Fluorescent and nonfluorescent crystals of N-(3,5-dihalogenosalicylidene)-2-methoxyaniline

Article abstract of DOI:10.1016/j.molstruc.2015.10.053

Crystals of N-(3,5-diXsalicylidene)-2-methoxyaniline (X = fluoro, chloro, bromo, and iodo) were prepared, and their structural and luminescence properties in the solid state were investigated. Only fluorescent orange crystals were obtained for N-(3,5-difl

Full text of DOI:10.1016/j.molstruc.2015.10.053

Accepted Manuscript
Fluorescent and nonfluorescent crystals of N-(3,5-dihalogenosalicylidene)-2-
methoxyaniline
Masanobu Tsuchimoto, Narihiro Yoshida, Atsuki Sugimoto, Naozumi Teramoto,
Kiyohiko Nakajima
PII:
S0022-2860(15)30355-0
10.1016/j.molstruc.2015.10.053
MOLSTR 21898
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 15 August 2015
Revised Date: 5 October 2015
Accepted Date: 19 October 2015
Please cite this article as: M. Tsuchimoto, N. Yoshida, A. Sugimoto, N. Teramoto, K. Nakajima,
Fluorescent and nonfluorescent crystals of N-(3,5-dihalogenosalicylidene)-2-methoxyaniline, Journal of
Molecular Structure (2015), doi: 10.1016/j.molstruc.2015.10.053.
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ACCEPTED MANUSCRIPT
Fluorescent and nonfluorescent crystals of
N-(3,5-dihalogenosalicylidene)-2-methoxyaniline
Masanobu Tsuchimoto,*a Narihiro Yoshida,b Atsuki Sugimoto,b Naozumi Teramoto,b Kiyohiko
Nakajimac
aDepartment of Chemistry, Chiba Institute of Technology, Shibazono 2-1-1, Narashino, Chiba
275-0023, Japan
bDepartment of Life and Environmental Sciences, Chiba Institute of Technology, Tsudanuma 2-17-1,
Narashino, Chiba 275-0016, Japan
cDepartment of Chemistry, Aichi University of Education, Kariya 448-8542, Japan
* Corresponding author. Tel.: +81-47-454-9638; Fax: +81-47-454-9689.
E-mail: tsuchimoto.masanobu@p.chibakoudai.jp (M. Tsuchimoto)
ABSTRACT
Crystals of N-(3,5-diXsalicylidene)-2-methoxyaniline (X = fluoro, chloro, bromo, and iodo) were
prepared, and their structural and luminescence properties in the solid state were investigated. Only
fluorescent orange crystals were obtained for N-(3,5-difluorosalicylidene)-2-methoxyaniline (1).
Both fluorescent orange crystals (2a) and nonfluorescent red-orange crystals (2b) were obtained for
N-(3,5-dichlorosalicylidene)-2-methoxyaniline. Slightly fluorescent orange crystals were obtained
for
N-(3,5-dibromosalicylidene)-2-methoxyaniline
(3)
and
N-(3,5-diiodosalicylidene)-2-methoxyaniline (4). Fluorescence spectra of 1, 2a, 3, and 4 in the solid
state show a fluorescence band at around 600 nm by excitation at 480 nm. The fluorescence
intensities were in the order 1 > 2a > 3 > 4. X-ray crystal-structure analyses of 1, 2a, and 2b were
obtained. The N-salicylideneaniline molecules in all of the crystals have planar structures. The
molecules in both 1 and 2b take the enol form, while the molecules in 2a take the cis-keto form.
The crystals 2a include water of crystallization, which is hydrogen bonded to the adjacent
N-salicylideneaniline molecules. Molecules in fluorescent 2a are arranged in stacks along the a-axis
to form a columnar structure in the crystals. In contrast, molecules in nonfluorescent 2b are strongly
π–π stacked to form dimers in crystals. The orange crystals 2a were transformed to the red-orange
crystals 2b by removal of water of crystallization upon heating at 70 °C.
