Mechanofluorochromism of carbazole-type D-π-A fluorescent dyes

Article abstract of DOI:10.1016/j.tet.2011.11.012

We have newly designed and synthesized unsymmetrical carbazole-type D-π-A fluorescent dyes. The dyes show a bathochromic shift-type mechanofluorochromism (MFC): grinding of as-recrystallized dyes induces a bathochromic shift of fluorescent color and the f

Full text of DOI:10.1016/j.tet.2011.11.012

Tetrahedron 68 (2012) 529e533
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Tetrahedron
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Mechanofluorochromism of carbazole-type De
p
eA fluorescent dyes
Yousuke Ooyama, Naoya Yamaguchi, Shogo Inoue, Tomoya Nagano, Eigo Miyazaki, Hiroshi Fukuoka,
*
Ichiro Imae, Kenji Komaguchi, Joji Ohshita, Yutaka Harima
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 August 2011
Received in revised form 4 November 2011
Accepted 5 November 2011
Available online 17 November 2011
We have newly designed and synthesized unsymmetrical carbazole-type DepeA fluorescent dyes. The
dyes show a bathochromic shift-type mechanofluorochromism (MFC): grinding of as-recrystallized dyes
induces a bathochromic shift of fluorescent color and the fluorescent color is recovered by heating or
exposure to solvent vapor. In order to clarify the MFC mechanism for the carbazole-type DepeA fluo-
rescent dyes, time-resolved fluorescence spectroscopy, X-ray powder diffractometry, single-crystal X-ray
structural analysis, IR spectroscopy, and differential scanning calorimetry are performed before and after
grinding of the solids. On the basis of experimental results and semi-empirical molecular orbital cal-
culations (AM1 and INDO/S), we have revealed that the MFC is attributed to a reversible switching be-
tween crystalline and amorphous states with changes of intermolecular hydrogen bonding and
interaction.
pep
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
(De
p
eA) fluorescent dyes with strong electron-withdrawing
substituents as acceptor and thus large dipole moments (ca. 5 D)
show a bathochromic shift-type MFC.6 We have ascribed the MFC
to a reversible switching between crystalline and amorphous
states with changes of dipoleedipole interaction and in-
Since Araki et al. reported mechanofluorochromism (MFC) of
amide-substituted tetraphenylpyrene C6TPPy in 2007,1 the MFC
organic dyes, which show a hypsochromic or a bathochromic shift
of fluorescent color induced by mechanical stress to the crystal,
being accompanied by a reversion to the original fluorescent color
by heating or exposure to solvent vapor, have received an in-
creasing interest as attractive materials both for the fundamental
research field of solid-state photochemistry and for the applied
field of optoelectronic devices.2e5 The hypsochromic shift-type
MFC of C6TPPy has been explained in terms of a reversible
change of intermolecular amide hydrogen bonds by grinding and
heating. More recently, Park et al. have found an interesting stimuli
luminescence of cyano distrylbenzene derivative DBDCS: a hyp-
sochromic shift of fluorescent color is induced by thermo-stimulus
to the as-recrystallized DBDCS, being accompanied by a reversion
to the original fluorescent color by mechano-stimulus.5 They have
demonstrated that the origin for the stimuli luminescence
switching is the two-directional shear-sliding capability of molec-
ular sheets, which are formed via intermolecular multiple CeH/N
and CeH/O hydrogen bonds. These MFC organic dyes so far re-
ported are symmetrical molecules, and the number of organic
fluorescent dyes exhibiting the MFC is still limited and the mech-
anism is a matter, which requires intensive debates.
termolecular
pep interaction.
In order to obtain further useful information on the effects of the
substituents and chromophore skeleton on the MFC of un-
symmetrical De
synthesized carbazole-type De
a diphenylamino group as D, p-pyridine ring as A, a carbazole
skeleton as -conjugated system, and a substituent (R) (Scheme 1,
peA fluorescent dyes, we have newly designed and
peA fluorescent dyes 1 and 2 with
p
detailed synthesis procedures for the dyes are shown in
Supplementary data). Herein, on the basis of experimental results
and semi-empirical molecular orbital calculations (AM1 and INDO/
S), we discuss the mechanism of bathochromic shift-type MFC of
unsymmetrical carbazole-type De
peA fluorescent dyes 1 and 2.
