Constitutional isomers of a C3-symmetric molecule showing different piezochromic behaviours: On-off switching and colour tuning

Article abstract of DOI:10.1039/c4tc00822g

The piezochromic behaviours of C₃-symmetric molecules with p-bromophenyl side groups connected to a phenyl ring core through cyano-vinylene bridges were studied. Upon grinding, α-BPAN-Br having cyano groups at the α-position to the phenyl ring core exhibited substantial quenching of a bluish green emission (on-off switching) and its constitutional isomer, β-BPAN-Br, showed an emission colour change from bluish green to deep blue (colour tuning). When the molecules were exposed to an organic vapour, the initial emission of each molecule was restored. The powder X-ray diffraction and DSC studies revealed that the as-synthesized and vapour-annealed samples had the same crystalline structures, while the ground samples had amorphous structures. Their structural analogues, α-BPAN-H and β-BPAN-H which had no bromo groups, did not show any piezochromic or vapochromic behaviour. Poly(β-BPAN) consisting of covalently linked β-BPAN units were prepared by Ullmann reaction of β-BPAN-Br. The polymer showed similar luminescence to that of crystalline β-BPAN-Br, but its initial emission was not changed by grinding. the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc00822g

Journal of
Materials Chemistry C
View Article Online
View Journal | View Issue
PAPER
Constitutional isomers of a C3-symmetric molecule
showing different piezochromic behaviours: on–
off switching and colour tuning
Cite this: J. Mater. Chem. C, 2014, 2,
5963
a
b
a
b
a
*
Yoonbin Lim, Insil Choi, Hyunpyo Lee, Il Won Kim and Ji Young Chang
The piezochromic behaviours of C3-symmetric molecules with p-bromophenyl side groups connected to a
phenyl ring core through cyano-vinylene bridges were studied. Upon grinding, a-BPAN-Br having cyano
groups at the a-position to the phenyl ring core exhibited substantial quenching of a bluish green
emission (on–off switching) and its constitutional isomer, b-BPAN-Br, showed an emission colour
change from bluish green to deep blue (colour tuning). When the molecules were exposed to an organic
vapour, the initial emission of each molecule was restored. The powder X-ray diffraction and DSC
studies revealed that the as-synthesized and vapour-annealed samples had the same crystalline
structures, while the ground samples had amorphous structures. Their structural analogues, a-BPAN-H
and b-BPAN-H which had no bromo groups, did not show any piezochromic or vapochromic behaviour.
Poly(b-BPAN) consisting of covalently linked b-BPAN units were prepared by Ullmann reaction of b-
BPAN-Br. The polymer showed similar luminescence to that of crystalline b-BPAN-Br, but its initial
emission was not changed by grinding.
Received 23rd April 2014
Accepted 16th May 2014
DOI: 10.1039/c4tc00822g
www.rsc.org/MaterialsC
for on–off switching. The uorescence of the co-crystallised
complex between 2,5-di(E)-distyrylfuran as an electron donor
Introduction
and N-alkyl substituted maleimide as an acceptor was quenched
when deposited on the substrate but recovered by applying
mechanical force.20 Kwon et al. achieved an on–off switching
with a donor–acceptor–donor triad molecule containing dicya-
nodistyrylbenzene and carbazole moieties.18
In this work, four types of C3-symmetric molecules having
cyano-vinylene bridges and a phenyl ring core were synthesized
(Scheme 1) and their structural effects on the piezochromism
Luminescent materials are of great interest because of their
potential applications such as in displays,1,2 sensors,3 and
memory devices.4 In recent years, considerable efforts have
been made to control the luminescence properties with external
stimuli such as temperature,5–8 light,8–10 vapour,11–14 or pH.14,15
Mechanical stress is one of the simplest stimuli that can be
applied to a responsive material. Piezochromic materials,
showing reversible and repeatable colour change induced by
mechanical stress, are being widely studied in this
context.5–7,11–26 In recent years, various piezochromic materials
with different luminophores such as anthracene,5 diphenyldi-
benzofulvene,19 cyanostyrylbenzene,12 pyrene,15,26 tetraphenyle-
thene,22 and triphenylamine have been investigated.23,24
Piezochromic materials exhibiting signicant change in
their luminescence intensities by mechanical shearing are
particularly attractive because they can be utilized for on–off
switching of digital signals. A few examples for on–off switch-
able piezochromic materials have been reported.18–20 Tang's
research group demonstrated the reversible switching of emis-
sion intensities of piezochromic materials based on diphe-
nyldibenzofulvenes.19 Luo et al. used a donor–acceptor system
aDepartment of Materials Science and Engineering, College of Engineering, Seoul
National University, Seoul 151-744, Korea. E-mail: jichang@snu.ac.kr; Fax: +82-2-
885-1748; Tel: +82-2-880-7190
bDepartment of Chemical Engineering, College of Engineering, Soongsil University,
Scheme 1 Synthesis of (a) a-BPAN-H and a-BPAN-Br and (b) b-
BPAN-H and b-BPAN-Br.
