A strap strategy for construction of an excited-state intramolecular proton transfer (ESIPT) system with dual fluorescence

Article abstract of DOI:10.1002/anie.201404867

An amine-embedded flexible alkyl strap has been incorporated into an emissive boryl-substituted dithienylpyrrole skeleton as a new entity of excited-state intramolecular proton transfer (ESIPT) chromophores. The π-electron system shows a dual emission, wh

Full text of DOI:10.1002/anie.201404867

Angewandte
Chemie
DOI: 10.1002/anie.201404867
Excited-State Design
A Strap Strategy for Construction of an Excited-State Intramolecular
Proton Transfer (ESIPT) System with Dual Fluorescence**
Naoya Suzuki, Aiko Fukazawa, Kazuhiko Nagura, Shohei Saito, Hirotaka Kitoh-Nishioka,
Daisuke Yokogawa, Stephan Irle,* and Shigehiro Yamaguchi*
Abstract: An amine-embedded flexible alkyl strap has been
incorporated into an emissive boryl-substituted dithienylpyr-
role skeleton as a new entity of excited-state intramolecular
proton transfer (ESIPT) chromophores. The p-electron system
shows a dual emission, which covers a wide range of the visible
region depending on the solvent polarity. The incorporation of
the aminoalkyl strap as well as the terminal boryl groups
efficiently stabilize the zwitterionic excited-state species result-
ing from the ESIPT even in an aqueous medium.
Luminescence is one of the fundamental properties that lead
to attractive molecular functions of organic compounds.
Construction of novel p-conjugated skeletons that can yield
characteristic luminescence properties would have a signifi-
cant impact on a broad spectrum of chemistry including not
only organic electronics, but also bioimaging and sensors.
Among the various fascinating luminescence properties, we
now focus the attention on a fluorescence based on the
excited-state intramolecular proton transfer (ESIPT).[1] The
ESIPT is a photochemical process that produces a tautomer
with a totally different electronic structure from the initial
excited form. Consequently, the ESIPT state gives rise to
a fluorescence with an anomalously large Stokes shift. In
addition, the ESIPT chromophores often exhibit a dual
emission originating from both the initial excited form and
the proton-transferred tautomer, which covers a broad wave-
length range. These features of the ESIPT can be the basis of
various applications including white light-emitting materi-
als,[2–4] polymorph-dependent light-emitting materials,[5] and
fluorescent probes.[6–9] However, as the formation of an
intramolecular hydrogen bond is a prerequisite of the ESIPT,
the ESIPT chromophores have been limited to only a few
variations, such as planar molecules that consist of a phenol or
aniline scaffold as a hydrogen-bond donor and an imino/azo-
nitrogen- or carbonyl-oxygen-containing ring skeleton as
a hydrogen-bond acceptor. The ESIPT in most of these
molecules is based on a keto–enol type tautomerization
Figure 1. Designs of the excited-state proton transfer (ESIPT) systems
based on a) a keto–enol type tautomerism and b) a strap strategy with
a basic flexible chain. c) Chemical structures of compounds 1–3.
(Figure 1a). An exception is 2-(hydroxyphenyl)imidazo[1,2-
a]pyridine, whose ESIPT state adopts a zwitterionic form
instead of a keto form.[5,10]
We now disclose a new design of the ESIPT chromophore,
which does not rely on the keto–enol tautomerism, but is
based on a strap strategy making use of a functional flexible
chain. To equip the chromophore with an alkyl chain strap has
been used as a simple strategy to constrain a conformation of
the p-conjugated skeleton[11–14] or to insulate it.[15–18] In
contrast, our idea is to employ an alkyl strap embedding an
amine moiety, thereby endowing it with the function of a base.
We install this strap in an emissive p-conjugated scaffold
bearing a relatively acidic proton (Figure 1b). As the p
skeleton, we chose a pyrrole-containing oligoarene. The
appropriate chain length of the aminoalkyl strap would
ensure the formation of an intramolecular hydrogen bond
[*] N. Suzuki, Prof. Dr. A. Fukazawa, Dr. K. Nagura, Dr. S. Saito,
Dr. H. Kitoh-Nishioka, Prof. Dr. D. Yokogawa, Prof. Dr. S. Irle,
Prof. Dr. S. Yamaguchi
ꢀ
between a nitrogen atom in the strap and the pyrrole N H
ꢀ
proton. In the excited state, the acidity of the pyrrole N H
would increase[19] similar to that of a phenol O H. This
Institute of Transformative Bio-molecules (WPI-ITbM) and
Department of Chemistry, Graduate School of Science
Nagoya University
Furo, Chikusa, Nagoya 464-8602 (Japan)
E-mail: sirle@chem.nagoya-u.ac.jp
[20]
ꢀ
behavior would be enhanced by introduction of an electron-
accepting group in the pyrrole-containing p-conjugated
skeleton to impart a donor–acceptor type character. Based
on this consideration, we designed compound 1 that has an
electron-donating dithienylpyrrole skeleton with electron-
accepting boryl groups at the termini. We found that the
ESIPT indeed occurred in this molecule to produce a zwitter-
yamaguchi@chem.nagoya-u.ac.jp
[**] This work was partly supported by JST, CREST (S.Y. and S.I.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404867.
