Crystal structures and triboluminescence based on trifluoromethyl and pentafluorosulfanyl substituted asymmetric N-phenyl imide compounds

Article abstract of DOI:10.1021/cm202650u

A series of asymmetric N-phenyl imides with trifluoromethyl and pentafluorosulfanyl substituents at the terminal phenyl position have been prepared. They are colorless both in solution and the solid state, and they show blue emission in the solid state. X-ray crystal structure analysis of trifluoromethyl and pentafluorosulfanyl substituted derivatives revealed highly ordered noncentrosymmetric molecular arrangements with large net-dipole moments related to the piezoelectric behavior. XRD measurements of the powders afforded intense sharp reflection peaks, indicating formation of the lamellar ordering. By grinding with a spatula, their solids showed vivid blue triboluminescence.

Full text of DOI:10.1021/cm202650u

Article
pubs.acs.org/cm
Crystal Structures and Triboluminescence Based
on Trifluoromethyl and Pentafluorosulfanyl Substituted
Asymmetric N-Phenyl Imide Compounds
Hikaru Nakayama,† Jun-ichi Nishida,*,† Noriyuki Takada,‡ Hiroyasu Sato,§ and Yoshiro Yamashita*,†
†Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
Yokohama, Kanagawa 226-8502, Japan
‡Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565,
Japan
§Rigaku Corporation, Akishima, Tokyo 196-8666, Japan
S
* Supporting Information
ABSTRACT: A series of asymmetric N-phenyl imides with trifluoromethyl and pentafluoro-
sulfanyl substituents at the terminal phenyl position have been prepared. They are colorless
both in solution and the solid state, and they show blue emission in the solid state. X-ray
crystal structure analysis of trifluoromethyl and pentafluorosulfanyl substituted derivatives
revealed highly ordered noncentrosymmetric molecular arrangements with large net-dipole
moments related to the piezoelectric behavior. XRD measurements of the powders afforded
intense sharp reflection peaks, indicating formation of the lamellar ordering. By grinding with
a spatula, their solids showed vivid blue triboluminescence.
KEYWORDS: crystal structure analysis, triboluminescence, imide compounds, piezoelectric properties, trifluoromethyl groups
Chart 1. Chemical Structures of Asymmetric N-Phenyl Imide
Compounds
INTRODUCTION
■
Light emission caused by mechanical stress (force) in solid
materials is known as triboluminescence (TL) (sometimes
called mechanoluminescence). TL has a long history1−3 and
has recently been applied to real-time sensors of mechanical
stress and structural damage4 and a resource of X-ray.5 As
typical TL materials, inorganic solids6 containing lanthanoids
such as europium and their complexes7−9 are known. Although
organic compounds such as a lump of sugar,10 coumarin,11 N-
isopropylcarbazole,12 9-anthracenecarboxylic acid and the
esters,13 and hexaphenylcarbodiphosphorane14 also show TL,
the number is limited, and the efficiency is generally low. The
TL mechanism still remains unclear, and further studies have
been desired. To make progress in this field, a discovery of a
new series of compounds affording vivid TL and the study on
the relationship between structure and properties are especially
important. According to previous reports, TL is closely related
to piezoelectric properties of solids.1,2,7 Dipolar structures and
noncentrosymmetric molecular arrangements in crystals are
necessary to achieve the piezoelectric properties. However, it is
generally difficult to obtain noncentrosymmetric organic
crystals systematically. Bearing this in mind, we have paid atten-
tion to asymmetric N-phenyl type imides (Chart 1)15,16 for the
following reasons. First, imide derivatives are excellent accep-
tors and very popular components in organic electronics.17 In-
vestigation of physical properties and crystal structures based
on simple N-phenyl imides are useful for the applications.
