Acid-Induced Shift of Intramolecular Hydrogen Bonding Responsible for Excited-State Intramolecular Proton Transfer

Article abstract of DOI:10.1002/asia.201800457

The significant progress recently achieved in designing smart acid-responsive materials based on intramolecular charge transfer inspired us to utilize excited-state intramolecular proton transfer (ESIPT) for developing a turn-on acid-responsive fluorescent system with an exceedingly large Stokes shift. Two ESIPT-active fluorophores, 2-(2-hydroxyphenyl)pyridine (HPP) and 2-(2-hydroxyphenyl)benzothiazole (HBT), were fused into a novel dye (HBT-HPP) fluorescent only in the protonated state. Moreover, we also synthesized three structurally relevant control compounds to compare their steady-state fluorescence spectra and optimized geometric structures in neutral and acidic media. The results suggest that the fluorescence turn-on was caused by the acid-induced shift of the ESIPT-responsible intramolecular hydrogen bond from the HPP to HBT moiety. This work presents a systematic comparison of the emission efficiencies and basicity of HBT and HPP for the first time, thereby utilizing their differences to construct an acid-responsive smart organic fluorescent material. As a practical application, red fluorescent letters can be written using the acid as an ink on polymer film.

Full text of DOI:10.1002/asia.201800457

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Acid-induced shift of intramolecular hydrogen bonding
responsible for excited-state intramolecular proton transfer
Authors: Qin Wang, Yahui Niu, Rong Wang, Haoran Wu, and Yanrong
zhang
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Acid-induced shift of intramolecular hydrogen bonding
responsible for excited-state intramolecular proton transfer
Qin Wang[a], Yahui Niu[a], Rong Wang[a], Haoran Wu[b] and Yanrong Zhang*[a]
Abstract: The significant progress recently achieved in designing
smart acid-responsive materials based on intramolecular charge
transfer inspired us to utilize excited-state intramolecular proton
shifted fluorescence emission. Thus, the development of novel
molecules featuring other emission mechanisms continues to be
an active area of research. When the N-heteroaromatic rings are
rationally designed to form intramolecular H-bond with free
hydroxyl group at suitable position, upon excitation these
molecules could undergo excited state intramolecular proton
transfer (ESIPT), which is known as photoinduced
tautomerization from enol (E*) to keto (K*) tautomers through
proton migration within the intramolecular H-bonding site.[4]
Based on the typical four-level photophysical process, the
ESIPT-active dyes present anomalously large Stokes shift
without self-absorption, which are extremely desirable features
in numerous applications including white light-emitting materials,
fluorescent probes and imaging.[5]
transfer (ESIPT) for developing
a
turn-on acid-responsive
fluorescent system with an exceedingly large Stokes shift. Two
ESIPT-active fluorophores, 2-(2-hydroxyphenyl)pyridine (HPP) and
2-(2-hydroxyphenyl)benzothiazole (HBT), were fused into a novel
dye (HBT-HPP) fluorescent only in the protonated state. Moreover,
we also synthesized three structurally relevant control compounds to
compare their steady-state fluorescent spectra and optimized
geometric structures in neutral and acidic media, suggesting that the
fluorescence turn-on was caused by the acid-induced shift of the
ESIPT-responsible intramolecular hydrogen bond from HPP to HBT
moiety. This work presents a first-time systematic comparison of the
emission efficiencies and basicity of HBT and HPP, utilizing their
differences to construct an acid-responsive smart organic
fluorescent material. As a practical application, red fluorescent
letters can be written using acid as ink in polymer film.
