Mechanochromism, Twisted/Folded Structure Determination, and Derivatization of (N-Phenylfluorenylidene)acridane

Article abstract of DOI:10.1002/anie.201902636

(N-Phenylfluorenylidene)acridane (Ph-FA) compounds with electron-withdrawing and -donating substituents (H, MeO, Ph, NO₂, Br, F) at the para position of the phenyl group were successfully synthesized by Barton–Kellogg reactions of N-aryl thioacridones and diazofluorene. By using the substituent on the nitrogen atom to alter the electronic properties, both the folded and twisted conformers of p-NO₂-C₆H₄-FA could be crystallographically characterized, which enabled the charge transfer from the electron-donating acridane moiety to the electron-accepting fluorenylidene moiety to be understood. Ground-state mechanochromism, thermochromism, vapochromism, and proton-induced chromism were demonstrated between the folded and twisted conformations of the conformers. Protonation and chemical oxidation of Ph-FA gave two stable acridinium compounds, namely, the fluorenylacridinium and acridinium radical cations. The present study will contribute to the development of functional dyes and organic semiconductors.

Full text of DOI:10.1002/anie.201902636

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Mechanochromism, Twisted/Folded Structures Determination,
and Derivatization of N-Phenyl Fluorenylidene-Acridane
Authors: Yutaka Matsuo, Ya Wang, Hiroshi Ueno, Takafumi
Nakagawa, and Hiroshi Okada
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902636
Angew. Chem. 10.1002/ange.201902636
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10.1002/anie.201902636
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Mechanochromism, Twisted/Folded Structures Determination,
and Derivatization of N-Phenyl Fluorenylidene-Acridane
Yutaka Matsuo*[a,b], Ya Wang[a], Hiroshi Ueno[c], Takafumi Nakagawa[b], and Hiroshi Okada[b]
Abstract: N-phenyl fluorenylidene-acridane (Ph-FA) compounds with
electron-withdrawing and -donating substituents (H, MeO, Ph, NO2,
Br, F) at the para position of the phenyl group were successfully
synthesized by the Barton-Kellogg reaction of N-aryl thioacridones
and diazofluorene. By using the substituent on nitrogen to alter
electronic properties, both the folded and twisted conformers of p-
NO2-C6H4-FA were crystallographically characterized to understand
charge transfer from the electron-donating acridane moiety to the
Agranat and co-workers reported both X-ray crystal structures of
the folded and twisted forms of fluorenylidene-xanthene (FX),
where crystals were obtained in different temperature regions in
a sublimation tube.3 Fujita and co-workers were able to confine a
single twisted form of FX in a cage molecule.10 In another example,
the structures of both the folded and twisted isomers of an
overcrowded alkene were determined by means of a quinoidal
structure with dicyanomethylene.11 The smallest BAE, 9,9′-
bifluorenylidene has two five-membered rings at the central part
and is known to be in the twisted form.12,13 However, it does not
exhibit chromic color change. Compounds with five- and six-
member rings linked via the double bond (Figure 1a) have a
special balance such that chromic properties are present and the
twisted form is dominant.
We recently reported fluorenylidene-acridane (FA), a BAE
with a nitrogen atom bearing alkyl groups.4 The choice of the alkyl
chain affected crystallinity (crystalline or amorphous) and ground
state mechanochromic properties.4 FAs have advantages over FX
in that their electronic properties and crystallinity can be easily
turned by changing the R substituent on the nitrogen atom (Figure
1a). In addition, the nitrogen atom is beneficial for application in
organic electronics and photocatalysis as triarylamines14 and
acridiniums15 are widely used in both of these research fields. In
this work, we investigated the substituent effect in depth by using
various N-phenyl groups and sought to understand the key factors
for control of the folded/twisted conformations to demonstrate
various chromic behavior. Herein we report mechanochromism
based on the conformation structure changing and heating-
induced reversion. We succeeded in a new molecular design for
the sterically condensed alkene compound. In addition, we
investigated the formation of two acridinium cations by
protonation and oxidation for use in future research. We expect
that the fundamental information presented here will contribute to
the development of the chemistry of BAEs and overcrowded
alkenes.
electron-accepting
fluorenylidene
moiety.