Seoul 156-743, Korea
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5963–5968 | 5963

View Article Online
Journal of Materials Chemistry C
Paper
were investigated. a-BPAN-H and a-BPAN-Br have cyano lattice energy and only the structures of the lowest energy per
groups at the a-position to the phenyl ring core and b-BPAN-H each run were compared (total 26). Finally, the multiple
and b-BPAN-Br have cyano groups at the b-position to the core. repeated structures were extracted as the nal results.
A network polymer was also prepared by Ullmann reaction
of b-BPAN-Br to compare its optical properties with those of
b-BPAN-Br.
(Benzene-1,3,5-triyl)tris(3-phenylacrylonitrile) (a-BPAN-H)
To a solution of (benzene-1,3,5-triyl)triacetonitrile (0.100 g,
0.512 mmol) and benzaldehyde (0.169 g, 1.59 mmol) in ethanol
(30 mL) and tetrahydrofuran (THF, 3 mL) was added sodium
hydroxide (0.123 g, 3.07 mmol) in ethanol (10 mL). Aer stirring
for 12 h at room temperature, the precipitate was ltered and
washed with ethanol. The product was obtained as a yellow
solid. Yield: 76%. 1H NMR (300 MHz, CDCl3, ppm): d 7.977 (m,
Ar-H, 6H), d 7.957 (s, Ar-H, 3H), d 7.696 (s, vinyl, 3H), d 7.521 (m,
Ar-H, 9H). 13C NMR (400 MHz, DMSO-d6, ppm): d 145.32,
135.83, 133.44, 131.13, 129.35, 129.07, 123.86, 117.59, 109.11.
Elemental analysis: calc. for C33H21N3: C, 86.25; H, 4.61; N, 9.14.
Found: C, 86.30; H, 4.45; N, 9.04%.
Experimental
Materials
Benzene-1,3,5-tricarbaldehyde27 and (benzene-1,3,5-triyl)tri-
acetonitrile28 were prepared according to the literature. All other
chemicals were purchased from Aldrich or Tokyo Chemical
Industry and used without any further purication.
Measurements
1H nuclear magnetic resonance (NMR) spectra were recorded on
a Bruker Advance 300 spectrometer (300 MHz). 13C NMR spectra
were recorded on a Jeol JNM-LA400 (100 MHz). Solid-state 13C
NMR spectra were recorded on a BrukerAvance 400WB spec-
trometer (100 MHz) equipped with a CP-MAS probe. Elemental
analyses were performed using a Flash EA 1112 elemental
analyzer. Differential scanning calorimetry (DSC) measure-
ments were performed on aꢁ1TA modulated DSC Q10 with a
scanning rate of 10 ꢀC min under nitrogen. Powder X-ray
diffraction (PXRD) patterns were obtained using a Bruker New
(Benzene-1,3,5-triyl)tris(3-(4-bromophenyl)acrylonitrile) (a-
BPAN-Br)
To a solution of (benzene-1,3,5-triyl)triacetonitrile (0.300 g, 1.54
mmol) and 4-bromobenzaldehyde (0.506 g, 4.76 mmol) in
ethanol (50 mL) and THF (6 mL) was added sodium hydroxide
(0.369 g, 9.22 mmol) in ethanol (20 mL). Aer stirring for 12 h at
room temperature, the precipitate was ltered and washed with
ethanol. The product was obtained as a pale yellow solid. Yield:
˚
D8 Advance (Cu Ka radiation, l ¼ 1.54 A). UV-Vis spectra were
1
63%. H NMR (300 MHz, CDCl3, ppm): d 7.936 (s, Ar-H, 3H), d
obtained using
a
Sinco S-3150 spectrometer. Photo-
7.837 (d, J ¼ 8.7 Hz, Ar-H, 6H), d 7.670 (d, J ¼ 8.7 Hz, Ar-H, 6H), d
7.612 (s, vinyl, 3H). 13C NMR (400 MHz, THF-d8, ppm): d 144.00,
137.76, 133.98, 133.29, 132.12, 126.14, 125.36, 118.00, 111.91.