2-(2-Hydroxyphenyl)benzothiazole
(HBT),
featuring
an
intramolecular H-bond between the phenolic hydroxyl and the
benzothiazolyl N atom, is one of the most extensively studied
ESIPT-active dyes due to its facile synthesis and stable optical
properties. [6] A number of studies including our previous ones[7,8]
demonstrated that the introduction of electron-withdrawing
substituent at the 4’-position or electron-donating substituent at
the 5’-position, or a combination thereof can result in red-shifted
fluorescence emission. Additionally, molecular planarization is
one of the key requirements for red-shifted fluorescence, being
easily realized via intramolecular H-bonding.[9] In view of the
above and considering our general interest in ESIPT-active dyes,
we herein designed and synthesized a novel HBT derivative
(HBT-HPP) bearing an electron-withdrawing pyridin-2-yl moiety
and an electron-donating phenolic hydroxyl at 4’- and 5’-
positions, respectively, with the simultaneous formation of
another intramolecular H-bond between the pyridine N atom and
the hydroxyl group helping to preserve the planar conformation
of HBT-HPP (Scheme 1). Unexpectedly, HBT-HPP was non-
fluorescent in solution, crystalline, and polymer film states,
exhibiting relatively strong red fluorescence with an exceedingly
large Stokes shift when exposed to acid in any of the above
states. Simultaneously, three structurally relevant control
compounds were exploited to shed light on this abnormal
behavior. The results obtained through analyzing their
fluorescent spectra and comparing optimized geometric
structures and frontier molecular orbitals in neutral and acidic
media, revealed that among the two intramolecular H-bonds in
HBT-HPP, the H-bond responsible for ESIPT is that in the 2-(2-
hydroxyphenyl)pyridine (HPP) moiety. That the inherently low
fluorescence efficiency of the HPP moiety and the decreased H-
bond strength therein observed for HBT-HPP,[10] resulted in
fluorescence quenching of HBT-HPP in neutral medium.
However, in acidic medium, protonation of the pyridin-2-yl group
blocked ESIPT originating from the HPP moiety and induced a
switch to HBT moiety, resulting in fluorescence turn-on. To the
best of our knowledge, this study offers a first-time comparison
Introduction
The design and development of smart fluorescent materials
capable of altering their emission intensity/wavelength in
response to external stimuli (such as mechanical pressure, light,
heat, guest molecules and pH, etc.) have recently attracted
increased attention.[1] Since these stimuli often result in
molecular structure/conformation or intermolecular interaction
changes, understanding of potential mechanisms responsible for
photophysical property changes is extremely important for
designing novel smart fluorophores for a wide range of practical
applications. Specifically, fluorophores containing N-heterocyclic
moieties are of particular interest for acid-responsive material
development, since their original photophysical properties can
be changed by protonation of N atoms. Although the last decade
has witnessed remarkable progress in this research field,[2] most
of the investigated dyes rely on intramolecular charge transfer.[3]
Formation of ammonium salts can reduce the electron density of
donor/acceptor moiety bearing N atoms, resulting in blue- or red-
[a]
[b]
Qin Wang, Yahui Niu, Rong Wang, and Yanrong Zhang
College of Chemistry & Pharmacy
Northwest A&F University
Yangling, Shaanxi 712100, P. R. China
E-mail: zhangyr@nwsuaf.edu.cn
Haoran Wu
Innovation Experimental College
Northwest A&F University
Yangling, Shaanxi 712100, P. R. China
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Scheme 1. Acid-induced shift of intramolecular H-bond responsible for ESIPT from lower-efficiency HPP moiety to higher-efficiency HBT moiety. The
structures of HBT and HPP are shown in the red and blue dashed frames, respectively. Potential H-bonds are shown as dotted lines.
of the ESIPT efficiencies of HPP and HBT (although both of
them have been investigated separately) and fuses them into
one molecule, providing experimental and theoretical guidelines
for the design of novel ESIPT-active dyes with unique
fluorescent properties. Moreover, the on-off fluorescence
switching of HBT-HPP upon treatment with acids (e.g.,
trifluoroacetic acid (TFA)) and bases (e.g., NH3) can be exploited
to fabricate self-erasable and rewritable fluorescent platform.