Keywords:
Salicylideneaniline crystal; Fluorescent crystal; Nonfluorescent crystal; Tautomeric isomer;
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Solid-state transformation
1. Introduction
Crystals of N-salicylideneanilines are well-known thermochromic and photochromic organic
compounds [1–12]. The thermochromic properties of N-salicylideneanilines have been extensively
studied for the past few decades. In general, most of the thermochromic N-salicylideneaniline
crystals are orange at room temperature. The crystals change to yellow upon cooling to liquid
nitrogen temperature, and return to orange upon returning to room temperature. The mechanism for
the thermochromic reaction of the N-salicylideneanilines was explained through a reversible
tautomeric isomerization reaction between the enol and cis-keto forms [1–3]. In general, the enol
isomer is thermodynamically more stable than the cis-keto isomer. Upon cooling, the proportion of
the enol form increases, and that of the cis-keto form decreases with the color change. Most of the
thermochromic N-salicylideneaniline crystals are fluorescent. The structures of the thermochromic
N-salicylideneaniline molecules are planar, and in many cases, the molecules are arranged in stacks
to form a columnar structure in the crystals [12–15]. Several years ago, Harada et al. explained that
the thermochromic color change of the N-salicylideneaniline crystals was due to the
temperature-induced change of fluorescence in addition to the change in light absorption of the
compounds [16].
Recently, we have found that N-(3,5-dichlorosalicylidene)-2-methoxyaniline crystallized into
two different shapes with different colors (2a and 2b). Orange needle crystals 2a were obtained by
recrystallization of the compound from wet methanol solution, and red-orange prismatic crystals 2b
were obtained by recrystallization from ethanol. The orange crystals 2a were fluorescent, but the
red-orange crystals 2b were completely nonfluorescent. The crystals of both 2a and 2b showed
general thermochromic behavior upon cooling; the orange 2a and red-orange 2b changed to
yellow-orange and yellow, respectively, upon cooling to –196 °C, and returned to their original
color upon returning to 25 °C. In contrast, only the orange crystals 2a showed a specific color
change upon heating; orange crystals 2a were irreversibly changed to red-orange upon heating at 70
°C. In this study, we prepared
a
set of halogen-substituted compounds
N-(3,5-diXsalicylidene)-2-methoxyaniline (X = fluoro (1), chloro (2a, 2b), bromo (3), and iodo (4);
Scheme 1), and investigated the luminescence properties of the compounds in the solid state. X-ray
crystal structures of 1, 2a, and 2b were determined, and the relation between the structures and
luminescence properties is discussed. Thermal transformation of 2a in the solid state is also
explained.
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X
X
O
X
X
OH
N
OCH3
NH
OCH3
enol
cis-keto
X = F, Cl, Br, I
Scheme 1. Structures of the isomers of N-(3,5-diXsalicylidene)-2-methoxyaniline.
2. Experimental
2.1. Synthesis
N-(3,5-diXsalicylidene)-2-methoxyaniline (X = fluoro, chloro, bromo, and iodo) was prepared
by the reaction of 3,5-diXsalicylaldehyde (2 mmol) with 2-methoxyaniline (2 mmol) in ethanol at
60 °C for 2 h. The resulting products were collected by filtration and washed with ethanol. Yield:
0.464 g (88%) for 1, 0.562 g (97%) for 2b, 0.719 g (93%) for 3, and 0.920 g (96%) for 4. As for
N-(3,5-dichlorosalicylidene)-2-methoxyaniline, orange needle crystals (2a) were obtained by
recrystallization of the compound from wet methanol solution containing 10 vol% of water.
Reddish-orange prismatic crystals (2b) were obtained by recrystallization of the compound from
ethanol.
In
contrast,
only
orange
crystals
were
obtained
for
N-(3,5-diXsalicylidene)-2-methoxyaniline (X = F (1), Br (3) and I (4)). Anal. Calc. for 1 =
C14H11F2NO2: C, 63.88; H, 4.21; N, 5.32%. Found: C, 63.99; H, 4.14; N, 5.23%. Calc. for 2a =
C14H13Cl2NO3: C, 53.52; H, 4.17; N, 4.46%. Found: C, 53.49; H, 3.99; N, 4.38%. Calc. for 2b =
C14H11Cl2NO2: C, 56.78; H, 3.74; N, 4.73%. Found: C, 56.75; H, 3.56; N, 4.63%. Calc. for 3 =
C14H11Br2NO2: C, 43.67; H, 2.88; N, 3.64%. Found: C, 43.63; H, 2.51; N, 3.58%. Calc. for 4 =
C14H11I2NO2: C, 35.10; H, 2.31; N, 2.92%. Found: C, 35.07; H, 2.02; N, 2.83%.
2.2. Physical measurements
IR spectra by a KBr method were recorded on a JASCO A-202 spectrometer. Diffuse reflectance
spectra of the ground samples without dilution were recorded on a JASCO V-570
spectrophotometer equipped with an integrating sphere. An optical filter was used for the
measurement of reflectance spectra to eliminate the influence of the fluorescence. Fluorescence
spectra by excitation at 480 or 360 nm in the solid state were measured on a Hitachi F-7000
fluorescence spectrophotometer equipped with an integrating sphere. The quantum yields in the
solid state were obtained by an absolute method using a Hitachi F-7000 fluorescence spectrometer
equipped with an integrating sphere by excitation at 480 nm. Powder X-ray diffraction (XRD)
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patterns were obtained on a Rigaku RINT-1500 diffractometer with monochromatized Cu Kα
radiation ( = 1.54056 Å).