Elemental analysis: calc. for C33H18Br3N3: C, 56.93; H, 2.61; N,
6.04. Found: C, 57.02; H, 2.60; N, 6.10%.
luminescence (PL) measurements were performed on a Shi-
madzu RF-5301PC spectrouorometer. The FT-IR spectra were
recorded with a Thermo Scientic Nicolet 6700 FT-IR spec-
trometer using KBr pellets. N2 uptake amounts were measured
using a Belsorp-Max (BEL Japan, Inc.) apparatus.
(Benzene-1,3,5-triyl)tris(2-phenylacrylonitrile) (b-BPAN-H)
Crystal structure prediction
To a solution of benzene-1,3,5-tricarbaldehyde (0.100 g, 0.617
mmol) and phenylacetonitrile (0.224 g, 1.91 mmol) in ethanol
(30 mL) and THF (3 mL) was added sodium hydroxide (0.148 g,
3.70 mmol) in ethanol (10 mL). Aer stirring for 12 h at room
temperature, the precipitate was ltered and washed with
ethanol. The product was obtained as a pale yellow solid. Yield:
The packing propensity of a-BPAN-Br and b-BPAN-Br was
investigated through crystal structure prediction using the
Polymorph Predictor module of the Materials Studio soware
package (version 6.1, 2012; Accelrys). The molecular structures
of two molecules were drawn and their structures were indi-
vidually optimized through energy minimisation in the
COMPASS force eld.29 The resulting structures were used as
the input for the Polymorph Predictor.30,31 The Polymorph
Predictor was performed in the COMPASS force eld with
integral charges, and the calculation of the van der Waals and
Coulomb interactions was done through the Ewald summation
method.32 No rigid body constraints were applied during the
process. The process of the Monte Carlo simulated annealing
(100 000 K to 300 K) was independently repeated 26 times for
each molecule to obtain reproducible results. The number of
1
81%. H NMR (300 MHz, CDCl3, ppm): d 8.443 (s, Ar-H, 3H),
7.764 (m, Ar-H, 6H), 7.738 (s, vinyl, 3H), 7.518 (m, Ar-H, 9H).
13C NMR (400 MHz, DMSO-d6, ppm): d 141.38, 135.21,
133.18, 130.54, 129.82, 129.34, 126.04, 117.19, 112.95.
Elemental analysis: calc. for C33H21N3: C, 86.25; H, 4.61; N, 9.14.
Found: C, 86.27; H, 4.57; N, 9.09%.
(Benzene-1,3,5-triyl)tris(2-(4-bromophenyl)acrylonitrile) (b-
BPAN-Br)
the trial structures for each run was set as 7000 and the number To a solution of benzene-1,3,5-tricarbaldehyde (0.100 g, 0.617
of the nal structures per run was between 24 and 159 aer mmol) and 4-bromophenylacetonitrile (0.375 g, 1.91 mmol) in
clustering. The explored space group was limited to P2/m based ethanol (30 mL) and THF (3 mL) was added sodium hydroxide
on the indexing of the powder XRD patterns through GSAS-II (0.148 g, 3.70 mmol) in ethanol (10 mL). Aer stirring for 12 h at
utilizing the index algorithm by singular value decomposi- room temperature, the precipitate was ltered and washed with
tion.33–35 The predicted structures were ranked according to the ethanol. The product was obtained as a pale yellow solid. Yield:
5964 | J. Mater. Chem. C, 2014, 2, 5963–5968
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
1
89%. H NMR (300 MHz, CDCl3, ppm): d 8.396 (s, Ar-H, 3H),
7.645 (d, J ¼ 9.0 Hz, Ar-H, 6H), 7.604 (d, J ¼ 9.0 Hz, Ar-H, 6H),
7.852 (s, vinyl, 3H). 13C NMR (400 MHz, DMSO-d6, ppm): d
141.93, 135.14, 132.44, 132.26, 130.74, 128.07, 123.20, 116.85,
111.94. Elemental analysis: calc. for C33H18Br3N3: C, 56.93; H,
2.61; N, 6.04. Found: C, 56.67; H, 2.67; N, 6.02%.