dimethoxybenzaldehyde,
4-iodo-2,5-dimethoxybenzalde-hyde
(compound 1) was first synthesized according to previously
reported method. [11] The reaction between compound 1 and 2-
aminothiophenol in the presence of HCl/H2O2 in methanol
yielded
2-(4-iodo-2,5-dimethoxyphenyl)benzo[d]thiazole
(compound 2), which was then boronized to yield 2-(2,5-
dimethoxy-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)phenyl)benzo[d]thia-zole (compound 3). Under conventional
Suzuki reaction condition, the reaction of this organoboron
compound with 2-bromopyridine afforded the key intermediate 2-
(2,5-dimethoxy-4-(pyridin-2-yl)phenyl)benzo[d]thiazole
(MBT-
MPP). Treatment of this intermediate with 3-fold equivalent BBr3
removed the two methyl groups simultaneously and yielded
HBT-HPP; while treatment of this intermediate with 1.2-fold
equivalent BBr3, two monomethylated compounds HBT-MPP
and MBT-HPP were obtained simultaneously. These new
compounds were unambiguously characterized by 1H and 13C
nuclear magnetic resonance spectroscopy (NMR) and high-
resolution mass spectrometry (HRMS). Additionally, single-
crystal X-ray diffractions were also performed on HBT-HPP,
MBT-MPP and HBT. All characterization details are given in
Supporting Information.
Photophysical
Property
and
Theoretical
Calculation of HBT-HPP in Neutral Medium
Since ESIPT is known to be largely affected by
microenvironmental factors, [12,4b] we first investigated the effect
of solvent on the absorption and fluorescence emission spectra
of HBT-HPP, showing that the former spectra exhibited solvent-
independent λmax (410 nm) (Fig. S1). Rather unexpectedly,
Scheme 2. Chemical structures and synthetic routes for HBT
derivatives.(a) I2, AgNO3, MeOH, r.t., 12 h; (b) 2-aminothiophenol,
C2H5OH, H2O2, HCl, RT, 0.5 h; (c) KOAc, bis(pinacolato)diboron,
Pd(dppf)2Cl2, dioxane, 120 oC; (d) 2-bromopyridine, K2CO3, toluene/
ethanol / H2O; (e) 3.0 eqiuv. BBr3, CH2Cl2; (f) 1.2 eqiuv. BBr3, CH2Cl2.
solvent-dependent fluorescence,
a characteristic feature of
ESIPT-active dyes, was not observed for HBT-HPP (Fig. 1A),
which only exhibited enol emission (470 nm) with a very low Φf
and a normal Stokes shift even in nonpolar PhMe and in
medium-polarity CH2Cl2 (0.015%, 60nm). It should be noted that
excitation in non-H-bonding solvents unable to effectively
compete for the phenolic hydroxyl proton with the H-bonding
Results and Discussion
acceptor[8] (herein, the two
N
atoms in two aromatic
Synthesis and Characterization
heterocycles), should result in keto emission with a large Stokes
shift (>150 nm). The existence of intramolecular H-bonds in
The synthetic strategy employed to prepare HBT-HPP was
depicted in Scheme 2. From the commercially available 2,5-
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Figure 1. Steady-state fluorescence spectra of (A) HBT-HPP, (B) MBT-MPP, (C) HBT-MPP, and (D) MBT-HPP recorded in various solvents at room
temperature.
HBT-HPP was confirmed by 1H NMR (Fig. S29), with the two
sharp peaks at 12.0 and 13.9 ppm ascribed to phenolic hydroxyl
protons at 2’- and 5’-positions, respectively. HBT-MPP (12.1
ppm, Fig.S32), MBT-HPP (13.9 ppm Fig.S35), HBT (12.6 ppm)
[8c]
and HPP (14.4 ppm, Fig.S38) were used as control
compounds to aid these assignments. All OH resonances were
significantly shifted downfield, being observed at >12 ppm for
HBT and at >13 ppm for HPP moieties, which clearly indicated
the formation of intramolecular H-bonds in HBT-HPP. [13]
To pinpoint the origin of the unexpected photophysical
properties of HBT-HPP, we characterized the absorption and
fluorescence spectra of three structurally relevant control
compounds. Thus, bimethylated MBT-MPP showed emission
Figure 2. (A) Optimized geometries of HBT-HPP in S0 (black text) and S1 (red
text) states. (B) HOMO and LUMO of HBT-HPP in CH2Cl2.
