Crystal structures and emitting properties of trifluoromethylaminoquinoline derivatives: Thermal single-crystal-to-single-crystal transformation of polymorphic crystals that emit different colors

Article abstract of DOI:10.1002/chem.201201213

2,4-Trifluoromethylquinoline (TFMAQ) derivatives that have amine (1), methylamine (2), phenylamine (3), and dimethylamine (4) substituents at the 7-position of the quinoline ring were prepared and crystallized. Six crystals including the crystal polymorph

Full text of DOI:10.1002/chem.201201213

DOI: 10.1002/chem.201201213
Crystal Structures and Emitting Properties of Trifluoromethylaminoquinoline
Derivatives: Thermal Single-Crystal-to-Single-Crystal Transformation of
Polymorphic Crystals That Emit Different Colors
Yuichiro Abe, Satoru Karasawa, and Noboru Koga*[a]
Abstract: 2,4-Trifluoromethylquinoline
(TFMAQ) derivatives that have amine
(1), methylamine (2), phenylamine (3),
and dimethylamine (4) substituents at
the 7-position of the quinoline ring
were prepared and crystallized. Six
crystals including the crystal poly-
morphs of 2 (crystal GB and YG) and
3 (crystal B and G) were obtained and
characterized by X-ray crystallography.
In solution, TFMAQ derivatives emit-
ted relatively strong fluorescence
(lfmax =418–469 nm and Ff(s)=0.23–
0.60) depending on the solvent polarity.
From Lippert–Mataga plots, Dm values
in the range of 7.8–14 D were obtained.
In the crystalline state, TFMAQ deriv-
atives emitted at longer wavelengths
(lfmax =464–530 nm) with lower intensi-
ty (Ff(c)=0.01–0.28) than those in n-
hexane solution. The polymorphous
crystals of 2 and 3 emitted different
colors: 2, lfmax =470 and 530 nm with
Ff(c)=0.04 and approximately 0.01 for
crystal GB and YG, respectively; and
3, lfmax =464 and 506 nm with Ff(c)=
0.28 and approximately 0.28 for crystal
B and G, respectively. In both crystal
polymorphs of 2 and 3, crystals GB
and G showed emission color changes
by heating/melting/cooling cycles that
were representative. By following the
color changes in heating at the temper-
ature below the melting point with X-
ray diffraction measurements and X-
ray crystallography, the single-crystal-
to-single-crystal transformations from
crystal GB to YG for 2 and from crys-
tal B to G for 3 were revealed.
Keywords: crystal structures · fluo-
rescence · polymorphism · solid-
state emission · X-ray diffraction
Introduction
semblies, and polymers that show responses to pressure,[1a,
heat,[1c,11] light,[12] and so on[13] have been reported.
b,2a,d,10]
Organic molecules[1] and metal-containing compounds[2] that
exhibit solid-state emissions have attracted significant atten-
tion not only in the field of fundamental research on photo-
physical properties but also in practical applications for elec-
troluminescent devices, light-emitting diodes (LED),[3] solid-
state dye lasers,[4] and light-emitting electrochemical cells.[5]
Therefore, many solid-state emitting molecules that origi-
nate from the intramolecular charge-transfer (ICT) state,[6]
excited-state intramolecular proton transfer (ESIPT),[7] ag-
gregation-induced enhanced emission (AIEE),[8] and
others[9] have been extensively studied. In the development
of new functional emitting materials, furthermore, some
emitting molecules were recently found to show responsive-
ness to external stimuli. Solid-state emitting molecules, as-
The characteristics of solid-state emission are that the emit-
ting behavior strongly depends on the molecular structure
and molecular arrangement in the solid state. Therefore, the
emitting wavelength and intensity can be reversibly changed
and tuned by subtly altering the molecular structure and ar-
rangement in the crystalline state by means of external stim-
uli. In many approaches to the development of emitting
molecules that are responsive to external stimuli, poly-
morph-dependent emissions were proposed as a promising
approach to control the emission properties by means of ex-
ternal stimuli.[7a,14] Based on this strategy, the alteration of
solid-state emissions using thermal crystal-to-crystal (C-to-
C) interconversion between crystal polymorphs has been re-
ported.[15] However, examples that demonstrate the crystal-
to-crystal interconversion by X-ray crystallography remain
limited.[15b] This time, we focused on a single-crystal emitting
molecule with thermal single-crystal-to-single-crystal (SC-to-
SC) interconversion. In the present study, we prepared
simple quinoline derivatives with donor and acceptor sub-
stituents as new fluorophores, and obtained their polymor-
phous single crystals that emitted different colors. Surpris-
ingly, the crystals demonstrated SC-to-SC transformations
between the polymorphs in response to heat, and the ther-
mal alteration of molecular rearrangements in the single-
crystal polymorphs were successfully revealed by X-ray dif-
fraction and X-ray crystallography. We report here the prep-
aration of the N-methyl and N-phenyl derivatives of 2,4-tri-
[a] Y. Abe, Dr. S. Karasawa, Dr. N. Koga
Graduate School of Pharmaceutical Sciences
Kyushu University
3-1-1 Maidashi, Higashi-ku
Fukuoka, 812-8582 (Japan)
Fax : (+81)92-642-6590
E-mail: koga@fc.phar.kyushu-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201213. It contains CIF files
for 1–4; molecular structures (1 and 3B) and crystal packing for 1
and 2; absorption and emission spectra in various solvents and in the
powder state; DSC and XRD patterns for 3; crystallographic data for
1–4; and movies for thermal SC-to-SC interconversion of 2 and 3.
