Alkylamide-substituted tetraphenylethylene: Three modes of fluorescence based on a hydrogen-bonded excimer

Article abstract of DOI:10.1039/c6ob01110a

Three modes of emission, with different intensities and wavelengths were observed in tetraphenylethylenetetraamide 1 depending on the solvent, concentration and continuous irradiation with light, where intermolecular hydrogen bonds between amide units pla

Full text of DOI:10.1039/c6ob01110a

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COMMUNICATION
Alkylamide-Substituted Tetraphenylethylene: Three Modes of
Fluorescence Based on a Hydrogen-Bonded Excimer
Takashi Takeda,*a Shunsuke Yamamoto, a Masaya Mitsuishi a and Tomoyuki Akutagawaa
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
LC phase.9 Upon the application of an electric field, the amide
Three modes of emission, with different intensities and
wavelengths were observed in tetraphenylethylenetetraamide
1 depending on the solvent, concentration and continuous
irradiation with light, where intermolecular hydrogen bonds
units in the LC phase invert, which provides ferroelectricity.10 In
50 each case, the intermolecular hydrogen bonding of amide units
plays an important role.
In this study, we introduced four tetradecylamide units to
10 between amide units played a significant role. Limitation of
the motility of the molecule in excited state by
intermolecular hydrogen bonding is the key for observation
of fluorescence even in the solution and transformation to the
corresponding 9,10-diphenylphenanthrene derivative.
tetraphenylethylene (TPE), which is
a representative AIE
1
luminophore.11 We expect that the initial aggregation would
55 occur in a well-organized fashion with molecular movement
limited less in good solvents due to intramolecular hydrogen
bonding, while aggregation with
a
random molecular
15 Materials that exhibit aggregation-induced emission (AIE)1 have
attracted considerable interest not only from a fundamental
perspective but also with regard to practical applications, such as
in organic light-emitting diode devices.2 In contrast to typical
emissive polyaromatic hydrocarbons (PAH), which lose
20 fluorescence with an increase in concentration, AIE luminophores
show increased fluorescence. Whereas PAH easily form excimer
in concentrated solution that lose photoexcitation energy by
nonradiative processes, the rotation of the substituents in AIE
luminophores is restricted in the aggregate, which suppresses
25 nonradiative deactivation to increase the fluorescence intensity. A
variety of AIE luminophores such as penta/hexaphenylsiloles,3
polyphenylenes,4 and arylenevinylenes5 have been reported.
Since the AIE phenomena reported so far have been examined
in mixtures of a good solvent (generally THF) and a poor solvent
30 (generally water),6 it is impossible to control the aggregation state
and the degree of molecular motion in the aggregate. In fact, the
luminescence spectrum of AIE is generally broad, which suggests
random molecular structures and the interaction of AIE molecules
in the aggregate. If the manner of the aggregation of AIE
35 molecules could be controlled, their fluorescence spectra may be
sharpened and the fluorescence wavelength and intensity could be
tuned depending on intermolecular interactions and the degree of
allowed molecular motion, respectively.
arrangement and more limited molecular motion would occur in
poor solvents. In addition, the intramolecular photocyclization of
60 TPE derivatives with an electron-donating group has been
reported.12,13 Thus, continuous photoirradiation might also alter
the photophysical properties of tetradecylTPEtetraamide 1.
Herein, we describe the preparation and photophysical properties
of 1. Fluorescence properties such as the wavelength and/or
65 intensity of 1 could be tuned by changing the concentration and
solvents. The high aggregation properties of this molecule
enabled us to observe the fluorescence of hydrogen-bonded and
well-organized aggregate 1 even in good solvents such as CHCl3
and THF as well as typical AIE emission not only in aqueous
70 medium but also in a mixed organic solvent of CHCl3 (good
solvent) and DMSO (poor solvent). Upon continuous irradiation
with UV-light, intramolecular photocyclization occurred to give a
new fluorescence peak in 1 even with an electron-deficient amide
group. Intermolecular hydrogen bonding between amide units in
75 the excited state decreased the motility of the molecule to induce
this photocyclization. Since the wavelengths and intensities of
these three modes of fluorescence of 1 are different, it may have
potential as a multi-stimulus-responsive fluorophore.
