Solvent-Modulated Emission Properties in a Superhydrophobic Oligo(p-phenyleneethynylene)-Based 3D Porous Supramolecular Framework

Article abstract of DOI:10.1021/acs.inorgchem.8b00584

A chromophoric oligo(p-phenyleneethynylene) (OPE) bola-amphiphile with dioxyoctyl side chains (H₂OPE-C₈) has been self-assembled with Cd^II to form a 1D coordination polymer, {Cd(OPE-C₈)(DMF)₂(H₂O)} (1), which is further interdigitated to form a 2D network. Such 2D networks are further interwoven to form a 3D supramolecular framework with surface-projected alkyl chains. The desolvated framework showed permanent porosity, as realized from the CO₂ adsorption profile. 1 showed high water contact angles, portraying its superhydrophobic nature. 1 also showed a linker-based cyan luminescence. Solvent removal led to a bathochromic shift in emission into the green region. Resolvation with N,N-dimethylformamide brought back the original cyan emission, whereas for tetrahydrofuran, ethanol, and methanol, it persisted at an intermediate state. Density functional theory calculations unraveled that, twisting of the OPE phenyl rings generated the red shift in emission.

Full text of DOI:10.1021/acs.inorgchem.8b00584

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Solvent-Modulated Emission Properties in a Superhydrophobic
Oligo(p‑phenyleneethynylene)-Based 3D Porous Supramolecular
Framework
Syamantak Roy,† Venkata M. Suresh,†,‡ Arpan Hazra, Arkamita Bandyopadhyay, Subhajit Laha,†
,‡
†
§
Swapan. K. Pati,§,⊥ and Tapas Kumar Maji*,†
†
§
⊥
Molecular Materials Laboratory, Chemistry and Physics of Materials Unit, New Chemistry Unit, and Theoretical Sciences Unit,
Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India
*
S Supporting Information
MOFs has also been achieved via the encapsulation of emissive
15−17
ABSTRACT: A chromophoric oligo(p-phenyleneethyny-
dyes in its pores.
nanoscale to fabricate thin-film devices for optoelectronics.
Several π-conjugated systems have been exploited to generate
They have also been scaled down to the
18,19
lene) (OPE) bola-amphiphile with dioxyoctyl side chains
II
(
H OPE-C ) has been self-assembled with Cd to form a
2
8
4
,6
1
D coordination polymer, {Cd(OPE-C )(DMF) (H O)}
8 2 2
MOFs with excellent luminescent properties. Oligo(p-phenyl-
eneethynylenes) (OPEs) are one such class of molecules that lead
to highly luminescent MOF structures. OPEs are chromophoric
systems whose structures are built up with aryl units connected
(
1), which is further interdigitated to form a 2D network.
Such 2D networks are further interwoven to form a 3D
supramolecular framework with surface-projected alkyl
chains. The desolvated framework showed permanent
20−22
via ethyne linkages.
This creates structural rigidity and
porosity, as realized from the CO adsorption profile. 1
2
extensive π conjugation, leading to its excellent luminescence and
showed high water contact angles, portraying its super-
hydrophobic nature. 1 also showed a linker-based cyan
luminescence. Solvent removal led to a bathochromic shift
in emission into the green region. Resolvation with N,N-
dimethylformamide brought back the original cyan
emission, whereas for tetrahydrofuran, ethanol, and
methanol, it persisted at an intermediate state. Density
functional theory calculations unraveled that, twisting of
the OPE phenyl rings generated the red shift in emission.
intrinsic conductivity. The simultaneous existence of both
properties is indispensable for organic light-emitting transistors
23−25
and device applications.
Additionally, the ease of function-
alization of OPE side chains with long alkyl groups would
decrease the surface energy, leading to internal porous and
external surface water-repellent properties in the MOF
structures. We have recently shown that linking bola-amphiphilic
OPE ligands with long alkyl chains to metal ions generated
luminescent, semiconducting, and superhydrophobic nanoscale
26,27
MOFs.
The nanoscale architecture, also generated through
this approach, allowed coating over large areas for self-cleaning,
device fabrication, and bioimaging applications.