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FULL PAPER
fluoromethyl-7-aminoqumoline (TFMAQ), their absorption
and emission properties in the solution and the crystalline
state, and the emission change with the thermal SC-to-SC
phase transition.
kinds of crystals were separately obtained as yellowish thin
plates from Et2O/n-hexane (10:3) at 48C for crystal GB and
yellowish blocks from Et2O/n-hexane (1:10) at 208C for
crystal YG of 2, respectively, and as yellowish thin blocks
from Et2O/n-hexane (10:3) at 48C for crystal B and yellow-
ish blocks from Et2O/n-hexane (1:10) at 208C for crystal G
of 3, respectively. The crystals of 1 and 4 were stable at am-
bient atmosphere, whereas crystal GB of 2 and crystal B of
3 were slightly labile and were kept below 08C. The IR
spectra of compound 1 showed absorption due to the NH
stretching (nCꢁH) at 3461 and 3333 cmꢁ1. Compound 2 also
showed absorptions at 3384 and 3328 cmꢁ1 for crystal GB
and crystal YG, respectively, whereas 3 showed absorptions
at 3397 cmꢁ1 for crystal B and at 3439, 3408, and 3397 cmꢁ1
Results and Discussion
Preparation:
2,4-Di(trifluoromethyl)-7-aminoquinoline
(TFMAQ, 1) with electron-donor and -acceptor substituents
was prepared as a new fluorophore. To increase the elec-
tron-donor ability, the methylamino (2), phenylamino (3),
and dimethylamino (4) groups were introduced at the 7-po-
sition of quinoline.
for crystal G. The observed low wavenumbers for nNꢁ in
H
the crystals of 1 and 2 suggested the formation of hydrogen
bonds.
Crystal-structure analysis: To understand the properties of
the solid-state emission, six single crystals of 1, 2, 3, and 4,
including crystal GB and YG for 2 and crystal B and G for
3, were analyzed by X-ray crystallography. The space groups
and Z values for 1, 2, 3, and 4 are listed in Table 1 and other
crystallographic data and experimental details are summar-
ized in Table S3 of the Supporting Information. Crystal GB
and YG for 2 and crystal B and G for 3 were confirmed to
be polymorphous. The crystal polymorphs for 2 were partic-
ularly interesting and crystal GB and YG were rotational
isomers in synperiplanar (sp) and antiperiplanar (ap) config-
urations, respectively. In contrast, both crystal B and G in 3
had only sp forms. The phenyl ring for crystal G was found
to have a disorder in the position that rotated by 908. Fur-
thermore, the crystal GB for 2 and crystal G for 3 had two
crystallographically independent molecules, which are
shown as gray and black in Figure 2. The molecular struc-
2,4-Di(trifluoromethyl)-7-aminoquinoline (1) was pre-
pared as a parent molecule by means of a one-step synthesis
(Combes quinoline synthesis[16]). The reaction of 1,3-diami-
nobenzene with hexafluoroacetylacetone in the presence of
Monmorillonite K10 was carried out, and 1 was obtained in
relatively high yield (ꢀ96%). The amine at the 7-position
of quinoline in 1 was methylated with methyl iodide in the
presence of potassium carbonate to give a mixture of 2 and
4, followed by separation with silica-gel column chromatog-
raphy. Fluorophore 3 was prepared by the procedure using
bromobenzene in the presence of dichloro
ACHTUNGTRENNUNG
phosphino)ferrocene]palladium [PdCl CHTUNGTRENNUNG
BuOK. TFMAQ derivatives 1, 2, 3, and 4 were easily crys-
tallized from the mixtures of n-hexane and diethyl ether to
afford yellowish single crystals. In 2 and 3 especially, two
Table 1. Space groups, Z values, selected bond lengths, dihedral angles, crystal packing modes, and various intermolecular distances for 1, 2 (crystal GB
and YG), 3 (crystal B and G), and 4.[a]
1
2
3
4
GB
YG
B
G
space group, Z
¯
Pna21 (no. 33),
P21/c (no. 14),
P212121 (no. 19),
Pbca (no. 61),
P1 (no. 2),
P21/c (no. 14),
4
8
4
8
4
4
ꢁ
bond length [ꢁ] of C7 Na
1.387
1.359, 1.360
1.352
1.384
1.378
1.372
dihedral angle [8]
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
q NHa1Ha2
q NCMeHa
19.18, 23.07
q NCMeHa
6.12
q NCPhHa
17.91
q NCPhHa
10.88, 11.68
q NCMe1CMe2
29.08
g-HB
3.182
4.69
HTC
–
crystal packing mode
SHB
g-HB
SC
SP
–
ꢁ
intermolecular distance of Nq Na [ꢁ]
3.216, 3.246
3.129
–
nearest distance between the quinoline planes [ꢁ] for p–p interactions
ꢁ
ꢁ
ꢁ
C6 C7’ 3.482
C8 N1’ 3.619,
C9 C9’ 3.600
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
C6 C9’ 3.535
C7 C6’ 3.482
C6 C9’ 3.712
C1 C5’ 3.660
C7’ C9’’ 3.612
C1’ C6’’ 3.609
ꢁ
C7’’ C7’’’ 3.459
CH–p interaction [ꢁ]
ꢁ
ꢁ
–
–
–
–
–
–
Hph5 Cph3’ 2.648,
Hph3 Cph3ꢂ 2.781,
–
–
ꢁ
ꢁ
Hph2 Cph6’’ 2.719
Hph5’’ Cph3 2.774
[a] g-HB: g-herringbone; SHB: sandwich herringbone; SC: slipped columnar; SP: slipped parallel; HTC: head-to-tail columnar; q: quinoline ring.
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15039

N. Koga et al.
tures are shown as ORTEP drawings of crystal GB and YG
for 2 and crystal G for 3 and 4 in Figure 1; and 1 and crystal
B for 3 in Figure S1 of the Supporting Information.
that contains the slipped-parallel and slipped-columnar
structures. The crystal packing modes for crystal G and B of
3 were slipped parallel and slipped columnar, respectively,
in the offset stacked type, and that for 4 was the head-to-tail
structure in the stacked type. The molecular arrangements
in the molecular packings for 2, 3, and 4 are shown in
Figure 2 and Figure S2 in the Supporting Information. That
for 1 is shown in Figure S2 in the Supporting Information.