Second, the imide derivatives have a dipole moment, and the
dipolar structures are readily constructed by introducing sub-
stituents. When suitable N-phenyl imides are arranged with
noncentrosymmetric space groups in crystals, they are expected
to show piezoelectric properties leading to TL. Third, imide
derivatives and the polymers have generally high chemical and
thermal stabilities, mechanical toughness, and flame resis-
tance.18 Moreover, various derivatives are easily obtained by
simple synthetic methods. We have now prepared asymmetric
N-phenyl imides 1 and 2 with trifluoromethyl19 and penta-
fluorosulfanyl20 groups and succeeded in achieving TL
behavior. The physical properties, crystal structures, and vivid
TL properties are presented here.
Received: September 6, 2011
Revised: January 18, 2012
Published: January 19, 2012
© 2012 American Chemical Society
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(20 mL) were added to a flask, and the mixture was refluxed for 24 h.
The reaction mixture was cooled to room temperature and precip-
itated with water. The white solid was filtered and dried. The product
was purified by sublimation to give 1c (1.14 g, 78%) as a white solid.
Mp 204−207 °C. MS/EI (70 eV): m/z 238 (M+, 100%). IR(KBr)
νmax/cm−1 3040 (CHAr), 2915 (CH3) 1747, 1714 (CO), 1515, 722
7.96 (m, 4H), 7.78 (m, 4H), 2.41 (s, 3H). 13C NMR (75 MHz,
CDCl3): δ 167.41, 138.15, 134.29, 131.79, 129.76, 128.93, 126.44,
123.65, 21.18. Anal. Calcd for C15H11NO2: C, 75.94; H, 4.67; N, 5.90.
Found: C, 75.76; H, 4.63; N, 5.82.
EXPERIMENTAL SECTION
■
Materials and Instrumentations. Tetrakis(triphenylphosphine)-
palladium(0), n-butyllithium in n-hexane, trimethyl borate, THF,
DMF, phthalic anhydride, and 4-bromoaniline were purchased from
Kanto Chemicals and used without further purification. 2,3-Naph-
thalene dicarboxylic anhydride, 4-aminobenzotrifluoride, and 4-
bromobenzotrifluoride were purchased from Tokyo Kasei Co. and
used without further purification.
Imide derivatives 1 and 2 were synthesized by a simple conden-
sation reaction of carboxylic anhydrides with the corresponding
aromatic amines in high yields (77−85%). Imides 1a,21 1c,22 1e,23 and
1f24 are known compounds. For the preparation of imides 1d and 2b,
4-(4-trifluoromethylphenyl)aniline was prepared by the Suzuki
coupling reaction of 4-(trifluoromethyl)phenylboronic acid with 4-
bromoaniline.25 All these imide derivatives were purified by sub-
limation. Experimental data were added for comparison.
1
(CCAr), 1385, 1119 (C−N−C). H NMR (300 MHz, CDCl3): δ
N-(4′-Trifluoromethylbiphenyl)phthalimide (1d). Phthalic an-
hydride (1.25 g, 8.43 mmol), 4′-(trifluoromethyl)-4-biphenylamine
(3)25 (2.00 g, 8.43 mmol), and DMF (30 mL) were added to a flask,
and the mixture was refluxed for 24 h. The reaction mixture was cool-
ed to room temperature and precipitated with water. The white solid
was filtered and dried. The product was purified by sublimation to give
2 (2.62 g, 85%) as a white solid. Mp 282−284 °C. MS/EI (70 eV): m/
z 367 (M+, 100%). IR(KBr) νmax/cm−1 3041 (CHAr), 1790, 1731
(CO), 1614, 1532, 713 (CCAr), 1403, 1114 (C−N−C), 1337,
Melting points were obtained on a SHIMADZU DSC-60. DI mass
were collected on a JEOL JMS-700 mass spectrometer. H NMR and
1
13C NMR spectra were recorded on a JEOL JNM-ECP300 NMR
spectrometer, and chemical shifts were referenced to tetramethyl-
silane (TMS). IR spectra were recorded as KBr discs on a PERKIN
ELMER FT-IR Spectrometer PARAGON 1000 spectrophotometer.
