2-(2-hydroxyphenyl)-benzothiazole (HBT)-rhodamine dyad: Acid-switchable absorption and fluorescence of excited-state intramolecular proton transfer (ESIPT)

Article abstract of DOI:10.1021/jp5068507

Dyad was prepared by link rhodamine and excited state intramolecular proton transfer (ESIPT) chromophore 2-(2-hydroxyphenyl)-benzothiazole (HBT) using Click reaction, with the goal to switch the absorption/emission property of ESIPT chromophore. The photophysical properties of the dyad were studied with steady state and time-resolved absorption and emission spectroscopy. In the absence of acid, that is, with rhodamine is in spirolactam structure, ESIPT was observed, the enol form emission of HBT unit was observed at 404 nm in protic solvents. In aprotic solvents, emission of the keto form of HBT was observed at 543 nm. With addition of acid such as trifluoroacetic acid, the rhodamine unit transforms to the opened amide structure, intense absorption band at 554 nm developed, as well as a strong fluorescence band at 579 nm; in EtOH, the enol emission of HBT at 406 nm was not quenched by the resonance energy transfer (RET), thus, dual fluorescence was observed. In dichloromethane, however, the fluorescence of the keto form of HBT unit was completely quenched. Thus, the absorption and emission of the ESIPT chromophore were switched by a acid/base-activatable rhodamine chromophore. Such studies will add additional modulability to the ESIPT chromophores.

Full text of DOI:10.1021/jp5068507

Article
pubs.acs.org/JPCB
2‑(2-Hydroxyphenyl)-benzothiazole (HBT)-Rhodamine Dyad: Acid-
Switchable Absorption and Fluorescence of Excited-State
Intramolecular Proton Transfer (ESIPT)
Poulomi Majumdar and Jianzhang Zhao*
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2
Ling-Gong Road, Dalian 116024, People’s Republic of China
S
* Supporting Information
ABSTRACT: Dyad was prepared by link rhodamine and excited state intramolecular proton
transfer (ESIPT) chromophore 2-(2-hydroxyphenyl)-benzothiazole (HBT) using Click
reaction, with the goal to switch the absorption/emission property of ESIPT chromophore.
The photophysical properties of the dyad were studied with steady state and time-resolved
absorption and emission spectroscopy. In the absence of acid, that is, with rhodamine is in
spirolactam structure, ESIPT was observed, the enol form emission of HBT unit was observed
at 404 nm in protic solvents. In aprotic solvents, emission of the keto form of HBT was
observed at 543 nm. With addition of acid such as trifluoroacetic acid, the rhodamine unit
transforms to the opened amide structure, intense absorption band at 554 nm developed, as
well as a strong fluorescence band at 579 nm; in EtOH, the enol emission of HBT at 406 nm
was not quenched by the resonance energy transfer (RET), thus, dual fluorescence was
observed. In dichloromethane, however, the fluorescence of the keto form of HBT unit was
completely quenched. Thus, the absorption and emission of the ESIPT chromophore were switched by a acid/base-activatable
rhodamine chromophore. Such studies will add additional modulability to the ESIPT chromophores.
well-known fluorophore that shows strong absorption and
emission in visible spectral region, high fluorescent quantum
yields, and high photostability. Notably, rhodamine is able to
response to external stimuli, such as pH, to undergo the
reversible closed rhodamine spirocyclic lactam ↔ opened
amide transformation.59−63 Accompanied with the structural
transformation, the photophysical properties change substan-
tially, that is, the switching ON and OFF of the absorption
band at about 550 nm, as well as the fluorescence at about 580
nm (for rhodamine B). Hence, rhodamine derivatives have
been widely used as fluorescence sensors and switches by
employing this switching ON/OFF effect.64−70 However, this
unique property of rhodamine was never coupled with the
ESIPT chromophore.
Concerning the aforementioned challenges, herein we
prepared 2-(2-hydroxyphenyl)-benzothiazole (HBT)-rhod-
amine dyad (Scheme 2) to investigate the ESIPT property
and fluorescence switching of the absorption and fluorescence
of the ESIPT chromophore with external stimuli such as acid/
base. The dyad exhibits switchable ESIPT emission property.