In 1 and crystal GB and YG of 2, which are g-HB, SHB,
and g-HB, respectively, the nearest distances (rN N ) be-
qꢁ
a
tween the nitrogen of the quinoline ring and the nitrogen of
ꢁ
amine, Nq Na, were 3.18 for 1, 3.22 (3.25) for crystal GB,
and 3.13 ꢁ for YG of 2. The observed short distances sug-
gested that a one-dimensional hydrogen bond formed along
the a axis for 1 and crystal GB and YG of 2 (the dashed
lines in Figure 2a and b, and Figure S2 in the Supporting In-
formation). The observed intermolecular hydrogen bond
might provide the double-bond character accompanied by
the hydrogen shift (Scheme S1 in the Supporting Informa-
ꢁ
tion) to the C7 Na single bond. In 2 especially, the hydrogen
bonds led to the interesting separation of sp and ap rotam-
ers in the crystalline state. The quinoline rings within the g-
HB structure were partially overlapped at distances of 3.56
ꢁ
and 3.71 ꢁ of C6 C9* for 1 and crystal YG for 2, respec-
Figure 1. ORTEP drawings of the molecular structures of crystals a) GB
and b) YG for 2, c) crystal G for 3, and d) 4.
tively, whereas the quinoline rings in the SHB structure for
crystal GB of 2 had no overlap and instead had edge-to-
edge short contacts with the neighboring herringbone chain
ꢁ
ꢁ
In the molecular structure of TFMAQ derivatives, the dis-
tance between C7 and Na changed in the range of 1.35–
1.39 ꢁ. The distances of 1.37–1.39 ꢁ for 1, 3 (crystal B and
G), and 4 were comparable with those of aromatic amine,
whereas those of polymorphs of 2 were 1.35–1.36 ꢁ and
were somewhat shorter, thereby suggesting double-bond
character. This character might be one of the factors in the
separation of the rotamer in the crystalline state (Scheme S1
in the Supporting Information). The dihedral angles be-
at distances of 3.48 and 3.46 ꢁ (C6 C7* and C7** C7***,
respectively) to form a dimer (Figure 2b and Figure S2 in
the Supporting Information). Accordingly, in 2, the p–p in-
teraction for crystal YG was more favorable than that for
GB. The observed difference of the crystal packing in poly-
morphs of 2 might affect the solid-state emission.
In polymorphous crystals B and G in 3, which have a hy-
drogen atom at the amino moiety, on the other hand, no
short distances of rN
were observed. The molecules of
ꢁ
Na
q
tween the two planes defined as C6-C8-Na and Na-R
G
crystal B and G showed different p–p stacking of quinoline
rings in the offset stacked type and slipped-parallel and slip-
ped-columnar modes,[9] respectively (Figure 2c and d). The
quinoline molecules for crystal B layered in parallel with
H)-R(C or H) varied depending on the substituent: R: 29.18
ACHTUNGTRENNUNG
for 1; 19.2, 23.1, and 6.18 for crystal GB and YG of 2, re-
spectively; 17.9 and 10.9 and 11.78 for crystal B and G of 3,
respectively; and 4.78 for 4. The observed small dihedral
angles, especially for 4, were expected to cause a strong in-
teraction between donor and acceptor units in the excited
state as well as in the ground state. Selected bond lengths
and dihedral angles for 1, 2, 3, and 4 are listed in Table 1.
In the molecular arrangements for six crystals of TFMAQ
derivatives, four kinds of packing modes were observed,
Generally, the crystal packing motifs for the aromatic com-
pounds were classified into the edge-to-face and the face-to-
face groups.[17] The edge-to-face motifs (herringbone family)
were classified into four structures: herringbone (HB), sand-
wich (SHB), b (b-HB), and g (g-HB) structure in HB by the
length of the shortest cell axis. The packing modes were
SHB for crystal GB of 2 and g-HB for 1 and crystal YG of
2. On the other hand, the face-to-face group was classified
into two types: the stacked type that contains the head-to-
tail and head-to-head structures, and the offset stacked type
ꢁ
the rotation of 738 in the C1 C5* distance of 3.66 ꢁ alter-
nately, whereas those for crystal G partially alternately over-
ꢁ
ꢁ
lapped at distances of 3.61 and 3.62 ꢁ (C7* C9** and C8
N1*, respectively; dashed lines in Figure 2c and d). The p–p
overlap of crystal G was better than that of crystal B. The
phenyl ring for crystals B and G had intra- and intercolumn
CH–p interaction from the neighboring molecules as indi-
cated by Hirshfeld surface analysis[18] (the dotted lines in
Figure 2c and d and Figure S3 in the Supporting Informa-
tion). The observed difference of crystal packing in poly-
morphs of 3 might also affect the solid-state emission.
The molecules of 4 in the stacked type were arranged
ꢁ
head-to-tail in parallel at distances of 3.60 and 3.61 ꢁ (C1
ꢁ
C6** and C7* C1**, respectively, and the dashed lines in
Figure 2e), alternately, to form a columnar structure with an
H-aggregate form (Figure 2e). The distance greater than
3.6 ꢁ suggested that the intermolecular interaction in the
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Trifluoromethylaminoquinoline Derivatives
FULL PAPER
Figure 2. Molecular packings with ball-and-stick model of crystal a) GB and b) YG for 2, crystals c) B and d) G for 3, and e) 4. In (a) and (d), gray and
black molecules are crystallographically independent. The dashed lines in (a) and (b) indicate hydrogen bonding. Dashed and dotted lines in (c), (d), and
(e) indicate p–p interactions and CH–p interactions, respectively. CF3 groups and hydrogen atoms are omitted for the sake of clarity.
solid-state emission for 4 might be weak. The intermolecular
nonpolar, medium, medium, and polar solvents, respectively,
are shown in Figure 3 for 1 and Figure S4 in the Supporting
Information for the others.
ꢁ
distances of quinoline–quinoline, and Nq Na for 1, 2 (crystal
GB and YG), 3 (crystal B and G), and 4 are summarized in
Table 1 together with space groups, Z values, and their se-
lected bond lengths and dihedral angles.
Absorption and emission properties: The absorption and
emission properties of 1, 2, 3, and 4 were investigated in sol-
ution by using various solvents and in their solid state. For
the fluorescence spectra, the absorption maxima (370–
400 nm) in the visible region were used as the wavelength
excitation.
In solution: Absorption and emission spectra of the solution
samples (100 and 10 mm, respectively) of 1, 2, 3, and 4 dis-
solved in various organic solvents (11 solvents) were meas-
ured. The steady-state absorption and emission spectra in n-
hexane, CHCl3, AcOEt, and DMSO, which were selected as
Figure 3. Absorption and emission spectra of 1 in n-hexane (black solid
line), chloroform (black dotted line), ethyl acetate (black dashed line),
and DMSO (gray solid line). Arrows in spectra indicate the vertical axes.
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N. Koga et al.