Elemental analyses were carried out with a LECO/CHNS-932
analyzer at the Tokyo Institute of Technology, Chemical Resources
Laboratory. UV−vis spectra were recorded on a JASCO V-650
spectrophotometer. PL spectra were collected on a JASCO FP-6600
spectrometer and JASCO FP-8500 spectrometer. Photoluminescence
quantum yields in the solid state were determined by using an
integrating sphere (in JASCO Corporation). X-ray diffraction (XRD)
measurements were carried out on a Rigaku RINT with a Cu Kα
source (λ = 1.541 Å). XRD patterns were obtained using Bragg−
Brentano geometry with Cu Kα 5 radiation as an X-ray source with an
acceleration voltage of 40 kV and a beam current of 30 mA. TL spectra
were measured with a polychromator-ICCD multichannel spectropho-
tometer system (Princeton Instruments Inc.) through a plastic optical
fiber after cooling in liquid N2. The transient properties of TL were
monitored by a photomultiplier connected to a storage oscilloscope
(HP 54542A), where emitted light was passed through a slit (1 mm).
The time resolution of the detection system was 20 ks.
1
786 (CF3). H NMR (300 MHz, CDCl3): δ 7.99 (m, 2H), 7.84 (m,
2H), 7.74 (d, 6H, J = 8.4 Hz), 7.58 (d, 2H, J = 8.4 Hz). Anal. Calcd
for C21H12F3NO2: C, 68.67; H, 3.29; N, 3.81. Found: C, 68.41; H,
3.26; N, 3.78.
N-Phenylphthalimide (1e23). Phthalic anhydride (4.00 g, 27.0
mmol), aniline (2.51 g, 27.0 mmol), and DMF (40 mL) were added to
a flask, and the mixture was refluxed for 24 h. The reaction mixture
was cooled to room temperature and precipitated with water. The
white solid was filtered and dried. The product was purified by sub-
limation to give 1e (4.21 g, 70%) as a white solid. Mp 209−212 °C.
IR(KBr) νmax/cm−1 3078 (CHAr), 1779, 1714 (CO), 1594, 1505,
718 (CCAr), 1385, 1117 (C−N−C). 1H NMR (300 MHz, CDCl3):
δ 7.96 (m, 2H), 7.79 (m, 2H), 7.51 (d, 2H, J = 7.2 Hz), 7.46 (s, 1H),
7.42 (d, 2H, J = 6.6 Hz). 13C NMR (75 MHz, CDCl3): δ 167.26,
134.38, 131.74, 131.65, 129.10, 128.09, 126.55, 123.73. Anal. Calcd
for C14H9NO2: C, 75.33; H, 4.06; N, 6.27. Found: C, 75.33; H, 3.81;
N, 6.20.
N-(4-Fluorophenyl)phthalimide (1f24). Phthalic anhydride
(4.00 g, 27.0 mmol), 4-fluoroaniline (3.00 g, 27.0 mmol), and DMF
(40 mL) were added to a flask, and the mixture was refluxed for 24 h.