with a normal Stokes shift (~50 nm, Φf = 35.5%) (Fig.1B and
Fig.S3-S4), which was in agreement with the lack of ESIPT due
to the absence of the phenolic hydroxyl proton.[14]
Subsequently, we employed theoretical calculations to find a
Monomethylated HBT-MPP (Fig.1C), featuring an intramolecular
suitable explanation for the absence of ESIPT in the case of
H-bond between the N atom of the benzothiazole ring and the
HBT-HPP, relying on their successful use for rationalizing
certain photophysical properties of fluorescent dyes.[13,15] The
phenolic hydroxyl in the 2’-position, showed dual emission in
PhMe and CH2Cl2 (Φf = 0.35%) and single emission in THF.
geometric structures of HBT-HPP, MBT-HPP and HBT-MPP
were optimized for S0 and S1 states in CH2Cl2 using DFT and
Moreover, the assignment of the lower-energy emission with an
exceedingly large Stokes shift (180 nm) at 560 nm to the ESIPT
TDDFT at the B3LYP/6-31+G(d) level. The obtained results
emission of the keto tautomer was further verified by excitation
demonstrated the existence of intramolecular H-bonds in all
spectra analysis (Fig.S5-S6). However, monomethylated MBT-
three compounds, in accordance with the conclusions drawn
from 1H NMR analyses. The structural parameters of
HPP, possessing an intramolecular H-bond between the pyridine
N atom and the phenolic hydroxyl in the 5’-position, showed
intramolecular H-bonds (O–H···N) are shown in Fig.2A and
emission similar (Fig.1D and Figs.S7-S8) (Φf = 0.02%) to that of
Figs.S9A-9B. Notably, upon S0→S1 excitation, the O–H bond
lengths increased, whereas the N···H and N···O distances and
HBT-HPP, i.e., no ESIPT effect.
the distortion angles between the heterocycle and the central
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phenol ring decreased. These structural changes indicated that
intramolecular H-bonds in these three compounds are
substantially reinforced in the S1 state, complying with the
prerequisite for ESIPT. Additionally, the three HBT derivatives
featured similar molecular orbital electron distributions, with the
HOMOs being mainly located on the central phenol ring and the
LUMOs being spread over the whole molecule (Fig.2B and
Figs.S9C-9D). Upon going from the HOMO to the LUMO, the
electron density of the O atom decreased, while that of the N
atom increased. Moreover, changes in electron density indicated
that both the acidity of the phenolic hydroxyl and the basicity of
the N atom improved upon going from S0 to S1, which allowed
O-to-N proton transfer to complete the enol-to-ketone
tautomerization. However, as mentioned above, the ESIPT
effect was only observed for HBT-MPP, which features an
intramolecular H-bond in the HBT moiety, whereas HBT-HPP
and MBT-HPP, containing intramolecular H-bonds in the HPP
moiety, exhibited dramatically dim fluorescence with normal
Stokes shifts.
Figure 3. Temperature-dependent emission spectra of (A) HBT and (B) HPP (10 M in CH2Cl2).
Figure 4. (A) Face-on view of HBT crystal, (B) Optimized geometries of HBT in S0 (black text) and S1 (red text) states. Optimized geometries of HPP in (C) S0
and (D) S1 states in CH2Cl2.
