Tuning of excited-state intramolecular proton transfer (ESIPT) fluorescence of imidazo[1,2-a]pyridine in rigid matrices by substitution effect

Article abstract of DOI:10.1021/jo302711t

2-(2′-Hydroxyphenyl)imidazo[1,2-a]pyridine (HPIP, 1) and its derivatives are synthesized, and their fluorescence properties are studied. Although all the compounds show faint dual emission (Φ ≈ 0.01), which is assigned to the normal and excited-state intr

Full text of DOI:10.1021/jo302711t

Article
pubs.acs.org/joc
Tuning of Excited-State Intramolecular Proton Transfer (ESIPT)
Fluorescence of Imidazo[1,2‑a]pyridine in Rigid Matrices by
Substitution Effect
Toshiki Mutai,* Hirotaka Sawatani, Toshihide Shida, Hideaki Shono, and Koji Araki*
Department of Materials and Environmental Science, Institute of Industrial Science, The University of Tokyo, 4-6-1, Komaba,
Meguro-ku, Tokyo 153-8505, Japan
S
* Supporting Information
ABSTRACT: 2-(2′-Hydroxyphenyl)imidazo[1,2-a]pyridine
(HPIP, 1) and its derivatives are synthesized, and their
fluorescence properties are studied. Although all the
compounds show faint dual emission (Φ ≈ 0.01), which is
assigned to the normal and excited-state intramolecular proton
transfer (ESIPT) fluorescence in a fluid solution, they generally
display efficient ESIPT fluorescence (Φ up to 0.6) in a
polymer matrix. The introduction of electron-donating and
electron-withdrawing groups into the phenyl ring causes blue
and red shifts of the ESIPT fluorescence emission band,
respectively. On the other hand, the introduction of such
groups into the imidazopyridine part results in fluorescence
shifts in the opposite directions. The results of ab initio quantum chemical calculations of the intramolecular proton-transferred
(IPT) state are well in line with the ESIPT fluorescence energies. The plots of the calculated highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels against the Hammett substituent constants (σ) show
good linearity with different slopes, which can rationalize the effect of the substituent and its position on the IPT state. Therefore,
we have developed a series of HPIPs as new ESIPT fluorescent compounds and demonstrate that ESIPT fluorescence properties
would be rationally tuned using quantum chemical methods.
To date, the most studied compounds, because of their efficient
ESIPT fluorescence, are 2-(2′-hydroxyphenyl)benzimidazole
(HBI),6 2-(2′-hydroxyphenyl)benzoxazole (HBO),7 2-(2′-
hydroxyphenyl)benzothiazole (HBT)8 (Scheme 1b), and their
analogues.
INTRODUCTION
■
Organic solid-state luminescent materials have been attracting
considerable attention because they have various applications.1
Whereas solid-state luminescent materials are generally
designed from the compounds that exhibit efficient fluores-
cence in solution, it is well-known that luminescence is
quenched in the solid state as a result of intermolecular
interactions, which enhance nonradiative deactivation of the
excited state.
In recent years, there has been growing interest in organic
compounds that are nonemissive in the solution state but
efficiently luminescent in a rigid environment.2 In general, such
compounds have two or more aromatic groups connected by a
single bond. Enhanced emission is caused by restricting the
intramolecular rotation around the single bond. These
compounds would be good candidates for new solid-state
luminophores, since they have not been considered as light
emitters.
A photoinduced proton transfer through an intramolecular
hydrogen bond is termed excited-state intramolecular proton
transfer (ESIPT),3 which is a remarkably fast process (rate
constant k ≈ 1 × 1013 s−1; Scheme 1a).4 Emission from the
ESIPT state5 is characterized by a large Stokes shift (≈ 10 000
cm−1), which enables long-wavelength fluorescence by UV
excitation, but the fluorescence quantum yield is generally low.
Luminescent behavior in the solid state is a major current
topic,9 and there have been some reports of solid-state ESIPT
luminescence6a,8a,10,11 and basic application to electrolumines-
cent devices8a,12,13 and white luminescent materials.13,14
However, the development of such systems remains a challenge
because compounds exhibiting efficient solid-state ESIPT
luminescence have been limited to date. In addition, it is
generally difficult to tune known fluorophores to give desirable
photophysical properties by chemical modification, because a
small structural alteration often impairs their fluorescence
properties.
We have previously reported11 that 2-(2′-hydroxyphenyl)-
imidazo[1,2-a]pyridine (HPIP, 1, Scheme 1c) exhibits efficient
ESIPT luminescence in the solid state, and we also found that 1
forms two crystal polymorphs that exhibit bright ESIPT
luminescences of different colors, namely, blue-green (496
nm, Φ = 0.50) and yellow (529 nm, Φ = 0.37). However, there
Received: December 13, 2012
© XXXX American Chemical Society
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a
forming the imidazo[1,2-a]pyridine core (Scheme 2). Starting
Scheme 1
from reactants with various R1, R2, and R3, the reaction yielded
a
Scheme 2. Synthetic Scheme of 1−14
a
(a) NaHCO3, CH3CN, reflux, 20 h; (b) BBr3, CH2Cl2, 15 h.
compounds 2−14. 5′-Substituted HPIPs 2−6 were synthesized
by coupling of 2-aminopyridine and 4-substituted 2-
(bromoacetyl)phenol (R2 = OMe, Me, Cl, Br, F). 6-Substituted
HPIPs (7−11) were obtained by reaction of 5-substituted 2-
aminopyridine (R3 = Me, Cl, Br, F, CF3, CN) and 2-
(bromoacetyl)anisole. The resultant methoxy intermediates
were demethylated by tribromoborane. 4′-Methoxy-12 and
5′,6-disubstituted HPIPs 13 and 14 were synthesized similarly
to 5′-substituted HPIPs. All compounds were obtained as
colorless microcrystalline powders.
