Switching of polymorph-dependent ESIPT luminescence of an imidazo[1,2-a]pyridine derivative

Article abstract of DOI:10.1002/anie.200803975

(Figure Presented) Reproducible switching of the polymorph-dependent excited-state intramolecular proton-transfer (ESIPT) luminescence of an imidazo[1,2-a]pyridine between blue-green and yellow (see picture) is achieved by thermal control of its solid-sta

Full text of DOI:10.1002/anie.200803975

Communications
DOI: 10.1002/anie.200803975
Luminescence Switching
Switching of Polymorph-Dependent ESIPT Luminescence of an
Imidazo[1,2-a]pyridine Derivative**
Toshiki Mutai,* Haruhiko Tomoda, Tatsuya Ohkawa, Yuji Yabe, and Koji Araki*
Organic solid-state luminescent materials have been attract-
ing considerable interest in various fields of application.[1]
Switching and tuning of solid-state luminescence properties,
particularly by controlling the mode of molecular packing
instead of chemical alteration of the molecules, is one of the
recent targets of both fundamental research and practical
applications. An increasing number of reports on this
subject[2] has appeared recently. However, successful exam-
ples of achieving efficient switching of solid-state lumines-
cence by means of a dry process are rather limited.[3,4] One
reason for this difficulty may be the absence of an effective
mechanism to transform the alteration of molecular packing
into different luminescence properties. Up to now, successful
systems have generally adopted transformation between the
monomeric and dimeric/excimeric states as the mechanism of
packing-induced emission switching.[3]
Figure 1. a) Absorption and fluorescence spectra of 1 in THF solution
Here we report a novel and effective mechanism of
switching organic solid-state luminescence based on an
excited-state intramolecular proton-transfer (ESIPT)[5] pro-
at room temperature (dashed line) and 77 K (solid line). b) Absorption
and luminescence spectra of polymorph BG (dotted line) and Y (solid
line). Inset: Pictures of both polymorphs under UV irradiation
(365 nm).
cess.
Two
crystal
polymorphs
of
2-(2’-hydroxy-
phenyl)imidazo[1,2-a]pyridine (1; Scheme 1) exhibit bright
photoluminescence of different colors—blue-green and
yellow—both of which are due to ESIPT (Figure 1). X-ray
crystallographic analysis showed different molecular confor-
mations and modes of packing in these two crystal poly-
morphs, and interconversion of the polymorphs by a thermal
dry process successfully and repeatedly realized switching of
ESIPT luminescence.
Slow and fast cooling of a hot solution of 1 in aqueous
ethanol gave crystal polymorphs BG and Y, respectively. In
À1
À
the IR spectra, an O H stretching band at around 3135 cm
for both crystals indicated formation of intramolecular
hydrogen bonds between the OH group and nitrogen atom
N1 in the imidazopyridine ring.
On excitation at 330 nm, 1 showed weak fluorescence
(quantum yield F = 0.01–0.08) in the visible region in dilute
solution: blue fluorescence at 375 nm in protic ethanol,
orange fluorescence with a large Stokes shift at 567 nm in
nonpolar cyclohexane, and both blue and orange fluorescence
at 377 and 602 nm in THF (Table 1). Figure 1a shows the
emission spectra of 1 in THF solution. In contrast, the 2’-
methoxy derivative 2, which does not form an intramolecular
hydrogen bond, shows only blue fluorescence, regardless of
the solvent. These results indicated that the blue fluorescence
is direct emission from a locally excited state, and the orange
fluorescence is the ESIPT fluorescence from 1a (Figure 1a).
These assignments are in good agreement with those reported
previously by Douhal et al.,[6] who also suggested that the
significant Stokes shift of the orange emission is due to
solvent rearrangement and/or further conformational change
of a zwitterionic ESIPT state.
Scheme 1. Molecular structures of 1, 1a, and 2.