27,28
uminescent materials have found widespread applications in
L
sensors, light-emitting diodes, bioimaging, light harvesting,
Keeping this in mind, we employed a dioxyoctyl-functionalized
1
,2
II
etc. Porous luminescent metal−organic frameworks (MOFs)
OPE dicarboxylate (H OPE-C ) for self-assembly with Cd into
2
8
cover all of these applicative areas, with a host of other
a 3D supramolecular porous framework, {Cd(OPE-C )-
8
3
,4
advantages. In general, emission in MOFs can be realized by
(DMF) (H O)} (1; CCDC1827675) in a N,N-dimethylforma-
2
2
the lanthanide metal ions, π-chromophoric linkers, ligand-to-
mide (DMF)/ethanol mixture. Photoluminescence (PL) meas-
urements indicated a linker-based cyan emission of 1. Degassed 1
(1a) showed a bathochromic shift in emission to the green
region. Such a change in emission is supported by density
functional theory calculations. Interestingly, the emission of 1
could be tuned based on the reintroduction of different solvent
vapors.
5
,6
metal charge transfer, or metal-to-ligand charge transfer.
Furthermore, the permanent porosity of MOFs allows for the
encapsulation of a variety of guest molecules, and this allows
7
further modulation of the emission properties. This can arise
because of either (a) inherent emission of the guest, (b) exciplex
formation, (c) charge-transfer complex formation, or (d)
8
−10
structural changes induced by guest removal/encapsulation.
H OPE-C was synthesized according to reported Sonoga-
2 8
29
Structural modification occurs when the MOF structure distorts
upon desolvation, leading to an emission different from that
initially originating from the as-synthesized structure. Further,
reversible tuning of this emission can occur when the solvent
molecules recoordinate to the unsaturated metal center or
occupy the pores of the MOFs. This versatility in tuning the
emission color offers excellent sensing applications of these
shira−Hagihara coupling procedures. Green, block-shaped
crystals of 1 were isolated in a crystal tube following the
procedures described in the Supporting Information. Single-
II
crystal structure determination of 1 revealed. The Cd center
here is heptacoordinated with four coordination sites made up of
the O atoms of two adjacent OPE carboxylates (Figure 1a). Two
O atoms from two DMF molecules make up the fifth and sixth
11
systems via vapochromism. Emission of MOFs has also been
successfully utilized in the sensing of metal cations and
Received: March 6, 2018
1
2−14
anions.
Furthermore, light harvesting using luminescent
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00584
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Inorganic Chemistry
Communication
3
cm /g, with negligible uptake at low pressure. Hence, the pore
surface has less affinity toward water (Figure S15b). For contact-
angle measurements, we drop-casted an ethanolic dispersion of 1
on a glass plate and put 5-μL-volume water droplets on the coated
surface. The contact angle measured at different positions of the
coated surface varied between 153 and 157°, proving that the
surface was indeed superhydrophobic. Interestingly, 1a also
revealed contact-angle values similar to those of 1 (Figure S9b,c).
Detailed photophysical investigations of 1 and 1a were then
performed. 1 absorbed at 400 nm in the solid state and emitted at
Figure 1. (a) Structural representations of 1: (a) view of the asymmetric
unit and 1D coordination chain; (b) view of the 3D structure formed via
interweaving of the 2D sheets.
4
88 nm, showing a strong cyan luminescence (Figure S10).
H OPE-C also showed the same emission maximum upon the
2
8
same excitation (Figure S11). This proved that the emission of 1
was ligand-centered. We then calculated the highest occupied
molecular orbital (HOMO)−lowest unoccupied molecular
orbital (LUMO) levels of the fragmented asymmetric unit of
the compound by HSE06. It was clearly seen that the both the
HOMO and LUMO of 1 were populated by the orbitals of π-
chromophoric OPE, further confirming our conjecture that the
emission originates from the ligand (Figure S12). 1 and 1a also
showed an absorption maximum centered around 400 nm
coordination sites. The seventh coordination site is filled by a
H O molecule. The three phenyl rings constituting the OPE
2
backbone lie in the same plane, and there exists minimal twisting
of the OPE core (Figure 1a). The 1D chain of 1 is generated by
coordination of the terminal carboxylates of adjacent OPE-C to
8
II
Cd , which extends in a zigzag fashion (Figure 1a). Two such 1D
chains pass each other in a crisscross fashion, meeting at the
II
respective Cd centers (Figure S1, highlighted in red and green).