Absorption Properties in Fluid and Rigid Media. As
representative examples of phenyl- and imidazopyridyl-
substituted HPIPs, the absorption and fluorescence spectra of
5′-bromo (5) and 6-bromo (9) HPIP in tetrahydrofuran
(THF) are presented in Figure 1. In a fluid THF solution,
π−π* absorption bands of both compounds appeared at around
340 nm. As shown in Table 1, the absorption spectra of 5′-
substituted (2−6), 6-substituted (7−11), 4′-substituted (12),
a
(a) The energy diagram of ESIPT process. (b) Molecular structure of
well-studied benzazoles exhibiting ESIPT fluorescence. (c) The enol,
IPT, and solvated enol (Enol·Sol) forms of 2-(2′-hydroxyphenyl)-
imidazo[1,2-a]pyridine (HPIP, 1).
has been no systematic study of the luminescent properties of
HPIP derivatives because of their very low emission quantum
yields in organic fluid solutions (Φ ≈ 0.01).15−17 Our recent
quantum chemical study18 on the potential energy surfaces in
the ground (S0) and excited (S1) states showed that the ESIPT
state (S1) smoothly approached the S0−S1 conical intersection,
coupled with twisting motion around the central C−C single
bond connecting the phenyl and imidazo[1,2-a]pyridine rings.
The S0−S1 energy gap was sufficiently small at the dihedral
angle of 60° for rapid radiationless decay in a fluid solution.
Therefore, the remarkable emission enhancement for solid
HPIP was assigned to suppression of the efficient radiationless
decay by fixing the dihedral angle at a nearly coplanar
orientation in a rigid environment.
In this study, we synthesized various HPIP derivatives and
studied their ESIPT fluorescence properties. Though all the
HPIPs showed quite weak fluorescence in fluid organic
solutions, they displayed efficient fluorescence in rigid media
(Φ ≈ 0.6). A wide range of solid-state ESIPT fluorescence was
achieved with good quantum efficiency. Furthermore, the
ESIPT fluorescence of HPIPs was simulated by molecular
orbital calculations, and the results were well in line with the
experimental data. The ESIPT fluorescence could be rationally
controlled by shifting the energy level of either the lowest
unoccupied molecular orbital (LUMO) or the highest occupied
molecular orbital (HOMO) of the intramolecular proton-
transferred (IPT) state.
RESULTS AND DISCUSSION
Synthesis. A series of HPIPs were synthesized through
reaction of 2-aminopyridine and α-bromoacetophenone,
■
Figure 1. Absorption and emission spectra of 5 (a) and 9 (b) in a
THF solution at room temperature (dotted line), at 77 K (broken
line), and in a PMMA film (solid line).
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Table 1. Absorption and Fluorescence Maxima of 1−14
THF (rt)
THF (77 K)
PMMA
λabs/nm
333 (1.02)
λem/nm
λem/nm
λabs/nm
λem/nm (Φ)
1
377, 602 (0.08)
384 (0.01)
370, 521
376, 571
370, 535
369, 513
370, 507
370, 531
368, 518
383, 549
383, 523
392, 543
401, 545
378, 529
388, 571
396
331
336
332
333
333, 348
332
331
340
340
331
351
336
343
342
520 (0.37)
2
352.5 (1.36), 339 (1.47)
334 (1.38)
381, 565 (0.06)
530 (0.31)
3
382, 620 (0.01)
378, 576 (0.02)
378, 573 (0.01)
379, 597 (0.02)
381, 588 (0.02)
392, 634 (<0.01)
390, 640 (<0.01)
398 (0.01)
4
333.5 (1.39)
333.5 (1.47), 348 (1.15)
333.5 (1.11)
331 (1.12)
509 (0.42)
5
510 (0.57)
6
514 (0.37)
7
378, 522 (0.35)
539 (0.23)
8
340 (1.05)
9
341 (1.08)
533 (0.16)
10
11
12
13
14
339 (1.08)
390, 541 (0.20)
399, 561 (0.13)
514 (0.61)
336.5 (1.01)
333.5 (1.44)
343 (1.52)
399, 569 (0.01)
386, 577 (0.01)
395 (0.01)
389, 571 (0.09)
390, 570 (0.06)
342 (1.18)
401 (<0.01)
and disubstituted (13, 14) HPIPs appeared in the near-UV
region in fluid THF solution, with a maximum wavelength in
the range 333−341 nm. Such marginal differences in the
absorption spectra indicate that substitution has little effect on
the absorption properties of HPIP 1.
The absorption properties of 1−14 in the solid state were
then examined by preparing a transparent poly(methyl
methacrylate) (PMMA) film containing 0.5 wt % of each
HPIP derivative by the spin-coating method. The transmission
absorption spectra of 1−14 were quite similar in terms of shape
and maximum wavelength, comparable to those observed in
dilute THF solution, indicating that the HPIPs existed as
monomers in the PMMA matrix and were insusceptible to a
rigid environment (Figure 1, Table 1).
Fluorescence Properties in Fluid and Rigid Media. On
excitation at 330 nm, compounds 5 and 9 exhibited weak dual
emission (Φ ≈ 0.01) at 378 and 573 nm for 5 and at 390 and
640 nm for 9 in dilute THF solution. The excitation spectra of
the two emission bands of 5 and 9 were similar to the
corresponding absorption spectra, indicating that both
emissions originated from single molecule species. By
comparing the dual emission bands with the reported
fluorescence spectra of the parent compound 1,15 the near-
UV emission was assigned to a normal fluorescence from the
enol species that is not intramolecularly hydrogen-bonded due
to solvation (1 Enol·Sol, Scheme 1c), and that with a large
Stokes shift (ca. 12 000 cm−1) is the ESIPT fluorescence
(Scheme 1a). Although the normal fluorescence emissions of 1,
5, and 9 appeared in a similar region, the ESIPT fluorescence
emissions differed significantly. Whereas a blue shift (ca. 30
nm/840 cm−1) of the ESIPT fluorescence was observed for the
5′-bromo derivative, an opposite red shift with a similar value
(ca. 40 nm/990 cm−1) was observed for the 6-bromo derivative
9, suggesting that the ESIPT fluorescence would be tunable not
only by substituent, but also by substituting position.