The reaction mixture was cooled to room temperature and
precipitated with water. The white solid was filtered and dried. The
product was purified by sublimation to give 1f (4.94 g, 76%) as a white
solid. Mp 183−187 °C. IR(KBr) νmax/cm−1 3062 (CHAr), 1715 (C
O), 1514, 716 (CCAr), 1394, 1110 (C−N−C). 1H NMR (300
MHz, CDCl3): δ 7.95 (m, 2H), 7.80 (m, 2H), 7.43 (m, 2H), 7.20
(t, 2H, J = 8.5 Hz). Anal. Calcd for C14H8FNO2: C, 69.71; H, 3.34; N,
5.81. Found: C, 70.06; H, 3.39; N, 5.76.
N-(4-Trifluoromethylphenyl)phthalimide (1a21). Phthalic an-
hydride (920 mg, 6.21 mmol), 4-aminobenzotrifluoride (1.00 g, 6.21
mmol), and DMF (20 mL) were added to a flask, and the mixture was
refluxed for 24 h. The reaction mixture was cooled to room
temperature and precipitated with water. The white solid was filtered
and dried. The product was purified by sublimation to give 1a (0.71 g,
77%) as a white solid. Mp 257−259 °C. MS/EI (70 eV): m/z 291
(M+, 100%). IR(KBr) νmax/cm−1 3067 (CHAr), 1787, 1722 (CO),
1615, 1526, 700 (CCAr), 1402, 1110 (C−N−C), 1338, 715 (CF3).
1H NMR (300 MHz, CDCl3): δ 8.00 (m, 2H), 7.83 (m, 2H), 7.78 (d,
2H, J = 9.0 Hz), 7.65 (d, 2H, J = 8.1 Hz). 13C NMR (75 MHz,
CDCl3): δ 166.77, 134.75, 131.50, 126.44, 126.29, 126.24, 126.19,
126.14, 124.01. Anal. Calcd for C15H8F3NO2: C, 61.86; H, 2.77; N,
4.81. Found: C, 62.07; H, 2.72; N, 4.78.
N-(4-Trifluoromethylphenyl)-2,3-naphthimide (2a). 2,3-Naph-
thalene dicarboxylic anhydride (6.14 g, 31 mmol), 4-aminobenzotri-
fluoride (5.00 g, 31.0 mmol), and DMF (70 mL) were added to a flask,
and the mixture was refluxed for 24 h. The reaction mixture was
cooled to room temperature and precipitated with water. The white
solid was filtered and dried. The product was purified by sublimation
to give 3 (8.60 g, 81%) as a cream-colored solid. Mp 349−351 °C.
MS/EI (70 eV): m/z 341 (M+, 100%). IR(KBr) νmax/cm−1 3068
(CHAr), 1788, 1714 (CO), 1614, 1525, 725 (CCAr), 1394, 1111
N-(4-Pentafluorosulfanylphenyl)phthalimide (1b). Phthalic
anhydride (0.34 g, 2.28 mmol), 4-aminophenylsulfurpentafluoride
(0.50 g, 2.28 mmol), and DMF (15 mL) were added to a flask, and the
mixture was refluxed for 24 h. The reaction mixture was cooled to
room temperature and precipitated with water. The white solid was
filtered and dried. The product was purified by sublimation to give 1b
(0.67 g, 84%) as a white solid. Mp 219−222 °C. MS/EI (70 eV): m/z
349 (M+, 100%). IR(KBr) νmax/cm−1 3070 (CHAr), 1760, 1731 (C
O), 1514, 715 (CCAr), 1402, 1101 (C−N−C), 821 (SF5). 1H NMR
(300 MHz, CDCl3): δ 7.99 (m, 2H), 7.91 (d, 2H, J = 9.0 Hz), 7.83 (m,
2H), 7.84 (m, 2H), 7.65 (d, 2H, J = 8.4 Hz). 13C NMR (75 MHz,
CDCl3): δ 166.60, 134.85, 131.41, 130.61, 126.96, 126.90, 126.06,
126.09. Anal. Calcd for C14H8F5NO2S: C, 48.14; H, 2.31; N, 4.01.
Found: C, 48.21; H, 2.31; N, 3.97.
1
(C−N−C), 1337, 763 (CF3). H NMR (300 MHz, CDCl3): δ 8.50
(s, 2H), 8.13 (m, 2H), 7.81 (d, 2H, J = 9.0 Hz), 7.77 (m, 2H), 7.71
(d, 2H, J = 9.0 Hz). Anal. Calcd for C19H10F3NO2: C, 66.87; H, 2.95;
N, 4.10. Found: C, 66.79; H, 3.00; N, 4.20.
