D-A-D type dinitriles with vapor-dependent luminescence in the solid state

Article abstract of DOI:10.1016/j.tetlet.2017.09.068

D-A-D (Donor-Acceptor-Donor) type dinitriles linked by a styryl or phenylethynyl group have been prepared. These groups were introduced to increase the flexibility and the size of the π-conjugation in the chromophores. Both compounds showed strong emission in the solid state, AIE (aggregation-induced emission) behavior, and mechanochromism. The fluorescence color of ground powder changed by organic solvent vapor (vapochromism). Especially, the emission color of the styryl dinitrile after exposures depends on the solvent, while that of the phenylethynyl dinitrile is the same after exposure to different solvents. These results were explained by single crystal and powder XRD measurements, which revealed that the flexible styryl linker leads to a loose crystal packing, resulting in a dinitrile with multi-state microcrystalline structures. This methodology based on the flexible linker allows for the detection of small organic molecules without transition metals.

Full text of DOI:10.1016/j.tetlet.2017.09.068

Accepted Manuscript
D-A-D type dinitriles with vapor-dependent luminescence in the solid state
Taniyuki Furuyama, Junichi Shinozaki, Thiago Teixeira Tasso, Hajime Maeda,
Masahito Segi, Nagao Kobayashi
PII:
S0040-4039(17)31204-2
https://doi.org/10.1016/j.tetlet.2017.09.068
TETL 49338
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Accepted Date:
20 August 2017
22 September 2017
Please cite this article as: Furuyama, T., Shinozaki, J., Tasso, T.T., Maeda, H., Segi, M., Kobayashi, N., D-A-D type
dinitriles with vapor-dependent luminescence in the solid state, Tetrahedron Letters (2017), doi: https://doi.org/
10.1016/j.tetlet.2017.09.068
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Taniyuki Furuyama*, Junichi Shinozaki, Thiago Teixeira Tasso, Hajime Maeda, Masahito Segi, Nagao
Kobayashi*

1
Tetrahedron Letters
journal homepage: www.els evier.com
D-A-D type dinitriles with vapor-dependent luminescence in the solid state
Taniyuki Furuyamaa,b, ∗, Junichi Shinozakia, Thiago Teixeira Tassob, Hajime Maedaa, Masahito Segia, and
Nagao Kobayashib,c,
∗
aGraduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan
bDepartment of Chemistry, Graduate School of Science, Tohoku University, 980-8578, Japan
cFaculty of Textile Science and Technology, Shinshu University, 386-8567, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
D-A-D (Donor-Acceptor-Donor) type dinitriles linked by a styryl (1) or phenylethynyl (2) group
have been prepared. A styryl or phenylethynyl group introduced to increase the flexibility and
the size of the π-conjugation. Both compounds showed strong emission in the solid state, AIE
(aggregation-induced emission) behavior, and mechanochromism. The fluorescence color of
ground powder changed by organic solvent vapor (vapochromism). Especially, the color of
styryl-linked 1 after exposures depends on the solvent, while the color of 2 is the same after
exposure to different solvents. Single crystal and powder XRD measurements revealed multi-
state microcrystalline structures of 1 by the flexible styryl linker that leads to a loose crystal
packing. This methodology based on the flexible linker allows for the detection of small organic
molecules without transition metals.
Available online
Keywords:
Cyanides
Aggregation
Fluorescence
Mechanochromism
Vapochromism
2009 Elsevier Ltd. All rights reserved.
Organic materials whose spectral arrangement of optical
properties is changed by weak external stimuli (chromic
responses) are one of the most attractive targets in chemistry, as
well as in material and biological sciences. In this regard, π-
conjugated organic compounds are good candidates, since their
properties can be fine-tuned by synthetic modifications. Various
materials with chromic responses, such as photochromism,
should be coupled with specific molecular inputs.3-4
Vapochromism, for instance, is still rare for organic conjugated
compounds without transition metals,5-6 with the exception of a
response with acid/base vapor fuming (a kind of halochromism).7
Tetraphenylethylene (TPE) is known to be a key moiety not only
for aggregation-induced emission (AIE) behavior, but also for
various chromic responses in organic materials.8-9 Recently, we
found that the TPE-linked dinitrile (TPE–CN2, Scheme 1)
showed strong emission in the solid state, AIE behavior and
chromic responses to mechanical forces.10 Although TPE–CN2
showed vapochromism as well, the colors after fuming did not
depend on the solvent used. Various dinitrile derivatives have
been synthesized as precursors of tetraazaporphyrins and dinitrile
derivatives with appropriate donor moieties show intense
emission in the solid state.11 The vapochromic response of
transition metal-containing organometallic materials originates
from changes in metal-metal interactions.12 In this sense, another
strategy is required to develop pure-organic vapochromic
materials which can respond to various kinds of vapor. While
host-guest chemistry is often efficient for overcoming this
problem,13-14 we focused on the flexibility of molecules. By
introducing flexible moieties into molecules with chromism in
the solid state, we anticipated that the microcrystalline structure
of the flexible π-conjugated chain might change on extra weak
external stimuli, such as simple, neutral solvent vapors. To
increase the flexibility and the size of the π-conjugation, styryl
solvatochromism,
halochromism,
thermochromism,
mechanochromism, and vapochromism have been synthesized.
Although some responses occur in solution, mechanochromism
and vapochromism appear only in the solid state. Chromic
responses in emission properties in the solid or aggregated state
are required for applications in solar cells, probes for bioimaging,
photosensitizing agents and sensors of toxic volatile organic
compounds (VOCs).1 However, the emission of typical π-
conjugated compounds is almost entirely quenched in the solid
state due to an aggregation-caused quenching (ACQ) effect,2 so
that reversibly controlling the emission properties by mechanical
forces and/or vapors is an attractive challenge in modern material
chemistry.
In this communication, we report the synthesis and multi-
chromic properties of D-A-D (Donor-Acceptor-Donor) type
dinitriles. Classical optical probes show only one response to
various kinds of external stimuli, however, for the detection of
small molecules such as VOCs in the air and biological signals in
c—an—cer—and other diseased tissues, more than two emissive states
∗ Corresponding author. Tel.: +81-76-234-4786; e-mail: tfuruyama@se.kanazawa-u.ac.jp (T.F).
∗ Corresponding author. e-mail: nagaok@shinshu-u.ac.jp (N.K).

2
Tetrahedron
(1) or phenylethynyl (2) groups15-16 were introduced between
showed a 1D-layer structure in the crystal, with THFs oriented
the TPE and dinitrile moieties of TPE-CN2.
between the layers, where the distances of the intermolecular
layers are 3.658 and 4.874 Å (Fig. S2, see Supporting
Infromation). The TPE moieties were oriented in a twisted
manner, indicating weak intermolecular π-π stacking interactions.
The flexible styryl linker of 1 may lead to a loose crystal
packing, resulting in solvent molecules delocalized on the entire
molecule. The single crystal structure of TPE-CN2 from a
chloroform/methanol mixture,10 on the other hand, contained
chloroform molecules localized only on the dicyanoethylene
moiety (Fig. S3, see Supporting Infromation) indicating a more
rigid packing structure.
Fig. 1. a) X-ray crystal structure of 1. The thermal ellipsoids were scaled
to the 50% probability level. Hydrogen atoms and solvent molecules have
been omitted for clarity. b) Packing diagram of 1⊃THF. Hydrogen atoms are
omitted for clarity.
Absorption and emission spectra of diluted solutions of these
compounds are shown in Fig. S4 (see Supporting Information),
with a summary of the absorption and emission properties in
solution listed in Table S3 (see Supporting Information). Both
compounds absorb light around 400 nm, and solvent effects in
the absorption properties are low. Calculated absorption spectra
(Fig. S5, see Supporting Infromation) reproduced their
absorption properties. The intense absorption band could be
assigned to an intramolecular charge-transfer (CT) transition
from TPE to dinitrile moieties. This is consistent with their
emission spectra in various solvents, which significantly
redshifted on increasing the solvent polarity. To quantify the
solvatofluorochromism of these compounds, Lippert-Mataga
plots18 were made (Fig. S4c, see Supporting Infromation). On
linear fitting, large slopes were found for 1 (6.1 x 103) and 2 (5.1
x 103), which represent similar solvatofluorochromism to
previously reported D-A type chromophores (TPE-CN2: 7.9 x
103).10 1 and 2 showed typical AIE behavior in THF/water mixed
solvents (Fig. S6, see Supporting Infromation). The fluorescence
intensity was sharply increased with increasing water content,
indicating that the molecules aggregate with increasing amounts
of poor solvent. The 5- and 58-fold increase in the intensity at the
fluorescence peak of compounds 1 and 2 observed in the mixture
containing 90% water substantiated a strong AIEE effect.