PMMA. The quantum yields of 5 and 9 rose to 0.57 and 0.16,
respectively, which is an order of magnitude larger than those in
a fluid THF solution.
Previously, we studied the polymorph-dependent ESIPT
luminescence of 1 and concluded that the blue-green (496 nm,
Φ = 0.50) and yellow (529 nm, Φ = 0.37) luminescence could
be assigned to the coplanar and twisted species, respectively.11
Therefore, the yellow ESIPT fluorescence emissions of 1 in a
frozen THF solution (521 nm) and PMMA matrix (520 nm)
indicate that 1 might form a planar conformation in these rigid
media.
As shown in Table 1, the ESIPT fluorescence was very weak
for all HPIPs in a fluid THF solution (Φ ≈ 0.01), but was quite
efficient in a PMMA matrix (Φ = 0.1−0.6). A blue shift (50−90
nm/2 100−3 600 cm−1) of the ESIPT fluorescence was
observed for all HPIPs, regardless of the substituent and its
position, suggesting that the blue shift could be attributed to
the effect of the rigid environment. The fluorescence decays of
1−14 in a PMMA matrix fitted reasonably with a biexponential
curve (τ1 = 0.8−1.6 ns, τ2 = 2.9−5.4 ns; χ2 = 0.9−1.2). The
weighted average lifetimes (τM) of all the compounds were
2.1−4.9 ns, indicating singlet emission.
It was reported by Douhal et al.15,16 that a zwitterionic
ESIPT species further transitioned to the twisted conformation
(Figure 2) along with rearrangement of surrounding solvent
molecules in a fluid solution, which could be the reason for the
significantly large Stokes shift. Our recent study on the
potential energy surfaces in the ground (S0) and excited (S1)
states of 1 supports this discussion;18 the S0−S1 gap energy
(i.e., the ESIPT fluorescence energy) decreases with increasing
the dihedral angle (θ) between phenyl and imidazo[1,2-
In a frozen THF solution at 77 K, the emission intensified
more than 50-fold. The ESIPT fluorescence bands of 5 and 9
shifted to significantly shorter wavelengths, 507 and 549 nm,
respectively, whereas the excitation spectrum appeared in the
same region as for a fluid solution. In contrast, the normal
fluorescence (ca. 380 nm) did not show a notable shift.
The emission upon excitation at 330 nm was similarly
intense and blue-shifted in a PMMA matrix. It is worth noting
that ESIPT fluorescence was the only or dominant emission in
Figure 2. The proton transfer and subsequent twisting motion of
HPIP (1) in the excited state.
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a]pyridine rings. Because such molecular motion processes are
suppressed in the rigid environments, the ESIPT species retains
the planar conformation and thereby emits the blue-shifted
ESIPT fluorescence. A similar environmental effect has been
found for twisted intramolecular charge-transfer (TICT)
emission,19 which has been reported20 to show a blue shift in
rigid media.
A gradual red shift of the ESIPT fluorescence was observed
when the electron-donating nature of the 5′-substituent
increased. On the other hand, the ESIPT fluorescence showed
a blue shift on introducing an electron-donating group at the 6
position, indicating the opposite substituent effect. In the case
of disubstituted HPIPs 13 and 14, the ESIPT fluorescence
emissions were observed at much longer wavelengths compared
with the corresponding monosubstituted HPIPs 3, 8, and 10.
The lower-energy fluorescence might be the result of the
cumulative effect of two substitutions.
core of HOMO and LUMO were qualitatively similar to that of
1.
Then the excited electronic state of the IPT form was
calculated using the CI-Single method (CIS/6-31+G(d)) after
geometry optimization by CIS/6-31G(d). The lowest-energy
transition band, which corresponds to the ESIPT fluorescence,
was calculated to be HOMO−LUMO. Unlike the enol form,
the HOMO and LUMO of the IPT form, HOMOIPT and
LUMOIPT, were localized mainly on the imidazo[1,2-a]pyridine
and phenyl ring, respectively (Figure 3).
The measured ESIPT emission energies of 1−14 in a PMMA
matrix were plotted against the calculated energies of the
HOMOIPT−LUMOIPT transitions (Figure 4). Although the
Chemical alteration of a fluorophore often causes undesirable
perturbation to its emissive electronic state and results in
impairment of its fluorescence properties. However, in the case
of HPIPs, substitution of either the phenyl or imidazo[1,2-
a]pyridine ring schematically altered the color of the ESIPT
fluorescence in the solid matrix, while retaining reasonable
quantum efficiency. Because substituted HPIPs are readily
available using various commercially available aminopyridines
and hydroxyacetophenones, substitution of HPIP is a simple
and useful method for tuning color without damaging the
luminescent nature in the solid state.
Theoretical Simulation of Substituent Effect. To
understand the effect of substituents further, the electronic
states of HPIPs were studied using ab initio quantum chemical
calculations. Geometry optimization of the enol form of 1 by an
RHF/6-31G(d) calculation resulted in a coplanar conformation
as the energy-minimum structure, which was further applied to
a single-point energy calculation including the diffuse function
(RHF/6-31+G(d)). The lowest-energy absorption band was
shown to be the HOMO−LUMO transition. Both the HOMO
and LUMO of the enol form were delocalized over a molecule
(Figure 3). The same calculation was also applied to 2−14, and
it was found that the electronic configurations of the aromatic
Figure 4. Plot of the calculated against measured energy of ESIPT
fluorescence of 1−14.
estimated values of the transition energy were somewhat larger
(ca. 0.9 eV) than the observed energies (Table S1, Supporting
Information), the linear correlation of the plots (R2 = 0.91)
confirmed that the quantum chemical simulation was effective
for qualitative reproduction of the observed ESIPT fluorescence
emissions of HPIPs.