N-(4′-Trifluoromethylbiphenyl)-2,3-naphthimide (2b). 2,3-
Naphthalene dicarboxylic anhydride (1.67 g, 8.43 mmol), 4′-(trifluoro-
methyl)-4-biphenylamine (3)25 (2.00 g, 8.43 mmol), and DMF
(40 mL) were added to a flask and the mixture was refluxed for 24 h.
The reaction mixture was cooled to room temperature and
precipitated with water. The white solid was filtered and dried. The
product was purified by sublimation to give 3 (2.93 g, 83%) as a
cream-colored solid. Mp 372−375 °C. MS/EI (70 eV): m/z 417 (M+,
100%). IR(KBr) νmax/cm−1 3041 (CHAr), 1787, 1714 (CO), 1614,
N-(4-Methylphenyl)phthalimide (1c22). Phthalic anhydride
(0.92 g, 6.21 mmol), p-toluidine (0.67 g, 6.21 mmol), and DMF
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1
1505 (CCAr), 1398, 1133 (C−N−C), 1337, 761 (CF3). H NMR
(300 MHz, CDCl3): δ 8.50 (s, 2H), 8.13 (m, 2H), 7.74 (m, 8H), 7.64
(m, 2H). Anal. Calcd for C25H14F3NO2: C, 71.94; H, 3.38; N, 3.36.
Found: C, 72.03; H, 3.26; N, 3.39.
to afford a fluorine layer. The crystal belongs to a noncentro-
symmetric space group Cc, resulting in a large net-dipole mo-
ment. On the other hand, single crystals of 1b with a penta-
fluorosulfanyl group were obtained by slow evapolation of the
chloroform/DMF solution. The dihedral angle between the
imide and phenyl rings is 60.1°, which is a little larger than that
in 1a due to the bulky substituent. The molecular arrangement
is almost the same with that of 1a with close contacts of 2.99
and 3.02 Å between the carbonyl groups. The crystal belongs to
a noncentrosymmetric space group Pna21. The crystal structure
of imide 1c with a methyl group is already reported with a
noncentrosymmetric space group Pna21.22 However, compared
with 1a and 1b, the imide 1c takes a half-slipped herringbone
packing along the molecular long axis (Figure S3, Supporting
Information). In contrast, nonsubstituted phenyl and 4-
fluorophenyl derivatives 1e and 1f have a center of symmetry
in crystals.23,24 This result suggests that the substituent on the
phenyl ring determines the crystal structure, and the bulkiness
seems important for formation of noncentrosymmetric crystals.
Optical and Electrochemical Properties. The asymmet-
ric N-phenyl imides 1 and 2 are colorless in solution and the
solid state. The absorption spectra in dichloromethane are
described in Figure 2 and Figure S6, Supporting Information.
X-ray Crystal Structure Analysis. X-ray measurements of single
crystals of 1a and 1b were made on a Rigaku Saturn724 diffrac-
tometer using multilayer mirror monochromated Mo Kα radiation
(λ = 0.71075 Å) at −180 °C. The structures were solved by the direct
method (SIR2008) and refined by the full-matrix least-squares
method on F2. The non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were refined using the riding model. Absorption
correction was applied using an empirical procedure. All calculations
were performed using the CrystalStructure crystallographic software
package except for refinement, which was performed using SHELXL-
97.26 Crystal data for 1a: C15H8F3NO2, M = 291.23, crystal dimen-
sions 0.10 × 0.08 × 0.08 mm, monoclinic, space group Cc, a =
27.60(2), b = 5.633(4), c = 7.961(7) Å, β = 95.498(12), V = 1232(2)
Å3, Z = 4, Dc = 1.570 g cm−3, 7566 reflections collected, 1382
independent (Rint = 0.0397), GOF = 1.089, R1 = 0.0615 (I > 2.00σ(I)),
wR2 = 0.1543 for all reflections. Crystal data for 1b: C14H8F5NO2S,
M = 349.27, crystal dimensions 0.10 × 0.10 × 0.10 mm, orthorhombic,
space group Pna21, a = 8.2084(8), b = 5.7718(6), c = 28.328(3) Å, V =
1342.1(3) Å3, Z = 4, Dc = 1.728 g cm−3, 8675 reflections collected,
2456 independent (Rint = 0.0880), GOF = 1.127, R1 = 0.0786 (I >
2.00σ(I)), wR2 = 0.2467 for all reflections. The CCDC reference
numbers are 815102 and 815103.