Scheme 1. Molecular Structure of dinitirles 1 and 2 in this work.
The synthetic procedure of 1 and 2 is shown in Scheme S1
(see Supporting Information). Dinitrile derivatives could be
synthesized by dimerization of phenylacetonitrile derivatives.17
Vinylene-bridged TPE-phenylacetonitrile 4 was obtained from
TPE-Br 5 and 4-cyanomethylstyrene by the Heck reaction. After
dimerization, 1 was obtained as a mixture of E and Z forms at a
dinitrile moiety. However, the Z-form 1 was isomerized to a
mixture of E and Z rapidly in solution under ambient light (data
is not shown). The pure E-form 1 suffered no isomerization after
irradiation with UV-light (365 nm) for 1 min in solution (Fig. S1,
see Supporting Information), so it was used for characterization
of the structure and investigation of the optical properties,
assuming negligible isomerization during these experiments.
When equilibrium was reached (ca. 1 hour), the E/Z ratio found
was 73/27. Hence the E form of 1 is more favorable. No
isomerization was observed on irradiation of the samples with
light in the solid state. 2 was synthesized from bis(4-
bromophenyl)fumaronitrile (8, E form) and TPE-ethynylene 7 by
the Sonogashira reaction. Only the E-form 2 was obtained, since
bis(4-bromophenyl)maleonitrile (Z form) could not be produced
by the dimerization of (4-bromophenyl)acetonitrile.17 1 and 2
1
could be fully characterized by H and 13C NMR and HR-FAB
Fig. 2 shows emission spectra of the compounds in the solid
state. In the as-prepared, dried powder, the emission peak of 1
(617 nm) appeared at a longer wavelength than that of 2 (574
nm). An additional intermolecular packing interaction may exist
for 1, because the obtained structure of 1 in 1⊃THF is slightly
different from the calculated structure (as gas phase) of 1 (Fig.
S7, see Supporting Infromation). After the powders of 1 and 2
were exposed to toluene vapor, the emission peaks were blue-
shifted, and visible color changes were observed. The emission
mass spectrometry.
The structure of 1 was unambiguously elucidated by X-ray
diffraction analysis of crystals obtained from the diffusion of
methanol into a THF solution (Fig. 1). Unfortunately, suitable
single crystals of 2 could not be obtained. All three vinylene
moieties were E form (Fig. 1a). A crystal packing diagram of the
crystals is shown in Fig. 1b, in which THF molecules were found
in the unit cell (the crystal is denoted as 1⊃THF). 1⊃THF

3
peaks were returned to the initial state after grinding the powder.
vapor, the fluorescent peak shifted to the blue on decreasing the
solvent polarity.
Fig. 2. Changes in the fluorescence spectra of a) 1 and b) 2 on external
stimuli. Excitation at 442 and 411 nm for 1 and 2, respectively. Photos were
taken under UV light irradiation (365 nm).
Therefore, compounds 1 and 2 showed mechano-/vapochromic
responses. For a demonstration of mechanochromic applicability,
the powder of 1 was dispersed on filter paper, and some
characters were written by scratching (Fig. S8, see Supporting
Infromation). On fuming the paper with toluene, the characters
became invisible, and this writing/erasing process could be
repeated reversibly. Powder XRD (PXRD) measurements
revealed a microcrystalline phase-transition of 1 and 2 during the
grinding/fuming cycle (Fig. S9, see Supporting Infromation). On
grinding, the reflection peaks became weaker and broader,
indicating an emergence of amorphous character as a result of the
application of an external mechanical force to the powder. After
fuming with toluene, the peaks were regenerated. The positions
and intensities of the peaks completely matched between the first
and second fuming cycles, thus indicating that the
microcrystalline structure was regenerated reversibly by fuming
with organic solvents. These chromic responses of 1 and 2 were
also induced by dichloromethane vapor. Interestingly, the color
of ground 1 was changed from red to reddish-yellow by
dichloromethane vapor, while the color was changed from red to
orange by toluene vapor (Fig. 2a, inset). On the other hand, the
Fig. 3. Experimental XRD patterns of dried 1, 1 with THF vapor and 1
with CHCl3 vapor and simulated pattern for the single crystal structure of
1⊃THF and 1⊃CHCl3.
