Prototropic tautomerism and solid-state photochromism of N-phenyl-2-aminotropones

Article abstract of DOI:10.1016/j.dyepig.2010.06.001

N-(4-X-phenyl)-2-aminotropones (X = methoxy, chloro, or nitro substituent) and their clathrate crystals with deoxycholic acid (DCA) were prepared and their prototropic phenomena were determined. Each 2-aminotropone derivative existed as a keto-amine form

Full text of DOI:10.1016/j.dyepig.2010.06.001

Dyes and Pigments 89 (2011) 319e323
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Prototropic tautomerism and solid-state photochromism
of N-phenyl-2-aminotropones
,
,
,
*
Yasuhiro Ito a 1, Kiichi Amimoto b 2, Toshio Kawato a
a Department of Chemistry, Faculty of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 810-8581, Japan
b Department of Science Education, Graduate School of Education, Hiroshima University, 1-1-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8524, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
N-(4-X-phenyl)-2-aminotropones (X ¼ methoxy, chloro, or nitro substituent) and their clathrate crystals
with deoxycholic acid (DCA) were prepared and their prototropic phenomena were determined. Each 2-
aminotropone derivative existed as a ketoeamine form in the crystal state irrespective of its substituent,
which influenced the stability of ketoeamine or enoleimine structure in solution. X-Ray crystal structure
analysis of N-(4-methoxyphenyl)-2-aminotropone revealed the existence of bimolecular hydrogen bonds
and close molecular packing, which would inhibit structural change necessary to photo-induced
prototropic tautomerization. Thus, N-phenyl-2-aminotropones did not show photocoloration in the
crystal state, while their clathrate crystals with DCA were found to exhibit photochromism.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 23 November 2009
Received in revised form
31 May 2010
Accepted 3 June 2010
Available online 16 June 2010
Keywords:
Photochromism
Prototropy
Tautomerism
Tropolone
Clathrate crystals
Deoxycholic acid
1. Introduction
structure and properties of the tropolone ring [12,13]. Tropolone
undergoes a proton tunneling from hydroxyl group to carbonyl
Photochromic materials have received considerable attention in
recent years because of their various kinds of potential usefulness
[1e5]. Although a large number of photochromic compounds have
been synthesized and studied, photochromic organic solids, which
are of high performance and easiness for handling superior to
photochromic solutions, are few in number [6,7]. Therefore, devel-
opment of new photochromic organic crystals is one of the first
important steps for the fundamental studies concerning develop-
ment of photoelectronic systems. N-Salicylideneamines (Schiff bases)
are well known to exhibit photochromic phenomena in the crystal
state [8e11]. The photocoloration of Schiff bases involves excited
state proton transfer, which is based on the alteration of basicity
(or acidity) of related functional groups (OH and NH) in the photo-
excited state. As a new candidate for such photosensitive organic
chromophores based on prototropic tautomerism, we focused
our interest on 2-hydroxytropone (tropolone) derivatives. Since their
discovery, tropolone natural products and synthetic tropolone
derivatives have attracted considerable interest due to the unique
oxygen atom [14,15]. Spectroscopic studies on some tropolone
derivatives have revealed that tautomerism due to five-membered
intramolecular prototropy is in very fast equilibrium in solution and
in gas phase [16e18]. Such tautomerism in the crystal state has been
less demonstrated in detail [19,20]; thus, photo-induced prototropic
features of tropolone analogues have been hardly demonstrated.
Some years ago aminotropone derivatives have been studied con-
cerning new optical systems [21]. In view of the structural resem-
blance to the central part of N-salicylideneaniline, we considered
that N-phenyl-2-aminotropone might be lead to new photochromic
compounds whose coloration would occur due to ESIPT. Herein, we
will report tautomeric behavior between NH form and OH form of
N-phenylaminotropones and their photochromic property in deox-
ycholic acid (DCA) crystal lattice for the first time.
2. Experimental
2.1. Methods and instruments
* Corresponding author. Tel./fax: þ81 92 642 3913.
E-mail addresses: kamimo@hiroshima-u.ac.jp (K. Amimoto), kawato@chem.
kyushu-univ.jp (T. Kawato).
