New two-photon absorption benzotriazole-coumarin dyads: The evidence of internal proton transfer in the excited state

Article abstract of DOI:10.1016/j.tetlet.2014.11.074

New near-infrared two-photon absorption π-extended benzotriazole-coumarin dyads as target molecules were synthesized. The corresponding coumarin derivatives and esterified dyads were also prepared as the references. The experimental evidence of internal p

Full text of DOI:10.1016/j.tetlet.2014.11.074

Tetrahedron Letters 56 (2015) 236–242
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
New two-photon absorption benzotriazole–coumarin dyads:
the evidence of internal proton transfer in the excited state
⇑
⇑
⇑
Yulong Gong, Yao Lu, Zhenqiang Wang, Shengtao Zhang, Ziping Luo , Hongru Li , Fang Gao
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 40044, China
a r t i c l e i n f o
a b s t r a c t
Article history:
New near-infrared two-photon absorption
p
-extended benzotriazole–coumarin dyads as target
Received 15 August 2014
Revised 14 November 2014
Accepted 17 November 2014
Available online 21 November 2014
molecules were synthesized. The corresponding coumarin derivatives and esterified dyads were also
prepared as the references. The experimental evidence of internal proton transfer in the excited state
of the benzotriazole-based chromophores under one and near-infrared two-photon irradiation are firstly
presented in this Letter.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Two-photon absorption
Organic dyads
H-bonding effect
Proton transfer
Excited state
There is a long-staying scientific controversy on internal proton
transfer in the excited state for o-hydroxy-phenyl-benzotriazole
family, which contains typical intramolecular proton transfer
groups. Unlike o-hydroxy-phenyl-benzothiazole and o-hydroxy-
phenyl-benzoxazole,1,2 excited state intramolecular proton trans-
fer (ESIPT) of o-hydroxy-phenyl-benzotriazole family is lack of the
substantial experimental evidence. Due to more electron-deficiency
in the heterocyclic ring of o-hydroxy-phenyl-benzotriazole
comparing with o-hydroxy-phenyl-benzothiazole and o-hydroxy-
phenyl-benzoxazole, the occurrence of internal proton transfer
in the excited state of o-hydroxy-phenyl-benzotriazole family
decreases.
were reported to exhibit ESIPT under near-IR laser excitation.9
Unfortunately, no characteristic proton transfer emission band
was detected under near-IR laser irradiation to these benzoxaz-
ole-based dyes. To the best of our knowledge, there is no
report on ESIPT of o-hydroxy-phenyl-benzotriazole-based organic
chromophores under one-photon and near-IR laser irradiation.
Extended organic chromophores with ESIPT possess more appli-
cation potentials.10,11 In particular,
p-extended organic chromoph-
ores with ESIPT can be utilized as TPA dyes by near-IR femtosecond
laser. Several groups developed enlarged organic molecules con-
taining proton transfer segments through the chemical covalent
bond.12–14 However, the insertion of
p-conjugation units was
So far, there have been different opinions on the occurrence of
internal proton transfer in the excited state for this family.3–5 For
instance, it was reported that intramolecular electron transfer led
to the deactivation of the excited state for o-aryl-benzotriazoles,3
while internal proton transfer was regarded as the main decay
pathway in the excited singlet state for 2-(20-hydroxy-phenyl)-
benzotriazole.4,5
Two-photon absorption (TPA) chromophores opened a door for
the application of near-IR laser,6–8 which demonstrated various
superior physical advantages such as low energy, deep penetration,
and negligible damage to normal biological tissues. A few 2-(20-
hydroxyphenyl)-benzoxazole-based dyes with TPA cross sections
reported to inhibit intramolecular proton transfer in the excited
state for an organic chromophore.12 Therefore, the development
of new p-extended o-hydroxy-phenyl-benzotriazole-based organic
chromophores with internal proton transfer in the excited state is
still a great challenge.