N
N
N
R
In 2009, we have found that newly developed
1 : R = H
2 : R = nBu
unsymmetrical heteropolycyclic donoreacceptor
p-conjugated
* Corresponding author. Tel.: þ81 82 424 6534; fax: þ81 82 424 5494; e-mail
Scheme 1. Carbazole-type DepeA fluorescent dyes.
address: harima@mls.ias.hiroshima-u.ac.jp (Y. Harima).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.11.012

530
Y. Ooyama et al. / Tetrahedron 68 (2012) 529e533
2. Results and discussion
formation of intermolecular pep
interactions7 or continuous in-
termolecular hydrogen bonding8 in the crystalline state leading to
delocalization of excitons or eximers.
2.1. Photophysical properties of DepeA fluorescent dyes in
solution and in the solid-state
Table 2
Solid-state photophysical data for 1 and 2 before and after grinding
The absorption and fluorescence spectra of 1 and 2 in 1,4-
dioxane are shown in Fig. 1 and their spectral data are summa-
rized in Table 1. The absorption and fluorescence spectra of 1 and 2
resemble very well each other, showing that the effect of N-alkyl-
ation of the carbazole ring on the photophysical properties of the
dye 1 is negligible. The dyes 1 and 2 show two absorption maxima:
a
Dye
Excitation
Excitation
FF
s
1/nsb
s
2/nsb
(A2/%)c
ex
em
l /nm
max
l /nm
max
(A1/%)c
1 (Before)
1 (After)
2 (Before)
2 (After)
441
450
397
424
463
477
434
456
0.02
0.02
0.17
0.16
0.2 (41)
0.2 (79)
0.4 (81)
0.4 (66)
2.6 (59)
4.2 (21)
2.2 (19)
3.6 (34)
one band appears at around 305 nm ascribed to p/p* transition,
a
Fluorescence quantum yield.
Fluorescence lifetime.
Fractional contribution.
and another band occurs at around 375 nm assigned to the intra-
molecular charge transfer (ICT) excitation from D (diphenylamino
group) to A (p-pyridine ring). The corresponding fluorescence
maximum occurs at around 420 nm and the fluorescence quantum
yields (FF) are ca. 0.8. The time-resolved fluorescence spectroscopy
of 1 and 2 indicated that the decay profile fitted satisfactorily
a single exponential function with sf¼1.9 ns for both 1 and 2. Both 1
and 2 in 1,4-dioxane were nearly-colorless, and their fluorescent
colors were blue for both 1 and 2.
b
c
Fig. 1. Absorption and fluorescence spectra of 1 and 2 in 1,4-dioxane.
Fig. 2. (a) Solid-state excitation and (b) emission spectra of the crystals of 1 and (c)
Solid-state excitation and (d) emission spectra of the crystals of 2.
Table 1
Spectroscopic properties of 1 and 2 in 1,4-dioxane and calculated absorption spectral data
Compound
In 1,4-Dioxane
Calculationc
max/Mꢀ1 cmꢀ1
)
lflmax/nm
FF
s
f/nsb
lmax/nm
fd
CI componente
abs
a
l
/nm (
3
max
1
2
305 (15,700), 372 (30,200)
304 (18,000), 375 (33,000)
423
0.83
0.84
1.9
306
345
306
345
0.23
0.96
0.25
0.87
HOMO/LUMOþ2 (66%)
HOMO/LUMO (65%)
HOMO/LUMOþ2 (68%)
HOMO/LUMO (61%)
423
1.9
a
Fluorescence quantum yields (
Fluorescence lifetime.
Calculated at INDO/S method after geometrical optimizations using the MOPAC/AM1 method.
Oscillator strength.
F
) were determined by using a calibrated integrating sphere system (lex¼370 nm).
b
c
d
e
The transition is shown by an arrow from one orbital to another, followed by its percentage CI (configuration interaction) component.
On the other hand, colors of the dyes recrystallized from acetone
2.2. Semi-empirical MO calculations (AM1, INDO/S)
were light-yellow for 1 and nearly-colorless for 2, and the fluo-
rescent colors were blue for both 1 and 2. The fluorescence exci-
The photophysical properties of 1 and 2 were analyzed by using
semi-empirical molecular orbital (MO) calculations. The molecular
structures were optimized by using the MOPAC/AM1 method,9 and
then the INDO/S method10 was used for spectroscopic calculations.