It has been reported that the Hammett substituent constant
(σ) could be a useful parameter for explaining the substituent
effect on the emission properties.21 Thus, the calculated energy
levels of HOMOIPT and LUMOIPT (Table S1, Supporting
Information) of 5′-substituted and 6-substituted HPIPs were
plotted against Hammett substituent constants, as shown in
Figure 5.
Figure 5. Plot of the calculated energy levels of HOMOIPT and
LUMOIPT against the Hammett substituent constants (σp or σm). (a)
5′-Substituted HPIP (2−6), and (b) 6-substituted HPIP (7−11).
Figure 3. Molecular orbitals of the enol and IPT forms of 1 that
participate in the ESIPT process.
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As shown in Figure 5a, the energy levels of 5′-substituted
HPIPs are well correlated with the Hammett para-substituent
constant (σp; R2 = 0.98 for HOMOIPT, 0.88 for LUMOIPT), but
not against the Hammett meta-substituent constant (σm; R2 =
0.46 for HOMOIPT, 0.83 for LUMOIPT). The energy gap
became larger when the value of σp increased, which is
consistent with the observed blue shift of the ESIPT
fluorescence. More importantly, the energy level of HOMOIPT
decreased more steeply than that of LUMOIPT, indicating that
substitution at the 5′ position had a larger electronic effect on
HOMOIPT. This could be ascribed to the localized electron
density on the phenyl ring in HOMOIPT compared with
LUMOIPT, which was supported by the ab initio calculation
(Figure 3).
introduction of these groups into the imidazopyridine part (7−
11) resulted in fluorescence shifts in the opposite direction.
The plots of the calculated HOMOIPT and LUMOIPT energy
levels against the Hammett substituent constants (σ) showed
good linearities with different slopes, which rationally explain
the effect of the substituent and its position on the IPT state.
Thus, a series of HPIPs as new ESIPT fluorescent compounds
were developed, and substituent effects on the ESIPT
fluorescence properties were successfully reproduced and
explained by the electronic configurations and energy levels
of HOMOIPT and LUMOIPT. It is suggested that ESIPT
fluorescence properties might be predictable using quantum
chemical simulations.
In the case of 6-substituted HPIPs (Figure 5b), the energy
EXPERIMENTAL SECTION
levels were linearly fitted against σm (R2 = 0.98 for HOMOIPT
,
■
1
0.99 for LUMOIPT) instead of σp (R2 = 0.90 for HOMOIPT
,
Methods. H NMR peaks were assigned using H−H COSY and
heteronuclear multiple quantum coherence (HMQC) methods. UV−
vis absorption and fluorescence spectra in organic solutions were
recorded using standard spectrophotometer and spectrofluoropho-
tometer, respectively. The fluorescence quantum yield in a fluid THF
solution was calculated using 2-aminopyridine (Φ = 0.37; ethanol;
excitation wavelength 285 nm) as a standard. Time-resolved emission
decay was recorded by exciting samples with a nitrogen laser pulse
(337 nm).
The fluorescence spectra in a PMMA matrix were measured in an
integral sphere attatched to a spectrofluorophotometer, and the
quantum yield was obtained using calculation software based on the
literature method,22 which was installed in the spectrofluoropho-
tometer.
0.80 for LUMOIPT). The smaller energy gap with increasing σm
value is consistent with the observed red shift of the ESIPT
fluorescence. Opposite to 5′-substituted HPIPs, the energy
level of LUMOIPT decreased more steeply than that of
HOMOIPT when the value of σm increased. The larger effect
of 6-substitution on the energy level of LUMOIPT was assigned
to the larger electron density on the imidazo[1,2-a]pyridine
ring in LUMOIPT, as was shown by the quantum chemical
calculations (Figure 3).
It is noteworthy that the energy levels of HOMOIPT and
LUMOIPT 5′,6-disubstituted HPIPs could be estimated by
summation of the substituent effect on each aromatic ring. In a
comparison of 5′-methyl-6-chloro-HPIP 13 and 1, the energy
level of HOMOIPT was higher (−6.11 eV for 13 and −6.14 eV
for 1), whereas that of LUMOIPT was lower (0.51 eV for 13 and
0.79 eV for 1; Table S1, Supporting Information). This result
indicates dominant effects of 5′-methyl and 6-chloro sub-
stituents with respect to the energy levels of HOMOIPT and
LUMOIPT, respectively, which is in good agreement with the
above-discussed effect of monosubstitution and with the
observed lower-energy fluorescence emissions of the disub-
stituted HPIPs. The same result was obtained by a comparison
of 5′-methyl-6-(trifluoromethyl)-HPIP 14 and 1.
The energy levels of HOMO and LUMO of the enol form were
calculated by the HF method after geometry optimization (HF/6-31G
+(d)//HF/6-31G(d)), and those of HOMOIPT and LUMOIPT
,
corresponding to ESIPT and IPT states, were calculated by the CI-
Single method (CIS/6-31+G(d)//CIS/6-31G(d)). These calculations
were performed using a Gaussian 03W, Gaussian Inc. (Revision C.02)
package,23 and the results were processed on a GaussView 4.1 or a
Fujitsu Scigress Explorer (Version 7).