Results and discussion
Fig. 2 PL spectra of (a) a-BPAN-Br and (b) b-BPAN-Br recorded in as-
synthesized and ground states (lex ¼ 365 nm).
C3-symmetric molecules having cyano-vinylene bridges and a
phenyl ring core were synthesized via Knoevenagel condensa-
tion reaction as shown in Scheme 1.36 For a-BPAN-H and a-
BPAN-Br, (benzene-1,3,5-triyl)triacetonitrile was reacted with
benzaldehyde and 4-bromobenzaldehyde, respectively, in
ethanol in the presence of sodium hydroxide. In the case of b-
BPAN-H and b-BPAN-Br, benzene-1,3,5-tricarbaldehyde was
reacted with phenylacetonitrile and 4-bromophenylacetonitrile,
respectively. The compounds obtained were bright yellow solids
and at 493 nm, respectively (Fig. 2). a-BPAN-H and b-BPAN-H
showed blue emissions with a peak maxima at 455 and 437 nm,
respectively (Fig. 3).
It is noteworthy that only the compounds bearing bromo
groups displayed piezochromism. Upon grinding, a-BPAN-Br
and b-BPAN-Br changed from opaque bright yellow solids to
colourless solids and their luminescence colour changed
considerably, while the colour and luminescence of a-BPAN-H
and b-BPAN-H showed no signicant change. More interest-
ingly, the two constitutional isomers, a-BPAN-Br and b-BPAN-
Br, showed very different piezochromic properties. When the
ground samples were excited at 365 nm, b-BPAN-Br showed
blue emission with a peak maximum at 446 nm, which was 47
nm blue-shied, while the luminescence of a-BPAN-Br was
almost quenched. The emission patterns of a-BPAN-Br and b-
BPAN-Br observed in the ground state were similar to those
observed in the monomer state.
When the ground samples of a-BPAN-Br and b-BPAN-Br were
exposed to the vapours of organic solvents, their initial lumi-
nescence colours were restored. Although common organic
solvents such as THF, ethyl acetate (EA), acetone, chloroform
and dichloromethane were useful for the luminescence resto-
ration, EA and THF were the most effective for a-BPAN-Br and b-
BPAN-Br, respectively. The original bright yellow colour
appeared within one minute when the ground compounds were
in contact with the vapour of EA or THF. Upon excitation at 365
nm, they showed bluish green luminescence as observed in the
as-synthesized compounds (Fig. 4). The restoration of the
luminescence as well as the solid colour suggested that the
organic solvent vapour provided sufficient driving force for the
rearrangement of the molecules into the ordered phase.
As a demonstration for practical applications of these pie-
zochromic and vapochromic properties of a-BPAN-Br and b-
BPAN-Br, we attempted piezochromic writing and vapochromic
1
and characterized by H NMR, 13C NMR and element analysis.
a-BPAN-Br and b-BPAN-Br showed no signicant difference
in the band gap, showing almost the same absorption spectra in
THF (5.0 ꢂ 10ꢁ5 M) with the absorption onset around 375 nm
(Fig. 1a); however, their emission behaviours signicantly
differed when excited at 365 nm. While no noticeable emission
was observed from a-BPAN-Br in THF, an intense blue emission
with a peak maximum at 415 nm was detected from b-BPAN-Br
in THF (Fig. 1b). a-BPAN-H and b-BPAN-H showed the absorp-
tion maxima at 318 and 324 nm, respectively in THF (5.0 ꢂ 10ꢁ5
M) (Fig. 1c). a-BPAN-H exhibited a weak blue emission with a
peak maximum at 412, while b-BPAN-H showed a strong blue
emission with a peak maximum at 427 nm when excited at 365
nm (Fig. 1d).