(
Figure S13). However, when the emission spectrum of 1a was
Each individual 1D chain is further packed into another via the
interdigitation of alkyl chains to form unique 2D sheets in the ac
plane (Figure S2). These 2D sheets (red and green) are further
interwoven, oblique to the ab plane, via C−H···π interactions and
C−H···O interactions (Figure S3). This leads tothe generation of
a 3D supramolecular architecture of 1. When viewed along the c
axis, inward projecting oxyoctyl chains and coordinated DMF
molecules are found in the pore (Figure 1b). This arrangement
also leads to the oxyoctyl chains projecting outward toward the
surface, which might lead to possible superhydrophobicity of the
surface and the pores (Figure 1b).
recorded at the same excitation wavelength, there was a red shift
of the emission wavelength to 525 nm (Figure 2a). It is known
The IR spectrum of 1 revealed the presence of H O molecules
2
(
Figure S4). Thermogravimetric analysis (TGA) of single crystals
of as-synthesized compound 1 revealed a 15.5% weight loss up to
50 °C corresponding to the loss of coordinated H O and DMF
2
2
molecules, after which the structure is stable up to 300 °C. TGA
of 1a expectedly showed no weight loss due to coordinated
solvents, with the framework also being stable up to 300 °C
Figure 2. PL plots: (a) experimental for 1, 1a, and DMF-exposed 1a; (c)
theoretical for 1 (black) and 1a (red). (b) Images of 1 and 1a under
normal and UV light. (d) HOMO (lower) and LUMO (upper) plots for
linear (left) and bent (right) structures of 1.
(Figure S5). Powder X-ray diffraction (PXRD) of 1 revealed a
good coherence with the simulated pattern from single-crystal X-
ray diffraction (SCXRD), suggesting phase purity of the
compound (Figure S6). Interestingly, there was a noticeable
change in the structure of 1 upon desolvation, observed via the
loss of numerous Bragg reflections and also a shift of the low-
angle peak at higher angles. This suggests an expected structural
reorganization (Figure S6). PXRD also reveals that the structure
comes back to the as-synthesized one (1) upon exposure to DMF
vapors, proving reversibility of the structure transformation
that OPE systems show a change in the emission color based on
the degree of twisting of the central phenyl ring, i.e., ligand
30
conformation. Previous reports suggest that a large bath-
ochromic shift occurs when the central phenyl ring is twisted out-
31
of-plane of the terminal phenyl rings. Excimer formation also
leads to a bathochromic shift in the emission. So, to rule out one
possibility over the other, we carried out lifetime measurements
on 1 and 1a. The lifetimes of both 1 and 1a monitored at 488 nm
and excited at 405 nm revealed minimal changes. 1 had an
excited-state lifetime of 1.6 ns, whereas 1a revealed a lifetime of
1.8 ns (Figure S14). These ruled out any possibility of excimer
formation. Additionally, there was a minimal change of 2 nm in
the UV spectra of 1 and 1a, suggesting no ground-state
interaction (Figure S13). Thus, the bathochromic shift in 1a
may be due to the twisting of OPE chains. To gain more insight
into this phenomenon, theoretical modeling was carried out. The
central benzene ring of 1 was rotated by 20° to check the effect of
the molecular plane rotation on the emission spectra of the
system. It was found that the emission spectra of the rotated
system was red-shifted by 44.8 nm, with peaks at 447.6 nm for the
twisted system and 402.8 nm for the linear system (Figure 2c). To
(
Figure S7). Adsorption experiments on 1, degassed at 200 °C
under vacuum, were carried out to investigate its porosity. The N2
adsorption curve at 77 K revealed a minimal uptake of 30 mL with
a type II profile (Figure S8). However, the CO adsorption profile
2
at 195 K revealed a type I profile, with a final uptake of 40 mL
indicating the microporosity in 1 (Figure S15a). The Langmuir
surface area calculated from the CO adsorption profile was 135
2
2
m /g.
As discussed before, SCXRD revealed surface-projected
periodic octyl chains (Figure S9a). This made us interested in
probing its pore hydrophobicity and surface contact angles
because long alkyl chains are reported to decrease the surface free
2
6
energy and hence increase the hydrophobicity. Water
adsorption measurements on 1a showed a final value of 25
B
DOI: 10.1021/acs.inorgchem.8b00584
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Inorganic Chemistry
Communication
find the reason, we further plotted the wave functions
corresponding to these transitions and saw that, in the rotated
system, the nature of the orbital overlap changed, leading to a red
shift of the spectra. In both cases, the transitions were of π−π in
nature, but for the linear case, the transition occurred from an
upper energy level (where the corresponding molecular orbital
has 9 nodes) to the lower energy level (where the molecular
orbital has 8 nodes). In the case of the twisted molecule, the
transition occurred from an energy level with the molecular
orbital having 9nodes to a lower energy level where the molecular
orbital also had 9 nodes. This leads to a lower energy gap or
higher wavelength transition for the twisted system (Figure 2d).