In this Letter, coumarin is used as the
benzotriazole–coumarin dyads, where the electron-accepting
,b-unsaturated carbonyl group acts as the -linker to increase
p-unit to construct new
a
p
the activity of the phenolic hydroxy group. Herein, we describe
our studies on the internal proton transfer of 2-(20-hydroxy-
phenyl)-benzotriazole-coumarin dyads in the excited state under
both one-photon and near-IR two-photon irradiation, respectively.
Several target benzotriazole–coumarin dyads (C1–C3) were
designed and synthesized as shown in Scheme 1. The corresponding
coumarins (C4–C6) and O-acetyl dyads (C7–C9) were also prepared
⇑
Corresponding authors. Tel./fax: +86 23 65102531.
E-mail addresses: zpluo@126.com (Z. Luo), hongruli1972@gmail.com (H. Li),
fanggao1971@gmail.com (F. Gao).
http://dx.doi.org/10.1016/j.tetlet.2014.11.074
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

Y. Gong et al. / Tetrahedron Letters 56 (2015) 236–242
OH
237
HO
HO
NH2
NO2
+
N2
HCl / NaNO2
N
N
CHO
Zn / NaOH
N
N
N
NO2
S1
NO2
CHO
CHO
A
N
c
O
O
A
c
O
O
/
D
C
M
(
R
E
OH
t
)
O
3
O
S2
Piperidine
O
R
CHO
AcO
N
N
N
S3
CHO
HO
O
O
N
N
N
R
O
C1, R=-Cl
C2, R=-CH3
C3, R=-H
C1/C2/C3
(a)
HO
O
O
O
OH
R
O
OHC
R
C4/C5/C6
Piperidine/EtOH
Piperidine/EtOH
O
O
C4/C7, R=-Cl
C5/C8, R=-CH3
C6/C9, R=-H
N
N
OAc
N
AcO
O
O
O
N
N
N
OHC
R
C7/C8/C9
(b)
HO
N
N
N
NH
N
N
S4
S5
(c)
Scheme 1. Chemical structures of the molecules studied in this work.
for reference. The detailed procedures for the target and reference
molecules are described in Supplementary material.
Compounds C2 and C3 exhibit similar 1H NMR nature as C1. 1H
NMR chemical shift of OH in C2 or C3 situates at downfield comparing
with those of C5 or C6 in D6-DMSO. 1H NMR chemical shift of OH in
the equal molar mixture of C5/1,2,3-benzotriazole or C6/1,2,3-benzo-
triazole is almost identical to that of C5 or C6, respectively. Further-
more, 1H NMR chemical shift of OH in the equal molar mixture of
C8/phenol or C9/phenol is similar to that of phenol, respectively.
The above results also show that there is no strong interaction
between OH and C@N groups in the ground state as they are
contained in different molecules respectively. The fact that 1H
As shown in 1H NMR (Fig. 1), the chemical shift of OH in C1
moves to downfield comparing with those of C4 and phenol in
D6-DMSO (C1, 11.23 ppm, C4, 10.20 ppm, phenol, 9.35 ppm). It is
also noticed that 1H NMR chemical shift of OH in the equal molar
mixture of C4/1,2,3-benzotriazole (S5) is almost the same as that
of C4 (Fig. S1, Supplementary material). In addition, 1H NMR chem-
ical shift of OH in the equal molar mixture of C7/phenol is almost
the same as that of phenol (Fig. S1).

238
Y. Gong et al. / Tetrahedron Letters 56 (2015) 236–242
Figure 1. 1H NMR spectra of C1, C4, and phenol.
NMR chemical shifts of OH in target molecules C1–C3 appeared at
lower magnetic field suggests that there is an intramolecular H-
bonding effect between the OH and the C@N group in the ground
states of C1–C3 respectively.15
C1, C4, and C7 only exhibit the absorption in the range of 250–
450 nm in benzene (Fig. 2a). While in DMF, in addition to the
absorption in the above range, C1 shows one more new absorption
band in the range of 450–700 nm (Fig. 2b). Similar absorption phe-
nomenon is also observed in DMSO. As contrast, the reference mol-
ecules such as C4 and C7 only show the absorption in the range of
250–420 nm in various solvents. The further study shows that the
equal molar mixture of C4/1,2,3-benzotriazole and C7/phenol do
not display the absorption in the range of 450–700 nm in DMF
and DMSO, respectively, (Fig. S2, Supplementary materials).