RESULTS AND DISCUSSION
■
X-ray Crystal Structure Analysis. Single crystals of 1a
were obtained by slow evapolation of the chloroform solution.
X-ray analysis revealed the twisted geometry with a dihedral
angle of 57.6° between the imide and phenyl rings (Figure S1,
Supporting Information). The dihedral angle is similar to those
of other N-phenyl substituted imide derivatives.22,23 Interest-
ingly, these molecules form a quite regular layer structure in the
crystal as depicted in Figure 1, in which they are arranged in the
Figure 2. Absorption spectra of imides 1a, 1b, 1d, 2a, and 2b in CH2Cl2.
Reflection spectra in the solid state are also collected using in-
tegrated sphere, and broad and red-shifted peaks are shown
in Figure 3 (after K-M transformation). The imides 1 and 2
Figure 3. Reflection spectra of imides 1a, 1b, 1d, 2a, and 2b in the
solid state (after K-M transformation).
exhibit blue fluorescence in the solid state, and the emission
maxima are summarized in Table 1. Although intensity of the
solid emission is low, the photoluminescence quantum yields of
compounds 1a and 1b could be determined and are described
in Table 1. Values of other derivatives are less.
Figure 1. Crystal structures of (a) 1a viewed along the b-axis and (b)
1b viewed along the b-axis.
same direction. The molecules are packed in a herringbone
manner with close contacts between the carbons of carbonyl
units and the oxygens of neighboring carbonyl units (2.87 and
2.93 Å). The trifluoromethyl groups are also arranged regularly
The cyclic voltammograms of 1 and 2 were measured to in-
vestigate their electron-accepting properties, and the potentials
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and estimated as an order of ten nanoseconds in Figure 4c. The
TL spectrum exhibited almost the same emission maximum as
the PL one as shown in Figure 5a, suggesting that the TL and
Table 1. Optical and Electrochemical Data
a
b
c
compd
λabs (nm) (log ε)
λem (nm) (Φ)
Ered (V)
1a
1b
1c
1d
1f
239 (4.48), 295 (3.22)
241 (4.51), 295 (3.23)
230 (4.40), 293 (3.24)
227 (4.32), 265 (4.50)
294 sh.
448 (4.0%)
444 (3.3%)
505
−1.26
−1.36
d
−1.36
−1.30
−1.38
−1.35
−1.40
485
495
1e
2a
2b
295 sh.
488
265 (4.84), 361 (3.63)
456
267, 361sh.
457
−1.43
c
a
b
In CH2Cl2. Emission maxima in solids, excited at 310 nm. 0.1 M
d
n-Bu4NPF6 in DMF, Pt electrode, V vs SCE. Half-wave potentials. An
irreversible wave, Epc. In CH2Cl2, the reduction peak was observed as a
reversible wave at Ered = −1.43 V.
are listed in Table 1. Almost all derivatives showed a clear re-
versible reduction wave in DMF (see Figure S12−S16,
Supporting Information). Only in the case of imide 1b with a
pentafluorosulfanyl group was the return peak not observed in
DMF. The anion radical of imide may react with the penta-
fluorosulfanyl group. The simple imide 1a with an electron-
withdrawing trifluoromethyl group showed the highest
reduction potential in these measurements.