Ground
state
mechanochromism, thermochromism, vapochromism, and proton-
induced chromism were demonstrated by utilizing folded/twisted
conformational change between the conformers. Protonation and
chemical oxidation of Ph-FA gave two stable acridiniums, namely,
fluorenyl acridinium and acridinium radical cation. The present studies
will contribute to development of functional dyes and organic
semiconductors.
Bistricyclic aromatic enes (BAEs)1 are
a
class of
overcrowded alkenes that have recently attracted interests as
functional molecules for molecular machines2 and chromism.3,4
BAEs are composed of two tricyclic aromatic systems connected
via an alkene, and have received special attention since they were
discovered more than 100 years ago.1,5 Among the various forms
of chromism, the thermochromism of BAEs was the first to be
discussed in-depth,6 and thus they are referred as thermochromic
ethylenes.6b Upon heating, the color changes from light to dark,
and this change can also be brought about through
piezochromism4,7 and photochromism.8 This color change is
triggered by a conformational change from a folded conformer to
a twisted conformer. The latter shows strong light absorption due
to a charge transfer band in the long-wavelength region.4 The
energy difference between the two conformers is generally small,9
and they are in equilibrium in solution.
The twisted conformer is more interesting as a functional
molecule because of its intense light absorption and high charge
carrier mobility.4 However, as it was originally obtained as the
high-temperature thermochromic form, strategies to obtain higher
ratios of the twisted conformer are still less explored. By using one
five-membered ring at the central ring of the tricyclic moiety,
We synthesized various N-aryl-FAs by the Barton-Kellogg
reaction16 of N-aryl thioacridones and diazofluorene (Figure 1b).
In previous syntheses of N-alkyl FAs,4 alkyl groups were installed
last because the Barton-Kellogg reaction requires an electron-
withdrawing group on thioacridones for 1,3-dipolar nucleophilic
attack by the diazofluorene. In this study, several of the phenyl
groups were electron-withdrawing, so Cu-I catalyzed amination17
was first performed to prepare N-aryl acridones 1 (Supporting
Information), followed by conversion into N-aryl thioacridones 2,
which were used for the subsequent Barton-Kellogg reaction with
diazofluorene 3 in xylene at 140 °C in the presence of PPh3 (1
equiv) as a desulfurizing reagent. Here, high temperature was
essential for this Barton-Kellogg reaction. At high temperature,
[a]
Prof. Dr. Y. Matsuo, Y. Wang
Department of Chemistry, School of Chemistry and Materials
Science, and Hefei National Laboratory for Physical Sciences at the
Microscale, University of Science and Technology of China, 96
Jinzhai Road, Hefei, Anhui 230026, China
E-mail: matsuo@ustc.edu.cn
[b]
[c]
Prof. Dr. Y. Matsuo, Dr. T. Nakagawa, Dr. H. Okada
Department of Mechanical Engineering, School of Engineering, The
University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656,
Japan
however, the homocoupling product of
3
(i.e., 9,9′-
bifluorenylidene) was also obtained. We therefore added 3 equiv
of 3 dropwise into the reaction mixture to maximize the chemical
yield. After separation by silica gel column chromatography, we
isolated several Ph-FAs with para-substitution of electron-
withdrawing or -donating groups on the phenyl groups in 53–71%
Dr. H. Ueno
School of Chemistry, Northeast Normal University, 5268 Renmin
Street, Changchun, Jilin 130024, China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201902636
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yield: X = H (4a), Ph (4b), OMe (4c), NO2 (4d), Br (4e), and F (4f).