In the absorption spectrum of the solution samples in n-
hexane, TFMAQ derivatives showed absorptions in two re-
The fluorescence quantum yields, Ff, for the four fluoro-
phores also largely depended on the solvent polarity. The
values of Ff for 1 and 2, which were 0.23 and 0.27 in n-
hexane, 0.45 and 0.56 in CHCl3, and 0.61 and 0.65 in
AcOEt, respectively, increased as the solvent polarity in-
creased and abruptly decreased to 0.23 and 0.24 in a high-
polarity solvent, DMSO. Similarly, fluorophore 4 showed Ff
values of 0.60, 0.73, 0.60, and 0.16, in n-hexane, CHCl3,
AcOEt, and DMSO, respectively. In these three fluoro-
phores, the Ff values increased as the solvent polarity was
changed from nonpolar to medium polarity and then de-
creased at high polarity. In contrast, the solvent–polarity de-
pendence of Ff for 3 with the phenyl ring at the amino
group showed different behavior. The value of Ff for 3 was
0.48 in n-hexane, drastically decreased at medium polarity
(0.19 and 0.03 for CHCl3 and AcOEt, respectively), and
reached 0.01 in DMSO. The large reduction of the Ff value
observed in the polar solvents for 3 might be due to the ef-
fective transfer from the planar PICT state to the twisted
photoinduced charge-transfer (TPICT) state,[25] which led to
the nonradiation decay process. The observed polarity-de-
pendent emission behaviors were often observed in the
PICT fluorophores.[26] Furthermore, the Ff values in n-
hexane for 1 and 2 seemed to be somewhat low compared
to that for 4. This result might be due to a flip-flop motion
of the amino group, which induces the nonradiative decay
from the photoexcited state. A similar reduction of the Ff
value in n-hexane has been reported for the analogous 7-
aminocoumarin derivatives.[23] All data for the eleven sol-
vents are listed in Table S1 in the Supporting Information
with Df and Dn (nabsꢁnf).
gions: approximately 280 and 380–400 nm. In the latter
ab
max
region, the absorption maxima (l ) in n-hexane were ob-
served at 370, 385, 392, and 404 nm for 1, 2, 3, and 4, respec-
tively, and shifted to longer wavelength with increasing the
solvent polarity. The observed solvent dependence of the
ab
max
l
values for the TFMAQ derivatives indicated that the
electronic interaction between the trifluoromethylquinoline
(acceptor) and the amine (donor) units within the molecules
in the ground state was significant and the molecules were
effectively polarized.
In the emission spectra of the irradiation at the charge-
transfer (CT) band (ꢀ370 nm), TFMAQ derivatives inten-
sively emitted in the range of 400–600 nm. In nonpolar sol-
vent, n-hexane, which had no significant solute–solvent in-
teraction, 1, 2, 3, and 4, showed broad emissions centered at
418 (430, sh), 443, 446, and 452 nm (470 nm, sh), respective-
ly. The difference in the emission maximum (lfmax) was due
to the inductive effect of the substituents. The lfmax values
shifted to a longer wavelength (bathochromic shift) as the
polarity of the solvent increased. Large differences between
ab
max
l
and lfmax that were dependent on the solvent polarity
were observed in the range of 48–195 nm from n-hexane to
DMSO. The observed broad fluorescent emissions with
ab
large differences between l and lfmax and their solvent-po-
max
larity dependence indicated the CT character in the excited
state was more polarized than that in the ground state. To
estimate quantitatively the degree of charge separation in
the ground state and the excited state, the absorption and
emission spectra that showed solvatochromic shifts were an-
alyzed by the Lippert–Mataga model.[19] The Lippert–
Mataga equation [Eq. (1)], in which h, c, a, and Df are
Planckꢂs constant, the velocity of light, the Onsager
radius,[20] and the solvent-polarity function, respectively,
fitted the data to give the difference between the ground-
and excited-state dipole moments (Dm) of 8.3, 7.8, 12, and
9.6 D, respectively.
In the solid state: For the solid-state absorption and emission
spectra measurements, crushed crystals and thoroughly
ground crystals were used as crystal and powder samples.
Both samples were confirmed to be microcrystals by X-ray
diffraction (XRD) measurements, in which the XRD pat-
terns for both samples were consistent with those simulated
from the results of X-ray crystallography. The solid-state ab-
sorption spectra were obtained by measurements of the dif-
fractive reflection spectra followed by Kubelka–Munk con-
version.
Dn ¼ ð2Df=hca3ÞDm2 þ Dn0
ð1Þ
Onsager cavity (a) values of 4.8, 4.9, 5.3, and 5.0 ꢁ for 1, 2,
3, and 4, respectively, estimated by DFT calculation using
the B3LYP/6-31G(d) basis set, were used.[19a,21] The Lippert–
Mataga plots are shown in Figure S4’ in the Supporting In-
formation. The large Dm values indicated that the species in
the photoexcited state were largely polarized in the order of
1, 2, 4, and 3. It was noted that the photoexcited states for 3
with a phenyl group were considerably affected by the con-
tribution of the stabilization from the polar solvent relative
to those for the derivatives with methyl substituents. The ob-
tained value of 8.3 D for 1 was much larger than that for the
typical photoinduced intramolecular charge-transfer (PICT)
compound aminophthalimide (4.1 D),[22] and comparable to
those for analogous 7-aminocumarine (7.3 D)[23] and 6-pro-
pinyl-2-(dimethylamino)naphthalene (PRODN) (7–8 D).[24]
The absorptions for the powder and the emission spectra
for the crystal samples of 2 (crystal GB and YG) and 3
(crystal B and G) are shown in Figure 4, and those for 1 and
4 are shown in Figure S5 in the Supporting Information.