In the solid states, the compounds showed different emis-
sion properties from those in the solution states where they are
considered to exist as monomers.12 When excited at 365 nm,
both the a-BPAN-Br and b-BPAN-Br compounds exhibited
bluish green luminescence with a peak maximum at 483 nm
Fig. 1 (a) Absorption and (b) PL spectra of a-BPAN-Br and b-BPAN-Br
in THF (5.0 ꢂ 10ꢁ5 M) (lex ¼ 365 nm). (c) Absorption and (d) PL spectra Fig. 3 PL spectra of (a) a-BPAN-H and (b) b-BPAN-H recorded in as-
of a-BPAN-H and b-BPAN-H in THF (5.0 ꢂ 10ꢁ5 M) (lex ¼ 365 nm).
synthesized and ground states (lex ¼ 365 nm).
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5963–5968 | 5965

View Article Online
Journal of Materials Chemistry C
Paper
Fig. 6 PXRD patterns of (a) a-BPAN-Br and (b) b-BPAN-Br.
ground samples to the EA or THF vapour, suggesting the
restoration of their initial phases. The crystallisation by the
solvent vapour treatment was also conrmed by DSC.11,26 As-
synthesized a-BPAN-Br and b-BPAN-Br showed melt transitions
near 210 ꢀC and 240 ꢀC, respectively, in the rst heating scan. In
contrast, the ground samples of a-BPAN-Br and b-BPANꢀ-Br
Fig. 4 PL spectra of the ground samples, (a) a-BPAN-Br and (b) b-
BPAN-Br, recorded after vapor-exposure (lex ¼ 365 nm). PL changes
of (c) a-BPAN-Br and (d) b-BPAN-Br by grinding and vapor exposure
(lex ¼ 365 nm).
ꢀ
showed exothermic crystallisation peaks at 98 C and 109 C,
respectively, followed by melt transition peaks (Fig. 7). The
ground sample of a-BPAN-Br showed a very small crystallisation
peak, indicating slow crystallisation. The lower crystallisability
of a-BPAN-Br compared to that of b-BPAN-Br could be associ-
ated with the steric effect of the a-nitrile groups on the
conformation of the molecule. The crystallisation peaks did not
appear in the DSC thermograms of the vapour-annealed
samples. These results were consistent with the PXRD results in
which the ordered crystalline phases of a-BPAN-Br and b-BPAN-
Br were changed to the amorphous phases by grinding and were
restored by the solvent vapour exposure.11,18
A considerable difference in the piezochromic behaviours of
the two constitutional isomers, a-BPAN-Br and b-BPAN-Br, was
also of signicant interest. To elucidate the origin of this
discrepancy, we attempted to grow crystals with single crystal X-
ray diffraction qualities, but failed to obtain good crystals
probably due to the three-branched complicated structures of
the two molecules. Instead, crystal structure prediction using
the Polymorph Predictor module of the Materials Studio was
adopted for the investigation of crystal structures of each
molecule.30,31 The optimized structures with minimum lattice
energy were obtained by systematic calculation of van der Waals
and Coulomb interactions, as explained in detail in the Exper-
imental section. As a result, a striking contrast was observed
between the optimized structures of a-BPAN-Br and b-BPAN-Br.
While a-BPAN-Br has twisted conformations between the
phenyl ring core and branches, b-BPAN-Br has a perfectly planar
erasing.20,25 Two non-uorescent lter papers were wetted with
solutions of a-BPAN-Br and b-BPAN-Br in EA and THF, respec-
tively.13,25 As the solvents had evaporated, the lter papers
emitted bluish green luminescence similar to the as-synthe-
sized compounds. When the letters ‘SNU’ were written on the
papers with a rod, distinct changes in the luminescence were
observed only at the regions where mechanical forces were
applied (Fig. 5). The letters written on the paper of a-BPAN-Br
appeared dark while those of b-BPAN-Br showed blue emission.