Ph-FA (4a) was dark blueish green (Figure 1b, inset), indicating
most the material was the twisted conformer. In contrast, for
compound 4e, after rotary evaporation to remove the solvent
(petroleum ether), we obtained a dark green solid with yellow
spots on the wall of the flask (Figure 1c). The folded conformers
were aggregated in the yellow spots. These yellow spots spread
very slowly over several hours. This color change occurred
because the electron-withdrawing group in 4e suppressed
formation of the twisted conformer as described in detail in the
showed complex absorption patterns around 400 nm, which
mainly involved absorption by the folded conformers and some by
the twisted conformers as well. Compared with the previously
reported spectrum of Me-FA,4 that of Ph-FA (4a, blue line in
Figure 2a) indicated a higher ratio of the twisted conformer. The
absorption band around 650 nm is clearly larger than that around
400 nm, except for 4d (red line in Figure 2a). This high twisted
isomer content is consistent with the above-mentioned DFT
calculation results (Figure S1). We attribute this to steric effects;
that is, the relatively bulky phenyl groups suppress folding of the
acridane moiety, and the phenyl group prefers a staggered
conformation with respect to the planar acridane moiety. In
comparing the spectra of the present six N-aryl FAs, we observed
electronic effects; in
1
following paragraph. All products were characterized by H and
13C NMR spectroscopy and high-resolution MALDI-TOF mass
spectrometry.
particular,
the
electron-
withdrawing
nitro
substituents (in 4d,
red line) lowers the
ratio of the twisted
conformer. This is
attributed
suppression
charge
to
of
transfer
from the electron-
donating acridane
moiety
to
the
electron-accepting
fluorenylidene
moiety
strongly electron-
withdrawing nitro
by
the
group (Figure 1d).
In addition, except
for
4d,
the
absorption of the
twisted conformer
was
This is evidence of
charge transfer
from the acridane
moiety to the
red-shifted.
fluorenylidene moiety creating partially zwitterionic character18
with fluorenyl anion and acridinium cation. Lowering the HOMO
energy level in the acridane moiety of 4d widens the energy gap
to the electron-accepting unoccupied molecular orbital (MO) of
the fluorenylidene moiety, thus suppressing the charge transfer.
Next, we investigated the effect of solvent on the absorption
spectra of the N-aryl -FAs (Figure 2b for 4a, Figures S2 and S3
for other compounds). Stabilization of the charge-separated form
of the twisted conformer of BAEs in polar solvents has been
discussed previously.18 As expected, we found that ratio of the
twisted conformer increased in polar solvents such as
dichloromethane and acetonitrile through stabilization of charge-
transferred form of FA. In non-polar solvents such as n-hexane,
the ratio of the folded conformer increased. In addition, spectra in
methanol and isopropanol also showed a high ratio of the folded
conformer, although methanol and isopropanol are polar solvents.
The solubility of 4 in methanol is low, and this might cause
aggregation of the folded forms. Indeed, a plot of the absorption
maxima around 650 nm against the solvent polarity parameter
Figure 1. Fluorenylidene-acridanes. (a) Design concept and features of R-FAs
among various BAEs with pentagon or hexagon at the central rings. In this work,
the R groups are substituted phenyl groups. (b) Synthesis of Ph-FAs with
various substituents on the phenyl group. Inset: Appearance of Ph-FA (4a). (c)
Appearance of p-Br-C6H4-FAs (4e) as a dark green solid with yellow spots. (d)
Suppression of charge transfer from the acridine donor to the fluorene acceptor
by the electron-withdrawing nitro group. (e) Formation of acridinium of 4a,
protonated form. (f) Chemical oxidation by AgBF4. Inset: Solution of 6 in
dichloromethane.
To examine the effect of the substituents on the equilibrium
of the folded and twisted conformers of compounds 4a–f, we first
measured their UV-Vis absorption spectra in dichloromethane at
room temperature (Figure 2a). All spectra exhibited broad
absorption maxima around 650 nm characteristic to a charge
transfer band in the twisted conformer. In addition, the spectra
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ET(30)19 clearly indicated that higher polarity of the solvent
stabilized the twisted conformer, except for the alcohols (Figure
S4).
almost same potential as the first reduction wave of other Ph-FAs,
which indicates this second one can be assigned to reduction of
the fluorene moiety giving fluorenyl anion. Compounds 4a–f have
slightly narrower bandgaps than Me-FA (Table 1).4 The stable
redox properties observed in this wide potential range suggest
that organic electronics applications21 as well as photocatalytic
redox applications15 are potentially fruitful areas of future research.
