for all models for the investigation of electron transfer between
diffusion times used. The observed lack of the diffusion organic molecules within confinement. Indeed, linear analogues
2
Fig. 4 129Xe NMR spectra of 1 acquired at 138.45 MHz (11.756 T) at (A) 295 K and (B) 243 K by accumulating 1920 and 960 transients respectively, with
a recycle delay of 40x and pulse length of 10 s. The dashed blue trace is the least-squares fit24 to an axially symmetric chemical shift anisotropy powder
pattern. (C) 129Xe NMR spectra measured using a stimulated echo PFG NMR sequence at 298K and diffusion time 5 ms.
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198; K. Yan, R. Dubey, T. Arai, Y. Inokuma, and M. Fujita, J.
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2
of host
1 showed stable and regenerable radical formation upon
17
UV-irradiation in the solid-state. Currently, we are evaluating
2
and hope to report on these in due time.
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This work was supported in part by the National Science
Foundation (NSF) CHE-1608874 and OIA-1655740. A portion of
this work was performed at the National High Magnetic Field
Laboratory’s AMRIS Facility, which is supported by NSF
Cooperative Agreement No. DMR-1644779 and the State of
Florida.
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Conflicts of interest
There are no conflicts to declare.
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Crystals of brominated triphenylamine bis-urea macrocycles are robust materials with can undergo
single-crystal-to-single-crystal guest exchange inside 1-dimensional columns.

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