All melting points (mp) were measured by a Yanaco model MP-
500V micro melting point apparatus and were uncorrected. 1H
NMR spectra were recorded on a JEOL GSX-270 (270 MHz) in CDCl3
1
Tel./fax: þ81 92 642 3913.
2
Tel./fax: þ81 82 424 7091.
and CD3COCD3 as the solvent. Chemical shifts are given as d-values
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.06.001

320
Y. Ito et al. / Dyes and Pigments 89 (2011) 319e323
Table 1
in ppm relative to tetramethylsilane (TMS) as the internal standard.
IR spectra were measured using JASCO FT/IR-420 spectrometer.
UV/VIS spectra were measured by JASCO UVIDEC-650 spectrometer
or JASCO V-560DS spectrometer. Elementary analyses were per-
formed by the Service Center of the Elementary Analysis of Organic
Compounds affiliated to the Faculty of Science in Kyushu University.
Lattice parameters of DCA clathrate crystals with N-salicylideneaniline (SA@DCA)
and 1 (1@DCA)
Compound
SA@DCA
1@DCA
Crystal system
Lattice parameters
a
b
orthorhombic
orthorhombic
13.212 Å
13.884 Å
25.709 Å
P212121
7.223 Å
12.464 Å
25.325 Å
P212121
2.2. General procedure of synthesis of N-phenyl-2-aminotropone
derivatives (1e3)
c
Space group
A
mixture of freshly prepared 2-chlorotropone (141 mg,
1.00 mmol) [22], 4-substituted aniline (1.00 mmol) and triethyl-
amine (104 mg, 1.03 mmol) in benzene (10 ml) was refluxed for
166 h with stirring. After being cooled, the solution was washed
with water and the organic layer was evaporated. The residue was
recrystallized from aqueous ethanol to give the product as yellow to
orange crystals.
the end of a glass fiber and coated with epoxy resin. X-Ray diffraction
experiments were performed on a Rigaku RAXIS-IV imaging plate
area detector with graphite monochromated Mo-K
a radiation
(
l
¼ 0.71069 Å). Data analyses were carried out with the WinGX
software package [23]. The structure was solved by direct methods
using SHELXS-97 and refined by full matrix least-squares on F2 with
SHELXL-97 [24]. All non-hydrogen atoms were refined anisotropi-
cally. The molecular figure was obtained by ORTEP3 [25] and
Raster3D [26]. Mercury software was used for the crystal packing
drawing [27].
2.2.1. N-(4-chlorophenyl)-2-aminotropone (1)
Yellow needles. Yield 97 mg (42%), mp 144.6e144.9 ꢀC, 1H NMR
(270 MHz, CDCl3); d 6.78e7.42 (m, 9H, AreH and CPeH), 8.67 (s, 1H,
NH). IR (KBr): 3208 (nNeH) cmꢁ1. Anal. Calc for C13H10ClNO: C, 67.40;
Crystal data. C14H13NO2, orthorhombic, space group Pbca (#61),
a ¼ 7.825(3) Å, b ¼ 15.379(7) Å, c ¼ 18.124(6) Å, V ¼ 2180.9(15) Å3,
H, 4.35; N, 6.05%. Found: C, 67.43; H, 4.30; N, 6.10%.
Z ¼ 8, Final R (I > 2 (I)), R1 ¼ 0.044, wR2 ¼ 0.0851.
s
2.2.2. N-(4-methoxyphenyl)-2-aminotropone (2)
Full crystallographic data for the structure of 2 in this paper have
been deposited with the Cambridge Crystallographic Data Centre
under deposition number ********. Copy of the data can be obtained,
free of charge, on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK, (fax: þ44-(0)1223-336033 or e-mail: deposit@ccdc.
cam.ac.UK).
Orange granular crystals. Yield 101 mg (44.4%), mp 104.0e106.4 ꢀC,
1H NMR (270 MHz, CDCl3);
d 3.85 (s, 3H, OCH3, 6.71e7.56) (m, 9H,
AreH and CPeH), 8.64 (s, 1H, NH). IR (KBr): 3220 (nNeH) cmꢁ1. Anal.
Calc for C14H13NO2: C, 73.99; H, 5.77; N, 6.16%. Found: C, 73.86; H, 5.79;
N, 6.02%.