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ionic excited state. Here we present the synthesis and
characteristic fluorescence properties of 1. The time-resolved
fluorescence spectroscopy and the quantum chemical calcu-
lations as well as the comparison with pertinent reference
compounds, a nonborylated (2) and an alkyl-strapped (3)
analogue, provide deeper insight into the ESIPT emission.
Compound 1 was synthesized in two steps from a dialkyl-
amine-tethered dithiophene 4 (Scheme 1). The Pd-catalyzed
cross-coupling of the dibromide 4 with pyrrole-2,5-diboronate
in 5 mm toluene/dimethyl formamide (DMF) produced the
strapped dithienylpyrrole 2 in 20% yield. Compound 2 was
dilithiated with 3.1 equiv of LDA followed by treatment with
Mes2BF to afford the diborylated derivative 1. This com-
pound was stable enough to purify by silica gel column
chromatography and 51% yield was obtained. The alkyl-
strapped reference compound 3 was obtained in a similar
manner (see Scheme S2 in the Supporting Information, SI).
The structure of 1 was determined by X-ray crystallog-
raphy (Figure 2). The aminoalkyl strap constrains the con-
formation of the dithienylpyrrole framework in a twisted
form. The dihedral angles between the central pyrrole ring
and the adjacent thiophene rings are 43.58 and ꢀ19.68. The
close proximity between the nitrogen atom in the alkyl strap
ꢀ
and the pyrrole N H is noteworthy. The distance between the
two nitrogen atoms N1 and N2 is 3.26 ꢀ, indicative of a weak
ꢀ
hydrogen bond between the amine moiety and the N H
proton of the pyrrole ring.[21] The resonance signal of the N H
proton in the H NMR spectrum of 1 in C6D6 appeared at
ꢀ
1
12.8 ppm, which is significantly downfield-shifted compared
to that of the alkyl-strapped analogue 3 (7.71 ppm). This
difference suggested that the hydrogen bonding is retained
even in solution (Figure S41).
Compound 1 showed remarkable solvent-dependent fluo-
rescence. In cyclohexane, 1 has a strong absorption band with
the maximum at 451 nm and exhibited a bluish-green
fluorescence at 500 nm with the quantum yield FF of 0.21
(Figure 3a and Table 1). When the solvent was changed to
benzene, the absorption maximum did not change signifi-
cantly, but the emission band was slightly shifted to 520 nm
and, most notably, a new intense band appeared at 581 nm. As
Scheme 1. Synthesis of compound 1.
Figure 3. a) Fluorescence spectra of 1 in various solvents excited at
390 nm (inset: photographs under the irradiation with a 365 nm lamp)
and b) fluorescence spectra of 1, 2, and 3 in THF (excitation wave-
length: 390 nm for 1 and 3, 330 nm for 2).
a result, the dual emission bands from a single molecule cover
a wide range in the visible region. As the solvent polarity was
further increased by using acetone or DMF, the intensity of
the shorter emission band significantly decreased and the
longer emission band became dominant, giving rise to an
orange or reddish-orange fluorescence (Figure 3a, inset). In
conjunction with these changes, the FF also increased in the
more polar solvents. The highest FF value of 0.46 was
obtained in acetone.
Figure 2. Thermal ellipsoid plot of 1 (50% probability). Hydrogen
To elucidate the origin of the dual emission, we conducted
several measurements. First, the excitation spectra of 1 were
ꢀ
atoms are omitted for clarity except for the N H proton on the pyrrole
ring.
2
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Table 1: Photophysical data of 1, 2, and 3 in various solvents.
(Figure 3a and Table 1). This is in contrast to the conventional
ESIPT systems based on the keto–enol tautomerism, which
generally shows a hypsochromic shift with increasing solvent
polarity due to the less polar nature of the keto form in the
ESIPT state than the corresponding geometry in the ground
state.[9a,22,23] The present result indicates that the ESIPT state
in 1 has a highly polarized character.