According to previous reports,[16,17] intramolecular rotation, a
non-radiative energy dissipation pathway, can be restricted at
decreased temperatures. Then we recorded low-temperature
emission spectra of HBT-HPP, MBT-HPP, HBT-MPP, HPP and
HBT in CH2Cl2. (Fig.S10 and Fig.3) For HBT-HPP and MBT-
HPP, non-emissive at room temperature, keto emission is
predicted to be switched on at low temperature if quenching is
caused by intramolecular rotation. However, for these two
compounds, the enol emission intensity gradually increased with
decreasing temperature, and no keto emission was observed
(Figs.S10A-S10B). Conversely, both HBT-MPP and HBT
showed keto emission even at room temperature, with its
intensity drastically increasing with decreasing temperature
(Figs.S10C and 3A). For HPP, enol emission was observed at
room temperature, with the intensity of the keto emission
observed below 0°C gradually increasing with decreasing
temperature (Fig.3B). Thus, the above results demonstrate that
intramolecular rotation in HBT-HPP is not the main contributor to
emission quenching, requiring other factors to be explored.
Additionally, comparison of the low-temperature emission
spectra of HBT and HPP allowed us to conclude that the ESIPT
efficiency of the latter should be lower than that of the former,
which has not been reported previously.
For HBT-HPP, featuring two different intramolecular H-bonds,
the H-bond involving the pyridine ring should be the first one to
be involved in the production of the keto tautomer, as concluded
by comparing the molecular structures and fluorescent spectra
of HBT-HPP, HBT-MPP (H-bond involving the benzothiazole
ring), and MBT-HPP (H-bond involving the pyridine ring). This
conclusion was verified by directly comparing the optimized
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geometries of HBT and HPP in CH2Cl2 (Fig.4), since the
intramolecular H-bond (its existence confirmed by the
corresponding crystal structure (Fig.4A and Table S1) of HBT in
S0 was only reinforced upon excitation to S1 (Fig. 4B).
Conversely, for HPP the direct enol-to-ketone conversion
observed for the S0 →S1 transition was accompanied by the
complete transfer of the phenolic OH proton from O to N (Figs.
4C-4D). Additionally, according to the unique four-level
photophysical ESIPT process and our previous research on the
effects of electronic factors on the keto emission efficiency of
ESIPT fluorophores, [8] the strength of the intramolecular H-bond
has a decisive influence on their photophysical properties.
Obviously, intramolecular H-bonds involving pyridine in HBT-
HPP and MBT-HPP are weakened by electron donation from
hydroxyl or methoxy groups at the 2’-position, as reflected by the
upfield shifts of the 1H NMR signals of 5’-OH and the decreased
O–H bond lengths compared to those of HPP.
corresponded to a thermodynamically disfavored configuration
rarely reported previously.[19] Yet another molecular
configuration was observed for HBT-HPP-R, which crystallized
in the monoclinic space group P2(1)/n (Table S4). Although one
kind of molecular spatial configuration was found in the crystal
lattice (Fig.7A), and the molecules were stacked along a- or b-
axes in anti-parallel mode, two different distances betwee-n
neighboring molecules (3.499 and 3.469 Å) were observed, with
the electron-rich phenol ring being sandwiched between phenol
and pyridine rings (Fig.7B).
Based on the results obtained in neutral medium, we concluded
that among the two intramolecular H-bonds in HBT-HPP, the
one responsible for ESIPT was formed by the HPP moiety. The
relatively lower ESIPT efficiency of HPP compared to that of
HBT and the weakened intramolecular H-bond strength were
determined as the two key reasons of HBT-HPP emission
quenching in the molecularly dispersed state.
Figure 5. (A) Face-on view of an HBT-HPP-Y1 crystal, (B) view of the HBT-HPP-Y1
crystal along the a- or b-axis with a pitch angle of 34.66°.
Photophysical Properties of Crystalline HBT-HPP
Considering that molecular spatial configurations and packing
modes play vital roles in determining solid-state fluorescence
properties, [18] we attempted to grow single crystals of HBT-HPP.
