Tetraarylsuccinonitriles as mechanochromophores to generate highly stable luminescent carbon-centered radicals

Article abstract of DOI:10.1039/c7cc06913h

This communication reports on the design and synthesis of mechanochromophores with a dynamic covalent system composed of a tetraarylsuccinonitrile skeleton that generate a metastable organic luminescent carbon radical. The mechanically generated radical species showed pink color and yellow light emission under UV irradiation. Unusually high stability of the luminescent carbon-centered radicals was also observed in a polymer system.

Full text of DOI:10.1039/c7cc06913h

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Tetraarylsuccinonitriles as Mechanochromophores to Generate
Highly Stable Luminescent Carbon-Centered Radicals
Toshikazu Sumia, Raita Gosekia,b, and Hideyuki Otsuka*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
photoexcitation.18–20 Therefore, developing a mechanochromic
luminescent molecule based on radical systems will expand the
scope of mechanochemistry21–23 and luminescent organic
radicals in scientific research.
This paper reports on the design and synthesis of
mechanochromophores with dynamic covalent system composed
of tetraarylsuccinonitrile skeleton that generate metastable
organic luminescent carbon radical. The mechanically generated
radical species showed pink color and yellow light emission under
UV irradiation. Unusually high stability of luminescent carbon-
centered radicals was also observed in a polymer system.
We focused on tetraarylsuccinonitrile (TASN) derivatives as
promising candidates for mechanochromophores, which can
potentially generate radical species (Scheme 1a). The simplest
TASN, tetraphenylsuccinonitrile (TPSN), usually exists in a
dimer form at room temperature but attains equilibrium with
Mechanochromic luminescent materials,1 which exhibit simple
and/or luminescent color changes in response to external the corresponding radical species at moderate temperatures in
solution (Scheme 1b).24,25 Structural characterization of TPSN
mechanical stimuli such as shearing, stretching, and
compressing, often in
a reversible manner, have been
and TASN derivatives have been investigated and their
reactivity have been studied.26–30 The carbon-centered radicals
generated are highly stable at room temperature unlike
conventional carbon-centered radicals, but they dimerize to
form more stable TASN structures. Furthermore, TPSN can act
as an “iniferter” (initiator–transfer agent–termination) agent
attracting considerable research interests because of their
potential applications in optoelectronic devices, sensors,
probes, and optical data storage devices.2-4 Many
mechanochromic luminescent materials have been developed
so far, utilizing organometallic complexes,5 organic crystalline
compounds,6,7 liquid crystalline compounds,8 and force-
sensitive molecules (i.e., mechanophores).9–11 In these
systems, color changes and/or luminescent color changes are
usually associated with changes in the chemical structures or
formation/deformation of aggregation structures owing to
non-covalent interactions, e.g., π-π interactions. These systems
are often irreversible because the luminescent changes
originate from changes to more stable states in response to
mechanical stress. In some cases, however, the changed
structure can be reverted to the original thermodynamically
stable one, although thermal energy or solvent assistance is
required. Dynamic covalent systems that generate metastable
organic radicals are promising candidates as mechanochromic
or mechanoluminescent systems, because the systems can
autonomously go back to the thermodynamically stable
original states without generating any byproduct.12–17 However,
the example of photostable radical with luminescent property
is extremely limited and easily degrades upon
a.Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-
12-1, Ookayma, Meguro-ku, Tokyo 152-8550, Japan.
b.Department of Chemical Science and Engineering, Tokyo Institute of Technology,
2-12-1, Ookayma, Meguro-ku, Tokyo 152-8550, Japan.
E-mail: otsuka@polymer.titech.ac.jp
† Electronic Supplementary Information (ESI) available: Synthesis, experimental
methods. See DOI: 10.1039/x0xx00000x
Scheme 1. (a) Chemical structure of TASN-diol, (b) equilibrium between
TPSN and corresponding radical species, (c) synthetic scheme for TASN-diol,
and (d) synthetic scheme for a TASN-centered polystyrene.
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for radical polymerization at elevated temperatures.25,26
Recently, Scaiano et al. revealed that the radical species
generated from TPSN has high tolerance to oxygen24,25 and that
the bond dissociation energy (BDE) of the central C–C bond in
TPSN is quite low (109.6 kJ mol–1).25 In addition, TASN
derivatives showed thermochromic behavior based on
dissociated radicals.25,31 However, the luminescent property of
the radicals generated and mechanochromic behavior of TASN
derivatives have never been investigated, despite the low BDE
of TASN derivatives being expected to induce mechanical
dissociation12 and the carbon-centered radicals being
analogous to photoluminescent diphenylmethyl radicals.18
Herein, we report the mechanochromic property of TASN
derivatives based on homolytic cleavage of central carbon–
carbon covalent bonds and chromic properties and stability of
the radical spices generated. A dihydroxy-functionalized TASN
derivative (TASN-diol) was designed and synthesized due to its
facile introduction into polymer systems, and the preparation
of a TASN-containing polystyrene and its mechanoresponsive
behavior were demonstrated (Scheme 1c,d).