λ
2.3. Thermal analysis
Thermal analyses of 2a were carried out using a Perkin-Elmer TGA 7 and Pyris 1 for
thermogravimetry (TG) measurements and differential scanning calorimetry (DSC) measurements,
respectively. TG measurements were carried out with heating rates of 20 °C min–1 up to 450 °C.
DSC measurements were carried out with heating rates of 10 °C min–1 up to 180 °C.
2.4. Thermal transformation of 2a in the solid state
Orange crystals of 2a were completely transformed into red-orange crystals upon heating at
70 °C for 10 h. The reaction time to complete the transformation depended on the crystal size of 2a.
The larger crystals needed a longer reaction time to complete the transformation. A powdered
sample of 2a was completely transformed upon heating at 60 °C within 2 h. Such thermal
transformations proceeded very slowly even at room temperature.
2.5. X-ray crystal structures
Crystal data of 1, 2a, and 2b are listed in Table 1. Orange crystals of 1 were grown from an
acetonitrile solution. Orange crystals of 2a were grown from a wet methanol solution containing 10
vol% of water. Red-orange crystals of 2b were grown from an ethanol solution. An orange prism
crystal (0.40 × 0.40 × 0.20 mm3) of 1, an orange needle crystal (0.50 × 0.06 × 0.06 mm3) of 2a, and
a red-orange prism crystal (0.40 × 0.30 × 0.20 mm3) of 2b were used for the X-ray measurements.
The intensity data were collected at 25 °C using a Rigaku R-AXIS RAPID imaging plate
diffractometer for 1 and a Rigaku SCXmini CCD diffractometer for 2a and 2b with
graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) up to 2θ = 55°. A total of 10736,
14582, and 13848 reflections were measured and 2676, 3240, and 3046 independent reflections
were obtained for 1, 2a, and 2b, respectively. Numerical absorption corrections were applied to the
reflection data [17]. The structures were solved by direct methods (SHELXL-97 [18] for 1 and
SIR2004 [19] for 2a and 2b) and refined on F2 by using the SHELXL-97 software package [18]. All
of the non-hydrogen atom positions were refined anisotropically, and all of the hydrogen atoms
were found by difference synthesis, and refined isotropically. The calculations were performed with
the Crystal Structure software package [20].
3. Results and discussion
3.1. Synthesis
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N-(3,5-diXsalicylidene)-2-methoxyaniline (X = fluoro, chloro, bromo, and iodo) was prepared
by the reaction of 3,5-dihalogenosalicylaldehyde with 2-methoxyaniline in ethanol. Only
fluorescent orange crystals were obtained for N-(3,5-difluorosalicylidene)-2-methoxyaniline (1). In
contrast, both fluorescent orange crystals (2a) and nonfluorescent red-orange crystals (2b) were
obtained for N-(3,5-dichlorosalicylidene)-2-methoxyaniline by recrystallization of the compound
from wet methanol and ethanol, respectively. Slightly fluorescent orange crystals were obtained for
N-(3,5-dibromosalicylidene)-2-methoxyaniline (3) and N-(3,5-diiodosalicylidene)-2-methoxyaniline
(4). Crystals of 2a contained water of crystallization, which was suggested by elemental analysis. In
contrast, no solvents of crystallization were contained in the other crystals. Pictures of the crystals
under irradiation with ambient light and UV light (black light; peak wavelength: 352 nm) at 25 °C
are shown in Fig. 1. Orange crystals of 1 and 2a showed orange fluorescence under irradiation with
UV light. In contrast, red-orange crystals of 2b were completely nonfluorescent. Orange crystals of
3 and 4 showed very weak orange fluorescence. All of the crystals (1, 2a, 2b, 3, and 4) showed
general thermochromic behavior upon cooling; the crystals changed to yellow or yellow-orange
upon cooling to –196 °C, and returned to their original color upon returning to 25 °C.
1
2a
2b
3
4
Under
ambient
light
Under
UV
light
Fig. 1. Pictures of the crystals under ambient light and UV light at 25 °C.