In the absorption spectra for the powder samples, the l
ab
max
values of the long-wavelength absorption, which were 398,
413 (414), 425 (427), and 434 nm for 1, 2, 3, and 4, respec-
ab
max
ab
max
ab
max
tively, showed a redshift Dl =l (c)ꢁl
(n-C6) of 27–
35 nm relative to the corresponding values in n-hexane, in
which the contribution of stabilization by the solvent was
negligible. The numbers in parentheses for 2 and 3 are those
for crystal YG and crystal G, respectively. The emission
maxima, lfmax(c), for the crystalline (c) samples were 467,
470 (530), 464 (506), and 507 nm for 1, 2, 3, and 4, respec-
tively. These values were largely different from those in n-
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Trifluoromethylaminoquinoline Derivatives
FULL PAPER
The large redshifts of lfmax(c) and reduction of Ff(c) ob-
served in the solid-state emission might be due to the inter-
molecular p–p interaction and the intermolecular hydrogen
bond. Although the p–p interaction at distances between
the quinoline rings greater than 3.6 ꢁ were limited,[27] the
p–p interaction was considered to stabilize the photoexcited
state and led to the redshift of lfmax(c) and the reduction of
Ff(c) in the solid state. In addition, the intermolecular hy-
drogen bonding might lead the nonradiative decay proc-
ess.[28] The molecular arrangement modes revealed by X-ray
crystallography were herringbone mode for 1 and 2, slipped-
parallel and slipped-columnar modes for crystal G and B, of
3, respectively, and a head-to-tail columnar mode for 4. The
herringbone, slipped-columnar, and slipped-parallel modes
showed partial overlapping of the quinoline rings with J-ag-
gregate form,[26,28] whereas the head-to-tail columnar mode
had a better overlap of the molecules with H-aggregate
form[27,30] and the p–p interaction was favorable.
In the parent fluorophore 1 with a herringbone structure
and an intermolecular hydrogen bond, the observed Dlfmax of
42 nm and DFf of 0.21 were large, thus indicating that the
solid-state emission was strongly affected by both the p–p
interaction[31] and the hydrogen bond. The effect of the in-
termolecular hydrogen bonding in addition to the p–p inter-
action was considered to contribute to the large reduction of
the intensity of the solid-state emission for 1.
Fluorophore 2 with a herringbone structure and an inter-
molecular hydrogen bond similar to those for 1 also showed
large redshift of Dlfmax and an intensity reduction of DFf.
The crystal polymorphs of 2, for which crystals GB and YG
have sp and ap rotamers, respectively, emitted different
colors at lfmax(c) of 470 and 530 nm, respectively. The ob-
served large difference (60 nm) of lfmax(c) between crystal
GB and crystal YG might be due to the difference in the in-
termolecular overlap of the quinoline plane (see Figure 2a
and b), in which those for YG were much better than those
Figure 4. Spectra of absorption for the powder and emission for the crys-
tal samples of a) 2 (crystal GB: solid line, and crystal YG: dashed line),
and b) 3 (crystal B: solid line, and crystal G: dashed line). Arrows in the
spectra indicate the vertical axes.
hexane and were redshifted Dlfmax =lmf ax(c)ꢁlfmax (n-C6) by
33–86 nm. Interestingly, the two crystal polymorphs ob-
tained in 2 and 3 emitted different colors by 60 and 42 nm,
respectively. The fluorescence intensities, Ff(c), for the crys-
talline samples of 1, 2, 3, and 4, were 0.02, 0.04 (0.01), 0.28
(0.28), and 0.21, respectively, which were substantially re-
duced relative to those in n-hexane. The differences in emis-
sion intensity, DFf =Ff
G
the crystalline state were 0.21, 0.23 (0.26), 0.20 (0.20), and
0.39 for 1, 2, 3, and 4, respectively. Furthermore, in 3 and 4,
reductions of the emission intensities by grinding the crystals
to a powder were observed. By grinding the crystal, notably,
the alteration of the emission for crystal B of 3 with the red-
ꢁ
for GB. In addition, the dihedral angle (q NCMeHa) for crys-
tal YG, which was 6.128 and affected the intramolecular CT
at both the ground and photoexcited states, was smaller
than those for crystal GB (19.2 and 23.18C). On the other
hand, the degree of reduction of the Ff values depended on
the hydrogen bond. The geometry of the hydrogen bond for
crystal YG, in which the angle of Nq-H-Na was nearly linear
(Nq-H-Na =1688), in addition to the short distance between
shift by 16 nm suggested a mechanochromic behavior. The
ab
values of l , lfmax, and Ff for the crystal and powder sam-
max
ples for 1, 2, 3, and 4 are summarized in Table 2 together
with the difference of those values between the crystal sam-
ples and the solution samples: Dlfmax and DFf.
Nq and Na (rN =3.13 ꢁ), was more favorable
ꢁ
Na
q
Table 2. Photophysical data for 1, 2, 3, and 4 in the solid state.[a]
than those for crystal GB (Nq-H-Na =146–1528 and
rN =3.22 and 3.25 ꢁ).
ꢁ
Na
q
1
2
3
4
Crystal polymorphs B and G for 3 had a slipped-
columnar and a slipped-parallel mode, respectively,
and those molecular arrangements caused the
solid-state emission to experience a large redshift:
Dlfmax =34 and 59 nm for crystals B and G, respec-
tively. Although 3 had a hydrogen atom at the
amine moiety, it had no intermolecular hydrogen
bond in the crystalline state. Therefore, the reduc-
tion in Ff values for 3 was not as large as those for
1 and 2. Crystal polymorphs (crystals B and G) for
GB
YG
B
G
ab
max
l
[nm]
P
398
28
467
472
42
0.02
0.03
0.21
413
28
470
476
33
0.04
0.04
0.23
414
29
530
529
86
0.01
0.02
0.26
425
33
464
480
34
0.28
0.07
0.20
427
35
506
505
59
0.28
0.13
0.20
434
27
507
504
35
0.21
0.13
0.39
ab
Dl [nm]
max
lfmax(c) [nm]
lfmax(p) [nm]
Dlfmax [nm]
Ff(c)
C
P
C
P
Ff(p)
DFf
[a] P: powder sample. C: crystal sample; Dlfmax =lmf ax(c)ꢁlmf ax
(nꢁC6) and DFf =Ff-
N
(nꢁC6)ꢁFf(c).
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15043

N. Koga et al.
3 also emitted different colors at 464 and 506 nm, respec-
tively. The difference observed in the emitted colors might
be due to the molecular arrangements. As shown in Fig-
ure 2c and d, the overlap of the quinoline planes for crystal
G was better than those for B. In addition, the dihedral
tion). The observed changes of the emission spectra after
heating suggested that crystal GB for 2 and B for 3 were
converted to crystals YG and G, respectively, by heating
below melting point and then returned to the starting crys-
tals GB and B, respectively, by heating over melting point
and then cooling. The observed thermal emission color
changes could be reproduced by the heating/melting/cooling
cycle.