Table 1. Oxidation and reduction potentials (E1/2 vs. Fc/Fc+) in the
electrochemical studies.[a]
Ox1
[V]
Ox2
[V]
Red1
(Red2)
[V]
HOMO
levels
[eV]
LUMO
levels
[eV]
Eg
[eV]
Compounds
4a
4b
–0.20
–0.20
0.72
0.71
–1.79
–4.60
–3.01
1.59
1.60
–1.80
–4.60
–3.00
4c
4d
–0.20
–0.09
0.74
0.73
–1.81
–1.50
(–1.82)
–1.79
–1.79
–1.82
–4.60
–4.71
–2.99
–3.30
1.61
1.41
4e
–0.17
–0.16
–0.17
0.70
-
0.74
–4.63
–4.64
–4.63
–3.01
–3.01
–2.98
1.62
1.63
1.65
4f
Me-FA[4]
[a]Estimated HOMO and LUMO energy levels were given by a following
ox1
red1
equation: E(HOMO) = –(E1/2
+ 4.80), E(LUMO) = –(E1/2
+ 4.80). Eg is
energy gap between HOMO and LUMO levels.
Figure 2. UV-Vis spectra. (a) The spectra of various N-aryl-FAs in
dichloromethane normalized to the peak around 650 nm. Red, green, yellow,
red, purple, and black lines represent spectra for 4a, 4b, 4c, 4d, 4e, and 4f,
respectively. (b) Normalized UV-Vis absorption spectra of Ph-FA (4a) in various
solvents.
Cyclic voltammograms of 4a in dichloromethane showed
two reversible oxidation waves, at E1/2 = –0.20 and 0.72 V (vs
Fc/Fc+) (Figure S8, Table 1). The first oxidation potential was
negative against the ferrocene/ferrocenium redox couple. Such
extreme ease of oxidation is one of the useful features of these
Ph-FAs. We suggest that the first oxidation gives the acridinium
cation followed by the second oxidation giving the fluorenyl cation.
The first reduction wave at –1.79 V was also reversible. It affords
a fluorenyl anion. A very small reduction wave around –0.9 V
might be assigned to reduction of the folded conformer, as
described in the literature.18,20 The electronic effect of substituents
was slightly evident. For example, 4e with an electron-
withdrawing bromine atom exhibited had a slight positive shift of
oxidation potentials. It should be noted that only 4d showed two
reduction waves. According to the theoretical calculation results
as described above, the LUMO of 4d was localized on the
nitrophenyl group and was lowered by its strongly electron-
withdrawing nitro group. The second wave was observed at
Figure 3. Single-crystal X-ray structures of 4d. The structures of (a-d) orange
crystals of the folded conformer and (e-h) dark crystals of the twisted conformer.
(a,e) Obtained single crystals from one batch. (b,f) Ball-and-stick structure. (c,g)
ORTEP drawing. (d,h) Bond alternation.
Single crystals suitable for X-ray crystallography were
obtained by recrystallization of 4 from bilayer systems using
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dichloromethane/methanol or dichloromethane/petroleum ether,
or by evaporation of dichloromethane solution. Fortunately, we
could obtain single-crystal X-ray structures of both the folded and
twisted conformers from the same compound by taking into
consideration the electronic effect of the N-substituents. Normally,
simple Ph-FA (4a) tends to form predominantly the twisted
conformer (Figure 1b, inset), as we designed this molecule to
obtain more of the twisted conformer in equilibrium. The use of a
nitro group in 4d suppressed charge transfer (Figure 1d) and
increased the ratio of the folded conformer, as seen in its UV-Vis
spectrum (Figure 2a). The higher ratio of crystals of the folded
compound could also be seen visually (as in Figure 1c). Thus, we
used compound 4d for recrystallization. Surprisingly, we could
obtain relatively large single crystals of both twisted and folded
conformers from one recrystallization batch (Figure 3) by slow
Figure 4. Ground State Mechanochromism. (a) Absorption color change by
grinding of 4d. From upper left to lower right. (b) Schematic drawing for
mechanochromic color change of N-aryl-FA from yellow (folded) to green
(twisted and folded). We surmise that the green color is derived from a mixture
of yellow folded conformer and bluish green twisted one.
