Polyimide and imide compound exhibiting bright red fluorescence with very large stokes shifts via excited-state intramolecular proton transfer

Article abstract of DOI:10.1021/ma502456f

A novel polyimide (PI) emitting a prominent red fluorescence was prepared based on 3,6-dihydroxypyromellitic dianhydride (P2HDA) and 4,4′-diaminocyclohexylmethane (DCHM). In order to investigate the fluorescence properties of the PI, an imide model compound, N,N′-dicyclohexyl-3,6-dihydroxypyromellitimide (P2H-Ch), corresponding to one repeating unit of the PI, was also synthesized. The UV-vis absorption and fluorescence spectra of P2H-Ch in CHCl₃ and the solid PI film demonstrated intense red fluorescence at around 640 nm with a very large Stokes shift (ν) of 7655 and 8994 cm^-1, respectively, via the excited-state intramolecular proton transfer (ESIPT). Moreover, the corresponding monoanion and dianion species were formed in basic conditions with an organic base (DBU) and basic salt (NaOH), which were characterized by highly visible halochromism. The introduction of -OH groups into the pyromellitic moiety of imide compounds or PIs at their para positions led to the long-wavelength fluorescent emission as well as to the high pH sensitivity.

Full text of DOI:10.1021/ma502456f

Article
pubs.acs.org/Macromolecules
Polyimide and Imide Compound Exhibiting Bright Red Fluorescence
with Very Large Stokes Shifts via Excited-State Intramolecular Proton
Transfer
Kenta Kanosue, Takamichi Shimosaka, Junji Wakita, and Shinji Ando*
Department of Chemistry & Materials Science, Tokyo Institute of Technology, Ookayama 2-12-1-E4-5, Meguro-ku, Tokyo 152-8552,
Japan
S
* Supporting Information
ABSTRACT: A novel polyimide (PI) emitting a prominent
red fluorescence was prepared based on 3,6-dihydroxy-
pyromellitic dianhydride (P2HDA) and 4,4′-diaminocyclohex-
ylmethane (DCHM). In order to investigate the fluorescence
properties of the PI, an imide model compound, N,N′-
dicyclohexyl-3,6-dihydroxypyromellitimide (P2H-Ch), corre-
sponding to one repeating unit of the PI, was also synthesized.
The UV−vis absorption and fluorescence spectra of P2H-Ch
in CHCl3 and the solid PI film demonstrated intense red
fluorescence at around 640 nm with a very large Stokes shift
(ν) of 7655 and 8994 cm−1, respectively, via the excited-state
intramolecular proton transfer (ESIPT). Moreover, the
corresponding monoanion and dianion species were formed in basic conditions with an organic base (DBU) and basic salt
(NaOH), which were characterized by highly visible halochromism. The introduction of −OH groups into the pyromellitic
moiety of imide compounds or PIs at their para positions led to the long-wavelength fluorescent emission as well as to the high
pH sensitivity.
intermolecular interactions. They have been widely applied in
the aerospace, electrical, electronics, and photonics industries.9
In particular, wholly aromatic polyimides (Ar-PI) synthesized
from (1) pyromellitic dianhydride and bis(4-aminophenyl)
ether (PMDA/ODA) and (2) 3,3′,4,4′-biphenyltetracarboxylic
dianhydride and p-phenylenediamine (s-BPDA/PDA) have
been commercialized as Kapton-H and Upilex-S, respectively.
Recently, fluorinated PIs and semialiphatic PIs (Al-PIs) have
also attracted much attention as a new class of electronic and
optical materials due to their colorlessness and high trans-
parency, low refractive index, and low birefringence.10−13 For
example, they can be used as waveguides and optical peripheral
components with highly controlled properties in the visible and
near-IR region.11
More recently, fluorescence properties of low-molecular-
weight imide compounds,14−18 Ar-PIs, and Al-PIs have been
widely studied.12,14−27 It is well-known that Ar-PIs undergo two
types of electronic transitions after irradiation by UV or short-
wavelength visible light.16,17,20,26,27 The first one is called
“locally excited (LE) transition” and involves the dianhydride
moieties; the second one is called “charge transfer (CT)
transition” and originates from CT complex formation between
electron-donating diamine and electron-accepting dianhydride
1. INTRODUCTION
Polymeric materials exhibiting fluorescent emission1,2 are
expected to be applied for wavelength down-conversion in a
wide range of applications such as flat panel displays,
photovoltaic devices, and crop cultivators.3,4 Wavelength
down-convertors can efficiently change short wavelength light
(UV or violet-blue light) to longer wavelength light (green,
yellow, red, or near-infrared). Polymeric fluorescent materials
have attracted significant interest due to their inherent
advantages, i.e., controllability of optical properties, high
efficiency, light weight, flexibility, facile formation, and
processing of films and thin lines by spin-coating or inkjet
printing.5,6 However, conventional fluorescent polymers
consisting of π-conjugated sequences, such as poly(phenylene
vinylene) and polyfluorene derivatives, do not have sufficient
thermal stability, i.e., glass transition temperatures (Tg) or
degradation temperatures (Td under air), and do not comply
with the temperatures required by device fabrication processes
in modern electronics and photonics applications.7,8 In
addition, these π-conjugated polymers are generally not
resistant to radiation, environmental factors, and long-term
exploitation, which is also an important requirement for
polymeric wavelength converting materials.
Polyimides (PIs) are a class of super engineering plastics
well-known for their high thermal and chemical stability, optical
and radiation resistance, and good mechanical properties
originating from their rigid molecular structure and strong
Received: December 6, 2014
Revised: February 7, 2015
© XXXX American Chemical Society
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moieties. Furthermore, LE transitions can be classified into
“π−π*” and “n−π*” transitions. The LE(π−π*) transitions in
PIs are generated at π-conjugated components of the
dianhydride moieties, whereas the LE(n−π*) transitions are
generated from the lone-pair electrons at the carbonyl oxygen
and the nitrogen atoms of the imide rings (n-orbital).