ꢁ
angles (q NCPhHa) of 10.9 and 11.78 for crystal G were
small compared to that (17.98) for crystal B, which might
have contributed to the difference in the redshift of the
emission.
In the solid-state emission of 4, the emission maximum
shifted to longer wavelength by Dlfmax =35 nm, and the in-
tensity drastically reduced by DFf =0.39. The difference in
the emission properties from those in n-hexane might be
due to the molecular arrangement (Figure 2e). The mole-
cules in 4 were layered in a head-to-tail columnar mode (H-
aggregate form),[27,30] which causes the donor–acceptor type
of intermolecular p–p interaction, this leading to the red-
shift and the nonradiative decay process.
DSC measurements: To investigate the thermal properties of
the crystal polymorphs of 2 and 3, differential scanning calo-
rimetry (DSC) was conducted over the temperature range
of 25–1708C during three cycles of heating and cooling (a
rate of 4 and 108Cminꢁ1 for heating and cooling, respective-
ly). Roughly crushed crystals and thoroughly ground crystals
were used as the crystal and powder samples for DSC meas-
urements.
The DSC curves obtained for the crystal samples of 2
with heating and cooling rates of 4 and 108Cminꢁ1, respec-
tively, are shown in Figure 6. In 2, the same endothermic
peaks at 1378C for crystal GB and YG were observed in the
heating process, whereas exothermic peaks at 106–107 and
108–1098C for crystal GB and YG, respectively, were ob-
served in the cooling process. In addition, crystal GB
showed a weak exothermic peak in the temperature range
of 100–1208C during heating in the three cycles. In contrast,
the additional peak for crystal YG was observed only in the
second and the third cycles. The additional exothermic
Thermal emission properties: The polymorphs of 2 and 3
showed interesting emission behaviors in response to heat.
When the powder sample of crystal GB for 2 was heated at
approximately 908C below the melting point (1388C), the
greenish blue emissions (lfmax =470 nm) changed to yellowish
green (530 nm). The obtained emission spectrum after heat-
ing was similar to that for crystal YG. Subsequently, when
the sample was heated to a temperature over melting point
and then cooled, it quickly crystallized. The obtained micro-
crystals had greenish blue emissions and the emission spec-
trum was similar to that for crystal GB. The emission spec-
tra for 2 at 258C before and after heating at 90 and 1408C
are shown in Figure 5 together with photographs.
Figure 5. Top: Emission spectra at 258C for the powder sample of crystal
GB for 2; before heating (solid line), after heating at 908C (dashed line),
and after heating at 1408C (dotted line). Bottom: photographs of the
emission color change.
Figure 6. DSC curves for a) crystal GB and b) crystal YG of 2 over three
cycles: first (solid line), second (dotted line), and third cycle (dashed
line). Insets show the expanded temperature region in which the exother-
mic peak appears. Temperature sweep rate: 4 and 108Cminꢁ1 for heating
and cooling, respectively.
The sample of crystal B for 3 also showed thermal spec-
tral alteration similar to those for crystal GB for 2 by heat-
ing at 110 and 1408C (Figure S6 in the Supporting Informa-
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Trifluoromethylaminoquinoline Derivatives
FULL PAPER
peaks observed at 100–1208C might be due to the crystal-to-
crystal phase transition. Accordingly, the observed DSC re-
sults for 2 would suggest that crystal GB was converted to
crystal YG in the heating process and returned to crystal
GB in the cooling process after melting at the temperature
of 1378C, whereas crystal YG melted at 1378C and convert-
ed to crystal GB during the cooling process.
DSC experiments for the crystal samples of 3 under the
same temperature sweep rate, on the other hand, showed no
weak exothermic peak as observed in 2. The DSC curves for
3 depended on the sample form (crystal or powder) and the
heating and cooling sweep rate. When the powder samples
and the temperature sweep rates of 1 and 308Cminꢁ1 for
heating and cooling, respectively, were used, DSC curves
that showed weak exothermic peaks due to the phase transi-
tion were obtained. The DSC curves of crystal B and G are
shown in Figure S7 in the Supporting Information. The same
endothermic peaks at 1268C for crystals B and G were ob-
served during the heating process, whereas exothermic
peaks at 92–97 and 97–1028C for crystals B and G, respec-
tively, were observed in the cooling process. In addition,
crystal B showed a weak broad exothermic peak at the tem-
perature of approximately 1108C upon heating in the three
cycles. In contrast, the additional broad peak for crystal G
was observed only in the second and the third cycles. These
DSC curves suggested the transformation from crystal B to
G at approximately 1108C and the crystallization of crystal
B after melting and then cooling.
The observation of exothermic peaks at the temperature
below melting point might indicate that crystals YG for 2
and G for 3 were thermodynamically more stable than the
corresponding polymorphs. Furthermore, the results of DSC
experiments suggested that the crystal transformation in 3
took more time than 2.
Figure 7. Changes in the XRD spectra at 258C for a) crystal GB for 2
and b) crystal B for 3 under various conditions as follows: i) powder
sample; ii) the sample after heating at 908C for 2 and 1108C for 3 for
10 min; iii) the sample after heating at 1408C (over m.p.) and then cooled
down to 258C; and iv) powder sample of the corresponding polymor-
phous crystal (crystal YG and G).
XRD measurements: To confirm the thermal transformation,
the XRD patterns after heating the two crystal polymorphs
in 2 and 3 were investigated. The samples for XRD meas-
urements were prepared as follows. The powder samples
were placed on cover glasses and heated on a heating block
for melting-point measurements. The XRD measurements
were carried out in the order of (1) before heating, (2) after
heating below melting point (90 and 1108C for 2 and 3, re-
spectively), and then (3) after heating above melting point
and rapid cooling. XRD patterns for 2 and 3 before and
after heating are shown in Figure 7.
Before heating, all diffraction peaks for crystals GB and
YG for 2 and crystals B and G for 3 (Figure 7i and iv) were
consistent with those simulated from the results of X-ray
analysis. After heating the powder samples of crystal GB of
2 at 908C and crystal B of 3 at 1108C, the XRD patterns
drastically changed (Figure 7ii) in a way that was consistent
with those (Figure 7iv) for the corresponding polymorphs,
crystals YG and G, respectively. Consequently, heating at
the temperature over melting point, followed by rapid cool-
ing, gave the patterns (Figure 7iii) for the starting crystals.