[*] Dr. T. Mutai, T. Ohkawa, Prof. Dr. K. Araki
Institute of Industrial Science, The University of Tokyo
4-6-1, Komaba, Meguro-ku, Tokyo 153-8505 (Japan)
Fax: (+81)3-5452-6364
E-mail: araki@iis.u-tokyo.ac.jp
mutai@iis.u-tokyo.ac.jp
Prof. Dr. H. Tomoda, Y. Yabe
Department of Applied Chemistry, Shibaura Institute of Technology,
Toyosu, Kouto-ku, Tokyo 108-8548 (Japan)
[**] This work was supported by a Grant-in-Aid (No. 18310076,
18710077, and 20510094) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan. ESIPT=excited-state
intramolecular proton-transfer.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803975.
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Chemie
À
Table 1: Absorption and emission of 1 and 2 in various states.
an O H···N1 intramolecular hydrogen bond (Figures S3 and
S4 in Supporting Information). The most notable conforma-
tional difference is the N1-C2-C1’-C2’ dihedral angle q
between the phenyl and imidazopyridine rings. While the
two aromatic rings are nearly coplanar (q = À1.0 and 1.38) in
polymorph Y, the conformation of 1 is twisted in polymorph
BG (q = 5.88). The distances O···N1 representative of the
length of the intramolecular hydrogen bond showed only a
slight difference (less than 0.02 ꢀ).
In frozen solutions, amorphous polymer matrices, or pure
amorphous solids, where no organized molecular assembly is
present, substrates tend to be in energetically stable con-
formation on average. Geometry optimization by HF/6-
31G(d) calculation indicated a coplanar conformation as the
lowest energy structure.
labs [nm]
lem [nm] (F)
t [ns]
1
THF (RT)
332, 346
377 (0.08)
602 (0.02)
370
2.63
0.52
2.69
5.26
5.91
5.84
6.39
THF (77 K)
332[a], 349[a]
521
polymorph BG
polymorph Y
amorphous
339[b]
337[b]
337[b]
496 (0.50)
529 (0.37)
527 (0.39)
2
THF (RT)
THF (77 K)
powder
332
378 (0.11)
348, 366, 383
382 (0.24)
2.56
2.81
2.75
329[a], 344[a]
312[b]
[a] Excitation spectrum. [b] Kubelka–Munk spectrum.
In polymorph Y, the molecules of 1 packed in the
structure very close to the coplanar conformation. Since
these solids showed identical yellow ESIPT luminescence, the
most stable conformation of 1 is likely to be the coplanar
structure. On the other hand, that blue-green ESIPT lumi-
nescence is emitted only from polymorph BG suggests a
substantial effect of the mode of the molecular packing. As
the molecular geometry is fixed in a twisted conformation in
polymorph BG, this packing-induced small deviation of the
molecular conformation may affect the ESIPT process and
lead to higher-energy ESIPT luminescence.
We then examined the thermal properties of both
polymorphs. In the first heating curve of differential scanning
calorimetry (DSC), polymorph BG showed an endothermic
peak at the melting point (1418C). On the other hand, both
polymorph Y and the amorphous solid displayed a small peak
at around 1318C in addition to the melting peak at 1418C
(Figure S5 in Supporting Information), and the former was
found to correspond to the phase transition to polymorph BG.
When powdered crystals of polymorph Y or the amorphous
solid were heated at 1358C for 1 min on a glass plate, the
XRD pattern of the heated powder (Figure 2d) was different
from that of starting polymorph Y (Figure 2c) or the
amorphous solid and was comparable to the pattern of
polymorph BG (Figure 2a). Accordingly, the luminescence
turned from yellow to blue-green. The resultant blue-green-
emitting powder was then melted and kept at 1508C for 1 min.
Quick cooling of the heated glass plate by ice cubes yielded a
yellow-emitting solid. The XRD pattern showed only a vague
diffraction image, that is the major state of the solid was
amorphous.