As a result, slow diffusion of CH2Cl2 from solution over several
days at room temperature simultaneously afforded two types of
yellow and one type of red HBT-HPP crystals (under visible
light), which were denoted as HBT-HPP-Y1, HBT-HPP-Y2, and
HBT-HPP-R, respectively. In the above crystals, almost all HBT-
HPP atoms lied in the same plane due to the formation of the
six-membered intramolecular H-bonds (Figs.5A–7A). All three
types of HBT-HPP crystals exhibited the observed three-
dimensional structures through π-π interactions along the a- and
b-axes (Figs.5B-7B) and weak intermolecular interactions (such
as C-H···π and O···H–C) along the c-axis (Figs.S11C-S13C).
However, the above crystal types displayed different packing
modes. HBT-HPP-Y1, crystallizing in the monoclinic space
group Pn (Table S2), featured stacking along a- or b-axes in
cross-parallel mode with an interplanar spacing of 3.544Å, a
pitch angle of 34.66°, and a roll angle of 74.47°, with the
electron-rich phenol ring being sandwiched between the
electron-deficient benzothiazole and pyridine rings of
neighboring molecules (Fig.5B). For HBT-HPP-Y2, crystallizing
in the tetragonal space group P4(3) (TableS3), the two different
configurations (purple and yellow molecules in Fig.6A) featured
anti-parallel stacking along a- or b-axes, with the distances
between two neighboring yellow/purple molecules equaling
7.118/7.163Å, respectively (Fig.6B). Moreover, the above
structures featured electron-rich phenol rings sandwiched
between those of neighboring molecules, exhibiting a pitch angle
Figure 6. (A) Face-on view of an HBT-HPP-Y2 crystal, (B) view of the HBT-
HPP-Y2 crystal along the a- or b-axis with a pitch angle of 67.76°.
Figure 7. (A) Face-on view of an HBT-HPP-R crystal, (B) view of the HBT-
HPP-R crystal along the a- or b-axis with a pitch angle of 63.44°
As discussed above, numerous intermolecular interactions
observed for the stacking modes were expected to restrict
intramolecular rotation. Although non-radiative energy loss by
intramolecular rotation was efficiently inhibited, no crystal form
exhibited fluorescence (Fig.S14), in agreement with the results
of temperature-dependent fluorescence measurements, proving
our hypothesis that intramolecular rotation is not the main
reason of the fluorescence quenching observed for HBT-HPP.
of 67.76°and
a roll angle of 87.43° (Fig.S12B), which
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Figure 8. Changes of fluorescence spectra observed for (A) HBT-HPP, (B) HBT-MPP, (C) MPP-HPP, and (D) MBT-MPP upon addition of TFA in CH2Cl2 (10 M).
Scheme 3. Conformations of HBT-based fluorophores in protonated and photochemically excited states.
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MBT-HPP-H and MBT-MPP-H emissions were close to those of
HBT-HPP-H and HBT-MPP-H enol emission.
Photophysical
Property
and
Theoretical
1H NMR spectroscopic measurements were exploited to confirm
the proposed structures of protonated compounds, since we
failed to obtain their single crystals. When designing NMR
experiments, we considered the following facts. (1) The basicity
of the pyridine N atom of HBT-HPP and MBT-HPP is reduced by
the intrinsic intramolecular H-bond, the large excess of acid was
required for complete protonation, which would affect all proton
signals.20 (2) Compared to those of HBT-HPP and MBT-HPP,
the pyridine N atoms of HBT-MPP and MBT-MPP are more
basic and can therefore be completely protonated by equimolar
amounts of TFA within a certain concentration range (Fig. S16).
(3) Knowing the crystal structure of MBT-MPP (Fig.9A and Table
S5) would facilitate the determination of its conformation before
protonation. In view of the above, we turned our attention to
MBT-MPP to indirectly characterize the structure of HBT-HPP-H.