The calculated absorption wavelengths and the transition charac-
ters of the absorption bands are collected in Table 1. The calculated
absorption wavelengths and the oscillator strength values of the
two dyes are comparable to the observed spectra in 1,4-dioxane;
the effect of N-alkylation of the carbazole ring on the photophysical
ex
em
tation and emission maxima (
l
and
l
max
) of the dyes in the
max
crystalline state were red-shifted by 69 and 40 nm for 1 and 22 nm
and 11 nm for 2, respectively, relative to those for the corre-
sponding dyes in 1,4-dioxane, being accompanied by the consid-
erable decrease in the FF value (‘before’ in Table 2 and Fig. 2). For
ex
em
De
peA fluorescent dyes, in general, the red-shifts of
l
and
l
,
max
max
and the lowering of FF value by changing from solution to the
crystalline state are quite common and explained in terms of the

Y. Ooyama et al. / Tetrahedron 68 (2012) 529e533
531
em
properties of 1 and 2 is negligible, in accord with the experimental
results. The calculations indicate that the longest excitation bands
are mainly assignable to the transition from HOMO to LUMO, where
HOMO is mostly localized on the diphenylaminoecarbazole moiety
and LUMO is mostly localized on the pyridinylecarbazole moiety.
The HOMO and LUMO of 1 and 2 are shown in Fig. 3a. The changes
in the calculated electron density accompanied by the first electron
excitation for 1 and 2 are shown in Fig. 3b, which reveal a strong
migration of an electron from the diphenylaminoecarbazole moi-
ety to the pyridine ring. The values of the dipole moment in the
ground state (mg) are ca. 3.6 D for both 1 and 2.
window and coincided with the emission wavelength
l
max
in the
normal fluorescence spectroscopy. The fluorescence decay profiles
indicate the existence of two distinct emitting states in both the
dyes 1 and 2 before and after grinding.
2.4. Measurements of XRD, DSC, and IR spectra for the solids
before and after grinding
The XRD patterns and DSC curves for 1 and 2 before and after
grinding of as-recrystallized dyes are shown in Figs. 4 and 5, re-
spectively. The XRD measurements with as-recrystallized dyes 1
and 2 exhibited diffraction peaks ascribable to well-defined mi-
crocrystalline structures. They almost disappeared after grinding,
showing that the crystal lattice was significantly disrupted. For both
the dyes 1 and 2, the diffraction peaks of the ground dyes after
being heated were quite similar to those before grinding, sugges-
tive of recovery of the microcrystalline structure. The DSC analysis
for dyes 1 and 2 indicated that the dyes 1 and 2 before grinding
showed only one sharp endothermic peak associated with melting.
On the other hand, the ground solids underwent an endothermic
glass transition (Tg) and then an exothermic recrystallization (Tc)
before melting (Tm). The DSC traces of the ground powders are
typical of amorphous solids.
Fig. 3. (a) HOMO and LUMO of 1 and 2. The red and blue lobes denote the positive and
negative phases of the coefficients of the molecular orbitals. The size of each lobe is
proportional to the MO coefficient. (b) Calculated electron density changes accompa-
nying the first electronic excitation of 1 and 2. The black and white lobes signify de-
crease and increase in electron density accompanying the electronic transition,
respectively. Their areas indicate the magnitude of the electron density change. (Light
blue, green, blue and red balls correspond to hydrogen, carbon, nitrogen, and oxygen
atoms, respectively.)
Fig. 4. XRD patterns of (a) 1 and (b) 2 before and after grinding, and after heating the
ground solids.
2.3. MFC characteristics of De
peA fluorescent dyes
The MFC characteristics of the carbazole-type De
p
eA fluores-
cent dyes were investigated according to the following procedure.
By grinding the as-recrystallized dyes 1 and 2 at a stress of
ex
em
50e100 N/cm2 in a mortar with a pestle, the
l
and
l
for 1 and
max
max
2 are red-shifted by 9 nm and 14 nm for 1 and 27 nm and 22 nm for
2, respectively. However, the FF values change little by grinding.
The photophysical data of the two dyes before and after grinding of
as-recrystallized dyes are summarized in Table 2 and the excitation
and fluorescence spectral changes are also shown in Fig. 2. When
the ground samples of 1 and 2 were heated at 150 ꢁC for 1 and
110 ꢁC for 2 (beyond the recrystallization temperature (Tc), de-
scribed later), respectively, or exposed for several minutes to or-
ganic solvents, such as acetone, the colors and fluorescent colors of
the dyes recovered to the original ones.
Fig. 5. DSC curves (scan rate: 10 ꢁC minꢀ1) of (a) 1 and (b) 2 before and after grinding.
The FT-IR study for the as-recrystallized dye 1 show that no free
NeH stretching of carbazole amino group is present, but the
broadening of the absorption band at 3417 cmꢀ1 indicates the for-
mation of the weaker hydrogen bondings between the proton on
carbazole nitrogen atom and pyridine nitrogen (Fig. 6), which is
completely consistent with the X-ray structural analysis of 1 (de-
scribed later). On the other hand, the NeH stretching of carbazole
amino group at 3403 cmꢀ1 observed in the IR spectrum of the
ground dye become sharper than that observed in the as-
recrystallized dye, indicative of the formation of the stronger hy-
drogen bonding.