Triboluminescence Measurements. By grinding the
powder of 1a with a microspatula at room temperature, blue
light emission was observed as shown in Figure 4a. The TL
Figure 5. PL and TL spectra of (a) 1a, (b) 1b, (c) 1d, (d) 2a, and (e) 2b.
PL arise from the similar excited state. The decay of the emis-
sion also supports that the TL is fluorescence. On the other
hand, sharp peaks of approximately 369 nm in Figure 5a can be
ascribed to the emission of the excited nitrogen gas, which has
often been observed in other TL materials.1−3
Besides 1a, the other imides 1 and 2, except for 1e and 1f,
exhibited TL (Table 2). The TL spectra of 1b, 1d, 2a, and 2b
Table 2. Triboluminescence Data and Calculated Dipole
Moments
a
c
c
c
compd
λem (nm)
HOMO
LUMO
dipole moment (debye)
1a
454
6.77
7.04
6.98
7.23
6.17
6.22
6.38
6.35
6.66
6.18
2.51
2.38
2.65
2.41
2.24
2.42
2.28
2.37
2.28
2.19
5.71
5.70
7.53
6.48
2.06
5.82
2.53
4.01
6.84
6.88
d
1a
1b
446
d
1b
Figure 4. TL of 1a (a) at room temperature in the dark and (b) after
cooling in liquid N2 under daylight. (c) Decay of light emission of 1a
at room temperature.
1c
1d
1e
1f
b
490
could be observed by eyes in dark. The intensity of the emis-
sion was remarkably increased at low temperature. Thus, after
the solid in a tube was cooled in a liquid nitrogen bath, the
sample was pulled out from the bath and immediately grinded
outside. The TL became strong enough to be clearly observed
in daylight as depicted in Figure 4b, indicating that a radi-
ationless deactivation process was effectively suppressed at low
temperature. The blue-light was collected by a polychromator-
ICCD multichannel spectrophotometer system through a plas-
tic optical fiber outside a glass sample tube to give the spectra.
The luminescence lifetime at room temperature could be moni-
tored by a photomultiplier connected to a storage oscilloscope
2a
2b
436
460
a
b
TL maxima after cooling in liquid N2. Too weak to decide the
c
maximum. Performed using Gaussian 03 program at B3LYP/6-
31G(d) levels of theory. Performed using CIF files.
d
are depicted in Figure 5. The biphenyl derivatives 1d and 2b
show the emission maxima at longer wavelengths than those of
phenyl derivatives 1a and 2a. It should be noted that imides 1e
and 1f, which did not exhibit detectable TL, have a center of sym-
metry in crystals.23,24 This fact suggests that noncentrosymmetric
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structures are necessary for the TL behavior based on these
simple N-phenyl imides.27 Although quantitative comparisons
of TL have not been accomplished, the intensity was obviously
dependent on the molecular and crystal structures. Thus, the
TL intensity of 1c was clearly weaker than those of other imide
derivatives as described in Table 2. To investigate their
electronic states, the molecular orbital calculations were carried
out using the Gaussian 03 program at B3LYP/6-31G(d) levels
of theory. Figure 6 depicts the calculated HOMOs and
patterns of imides showed that TL little changed through grind-
ing, although the relative peak intensity and shape changed
slightly as seen in Figures 7a−d and S18−S23, Supporting
Information. In addition, the patterns were also kept through
sublimation. This result suggests that a phase transition did not
occur through the grinding and sublimation. Sharp reflection
peaks are observed in all compounds, and those in 1a, 1b, 1d,
2a, and 2b indicate the formation of the highly ordered lamella
structures. The d-spacings obtained from the first reflection
peaks were 13.97 Å for 1a, 14.02 Å for 1b, 17.52 Å for 1d,
16.29 Å for 2a, 19.45 Å for 2b. Since the molecular lengths of
the imides obtained from the single-crystal X-ray analysis and
the molecular orbital calculations are 11.60 Å for 1a, 12.99 Å
for 1b, 16.17 Å for 1d, 14.28 for 2a, and 18.62 Å for 2b, these
molecules are considered to form the lamella structures cor-
responding to each molecular length. On the other hand, the
d-spacing of 1c with a half-slipped herringbone type packing
was different from the molecular length (d-spacing: 8.53 Å,
molecular length: 11.45 Å). The weak TL property of 1c may
be also ascribed to the crystal structure. Trifluoromethyl and
pentafluorosulfanyl substituents seem to play an important role
in making the layer structures. Introduction of extended
π-conjugation in imides 1d, 2a, and 2b keeps the layer
structures and the strong TL, although their emission spectra
are red-shifted and broadened.