Hasegawa et al.19,20 first demonstrated that PMDA/ODA and
s-BPDA/PDA PIs exhibit weak fluorescent emission at 400−
700 nm in response to optical excitation at the CT absorption
bands in the UV/short-visible region. However, their quantum
efficiency (Φ) of CT fluorescence is very low due to the very
small oscillator strengths of the CT transitions; for example, a
Φ value of 9.7 × 10−7 was reported for the PMDA/ODA PI
film.21 On the other hand, Horie et al.12,13 have further
investigated absorption and fluorescence properties of Al-PIs
based on 4,4′-diaminocyclohexylmethane (DCHM) with
PMDA and 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane
dianhydride (6FDA). These PIs showed high optical trans-
parency in the visible region due to the effective suppression of
intramolecular CT interactions due to the weak electron-
donating property of the alicyclic diamine. However, these PIs
exhibited CT emission peaks at around 500 nm in addition to
the LE emission peaks at around 400 nm, although they had no
apparent CT absorption peaks in the absorption spectra. The
authors concluded that the CT emission was originated from
the interchain CT complexes formed between the alicyclic
diamine moieties of one molecular chain and the dianhydride
moieties of the other. This conclusion was supported by the
enhancement of CT fluorescence with increasing thermal
imidization temperature. Meanwhile, Ishii et al.26 have reported
that an Al-PI prepared from s-BPDA and trans-1,4-diaminocy-
clohexane (t-CHDA) showed an LE absorption peak at 320 nm
and an intense emission peak at 385 nm, whereas no CT
absorption and fluorescence were observed. The Φ value of s-
BPDA/t-CHDA was reported to be 0.05, which was much
higher than that of Ar-PIs, such as PMDA/ODA. This large Φ
value could be explained by the extended π-conjugation in the
aromatic dianhydride moiety in addition to the suppression of
intra/intermolecular CT interactions by introducing alicyclic
diamine with weak electron-donating properties.
Optical measurements of imide model compounds in
solution are straightforward and useful to understand and
predict the fluorescence properties of PIs because PIs are
generally insoluble in common organic solvents due to their
rigid and linear molecular chains and strong intermolecular
interactions. Recently, we have proposed and confirmed a novel
molecular design concept for “highly fluorescent PIs” based on
the DFT calculations and optical measurements of imide model
compounds.16 The principles for designing highly fluorescent
PIs can be summarized as follows: (1) use alicyclic diamines
and (2) use aromatic dianhydrides that have flexible linkages
with extended π-conjugation. According to this concept, we
have successfully synthesized a strongly blue fluorescent Al-PI
derived from 1,4-bis(3,4-dicarboxyphenoxy)benzene dianhy-
dride (HQDEA) and DCHM, which has a Φ value of 0.11.16
We have also reported a series of Al-PIs synthesized from
perfluorinated aromatic dianhydrides, such as 1,4-bis(3,4-
dicarboxytrifluorophenoxy)tetrafluorobenzene dianhydride and
difluoropyromellitic dianhydride, which exhibited strong bluish-
green and red fluorescence, respectively.25 Although their
fluorescence intensity was significantly enhanced, these PIs
could not be used in wavelength converters because the energy
gap between the absorption and fluorescence, which corre-
sponds to the difference between the excitation and emission
wavelengths called a Stokes shift, was not large enough. If much
larger Stokes shift values could be achieved together with the
high Φ value, UV/short-visible light from solar radiation or
LED sources could be efficiently converted to yellow, orange,
or red light. To significantly enhance the Stokes shift of PIs,
further innovative concepts for molecular design of fluorescent
PIs are demanded.
In our previous study, we reported that 3-hydroxy-N-
cyclohexylphthalimide (3HNHPI), an imide compound con-
taining an −OH group and forming an ideal intramolecular
hydrogen bonding (intra-HB) between the −OH and CO
groups, exhibits strong green fluorescence with a large Stokes
shift (ν = 11 394 cm−1, 1.412 eV).17 It has been reported that
molecules with intra-HB structures similar to 3HNHPI undergo
excited-state intramolecular proton transfer (ESIPT) as a result
of the increased proton-donating and accepting properties of
−OH and CO groups, respectively, upon excitation.28−30
Flavone,31 anthraquinone,32 quinoline,33 benzoxazole,34 and
salicylidene aniline35 derivatives are the representative ESIPT
compounds that have been applied in laser dyes,36 high-energy
radiation detectors,37 UV-photostabilizers,38 and fluorescent
probes.39 In the case of 3HNHPI, the ESIPT phenomenon was
strongly suggested to take place in the excited state. Excitation
of an enol by UV radiation readily generates the Franck−
Condon excited state, which rapidly undergoes the ESIPT
process to generate an excited-state keto tautomer. During the
subsequent relaxation to the ground state by radiative or
nonradiative processes, a reverse proton transfer occurs to yield
the original ground-state enol. Because of the large energy
difference between the enol and keto tautomer, the excited-
state keto tautomer exhibits an unusual green emission with a
large Stokes shift. It has been reported that a wide color range
of fluorescence can be generated through ESIPT processes by
adjusting the relative emission intensities of enol and keto.
Tang et al.40 obtained a white-light emitting compound with a
single ESIPT unit by rational tuning of the energies of the enol
and the keto tautomer. In addition, polymers containing ESIPT
units in their main or side chains have also been reported. Park
et al.41 reported that polybenzoxazole (PBO) having two −OH
groups in the repeating units exhibits an ESIPT emission at 610
nm with excitation at 380 nm (ν = 9939 cm−1). Chu et al.42
synthesized a polymer having two 2-(2′-hydroxyphenyl)-
benzoxazole (HBO) moieties in the repeating unit which
exhibits an ESIPT emission at 616 nm with a large Stokes shift
(ν = 7520 cm−1). Campo et al.43 reported that poly(methyl
methacrylate) (PMMA) with benzazole units in its side chains
exhibits various-color ESIPT emission depending on the kind
of solvent or chemical structure of the benzazole moieties.