The photophysical properties of the dyad were studied in detail
INTRODUCTION
■
Excited-state intramolecular proton transfer (ESIPT) have
drawn considerable attention because of its unique four-level
photophysical scheme, spectral sensitivity to the surrounding
medium, large Stokes shifts, dual emission featured fluores-
cence, and so on.1−15 These chromophores have been used in
fluorescent molecular probes,1,3−15 molecular logic gates,4,15−17
fluorescent bioimaging,18−26 ultraviolet stabilizers,27 and for
fundamental photophysical studies.20,28−44
ESIPT is phototautomerization with enol form (E) be
changed to keto form (K) upon photoexcitation, by migration
of a proton to the neighboring electronegative atom via
intramolecular hydrogen bonding (Scheme 1). On relaxation of
the keto form to the ground state, the enol form is recovered by
reverse proton transfer.45 The presence of intramolecular
hydrogen bonding between the acidic proton and basic moiety
is crucial. Acidic protons are usually −OH and −NH2 and basic
centers are N− and carbonyl oxygen (CO).46,47 2-(2-
Hydroxyphenyl) benzothiazole (HBT) and 2-(2-hydroxyphen-
yl) benzoxazole (HBO) are well-known ESIPT dyes.48
However, ESIPT units are rarely combined with external
stimuli-responsive chromophores to attain switching effect on
the spectral properties.
On the other hand, fluorescence is ideal for nondestructive
tracking or analysis of chemical/biochemical substances with
high sensitivity. Fluorescence as a spectral signal relay has been
widely used in molecular sensors,49−52 molecular switches,53
pH- or enzyme-activatable photodynamic therapy (PDT)
reagents,54−56 and molecular logic gates.57,58 Rhodamine is a
Special Issue: Photoinduced Proton Transfer in Chemistry and
Biology Symposium
Received: July 9, 2014
Revised: September 10, 2014
© XXXX American Chemical Society
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TOF MS spectrometer. Fluorescence spectra were recorded on
a RF-5301PC spectrofluorometer (Shimadzu, Japan). Fluo-
rescence quantum yields were measured with 5,10,15,20-
tetraphenylporphyrin (TPP; ΦF = 11% in toluene) and 2,6-
diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazainda-
cene (ΦF = 2.7% in MeCN) as standard. Fluorescence lifetimes
were measured with OB920 luminescence lifetime spectrom-
eter (Edinburgh, U.K.). Absorption spectra were recorded on
an Agilent 8453A UV−vis spectrophotometer (U.S.A.).
Synthesis and Characterization. 5-Ethynyl-2-hydroxy-
benzaldehyde (1),71 and 2-(5′-ethnyl-2′-hydroxyphenyl)-
benzothiazole (HBT)72,73 were synthesized according to the
literature methods.
Scheme 1. Schematic Representation of the Four Levels
ESIPT Photocycle for RH-HBT Dyad
Synthesis of Rhodamine Spirolactam (RH). Under an Ar
atmosphere, to a stirred solution of rhodamine B (402 mg, 0.84
mmol) in 1,2-dichloroethane (10 mL), 2-azidoethanamine (217
mg, 2.52 mmol) was added through syringe at 0 °C. To this
reaction mixture phosphorus oxychloride (0.3 mL, 3 mmol)
was added dropwise. The mixture was stirred at 0 °C for 15
min, heated at 85 °C for 5 h, and then stirred at 25 °C for 24 h.
The solution was diluted with dichloromethane (20 mL) and
acidified with 2 M HCl (30 mL). The organic layer was washed
with additional 2 M HCl (2 × 30 mL), 2 M NaOH (3 × 30
mL) and brine (30 mL). The organic layer was dried over
Na2SO4 and the solvent was evaporated under reduced
pressure. The product was purified by column chromatography
(silica gel, petroleum ether/EtOAc = 3:1, v/v) to give a light
pink solid RH. Yield: 230.0 mg (53.6%); mp 186−188 °C. For
1
the molecular structure verification with H NMR and MS
spectra, please refer to the Supporting Information.
with steady state and time-resolved spectroscopy, as well as
DFT/TDDFT computations.
Synthesis of Compound RH-HBT. Under an Ar atmosphere,
to a stirred solution of RH (51.0 mg, 0.1 mmol) and HBT (25
mg, 0.1 mmol) in THF (10 mL), one drop of TEA was added,
and the reaction mixture was stirred for 5 min at RT. Then
CuSO4·5H2O (13 mg) in water (4 mL) and sodium ascorbate
(28 mg) in water (4 mL) was added through syringe
consecutively. The mixture was stirred at 30 °C for 12 h.
The progress of the reaction was monitored by TLC. The
reaction mixture was extracted with CH2Cl2. The organic layer
EXPERIMENTAL SECTION
Materials and Equipment. All the chemicals are analyti-
■
cally pure and were used as received. Solvents were dried and
1
distilled prior to use. H and 13C NMR spectra were recorded
on a Bruker 400/500 MHz spectrophotometer (CDCl3 as
solvent, TMS as standard for which δ = 0.00 ppm). High
resolution mass spectra (HRMS) were determined with ESI-Q-
a
Scheme 2. Synthesis of HBT, HBT-1, RH, and RH-HBT
a
Reagents and conditions: (a) Ar, ethynyltrimethylsilane, PdCl2(PPh3)2, PPh3, CuI, NEt3, 80 °C, reflux, 5 h. (b) (n-Bu)4NF/THF, room
temperature. (c) 2-Aminothiophenol, MeOH, room temperature, 12 h. (d) Under Ar, 1,2-dichloroethane, POCl3, 85 °C, reflux, 4 h. (e) Under Ar,
1,2-dichloroethane, rhodamine B acid chloride, room temperature, 24 h. (f) Ar, THF, Et3N (one drop), CuSO4·5H2O, sodium ascorbate, 30 °C, 12
h.