Materials. 2-Bromo-1-(2-methoxyphenyl)ethanone was purchased
from Wako Chemical Co. 3-Bromo-, 5-bromo-, and 3,5-dibromo-2-
aminopyridines, and other chemicals, were also commercially available
and used as received. The syntheses of 2-(2′-hydroxyphenyl)imidazo-
[1,2-a]pyridine (1) and 2-(2′-methoxyphenyl)imidazo[1,2-a]pyridine
(10) have been described elsewhere.11
Transparent polymer films were prepared from a benzene solution
of PMMA (200 mg in 2 mL) containing 0.5 wt % of each HPIP
derivative by spin-coating method (500 μL, 1000 rpm, 15 s.).
General Synthetic Procedure for 5′-Substituted 2-(2′-
Hydroxyphenyl)imidazo[1,2-a]pyridine (5′-Substituted HPIP).
A chloroform solution of corresponding 4-substituted 2-acetylphenol
and copper(II) bromide (1.5 equiv) was refluxed for 16 h. After
cooling, filtering off insoluble solid, evaporation of the filtrate yielded
crude 4-substituted 2-(bromoacetyl)phenol, which was applied to a
silica gel column (CHCl3). Then an acetonitrile solution of 4-
substituted 2-(bromoacetyl)phenol, 2-aminopyridine, and NaHCO3 (2
equiv) was refluxed for 18 h. After filtering off insoluble solid and
evaporation, the crude product was purified by a silica gel column
chromatography (eluent for each compound shown below).
Thus, the substituent effects on the ESIPT fluorescence
emissions of monosubstituted HPIPs were reasonably explained
by the ab initio quantum chemical calculations. A noteworthy
correlation between the energy levels and Hammett substituent
constants (σm, σp) indicates that the orbital energies were
principally determined by the electronic properties of those
substituents.
The key to such schematically controllable properties of the
ESIPT fluorescence emissions of HPIPs is HOMOIPT and
LUMOIPT, in which the electrons are localized on the phenyl
ring and imidazo[1,2-a]pyridine ring, respectively.
CONCLUSION
■
The synthesis of 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridine
(HPIP, 1) and its derivatives (2−14) was described, and
substituent effects on the ESIPT fluorescence properties were
studied. Although all compounds showed faint dual emission
(Φ ≈ 0.01), ascribed to normal and ESIPT fluorescences in a
fluid solution, they generally displayed bright ESIPT
fluorescence in a polymer matrix (Φ = 0.1−0.6). The
introduction of electron-donating and electron-withdrawing
groups into the phenyl ring (2−6) caused blue and red shifts of
the ESIPT fluorescence, respectively. On the other hand,
5′-Methoxy HPIP (2). Eluent: CHCl3/hexane = 1:2. Further
recrystallization from ethanol afforded white crystal (235 mg, 69%):
1
mp 126.4−126.9 °C; H NMR (CDCl3, 400 MHz) δ 12.26 (1H, s,
OH), 8.16 (1H, dd, J = 5.5, 1.1 Hz, 5-H), 7.86 (1H, s, 3-H), 7.60 (1H,
d, J = 8.2 Hz, 8-H), 7.22−7.26 (1H, m, 7-H), 7.12 (1H, d, J = 2.7 Hz,
6-H), 6.98 (1H, d, J = 8.8 Hz, 3′-H), 6.83−6.88 (2H, m, 4′,6-H), 3.82
(3H, s, OCH3); 13C NMR (CDCl3, 100 MHz) δ 152.2, 151.5, 145.1,
143.5, 125.4, 125.2, 118.17, 116.8, 116.2, 115.7, 113.1, 110.5, 106.8,
55.9. Anal. Calcd for C14H12N2O2: C,69.99; H,5.03; N,11.66%. Found:
C,69.64; H,5.02; N,11.39%.
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5′-Methyl HPIP (3). Eluent: CHCl3/hexane = 5:1. Further
recrystallization from ethanol afforded white crystal (413 mg, 39%):
mp 150.9−151.2 °C; H NMR (CDCl3, 400 MHz) δ 12.48 (1H, s,
Calcd for C14H12N2O: C,74.98; H,5.39; N,12.49. Found: C,75.19;
H,5.26; N,12.42.
6-Chloro HPIP (8). Purification by a silica gel column chromatog-
1
OH), 8.14 (1H, d, J = 6.6 Hz, 5-H), 7.85 (1H, s, 3-H), 7.57 (1H, t, J =
4.9 Hz, 8-H), 7.39 (1H, d, J = 1.6 Hz, 6′-H), 7.22 (1H, ddd, J = 9.1,
6.9, 1.4 Hz, 7-H), 7.04 (1H, dd, J = 8.2, 1.6 Hz, 4′-H), 6.94 (1H, d, J =
8.2 Hz, 3′-H), 6.84 (1H, td, J = 7.0, 1.2 Hz, 6-H), 2.32 (3H, s, CH3);
13C NMR (CDCl3, 100 MHz) δ 155.1, 145.4, 143.4, 130.4, 127.9,
125.9, 125.3, 125.0, 117.4, 116.7, 115.7, 113.1, 106.6, 20.6. Anal. Calcd
for C14H12N2O: C,74.98; H,5.39; N,12.49%. Found: C,75.02; H,5.35;
N,12.34%.
raphy (CHCl3) and recrystallization from dimethyl sulfoxide yielded 8
1
as white solid (1.26 g, 76%): mp 197.6−198.5 °C; H NMR (CDCl3,
400 MHz) δ 12.41 (1H, s, OH), 8.19 (1H, dd, J = 1.8, 0.9 Hz, 5-H),
7.83 (1H, d, J = 0.7 Hz, 3-H), 7.58−7.52 (2H, m, 6′,8-H), 7.23−7.27
(1H, m, 4′-H), 7.20 (1H, dd, J = 9.5, 1.9 Hz, 7-H), 7.04 (1H, dd, J =
8.2, 1.1 Hz, 3′-H), 6.89 (1H, ddd, J = 8.1, 6.8, 0.9 Hz, 5′-H); 13C NMR
(CDCl3, 100 MHz) δ 157.2, 146.3, 141.9, 130.0, 126.5, 125.8, 123.2,
121.3, 119.1, 117.8, 117.0, 115.7, 107.1. Anal. Calcd for C13H9ClN2O:
C,63.81; H,3.71; N,11.45%. Found: C,63.70; H,3.67; N,11.31%.