However, the thermal stability of these polymers would be
insufficient for optical or photonic applications due to the lower
Tg of the main chains. To the best of our knowledge, thermally
stable polymers such as PIs with ESIPT moieties incorporated
in their main chain have never been reported.
First, 3,6-dihydroxypyromellitic dianhydride (P2HDA),
which has two −OH groups in the para positions of the
benzene ring of pyromellitic dianhydride, was synthesized.
Additionally a PI (P2H-DC; Chart 1a) and an imide model
compound (P2H-Ch; Chart 1b) were newly synthesized to
examine their fluorescence properties. Comparative experi-
ments for related molecules with the same skeleton but without
−OH groups (PM-DC and PM-Ch; Chart 1c,d) were
performed to clarify the substituent effect of the −OH groups
B
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at 100 °C for 36 h. After cooling to room temperature, yellow
precipitate was filtrated and dried. Faint yellow crystals were obtained
by recrystallization from water (yield 32%). 13C NMR (100 MHz,
DMSO-d6, ppm) δ = 121.81, 148.16, 167.91. Elemental Analysis (%)
Calcd for C10H6O8·2H2O: C 37.28, H 3.13, O 59.59. Found: C 37.28,
H 3.05, O 59.67.
Chart 1. Chemical Structures of (a) P2H-DC, (b) P2H-Ch,
(c) PM-DC, and (d) PM-Ch
P2HDA. 3,6-Dihydroxypyromellitic acid (1.14g, 4 mmol) was heated
at 180 °C under reduced pressure and resulted in yellow solid (yield
72%). 13C NMR (100 MHz, acetone-d6, ppm) δ = 125.93, 147.53,
160.70.
P2H-Ch. The precursor of P2H-Ch, amic acid silyl ester, was
prepared by the in situ silylation method reported by Oishi et al.45,46
Cyclohexylamine (595 mg, 6 mmol) and BSTFA (771 mg, 3 mmol)
were stirred for 30 min in DMAc (6 mL) in an ice bath (solution I).
P2HDA (321 mg, 1.28 mmol) was stirred for 30 min in DMAc (6 mL)
in an ice bath (solution II). Solutions I and II were mixed and then
stirred overnight in an ice bath. After distillation of DMAc under
reduced pressure, propionic acid (15 mL) was added and refluxed for 6
h under a nitrogen atmosphere. After cooling to room temperature,
yellow solid reprecipitated by excess methanol was filtrated and dried.
Yellow crystals were obtained by recrystallization from o-xylene (yield
72%). A melting point of P2H-Ch could not be accurately obtained
due to an oxidation reaction in the −OH groups moieties at around
and their role in the ESIPT behaviors. Second, to investigate
the sensitivity of the imide compound and PI to basicity of the
environment (pH), optical absorption spectra were measured
in basic conditions by using an organic base (DBU) and basic
salt (NaOH). This study aims to design new types of highly
fluorescent PIs by introducing “ESIPT units” in the main chain,
which would enable to control the absorption and fluorescence
wavelength over a very wide range.
1
230 °C. H NMR (400 MHz, DMSO-d6, ppm) δ = 1.1−1.4 (m, 6H),
1.6−1.9 (m, 10H), 2.0−2.2 (m, 4H), 3.94 (m, 2H). 13C NMR (100
MHz, DMSO-d6, ppm) δ = 24.13, 28.57, 49.27, 53.78, 122.95, 143.52,
164.61. Elemental Analysis (%) Calcd for C22H24N2O6: C 64.07, H
5.87, N 6.79, O 23.28. Found: C 64.36, H 5.90, N 6.66, O 23.08.
HRMS (EI) m/z [M + H]+ calcd for C22H24N2O6 412.1634; found
412.1624. The HRMS spectrum of P2H-Ch is shown in Figure S1 of
the Supporting Information.
2. EXPERIMENTAL SECTION
P2H-DC PI. The precursor of P2H-DC PI, poly(amic acid) silyl
ester (PASE), was also prepared by the in situ silylation method.
DCHM (84 mg, 0.4 mmol) and BSTFA (108 mg, 0.4 mmol) were
stirred in DMAc (1 mL) for 30 min in an ice bath. P2HDA (100 mg,
0.4 mmol) was added to the solution and then stirred overnight. The
resulting viscous yellow solution was spin-coated onto a fused silica
(amorphous SiO2) substrate, followed by soft-baking at 70 °C for 1 h
and subsequent thermal imidization by a one-step imidization
procedure: the final curing conditions were 220 °C for 1.5 h under
nitrogen flow. The heating rate from 70 to 220 °C was 4.6 °C/min.
Measurements. Solutions of the imide compounds for optical
measurements had concentrations in the range from 10−5 to 10−6 M.
The solvents, chloroform (CHCl3, 99.9%, Kanto Chemical Co., Inc.,
fluorescence grade) and trifluoroacetic acid (TFA, 99.9%, Aldrich,
spectroscopic grade), were used without further purification. UV/vis
absorption spectra of the solutions and PI films were measured with a
Hitachi U-3500 spectrophotometer. Fluorescent excitation/emission
spectra of the same samples were measured with a Hitachi F-4500
fluorescence spectrometer equipped with a HAMAMATSU R928
photomultiplier tube. The fluorescence spectra of solutions were
measured without degassing. The front-face method was adopted for
film samples to reduce self-absorption of the emitted fluorescence.