The results suggest that the new absorption band is not origi-
nated by intermolecular interaction. The absorption of C1 in
450–700 nm in DMSO and DMF is assigned to internal molecular
H-bonding effect.3 It is considered that the great polarity of DMSO
and DMF is beneficial for the enhancement of the activity of the
hydroxy group,16 so intramolecular hydrogen bonding strength of
C1 between the hydroxy group and imino group increases, while
leading to the formation of the absorption band in 450–700 nm.
Therefore, it is noticed that the absorption band of C1 in the
range of 450–700 nm gradually decreases when lower polar sol-
vents such as CH2Cl2 is added into DMF or DMSO solution
(Fig. S3, Supplementary materials).
Comparing with the target molecules, 1H NMR chemical shift of
OH in 4-methyl-2-(2H-benzotriazol-2-yl)-phenol (S4) is located at
the upfield (Fig. S1). This reflects the electron-withdrawing effect
of the a,b-unsaturated carbonyl group on internal H-bonding role
in the ground state.
By comparison of the 1H NMR of C1, C2, and C3, the chemical
shift of OH in C1 moves to lower magnetic field (C1, 11.23 ppm,
C2, 11.19 ppm, and C3, 11.15 ppm), indicating that there is stron-
ger internal H-bonding effect in the ground state of C1.
The ultraviolet/visible absorption spectra of the target and
reference molecules were determined in various solvents.
Typical absorption spectra of C1, C4, and C7 in benzene and
DMF are shown in Figure 2a and b, respectively. Because
1,2,3-benzotriazole shows quite weak optical density, coumarin
moiety makes a major contribution to the absorption of the
entire chromophore molecule. As a result, the maximal molar
extinction coefficients and the absorption maxima of C1
and C4 are quite close (such as in benzene, C1, kmax, 361 nm,
emax
,
0.208 cmꢀ1 molꢀ1 L, C4, kmax
molꢀ1 L, Table 1).
,
355 nm, emax
,
0.242 cmꢀ1
The study shows that the absorption of C1 in the range of
450–700 nm decreases in triethylamine (TEA) comparing with that
0.4
0.3
0.2
0.1
0.0
C1
C4
C7
C1
C4
C7
0.3
0.2
0.1
0.0
(a)
(b)
300
400
500
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Figure 2. UV/visible absorption spectra (a) and (b) of C1, C4, and C7 in benzene and DMF, respectively, the concentration is 10ꢀ5 mol/L for the absorption spectra.

Y. Gong et al. / Tetrahedron Letters 56 (2015) 236–242
239
Table 1
The spectral parameters of C1, C4, and C7 in various solvents, ka,max: the absorption maximum (nm), kf,max: the emission maximum (nm).
e
max, the maximal molar extinction
coefficient (cmꢀ1 molꢀ1 L),
Solvents
U
, the fluorescence quantum yield
C1
C4
ka,max (nm) kf,max (nm)
C7
ka,max (nm) kf,max (nm)
U
emax (ꢁ10ꢀ5
)
U
emax (ꢁ10ꢀ5
)
ka,max (nm) kf,max (nm)
U
emax (ꢁ10ꢀ5
)
Benzene
THF
361
359
351
354
333
344
381
373
431
431
433
444
450
448
425
419
432
0.081 0.208
0.080 0.276
0.105 0.291
0.093 0.265
0.080 0.175
0.077 0.320
0.010 0.245
0.013 0.148
0.007 0.152
360
360
351
355
336
350
382
383
353
431
438
403
434
435
436
433
430
435
0.016 0.242
0.025 0.256
0.121 0.107
0.015 0.275
0.013 0.168
0.011 0.324
0.006 0.237
0.007 0.221
0.009 0.197
365
373
368
360
363
354
340
359
377
439
434
429
432
433
437
434
413
435
0.060 0.268
0.070 0.289
0.151 0.293
0.115 0.285
0.104 0.099
0.054 0.270
0.091 0.298
0.093 0.195
0.013 0.325
EtOAc
CH2Cl2
Acetone
CH3CN
DMSO
DMF
Methanol 307
in DMSO or DMF (Fig. S4, Supplementary materials). As a matter of
fact, that absorption decreases as well when aqueous potassium
carbonate solution is added (Fig. S5, Supplementary materials).