The time-resolved fluorescence spectroscopy with the dyes 1
and 2 revealed that, irrespective of grinding, the fluorescence decay
profiles fitted biexponential curves with fluorescence lifetimes of
s1¼0.2e0.4 ns and s2¼2.2e4.2 ns (Table 2). More interestingly, the
trs
emission wavelengths (
l
) for 1 in the time-resolved measure-
max
ments were dependent on the time window: for the as-
recrystallized dye, 462 nm for 0e4 ns and 517 nm for 5e13 ns;
for the ground dye, 475 nm for 0e4 ns and 500 nm for 5e10 ns. For
trs
the dye 2, however, the
l
value was independent of the time
max

532
Y. Ooyama et al. / Tetrahedron 68 (2012) 529e533
a continuous intermolecular hydrogen bonding between the fluo-
rophores, although the formation of intermolecular in-
pep
teractions between the fluorophores is responsible for the red-shift
of the absorption and fluorescence maxima and a decrease in the
solid-state fluorescence for 2 from 1,4-dioxane to the crystalline
state.
2.6. Mechanism of MFC observed with DepeA fluorescent
dyes
In some pigments, the color tone of a pigment powder is known
to depend on the size of the particles due to the difference in the
degree of light scattering on the pigment surfaces. In the present
experiments with the carbazole-type DepeA fluorescent dyes,
however, grinding of their powders induces shifts in peak wave-
length of fluorescence spectra as well as absorption spectra. These
spectroscopic observations rule out the possibility that the change
in particle size is responsible for the color and fluorescent color
changes observed here. Moreover, the XRD and DSC measurements
Fig. 6. IR spectra of 1 before and after grinding.
2.5. X-ray crystal structure of DepeA fluorescent dye
A single-crystal X-ray structural analysis was successfully made
for 1. As shown in Fig. 7, dye molecules are linked by intermolecular
NH/N hydrogen bonds between the proton on carbazole nitrogen
atom and pyridine nitrogen to form one-dimensional molecular
chain: a proton on carbazole nitrogen atom in the dye molecule is
directing toward the pyridine nitrogen of the neighboring dye (N(1)
demonstrate that the carbazole-type DepeA fluorescent dyes in-
terconvert between microcrystalline and amorphous states by
grinding and heating. Thus, based on the above results, in the case
ex
of the De
p
eA fluorescent dyes 1 and 2, the red-shifts of
l
max
and
em
l
max
, and lowering of FF by changing from solution to the crystal-
H(1)/N(3)* angle¼161(1)ꢁ, N(1)/N(3)* distance¼2.924(2) A).
ꢀ
line state are explained in terms of a continuous intermolecular
hydrogen bonding for 1 and the intermolecular interactions
for 2 in the crystalline state. By grinding the solids of 1 and 2, the
ꢀ
There are no short
pe
p
contacts of less than 3.60 A between the
pep
neighboring fluorophores, which indicates the absence of the pep
ex
em
interactions between the fluorophores. The X-ray structural anal-
ysis reveals that the formation of a continuous intermolecular hy-
drogen bonding between the fluorophores is a principal factor of
a large red-shift of the absorption and fluorescence maxima and the
drastic solid-state fluorescence quenching for the crystal of 1 from
1,4-dioxane to the solid state.8 Unfortunately, we could not obtain
sufficient sizes of single crystals for 2 to make the X-ray structural
analysis possible, however, the relatively strong fluorescence in-
tensity for the crystal of 2 demonstrated that the N-butylation of
carbazole moiety can effectively remove the formation of
l
and
l
are slightly red-shifted. The XRD and DSC measure-
max
max
ments demonstrate that the dyes 1 and 2 interconvert between
microcrystalline and amorphous states by grinding and heating.
Moreover, it was found for the dye 1 that the intermolecular hy-
drogen bonding for the ground dye becomes stronger than that
observed in the as-recrystallized dye. On these bases, it may be
inferred that the bathochromic shift-type MFC observed with 1 and
2 by changing from the crystalline state to the amorphous state are
caused by the formations of the stronger hydrogen bonding for 1
and intermolecular
study has demonstrated that a large bathochromic shift-type MFC
observed with De
pep interactions for 2 (Fig. 8). Our previous
peA fluorescent dyes (mg¼ca. 5 D) are caused by
strong dipoleedipole interactions. Thus, it was suggested that
a relatively small MFC for 1 and 2 is ascribable to their weak dipole
characters (mg¼ca. 3.5 D). On the other hand, the FF values undergo
very little change from the crystalline state to the amorphous state.