Emission spectra were measured with the excitation at the peak
wavelength (λex) of the corresponding excitation spectra. In contrast,
excitation spectra were measured by monitoring the fluorescence
intensity at the peak wavelength (λem) of the emission spectra. The
measured spectra were not corrected for the sensitivity of the
photomultiplier tubes to the fluorescence wavelength. The photo-
luminescence quantum efficiencies of the solution and film samples
were measured in another way by using a calibrated integrating sphere
(HAMAMATSU C9920) connected to a multichannel analyzer
(HAMAMATSU C7473) via an optical fiber link. In this measure-
ment, the samples were excited at a controlled λex using a
monochromated xenon light source, and the solution sample was
degassed by argon prior to measurement. The single crystal of P2H-
Ch was obtained from o-xylene solution. The single crystal XRD data
of P2H-Ch was recorded on a Rigaku R-AXIS RAPID-II (Cu Kα). The
structure was solved by the direct methods with the program
SHELXS-97 and was refined by the full-matrix least-squares method
Materials. Difluoropyromellitic dianhydride, generously supplied
by NTT corporation, was used as received. Cyclohexylamine (Kanto
Chemical Co., Inc.) was purified by distillation under reduced
pressure. 4,4′-Diaminocyclohexylmethane (DCHM), purchased from
Tokyo Kasei Kogyo Co., Ltd., was recrystallized from n-hexane and
sublimed under reduced pressure. N,O-Bis(trimethylsilyl)-
trifluoroacetamide (≥99%, BSTFA, Aldrich) and N,N-dimethylaceta-
mide (anhydrous, DMAc, Aldrich) were used as received. 1,8-
Diazabicyclo[5.4.0]undec-7-ene (DBU, Tokyo Kasei Kogyo Co.,
Ltd.) was purified by distillation under reduced pressure. o-Xylene
and sodium hydroxide (NaOH), purchased from Kanto Chemical Co.,
Inc., were used as received.
Synthesis. 3,6-Difluoropyromellitic Acid Tetramethyl Ester.
Difluoropyromellitic dianhydride (12.70 g, 50 mmol) was refluxed in
methanol (250 mL) and concentrated sulfuric acid (7.5 mL) for 48 h.
After cooling to room temperature, white powder reprecipitated by
excess water was filtrated and dried. Needle-like crystals were obtained
by recrystallization from methanol/water solution (yield 71%). 1H
NMR (400 MHz, DMSO-d6, ppm): δ = 3.90 (s, 12H). 19F NMR (377
MHz, DMSO-d6, ppm): δ = −116.58. 13C NMR (100 MHz, DMSO-
d6, ppm): δ = 53.78, 124.12, 152.21, 161.51. Elemental Analysis (%)
Calcd for C14H12F2O8: C 48.56, H 3.49, F 10.97, O 36.97. Found: C
48.33, H 3.46, F 11.17, O 37.04.
3,6-Dimethoxypyromellitic Acid Tetramethyl Ester. 3,6-Difluor-
opyromellitic acid tetramethyl ester (6.92 g, 20 mmol) and sodium
methoxide (2.70 g, 50 mmol) dissolved in DMAc (20 mL) were
heated and kept at 140 °C for 4 h under a nitrogen atmosphere. After
cooling to room temperature, brown solid reprecipitated by excess
water was acidified by hydrochloric acid and then filtrated and dried.
Plate-like crystals were obtained by recrystallization from methanol/
water solution (yield 33%). 1H NMR (400 MHz, DMSO-d6, ppm) δ =
3.76 (s, 6H), 3.86 (s, 12H). 13C NMR (100 MHz, DMSO-d6, ppm) δ
= 53.28, 64.08, 129.55, 150.63, 164.19. Elemental Analysis (%) Calcd
for C16H18O10: C 51.89, H 4.90, O 43.21. Found: C 51.76, H 4.80, O
43.44.
3,6-Dihydroxypyromellitic Acid.44 3,6-Dimethoxypyromellitic acid
tetramethyl ester (5.18 g, 14 mmol) was dissolved in a mixture of
acetic acid (44 mL) and hydrobromic acid (9.56 mL) and then heated
C
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with SHELXL-97.47 All the non-hydrogen atoms were refined with
anisotropic temperature factors. The H atoms of the −OH groups
were located from the difference electron density map. The other H
atoms were inserted in standard positions and refined by a riding
model. The Fourier transformed infrared (FT-IR) absorption spectra
of PI films were measured in the range of 650−4000 cm−1 using a
Thermo-Fisher Avatar-320 spectrometer equipped with a Thunder-
dome attenuated total reflection (ATR) attachment (incident angle
45°). The prism (internal reflection element) was made of germanium
crystal with a refractive index of 4.0. All optical measurements were
conducted at ambient temperature without humidity control.
Thermogravimetric analysis (TGA) was conducted using a Shimadzu
DTG-60 analyzer with a heating rate of 5 °C/min.
Quantum Chemical Calculation. The density functional theory
(DFT) with the three-parameter Becke-style hybrid functional
(B3LYP)48−50 was adopted for calculating electronic structures and
spectroscopic properties of the imide compounds. Geometry
optimization was independently performed with the 6-311G(d) basis
set for the S0 and S1 states, respectively. The 6-311++G(d,p) basis set
was used for calculating vertical excitation wavelength and oscillator
strength (f) at each of the S0 and S1 geometries. The S0 → S1
transition at the S0 geometry corresponds to optical absorption of the
ground state; on the other hand, that at the S1 geometry corresponds
to fluorescent emission according to Kasha’s rule. All calculations were
performed with the Gaussian-09 B.01 program package,51 which
implements analytical gradients at the time-dependent (TD)-DFT
level.
Figure 1. Optical absorption spectra of PM-Ch and P2H-Ch (1 × 10−5
M) in CHCl3 solution. The arrow denotes a weak absorption tailing of
PM-Ch solution.