6-Bromo HPIP (9). Purification by a silica gel column chromatog-
raphy (CHCl3) afforded 9 as white powder (166 mg, 72%): mp
5′-Chloro HPIP (4). Eluent: CHCl3/hexane = 2:1. Further
recrystallization from ethanol afforded white crystal (616 mg, 61%):
1
mp 188.0−188.8 °C; H NMR (CDCl3, 400 MHz) δ 12.74 (1H, s,
1
207.5−208.6 °C; H NMR (CDCl3, 400 MHz) δ 12.39 (1H, s, OH),
OH), 8.17 (1H, d, J = 7.1 Hz, 5-H), 7.87 (1H, s, 3-H), 7.61 (1H, d, J =
9.3 Hz, 8-H), 7.55 (1H, d, J = 2.7 Hz, 6′-H), 7.25−7.32 (1H, m, 7-H),
7.17 (1H, dd, J = 8.5, 2.5 Hz, 4′-H), 6.97 (1H, d, J = 8.8 Hz, 3′-H),
6.89 (1H, td, J = 6.8, 1.2 Hz, 6-H); 13C NMR (CDCl3, 100 MHz) δ
155.9, 144.1, 143.5, 129.3, 126.6, 125.5, 125.2, 123.5, 119.1, 117.4,
116.9, 113.4, 107.0. Anal. Calcd for C13H9ClN2O: C,63.81; H,3.71;
N,11.45%. Found: C,63.54; H,3.69; N,11.23%.
8.30 (1H, d, J = 1.1 Hz, 5-H), 7.83 (1H, s, 3-H), 7.57 (1H, dd, J = 7.7,
1.6 Hz, 6′-H), 7.49 (1H, t, J = 7.7 Hz, 8-H), 7.23−7.31 (2H, m, 4′,7-
H), 7.04 (1H, dd, J = 8.2, 1.1 Hz, 3′-H), 6.89 (1H, td, J = 7.4, 1.2 Hz,
5′-H); 13C NMR (CDCl3, 100 MHz) δ 157.3, 146.2, 141.9, 130.0,
128.6, 125.8, 125.3, 119.1, 117.8, 117.3, 115.7, 107.7, 106.9. Anal.
Calcd for C13H9BrN2O: C,54.00; H,3.14; N,9.69%. Found: C,53.82;
H,3.07; N,9.43%.
5′-Bromo HPIP (5). Eluent: CHCl3/hexane = 1:1. Further
6-Trifluoromethyl HPIP (10). Purification by a silica gel column
recrystallization from ethanol afforded white crystal (1.78 g, 60%):
1
chromatography (CHCl3) yielded 10 as white powder (423 mg, 44%):
mp 198.6−199.5 °C; H NMR (CDCl3, 400 MHz) δ 12.77 (1H, s,
1
mp 229.9−231.0 °C; H NMR (CDCl3, 400 MHz) δ 12.27(1H, s,
OH), 8.17 (1H, d, J = 7.1 Hz, 5-H), 7.86 (1H, s, 3-H), 7.69 (1H, d, J =
2.2 Hz6′-H), 7.60 (1H, d, J = 9.3 Hz, 8-H), 7.25−7.31 (2H, m, 4′,7-
H), 6.90 (2H, dd, J = 14.8, 7.7 Hz, 3′,6-H); 13C NMR (CDCl3, 100
MHz) δ 156.4, 143.9, 143.4, 132.1, 128.1, 125.6, 125.5, 119.5, 118.0,
116.8, 113.4, 110.6, 107.0. Anal. Calcd for C13H9BrN2O: C,54.00;
H,3.14; N,9.69%. Found: C,53.90; H,3.11; N,9.39%.
OH), 8.55(1H, s, 5-H), 7.97(1H, s, 3-H), 7.71(1H, d, J = 7.8 Hz, 8-H),
7.60(1H, dd, J = 6.4, 1.1 Hz, 6′-H), 7.40(1H, dd, J = 7.6, 1.1 Hz, 7-H),
7.27(1H, td, J = 5.6, 1.0 Hz, 4′-H), 7.05(1H, d, J = 7.2 Hz, 3′-H),
6.91(1H, td, J = 7.2, 1.1 Hz, 5′-H); 13C NMR (CDCl3, 100 MHz) δ
157.3, 147.2, 143.3, 130.4, 126.0, 124.3, 124.3, 121.2, 121.1, 119.2,
117.9, 117.4, 115.4, 107.9; HRMS (FAB/Double-focusing magnetic
sector), m/z Calcd. For C14H10F3N2O 279.0745, found 279.0737.
6-Cyano HPIP (11). Purification by a silica gel column
chromatography (CHCl3) and recrystallized from ethanol afforded
5′-Fluoro HPIP (6). Eluent: CHCl3. Further recrystallization from
ethanol afforded white crystal (346 mg, 44%): mp 161.6−162.6 °C;
1H NMR (CDCl3, 400 MHz) δ 12.51 (1H, s, OH), 8.18 (1H, d, J =
6.9 Hz, 5-H), 7.85 (1H, s, 3-H), 7.61 (1H, d, J = 9.2 Hz, 8-H), 7.25−
7.29 (2H, m, 6′,7-H), 6.93−6.99 (2H, m, 3′,4′-H), 6.89 (1H, t, J = 6.9
Hz, 6-H); 13C NMR (CDCl3, 100 MHz) δ 157.0, 154.7, 153.4, 144.4,
143.5, 125.5, 118.5, 116.9, 116.4, 116.2, 113.4, 111.5, 111.3, 107.1.