color difference of
2
after exposure to toluene or
dichloromethane vapor was negligible. The fluorescence spectra
of 1 after exposure to various solvents revealed that the color
after fuming depended on the type of solvent (Fig. S10, see
Supporting Infromation). Among five solvents which gave a
vapochromic response, the emission peak shifted from
dichloromethane (610 nm) to toluene (589 nm), which was
enough to produce a visible color change in the powder. The
solubility of 1 appeared important for the color change, so that
the exposure of hexane vapor to ground 1 did not change the
color (note that 1 does not dissolve in hexane). With respect to
the solvatofluorochromism of 1 in solutions, after exposure to
The difference in vapochromic responses was clarified by
PXRD measurements. In the case of 2, the pattern after exposure
to dichloromethane vapor became identical to that after exposure
to toluene vapor (Fig. S11, see Supporting Infromation). Thus,
solvent molecules were essential for the regeneration of the
microcrystalline structure of 2, but the solvent molecules did not
affect the final microcrystalline structure due to the rigid
ethynylene linkage in 2. Fig. S12 (see Supporting Infromation)
shows PXRD patterns of powder 1 after exposure to solvents that
give vapochromic behavior. The differences in the peaks are
relatively small; however, each pattern can be distinguished. The

4
Tetrahedron
contact with organic solvent vapor may form thin liquid layers
Foundation and Kanazawa University SAKIGAKE Project. The
authors thank Prof. Shigehisa Akine, Prof. Tetsuya Taima, Prof.
Tomoki Ogoshi, Prof. Makoto Karakawa, Dr. Yoko Sakata and
Dr. Takahiro Kakuta (Kanazawa University) for physicochemical
measurements.
on the surface of 1,19 then the solvent molecules penetrate into
the amorphous structure, generating the microcrystalline
structure. Simulated PXRD patterns determined from single
crystal X-ray structures obtained from THF/methanol (1⊃THF)
or chloroform/methanol (1⊃CHCl3)20 solution are shown in Fig.
3 in order to understand the vapor-dependent color change in 1.
The simulated PXRD pattern of 1⊃CHCl3 is different from that
of 1⊃THF, which agreed with the experimental PXRD pattern of
powder 1 after exposure to fuming chloroform or THF. The
packing structure of 1⊃CHCl3 also showed a 1D-layer structure,
with chloroform molecules oriented between the layers. On the
other hand, the distances of the intermolecular layers were
changed (4.862 Å and 3.621 Å) in relation to the size of the
solvent molecules (Fig. S13, see Supporting Infromation).
Previously reported single crystal structures of TPE-based
vapochromic molecules (solvent dependency was not found or
discussed) showed no solvent molecule on specific sites.10,21-22
Hence, the styryl linker is considered to be important for
obtaining the solvent dependent multi-microcrystalline structures
in this study. Since the position of the fluorescence peak depends
on the stacked structure of π-conjugated molecules,23 the color
changes after fuming can be assigned to the difference in spacing
between the layers in microcrystalline arrays.
Supplementary data
Supplementary data associated with this article can be found,
in
the
online
version,
at
http://dx.doi.org/10.1016/j.tetlet.XXXX.XX.XXX.
References and notes
1. Bamfield P, Hutchings MG. ed. Chromic Phenomena:
Technological Applications of Colour Chemistry 2nd ed. London,
Royal Society of Chemistry:2010.
2. Hong Y, Lam JWY, Tang BZ. Chem. Soc. Rev. 2011;40:5361-
5388.
3. Andréasson J, Pischel U. Chem. Soc. Rev. 2015;44:1053-1069.
4. Winckel Evd, Schneider RJ, Escosura Adl, Torres T. Chem. Eur.
J. 2015;21:18551-18556.
5. Sakon A, Sekine A, Uekusa H. Cryst. Growth Des. 2016;16:4635-
4645.
6. Peterson JJ, Davis AR, Werre M, Coughlin EB, Carter KR. ACS
Appl. Mater. Interfaces 2011;3:1796-1799.
7. Cheng X, Li D, Zhang Z, Zhang H, Wang Y. Org. Lett.
2014;16:880-883.