3.2. Spectral studies
Fluorescence spectra of the crystalline powders of 1, 2a, 3, and 4 by excitation at 480 nm at
room temperature are shown in Fig. 2. All of the spectra in Fig. 2 were measured with the same
measurement conditions with slit widths of 5 nm and voltage of 600 V. Each of the crystals shows a
broad emission band at around 600 nm (λmax: 596 nm for 1, 606 nm for 2a, 617 nm for 3, and 619
nm for 4). However, the intensities of the fluorescence decrease in the order 1 > 2a > 3 > 4. The
fluorescence quantum yields of 1 and 2a at 25 °C measured by an absolute method are 0.070 and
0.025, respectively. The fluorescence quantum yields of 3 and 4 were too small to measure
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accurately. The order of the emission intensities is coincident with the pictures of the compounds
under UV light in Fig. 1 [21]. The decrease in the fluorescence intensities for the compounds
containing heavier halogen atoms (1 > 2a > 3 > 4) would be explained by the heavy atom effect of
the halogen atoms on the intersystem crossing [22]. In contrast, crystals 2b are completely
nonfluorescent. As the crystals 2b are nonfluorescent both in air and under nitrogen atmosphere,
quenching by molecular oxygen is not important for the nonfluorescent property of 2b.
2500
2000
1500
1000
500
0
500 550 600 650 700 750 800
Wavelength / nm
Fig. 2. Fluorescence spectra of 1 (−−−), 2a (−−−), 3 (−−−), and 4 (−−−) in the solid state.
Diffuse reflectance spectra of orange 2a and red-orange 2b are shown in Fig. 3. Absorbance
of both 2a and 2b decreases rapidly in the wavelength region longer than 550 nm. The spectrum of
2a shows another weak shoulder band at around 600 nm. The spectral patterns of compounds 1, 3,
and 4 are similar to those of 2b [23].
300
400
500
600
700
Wavelength / nm
Fig. 3. Diffuse reflectance spectra of 2a (−−−) and 2b (−−−).
3.3. X-ray crystal structures
X-ray crystal-structure analyses of 1, 2a, and 2b were performed. Crystal data and selected bond
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lengths are shown in Tables 1 and 2, respectively. The molecular structures are shown in Fig. 4. All
of the N-salicylideneaniline molecules in the crystals of 1, 2a, and 2b have planar structures.
However, the molecules in each of the crystals take different tautomeric structures. In both 1 and 2b,
the H1 atom is attached to the O1 atom. In contrast, the H1 atom is attached to the N1 atom in 2a.
The O1–C2 bond length in 1 (1.335(2) Å) and 2b (1.329(2) Å) is longer than that in 2a (1.279(2) Å).
The N1–C7 bond length in 1 (1.279(2) Å) and 2b (1.282(2) Å) is shorter than that in 2a (1.304(2)
Å). Therefore, the molecules in 1 and 2b are assigned to the enol form, and that in 2a to the cis-keto
form. In general, the enol isomer is more stable than the cis-keto isomer, and the enol form is
usually observed in the crystal structures of most N-salicylideneanilines at room temperature.
However, the cis-keto form is observed in the crystal structure of 2a. There is water of
crystallization in 2a, which is hydrogen bonded to the O1 atoms of the adjacent
N-salicylideneaniline molecules. The hydrogen bonds between the N-salicylideneanilines and the
water of crystallization in 2a would affect the stabilization of the cis-keto form. Hydrogen bonds in
N-salicylideneaniline crystals sometimes affect the tautomeric structures of the
N-salicylideneaniline molecules. N-(3,5-dichlorosalicylidene)-2-methoxyaniline in this study also
forms a trifluoroacetic acid adduct (2c), which has intermolecular hydrogen bonds between the
trifluoroacetic acid and the N-salicylideneaniline molecules in the cis-keto form [24]. Moreover,
several crystal structures of other N-salicylideneaniline compounds in the cis-keto form that contain
intermolecular hydrogen bonds between the adjacent molecules have been reported [8–10, 25–28].
It is notable that we obtained both the enol and cis-keto tautomeric isomers of
N-(3,5-dichlorosalicylidene)-2-methoxyaniline and determined their structures. As mentioned above,
the crystals of 1 (enol form) and 2a (cis-keto form) are fluorescent, and those of 2b (enol form) are
nonfluorescent. The difference between the fluorescent and nonfluorescent forms of the crystals is
not caused by the different tautomeric structures.
1
2a
2b
Fig. 4. ORTEP drawings of 1, 2a, and 2b with 30% probability ellipsoids.