Anal. Calcd for C13H9FN2O: C,68.42; H,3.97; N,12.27%. Found:
C,68.16; H,3.94; N,12.16%.
1
11 as white solid (419 mg, 44%): mp 246.2−247.2 °C; H NMR
(CDCl3, 400 MHz) δ 12.06 (1H, s, OH), 8.61(1H, s, 5-H), 7.97(1H,
s, 3-H), 7.70(1H, d, J = 6.0 Hz, 8-H), 7.60(1H, dd, J = 5.3, 1.0 Hz, 6′-
H), 7.37(1H, dd, J = 5.8, 1.0 Hz, 7-H), 7.30(1H, td, J = 7.0, 1.0 Hz, 4′-
H), 7.05(1H, d, J = 5.3 Hz, 3′-H), 6.92(1H, td, J = 6.8, 1.0 Hz, 5′-H);
13C NMR (CDCl3, 100 MHz) δ 157.4, 147.8, 142.9, 131.0, 130.8,
126.1, 125.2, 119.4, 118.1, 117.7, 116.2, 115.0, 107.6, 99.6. Anal. Calcd
for C14H9N3O: C,71.48; H,3.86; N,17.86%. Found: C,71.79; H,3.83;
N,18.04%.
General Synthetic Procedure for 6-Substituted 2-(2′-
Hydroxyphenyl)imidazo[1,2-a]pyridine (6-Substituted HPIP).
An acetonitrile solution of 2-(bromoacetyl)anisol, 5-substituted 2-
amino-pyridine and NaHCO3 (2 equiv) was refluxed for 20 h. After
filtering off insoluble solid, the filtrate was evaporated, and the residue
was applied to a silica gel column (CHCl3/ethyl acetate =10:1) to
afforded 6-substituted 2-(2′-methoxyphenyl) imidazo[1,2-a]pyridine.
A dichloromethane solution of boron tribromide (1.0 M, 4 equiv) was
dropwise added to a cooled anhydrous dichloromethane solution of
well-dried 6-substituted 2-(2′-methoxyphenyl) imidazo[1,2-a]pyridine.
The reaction mixture was allowed to reach room temperature and
further stirred for 15 h. A saturated aqueous NaHCO3 was slowly
added with stirring, and then was separated with water and
chloroform. After washing the organic layer with saturated aqueous
NaHCO3 and water, and drying over Na2SO4, the organic layer was
evaporated to give crude product.
HPIP (1). Purification by recrystallization from ethanol (3.65 g,
43%): mp 142−143 °C; 1H NMR (CDCl3, 400 MHz) δ 12.70 (1H, s,
OH), 8.16 (1H, dd, J = 5.5, 1.1 Hz, 5-H), 7.88 (1H, s, 3-H), 7.60 (2H,
dd, J = 8.0, 1.4 Hz, 6′,8-H), 7.21−7.25 (2H, m, 4′,7-H), 7.04 (1H, dd, J
= 8.2, 1.1 Hz, 3′-H), 6.85−6.91 (2H, m, 5′,6-H); 13C NMR (CDCl3,
100 MHz) δ 157.3, 145.3, 143.4, 129.7, 125.7, 125.4, 125.2, 119.0,
117.7, 116.8, 116.2, 113.2, 106.7. Anal. Calcd for C13H10N2O: C,74.27;
H,4.79; N,13.33%. Found: C,74.02; H,4.81; N,13.30%.
4′-Methoxy HPIP (12). A chloroform (50 mL) solution of 2-acetyl-
5-methoxyphenol (1.22 g, 7.34 mmol) and copper(II) bromide (2.95
g, 13.2 mmol) was refluxed for 24 h. After cooling, filtering off
insoluble solid, evaporation of the filtrate yielded crude 2-
(bromoacetyl)-5-methoxyphenol, which was applied to a silica gel
column (ethyl acetate/hexane = 1:5; 50%). Then an acetonitrile (40
mL) solution of 2-(bromoacetyl)-5-methoxyphenol (869 mg, 3.55
mmol), 2-aminopyridine (334 mg, 3.55 mmol) and NaHCO3 (596
mg, 7.10 mmol) was refluxed for 20 h. After filtering off insoluble solid
and evaporation, the crude product was purified by a silica gel column
chromatography (ethyl acetate/hexane = 1:1). Recrystallization from
ethanol gave 12 as a white crystal (342 mg, 40%): mp 158.0−158.3
°C; 1H NMR (CDCl3, 400 MHz) δ 12.87 (1H, s, OH), 8.14 (1H, dt, J
= 6.7, 1.1 Hz, 5-H), 7.76 (1H, s, 3-H), 7.57 (1H, dd, J = 9.2, 0.9 Hz, 8-
H), 7.49 (1H, d, J = 8.2 Hz, 6′-H), 7.21 (1H, ddd, J = 8.9, 6.9, 1.1 Hz,
7-H), 6.84 (1H, td, J = 6.9, 1.4 Hz, 6-H), 6.59 (1H, d, J = 2.7 Hz, 3′-
H), 6.48 (1H, dd, J = 8.7, 2.7 Hz5′-H), 3.83 (3H, s, OCH3); 13C NMR
(CDCl3, 100 MHz) δ 161.0, 158.9, 145.4, 143.3, 126.6, 125.2, 124.9,
116.5, 112.9, 109.3, 106.5, 105.5, 101.7, 55.3. Anal. Calcd for
C14H12N2O2: C,69.99; H,5.03; N,11.66%. Found: C,70.17; H,5.03;
N,11.50%.