8. Mei J, Leung NLC, Kwok RTK, Lam JWY, Tang BZ, Chem. Rev.
2015;115:11718-11940.
9. Ding A-X, Hao H-J, Gao Y-G, Shi Y-D, Tang Q, Lu Z-L. J.
Mater. Chem. C 2016;4:5379-5389.
10. Tasso TT, Furuyama T, Kobayashi N. Chem. Eur. J.
2015;21:4817-4824.
11. Lee Y-T, Chiang C-L, Chen C-T. Chem. Commun. 2008;217-219.
12. Wenger OS. Chem. Rev. 2016;113:3686-3733.
13. Hatanaka S, Ono T, Hisaeda Y. Chem. Eur. J. 2016;22:10346-
10350.
14. Ogoshi T, Shimada Y, Sakata Y, Akine S, Yamagishi T. J. Am.
Chem. Soc. 2017;139:5664-5667.
15. Jagadesan P, Whittemore T, Beirl T, Turro C, McGrier PL. Chem.
Eur. J. 2017;23:917-925.
16. Sadowski B, Kita H, Grzybowski M, Kamada K, Gryko DT. J.
Org. Chem. 2017;82:7254-7264.
17. Yeh H-C, Wu W-C, Wen Y-S, Dai D-C, Wang J-K, Chen C-T. J.
Org. Chem. 2004;69:6455-6462.
18. Lakowicz JR. ed. Principles of Fluorescence Spectroscopy,
Baltimore, Springer:2006.
19. Dong Y, Lam JWY, Qin A, Liu J, Li Z, Tang BZ. Appl. Phys.
Lett. 2007;91:011111.
20. Although the crystallographic structure of crystals of 1 obtained
from chloroform/methanol solution could not be solved
completely (a portion of electron density around disordered
chloroforms could not be assigned), the structure of 1 and the site
of chloroforms could be determined ambiguously. Thus we used
the data as a model of the microcrystalline structure of 1 after
fuming chloroform.
21. Luo X, Zhao W, Shi J, Li C, Liu Z, Bo Z, Dong YQ, Tang BZ. J.
Phys. Chem. C 2012;116:21967-21972.
22. Zhang G-F, Wang H, Aldred MP, Chen T, Chen Z-Q, Meng X,
Zhu M-Q. Chem. Mater. 2014;26:4433-4446.
23. Rösch U, Yao S, Wortmann R, Würthner F. Angew. Chem. Int.
Ed. 2006;45:7026-7030.
In summary, dinitrile derivatives exhibiting multi-
luminescence responses with mechanical grinding and exposure
to vapors of organic solvents have been synthesized. The D-A-D
system and TPE moieties show efficient emission in the visible
region and AIE properties. Phenylethynyl-linked 2 exhibits an
obvious AIEE effect, and a 58-fold increase in fluorescence
intensity after aggregation. Mechanical grinding of powder
samples changes the emission color of the samples, and exposure
to an appropriate organic vapor for several minutes was sufficient
to reversibly regenerate the original color. A flexible π-
conjugated linkage is important to distinguish the kind of organic
solvent. The color of styryl-linked 1 after exposures depends on
the solvent, while the color of 2 is the same after exposure to
different solvents that show a chromic response. The differences
between 1 and 2 could be clearly explained by the PXRD spectra
of grinding and fuming samples. PXRD patterns of 1 after
exposure to solvents showed multi-microcrystalline arrays that
depend on the solvent. The single crystal structure of 1 supported
the PXRD patterns, and the spacings of the intermolecular layers
were changed by solvent molecules. Compound 1 is easy to
synthesize and free from expensive and/or toxic transition metals,
being a so-called “smart” material which is able to distinguish the
type of organic solvent by visible color changes. This kind of
material will be applied to novel sensors for the detection of
small organic molecules in the fields of environmental and
agricultural sciences. Further work is currently underway to
expand the scope of substrates to change the colors by the
introduction of additional functional moieties and to shift
emission colors to the near-IR region for biological applications.
Acknowledgments
This work was partly supported by a JSPS KAKENHI Grant
(No. 15K05409), The Foundation for Japanese Chemical
Research, Hokuriku Bank Foundation, The Murata Science
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5
Highlights:
D-A-D type dinitriles linked by flexible linkers
have been synthesized.
The vapochromic response of the styryl-linked
compound is solvent dependent.
These materials can distinguish different organic
solvents without metals.