80
To achieve this control, we envisioned the attachment of amide
40 units with strong aggregation ability to the terminal positions of
AIE luminophores. Benzeneoligoamide derivatives have been
extensively studied from the perspectives of self-aggregation in
solution, the formation of a liquid crystal (LC) and unique
ferroelectric properties in the LC phase. For example, benzene-
45 1,3,5-tricaboxamide forms columnar self-aggregation even in
dilute solution, which has been well characterized
experimentally.8 It also forms a thermotropic hexagonal columnar
85
90
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3250 cm-1.16 This indicates that TPEtetraamide 1 is isolated in
60 dilute solution (less than 10-3 M) while it possesses hydrogen
bonding units at the terminal position. The difference in
aggregation properties between 1 and benzeneoligoamides and
pyrenetetraamide is due to the difference in the degree of
molecular motion of the central aromatic units. While the central
65 units in planar benzeneamides and pyrenetetraamide are fixed by
hydrogen bonds, the benzene rings of TPEcarboxamide can twist
along the Csp2-Cbenzene bond axis. (See Figure S5 in ESI.)
5
10
15
20
70
Figure 1 Molecular structure of 1 and illustration of the concept
of dual enhancement of the fluorescence of 1 based on an
aggregation/precipitation/photoisomerization strategy.
75
3500
3400
3300
3200
Wavenumber / cm-1
1
Figure 2 H NMR (left, in CDCl3) and IR [right, solid state (red)
The preparation of tetraamide 1 is summarized in Scheme 1.
and in CHCl3 (blue)] spectra of 1.
We started the synthesis of
1
from tetraphenylethylene.
80
Tetrabromide 2 was prepared as described in the literature.14
While the preparation of TPEtetracyanide 3 from its tetrabromide
Next, we examined the absorption and fluorescence properties
of 1. Figure S1 in ESI shows the concentration-dependent UV-
Vis spectrum of 1 in CHCl3. The UV-Vis spectrum of 1 showed
long wavelength absorption at 330 nm. No concentration-
25 215 has already been reported, modification of the work-up was
necessary to obtain pure 3 (details in ESI). Hydrolysis of
tetracyanide 3 under basic conditions15a gave tetracarboxylic acid 85 dependence was observed in the UV-Vis spectrum of 1, which is
4, which was converted to the corresponding acid chloride by the
reaction with thionyl chloride. The generated acid chloride was
30 directly reacted with tetradecylamine in the presence of
triethylamine to give tetraamide 1 as a white solid. For a
consistent with the results in the 1H NMR and IR spectra. Figure
3 shows the fluorescent spectra of 1 in CHCl3 and THF. It is well
known that the parent TPE does not emit fluorescence because of
efficient internal conversion due to high motility in solution. In
comparison of photophysical properties, tetraester 5, which is 90 fact, nonsubstituted TPE and its ester derivative 5 did not emit
unable to form hydrogen bonds, was also prepared.
any fluorescence at a concentration of 1x10-4 M. On the other
hand, TPEtetraamide 1 showed significant, albeit very weak,
fluorescence at around 500 nm even at a low concentration on the
order of 10-5 M (Figure 3 and Figure S2 in ESI). The fluorescence
95 intensity against absorption 1-T (T: transmittance in the UV-Vis
spectra, Figure S3 in ESI) shows linear relationship, indicating
the emissive species are the same all over the concentration
regime. In addition to this, we also observed large Stokes shift
(170 nm, 1.26 eV; see Figure S2 in ESI) and the fluorescence
100 spectrum without vibronic structure in the solution of 1 in
THF/CHCl3. Based on these experimental results, we can
conclude that the emission is solely derived from excimer.
Dominant excimer emission even at lower concentration (~10-6
M) was also observed in the structurally similar pyrenetetraamide
105 with hydrogen bonding ability.16 If we consider the NMR and IR
spectra (vide ante), TPEtetraamide 1 should form a dimer or
oligomer only in the excited state, where its motility is restricted
by intermolecular hydrogen bonding to allow radiative irradiation.