When the letters on the a-BPAN-Br and b-BPAN-Br solution-
wetted papers were exposed to EA and THF vapour, respectively,
they were erased as the initial luminescences were recovered.
These piezochromic writing and vapochromic erasing behav-
iours agreed well with the piezochromic and vapochromic
properties of the solid compounds as shown in Fig. 4.
It has been proposed that piezochromism occurs by
destruction of an ordered structure by external mechanical
forces although its denite mechanism is still under contro-
versy.16 The idea that macroscopic mechanical forces substan-
tially alter the microscopic nanostructure seemed to be
acceptable at least based on PXRD and DSC analysis results for
a-BPAN-Br and b-BPAN-Br.11,15,18 The as-synthesized samples of
a-BPAN-Br and b-BPAN-Br exhibited crystalline PXRD patterns
with sharp and strong peaks. When these compounds were
ground, they showed no noticeable peaks, indicating the tran-
sition from the crystalline states to amorphous states (Fig. 6).
The crystalline peaks appeared again aer exposure of the
Fig. 5 Piezochromic writing and vapochromic erasing of the letters
‘SNU’ written on filter papers wetted with solutions of (a) a-BPAN-Br Fig. 7 DSC curves of (a) a-BPAN-Br (inset: a magnified curve of the
and (b) b-BPAN-Br in EA and THF, respectively.
ground sample) and (b) b-BPAN-Br.
5966 | J. Mater. Chem. C, 2014, 2, 5963–5968
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
structure over the entire molecule (Fig. 8). This difference in
molecular structures in crystalline states is ascribed to the
different positions and steric effects of the nitrile groups in each
molecule. In a-BPAN-Br, three nitrile groups are located at the
a-positions to the phenyl ring core, making the molecular
centre more crowded than in the case of b-BPAN-Br. This
crowdedness causes higher conformational torsion for a-BPAN-
Br relative to b-BPAN-Br, promoting molecular motions.37 The
luminescence quenching of a-BPAN-Br in the monomer and
ground states is likely because the excited energy was mostly
lost by non-radiative pathways such as vibrational and rota-
tional motions.12 On the other hand, the perfectly planar
structure of b-BPAN-Br in the crystalline state suppresses
molecular motions, and the excited energies are consumed in
the form of luminescence. With this relationship between
molecular structures and piezochromic behaviours observed in
a-BPAN-Br and b-BPAN-Br, we found that it is possible to
modulate on–off switching and colour tuning by changing the
position of bulky groups.
Fig. 9 (a) Solid-state 13C NMR spectrum of poly(b-BPAN) and (b) FT-IR
spectra of b-BPAN-Br and poly(b-BPAN).
To further understand the role of the weak intermolecular
bondings for piezochromism, we prepared a polymer, poly(b-
BPAN), by Ullmann reaction of b-BPAN-Br.38 Poly(b-BPAN)
consisted of b-BPAN units, which were linked covalently to each
other. The polymer structure was characterized by solid-state
Fig. 10 (a) Nitrogen sorption isotherm curves of poly(b-BPAN-Br) and
(b) PL spectra of as-synthesized and ground poly(b-BPAN-Br).
13C NMR and FT-IR spectroscopy (Fig. 9). In the solid-state 13C polymerisation.38 The polymer was isolated as yellow powders.
NMR spectrum, the carbon peaks for the cyano groups and Since b-BPAN-Br had three reactive sites, poly(b-BPAN) had a
phenyl groups appeared at 115 and 119–136 ppm, respectively. network structure. The Brunauer–Emmett–Teller (BET) surface
FT-IR spectroscopy showed that a characteristic absorption area of the polymer calculated from the N2 adsorption isotherm
band for a carbon–bromine bond at 488 cmꢁ1 disappeared aer at 77 K was 42.5 m2 gꢁ1 (Fig. 10a).
The polymer showed blue luminescence at 365 nm excitation
as crystalline b-BPAN-Br. However, its initial emission was not
changed by grinding (Fig. 10b), which was a completely
different behaviour compared with the crystalline monomer.