B
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was washed with brine (30 mL), dried over Na2SO4 and
evaporated in vacuum. The product was purified by column
chromatography (silica gel, petroleum ether/EtOAc = 1:1, v/v)
to give RH-HBT as a light pink solid. Yield: 55.0 mg, 72.4%;
mp 165−167 °C. For the molecular structure verification with
1H NMR and MS spectra, please refer to the Supporting
Information.
Scheme 3. Molecular Structures of Conformers of RH-HBT
Synthesis of Compound HBT-1. Under an Ar atmosphere,
to a stirred solution of 1-azidobutane (10 mg, 0.1 mmol) and
HBT (25 mg, 0.1 mmol) in THF (10 mL), one drop of TEA
was added, and the reaction mixture was stirred for 5 min at
RT. Then CuSO4·5H2O (13 mg) in water (4 mL) and sodium
ascorbate (28 mg) in water (4 mL) was added through syringe,
consecutively and allowed to stir at 30 °C for 12 h. The
progress of the reaction was monitored by TLC. The reaction
mixture was extracted with CH2Cl2. The organic layer was
washed with brine (30 mL), dried over Na2SO4, and evaporated
in vacuum. The product was purified by column chromatog-
raphy (silica gel, petroleum ether/EtOAc = 1:1, v/v) to give a
25.0 mg off-white solid HBT-1. Yield: 71.4%; mp 173−175 °C.
For the molecular structure verification with 1H NMR and MS
spectra, please refer to the Supporting Information.
DFT Calculations. The geometries of the compounds were
optimized using density functional theory (DFT) with B3LYP
functional and 6-31G(d) basis set. There are no imaginary
frequencies for all optimized structures. Time-dependent DFT
(TDDFT) was used for the calculation of the electronic spectra
(UV−vis absorption and fluorescence emission) of the
compounds. The steady-state UV−vis absorption of the organic
dyes was calculated with the optimized S0 state geometry. The
fluorescence of the compounds was calculated with the
optimized S1 state geometry. All these calculations were
performed with Gaussian 09W.74
The molecular structures were fully characterized by 1H NMR,
13C NMR, and HRMS (see Supporting Information).
UV−Vis Absorption and Fluorescence Spectra. The
absorption spectra of HBT, HBT-1, and RH-HBT were
studied (Figures 1 and S11). These compounds exhibit
absorption bands at about 290−300 nm and about 343−360
nm. The low energy absorption band are due to a π−π*
transition, which is assigned to the respective closed conformer
or the E-form (Schemes 3 and S1) of HBT moieties. The
absorption band at about 300 nm is probably due to neutral/
unsolvated open conformer of HBT, HBT-1, and RH-HBT.
The absorption maxima are negligibly sensitive to solvent
polarity,75−78 suggesting the presence of a weak influence of
inter- and intramolecular hydrogen bonding on the absorption
spectra and the absence of proton transfer (PT) processes in
the HBT at the ground state (S0).79−82 In polar protic solvent,
such as methanol or ethanol, HBT exhibited one weak low
energy band at about 400 nm (Scheme S1, open conformer
IV).79
RESULTS AND DISCUSSION
■
Design and Synthesis of the Dyads. Our approach to
switch the absorption/emission feature of an ESIPT
The fluorescence emission spectra and quantum yield of
HBT, HBT-1, and RH-HBT in different polar and non polar
solvents are summarized in (Table 1). In RH-HBT dual
emissions from the enol and keto forms were observed in
dichloromethane, ethyl acetate, tetrahydrofuran, methanol, and
ethanol (Figure 2), the enol tautomer gives emission at shorter
wavelength and the keto tautomer gives emission at longer
wavelength (the fluorescence quantum yields were presented in
Table S1).82 RH-HBT also exhibits ESIPT in toluene, as
evidenced from the strong emission at 548 nm with large
Stokes shift of about 189 nm (Figure 3a). In contrast, strong
enol emission with normal Stokes shift of 56 nm was observed
in protic solvent (Figure 3d) because the intramolecular
hydrogen bonding (−OH···N−) needed for the ESIPT process
is interrupted in this solvent. A similar trend was observed for
HBT and HBT-1 (Figure S12).