The green color of 4d could be returned to yellow by heating
to around 200 °C in air (Figure S13). This phenomenon is
opposite to the behavior of bianthrone, which has a dark color in
its high-temperature form.6 We surmise that the present color
change is due to thermally driven recrystallization over the glass
transition temperature into the folded crystal of 4d, which has a
stronger packing force than the twisted crystal of 4d. Here, the
mechanically induced color change reversed by the thermally
induced color change, that is, the combination of
mechanochromism and thermochromism, should be identical
because no special chemicals or instruments were needed.
Next, we examined color change by solvent vapor exposure.
When yellow powder of the folded conformer of 4d was exposed
to solvent vapor (dichloromethane), the color changed to green in
40 min (Figure S14a). The obtained green powder returned to a
yellow powder in 5 min when it was dried in air (Figure S14b).
These vapochromic phenomena can be explained by penetration
of solvent molecules into crystalline aggregates of the folded
conformer to trigger equilibrium in a miscible system showing
green color. Subsequent removal of the solvent molecules results
in recrystallization into the yellow folded microcrystalline powder.
Whereas the green powder can be obtained very easily by manual
grinding, the yellow powder can be regenerated by heating or
exposure to solvent vapor followed by drying in air. For a large
quantity of 4d powder in the laboratory, the easiest way to change
the color back to yellow was ultrasonication of a suspension of the
green powder in methanol (Figure S15). This treatment
immediately gave a suspension of a yellow powder of 4d, which
could be collected by filtration. The recovered powder was
checked by NMR to confirm that 4d had not decomposed. This is
also related to the crystallization of 4d into the folded crystals.
Reversible protonation-induced chromism was also
observed. When acetic acid was added to a solution of 4a in
dichloromethane in air, a color change was observed from blue to
green and then yellow (Figure S16a). In this reaction, protonation
occurred at the fluorene moiety to produce fluorenyl acridinium 5
(Figure 1e). This acridinium solution was stable in air. The UV-Vis
spectrum of 5 exhibited absorption at only short-wavelengths up
to 500 nm (Figure S17), indicating loss of the charge transfer band.
This solution could be changed from yellow back to blue by
addition of triethylamine (Figure S16b). This reversible behavior
would be useful in a proton sensor. Stable formation of acridinium
is an advantage of this compound.
evaporation
of
dichloromethane.
This
recrystallization
reproducibly gave both kinds of crystals.
With photophysical, electrochemical, and structural data of
the N-aryl-FAs in hand, we next investigated various chromic
properties of these compounds. Yellow powder of the folded
conformer of 4d was ground with a mortar and pestle (Figure 4a),
causing a color change from yellow to green. Also, we can draw
a
picture on the paper absorbing 4d by a glass stick
(Supplementary Movies). We surmise that this color change by
grinding was caused by breaking down the crystalline state of the
folded conformer into
a partially amorphous state, with
simultaneous conversion from the folded to twisted conformer
through equilibrium in a quasi-solid state. With this mechanical
stimulus, crystalline powder of the twisted conformer can partially
form to give a green mixture of folded and twisted conformers.
Figure 4b illustrates this grinding-induced ground state
mechanochromism for absorption color change. Not only
mechanical grinding, but also heating, solvent vapor exposure,
and ultrasonication could also induce color change.
As an advantage of nitrogen-containing FAs, cationic
species were very stable in air. Next, chemical oxidation of 4a was
carried out by treatment with AgBF4 salt to give acridinium 6 as a
radical cation (Figure 1f). This facile oxidation was predicted from
the cyclic voltammetry data showing a very low oxidation potential
(Figure S8). The obtained radical cation 6 was blackish green in
solution (Figure 1f, inset), and showed a UV-Vis-NIR absorption
spectrum extending to 1,200 nm (Figure S18). This NIR
absorption reflects the existence of a singly occupied MO in the
fluorenyl radical moiety. This acridinium radical cation 6 was also
air-stable in solution for several weeks. Considering that stable
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COMMUNICATION
acridinium cations have often been used in photocatalytic redox
applications,15 the present molecules might provide new building
blocks for electrochemically active functional materials.