These temperature-dependent XRD patterns clearly indicat-
ed that crystal GB for 2 and B for 3 were converted to crys-
tal YG and G by heating below melting point, and then re-
turned to the starting crystal GB and B after melting and
rapid cooling, respectively. Furthermore, the dependence of
the formation ratio of crystal B and G on cooling speed
after melting of 3, suggested by DSC experiments, was in-
vestigated. The XRD patterns after melting and slow cool-
ing were confirmed to be due to the mixture of crystal B
and G (Figure S8b in the Supporting Information). The
rapid cooling predominantly produced crystal B and the for-
mation ratio of crystal G increased as cooling rate de-
creased.
In the plate-shaped single crystals for GB of 2, the direc-
tion of the thickness was the c axis, which was determined
by X-ray crystallography. The alterations of the c axis in the
direction of the thickness were investigated by XRD pat-
terns before and after heating at the temperature below
melting point (Figure S8a in the Supporting Information).
The comparison of XRD patterns before and after heating
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15045

N. Koga et al.
with the simulation patterns on the basis of the results of X-
ray crystallography indicated that the c axis of crystal GB
corresponded to the c axis of crystal YG after transforma-
tion.
formation), the emission color change did not begin to take
place at once over the entirety of the crystal. Rather, the
green emission appeared at certain points, which might have
been defects of the crystal structure, and gradually expanded
from there. In the case of crystals GB and crystal B in the
video, it took approximately 17 and 57 min to change the
color entirely. The long transformation time for B was con-
sistent with the results of DSC experiments.
Single-crystal-to-single-crystal interconversion: Strikingly,
thermal transformations from crystals GB to YG for 2 and
from B to G for 3 were observed without the destruction of
single crystals. Single crystals of crystal GB for 2 and crystal
B for 3 were heated at 90 and 1108C, respectively, on a hot
plate attached to a temperature controller and the thermal
emission color change was recorded on video under a micro-
scope. Photographs of crystal GB for 2 and crystal B for 3
under heating are shown in Figure 8, and the movies are
available in the Supporting Information.
To confirm the single-crystal-to-single-crystal (SC-to-SC)
transformation, X-ray crystallography for samples obtained
after heating was carried out where possible. After heating
crystal GB for 2 at 908C, a crystal sample of the thin plate
was shattered in one direction (along the a axis) to afford
the small pieces of needle crystals. The obtained crystal was
measured by X-ray crystallography, and the long axis in the
crystal was determined to be the a axis (Figure S9 in the
Supporting Information). Since the long axis of crystal YG
was the a axis, this result in addition to the one in XRD ex-
periments might suggest that each a, b, and c axis for crystal
GB was consistent with those for crystal YG. However, a
single crystal of B of 3 after heating at 1108C was cut to
remove the unchanged parts by using the blue emission as
visual guide, and the obtained piece (crystal B’) in 0.2ꢃ0.1ꢃ
0.05 mm was used as the sample for X-ray crystallography.
The resulting crystallographic data for crystal B’ are sum-
marized in Table S2 together with those for starting crystal
B and crystal G. Although the R1 value was somehow large
(0.10), the X-ray crystallographic parameters for crystal B’
were in good agreement with those for crystal G, which
strongly supports the notion that the thermal transformation
from crystal B to crystal G took place in the single-crystal-
line state without destruction of the bulk crystal structure.
Conclusion
2,4-Di(trifluoromethyl)quinolines with amine (1), methyla-
mine (2), phenylamine (3), and dimethylamine (4) at the 7-
position were prepared as new fluorophores. Six crystals in-
cluding the crystal polymorphs of 2 (crystal GB and YG)
and 3 (crystal B and G) were obtained and their molecular
arrangements in the crystal packings were classified into
four modes: herringbone modes for 1 and 2, slipped-parallel
and slipped-columnar modes for crystal G and B, respective-
ly, of 3, and a head-to-tail columnar mode for 4. It is worth
noting that the polymorphs, crystal GB, and crystal YG in 2,
were rotational isomers, synperiplanar configurations, and
antiperiplanar configurations, respectively. In solution, four
fluorophores intensively emitted and their emitting proper-
ties strongly depended on the solvent polarity. The values of
fluorescence wavelength, lfmax, and intensity, Ff, increased
and decreased, respectively, with increasing solvent polarity.
The observed strong solvatochromism with Dm values in the
range of 7.8–14 D indicated that the emissions were due to a
photoinduced intramolecular charge-transfer (PICT) mecha-
nism. In the crystalline state, their emission exhibited a red-
shift by 33–86 nm relative to those in a solution of n-hexane,
Figure 8. Time dependence of the views of a) crystal GB of 2 at 908C and
b) crystal B of 3 at 1108C. Top: irradiation at 365 nm. Bottom: Under
ambient lighting. Scale bar beside 0 min indicates 1 mm.
The emission colors for the single crystals slowly changed
with heating, and the blue emissions for crystals GB and B
gradually turned to yellowish green and green, respectively.
As shown in Figure 8 and the movie (in the Supporting In-
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Trifluoromethylaminoquinoline Derivatives
FULL PAPER
8.1, 2.7 Hz, 1H), 7.28 (d, J=2.7 Hz, 1H), 3.17 ppm (s, 6H); FAB MS (in
m-NBA matrix): m/z: 308 [M]+; elemental analysis calcd (%) for
C13H10N2F6: C 50.62, H 3.30, N 9.11; found: C 50.66, H 3.27, N 9.09.
whereas the intensity of the emission was reduced (DFf =
0.2–0.39). The large reduction of the Ff value for 1 and 2
might be due to intermolecular hydrogen bonds in addition
to the p–p interactions. Interestingly, the polymorphic crys-
tals, crystal GB and YG for 2 and crystal B and G for 3,
emitted different colors with Dlfmax =60 and 42 nm, respec-
tively. In the solid-state emission, polymorphic crystals of 2
and 3 showed thermal responsiveness. The emitted blue
color of GB for 2 and B for 3 turned green at temperatures
below melting point and returned to blue after melting and
then rapid cooling. The observed emitted color changes
were followed by XRD measurements and were found to
accompany the SC-to-SC transformations from crystal GB
to YG for 2 and crystal B to G for 3. In particular, thermal
SC-to-SC transformation for 3 was confirmed by single-crys-
tal X-ray crystallography.