Figure 1. UV−vis absorption spectra of RH, HBT, HBT-1, and RH-
HBT in dichloromethane; c = 1.0 × 10−5 M; 20 °C.
chromophore is to link 2-(2-hydroxyphenyl)-benzothiazole
(HBT) with rhodamine via triazole unit (Scheme 2). Thus,
RH-HBT (Scheme 2) was obtained, which is expected to show
fluorescence switch on/off effect upon addition of acid/base.
Reaction of 2-azidoethanamine with commercially available
rhodamine B formed the light pink color rhodamine spirocyclic
lactam RH (Scheme 2). The ethynylated HBT unit was
obtained from 5-bromosalicylaldehyde through Sonogashira
cross-coupling reactions. 2-(2-hydroxyphenyl)-benzothiazole
(HBT) analogue HBT-1 was obtained through click reaction
of HBT and used as reference ESIPT compound. All the
compounds were obtained in moderate to satisfactory yields.
C
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Table 1. Photophysical Properties of Compounds in the Presence and Absence of Acid
a
b
c
d
h
compd
solvents
λabs (nm)
ε
λem (nm)
ΦF (%)
τF (ns)
f
f
f
f
f
f
f
g
g
g
g
g
g
g
g
g
g
g
g
g
g
RH
toluene
DCM
311
314
315
313
315
311
312
349
347
348
345
343
347
347
357
353
357
355
352
348
348
1.50
1.50
1.54
1.50
1.49
1.53
1.57
1.59
1.60
1.62
1.62
1.59
1.61
1.64
1.37
1.34
1.29
1.25
1.33
1.38
1.36
THF
EtOAc
MeCN
MeOH
EtOH
toluene
DCM
d
HBT
531
18.3
1.65
3.20
d
381/528
376/533
375/532
377/530
387/462
386/523
541
12.5
d
THF
8.1
4.62 (λem = 533)
3.76
d
EtOAc
MeCN
MeOH
EtOH
toluene
DCM
4.5
d
3.1
4.45
d
33.8
3.89
d
7.4
4.67
d
HBT-1
10.3
0.91
d
398/539
390/540
390/546
396/541
409/530
405/533
6.5
2.35 (λem = 539)
5.5 (λem = 540)
1.82 (λem = 546)
d
THF
9.8
d
EtOAc
MeCN
MeOH
EtOH
3.3
d
1.8
5.96 (λem= 541 nm)
0.19/4.43 (λem = 530)
5.05 (λem = 533)
0.22 (λem = 405)
0.72
d
16.9
d
15.0
e
4.1
d
RH-HBT
toluene
DCM
360
358
354
357
356
350
353
347
1.03
1.11
1.02
0.97
1.00
1.12
1.11
1.47
548
9.5
d
394/543
386/539
374/547
540
8.1
1.93
d
THF
4.9
4.77 (λem = 539)
1.36 (λem = 547)
6.41 (λem = 540)
0.11/4.51 (λem= 528)
4.10 (λem = 538)
2.08 (λem = 534)
0.15 (λem = 407)
2.22
d
EtOAc
MeCN
MeOH
EtOH
EtOH
2.6
d
2.0
d
404/528
404/538
407/534
5.6
d
4.5
e
HBT-1 + TFA
3.3
e
RH + TFA
EtOH
EtOH
554
4.30/4.56
1.25/2.85
579
25.5
e
RH-HBT + TFA
352/554
406/579
13.03
1.87 (λem = 579)
0.17 (λem = 406)
1.33/3.45
a
b
Maxima UV−vis absorption wavelength (c = 1.0 × 10−5 M, 20 °C). Molar extinction coefficient at the absorption maxima. ε: 104 M−1 cm−1.
c
d
Maxima emission wavelength (c = 1.0 × 10−5 M, 20 °C). Fluorescence quantum yields with 5,10,15,20-tetraphenylporphyrin (TPP; ΦF = 11% in
e
toluene) as standard. Fluorescence quantum yields with 2,6-diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (ΦF = 2.7% in
f
g
h
MeCN) as the standard. No fluorescence. Not applicable. Fluorescence lifetimes under air atmosphere (c = 1.0 × 10−5 M, 20 °C).
was observed in the range of 465−485 nm (Figure 4b) on
addition of TEA due to the formation of the respective anions
(Scheme S1). A gradual increase in the concentration of TEA
results in an isosbestic point (Figure 4a), indicating equilibrium
between the spirolactam conformer (λabs = ∼343−360 nm) and
its anion (λabs = ∼400 nm; Schemes 3 and S1). On the other
hand, on gradual addition of acid like trifluoroacetic acid (TFA)
to the HBT-1 (Figure S13 and S14) in ethanol results in the
disappearance of the band at 400 and 465−485 nm in the
absorption spectra and emission spectra, respectively, due to
the interruption of intermolecular hydrogen bonding between
solute and solvent in polar protic solvents. A similar trend was
observed for HBT. Due to the presence of strong intra-
molecular hydrogen bonding in RH-HBT in nonpolar solvents,
addition of base or acid has no effect on the absorption spectra/
emission spectra in nonpolar solvents.