3. RESULTS AND DISCUSSION
3.1. Imide Model Compounds. Crystal Structure of P2H-
Ch. The crystal structure and the details of the crystal lattice of
P2H-Ch analyzed by single crystal XRD technique are shown in
Figure S2 of the Supporting Information. Bilton et al.52−54
reported that one of the most probable intramolecular
hydrogen bonding (intra-HB) motifs is a planar conjugated
six-membered ring with a propensity for resonance-assisted
hydrogen bonding (RAHB), in which short contacts are formed
between hydrogen donors and acceptors. P2H-Ch apparently
has a typical RAHB structure in which electrons alleviate the
bias of charges by the resonance, which is well demonstrated by
the short distances between the atoms forming a pair of intra-
HBs: H1−O6 (2.240(2) Å) and O2−O6 (2.941(2) Å) and a
typical angle O2−H1−O6 (140(2)°).
UV/Vis Absorption and Fluorescence Spectra of the Imide
Compounds. First, the fluorescence properties of solutions of
the imide model compounds, PM-Ch and P2H-Ch, were
examined to clarify the effects of the −OH groups. The UV/vis
absorption and fluorescence excitation/emission spectra of PM-
Ch and P2H-Ch dissolved in CHCl3 (1 × 10−5 M) are shown
in Figures 1, 2a, and 2b, respectively. Table 1 summarizes the
peak wavelengths in the optical absorption (λabs) and
excitation/emission (λex, λem) spectra, the Stokes shifts (ν),
and photoluminescence quantum efficiencies (Φ) observed for
these solutions. The calculated vertical excitation wavelengths,
oscillator strengths (f), contributing molecular orbitals (MO),
assignment of the electronic transitions, and the contribution of
each transition for the compounds at the S0 geometry are listed
in Table 2. Here, HOMO−m and LUMO+m denote the (m +
1)-th highest occupied MO and the (m + 1)-th lowest
unoccupied MO, respectively. In addition, the calculated
emission wavelengths (λem,cal) and the assignment of electronic
transitions for the compounds in the excited state with the
optimized S1 geometry are listed in Table S1 of the Supporting
Information.
Figure 2. Fluorescence spectra of PM-Ch and P2H-Ch (1 × 10−5 M)
in CHCl3 solution: (a) excitation spectra monitored at λem; (b)
emission spectra excited at λex and photo images of these solutions
under UV (λ = 365 nm) irradiation.
Table 1. Experimental Absorption Wavelengths (λabs),
Excitation/Emission Wavelengths (λex, λem), Stokes Shifts
(ν), and Photoluminescence Quantum Efficiencies (Φ) of
PM-Ch and P2H-Ch (1 × 10−5 M) in CHCl3 Solutions
a
imide
compound
λabs
(nm)
λex
(nm)
λem
(nm)
ν
(cm−1
)
Φ
b
PM-Ch
320
421
316
436
457, 641
8710
<0.01
0.16
P2H-Ch
430
1374, 7655
a
b
ν = 107 (1/λex − 1/λem). In the case of PM-Ch, λabs and λex are not
corresponding to S0 → S1 but S0 → S2 transition wavelength.
As shown in Figure 1, PM-Ch, an OH-free imide compound,
not forming intra-HB, exhibits an absorption peak at 320 nm
with a weak tailing at around 360 nm (depicted by an arrow).
The spatial distributions of the calculated MOs are illustrated in
Figure S3 of the Supporting Information. The HOMO−3 of
PM-Ch, which is localized around the lone pair of electrons of
the oxygen and nitrogen atoms of the imide rings, is a
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a
Table 2. Calculated Electronic Transitions of PM-Ch and P2H-Ch at the Optimized S0 Geometry
imide compound
PM-Ch
state
S1
transition wavelength (nm)
356.6
oscillator strength
0.0001
orbitals
character of transition contribution
HOMO−1
HOMO−3
HOMO
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
LUMO
n−π*
n−π*
π−π*
π−π*
π−π*
π−π*
π−π*
n−π*
n−π*
π−π*
π−π*
π−π*
π−π*
n−π*
n−π*
0.95
0.05
1.00
0.76
0.18
0.06
1.00
0.97
0.03
1.00
1.00
0.98
0.02
0.97
0.03
LUMO+2
LUMO
S2
S7
348.9
291.7
0.0023
0.0391
HOMO−5
HOMO
LUMO
LUMO+1
LUMO+1
LUMO
HOMO−7
HOMO
P2H-Ch enol
P2H-Ch keto
S1
S2
412.7
371.3
0.2368
0.0001
HOMO−2
HOMO−4
HOMO−1
HOMO
LUMO
LUMO+1
LUMO
S3
S1
S3
357.6
557.9
386.9
0.0024
0.2329
0.0012
LUMO
HOMO−2
HOMO−3
HOMO−4
HOMO−1
LUMO
LUMO
S4
362.5
0.0001
LUMO
LUMO+1
a
Prohibited transitions (f = 0) are not listed.
nonbonding (n) orbital, and the HOMO, LUMO, LUMO+1,
and LUMO+2, which are delocalized over the dianhydride
moiety, are π orbitals. Accordingly, the calculated transition at
356.6 nm (S0 → S1) is assignable to an LE(n−π*), and those at
348.9 nm (S0 → S2) and 291.7 nm (S0 → S7) are assignable to
an LE(π−π*) transition (Table 2). The wavelength of the S0 →
S1 transition with a small f value (0.0001) agrees well with the
weak absorption tailing at longer wavelengths in the
experimental absorption spectrum (around 360 nm), in
which the weak tailing is attributable to the LE(n−π*) band.
In contrast, the experimental absorption peak (320 nm) is in
agreement with the calculated S0 → S2 transition with a
relatively large f value (0.0023), which indicates that this peak is
attributable to the LE(π−π*) band. The excitation and
emission peaks of PM-Ch are observed at 316 and 436 nm,
respectively, as shown in Figure 2. The excitation peak is
attributable to an LE(π−π*) band because its wavelength (λex)
is in agreement with that of the LE(π−π*) band in the
absorption spectrum (320 nm). When PM-Ch in CHCl3 is
excited at 316 nm, a fast internal conversion from the Franck−
Condon S2 states to the lowest energy S1 state takes place
(Table 2). Hence, the emission peak at 436 nm is attributable
to an LE(n−π*) fluorescence, which is verified by the fact that
λem agrees well with the calculated wavelength of the LE(n−π*)
emission (440.2 nm, see Table S1 of the Supporting
Information). Since the f values for LE(n−π*) transitions are
generally very small, the LE(n−π*) fluorescence of PM-Ch is
too weak to be seen with the eyes (Φ < 0.01) as demonstrated
by the photo image in Figure 2b.