This could be the result of partial neutralization of the hydroxy
group by TEA or potassium carbonate, and intramolecular
hydrogen bonding effect in C1 decreases. It is further observed that
the diminished absorption in 450–700 nm is recovered as the
further addition of aqueous HCl solution.
Compounds C2 and C3 show the similar absorption spectral
properties as C1. In lower polar solvents comparing with DMSO
and DMF, C2, and C3 only display the absorption below 450 nm.
In contrast, C2 and C3 exhibit one more new absorption band
above 450 nm in DMSO and DMF, respectively.
The further investigation also shows that the absorption band of
C1 in 450–700 nm decreases in methanol (Fig. S6, Supplementary
materials), and it nearly disappears for C2 and C3. The results show
that intermolecular H-bonding effect between the methanol
molecules and the solute molecules inhibit internal H-bonding
effect in these target chromophores. The results also indicate that
C1 displays stronger internal H-bonding effect than C2 and C3.
Normally, internal H-bonding effect in the ground state of an
organic molecule is considered as the prerequisite to undergo
intramolecular proton transfer in the excited state. Hence, the
presence of internal H-bonding effect in the target molecules
C1–C3 means that intramolecular proton transfer in the excite
state can occur. The fluorescence spectra of the molecules were
measured in various solvents to get the experimental evidences
of internal proton transfer in the excited singlet state.
Typical linear fluorescence emission spectra of C1, C4, and C7 in
benzene and DMF are shown in Figure 3. In benzene, C1, C4, and C7
show single emission band in 380–600 nm, and the maximal emis-
sion wavelengths are close (Table 1, C1, 361 nm, C4, 360 nm, and
C7, 365 nm). In DMF, C4 and C7 still show single emission band
in 380–600 nm. In contrast, C1 exhibits dual emission bands in
380–700 nm in DMF.
In DMF, the maximal emission wavelength of the first emission
band of C1 is close to those of C4 and C7 (ca. 430 nm). The maximal
wavelength of the second emission band of C1 is much red-shifted
(100 nm, in DMF), and the second emission band possesses large
Stokes shift (140 nm, in DMF). Similar experimental phenomena
are also observed in DMSO. In lower polar solvents such as benzene
and CH2Cl2 comparing with DMSO and DMF, C1 shows only single
emission band in 380–600 nm. However, C4 and C7 exhibit the
similar single emission band in 380–600 nm in various solvents.
The equal molar mixtures of C4/S5 and C7/phenol also show the
single emission band in 380–600 nm as C4 and C7 in various sol-
vents (Fig. S7, Supplementary materials). This shows the second
emission band of C1 in DMF is not yielded by intermolecular inter-
action. It is found that the new emission band in 480–700 nm of C1
gradually decreases as lower polar solvents such as CH2Cl2 is added
to DMSO or DMF (Fig. S8, Supplementary materials). In TEA, the
second emission band in 480–700 nm greatly decreases (Fig. S9,
Supplementary materials). It is further observed that the second
emission band in 480–700 nm in DMF and DMSO solution also
decreases as aqueous potassium carbonate solution is added
(Fig. S10, Supplementary materials). On the other hand, the
reduced second emission band by base is recovered as aqueous
HCl solution is added.