Crystals of 1, 2a, and 2b have different crystal-packing arrangements of the molecules. Fig. 5
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shows the crystal packing diagram of N-(3,5-difluorosalicylidene)-2-methoxyaniline in 1. There are
C–H•••F intermolecular hydrogen bonds between the fluorine atoms and the phenyl protons of the
adjacent molecules with distances of 3.492(3), 3.539(3) and 3.635(3) Å for F1•••C7′, F1•••C13′ and
F2•••C4″, respectively. The N-salicylideneaniline molecules bonded by the intermolecular hydrogen
bonds are on the same plane running along the a-axis. The planes consisting of the
hydrogen-bonded molecules are arranged in a herringbone packing pattern.
C7
’
C13’
F1
C4’’
F2
(a)
b
c
(b)
Fig. 5. (a) Intermolecular hydrogen bonds in 1. (b) Projection of the crystal structure of 1 along the
a-axis.
Fig. 6 shows crystal-packing diagrams of N-(3,5-dichlorosalicylidene)-2-methoxyaniline in 2a.
There are O–H•••O and C–H•••O intermolecular hydrogen bonds between the water of
crystallization and the N-salicylideneaniline molecules. The O1•••O3 and O1′•••O3 distances are
2.972(3) and 2.881(3) Å, respectively. The O3•••C6″, O3•••C7″, and O3•••C13″ distances are
3.720(3), 3.196(3), and 3.643(3) Å, respectively. The N-salicylideneaniline molecules are arranged
in stacks along the a-axis in head-to-tail fashion to form a columnar packing structure with the
interplanar distance of 3.55 Å. One-dimensional stacked columnar packing structures such as 2a
can be seen in other thermochromic N-salicylideneaniline crystals [12–15]. Although the molecules
are stacked in 2a, the overlap of the atoms between the adjacent stacked molecules in a column is
very small in 2a (Fig. 6(c)). The stacking structure of the N-salicylideneaniline molecules in the
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crystals of 2a is retained by the hydrogen bonds between the N-salicylideneaniline molecules and
water of crystallization. As will be described later, the removal of the water of crystallization of 2a
upon heating causes a transformation in the solid state. The presence of water molecules around
organic chromophores sometimes weakens the fluorescence intensity by vibrational deactivation.
However, vibrational deactivation by the crystal water seems to be small in crystals of fluorescent
2a.
O1’
C6’’
C7’’
O3
C13’’
O1
(a)
a
b
(b)
(c)
Fig. 6. (a) Intermolecular hydrogen bonds in 2a. (b) Projection of the crystal structure of 2a along
the c-axis. (c) Molecular overlap between the adjacent stacked molecules in a column of 2a viewed
from the vertical direction of the molecular plane.
Fig. 7 shows the crystal packing diagram of N-(3,5-dichlorosalicylidene)-2-methoxyaniline in 2b.
The molecules in 2b are strongly π–π stacked to form dimers with an interplanar distance of 3.40 Å.
The overlap of the atoms between the adjacent stacked molecules in a dimer is very large in 2b. The
C2, C4, C7, and C8 atoms are overlapped with the C7′, C8′, C2′, and C4′ atoms of the adjacent
molecules, respectively, with C2•••C7′ and C4•••C8′ distances of 3.286(2) and 3.417(2) Å. The
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dimers are π–π stacked with the adjacent dimers (the C4•••C6′′ distance: 3.592 (3) Å) to form a
herringbone packing pattern. As mentioned above, crystals of 2b are completely nonfluorescent.
The nonfluorescent nature of the crystals of 2b will be due to the proximity of the molecules caused
by the strong π–π stacking in the dimers. The proximity of the organic chromophores in the solid
state often induces self-quenching, which is called “aggregation-caused emission quenching
(ACQ)” [29]. Although both the molecules in fluorescent 2a and in nonfluorescent 2b are stacked in
the crystals, the overlap of the atoms between the adjacent stacked molecules is different. Large
overlap of atoms between the adjacent molecules is observed in nonfluorescent 2b, but not in
fluorescent 2a. The difference in the π–π stacking manner would influence the fluorescent and
nonfluorescent nature of the crystals. Recently, fluorescent quenching of a Schiff base compound
that has strong π–π stacking between the molecules in the crystals was reported [30].
C8
C2
C7
C4
(a)
b
c
(b)
Fig. 7. (a) Molecular overlap of the dimer in 2b viewed from the vertical direction of the molecular
plane. (b) Projection of the crystal structure of 2b along the a-axis.