Absorption
Fluorescence
e [ꢀ104
lem
[nm]
FF
t1
t2
[a]
[b]
[c]
[d]
[e]
Solvent
labs
[nm] mꢀ1 cmꢀ1
]
[ns] [ns]
1
cyclohexane 451
4.78
4.81
5.05
4.80
4.60
500, 530 0.21 0.46 1.1
520, 581 0.21 0.41 0.71
520, 594 0.37 0.37 1.5
520, 602 0.46 1.0
525, 609 0.41 0.87 2.6
benzene
THF
acetone
DMF
456
451
450
453
2.3
To gain deeper insights into the character of the excited
state of 1, calculations on both the ground state (S0) and the
lowest-energy excited singlet state (S1) in the gas phase and in
acetone were conducted using the CAM-B3LYP functional
with the 6-31G + (d) (for N atoms), 6-31G(d,p) (for the
migrating H atom), and 6-31G(d) basis sets (for the other
atoms). Truhlarꢁs SMD solvation model[24] was used to take
the solvent effect into account. The TD-DFT vertical
excitation calculations[25] suggested that the lowest-energy
transition from S0 to S1 is attributable to the HOMO!
LUMO transition. The HOMO and LUMO are inherently
the p and p* orbitals delocalized on the 2,5-dithienylpyrrole
moiety. The LUMO also has a significant contribution of the
vacant p orbitals of the boron atoms. Accordingly, the
HOMO!LUMO transition is the p–p* transition with
a pronounced charge transfer character similar to the
known boron-containing p-electron systems (Figure S22).[26]
Geometry optimizations in S1 starting from the Franck–
Condon state and the proton-transferred structure as initial
guesses indeed gave two local minima corresponding to the
LE and ESIPT states. The plausible potential energy surfaces
are shown in Figure 4. The energies of the LE and ESIPT
states relative to the optimized geometry in S0 are + 2.94 eV
and + 3.47 eV in the gas phase, and + 2.78 eVand + 2.81 eV in
acetone, respectively. Notably, the ESIPT state is significantly
stabilized in the polar solvent and consequently comparable
to the LE state in energy. The dipole moments of the LE and
ESIPT states are 4.07 D and 10.4 D in the gas phase, and
6.65 D and 17.1 D in acetone, respectively. The significant
increase in the dipole moment of the ESIPT state in the polar
solvent is consistent with the solvent effect observed in the
fluorescence spectra. The large magnitude of the dipole
moment vector and its direction in the ESIPT state of
1 strongly support its zwitterionic character (Figures S19 and
S21).
[f]
2
3
THF
THF
334
428
2.12
2.79
396
514
0.29 0.85
0.14 0.84
–
–
[f]
[a] Only the longest absorption maximum wavelengths are shown.
[b] Emission maxima upon excitation at 390 nm. [c] Absolute fluores-
cence quantum yields determined by a calibrated integrating sphere
system within ꢁ3% error. [d] Fluorescence lifetimes of the shorter-
wavelength emission bands. [e] Fluorescence lifetimes of the longer-
wavelength emission bands. [f] Not observed.
similar to the absorption spectra regardless of the monitored
wavelengths, 510 nm or 610 nm, as well as the solvents used
(Figures S2–S6). This fact demonstrates that the two emission
bands originate from the identical ground state. Secondly, the
picosecond time-resolved fluorescence spectroscopy revealed
that these two emission bands have distinct lifetimes from
each other. Thus, the lifetimes of the shorter-wavelength and
longer-wavelength emission bands range from 0.37 to 1.0 ns
and from 0.71 to 2.6 ns, respectively. These results lead to the
conclusion that these two emission bands originate from two
distinct states.
The comparison of 1 with the structurally relevant
compounds 2 and 3 demonstrated the necessity of the
amino group in the strap and the terminal boryl groups to
obtain the dual emission (Figures 3b, S7, and S8). Thus, in
stark contrast to the dual emission of 1, the nonborylated
derivative 2 only showed a single-component emission at
lem = 396 nm (tFL = 0.85 ns) in THF. This fluorescence did not
show a significant solvent dependence (Figure S7). These
results demonstrate that the introduction of the electron-
accepting boryl groups to the terminal positions is crucial for
the dual emission. The alkyl-tethered analogue 3 also
exhibited a single-component fluorescence with the lem of
514 nm in THF, whose fluorescence lifetime tFL was 0.84 ns.