Fig. 1 (a) EPR spectra for TASN-diol before and after grinding. (b) Reversible
change in dissociation–association equilibrium during heating–cooling cycles
from rt to 100 °C and the UV-vis spectra for TASN-diol in DMAc.
dissociation ratio of 0.028 % for TASN-diol after grinding,
nearly 5 times as large as that of TPSN (5.5×10–3%) with good
reproducibility
(Figure
S7).
The
difference
in
mechanoresponsiveness might be due to the so-called
“captodative stabilization effect,”32 thus, the electron-donating
alkoxy functions at the para-position of phenyl rings in TASN-
diol was considered to play a key role in stabilizing the
generated radical. These results suggested that the observed
disappearance of pink color was due to the recombination of
mechanically generated radicals to afford the original dimer
form, TASN-diol. Indeed, the pink color disappeared
immediately on adding a good solvent for TASN-diol.
The designed TASN-diol was synthesized according to
Scheme 1c. 4-Methoxymandelonitrile (
1), prepared from p-
anisaldehyde and KCN, was condensed with phenol under
acidic conditions; the obtained compound (
hydroxy group was converted to precursor
2
) with a phenolic
3
with an aliphatic
hydroxy group to extinguish the reactivity against radicals.
Finally, oxidative coupling of was performed to afford TASN-
3
diol as a white powder in 50% overall yield. The chemical
structure of TASN-diol was fully characterized by 1H and 13C
NMR and FT-IR spectroscopies and fast atom bombardment
mass spectrometry (FAB-MS) (Figures S1 and S2).
To evaluate the mechanoresponsive color change and
estimate the BDE of the central C–C bond of TASN-diol, UV-Vis
absorption spectroscopy was used in DMAc at various
temperatures between 20 and 100 °C. With increasing
temperature, new absorption peaks arose at around λmax = 350
nm and 550 nm and increased (Figure 1b); these peaks showed
reversibility on increasing/decreasing the temperatures. The
absorption peak at 550 nm corresponds to the pink color
based on the dissociated radical owing to extension of the
conjugated system. The BDE of the central C–C bond in TASN-
TASN-diol, a white powder was ground using a mixing mill
at room temperature to investigate its mechanoresponsive
property. The powder changed in color from white to pink after
grinding (Figure 1a) and then the color gradually faded within a
few hours (Figure S5). Importantly, the pink color could be
induced repeatedly by regrinding after complete
disappearance of the previous color. On the other hand, no
color change was observed with heating from 20 °C to 70 °C.
Therefore, this chromic behavior is not derived from local
thermal stimulus but from mechanical one. To confirm the
generation of a radical species by the grinding process,
electron paramagnetic resonance (EPR) spectroscopic analysis
was undertaken before and after grinding. As shown in Figure
1a, drastic increase in the peak intensity was observed after
grinding. Furthermore, the estimated g value (2.003) was
consistent with the generation of a carbon-centered radical,
indicating that the central carbon–carbon bond in TASN-diol
can undergo homolytic bond cleavage in response to
mechanical stress and generate the corresponding radical
species with pink color. The simulated stable structure by
density-functional theory (DFT) calculations (Figure S6)
strongly supported this result, and the spin density is
distributed on the central carbon atom and also on the cyano
and phenyl moieties. On the other hand, TPSN powder showed
no significant color change after being ground by the same
procedure (Figure S7). EPR measurements suggested a
diol was estimated as the enthalpy change (ꢀHUV) in the van’t
Hoff plot to be 92.9 kJ mol–1 (Figure S9). It value was slightly
smaller than that previously reported for TPSN (109.6 kJ mol–
1), in accordance with the abovementioned EPR results. To
further verify the obtained value, variable-temperature EPR
measurements were obtained in solution, and the g value of
2.003 completely agreed with that for a mechanically induced
one in solid state (Figures 2a, S10, and S11). The calculated
parameters are summarized in Table S1. It was concluded that
the estimated BDE value is authentic and selective scission of
the central C–C bond in TASN-diol occurred both in solution
and the bulk state.
Considering previously reported diphenylmethyl radical
systems,18–20 the absorption peak observed at around λmax
=
350 nm in UV-vis spectrum was ascribed to the excitation
absorption of the generated radical species. Indeed, the TASN-
diol solution emitted yellow photoluminescence under UV
irradiation at 365 nm (Figure S12). The fluorescence spectrum
of the DMAc solution (1×10–5 M) of TASN-diol exhibited an
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calculated from the H NMR spectrum (MnNMR = 63.4 kg/mol)
(Figure S3, S4).