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3.4. Thermal transformation of 2a in the solid state
Among the N-salicylideneaniline crystals (1, 2a, 2b, 3, and 4) in this study, only orange crystals
2a irreversibly changed to reddish-orange upon heating at 70 °C. The reddish-orange heated
product (2ah) was nonfluorescent, and the color of the heated product did not return to orange upon
returning to room temperature. Such an irreversible color change of the compound proceeded very
slowly, even at room temperature. The powder XRD patterns of crystals 2a, 2b, and 2ah are shown
in Fig. 8. The XRD patterns of 2a and 2b are different in the range of 2θ (Cu Kα) = 5 to 45°. On the
other hand, the XRD pattern of 2ah is almost identical to that of 2b. Therefore, the reddish-orange
heated product (2ah) is consistent with 2b. The IR spectrum of 2ah was also consistent with that of
2b [31]. These results indicate that the orange crystals of 2a were transformed to the red-orange
crystals of 2b upon heating at 70 °C. No intermediate compound during the transformation was
observed in the XRD patterns and IR data. TG measurement of 2a was performed. The data showed
5.6% weight loss between 48 °C and 97 °C, which corresponds to 0.98 molecules of water per
N-salicylideneaniline molecule. DSC measurement of 2a showed two endothermic peaks (∆H1 =
3.63 kJ mol–1; ∆H2 = 35.9 kJ mol–1) at 77 °C and 140 °C, respectively. The former endothermic
anomaly at 77 °C is assignable to the loss of crystal water and thermal transformation in the crystals.
The latter endothermic anomaly at 140 °C is assignable to melting of the compound. The removal
of water of crystallization in 2a will be followed by a tautomeric isomerization reaction and
transformation of the crystal structure. As the molecular arrangements and crystal structures of 2a
and 2b are very different, a large rearrangement of molecules is supposed to occur during the
thermal transformation in the solid state. It is notable that such a large rearrangement of molecules
occurs even in the solid state at 70 °C.
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2a
5
10 15 20 25 30 35 40 45
2b
5
10 15 20 25 30 35 40 45
2ah
5
10 15 20 25 30 35 40 45
2θ/°
Fig. 8. XRD patterns of 2a, 2b, and 2ah with Cu Kα. The compound 2ah was prepared upon
heating 2a at 70 °C for 10 h.
Conclusions
Crystals of N-(3,5-diXsalicylidene)-2-methoxyaniline (X = fluoro, chloro, bromo, and iodo)
were
N-(3,5-difluorosalicylidene)-2-methoxyaniline (1). Both fluorescent orange crystals (2a) and
nonfluorescent red-orange crystals (2b) were obtained for
N-(3,5-dichlorosalicylidene)-2-methoxyaniline. Slightly fluorescent orange crystals were obtained
for N-(3,5-dibromosalicylidene)-2-methoxyaniline (3) and
prepared.
Only
fluorescent
orange
crystals
were
obtained
for
N-(3,5-diiodosalicylidene)-2-methoxyaniline (4). Fluorescence spectra of 1, 2a, 3, and 4 in the solid
state show a fluorescence band at around 600 nm by excitation at 480 nm. The fluorescence
intensities were in the order 1 > 2a > 3 > 4. In contrast, crystals 2b are completely nonfluorescent.
X-ray crystal-structure analyses of 1, 2a, and 2b were performed. All of the N-salicylideneaniline
molecules in the crystals have planar structures. The molecules in 1 and 2b take the enol form,
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while the molecules in 2a take the cis-keto form. The crystals 2a include water of crystallization,
which is hydrogen bonded to the adjacent N-salicylideneaniline molecules. The hydrogen bonds
between the N-salicylideneanilines and the water of crystallization in 2a would affect the
stabilization of the cis-keto form in 2a. The crystals of 1 (enol form) and 2a (cis-keto form) are
fluorescent, and those of 2b (enol form) are nonfluorescent. Therefore, the fluorescent or
nonfluorescent character of the crystals is independent of the tautomeric structures. Molecules in
fluorescent 2a are arranged in stacks along the a-axis to form a columnar structure in the crystals. In
contrast, molecules in nonfluorescent 2b are strongly π–π stacked to form dimers in crystals. The
proximity of the organic chromophores by the strong π–π stacking in 2b would induce
self-quenching and nonfluorescent nature of the crystals. The difference in the π–π stacking manner
would influence the fluorescent and nonfluorescent nature of the crystals. The orange crystals 2a
were transformed to the red-orange crystals 2b by removal of water of crystallization upon heating
at 70 °C.