When solutions of 1 and 2 in THF were frozen at 77 K, the
fine-structured blue fluorescence from the locally excited
state remained unchanged, but the ESIPT fluorescence of 1
shifted to 521 nm. Since this dual blue and yellow emission of
1 was also observed in a polystyrene matrix at room temper-
ature, the shift of the ESIPT fluorescence of 1 from orange to
yellow in rigid matrices is ascribed to suppression of the
stabilization of the excited zwitterionic species through
solvent rearrangement and/or further conformational
changes of the substrate. A similar color change of the
ESIPT fluorescence of 1 from orange to yellow in poly-
(methyl methacrylate) was also reported.[6b]
Absorption bands of both crystal polymorphs appeared at
around 340 nm, close to that observed in a dilute THF
solution (Figure 1b). On excitation at 330 nm, the BG and Y
polymorphs exhibited blue-green (496 nm) and yellow lumi-
nescence (529 nm), respectively, with considerable quantum
yield (F = 0.37–0.50). Amorphous solid 1, prepared by the
freeze-drying method, showed a yellow luminescence similar
to that of polymorph Y. Though their luminescence colors
were different, their Stokes shifts of 9300–10800 cmÀ1 were
large enough to be well within the range of typical ESIPT
fluorescence.[5] Since the structureless spectral shape and
luminescence lifetime of the crystalline and amorphous solids
of 1 were essentially identical to those observed in the frozen
dilute solution, the observed yellow and blue-green lumines-
cence of the solids with significant Stokes shifts were assigned
as ESIPT luminescence. Thus, ESIPT luminescence is sensi-
tive to the environment, not only to the type of solvent and
matrix rigidity, but also to the mode of molecular packing in
the solid state. On the other hand, the fine-structured blue
luminescence (ca. 380 nm) emitted directly from the locally
excited state was little affected by the environment, either in
solution or in the solid. Moreover, the ESIPT luminescence is
a monomeric emission, since no apparent difference was
observed between the diluted and condensed states. To
understand the role of the molecular packing in the solid-
state luminescence of 1, X-ray crystallographic analyses of
crystal polymorphs BG (Pbca, Z = 8) and Y (P21/c, Z = 8)
were performed at room temperature.[7] The asymmetric unit
is composed of one and two molecules in polymorphs BG and
Y, respectively, and each molecule clearly shows formation of
Based on the above findings, we examined the switching
ability of the polymorph-dependent luminescence of 1.
Starting from polymorph Y, powdered crystals were kept at
1358C for 1 min to convert them to the blue-green-emitting
state. They were then heated at 1508C for 3 min and rapidly
cooled to give the yellow-emitting state. Performing the
switching cycle six times without deterioration (Figure 3)
demonstrated that the color of the efficient luminescence of 1
is switched easily and reproducibly by the thermal dry
process.
In summary, reproducible switching of organic solid-state
luminescence was achieved on the basis of the highly efficient
ESIPT luminescence of 1 (F = 0.37–0.50) by thermal control
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Communications
Keywords: hydrogen bonds · luminescence ·
.
nitrogen heterocycles · polymorphism · solid-state structures
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Figure 2. XRD profiles of crystal polymorphs BG (a) and Y (b)
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Figure 3. Plot of the intensity ratio of yellow and blue-green lumines-
cence (I527/I496) against the number of switching cycles.
of the molecular packing. The clear switching of the
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[7] a) Crystallographic data for polymorph BG: C13H10N2O, M =
210.23, orthorhombic, Pbca, a = 7.6560(2), b = 12.1630(3), c =
22.2380(5) ꢀ, V= 2070.80(9) ꢀ3, Z = 8, 1calcd = 1.349 gcmÀ3, T=
298(2) K, 18651 measured and 2030 independent reflections, R1 =
0.0548, wR2 = 0.1297, GOF = 1.104; b) crystallographic data for
polymorph Y: C13H10N2O, M = 210.23, monoclinic, P21/c, a =
18.2400(4), b = 5.77300(10), c = 20.3010(5) ꢀ, b = 108.1650(10)8,
V= 2031.15(8) ꢀ3, Z = 8, 1calcd = 1.375 gcmÀ3
,
T= 298(2) K,
18482 measured and 3733 independent reflections, R1 = 0.0624,
wR2 = 0.1732, GOF = 1.116. CCDC 698144 (polymorph BG) and
698145 (polymorph Y) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Received: August 12, 2008
Published online: October 29, 2008
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