An increase in the concentration increases the population of
X
R
R
X
35
a
d,e
X
R
R
X
40
2: X = Br
1: R = CONHC14H29
5: R = COOC14H29
b
3: X = CN
c
4: X = COOH
Scheme 1 Preparation of tetradecylTPEtetraamide 1 and tetraester
45 5. (a) Br2; (b) CuCN, DMF; (c) KOH, ethyleneglycol; (d) SOCl2,
(e) C14H29NH2 (for 1) or C14H29OH (for 5), Et3N, THF.
The nature of the aggregation of 1 was examined by 1H NMR and
1
IR spectra. The H NMR spectrum of 1 showed sharp signals of
50 both aromatic and aliphatic protons. No concentration-
1
dependence was observed in the H NMR spectrum of 1, which
indicates that 1 has limited aggregation ability in the ground state. 110 excimer to increase fluorescence. It is worth noting that the
This is in sharp contrast to benzeneoligoamides10 and
pyrenetetraamide,16 which exhibit broad signals of aromatic
55 protons due to aggregation induced by strong intramolecular
hydrogen bonds. This difference can also be confirmed by the IR
fluorescence spectrum of 1 in good solvents (THF, CHCl3) was
sharper and more bathochromic than that in a poor solvent (vide
infra), which suggest that
a well-ordered intermolecular
hydrogen-bonded excimer was formed in equilibrium in a good
spectrum in CHCl3, where N-H stretching vibration of 1 was 115 solvent.
observed at 3450 cm-1, whereas that of pyrenetetraamide was
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Organic & Biomolecular Chemistry
5.9810-5 M
4.7910-5 M
3.5910-5 M
2.3910-5 M
1.2010-5 M
5.9810-6 M
6000
5000
4000
3000
2000
1000
0
5000
4000
3000
2000
1000
0
6.2210-5 M
4.9710-5 M
60 An increase in fluorescence could also be triggered by continuous
irradiation with UV light. Figure 5 shows changes in the
fluorescence spectra of 1 upon irradiation with UV light.
Moderate fluorescence at around 400 nm was continuously
increased. This fluorescence was completely different from both
65 the weak fluorescence observed in good solvent at a high
concentration and the strong fluorescence in a mixture of
good/poor solvents. Since this emission property was similar to
that of 9,10-diphenylphenanthrene, this change in the
fluorescence spectrum of 1 upon irradiation with UV light could
70 be accounted for by a photocyclization reaction between the two
benzene rings.12,13 Upon photocyclization, motility of the
molecule decreased due to the connection between the two
benzene rings by a covalent bond, which decreases the internal
conversion of excited species. As a result, the intensity of
3.7310-5 M
2.4910-5 M
1.2410-5 M
6.2210-6 M
2.4910-6 M
5
350
400
450
500
550
600
400
450
500
550
600
Wavelength / nm
Wavelength / nm
10 Figure 3 Concentration-dependent fluorescence spectra of 1 in
CHCl3 (left) and THF (right).
A drastic increase in fluorescence was observed upon the addition
of a poor solvent (H2O) to a solution of 1 in a good solvent (THF).
15 While weak, concentration-dependent fluorescence was observed
at 500 nm in THF, broad and intense fluorescence emerged at
around 450 nm upon the addition of H2O. This observation is 75 fluorescence is increased. It is worth noting that the
similar to the AIE of the parent TPE molecule. Since the emission
of 1 in THF/H2O was broad and strong, and the N-H stretching
20 vibration of 1 in the solid state was at a lower wavenumber (3307
cm-1) than that in good solvent, aggregation induced by the
corresponding ester 5 without a hydrogen-bonding unit did not
show a change in its UV-Vis spectrum or exhibit fluorescence
even after irradiation with UV light. This clearly indicates that
the limited motility in the hydrogen-bonded excimer of 1
addition of a poor solvent should be unarranged with a more 80 accelerated its photocyclization reaction. The photocyclization of
severe restriction of molecular motility and stronger hydrogen
bonding between amide units compared with a hydrogen-bonded
TPEtetracarboxylate with limited motility on a cray surface, and
the fluorescence spectrum of the cyclized product with an
emission maximum of 400 nm were reported by George and
coworkers.13 Although the clear changes of the UV-Vis and
25 excimer formed in
a good solvent. Thanks to increased
aggregation properties due to amide hydrogen bonding, the
emission intensity of 1 was saturated at THF:H2O =1:1, whereas 85 fluorescence spectra of 1 upon irradiation of UV-light were
90% H2O was necessary in the original TPE molecule. The strong
aggregation properties of 1 enabled the observation of AIE even
30 in the mixed organic solvent CHCl3:DMSO (1:9). Distinct
fluorescence of 1 in the solid state was also observed (Figure S4
in ESI).