Visual observations under normal and UV light for 1 and 1a also
corroborated the red shift in emission (Figure 2b).
(Figure S7). This further proves different levels of structural
reorganization upon solvent reexposure.
To conclusively prove the solvent-tuned emission behavior of
1, we then carried out solvent-vapor adsorption isotherms on 1a.
The adsorption profiles of water, methanol, and THF vapors
were analyzed (Figure S15b). The uptake profile for methanol
initially showed minimal uptake at lower pressures. However, it
portrayed a gated behavior at higher pressures, leading to a final
uptake of 64 mL/g. The adsorption profile of THF showed a
gradual uptake of 60 mL/g. It was discussed in earlier sections
that water showed a minimal uptake at low pressure. Therefore, it
can be inferred that, at ambient conditions, the diffusion of
solvent molecules affects the degree of rotation of OPE chains,
leading to the variable emission seen in 1.
To summarize, we have crystallized and structurally charac-
terized a luminescent and superhydrophobic 3D supramolecular
porous framework (1). Investigations revealed that super-
hydrophobicity arose due to surface-projected alkyl chains,
which are known to decrease the surface energy. Additionally, 1
shows solvent-modulated emission behavior based on desolva-
tion and resolvation. Solvent-vapor adsorption studies and
theoretical calculations provide explanations for such behavior.
We believe that this work could act as a guiding principle for the
design of porous materials showing water-repellent and tunable
emission behavior.
Upon resolvation with DMF, the emission of 1 came back
again to the original emission peak position of 488 nm (Figures 2a
and 3b). Visual observation also corroborated the above
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00584.
Figure 3. (a) Images showing the emission color changes and
corresponding wavelengths of emission when 1a was exposed to
different solvent vapors. (b) Corresponding PL profile showing the
associated changes in emission.
Detailed synthetic procedure, structural figures, IR, TGA,
crystallographic tables, PXRD, and adsorption and lifetime
data (PDF)
Accession Codes
wavelength shift (Figure 3a). It can be clearly seen in the figure
that 1 emits cyan fluorescence, whereas 1a emits a strong green
emission. However, upon exposure to water, the emission
maxima did not shift back to the original one nor was there a
change in the green emission color (Figure 3a). To test whether
other organic solvent vapors can indeed be reintroduced using 1a,
we exposed it to chloroform, tetrahydrofuran (THF), acetoni-
trile, acetone, ethanol, and methanol. Interestingly, the emission
color reversed back to the cyan color, only for DMF and
acetonitrile. Figure 3a summarizes the emission wavelengths
observed upon exposure to the above-mentioned solvent vapors.
It was also clear that ethanol, methanol, acetone, and THF only
brought back the emission wavelength to an intermediate value
between the as-synthesized and degassed samples. Visual
observation also suggests the appearance of an intermediate
emission color. It may be concluded that the incorporation of
these solvents into the pore induces twisting of the phenyl rings of
the OPE chains, only up to a certain degree, after which it gets
fixed. This, in turn, shifts the emission color only up to an
intermediate state. This is further proven by observing the PXRD
patterns of all of the solvent-reexposed structures of 1a. The
PXRD patterns of DMF@1a and acetonitrile@1a match almost
exactly with that of 1, revealing the reversibility in the structure.
Expectedly, water@1a matches closely with 1a, indicating that
the degassed structure remains almost intact after water exposure.
The PXRD pattern of methanol@1a revealed no correspondence
with the pattern of either 1 or 1a, revealing an intermediacy
CCDC 1827675 contains the supplementary crystallographic
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 Author
*E-mail: tmaji@jncasr.ac.in.
ORCID
Arkamita Bandyopadhyay: 0000-0002-8511-4925
Swapan. K. Pati: 0000-0002-5124-7455
Tapas Kumar Maji: 0000-0002-7700-1146
Author Contributions
‡
S.R. and V.M.S. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.R. acknowledges the UGC (Government of India), and T.K.M.
is grateful to the Department of Science and Technology (Project
MR-2015/001019), Government of India, and JNCASR (Project
TRC-DST/C.14.10/16-2724) for financial support.
C
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22) Bunz, U. H. F.; Seehafer, K.; Bender, M.; Porz, M. Poly-
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