For RH-HBT, absorption bands at 312 and 353 nm were
observed in ethanol, respectively, without any absorption band
in the visible region (Figure 5). Upon addition of 5000 equiv of
trifluoroacetic acid (TFA), a new strong visible absorption band
at 554 nm developed due to the formation of the open-ring
Figure 2. Fluorescence emission spectra of RH-HBT in different
solvents; λex = 335 nm; c = 1.0 × 10−5 M; 20 °C.
Effect of Acid and Base. Absorption spectra of RH-HBT
in protic polar solvent ethanol exhibited a new absorption band
at about 400 nm upon addition of base (triethylamine, TEA;
Figure 4a). Similar results were observed for HBT and HBT-1
(Supporting Information, Figure S14). We assume that the
change is due to the formation of the respective anions of the
open conformers of HBT, HBT-1, and RH-HBT (Schemes 3
and S1). In emission spectra of RH-HBT, a new emission band
D
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Figure 6. Fluorescence emission spectra of HBT-1, RH-HBT, RH-
HBT + TFA (5000 equiv and 60 min standing time), and RH-HBT +
TFA (5000 equiv) + TEA (pure 20 μL; λex = 335 nm, where both
HBT-1 and RH-HBT + TFA solution give same absorbance); c = 1.0
× 10−5 M in EtOH; 20 °C.
Figure 3. Normalized absorption and fluorescence spectra of RH-
HBT in different solvents: (a) PhCH3 and DCM; (b) THF and
EtOAc; (c) MeCN and MeOH; (d) EtOH (c = 1.0 × 10−5 M); λex
360 nm; 20 °C.
=
Figure 4. (a) UV−vis absorption spectra of RH-HBT in the presence
of triethylamine (TEA). (b) Fluorescence emission spectra of RH-
HBT in the presence of TEA; λex = 360 nm; c = 1.0 × 10−5 M in
ethanol; 20 °C.
Figure 7. Fluorescence spectra of the compounds in CH2Cl2. (a)
HBT-1 and HBT-1 + TFA (10000 equiv; λex = 367 nm); (b) RH +
TFA (10000 equiv) and RH-HBT + TFA (10000 equiv; λex = 398
nm); (c) HBT-1 + TFA (10000 equiv) and RH-HBT + TFA (10000
equiv; λex = 367 nm); and (d) RH-HBT, RH-HBT + TFA (10000
equiv) and RH-HBT + TFA + TEA (λex = 360 nm). In each figure, the
solution of the two sample give same absorbance at the excitation
wavelength so that the fluorescence emission intensity can be
compared; c = 1.0 × 10−5 M in CH2Cl2; 20 °C.
The reversible switching of fluorescence emission of the
dyads upon addition of acid and base were investigated (Figure
6). Initially, RH-HBT dyads exhibits an emission band at 404
nm (Figure 6b) due to the emission of the respective enol-
form. But on addition of TFA, RH-HBT displayed an
additional distinct strong emission band at ∼579 nm (Figure
6) due to the open-ring amide form. These bands can be
switched OFF by addition of base, such as TEA (Figure 6).
HBT-1 alone in the presence of TFA does not show such
switching effect. Thus, the emission of RH-HBT at 579 nm in
the presence of TFA is due to the open-ring amide form of the
rhodamine unit. Interestingly, dual emission was observed for
RH-HBT in the presence of TFA (Figure 6), that is, the enol
form emission of the HBT moiety and the rhodamine emission
in RH-HBT were observed simultaneously (Figure 7). The
nonefficient FRET may be due to the poor overlap of the
emission of HBT and the absorption of rhodamine moiety. The
Figure 5. UV−vis absorption spectra of RH-HBT, RH-HBT + TFA
(5000 equiv and 60 min standing time), RH-HBT + TFA (5000
equiv) + TEA (20 μL); c = 1.0 × 10−5 M in EtOH; 20 °C.
amide form of rhodamine B (RH′ and RH-HBT′) from closed
spirocyclic lactam form (RH and RH-HBT), correspondingly
the color of the solution changed from colorless to pink (Figure
5). The sequential addition of the base such as TEA regenerates
the spirocyclic lactam form of rhodamine (Figure 5), and the
pink solution turns to colorless. Thus, the absorption of RH
and RH-HBT in visible region can be switched ON and OFF
by the spirocyclic lactam ↔ opened amide transformation of
the rhodamine moiety.