2.2.3. N-(4-nitrophenyl)-2-aminotropone (3)
3. Results and discussion
Orange granular crystals. Yield 39 mg (16%), mp 224.5e228.0 ꢀC,
1H NMR (270 MHz, CDCl3);
d 6.94e7.49 (m, 9H, AreH and CPeH),
3.1. Synthesis and tautomerization of N-(4-substituted-phenyl)-2-
aminotropones
8.93 (s,1H, OH). IR (KBr): 3190 (nNeH) cmꢁ1. Anal. Calc for C13H10NO3:
C, 64.46; H, 4.16; N, 11.56%. Found: C, 64.19; H, 4.36; N, 11.33%.
Desired
N-(4-substituted-phenyl)-2-aminotropones
1e3
2.3. Formation of DCA clathrate crystals 1@DCA
(Scheme 1) were synthesized according to the amination of
2-chrolotropone with the corresponding anilines. The yielded
compounds were identified by elemental analyses and 1H NMR
A mixture of 1 (93 mg, 0.40 mmol) and DCA (628 mg,1.60 mmol)
was recrystallized from a mixed solvent of methanol and ethyl
acetate (1:1). Resultant white-yellow fine crystals were washed by
the mixed solvent several times and used for photocoloration
measurement without further purification.
spectroscopy. Each resonance signal at
¼ 8.64 ppm for 2 in the NMR spectra in CDCl3 was assigned to the
NeH proton for a ketoeamine chelation form, while a singlet peak
at
d
¼ 8.67 ppm for 1 and at
d
d
¼ 8.93 ppm for 3 was reasonably assigned to the OH proton for
an enoleimine chelation form. Such a stable form was confirmed
by IR spectra of 1e3 in CCl4 solution (Fig. 1). Compounds 1 and 2
showed characteristic bands at 3209 cmꢁ1 and 3220 cmꢁ1 due to
the NH stretching vibration. In the IR spectrum of 3, however, an
absorption band due to the OH stretching vibration appeared at
3419 cmꢁ1. These results can be interpreted by the resonance effect
2.4. Photocoloration measurement
Crystalline powders of DCA clathrate compounds were placed
between two glass plates and were stored in the dark at 30 ꢀC before
the experiments were carried out. Photocoloration was accom-
plished by irradiating the sample with 365 nm light using a hand-
held UV lamp (UVP MINERALLIGHTÒLAMP UVGL-25). Reaction rate
of thermal fading of resultant photochrome was monitored by
measuring electronic reflectance spectra at 30 ꢀC at appropriate
intervals. The first-order rate constants k for thermal fading reac-
tions of the photocolored species were calculated from the variation
of substituents at the 4-position of benzene ring.
donating group can enhance the basicity of nitrogen atom of the
p-Electron-
in absorbance data at the optimum wavelength l (Table 1) with time
using the expression kt ¼ ln{(A0 ꢁ AN)/(At ꢁ AN)}, where A0, AN, and
At are the observed absorbance data at zero time, at the end of the
reaction (8e10 half-lives) and at time t (s), respectively.
2.5. X-Ray crystallography of 2
A single crystal suitable for X-ray diffraction was obtained by
recrystallization of 2 from cyclohexane. The crystal was mounted at
Scheme 1. Prototropic isomerism of N-phenyl-2-aminotropone 1e3.

Y. Ito et al. / Dyes and Pigments 89 (2011) 319e323
321
Fig. 1. IR spectra of 1e3 (i) in CCl4 solution and (ii) in KBr disk.
amino group to stabilize NH form. On the other hand, OH form is
predominant in the case of 3 with electron-withdrawing substit-
uent. Thus, distribution of the tautomeric isomers is dependent on
the electronic effect of substituents in solution. It should be noted,
however, that all the compounds in KBr disks showed characteristic
single bands at around 3200 cmꢁ1 due to NH stretching vibration,
which exhibited that the molecules existed as ketoeamine forms in
the crystal state.
predominant in CCl4 solution. Neither crystals nor solutions of
N-phenyl-2-aminotropone exhibited photocoloration; thus, amino-
tropone 1e3 are not photochromic.
3.3. Photochromism of N-phenyl-2-aminotropone
in DCA clathrate crystal lattice
The mechanism of photocoloration of Schiff bases involves
excited state intramolecular proton transfer (ESIPT) from the
phenolic hydroxyl group to the imine nitrogen atom through six-
membered chelating structure, followed by molecular structural
alteration to stabilize a resultant ketoeamine structure [8e11].