This suggests that a non-radiative decay route for the excited states
is relatively discouraged by the hydrogen bonding and the
interactions in the amorphous states for this series of De
fluorescent dyes.
p
p
ep
eA
3. Conclusion
We have found a new class of unsymmetrical carbazole-type
De eA fluorescent dyes displaying a bathochromic shift-type
mechanofluorochromism (MFC). This study demonstrates that the
MFC of the De eA fluorescent dyes is attributed to a reversible
switching between crystalline and amorphous states with changes
of intermolecular hydrogen bonding and interaction before
p
p
pep
and after grinding. We believe that these mechanofluorochromic
dyes can be a promising class of organic fluorescent dyes for
rewritable photoimaging and electroluminescence devices.
4. Experimental
4.1. General
IR spectra were recorded on a PerkineElmer Spectrum One FT-
Fig. 7. (a) Crystal packing and (b) hydrogen bonding pattern of 1.
IR spectrometer by ATR method. The load measurement was

Y. Ooyama et al. / Tetrahedron 68 (2012) 529e533
533
4.3. X-ray crystallographic studies
Crystals of 1 were recrystallized from a mixture of CH2Cl2en-
hexane as a light-yellow prism, air stable. The one selected had
approximate dimensions of 0.80ꢂ0.30ꢂ0.30 mm. The data sets
were collected at 20ꢃ1 ꢁC on a Rigaku RAXIS-RAPID Imaging
Plate diffractometer using graphite-monochromated Mo
Ka
ꢀ
(
l
¼0.71075 A) radiation at 50 kV and 40 mA. In all cases, the data
were corrected for Lorentz and polarization effects. All calcula-
tions were performed using the teXsan11 crystallographic soft-
ware package of Molecular Structure Corporation. The crystal
structure was solved by direct methods using SIR 92.12 The
structures were expanded using Fourier techniques.13 All non-
hydrogen atoms were refined anisotropically. All hydrogen
atoms were fixed geometrically and not refined. Compound
1: C29N3H21, M¼411.51, monoclinic, space group P21/c (no.14),
¼92.211(3)ꢁ,
ꢀ
a¼7.4450(7), b¼28.080(3), c¼10.787(1) A,
b
V¼2253.3(4) A , Dc¼1.213 g cmꢀ3, Z¼4, 21,242 reflections mea-
3
ꢀ
sured, 5023 unique (Rint¼0.031). The final
R indices were
R1¼0.048, wR¼0.161 (I>2 (I)), GOF¼1.11. Crystallographic data
s
(excluding structure factors) has been deposited with Cambridge
Crystallographic Data Centre as supplementary publication
number CCDC 810771. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.au.uk/data_request/cif.
Acknowledgements
This work was supported by Grants-in-Aid for Young Scientist
(B) (22750179) from the Japan Society for the Promotion of Science
(JSPS) and by a research grant from the Mikiya Science and Tech-
nology Foundation.
Fig. 8. Proposed mechanisms of MFC observed with (a) Carbazole-type De
rescent dyes 1 and (b) 2.
peA fluo-
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tet.2011.11.012.
performed with IMADA push-pull scale and digital force gauge (ZP-
200/V). Absorption spectra were observed with a Shimadzu UV-
3150 spectrophotometer and fluorescence spectra were measured
with a Hitachi F-4500 spectrophotometer. The fluorescence quan-
tum yields in solution and in the solid state were determined by
a Hamamatsu C9920-01 equipped with CCD by using a calibrated
integrating sphere system (lex¼370 nm). Fluorescence lifetimes
were determined with a HAMAMATSU Photonics C4334/C8898
time-resolved spectrophotometer by excitation at 375 nm (laser
diode). Powder X-ray diffraction measurements were performed on
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The semi-empirical calculations were carried out with the
WinMOPAC Ver. 3.9 package (Fujitsu, Chiba, Japan). Geometry cal-
culations in the ground state were made using the AM1 method. All
geometries were completely optimized (keyword PRECISE) by the
eigenvector following routine (keyword EF). Experimental ab-
sorption spectra of the compounds were compared with their ab-
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neglect of differential overlap/spectroscopic). Dipole moments of
the compounds were also evaluated from INDO/S calculations. All
INDO/S calculations were performed using single excitation full
SCF/CI (self-consistent field/configuration interaction), which in-
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were considered [keyword CI (15 15)].