Peak assignments performed based on the results of a two-
dimensional COSY measurement (Figs. S18-S19) revealed that
the peaks of pyridine protons (a–d) were shifted downfield in the
presence of TFA due to the deshielding effect of the pyridinium
cation, whereas the resonances of benzothiazole ring protons
(e–g) were not significantly shifted (Fig.9B). Based on the
identical emission peaks of protonated MBT-HPP and MBT-MPP,
one can easily conclude that in MBT-HPP, the pyridinic N atom
is protonated in preference to that in the benzothiazole ring,
although the basicity of the former is reduced by the inherent
intramolecular H-bond. Therefore, in HBT-HPP, the pyridinic N is
indeed protonated upon the addition of TFA, which results in the
activation of the ESIPT-responsible intramolecular H-bond in the
HBT moiety.
Calculation of HBT-HPP in Acid Media
To confirm our assumption concerning the mechanism of HBT-
HPP fluorescence quenching, we attempted to increase the
strength and induce the shift of the ESIPT-responsible
intramolecular H-bond from the HPP moiety to the HBT moiety
in HBT-HPP. Considering the facts that the pyridine N atom is
more basic than that of the benzothiazole ring and that the
protonation of the former should reinforce the intramolecular H-
bond of the HBT moiety, we measured the fluorescence spectra
of HBT-HPP in acidic medium. With increasing TFA
concentration, the intensity of fluorescence at 472 nm increased,
being accompanied by the appearance of emission at 595 nm
(Fig. 8A), with the above wavelengths corresponding to enol and
keto emissions of protonated HBT-HPP (HBT-HPP-H, structure
shown in Scheme 3), respectively. Thus, the obtained results
confirmed our hypothesis. To verify the above assignment, we
also observed fluorescence spectrum changes for the three
control compounds, noting the appearance of two new peaks at
472 and 595 nm for HBT-MPP (Fig. 8B) (in line with the behavior
of HBT-HPP). Moreover, in the presence of TFA, new peaks at
475 and 474 nm were observed for MBT-HPP and MBT-MPP
(Figs. 8C-8D), respectively, with their positions being very close
to that of the enol emission peak of protonated HBT-HPP. It
should be noted that the suggested structures of protonated
HBT-HPP and HBT-MPP were very similar, just as those of
protonated MBT-HPP and MBT-MPP (MBT-HPP-H and MBT-
MPP-H, i.e., methylated derivatives of protonated HBT-HPP and
HBT-MPP, respectively). Consequently, the wavelengths of
Figure 9. (A) Face-on view of an MBT-MPP crystal. (B) Selected parts of assigned 1H NMR spectra of MBT-MPP and MBT-MPP-H (CDCl3)
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After determining the protonation position of HBT-HPP, we
optimized the conformations of protonated compounds (Fig.
S20), demonstrating that a new intramolecular H-bond is formed
in all cases between the pyridine N atom and the O atom at the
5’-position with the exterior proton acting as a bridge.21 The
theoretical UV/Vis spectrum of HBT-HPP upon protonation,
which was based on the optimized conformation of HBT-HPP-H
(Fig. S17), resembled the experimentally obtained one. The
optimized conformations again verified that HBT-HPP-H and
HBT-MPP-H featured dramatically similar geometrical structures,
as did MBT-HPP-H and MBT-MPP-H, which further explained
the similarities between the fluorescence spectra of HBT-HPP
and HBT-MPP and those of MBT-HPP and MBT-MPP after
protonation. It should be noted that the inherent intramolecular
H-bond of the HBT moiety in HBT-HPP is indeed strengthened
by protonation. In the optimized ground state of HBT-HPP-H,
compared with that of HBT-HPP, the N···O and N···H distances
are shortened from 2.64871 to 2.63374 Å and from 1.76374 to
1.74782 Å, respectively, and the O–H bond length is increased
from 0.99249 to 0.99530 Å, which facilitates the ESIPT of the
HBT moiety in HBT-HPP-H.