In contrast, P2H-Ch, which can form intra-HBs, dissolved in
CHCl3, displays an intense absorption peak at 421 nm with a
shoulder at 400 nm (Figure 1). This peak is unambiguously
attributable to the LE(π−π*) band of the enol because it has a
calculated value of 412.7 nm (S0 → S1) with a very large f value
(0.2368). This intense absorption peak shows a significant
bathochromic shift compared to PM-Ch, which is explainable
by the electron donating effects of the two −OH groups in the
para-positions of P2H-Ch. A similar phenomenon was also
observed for 2,6-dihydroxynaphthalene carbaldehyde which
also has two intra-HBs.55 We have reported that introduction of
an intra-HB into the anhydride moiety of imide compounds
effectively lowers the energy level of the n orbital, and then the
S0 → S1 transition becomes an LE(π−π*) transition, not an
LE(n−π*) transition. This leads to a significant enhancement
of the fluorescent intensity of the LE emission from the π*
orbital.17 For example, the HOMO−1 of PM-Ch, which is
assignable to an n orbital, is stabilized to a lower energy state
(HOMO−2) in P2H-Ch by the introduction of −OH groups
(see Figure S3 of the Supporting Information). Moreover, it is
very likely that P2H-Ch forms dual intra-HBs in solution as
well as in crystalline state, which also stabilizes the n orbitals,
leading to a decrease in the LE(π−π*) transition energy relative
to that for LE(n−π*).56,57
For P2H-Ch dissolved in CHCl3, dual fluorescent emission
peaks are observed at 457 and 641 nm upon excitation at 430
nm. These peaks demonstrate relatively small (1374 cm−1, 0.17
eV) and very large Stokes shifts (7655 cm−1, 0.95 eV),
respectively. The electronic transition caused by excitation is
attributable to the lowest energy LE(π−π*) transition of the
enol because the λex value agrees well with that of the
LE(π−π*) absorption band (421 nm). The first light-blue
fluorescence peak at 457 nm is attributable to the LE(π−π*)
fluorescence from the enol because the λem value is close to the
calculated value (454.4 nm; see Table S1 of the Supporting
Information). In contrast, the bright red fluorescence peak at
641 nm with the large Stokes shift should be emitted via a
photophysical process with excited-state intramolecular proton
transfer (ESIPT) caused by excitation at 430 nm. The
occurrence of ESIPT in the excited state can be verified by
the following facts. First, the calculated λem of the keto
tautomer (630.3 nm, see Table S1 of the Supporting
Information) is comparable to the experimental value (640
nm). Second, P2H-Ch forms six-membered-ring planar
structures with intra-HBs in the ground state, which are typical
RAHB structures (see Figure S2 of the Supporting
Information). Similar structure is frequently found in ESIPT
compounds.28−30 Up to now, several molecules with a structure
similar to P2H-Ch have been reported to exhibit ESIPT
emission. Hanson et al.58 reported that a 1,3-bis(2-
pyridylimino)isoindoline (BPI) derivative containing −OH
groups exhibits an ESIPT emission peak at 597 nm with a
large Stokes shift (6600 cm−1, 0.82 eV) in a CH2Cl2 solution.
Fin et al.59 reported that a naphthalenediimide derivative
containing −OH groups exhibits dual emission peaks at 500
and 630 nm in a DMSO/water 4:1 solution, and the latter band
was attributed to an ESIPT emission. Third, the relative
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Scheme 1. Schematic Representation of Excitation and Emission Mechanism with ESIPT Photocycle Scheme of P2H-Ch in
CHCl3 and TFA Solutions
intensity of the emission peaks at 477 and 651 nm in a TFA
solution is much larger than that in a CHCl3 solution (see
Figure S4 of the Supporting Information). Because of the
strong hydrogen-donating property of TFA, intra-HBs in P2H-
Ch were destroyed, and the ESIPT process was suppressed
because of the formation of inter-HBs with TFA. Roberts and
co-workers60 investigated the effect of solvents on the ESIPT
emission of 2-(2′-hydroxyphenyl)benzimidazole (HBI). With
increasing concentrations of trifluoroethanol (TFE), which has
strong hydrogen-donating properties, the emission originating
from the keto tautomer at 470 nm gradually faded out
producing of a weak emission band between 350 and 400 nm.
This emission band was attributed to the enol, which forms
inter-HBs with TFE. In our previous report,17 the emission
from the enol of 3-hydroxy-N-cyclohexylphthalimide
(3HNHPI) having an ESIPT property was observed in its
TFA solution. In the case of P2H-Ch, which has a structure
similar to 3HNHPI, suppression of the proton transfer in the
excited state in TFA should occur in a similar manner. The
emission peak at 641 nm in CHCl3 is readily attributable to the
excited-state keto tautomer formed as a result of the ESIPT
process. The Φ value of 0.16 is significantly larger than that of
PM-Ch (Φ < 0.01), which mainly originates from the very large
f value (0.2329) of LE(π−π*) transition (S0 → S1) of the keto
tautomer of P2H-Ch. The expected photophysical behavior of
the compounds under investigation in the CHCl3 and TFA
solutions is illustrated in Scheme 1. It is obvious that the
prominent red emission observed in CHCl3 solution is
generated via the ESIPT process, whereas the light-blue
emission of the TFA solution is due to the prohibition of
ESIPT by the formation of inter-HBs with solvent molecules.