Figure 6. (a, c) HOMO and (b, d) LUMO of imides 1a and 2a
calculated using Gaussian 03 at B3LYP/6-31G(d) levels of theory.
LUMOs, showing that the LUMOs are located on the imide
parts. The dipole moments were also calculated and are sum-
marized in Table 2. Introduction of strong electron-withdrawing
substituents such as trifluoromethyl and pentafluorosulfanyl
groups in 1a and 1b makes dipole moments larger than that of
the methyl derivative 1c. The fact that the TL intensity of 1c was
not strong suggests that the dipole moment is related to TL.
X-ray Diffraction Patterns. To investigate the molecular
arrangement in the solid, X-ray diffraction (XRD) of powders
was measured in reflection mode. The XRD diffraction patterns
based on the powders are depicted in Figure 7. The XRD
CONCLUSIONS
■
From these experimental data, we have deduced the following
mechanism for the TL observed here. When the piezoelectric
crystal breaks, the crack surface is electrified. Electronic
discharge at the crack surface makes the excited states of
compounds and the surrounding N2 gas resulting in the
emission. The dipolar structures and noncentrosymmetric
molecular arrangements in crystals are necessary to achieve
the strong piezoelectric properties (space groups of Cc and
Pna21 belong to the piezo- and pyro-electroric groups). These
agree with the TL mechanism reported by Zink et al.1 On the
other hand, we have found the trifluoromethyl and
pentafluorosulfanyl substituted asymmetric N-phenyl imides
form noncentrosymmetric crystals with high possibility and
demonstrated TL using imides for the first time. Although the
photoluminescence quantum efficiencies of imides shown in
Table 1 are not so high, the TL behavior has high durability and
the TL intensity is considerably maintained, even after they are
well-ground. This is attributed to the chemical and electro-
chemical stability of the imide derivatives. In addition, the twist
conformation and herringbone-type packing are important to
achieve the solid emission and the low conductivity leading to
high dielectric properties.
In summary, imide derivatives with asymmetric structures
were found to show vivid TL. Investigation on the relationship
between the TL behavior and structure revealed that the non-
centrosymmetric molecular arrangement is an important factor
for TL, and the desired crystal structures were successfully
obtained by using simple N-phenyl imides. The trifluoromethyl
and pentafluorosulfanyl substituents can assist the molecules in
achieving high dipole moments and the layer structures.
Further studies on the TL mechanism and development of
high-performance TL materials are currently under way.
Figure 7. XRD patterns of powders of (a) pre- and (b) postgrinding
1a, (c) pre- and (d) postgrinding 1b, and (e−h) postgrinding 1c, 1d,
2a, and 2b.
675
dx.doi.org/10.1021/cm202650u | Chem. Mater. 2012, 24, 671−676

Chemistry of Materials
Article
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ACKNOWLEDGMENTS
■
This work is supported by a Grant-in-Aid for Scientific Research
(No. 19350092 and 22550162) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, and by the
Global COE program “Education and Research Center for
Emergence of New Molecular Chemistry”. We thank Center for
Advanced Materials Analysis (Suzukakedai), Technical Department,
Tokyo Institute of Technology, for elemental analysis and MS
measurements. We also thank JASCO Corporation for
measurements of photoluminescence quantum yields in solid
states.
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