Figure 10. (A) Reversible fluorescence switching cycles and (B) fluorescence spectra of HBT-HPP-doped PMMA films upon alternating exposure to TFA and NH3
gas, with the A- and B-numbers corresponding to the respective cycle numbers (e.g., B-2 = second exposure to NH3). (C) The word “TFA” written on HBT-HPP-
doped PMMA film using TFA as an ink under UV light.
Fluorescence Imaging in Polymer Films
fluorescent letters could be written on HBT-HPP-doped PMMA
film using TFA as ink (Fig.10C), with a movie of this process
provided in ESI.
Fluorescence imaging in polymer films is attracting increased
interest owing to its numerous potential applications.22 Herein,
we used PMMA as a polymer matrix, since it allows the desired
photochemical reaction to occur while providing a high-quality
film for fluorescence imaging, additionally being well soluble in
common organic solvents.23 Polymer films were prepared by
dropping a solution of HBT-HPP and PMMA (1:10 w/w) in
CH2Cl2 onto glass and tilting the glass to remove excess liquid.
Similarly to crystalline HBT-HPP, the HBT-HPP film was non-
fluorescent under 365 nm irradiation, with protonation upon
exposure to TFA vapor leading to fluorescence turn-on at 610
Conclusions
In summary, we proposed a novel strategy for fabricating acid-
responsive smart fluorescent materials with exceedingly large
Stokes shift based on the ESIPT mechanism. The negligible
emission of HBT-HPP in neutral medium stemmed from the
inherently lower emission efficiency of the HPP moiety and the
decreased H-bonding strength, whereas the switched-on
emission of protonated HBT-HPP originated from the inherently
higher emission efficiency of the HBT moiety and increased H-
bonding strength. The above inference was fully supported by
contrasting the photophysical properties of three structurally
relevant control compounds and the results of their theoretical
calculations in neutral and acidic media. Therefore, utilizing the
differences in the emission efficiencies and basicity of HBT and
HPP, we fabricated an acid-responsive fluorescent material and
exploited it to realize fluorescence imaging in a polymer film,
demonstrating that our molecular design concept presents a
new platform for developing acid-responsive smart fluorescent
materials for various imaging applications.
nm with
a
~30-fold improvement (Figs.10A and 10B).
Subsequent NH3 gas exposure induced complete fluorescence
quenching, almost reaching the values observed for the initial
film due to the reversible deprotonation of the pyridine moiety.
The on-off reversible fluorescence switching of HBT-HPP-doped
PMMA by alternating exposure to TFA/NH3 was performed for
six cycles. Although the fluorescence intensity was gradually
reduced with each acid/base treatment cycle, the fluorescence
intensity of the turn-on state was still ~25 times than that of the
turn-off state after the sixth cycle. Thus, the reversible on-off
fluorescence switching might be exploited for fabricating self-
erasable and rewritable fluorescent platforms. Furthermore, red
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Keywords: Excited state intramolecular proton transfer •
Experimental Section
Theoretical calculation • Protonation • intramolecular hydrogen
bond • fluorescent imaging
Materials
2,5-Dimethoxybenzaldehyde, 2-aminothiophenol, bis(pinacolato)diboron,
Pd(dppf)2Cl2, 2-bromopyridine, Pd(PPh3)4 and BBr3 were purchased from
Energy Chemical (Shanghai, China). All other materials were purchased
from local commercial suppliers and were of analytical reagent grade,
unless otherwise stated. Solvents were purified by standard procedures.
All the reactions were monitored by thin-layer chromatography (TLC) with
detection by UV.
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Entry for the Table of Contents
FULL PAPER
Qin Wang, Yahui Niu, Rong Wang,
Haoran Wu and Yanrong Zhang*
Page 1 – Page 10.
Acid-induced shift of intramolecular
hydrogen bonding responsible for
excited-state intramolecular proton
transfer
Acid–induced ESIPT activation: intramolecular H-bonding shift from 2-(2-
hydroxyphenyl)pyridine moiety to 2-(2-hydroxyphenyl)benzothiazole moiety
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