observed at low concentration (Figure 5), we could not complete
this photocyclization at higher concentration enough to take H
NMR spectrum (Figure S8 in ESI). Because of incompletion of
photocyclization at high concentration, limited solubility of 1 and
90 difficulty in scale-up due to light source, we could not isolate the
photocyclized product in pure form.
1
(a)
(b)
(a)
(b)
35
40
45
50
0 h
2 h
4 h
6 h
1.0
0 h
2 h
4 h
6 h
80000
95
100
105
110
0.8
0.6
0.4
0.2
0.0
60000
40000
20000
0
250
300
350
400
450
500
350
400
450
500
550
Wavelength / nm
Wavelength / nm
(c)
(d)
(c)
55 Figure 4 Changes in the fluorescence spectra of 1 (a) in THF
upon the addition of H2O and (b) in CHCl3 upon the addition of
DMSO. Photographic demonstration of change of fluorescence of 115 Figure 5 Changes in (a) UV-Vis and (b) fluorescence spectra of 1
1 in (c) THF/H2O and (d) in CHCl3/DMSO.
in CHCl3 upon irradiation with UV light (>290 nm, 112 mW: see
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solvent).7
ESI for detail). (c) Photographic demonstration of change of
fluorescence of 1 in CHCl3 upon irradiation of UV light.
In summary, we investigated the fluorescence properties of
TPEtetraamide 1, which can exhibit aggregation. While molecule
1 in solution is isolated in the ground state, it forms a well-
ordered hydrogen-bonded excimer upon irradiation with UV light.
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As
a
result, weak, sharp and concentration-dependent
fluorescence was observed at around 500 nm even in a good
10 solvent. Upon the addition of a poor solvent, broad and intense
fluorescence was induced, which was ascribed to the AIE
phenomenon of TPE derivatives. Continuous irradiation of a
solution of 1 with UV light induced a photocyclization reaction to
produce moderate emission at a shorter wavelength (around 400
15 nm). The hydrogen-bonded excimer of 1 plays an important role
in limiting the motility of the molecule to induce photocyclization.
Since the wavelength and intensity of the fluorescence emissions
of 1 can be manipulated by the external environment, such as by
solvents and light, it could serve as a multi-stimulus-responsible
20 fluorophore. The modulation of physical properties based on the
control of molecular motility should be an interesting
approach16,17 for the creation of advanced materials, and is
currently under investigation in our laboratory.
25 This work was partially supported by JSPS KAKENHI Grant
Number 26410034 and an IMRAM project grant for intramural
cooperative study of young scientists (FY 2015).
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Notes and references
a Institute of Multidisciplinary Research for Advanced Materials, Tohoku
30 University, Katahira 2-1-1, Aoba-ku, Sendai, 980-8577 Miyagi, Japan
Tel: +81-22-217-5654; Fax: +81-22-217-5655; E-mail:
takeda@tagen.tohoku.ac.jp
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† Electronic Supplementary Information (ESI) available: Experimental
procedures and characterization data for new compounds. UV-Vis and
35 fluorescence spectra of 1 in CHCl3 and THF. Plots of fluorescence against
1-T. Calculated dimer structure of 1. UV-Vis spectrum of 5. 1H NMR
1
spectra of 1 before and after photoirradiation. H NMR spectrum of 3. 1H
and 13C NMR spectra of 5. See DOI: 10.1039/b000000x/
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