These values are comparable to those of the shorter-wave-
length emission band of 1. Thus, the shorter-wavelength
emission band of 1 should be correlated to the diborylated
dithienylpyrrole skeleton itself and the longer-wavelength
emission band of 1 should result from the contribution of the
In the optimized structure of 1 in the ESIPT state in
acetone, the N···H distance between the strap amine and the
pyrrole hydrogen (1.041 ꢀ) indeed becomes shorter to
a considerable extent compared to that (2.301 ꢀ) in the LE
ꢀ
state. In agreement with this change, the B C bond between
the boryl group and the thienyl group is also shortened from
1.534 and 1.528 ꢀ in the LE state compared to 1.523 ꢀ in the
ESIPT state. According to the natural population analysis
(NPA), upon the change from the LE to ESIPT state, the
sums of the natural charges in the 2,5-dithienylpyrrole moiety
and in the two Mes2B moieties become more negative by
ꢀ0.213 and ꢀ0.098 in the gas phase, and ꢀ0.263 and ꢀ0.121 in
acetone, respectively.[27] Namely, the total negative charge of
the diborylated dithienylpyrrole moiety increases in the polar
solvent, and a substantial amount of the negative charge is
accommodated on the Mes2B moiety. These results clearly
demonstrate that the terminal boryl groups play a pivotal role
ꢀ
intramolecular N H···N hydrogen bond. Hence, we assigned
the two bands to the emissions from a locally excited state
(LE) and an ESIPT state, respectively. The temperature-
dependent fluorescence measurement of 1 in 2-methyltetra-
hydrofuran showed that the longer-wavelength emission band
became dominant as the temperature decreased from 294 K
to 170 K (Figure S16). Thus, the two distinct states respon-
sible for each emission band are in thermal equilibrium.
A characteristic feature of the ESIPT emission of 1 is its
solvent dependence. The longer emission band of 1 at 581 nm
in benzene showed a bathochromic shift to 609 nm in DMF
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Communications
water mixture from 0% to 40%,[33] the
intensity of the longer-wavelength emis-
sion band continuously increased and,
eventually, the fluorescence quantum
yield FF in the 60:40 (v/v) THF/water
solution reached 54%. The relative inten-
sities of the two emission bands were
almost proportional to the water content
of the THF/water mixture (for the
detailed discussion, see SI and Fig-
ure S17). This characteristic feature is
likely due to either the structural con-
straint of the pyrrole and alkylamine
moieties, or the steric protection of the
ꢀ
hydrogen-bonded N H···N moiety from
the solvent molecules by the aliphatic
aminoalkyl strap. This feature would be
beneficial to various applications that are
exerted in an aqueous medium, such as
bioimaging or sensory materials.
In summary, we have developed
a new ESIPT chromophore consisting of
an aminoalkyl-strapped dithienylpyrrole
bearing boryl groups. The p-electron
system shows dual emission bands with
high luminescence quantum yields, which
cover a wide range in the visible region
depending on the solvent polarity. Our
“functional strap” strategy endows two
Figure 4. a) Schematic representation of 1 in the LE and ESIPT states. Potential energy
surfaces b) in the gas phase and c) in acetone. The relative energies calculated with the CAM-
B3LYP functional are given in kcalmolꢀ1 relative to the optimized geometry in S0. The TD-DFT
vertical excitation energies and oscillator strengths (f) are calculated with the PBE0 functional.
Solvents are considered using the SMD model.
as the electron-accepting groups in the excited state, giving
rise to the significant stabilization of the zwitterionic ESIPT
state.
remarkable features totally different from those of the
conventional ESIPT system based on the keto–enol tauto-
merism: 1) the significant zwitterionic character, and 2) no
disruption of the ESIPTemission even in an aqueous medium.
These features are potential advantages for the application as
environment-responsive fluorophores. Further investigation
along this line as well as the systematic study regarding the
effect of the strap length, the substituents on the aminoalkyl
moiety, and the terminal boryl groups are now in progress in
our group.
In most of the conventional ESIPT systems, the ESIPT is
prohibited in an aqueous medium due to the formation of
intermolecular hydrogen bonding with the protic sol-
vent.[1,28,29] To retain the ESIPT emission even in the aqueous
medium, it requires a specific structure that significantly
favors the intramolecular hydrogen-bonded conformer[24] or
a special treatment to prevent the interaction with the solvent,
such as the formation of an inclusion complex of the ESIPT
system with a host molecule like cyclodextrin.[30–32] In
stark contrast, the present aminoalkyl-strapped 1 showed
the ESIPT emission even in water-containing solvents
(Figure 5). Upon increasing the water content in the THF/
Received: May 1, 2014
Published online: && &&, &&&&
Keywords: boron · ESIPT · fluorescence · hydrogen bonds ·
.
p conjugation
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4
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[33] Compound
1 became insoluble when the water content
exceeded 50%.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Excited-State Design
N. Suzuki, A. Fukazawa, K. Nagura,
S. Saito, H. Kitoh-Nishioka, D. Yokogawa,
S. Irle,* S. Yamaguchi*
&&&& — &&&&
A Strap Strategy for Construction of an
Excited-State Intramolecular Proton
Transfer (ESIPT) System with Dual
Fluorescence
Hold on to the strap: A new type of
excited-state intramolecular proton
transfer (ESIPT) chromophores has been
developed by incorporation of an amine-
embedded alkyl strap into an emissive
boryl-substituted dithienylpyrrole skele-
ton. The product’s dual fluorescence
covers a wide range in the visible region
depending on the solvent polarity. The
zwitterionic ESIPT state is efficiently sta-
bilized by the aminoalkyl strap and the
terminal boryl groups.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!