Perfluorophenyl-Bifuran: A Stable and Fluorescent Material Exhibiting Mechanofluorochromic Behavior

Article abstract of DOI:10.1002/hlca.201900027

π-Conjugated oligomers and polymers consisting of bifuran units are applied in optoelectronic devices, because bifuran units endow such devices with superior properties compared with their thiophene analogs. However, as is true for most furan oligomers, b

Full text of DOI:10.1002/hlca.201900027

HELVETICA
chimica acta
Accepted Article
Title: Perfluorophenyl-Bifuran: A Stable and Fluorescent Material
Exhibiting Mechanofluorochromic Behavior
Authors: Hadar R. Yakir, Linda J. W. Shimon, and Ori Gidron
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10.1002/hlca.201900027
Helvetica Chimica Acta
HELVETICA
Perfluorophenyl-Bifuran: A Stable and Fluorescent
Material Exhibiting Mechanofluorochromic Behavior
Hadar R. Yakir,a Linda J. W. Shimon,b and Ori Gidron*a
aInstitute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Jerusalem 91904, Israel, e-mail: ori.gidron@mail.huji.ac.il
bChemical Research Support Unit, Weizmann Institute of Science, Rehovot 76100, Israel
Dedicated to François Diederich on the occasion of his retirement
π-Conjugated oligomers and polymers consisting of bifuran units are applied in optoelectronic devices, because bifuran units endow such devices with
superior properties compared with their thiophene analogs. However, as is true for most furan oligomers, bifuran oligomers suffer from low photostability,
which restricts their application. In this work, we present the synthesis and the photophysical and structural characterization of perfluorinated phenyl
bifuran (PFB-2F), which displays high photostability, while maintaining strong fluorescence quantum efficiency in both solution and the solid state. X-ray
crystallography reveals that, unlike its thiophene analog, PFB-2F has a completely planar backbone, with slip-stacked packing and short interplanar
distances. PFB-2F crystals display mechanofluorochromic behavior, which renders perfluorophenyl-substituted oligofurans potential candidates for both
stable optoelectronic devices and responsive optical materials.
Keywords: π-Conjugated materials • Organic electronic materials • Oligothiophenes • Oligofurans • Mechanofluorochromism
Since the discovery of conductivity in organic crystals and polymers, the
field of organic electronics has evolved quickly.[1, 2] In recent years,organic
materialshavefoundapplicationin electronic devices,[3] such as organic field-
effect transistors (OFETs),[4] organic light-emitting diodes,[5] and organic
solar cells,[6] as well as in highly efficient organic light-emittingtransistors
(OLETs),[7] flexibledisplays, andspecificchemicalsensors.
diphenyl 5,5’-diphenyl-2, 2’-difuran (Ph-2F, Figure 1) and found them to
display strong fluorescence as well as good field effect mobility.[19]
However, the question of the photostability of such furan-containing
materials remains open. It was previously reported that adding
perfluorinated phenyl rings to conjugated systems, such as
oligothiophenes, often confers desired properties, such as greater
stability, lower sublimation temperature, as well as lower HOMO and
LUMO levels.[20] Perfluorinated rings often induce π–π stacking, and
therefore greatly affect solid state properties, such as field effect
mobility.[21]
π-Conjugated backbones can also function as responsive materials, in
which an observable property (such as color) changes in response to an
external stimulus. Mechanofluorochromic materials exemplify such
responsiveness, with a change in the color of their fluorescence induced
by mechanical grinding, crushing, or rubbing.[8, 9] Mechanofluorochromic
materials can be used in memory chips and sensors, among other
applications.[10],[11] Ideally, mechanofluorochromic behavior should be
combined with strong solid-state fluorescence in at least one form
(crystalline or powder).
Ph-2F
PFB-2F
We have previously introduced α-oligofurans (nF), which are the oxygen
containing analogs of the extensively studied α-oligothiophenes.[12] While
oligofurans show OFET properties similar to those of their thiophene
analogs, they possess several advantages, such as strong fluorescence,
rigidity/planarity, good conjugation and charge delocalization, and
increased solubility.[13-16] Moreover, we have recently demonstrated that
oligofurans can undergo multiple Diels-Alder cycloadditions to transform
LUMO
HOMO
HLG
-2.04
-1.36
-4.87
3.51
-5.47
3.43
Figure 1. Calculated (B3LYP/6-31G (d)) energies of the HOMO and LUMO levels and
calculated HOMO–LUMO gaps (HLGs) of Ph-2F and PFB-2F (in eV).
the π-conjugated backbone in
a a
regioselective manner.[17] In
In this work, we report the synthesis and properties of 5,5’-
diperfluorophenyl-2,2’-difuran (PFB-2F, Figure 1). The X-ray structure
reveals that, unlike its thiophene analog (PFB-2T), PFB-2F is planar, with
strong conjugation. We find that, unlike quarterfuran (4F) and Ph-2F, PFB-
2F is stable under ambient conditions, highly fluorescent in both solution
and the solid state, and displays mechanofluorochromic behavior.