N-Salicylideneamines as well as 2-aminotropones do not exhibit
photochromic phenomenon in solution due to the fast thermal
relaxation of the photochrome in solution. In every crystal of such
a compound, ESIPT occurs. A decisive condition for organic dyes
to exhibit crystalline photochromism is to secure reaction room
for photo-induced reactions in the crystal lattice [10]. From the
inspection of crystal packing diagram of 2, intermolecular interac-
tion via hydrogen bonding and molecular aggregation by aromatic
rings led to close structure, which inhibits molecular structure
alteration induced by prototropic tautomerization. Accordingly,
these crystals do not show any color change after photo-irradiation.
3.2. X-Ray crystal structure of 2
In order to obtain accurate structural information of N-phenyl-2-
aminotropone in the crystal state, X-ray crystal structure of 2 was
determined. The particular hydrogen atom considered is located at
the nitrogen atom in the five-membered chelating structure (Fig. 2):
the bond distance of NeH is 0.92 Å. Thus, it is confirmed that the
molecule exists as NH form. The bond angle of NeH/O is 110.4ꢀ and
the interatomic N(1)/O(1) distance is 2.54 Å, which is close enough
for the proton to transfer between the nitrogen and oxygen atoms.
Crystal structure of 2 is shown in Fig. 3, in which a pair of
molecules form a firm dimer connected by intermolecular double
hydrogen bonds between the NeH and carbonyl oxygen atoms
(intermolecular N/O distance: 3.04 Å and NeH/O angle: 148.6ꢀ).
Each dimer is in contact with proximate dimers. According to partial
packing diagrams around troponyl and benzene rings, in which
symmetry codes are (i) X, Y, Z, (ii) ꢁ1 þ1/2 þ X, þY, 1/2 ꢁ Z, and (iii)
1/2 þ X, þY, 1/2 ꢁ Z, the distance between the least-squares plane
defined by the troponyl ring(i) and adjoining H(8) of (ii) is 2.92 Å, and
the distance between the least-squares plane of the troponyl ring(i)
and adjoining C(4) of (iii) is 3.42 Å, respectively. Similarly, the
shortest distances from the benzene ring (i) to neighboring H(9) of
(ii) and (iii) are 3.09 Å and 2.82 Å, respectively. It is worthwhile to
note that intermolecular hydrogen bonds stabilize ketoeamine
structure in the crystal state, while intramolecular hydrogen bond is
Fig. 2. Molecular structure of 2 with numbering scheme.
Fig. 3. Crystal packing diagram of 2 along the a axis.

322
Y. Ito et al. / Dyes and Pigments 89 (2011) 319e323
In the previous papers, we have reported that utilization of crystal
lattice cavities in deoxycholic acid (DCA) clathrate crystals was one
of the most effective methods for producing photochromic crystals
[28e31]. Therefore, DCA clathrate compound with 1 (1@DCA) were
prepared by mixing 1 with DCA in a mixed solvent of methanol and
ethyl acetate. According to NMR peak analysis, yielded white-yellow
fine crystals were found to contain 1 as well as solvent methanol as
guest molecules. Such a phenomenon is observed in the case that
desired guest molecules are not so desirable as N-salicylideneaniline
[29]. The walls of guest rooms in deoxycholic acid crystal lattice to
accommodate organic molecules are made of hydrocarbon moieties,
and neighbouring molecules coexisting in the reaction room affect
the rate of thermal bleaching reaction of the guest photochrome.
X-Ray diffraction experiments on the best single crystal of 1@DCA
revealed that the clathrate compound crystallized in the ortho-
rhombic system of the space group of P212121 (#19). Although we
could not see the exact structure of 1@DCA because of difficulties in
refinement due to severe disorder of guest molecules, guest 1 was
suggested to be enclathrated within the cavities formed by host
DCA molecules according to the comparison with the crystal data of
already known DCA clathrate crystals with N-salicylideneaniline
(SA@DCA) (Table 1) [31].
Fig. 5. Kinetic plot of optical density decrease for the photocolored species derived
from 1@DCA.