Acknowledgement
This work was partially supported by a Grant-in Aid for Scientific Research (No. 24550076)
from the Ministry of Education, Culture, Sports, Science and Technology of Japan
Appendix A. Supplementary data
The X-ray crystallographic data in CIF format have been deposited with CCDC (reference
numbers CCDC 1418538, 1418539, 1418540 and 1418541 for 1, 2a, 2b and 2c). These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated with this article
can be found, in the online version.
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References
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M. Schmidt, S. Flavian, J. Chem. Soc. (1964) 2041–2051.
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AND RELATED COMPOUNDS, in: Photochromism: Molecules and Systems, ed. by H. Dürr,
H. B. Laurent, Elsevier, Amsterdam, pp. 685–712, 1990.
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[8] K. Ogawa, Y. Kasahara, Y. Ohtani, J. Harada, J. Am. Chem. Soc. 120 (1998) 7107–7108.
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[10] K. Ogawa, J. Harada, J. Mol. Struct. 647 (2003) 211–216.
[11] T. Fujiwara, K. Ogawa, J. Harada, J. Phys. Chem. B 108 (2004) 4035–4038.
[12] M. D. Cohen, S. Flavian, J. Chem. Soc. (1967) 334–340.
[13] J. Bregman, L. Leiserowitz, G. M. J. Schmidt, J. Chem. Soc. (1964) 2068–2085.
[14] I. Moustakaki-Mavridis, E. Hadjoudis, Acta Crystallogr., Sect. B 34 (1978) 3709–3715.
[15] N. Hoshino, T. Inabe, T. Mitani, Y. Maruyama, Bull. Chem. Soc. Jpn. 61 (1988) 4207–4214.
[16] J. Harada, T. Fujiwara, K. Ogawa, J. Am. Chem. Soc. 129 (2007) 16216–16221.
[17] T. Higashi, SHAPE, Rigaku Corporation, Akishima, Tokyo, 1999.
[18] G. M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112–122.
[19] M. C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G. L. Cascarano, L. De Caro, C.
Giacovazzo, G. Polidori, R. Spagna, J. Appl. Cryst. 38 (2005) 381–388.
[20] Crystal Structure Analysis Package, Rigaku Corporation, Tokyo 196-8666, Japan, 2000–2010.
[21] The fluorescence spectra of 1, 2a, 3, and 4 obtained by excitation at 360 nm at 25 °C show
similar tendencies to those obtained by excitation at 480 nm in Fig. 2. The fluorescence
spectra by excitation at 360 nm are deposited.
[22] N. J. Turro, V. Ramamurthy, J. C. Scaiano, Principles of Molecular Photochemistry: An
Introduction, University Science Books, pp. 313–314, 2009.
[23] Diffuse reflectance spectra of 1, 3, and 4 are deposited.
[24] The crystals of orange fluorescent 2c were obtained by recrystallization of the compound from
trifluoroacetic acid. The recrystallization from trifluoroacetic acid was accompanied by
decomposition, so it was difficult to obtain a sufficient amount of pure 2c to measure
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fluorescence spectra, diffuse reflectance spectra, XRD patterns, and so on. Anal. Calc. for 2c =
C16H12NO4Cl2F3: C, 46.85; H, 2.95; N, 3.42%. Found: C, 46.85; H, 2.80; N, 3.36%. Crystal
data of C14H11NO2Cl2•CF3COOH: C16H12Cl2F3NO4, M = 410.18, crystal size: 0.50 × 0.06 ×
0.06 mm3, monoclinic, space group P21/n (No. 14), a = 7.3776(2), b = 14.5464(4), c =
16.9005(5) Å,
reflections measured, 4029 independent reflections were obtained. R = 0.046 against the 3078
reflections with I > 2 (I), Rw = 0.1104 against all 4029 reflections. There were large disorders
β µ
= 103.8629(9)°, V = 1760.89(9) Å3, Z = 4, (Mo Kα) = 0.420 mm–1, 18243
σ
on the fluorine atoms of the trifluoroacetic acid. ORTEP drawings and selected bond lengths
are deposited.
[25] H. Karabıyık, N. O. İskeleli, H. Petek, C. Albayrak, E. Ağar, J. Mol. Struct. 873 (2008)
130–136.
[26] M. Kabak, A. Elmali, Y. Elerman, J. Mol. Struct. 477 (1999) 151–158.
[27] Y. Elerman, M. Kabak, A. Elmali, Z. Naturforsch. 57b (2002) 651–656.
[28] B. Kaitner, M. Zbacnik, Acta Chim. Slov. 59 (2012) 670–679.
[29] (a) T. Ozdemir, S. Atilgan, I. Kutuk, L. T. Yildirim, A. Tulek, M. Bayindir, E. U. Akkaya, Org.