comparative study between two constitutional isomers containing
bifuran and bithiophene units, we found that the bifuran unit is sufficient
for obtaining many of the favorable properties (e.g. planarity, solubility,
and fluorescence) observed for longer oligofurans.[18]
Notwithstanding their advantages, oligofurans often suffer from low
stability, thus limiting their applications as organic electronic materials.[13]
Kazantsev's group previously studied the properties of single crystals of
The electronic structure of PFB-2F was first calculated using DFT (B3LYP/6-
31G(d) level) and compared with that of Ph-2F (Figure 1). Both Ph-2F and
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PFB-2F are planar, while the interring furan–furan bond is slightly shorter
in Ph-2F, indicating its stronger quinoid character. The perfluorinated rings
reduce the electron density on the bifuran unit, which decreases the HOMO
energy from -4.87 eV for Ph-2F to -5.47 eV for PFB-2F. This decrease is
expected to increase the stability of PFB-2F toward oxidation. The LUMO
is also reduced from -1.36 eV for Ph-2F to -2.04 eV for PFB-2F.
favored by the planar backbone of PFB-2F than by the twisted backbone
of PFB-2T. In addition, a strong interaction is expected between the
electron rich furan and the electron poor perfluorinated phenyl ring of the
neighboring molecule. Overall, such
a close packing arrangement
highlights the potential of the perfluorinated furan molecule as the active
material in OFETs, as good intermolecular interactions are considered a
prerequisite for good field effect mobility.
Following the above mentioned computational study, PFB-2F was
synthesized using Stille coupling between 5,5-bis(tributylstannyl)-2,2’-
bifuran and bromopentafluorobenzene to obtain PFB-2F as white
crystalline solid in 60% yield (Scheme 1). Single crystals of PFB-2F were
grown by evaporation from a solution of chloroform and methanol. The
X-ray crystal structure shows that the PFB-2F molecule is planar (Figure
2a), as opposed to its thiophene analog, PFB-2T, in which one of the
Scheme 1. Synthesis of PFB-2F using Stille coupling.
◦
perfluorinated phenyl rings is twisted by 36 (Figure 2c).[21] The observed
PFB-2F and Ph-2F display similar absorption and emission spectra in
dioxane (Figure 3). Absorption consists of a π–π* transition, with a slight
bathochromic shift for Ph-2F.[22] However, the vibronic features in the
emission spectra are sharper for PFB-2F compared with Ph-2F, indicating
the more rigid backbone of PFB-2F in the excited state. Both compounds
display similar Stokes shift (0.25–0.29 eV; Table 1). The fluorescence
lifetime (τf) of PFB-2F (1.29 ns) is only slightly shorter than those of Ph-2F
(1.38 ns) and 4F (1.48 ns). However, these lifetimes are significantly longer
compared with thiophene analogs, such as 4T (0.38 ns), which we
previously explained by the greater rigidity of the bifuran backbone,
which decreases out-of-plane vibrations.[18] Cyclic voltammetry (CV)
measurements of PFB-2F reveal the first redox at 1.28 V, corresponding
to a HOMO of ca. - 5.6 eV (see ESI).
planarity can be explained by the short distance between the β-hydrogens
in thiophene and the fluorine atoms in the ortho positions, resulting from
the smaller exo angle of PFB-2T (127◦) compared with PFB-2F (134◦) and
Ph-2F (133◦). The interring furan–furan bond is slightly longer for PFB-2F
(1.44 Å) compared with Ph-2F (1.43 Å), indicating a more quinoid
character for the latter, as predicted by our calculations.
Comparison of the crystal packing of PFB-2F withthose of Ph-2F and PFB-
2T (Figure 2, bottom) provides insight into how fluorinated phenyl rings
affect molecular structure and packing characteristics. Whereas PFB-2F
and PFB-2T[21] stack in a parallel-displaced manner, Ph-2F[19] exhibits a
herringbone packing motif (Figure 2, bottom). The interplanar distance
for PFB-2F is extremely short (3.25 Å) in contrast to the significantly
longer interplanar distance for the thiophene analog, PFB-2T (3.64 Å).
One reason for this difference is that intermolecular interactions are more
Figure 2. The single crystal molecular conformations (top) and packing (bottom) of (a) PFB-2F, compared with (b) Ph-2F,[19] and (c) PFB-2T[21]
.
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The fluorescence quantum yield (Φf) in dioxane is stronger for PFB-2F
(97%) compared with 4F (80%) and is similar to Ph-2F (96%). Crystals of
PFB-2F and Ph-2F display strong fluorescence (Φf values of 53% and 51%
for PFB-2F and Ph-2F, respectively) while 4F displays low fluorescence of
only 10% (Table 1). The combination of slipped stacked packing with a
very short interplanar distance and with strong fluorescence indicates
that PFB-2F can potentially be applied as the active material for OLETs.