Apparently, the external mechanical force could not alter the
covalently linked network structure of poly(b-BPAN).
Conclusions
We prepared four types of conjugated C3-symmetric molecules
with a phenyl ring core and investigated their piezochromism.
Among them, two constitutional isomers a-BPAN-Br and b-
BPAN-Br having bromine atoms at the branched phenyl groups
showed piezochromism. Upon grinding, a-BPAN-Br exhibited
substantial quenching of bluish green emission while b-BPAN-
Br showed luminescence colour change from bluish green to
deep blue. This discrepancy was ascribed to the difference of
crowdedness in the vicinities of cores, which caused a different
degree of molecular torsion. The initial emission of each
molecule was restored by exposure to an organic solvent vapour.
PXRD and DSC studies revealed that the piezochromic and
vapochromic behaviours were operated by the destruction and
reconstruction of the ordered structures of the compounds.
Their structural analogues, a-BPAN-H and b-BPAN-H, which
had no bromo groups, however, did not show any piezochromic
or vapochromic behavior. Poly(b-BPAN), comprised of cova-
lently linked b-BPAN units, also did not show any piezochromic
Fig. 8 Optimized structures of (a) a-BPAN-Br and (b) b-BPAN-Br
calculated with the Polymorph Predictor in Materials Studio 6.1.
or vapochromic behavior. These results led to the conclusion
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5963–5968 | 5967

View Article Online
Journal of Materials Chemistry C
Paper
that the transitions between ordered and disordered phases by 15 S. Yamaguchi, I. Yoshikawa, T. Mutai and K. Araki, J. Mater.
external stimuli were enabled due to the weak intermolecular Chem., 2012, 22, 20065.
non-covalent bonds formed in crystalline a-BPAN-Br and b- 16 Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu and
BPAN-Br. We believe that the present work demonstrates an J. Xu, Chem. Soc. Rev., 2012, 41, 3878.
efficient method of modulating the properties of piezochromic 17 K. Ariga, T. Mori and J. P. Hill, Adv. Mater., 2012, 24, 158.
materials.
18 M. S. Kwon, J. Gierschner, S.-J. Yoon and S. Y. Park, Adv.
Mater., 2012, 24, 5487.
19 X. Luo, J. Li, C. Li, L. Heng, Y. Q. Dong, Z. Liu, Z. Bo and
B. Z. Tang, Adv. Mater., 2011, 23, 3261.
Acknowledgements
20 J. Luo, L.-Y. Li, Y. Song and J. Pei, Chem.–Eur. J., 2011, 17,
10515.
21 J. W. Chung, Y. You, H. S. Huh, B.-K. An, S.-J. Yoon,
S. H. Kim, S. W. Lee and S. Y. Park, J. Am. Chem. Soc.,
2009, 131, 8163.
22 X. Luo, W. Zhao, J. Shi, C. Li, Z. Liu, Z. Bo, Y. Q. Dong and
B. Z. Tang, J. Phys. Chem. C, 2012, 116, 21967.
23 K. Mizuguchi, H. Kageyama and H. Nakano, Mater. Lett.,
2011, 65, 2658.
This research was supported by grants from the Mid-career
Researcher Program through NRF grant (no. 2010-0017552)
funded by the National Research Foundation of Korea. I. Choi
and I. W. Kim appreciate the nancial support from the Human
Resources Development program (no. 20124030200070) of the
Korea Institute of Energy Technology Evaluation and Planning
(KETEP) grant funded by the Korea government Ministry of
Trade, Industry & Energy.
24 K. Mizuguchi and H. Nakano, Dyes Pigm., 2013, 96, 76.
25 T. Han, Y. Zhang, X. Feng, Z. Lin, B. Tong, J. Shi, J. Zhi and
Y. Dong, Chem. Commun., 2013, 49, 7049.
26 M.-J. Teng, X.-R. Jia, S. Yang, X.-F. Chen and Y. Wei, Adv.
Mater., 2012, 24, 1255.
Notes and references
1 C. D. Muller, A. Falcou, N. Reckefuss, M. Rojahn,
V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken, H. Becker
and K. Meerholz, Nature, 2003, 421, 829.