In this study, thermal SC-to-SC transformation between
the crystal polymorphs that emitted different colors were
successfully characterized by X-ray crystallography. Howev-
er, the observed thermal emission color changes were irre-
versible. Attempts are in progress to prepare a molecule
that exhibits reversible SC-to-SC interconversions in re-
sponse to other external stimuli.
Compound 3: Compound 1 (1.0 g, 3.6 mmol), 1,1’-bis(diphenylphosphi-
no)ferrocene, dppf (0.3 g, 0.54 mmol), [PdCl2ACHTNUTRGNE(UGN dppf)CH2Cl2] (0.15 g,
0.18 mmol), and potassium tert-butoxide (0.44 g, 4.0 mmol) were dis-
solved in degassed toluene (5 mL) in a Schlenk flask. Bromobenzene
(0.75 mL, 7.1 mmol) was added and then heated at 1008C for 6 h under a
nitrogen atmosphere. The reaction mixture was poured into water and
extracted three times with ethyl acetate (50 mL). The organic layers were
combined, dried over MgSO4, and evaporated under reduced pressure.
The crude residue was subjected to chromatography on silica gel by using
n-hexane/ethyl acetate (100:1) as eluent to afford a pale yellow solid
(0.91 g, 2.56 mmol) in 72% yield. Crystals B and G of 3 were obtained
from Et2O/n-hexane (10:3) at 48C and Et2O/n-hexane (1:10) at 208C, re-
spectively. M.p. 121–1228C and 124–1258C for crystal B and G, respec-
tively; 1H NMR (270 MHz, CD2Cl2): d=8.06 (d, J=9.4 Hz, 1H), 7.75 (s,
1H), 7.71 (d, J=2.7 Hz, 1H), 7.47 (dd, J=9.4, 2.7 Hz, 1H),7.39 (t, J=
7.4 Hz, 2H), 7.31 (d, J=7.4 Hz, 2H), 7.18 (t, J=7.4 Hz, 1H), 6.39 ppm (s,
1H); IR (KBr disk): n˜ =3397 cmꢁ1 (nNH) for crystal B; 3439, 3408, and
3397 cmꢁ1 (nNH) for crystal G; FAB MS (in m-NBA matrix): m/z: 357
[M+H]+; elemental analysis calcd (%) for C17H10N2F6: C 57.31, H 2.83,
N 7.86; found: C57.31, H 2.85, N 7.92.
CCDC-831845 (1); CCDC-831846 (GB of 2), CCDC-831847 (YG of 2),
CCDC-831849 (B of 3), CCDC-831850 (G of 3), and CCDC-831851 (4)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Experimental Section
The X-ray crystallography, differential scanning calorimetry (DSC), and
X-ray diffraction (XRD) measurements, as well as the absorption and
emission spectra are described in the Supporting Information.
Acknowledgements
This work was partially supported by the Iketani Science and Technology
Foundation (S. K.).
Compound 1: Hexafluoroacetylacetone (2.35 g, 11.35 mmol) was added
to a solution of m-phenylenediamine (1.0 g, 9.26 mmol) and Montmoril-
lontite K10 (1.5 g) dissolved in toluene (30 mL) with heating. The solu-
tion was heated at 908C for 1 h. The hot solution was filtered and the fil-
trate was condensed to approximately 10 mL under reduced pressure.
After cooling, the resulting precipitate was collected by filtration.
TFMAQ was obtained as a yellow solid (2.5 g, 8.93 mmol) in 96% yield.
The crystal of 1 was obtained by recrystallization from Et2O/n-hexane.
M.p. 187–1888C; 1H NMR (270 MHz, CDCl3): d=8.00 (d, J=8.7 Hz,
1H), 7.71 (s, 1H), 7.37 (d, J=2.0 Hz, 1H), 7.21 (d, J=2.0, 8.7 Hz, 1H),
4.34 ppm (s, 2H); FAB MS (in m-NBA matrix): m/z: 281 [M+H]+; IR
(KBr disk): n˜ =3333 cmꢁ1 (nNH); elemental analysis calcd (%) for
C11H6N2F6: C 47.16, H 2.16, N 10.00; found: C 47.33, H 2.13, N 9.87.
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Compounds 2 and 4: Methyl iodide (0.64 mL, 10 mmol) was added to a
solution of
1 (0.28 g, 1.0 mmol) and potassium carbonate (0.10 g,
0.72 mmol) in acetonitrile (4 mL). The solution was heated at 608C for
20 h. The reaction mixture was poured into water and extracted three
times with ethyl acetate (60 mL). Organic layers were combined, dried
over MgSO4, and evaporated under reduced pressure. Crude residue was
subjected to chromatography on silica gel by using n-hexane/ethyl acetate
(10:1–5:1) as eluents to separately afford pale yellow solids 2 (0.12 g,
0.41 mmol) and 4 (0.12, 0.39 mmol) in 41 and 39% yield, respectively.
Compound 2 was crystallized from Et2O/n-hexane (10:3) at 48C and
from CHCl3/n-hexane (1:10) at 208C to afford the crystal polymorphs,
crystal GB and crystal YG, respectively. The crystal of 4 was obtained by
the recrystallization from Et2O/n-hexane (10:3). Compound 2: M.p. 137–
1388C; 1H NMR (270 MHz, CDCl3): d=7.94 (d, J=8.1 Hz, 1H), 7.67 (s,
1H), 7.19 (d, J=2.7 Hz, 1H), 7.13 (dd, J=8.1, 2.7 Hz, 1H), 4.44 (s, 1H),
3.00 ppm (s, 3H); FAB MS (in m-NBA matrix): m/z: 294 [M]+; IR (KBr
disk): n˜ =3384, 3328 cmꢁ1 (nNH) for crystals GB and YG, respectively; ele-
mental analysis calcd (%) for C12H8N2F6: C 49.06, H 2.68, N 9.53; found:
C
48.99,
H 2.74, N
9.52. Compound 4: M.p. 108–1098C; 1H NMR
(270 MHz, CDCl3): d=8.02 (d, J=8.1 Hz, 1H), 7.64 (s, 1H), 7.38 (dd, J=
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Received: April 10, 2012
Revised: July 10, 2012
Published online: October 2, 2012
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