6-Methyl HPIP (7). Purification by recrystallization from ethanol
(820 mg, 38%): mp 180.6−181.5 °C; 1H NMR (CDCl3, 400 MHz) δ
7.92(1H, s, 5-H), 7.77(1H, s, 3-H), 7.57(1H, dd, J = 6.0, 1.0 Hz, 6′-
H), 7.49(1H, d, J = 9.2 Hz, 8-H), 7.21(1H, td, J = 5.4, 1.0 Hz, 4′-H),
7.02−09(2H, m, 3′,7-H), 6.87(1H, td, J = 4.8, 0.8 Hz, 5′-H), 2.34(s,
3H, CH3); 13C NMR (CDCl3, 100 MHz) δ 157.3, 145.0, 142.4, 129.4,
128.3, 125.6, 123.1, 122.8, 118.9, 117.6, 116.4, 116.1, 106.4, 18.2. Anal.
5′-Methyl-6-chloro HPIP (13). A chloroform (50 mL) solution of 2-
acetyl-4-methylphenol (3.00 g, 20.0 mmol) and copper(II) bromide
(6.70 g, 30.0 mmol) was refluxed for 22 h. After cooling, filtering off
insoluble solid, evaporation of the filtrate yielded crude 2-
(bromoacetyl)-4-methylphenol, which was applied to a silica gel
column (CHCl3/hexane = 1:1; 66%). Then an acetonitrile (30 mL)
F
dx.doi.org/10.1021/jo302711t | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
solution of 2-(bromoacetyl)-4-methylphenol (417 mg, 1.82 mmol), 2-
amino-5-chloropyridine (234 mg, 1.82 mmol) and NaHCO3 (306 mg,
3.64 mmol) was refluxed for 26 h. After cooling, insoluble solid was
filtered off. Evaporation of the filtrate afforded crude product, which
was then purified by a silica gel column chromatography (CHCl3).
Recrystallization from ethanol gave 13 as a white crystal (246 mg,
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1
52%): mp 199.6−200.4 °C; H NMR (CDCl3, 400 MHz) δ 12.18
(1H, s, OH), 8.20 (1H, d, J = 1.1 Hz, 5-H), 7.84 (1H, s, 3-H), 7.54
(1H, d, J = 9.9 Hz, 8-H), 7.37 (1H, d, J = 1.6 Hz, 6′-H), 7.20 (1H, dd,
J = 9.3, 2.2 Hz, 7-H), 7.06 (1H, dd, J = 8.2, 2.2 Hz, 4′-H), 6.94 (1H, d,
J = 8.2 Hz, 3′-H), 2.32 (3H, s, CH3); 13C NMR (CDCl3, 100 MHz) δ
155.0, 146.4, 141.8, 130.8, 128.1, 126.4, 125.9, 123.1, 121.2, 117.5,
117.0, 115.2, 107.0, 20.6. Anal. Calcd for C14H11ClN2O: C,65.00;
H,4.29; N,10.83%. Found: C,64.89; H,4.27; N,10.65%.
5′-Methyl-6-trifluoromethyl HPIP (14). A chloroform (50 mL)
solution of 2-acetyl-4-methylphenol (3.00 g, 20.0 mmol) and
copper(II) bromide (6.70 g, 30.0 mmol) was refluxed for 22 h.
After cooling, filtering off insoluble solid, evaporation of the filtrate
yielded crude 2-(bromoacetyl)-4-methylphenol, which was applied to a
silica gel column (CHCl3/hexane = 1:1; 66%). Then an acetonitrile
(30 mL) solution of 2-(bromoacetyl)-4-methylphenol (1.29 g, 5.63
mmol), 2-amino-5-chloropyridine (915 mg, 5.64 mmol) and NaHCO3
(576 mg, 6.85 mmol) was refluxed for 15 h. After cooling, insoluble
solid was filtered off. Evaporation of the filtrate afforded crude product,
which was then purified by a silica gel column chromatography
(CHCl3). Recrystallization from ethanol gave 14 as a white crystal
1
(1.12 g, 68%): mp 181.2−182.2 °C; H NMR (CDCl3, 400 MHz) δ
12.04 (1H, s, OH), 8.53 (1H, s, 5-H), 7.96 (1H, s, 3-H), 7.70 (1H, d, J
= 9.6 Hz, 8-H), 7.38−7.52 (2H, m, 6′,7-H), 7.08 (1H, d, J = 8.4 Hz,
4′-H), 6.96 (1H, d, J = 8.4 Hz, 3′-H), 2.33 (3H, s, CH3); 13C NMR
(CDCl3, 100 MHz) δ 155.0, 147.2, 143.2, 131.2, 128.2, 126.1, 124.7,
124.2, 124.2, 122.0, 121.0, 117.6, 117.2, 114.9, 107.8, 20.5. Anal. Calcd
for C15H11F3N2O: C,61.64; H,3.79; N,9.59%. Found: C,61.47; H,3.76;
N,9.42%.
ASSOCIATED CONTENT
* Supporting Information
■
S
1H and 13C NMR spectra of 2−14 (Figures S1−S26), the
calculated energy levels of HOMOIPT and LUMOIPT, and the
lowest transition energy (ΔE) obtained using the CIS-method
in the intramolecular hydrogen-transferred (IPT) state of 1−14
(Table S1). This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: araki@iis.u-tokyo.ac.jp, mutai@iis.u-tokyo.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid for Scientific
Research (B) (No. 21350109) and (C) (No. 24550222) from
the Japan Society for the Promotion of Science (JSPS), and
Promotion of Scientific Research (0241027-A) from the Iketani
Science and Technology Foundation, Japan.
(11) Mutai, T.; Tomoda, H.; Ohkawa, T.; Yabe, Y.; Araki, K. Angew.
Chem., Int. Ed. 2008, 47, 9522−9524.
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Jing, X. Synth. Met. 2003, 137, 1123−1124.
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ACS Appl. Mater. Interfaces 2011, 3, 654−657. (b) Tang, K.-C.; Chang,
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