The color of PS-TASN-PS dramatically turned from white to
passion pink in response to mechanical force exerted using a
mixing mill at room temperature similar to TASN-diol, and the
pink-colored
polymer
showed
strong
yellow
photoluminescence under UV irradiation (Figure 3a and S15).
Unlike the case of TASN-diol, the pink color was retained even
after several months, but it disappeared immediately on the
addition of THF, a good solvent for PS, due to rapid increase in
chain mobility and radical recombination. The white powder
reprecipitated
from
this
THF
solution
showed
mechanochromism to passion pink again on grinding. The
ground sample displayed EPR signals, and the estimated g
value was 2.003, indicating the generation of carbon-centered
radical similar to as for the ground TASN-diol sample (Figure
3b). On the other hand, no EPR signal was observed for the
ground PS homopolymer (Mn = 64.0 kg/mol), indicating that
TASN moieties incorporated into the mid-chains were
selectively cleaved. An interesting comparison can be made
between the TASN-containing PS and TASN-diol. PS-TASN-PS
had only one TASN unit in each chain, but the dissociated TASN
ratio for PS-TASN-PS (5.08%) was 168 times of that of TASN-diol
(2.79 × 10–2 %). Moreover, the ratio of dissociated TASN radical
in a blend sample of TASN-diol and PS homopolymer was
comparable to that of TASN-diol (Figure S8). Therefore, the
TASN-containing polymer system successfully functioned as a
mechanochromic polymer and mechanical stress effectively
transferred to the TASN unit through the polymer chains due to
an increase of intermolecular entanglements compared with
Fig. 2. (a) EPR spectra for TASN-diol in DMAc at various temperatures. (b)
Fluorescence spectra for TASN-diol in DMAc. (c) Images of ground TASN-diol
under UV irradiation at 365 nm.
emission peak at λmax,em = 560 nm; the heating obviously
augmented the emission (Figure 2b, Figure S13). In accordance
with the variable-temperature EPR measurements, the yellow
photoluminescence could be attributed to the activated
radicals arising from TASN-diol because the emission signal
intensity strongly depended on the amount of radicals
generated. In fact, the yellow photoluminescence was only
observed from the grinding part of the pink-colored region
(Figure 2c). Moreover, the luminescence spectrum of TASN-diol
before and after grinding was measured at room temperature.
The luminescence was only observed from ground TASN-diol,
and the maximum emission peak exhibited at 565 nm as
similar to solution state (Figure S14). Here, the absolute
photoluminescence quantum yield of the ground TASN-diol
was measured to be 39%. Thus, TASN-diol not only shows
mechanochromism with color change, but also potentially
shows yellow-emitting photoluminescence under UV
irradiation.
TASN-diol33,35–38
.
Based on these findings, we designed and synthesized a
mechanochromic polymer including a TASN unit at the center
of the polymer chain (Scheme 1d). To evaluate the stability of
the yellow-emitting activated radicals in solid state, the glassy
polystyrene (PS) chain was chosen as it was expected to
suppress radical recombination.33,34 TASN-containing PS was
prepared by combining atom transfer radical polymerization
(ATRP) and a click reaction. Alkyne-difunctionalized TASN was
prepared by esterification of TASN-diol and 5-hexynoyl
chloride;
ω-terminated azide polystyrene (PS-N3, Mn = 31.8
kg/mol) was prepared by ATRP of styrene and subsequent
conversion of the bromo-terminal by treatment with sodium
azide. After the click reaction of TASN with two alkyne groups
and PS-N3, the SEC showed two distinct peaks for the linked
product and the residual PS-N3. PS with one TASN at the center
of the polymer chain (PS-TASN-PS) was successfully isolated by
Fig. 3. (a) Photographs of PS-TASN-PS before and after grinding and ground
fractional precipitation using tetrahydrofuran (THF)/acetone sample under UV irradiation. (b) EPR spectra for PS-TASN-PS before and after
grinding. (c) Schematic representing cases before and after grinding PS-
TASN-PS. (d) Radical intensity transitions in EPR measurements and (e) those
under UV irradiation after grinding PS-TASN-PS.
and showed a sharp SEC peak (Mw/Mn = 1.09) and an expected
molecular weight (Mn= 63.8 kg/mol), in good agreement with
the theoretical value (Mntheo.
= 64.1 kg/mol) and that
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This work was supported by ImPACT Program of Council for
Science, Technology and Innovation (Cabinet Office,
Government of Japan) and KAKENHI (No. 16K14076 and No.
17H01205 for H.O. and No. 15K17907 for R.G.) from Japan
Society of the Promotion of Science (JSPS). R.G. gratefully
acknowledges funding from the Japan Prize Foundation. The
authors thank Prof. Takashi Ishizone, Prof. Toshikazu Takata,
Prof. Ken Tanaka for the help of UV-vis spectroscopy,
fluorescent spectroscopy, and absolute photoluminescence
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Conflicts of interest
There are no conflicts to declare.
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