E
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emission. Upon addition of TFA, the keto emission does not
change substantially. The enol form emission was enhanced
(Figure 7a). For RH-HBT, strong emission band at 579 nm
was observed upon addition of TFA, which is drastically
different from the weak emission of RH-HBT in the absence of
TFA. The emission of RH-HBT + TFA can be assigned to the
rhodamine moiety. This new emission band upon addition of
TFA was switched off by addition of base such as TEA (Figure
7d). Interestingly, the keto emission of HBT moiety was
quenched in RH-HBT + TFA, which is different from the
experiments in EtOH, for which the enol-emission band was
not quenched (Figure 6). The efficient FRET in RH-HBT +
TFA in DCM may be due to the better match of the keto
emission of HBT moiety and the absorption band of
rhodamine moiety. Thus, by selection of different solvents,
the emission of the ESIPT chromophore is able to be either
quenched or unaltered. It should be noted that there are two
possibilities for this result, that is, either the ESIPT was
inhibited by FRET, or ESIPT does occur, but the emission of
the keto form was quenched by FRET. Considering that the
ESIPT and the FRET may occur on the same time scale (ps), a
competing ESIPT and FRET is plausible.2 To the best of our
knowledge, the ESIPT chromophore was rarely switched with a
covalently attached external stimuli-activatable chromophore.
The cyclic spirolactam → open amide transformation of the
rhodamine moiety in the dyads is found with slow kinetics.
Although rhodamine has been intensively used in fluorescent
molecular sensors by employing the closed spirocyclic lactam
↔ open amide form transformation, the kinetic of the closed
lactam structure ↔ open amide structure was rarely studied.
The kinetics of the closed lactam structure ↔ open amide
structure transformation of RH-HBT at 554 nm upon addition
of TFA were studied (Figure S17). Slow kinetics were observed
for all the rhodamine-containing compounds. For RH and RH-
HBT, the transformation rate constants are k = (1.3 0.02) ×
10−3 min−1 and k = (0.9 0.02) × 10−3 min−1, respectively.
Therefore, sufficient standing time was used in all the
measurements in this paper. Interestingly, the reverse open
amide → the cyclic spirolactam transformation of the
rhodamine B moiety is much faster upon addition of base
such as TEA and a few seconds is sufficient. This reaction
kinetics was rarely reported. The kinetics is clearly important
for applications based on the closed spiro lactam ↔ open amide
transformations.
Figure 8. Selected frontier molecular orbitals involved in the vertical
excitation and the singlet excited state (S1) (both enol form and keto
form were calculated) of RH-HBT in closed form in toluene. The
calculations are based on the optimized ground state geometry (S0
state, excitation) and exited state geometry (S1 state, emission) at the
B3LYP/6-31G(d) level using Gaussian 09W.
Table 2. Selected Electronic Excitation Energies (eV) and
Corresponding Oscillator Strengths (f), Main
Configurations, and CI Coefficients of the Low-Lying
Electronic Excited States of the Closed RH-HBT Calculated
by TDDFT//B3LYP/6-31g(d) Based on the DFT//B3LYP/
6-31g(d) Optimized Ground State Geometries (for the UV-
Vis Absorption, i.e., Excitation) and Optimized S1 State
Geometry (for the Fluorescence Emission)
a
electronic
transition
energy
b
c
d
(eV/nm)
f
composition
CI
UV−vis
S0 → S1
S0 → S2
3.20/388
3.33/373
0.0030
0.0258
H → L
0.7051
0.1970
0.6777
0.6682
0.2006
0.7050
0.7027
H−2 → L
H−1 → L
H−2 → L
H−1 → L
H → L
S0 → S3
3.33/372
0.2726
RH-HBT in its open amide form shows small Stokes shifts
(ca. 25 nm; Figure S18) and high fluorescence quantum yields
(ΦF = 13.0%; Table 1), which are in stark contrast to the
typical features of the conventional ESIPT dyes (Stokes shift >
100 nm).84 Thus, acid- or base-activatable absorption and
emission properties of the ESIPT chromophore HBT were
achieved with RH-HBT. To the best of our knowledge, such a
method was rarely used for switching the absorption and
fluorescence of ESIPT chromophores.
DFT Calculations on the UV−Vis Absorption and
Fluorescence Emission. DFT/TDDFT calculations have
been used to study the photophysical properties of
fluorophores.85−97 The ground state geometries of the
fluorophores was optimized with DFT method. The UV−vis
absorption and the fluorescence emission of the compounds
were calculated with TDDFT methods.