Since oligofurans were found to undergo photooxidation readily, we sought
to check whether the addition of perfluorinated phenyls affects their
stability. Figure 4 displays the absorption spectra in dioxane of PFB-2F, Ph-
2F, and 4F under ambient conditions. 4F is the least stable compound,
showing a 70% decrease in absorption after 1 day, and complete bleaching
after 3 days. Ph-2F shows greater stability, but also bleached after 3 days
under ambient conditions. In sharp contrast, the absorption spectrum for
PFB-2F decreases less than 3% even after exposure to ambient conditions
for 3 days. This can be explained by the lower HOMO for this molecule, as
indicated by the abovementioned DFT calculations. PFB-2F was found to
be more stable toward photooxidation than any oligofuran-based material
previously tested in our group, including the recently introduced bifuran-
imide.[23] We can therefore conclude that the addition of electron
withdrawing groups at the α-positions has a significant stabilizing effect on
the oligofuran backbone.
be observed clearly upon grinding the crystalline sample. Such
mechanofluorochromic behavior, which has not been observed previously
for oligofurans, is particularly interesting in light of their strong solid-state
photoluminescence. Differential scanning calorimetry (DSC) shows only
melting and crystallization with no additional phase transitions (see ESI).
The quantum efficiency for the powdered sample is 36%, which is lower
compared with the crystalline phase, but still relatively strong for organic
materials in the solid-state. It is likely that the grinding results in a
significant change in molecular ordering from crystalline to amorphous.[24]
Grinding
1
Crystal
Powder
0.8
0.6
0.4
0.2
0
400
450
500
550
Wavelength (nm)
Figure 5. Photographs of PFB-2F under UV irradiation before and after grinding (top).
Fluorescence spectra of PFB-2F crystals and powder (bottom). Excitation wavelength:
355 nm.
Table 1. Comparison of photophysical properties a
a
a
λabs
λem
τf (ns)a
Φfa
Φf
Stokes
(nm)
(nm)
(Solid)
Shift (eV)
PFB-2F
Ph-2F
4Fb
355, 377
362, 381
364
409, 387
415, 391
413, 391
1.29
1.38
1.48
0.97
0.96c
0.80
0.53
0.58c
0.10
0.29
0.25
0.23
Figure 3. Normalized absorption (solid line) and emission (dashed line) spectra of
PFB-2F (green) and Ph-2F (blue) in 1,4-dioxane.
a Measured in 1,4-dioxane. bPhotophysical values for 4F are taken from reference
18. cIndicated values for Ph-2F are taken from reference 19.
In summary, we have investigated the photophysical and structural
properties of perfluorinated phenyl bifuran, PFB-2F, both computationally
and experimentally. We found that PFB-2F shows higher photostability
than phenyl-terminated bifuran under ambient conditions. DFT
calculations indicate that this stability may originate from its lower HOMO
energy reducing its reactivity toward oxidation. Single-crystals of PFB-2F
pack in a slip-stacked morphology with a very short interplanar distance,
owing to the planar backbone, which is in contrast to the twisted
conformation observed for its thiophene analog. The strong observed
fluorescence of PFB-2F in solution and in the crystalline state is
accompanied by reversible color-switching behavior. We therefore suggest
that endowing oligofuran units with strong acceptors, such as
perfluorophenyl units, can enable them to serve as stable candidates for
optoelectronic devices and smart materials.
Figure 4. Absorption spectra of PFB-2F, Ph-2F, and 4F in 1,4-dioxane under ambient
conditions after 1 hour (red), 1 day (blue), and 3 days (green).
When handling solid samples of PFB-2F, we observed a different color
under 365 nm illumination for the powdered samples (blue) compared with
crystalline samples (green). Indeed, after crushing the crystals, there is a
noticeable difference in color under UV light (Figure 5, top). Reversion to
the original color was obtained by recrystallization. The difference was also
measured by emission spectroscopy (Figure 5, bottom), and a blue shift can
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Experimental Section
Author Contribution Statement
The manuscript was written by H. R. Y. and O. G. through contribution of all
authors. H. R. Y. synthesized and characterized the compounds, and
performed the DFT calculation. L. J. W. S. performed the X-ray
crystallography. O. G. supervised the project. All the authors discussed the
results and revised the manuscript. Authors declare no competing interest.
General Information
The reagents and chemicals used here are commercially available and were
used without further purification. Flash chromatography was performed
using CombiFlash SiO2 columns. The compound 5,5-bis(tributylstannyl)-
2,2’-bifuran was synthesized according to the previously reported
procedure.[25] 1H and 13C NMR spectra were recorded in solution on a Bruker-
AVIII 500 MHz spectrometer using tetramethylsilane (TMS) as the external
standard. The spectra were recorded using chloroform-d as the solvent.
Chemical shifts are expressed in δ units. Elemental analysis was measured
on Thermo Flash 2000 CHN-O Elemental Analyzer. Cyclic voltammetry (CV)
measurements were performed using a Bio-Logic SP-150 Potentiostat in a
standard three-electrode setup. Pt wire, Ag/AgCl wire, and a Pt disk were
used as the counter electrode, pseudo-reference electrode, and working
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10.1002/hlca.201900027
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5
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