Variations in Absorption Spectra under Basic Conditions.
The pKa of phenolic −OH groups is generally lower than that
of alkyl alcohol −OH groups. This is due to the resonance
effect with the neighboring aromatic rings, which effectively
stabilizes the conjugate base of the phenolic compounds.
Therefore, deprotonation of −OH groups easily occurs in the
presence of basic reagents (e.g., NaOH and DBU). It is also
well known that hydroquinone derivatives can be in a
monoanion or dianion form depending on pH.59,61 Zahid et
al.61 reported the absorption properties of 2,3,5,6-tetracyano-
1,4-hydroquinone (TCHQ) under conditions with various pH
using citric acid and Na2HPO4 aqueous solutions. Under pH 2
and 8, absorption peaks attributed to its monoanion and
dianion forms were newly observed at 461 and 520 nm,
respectively. Since P2H-Ch can form a hydroquinonoid
structure, its UV/vis absorption spectrum can change depend-
ing on the basic conditions. In this study, DBU dissolved in
CHCl3 was used to adjust the basicity of the solutions. Figure 3
Figure 3. Optical absorption spectra of P2H-Ch (1 × 10−5 M) under
basic conditions using CHCl3/DBU mixed solvents.
shows variations in absorption spectra of P2H-Ch depending
on the DBU concentration ([DBU]). When [DBU] reached
10−3 M, the color of the solution immediately changed from
pale yellow to bright pink. In addition, a new strong absorption
peak was observed at 540 nm while the original absorption
band at 421 nm disappeared. This spectral change clearly
indicates that deprotonation was promoted by DBU, and the
new absorption band is attributed to the monoanion form.
Furthermore, when [DBU] is increased up to 1 M, the color of
the solution gradually changed from bright pink to light blue. In
addition, another new absorption peak was observed at a much
longer wavelength (638 nm), and the band at 540 nm
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significantly weakened. This indicates that the second
deprotonation step took place, and the new absorption band
is attributable to the dianion form. As seen in the photos in
Figure 3, the pH-dependent color change, i.e., halochromism,
from pale yellow (enol, neutral) to light blue (dianion) through
bright pink (monoanion) is very vivid and prominent. These
results clearly indicate that the introduction of −OH groups
into imide compounds make them more sensitive to a basic
environment.
3.2. Highly Fluorescent PI Containing −OH Groups.
Characterizations of the PIs. The ATR FT-IR spectra of the
PM-DC and P2H-DC films are shown in Figure S5 of the
Supporting Information. The completion of thermal imidiza-
tion was confirmed by the CO and C−N stretching bands of
the imide rings at 1700 and 1360 cm−1, respectively. Notably,
the CO stretching peak of P2H-DC is significantly shifted to
a lower wavenumber by 8 cm−1 relative to that of PM-DC,
which could be caused by the formation of intra-HBs between
the phenolic −OH and imide CO groups in P2H-DC.17,54
Figure 5. Photo images of PM-DC and P2H-DC PI films under white
light (left) and UV (λ = 365 nm) irradiation (right).
relative to PM-DC (by 90 nm) is comparable to that of P2H-
Ch and PM-Ch (by 101 nm). Note that the shoulder peak of
the PI (P2H-DC) is located at a slightly shorter wavelength
than the main peak of the imide model compound (P2H-Ch),
although the area of the absorption band in the visible region
for the PI is larger than that for the model compound.
5
The decomposition temperatures at a 5% weight loss (Td ) for
Figure 6 shows the fluorescence spectra of the PM-DC and
P2H-DC films, in which relatively wide bands of fluorescent
PM-DC and P2H-DC were found to be 462 and 364 °C,
respectively. The TGA curve of P2H-DC is shown in Figure S6
5
of the Supporting Information. Despite the lower Td
temperature for P2H-DC, which may be due to the
intermolecular dehydration reaction at the phenolic −OH
moiety, both PIs demonstrated good thermal stability without
significant weight losses below 350 °C. The high thermal
stability of the PIs originating from the rigid molecular
structure without degradable side chains is preferable and
advantageous for fabrication processes in modern electronics
and photonics applications.
UV/Vis and Fluorescence Spectra of the PI Films. The UV/
vis absorption spectra of the PM-DC and P2H-DC solid films
are shown in Figure 4. These PI films exhibit absorption bands
Figure 6. Fluorescence spectra of PM-DC and P2H-DC PI films.
emission are observed at 439 and 642 nm as a result of
excitation at 315 and 418 nm, respectively. These bands also
coincide well with the emission bands of PM-Ch and the keto
form of P2H-Ch (440 and 630 nm, respectively) dissolved in
CHCl3 (Figure 2b and Table 1). The intense red emission band
of P2H-DC clearly indicates that, even in the solid state, ESIPT
is a dominant process in the excited state of the PI and causes a
very large Stokes shift (8994 cm−1, 1.12 eV). This value is
comparable with the Stokes shifts of benzoxazole-type ESIPT
polymers.41,42 Although dual emission peaks from the enol and
keto forms were observed at 457 and 641 nm for P2H-Ch in
solution, the emission from the enol form was not clearly
observed for P2H-DC due to overlap with the excitation
radiation (not shown). As seen in Figure 5 (right), the P2H-
DC film exhibits prominent red fluorescent emission under UV
light irradiation (LED light source at 365 nm). The Φ value of
P2H-DC was not so high (0.01), though this value is
significantly enhanced compared with conventional PIs such
as PMDA/ODA (Φ = 9.7 × 10−7). These results clearly
demonstrate that ESIPT can be efficiently induced upon
excitation of the solid PI films, producing a significantly large
Stokes shift. The ESIPT phenomenon is a powerful and
versatile tool to control the fluorescence wavelengths and
Stokes shifts of imide compounds and PIs.