Keywords: Bistricyclic aromatic enes • Overcrowded alkenes •
Ground state mechanochromism • Conformer • Acridinium
Finally, we measured the charge carrier transport properties
of N-aryl-FA by space-charge limited current (SCLC)
measurements (Figures S19–S21).24 Hole and electron devices
were fabricated with configurations of ITO/PEDOT:PSS/N-aryl-
FA/MoO3/Al and ITO/4a/Al, respectively. The hole and electron
mobilities of 4a were 1.1 × 10–6 and 9.8 × 10–5 cm2/Vs,
respectively. This ambipolar transport property is attributable to
the formation of acridinium cation and fluorenyl anion as positive
and negative charge carriers, respectively. With electron-donating
OMe (4c) and electron-withdrawing F (4f) groups on the phenyl
group, hole mobilities increased and decreased to 3.8 × 10–6 and
9.0 × 10–8 cm2/Vs, respectively. These data imply that enhancing
charge transfer character stabilizes the twisted conformer and
thereby increases hole transport ability. Hole mobility was further
increased by doping of the oxidized form (acridinium 6) into 4a.
The thin film of the mixture of 4a and 10 wt% 6 had hole mobility
of μh = 9.9 x 10–5 cm2/Vs. Controllable charge carrier mobility via
tuning of the conformer ratio by changing the substituent and via
application of external stimuli will be a fascinating feature of these
semiconducting molecules to explore.
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In summary, we have successfully synthesized N-aryl FAs
through the CuI-catalyzed amination and Barton-Kellogg olefin
formation reaction from N-aryl thioacridone and diazofluorene.
The N-aryl substituents provide a facile way to tune the electronic
properties of the N-aryl FAs to find a good balance between their
folded and twisted conformers. By using electron-donating phenyl
groups, we could produce predominantly the twisted conformer,
which is more intriguing for functional material applications. On
the other hand, the presence of the folded conformer is essential
for various chromic functions. By using an electron-withdrawing
group such as a nitro group, the folded conformer can be
stabilized for realizing mechanochromism, thermochromism, and
vapochromism. Proton-induced chromism is facilitated by stable
formation of acridinium, which suggests new functions such as
acridinium-based oxidation ability switched by protons. X-ray
crystallographic analyses of both the folded and twisted
conformers were accomplished by substituent control.
Comparison of both structures provided a clue to understanding
the charge transfer from the electron-donating acridane moiety to
the electron-accepting fluorenylidene moiety. The advantage of
the nitrogen atom was also seen in the stable oxidation to
acridinium as a radical cation and the fair charge carrier transport
properties. These functions could play a crucial role in catalytic
and electronic applications. More importantly, the present studies
will contribute to understanding the chemistry of BAEs and
overcrowded alkenes to provide deeper knowledge of
fundamental physical organic and structural organic chemistry.
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Acknowledgements
This work was also supported by Grants-in-Aid for Scientific
Research (JSPS KAKENHI Grant Numbers JP15H05760,
JP16H04187, and JP17K19116) from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan. The
computations were performed using Research Center for
Computational Science, Okazaki, Japan.
This article is protected by copyright. All rights reserved.

10.1002/anie.201902636
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Yutaka Matsuo*, Ya Wang, Hiroshi
Ueno, Takafumi Nakagawa, Hiroshi
Okada
Page No. – Page No.
Mechanochromism, Twisted/Folded
Structures Determination, and
Derivatization of N-Phenyl
Ground state mechanochromism for absorption color change has been further
understood by obtaining two X-ray crystal structures for both folded and twisted
conformers. Thermochromism, vapochromism, chromism by ultrasonication, proton-
induced chromism, and carrier mobility change were also demonstrated.
Fluorenylidene-Acridane
This article is protected by copyright. All rights reserved.