27 N. Kaur, J.-G. Delcros, J. Imran, A. Khaled, M. Chehtane,
N. Tschammer, B. Martin and O. Phanstiel, J. Med. Chem.,
2008, 51, 1393.
28 P. Langer and J. T. Anders, Eur. J. Org. Chem., 2002, 686.
29 H. J. Sun, J. Phys. Chem. B, 1998, 102, 7338.
30 N. Panina, F. J. J. Leusen, F. F. B. J. Janssen, P. Verwer,
H. Meekes, E. Vlieg and G. Deroover, J. Appl. Crystallogr.,
2007, 40, 105.
¨
2 M. C. Gather, A. Kohnen, A. Falcou, H. Becker and
K. Meerholz, Adv. Funct. Mater., 2007, 17, 191.
3 S. J. Toal, K. A. Jones, D. Magde and W. C. Trogler, J. Am.
Chem. Soc., 2005, 127, 11661.
4 C. E. Olson, M. J. R. Previte and J. T. Fourkas, Nat. Mater.,
2002, 1, 225.
5 X. Zhang, Z. Chi, H. Li, B. Xu, X. Li, W. Zhou, S. Liu, Y. Zhang
and J. Xu, Chem. – Asian J., 2011, 6, 808.
31 K.-H. Kim, D. H. Jung, D. Kim, A. Lee, K. Choi, Y. Kim and
S.-H. Choi, Dyes Pigm., 2011, 89, 37.
6 G. Zhang, J. Lu, M. Sabat and C. L. Fraser, J. Am. Chem. Soc.,
2010, 132, 2160.
¨
32 M. Svard and A. C. Rasmuson, Ind. Eng. Chem. Res., 2009, 48,
7 Y. Sagara and T. Kato, Angew. Chem., Int. Ed., 2011, 50, 9128.
8 J. W. Chung, S.-J. Yoon, S.-J. Lim, B.-K. An and S. Y. Park,
Angew. Chem., Int. Ed., 2009, 48, 7030.
9 K. Rameshbabu, L. Zou, C. Kim, A. Urbas and Q. Li, J. Mater.
Chem., 2011, 21, 15673.
10 M. Han, D. Ishikawa, T. Honda, E. Ito and M. Hara, Chem.
Commun., 2010, 46, 3598.
11 C. Dou, D. Chen, J. Iqbal, Y. Yuan, H. Zhang and Y. Wang,
Langmuir, 2011, 27, 6323.
12 S.-J. Yoon, J. W. Chung, J. Gierschner, K. S. Kim, M.-G. Choi,
D. Kim and S. Y. Park, J. Am. Chem. Soc., 2010, 132, 13675.
13 X. Zhou, H. Li, Z. Chi, X. Zhang, J. Zhang, B. Xu, Y. Zhang,
S. Liu and J. Xu, New J. Chem., 2012, 36, 685.
14 C. Dou, L. Han, S. Zhao, H. Zhang and Y. Wang, J. Phys.
Chem. Lett., 2011, 2, 666.
2899.
33 A. A. Coelho, J. Appl. Crystallogr., 2003, 36, 86.
34 A. C. Larson and R. B. V. Dreele, General Structure Analysis
System (GSAS), Los Alamos National Laboratory Report,
LAUR, 2004, pp. 86–748.
35 B. H. Toby and R. B. V. Dreele, J. Appl. Crystallogr., 2013, 46,
544.
36 J. A. Mikroyannidis, S. S. Sharma, Y. K. Vijay and
G. D. Sharma, ACS Appl. Mater. Interfaces, 2010, 2, 270.
37 Y. Morisaki, L. Lin and Y. Chujo, J. Polym. Sci., Part A: Polym.
Chem., 2009, 47, 5979.
38 T. Ben, H. Ren, S. Ma, D. Cao, J. Lan, X. Jing, W. Wang, J. Xu,
F. Deng, J. M. Simmons, S. Qiu and G. Zhu, Angew. Chem.,
Int. Ed., 2009, 48, 9457.
5968 | J. Mater. Chem. C, 2014, 2, 5963–5968
This journal is © The Royal Society of Chemistry 2014