Figure 4. Optical absorption spectra of PM-DC and P2H-DC PI films.
with peak maxima at 317 and 407 nm, respectively. These peaks
coincide well with those of the corresponding imide model
compounds (320 nm for PM-Ch and 421 nm for P2H-Ch),
indicating that the PIs have electronic structures similar to the
model compounds in the ground states. Accordingly, both of
the absorption peaks are readily attributable to the LE(π−π*)
bands. Figure 5 represents photo images of the PM-DC and
P2H-DC films under white light exposure (left side). The
highly transparent colorless PM-DC film shows only a weak
absorption peak at 317 nm with its main absorption peak being
in the UV region, whereas the P2H-DC film is pale yellow in
color and has an absorption peak with a maxima at 407 nm, a
shoulder at 418 nm, and a tailing up to 500 nm (Figure 4). The
long wavelength shift of the absorption band of P2H-DC
Variations in the Optical Absorption Properties of the
P2H-DC Film under Basic Conditions. Figure 7 shows the UV/
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energy of the LE(π−π*) transition and enhance the LE
fluorescence from the π* orbital. In addition, P2H-Ch, which
forms intra-HBs, exhibited a strong emission from the excited
keto-form at 641 nm with a very large Stokes shift (7655 cm−1)
via the ESIPT process. Under basic conditions prepared by
mixed solvents of CHCl3/DBU, the UV/vis absorption
spectrum of P2H-Ch was significantly altered in response to
[DBU]. Strong absorption peaks appeared at 540 and 638 nm
at [DBU] = 10−3 and 1 M and are attributable to monoanion
and dianion forms resulting from the first and second
deprotonations, respectively.
The novel highly fluorescent PI, P2H-DC, also exhibited
prominent red emission at 642 nm with a very large Stokes shift
of 8994 cm−1 due to the efficient ESIPT process in the solid
state. In addition, the thermal degradation temperature of the
PI exceeds 350 °C. After soaking in NaOH aqueous solutions at
pH 11 and 14, intense absorption peaks corresponding to the
monoanion and dianion forms of P2H-DC were observed at
529 and 617 nm, respectively. It was verified that rational
introduction of −OH groups into the PI and imide compound
enabled to generate and control the characteristic red
fluorescence, its high sensitivity to basic conditions, and visible
halochromism. This study proposes a new molecular design
concept for enhancement and control of the fluorescence
properties of PIs and imide compounds based on the ESIPT
phenomenon. The highly red fluorescent PIs derived from
P2HDA are promising for new electronics and photonics
applications as well as for the manufacture of wavelength
converters and pH sensors.
Figure 7. Optical absorption spectra of P2H-DC PI film under basic
conditions using various concentrated NaOH(aq).
vis absorption spectra of the P2H-DC film under basic
conditions. The spectra were measured after soaking the PI
film in NaOH aqueous solutions (pH 11 and 14). Variations of
the absorption spectra with the elapsed time are shown in
Figure S7 of the Supporting Information. At pH 11, the
intensity of the absorption band at 407 nm for P2H-DC
gradually decreased and the intensity of the new band at 529
nm increased. The wavelength of the new band agrees well with
the absorption band of P2H-Ch dissolved in CHCl3/DBU
([DBU] = 10−3 M), indicating that the band is attributable to
the monoanion form of P2H-DC. After soaking in a basic
solution, the film color gradually changed from pale yellow to
deep red due to the first deprotonation of the −OH group in
the dianhydride moiety. Furthermore, at pH 14, another new
absorption band was observed at a much longer wavelength
around 630 nm. This also agrees well with the absorption band
of P2H-Ch dissolved in CHCl3/DBU ([DBU] = 1 M),
indicating that the new band is attributable to the dianion form
of P2H-DC. The film color then changed to deep blue due to
the second deprotonation of the residual −OH groups. These
facts indicate that PIs with −OH groups at the pyromellitic
moiety are also sensitive to the basic conditions even in the
solid state, exhibiting significant halochromism. Therefore, the
highly fluorescent PIs derived from P2HDA can be potential
candidates for the innovative wavelength converting materials
which can convert short wavelength light to long wavelength
light with high efficiency. In addition, they can be used not only
in electronics and photonics applications, but also as pH
sensors with color changes from yellow through deep red (at
pH 11) to deep blue (pH 14).
ASSOCIATED CONTENT
* Supporting Information
■
S
HRMS spectrum of P2H-Ch, single-crystal XRD data for P2H-
Ch, calculated molecular orbitals of PM-Ch and P2H-Ch (TD-
DFT method at the B3LYP/6-311++G(d,p) level), absorption
and fluorescence spectra of P2H-Ch in a TFA solution, ATR
FT-IR spectra of PM-DC and P2H-DC film, TGA curve of the
P2H-DC film, and elapsed time variations of the absorption
spectra of P2H-DC soaked in NaOH aqueous solutions at pH
11 and 14; X-ray crystallographic information file (CIF) of the
crystal structure of P2H-Ch. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sando@polymer.titech.ac.jp (S.A.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
4. CONCLUSION
Single-crystal XRD data for P2H-Ch were collected and
analyzed by Dr. Kohei Johmoto and Prof. Hidehiro Uekusa
at Tokyo Institute of Technology. Their cooperation and
efforts are greatly acknowledged.
A highly fluorescent PI and imide compound emitting
prominent red fluorescence at long wavelengths were rationally
designed and synthesized. Two −OH groups were introduced
to the pyromellitic dianhydride moiety of the compounds to
form intra-HBs with the imide CO groups. To investigate
the effects of the introduced −OH groups, fluorescence
properties of the PIs and imide compounds with the same
structure but without −OH groups were compared. In CHCl3
solutions, only a weak LE(n−π*) fluorescence peak was
observed for PM-Ch (Φ < 0.01), whereas an intense LE(π−π*)
fluorescence peak was observed for P2H-Ch (Φ = 0.16). The
−OH groups in P2H-Ch significantly lower the transition
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DOI: 10.1021/ma502456f
Macromolecules XXXX, XXX, XXX−XXX