FL(enol)
FL(keto)
S1 → S0
S1 → S0
3.20/388
2.74/457
0.0030
0.4218
H → L
a
b
Only the selected low-lying excited states are presented. Oscillator
c
d
strengths. Only the main configurations are presented. The CI
coefficients are in absolute values. H, L, and FL stands for HOMO,
LUMO, and fluorescence, respectively.
emission of HBT-1 was quenched in RH-HBT (Figure 6),
probably due to the electron transfer. In a simplified
consideration by assuming that the electron transfer is the
main cause of the quenching, the rate constant (kET) for the
electron-transfer reaction was estimated with the following
equation as to be kET = 1.6× 1010 s−1 (eq 1).83
⎡
⎤
ΦF(HBT‐1)
⎢
⎥
kET
=
/τ(HBT‐1)
⎢
⎣
⎥
⎦
Φ
F(RH‐HBT)
(1)
The ground state geometry of HBT-1 was optimized (Figure
S19). The calculated excitation wavelength for the S0 → S1
transition is 371 nm, which is close to the experimental result
The spectral changes upon addition of TFA in CH2Cl2 were
also studied (Figure 7). In DCM HBT-1 shows substantial keto
F
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Table 3. Selected Electronic Excitation Energies (eV) and Corresponding Oscillator Strengths (f), Main Configurations, and CI
Coefficients of the Low-Lying Electronic Excited States of the Opened Amide RH-HBT (after Addition of TFA) Calculated by
TDDFT//B3LYP/6-31g(d) Based on the DFT//B3LYP/6-31g(d) Optimized Ground State Geometries (for the UV−Vis
absorption, i.e., Excitation) and Optimized S1 State Geometry (for the Fluorescence Emission)
a
b
c
d
electronic transition
energy (eV/nm)
f
composition
CI
UV−vis
S0 → S1
S0 → S6
2.62/473
3.59/345
0.9816
0.1150
H → L
0.7043
0.6091
0.2355
0.6046
0.5476
0.7055
H−5 → L
H−6 → L
H−1 → L+1
H−2 → L+1
H → L
S0 → S8
S0 → S19
S1 → S0
3.70/335
4.38/283
2.46/503
0.2886
0.4560
1.2383
FL(enol)
a
b
c
d
Only the selected low-lying excited states are presented. Oscillator strengths. Only the main configurations are presented. The CI coefficients are
in absolute values. H, L, and FL stands for HOMO, LUMO, and fluorescence, respectively.
Figure 9. Selected frontier molecular orbitals involved in the vertical excitation and the singlet excited state (S1) (both enol form and keto form were
calculated) of compound RH-HBT open amide form in closed form in toluene. CT stands for conformation transformation. The calculations are
based on the optimized ground state geometry (S0 state, excitation) and exited state geometry (S1 state, emission) at the B3LYP/6-31G(d)/ level
using Gaussian 09W.
(357 nm, Figure 1). The electron density on the hydroxyl
group decreased upon excitation, whereas the electron density
on the N atom of benzothiazole moiety increased upon
photoexcitation (Figure S19). The change in the electron
density indicates that both the acidity of the hydroxyl group
and the basicity of N atom will increase upon excitation, which
coincides with the occurrence of ESIPT. The excited states
geometries of both the enol and keto form of HBT-1 were
calculated (Table S2). The emission of the keto tautomer was
calculated as 459 nm (the experimental results of 541 nm;
Figure 2).
RH-HBT was studied with similar method (Figure 8 and
Table 2). The calculated excitation wavelength for the S0 → S1
transition is 388 nm with oscillator strength of 0.0030, which is
a charge transfer state, thus can be considered as a dark state.91
As a result, the emission of HBT may be quenched in RH-
HBT. This postulation was supported by the experimental
results (Figure 6). The low-lying allowed transition for RH-
HBT is S0 → S3 (372 nm) with oscillator strength of 0.2726
which is close to the experimental results of 360 nm. HOMO
and LUMO have overlapped orbitals and localized on HBT
unit. In RH-HBT also, upon photoexcitation the electron
density of benzothiazole moiety increased. Due to the
involvement of the frontier molecular orbitals of benzothiazole
moiety, the basicity of N atom will increase upon excitation,
which corresponds the occurrence of ESIPT. The calculated
emission peak is at 459 nm (the experimental emission band is
at 547 nm; Table 2).
Upon addition of TFA, the spirocyclic rhodamine lactam →
open amide transformation, the photophysical property of the
RH-HBT changed (Figures 5−7). With TDDFT calculation on
the singlet excited state (UV−vis absorption), a new absorption
band at 473 nm, with a large oscillator strength (f) of 0.9816
was observed (Table 3). HOMO → LUMO transition is
involved in this absorption band, which is localized on the
rhodamine open amide part (Figure 9). The calculated
emission peak is at 503 nm (the experimental results of 579
nm. Figure 3). It is known that the B3LYP hybrids overestimate
the excitation energy for the opened form of Rhodamine
chromophore.98 The DFT calculations indicate that the
benzothiazole moiety is not involved in the frontier molecular
orbitals of the absorption and emission transitions of the
G
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Article
(2) Zhao, J.; Ji, S.; Chen, Y.; Guo, H.; Yang, P. Excited State
Intramolecular Proton Transfer (ESIPT): From Principal Photo-
physics to the Development of New Chromophores and Applications
in Fluorescent Molecular Probes and Luminescent Materials. Phys.