Upon exposure of 1@DCA crystals to UV light, the white-yellow
color of the crystals turned to pale orange, which returned slowly to
the original yellow in the dark. Reflectance spectra of 1@DCA crystals
before and after UV irradiation are shown in Fig. 4. It should be noted
that this photocoloration is the first observation for the tropolone
family and N-phenyl-2-aminotropones exhibit photochromism only
within the DCA inclusion environment. The thermal fading reaction
of the photoproduct was then monitored by the decreasing rate of
absorption band at 490 nm due to the photocolored species at 30 ꢀC.
Kinetic analysis exhibited a good fit to a first-order equation and two
consecutive first-order decays (k1 and k2) of the photochrome, cor-
responding to two-step isomerization to the stable form (Fig. 5). Rate
constants of thermal fading of the photochrome were measured to be
k1 ¼5.9 ꢂ 10ꢁ3 sꢁ1 and k2 ¼ 3.3 ꢂ 10ꢁ4 sꢁ1 for 1@DCA. DCA inclusion
crystal of 2 (2@DCA) was similarly prepared to exhibit similar spec-
tral change before and after UV irradiation. By monitoring the decay
of absorption band at 484 nm at 30 ꢀC, rate constants (k1 and k2)
of thermal fading of the photochrome were measured to be
k1 ¼ 4.8 ꢂ 10ꢁ3 sꢁ1 and k2 ¼ 5.1 ꢂ10ꢁ5 sꢁ1 for 2@DCA. These results
indicate that the thermal isomerization proceeds through an
unimolecular reaction pathway without any side reaction. Such
a photochromic behavior is similar to that of N-salicylideneaniline,
suggesting that the photocoloration of 1@DCA and 2@DCA would be
based on photo-induced intramolecular prototropic tautomerism of
1 and 2. Since N-phenyl-2-aminotropone exists as an NH form in the
ground state and prototropic tautomerism is influenced by electronic
state of the molecule, the photocolored species is reasonably
assigned to an OH form with some distortion.
At this stage, two-step fading process for the photochrome of
1@DCA and 2@DCA is not understood unambiguously. In the case of
Schiff base, such a phenomenon has been explained by assuming
the framework relaxation from two types of photochromes, cis-
ketoeamine form and trans-ketoeamine form, coexisting in a pho-
tocolored crystal [32,33]. As for the system in this study, however,
isomerization from two types of photochromes like syn-imino OH
form and anti-imino OH form is not probable to occur in a limited size
of reaction room constructed by DCA molecules. In contrast with
these aspects, we also have reported that photochromicity of Schiff
base was very sensitive to the size of reaction room in the crystal
[34e36]. Consequently, stability of photochrome depended on the
circumstances of molecules under consideration in the crystal lattice
containing solvent molecules [37e39]. Both 1@DCA and 2@DCA
crystals include methanol molecules along with 1 and 2; thus, some
coexisting pseudopolymorphic structure due to enclathrated meth-
anol molecules in the crystal might be responsible in part for the two
consecutive decays of photochrome. Since the reflectance spectra
had a poor quality in the UV region, mainly due to the light scattering
in the solid-state, the detail of molecular structure could not be
determined from the spectra due to
pep* electronic excitation.
4. Conclusion
N-Phenyl-2-aminotropone exhibited photochromism only
within the DCA inclusion environment. The photocoloration in this
study was small; however, the change of color was repeatable
without any side alteration and the degree of spectral change and
the fading rate constants measured were intact for the repeated
experiments. Thus, effectiveness of DCA clathrate method for
Fig. 4. Solid state reflectance spectra of 1@DCA before (full line) and after (dotted line)
UV irradiation.

Y. Ito et al. / Dyes and Pigments 89 (2011) 319e323
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preparation of new series of photochromic crystals was confirmed
in this study and photochromic properties of tropolone families
could be revealed for the first time. Further works on the mecha-
nism of the photocoloration reaction and other chromotropic
behavior of aminotropone derivatives are under investigation.
Acknowledgements
This work was supported by the Grant-in-Aid for Scientific
Research (No. 17029049) on Priority Areas (417) from the Ministry
of Education, Culture, Sports, Science and Technology (MEXT) of
Japan. We thank Dr. Hiroyuki Koyama of Kyushu University for his
support throughout this work.
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