Lett. 11 (2009) 2105–2107; (b) W. Z. Yuan, P. Lu, S. Chen, J. W. Y. Lam, Z. Wang, Y. Liu, H.
S. Kwok, Y. Ma, B. Z. Tang, Adv. Mater. 22 (2010) 2159–2163; (c) R. Jakubiak, C. J. Collison,
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[30] L. Wang, Z. Zheng, Z. Yu, J. Zheng, M. Fang, J. Wu, Y. Tian, H. Zhou, J. Mater. Chem. C 1
(2013) 6952–6959.
[31] The IR data of 2a, 2b, and 2ah by a KBr method are deposited.
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Table 1. Crystal Data
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
Compound 2a 2b
──────────────────────────────────────────────────────
1
Formula
Fw
C14H11F2NO2
263.24
C14H13Cl2NO3
314.17
C14H11Cl2NO2
296.15
Crystal size/mm
Crystal system
Space group
0.40×0.40×0.20
monoclinic
P21/n
0.50×0.06×0.06
monoclinic
P21/a
0.40×0.30×0.20
monoclinic
P21/n
Z
4
4
4
a/Å
b/Å
c/Å
7.944(3)
6.237(3)
24.175(14)
97.11(2)
1188.6(10)
1.471
7.4929(3)
24.2132(8)
7.8025(3)
92.312(1)
1414.43(9)
1.475
7.8156(2)
19.9751(5)
8.5711(2)
92.973(1)
1336.30(6)
1.472
β
/º
V/ Å3
Density/gcm−3
µ/mm−1
0.120
0.464
0.481
Temperature/°C
25
0.046
0.119
25
0.054
0.118
25
0.044
0.115
Ra) (I > 2σ(I))
b)
Rw
──────────────────────────────────────────────────────
2
2
2
2 2 1/2
a) R = Σ|Fo − Fc2| /ΣFo . b) Rw = [Σ w (Fo - Fc2)2/Σw(Fo ) ] ,
w−1 = σ2(Fo ) + (0.0440P)2 + 0.350P for 1a, w−1 = σ2(Fo ) + (0.0468P)2 + 0.3471P for 2a,
2
2
2
2
w−1 = σ2(Fo ) + (0.0578P)2 + 0.3123P for 2b, where P = (Fo + 2Fc2)/3.
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Table 2. Selected Bond Lengths and Contact lengths (l /Å).
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
Compound
────────────────────────────────────────────────
1a)
2ab)
2bc)
O1–C2
C1–C2
C1–C6
C1−C7
C2−C3
C3−C4
C4−C5
C5−C6
1.335(2)
1.411(3)
1.402(3)
1.449(3)
1.390(3)
1.361(3)
1.383(3)
1.355(3)
1.279(2)
3.492(3)
3.539(3)
3.635(3)
1.279(2)
1.438(2)
1.415(3)
1.416(3)
1.427(3)
1.366(3)
1.401(3)
1.354(3)
1.304(2)
1.329(2)
1.412(2)
1.405(2)
1.445(2)
1.405(3)
1.378(3)
1.380(3)
1.376(3)
1.282(2)
N1−C7
F1•••C7′
F1•••C13′
F2•••C4′′
O1•••O3
O1′•••O3
O3•••C6′′
O3•••C7′′
O3•••C13′′
C2•••O7′
C2•••C7′
C4•••C8′
C4•••C6′′
2.972(3)
2.881(3)
3.720(3)
3.196(3)
3.643(4)
3.286(2)
3.286(2)
3.417(2)
3.592(2)
────────────────────────────────────────────────
a) Primed and double primed atoms of 1 are generated by –1 + x, y, z,
and –x, –1 – y, –z, respectively.
b) Primed and double primed atoms of 2a are generated by –1/2 + x, 1/2 – y, z,
and x, y, 1 + z , respectively.
c) Primed and double primed atoms of 2b are generated by 1 – x, –y, 1 – z, and 1–x, –y, – z,
respectively.
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Graphical abstract
2b
2a
Highlights
• Fluorescent and nonfluorescent crystals of N-salicylideneaniline were prepared.
• The molecules in the crystals have different tautomeric structures.
• The molecules in the nonfluorescent crystals are strongly π–π stacked to form dimers.
• Crystal transformation between the crystals was observed.
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Highlights
• Fluorescent and nonfluorescent crystals of N-salicylideneaniline were prepared.
• The molecules in the crystals have different tautomeric structures.
• The molecules in the nonfluorescent crystals are strongly π–π stacked to form dimers.
• Crystal transformation between the crystals was observed.