Chem. Chem. Phys. 2012, 14, 8803−8817.
rhodamine open amide part. Molecular orbitals of the HBT
unit are involved in higher singlet excited states of RH-HBT,
such as S8 and S19 state (Figure 9 and Table 3). According to
Kasha’s rule, these high singlet excited states may be relaxed to
the low-lying S1 state on the rhodamine part before any
fluorescence emission occurs.
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(ESIPT) Process. Adv. Mater. 2011, 23, 3615−3642.
CONCLUSION
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Mechanisms for Design of Fluorescent Chemosensors Emerging in
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Chen, K- Y.; Hung, W- Y.; Hsu, Y- H.; Chou, P- T. Fine Tuning the
Energetics of Excited-State Intramolecular Proton Transfer (ESIPT):
White Light Generation in a Single ESIPT System. J. Am. Chem. Soc.
2011, 133, 17738−17745.
■
In summary, 2-(2-hydroxyphenyl)-benzothiazole (HBT)-rhod-
amine dyad was prepared as a new excited state intramolecular
proton transfer (ESIPT) chromophore, with the goal to switch
the absorption/emission property of ESIPT chromophore by
external stimuli, such as acid/base. The photophysical processes
were studied with absorption and emission spectroscopies. The
closed spirocyclic lactam form of rhodamine moiety shows no
visible light absorption whereas the opened amide form of
rhodamine moiety show strong absorption at 554 nm. The
switching of the fluorescence of 2-(2-hydroxyphenyl)-benzo-
thiazole (HBT)-rhodamine dyad RH-HBT were achieved on
addition of acid/base, with which the reversible closed
spirocyclic lactam form ↔ opened amide structure trans-
formation of the rhodamine moiety takes place. The closed
spirocyclic lactam form of rhodamine moiety RH-HBT shows
the characteristics of ESIPT. Upon addition of acid in EtOH,
the enol form emission band at 406 nm was not quenched by
the FRET, thus dual emission band was observed. In
dichloromethane, however, the keto form fluorescence
emission of the HBT unit was completely quenched. Thus,
the absorption and emission features of the ESIPT
chromophore were successfully switched by a acid/base-
activatable chromophore. These investigations will be useful
to design new ESIPT chromophore as luminescent molecular
probes and external stimuli-activatable fluorescent switching
probes.
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ASSOCIATED CONTENT
■
(13) Kim, S. K.; Lee, D. H.; Hong, J.-I.; Yoon, J. Chemosensors for
Pyrophosphate. Acc. Chem. Res. 2009, 42, 23−31.
S
* Supporting Information
1H NMR, 13C NMR, and HRMS spectra of RH, HBT, HBT-1,
and RH-HBT. This material is available free of charge via the
Internet at http://pubs.acs.org.
(14) Liu, B.; Wang, H.; Wang, T.; Bao, Y.; Du, F.; Tian, J.; Li, Q.; Bai,
R. A New Ratiometric ESIPT Sensor for Detection of Palladium
Species in Aqueous Solution. Chem. Commun. 2012, 48, 2867−2869.
(15) Xu, Y.; Pang, Y. Zn2+-Triggered Excited-State Intramolecular
Proton Transfer: A Sensitive Probe with Near-Infrared Emission from
Bis(benzoxazole) Derivative. Dalton Trans. 2011, 40, 1503−1509.
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AUTHOR INFORMATION
■
Corresponding Author
*Tel.: +86 411 8498 6236. E-mail: zhaojzh@dlut.edu.cn.
Notes
(17) Wu, Y.; Peng, X.; Fan, J.; Gao, S.; Tian, M.; Zhao, J.; Sun, S.
Fluorescence Sensing of Anions Based on Inhibition of Excited-State
Intramolecular Proton Transfer. J. Org. Chem. 2007, 72, 62−70.
(18) Zamotaiev, O. M.; Postupalenko, V. Y.; Shvadchak, V. V.;
Pivovarenko, V. G.; Klymchenko, A. S.; Mely, Y. Improved Hydration-
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Sensitive Dual-Fluorescence Labels For Monitoring Peptide−Nucleic
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Fluorescence of Methylated Derivatives of Hydroxyphenylimidazopyr-
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSFC (21073028, 21273028, 21473020, and
21421005), the Royal Society (U.K.) and NSFC (China-U.K.
Cost-Share Science Networks, 21011130154), Ministry of
Education (SRFDP-20120041130005), Program for Chang-
jiang Scholars and Innovative Research Team in University
[IRT_13R06], the Fundamental Research Funds for the
Central Universities (DUT14ZD226), and Dalian University
of Technology (DUT2013TB07) for financial support.
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