Compounds C4 and C7 also exhibit normal Stokes shift in vari-
ous solvents as shown in Table 1(such as 70 nm in benzene). The
first emission band of C1 in DMSO and DMF exhibits a similar
Stokes shift (44 nm in DMF and 46 nm in DMSO). The results
suggest that the first emission band of C1 in DMSO and DMF is
from the normal enol* to enol decay, which is almost effectively
the same as S0 ? S1 normal absorption spectral feature.17
However, the second emission band possessing larger Stoke shift
(such as 140 nm in DMF) is ascribed to internal proton transfer
in the excited state (keto* to keto decay, Scheme 2).18 It is
considered that ESIPT means the occurrence of real chemical
reaction, and thus polar organic solvents such as DMSO and DMF
800
400
C1
C1
C4
C7
C4
C7
(a)
(b)
600
300
400
200
0
200
100
0
400
500
600
400
500
600
Wavelength(nm)
Wavelength(nm)
Figure 3. Emission spectra (a) and (b) of C1, C4, and C7 in benzene and DMF, respectively, the concentration is 5 ꢁ 10ꢀ6 mol/L for the emission spectra, excitation at 350 nm.

240
Y. Gong et al. / Tetrahedron Letters 56 (2015) 236–242
O
H
O
H
N
N
O
O
O
O
O
O
N
N
N
UV absorption
N
Cl
Cl
Normal emission
Enol
Enol*
H-transfer
H-transfer
Keto*
O
O
Keto
H
H
N
N
O
O
O
O
O
O
ion
emiss
SIPT
E
N
N
N
N
Cl
Cl
Scheme 2. A four-level cycle enol–keto phototautomerization during internal proton transfer in the excited state of C1.
are favorable for the realization of such process.19 However, the
hydroxy group in C1 can be neutralized by TEA or potassium car-
bonate. Thus, internal proton transfer in the excited state of C1 is
inhibited by TEA or potassium carbonate, and the second emission
band in 480–700 nm decreases.
Compounds C2 and C3 show the similar linear emission spectral
properties as C1. It suggests that the target dyads C1–C3 can
undergo intramolecular proton transfer in DMSO and DMF in the
excited state. The further determination shows that the second
emission band of the target dyads C1–C3 in 480–700 nm decreases
in methanol (Fig. S11, Supplementary materials). The results
suggest that intermolecular H-bonding effect between the solvents
and the solute inhibits intramolecular proton transfer in the
excited state of these target molecules.17
It is observed that IE*/IN* ratio (the ratio of the maximal
internal proton transfer emission intensity to the maximal normal
emission intensity) of C1 in DMF is higher than in DMSO (DMF,
0.820, DMSO, 0.381). C2 and C3 exhibit the same phenomena as
well. This can be ascribed to the high polarity in DMF, which favors
internal proton transfer in the excited state of benzotriazole–
coumarin dyads.
It is also noticed that IE*/IN* ratio of C1 is greater than those of C2
and C3 in DMSO or DMF respectively (such as in DMF, C1, 0.820,
C2, 0.470, and C3, 0.684), which can be ascribed to more intensive
H-bonding effect due to electron-withdrawing effect of the
chlorine group in the coumarin ring of C1.
The target dyads C1–C3 exhibit TPA upconverted fluorescence
emission under near-IR femtosecond laser excitation. Typical TPA
fluorescence emission spectra of C1, C4, and C7 in benzene and
DMF under 730 nm laser are shown in Figure 4. It is surprising that
C1 shows dual TPA emission bands in the range of 400–700 nm in
both benzene and DMF solvents, which is different from the linear
emission nature. However, C4 and C7 still display single emission
band in the range of 400–600 nm under near-IR laser irradiation.
Further study shows that the mixture of C4/1,2,3-benzotriazole
or C7/phenol exhibits the same TPA emission spectra as C4 or C7,
respectively, (Fig. S12, Supplementary materials). In other solvents,
C1 exhibits dual emission bands 400–700 nm as excited by 730 nm
femtosecond laser as well. C4 and C7 show single emission band
from 400 to 600 nm in various solvents.
The maximum emission wavelength of the first TPA emission
band of C1 is close to those of C4 and C7 (ca. 450 nm) in various
solvents. The maximal wavelength of the second emission band
of C1 under near-IR laser irradiation is similar to that of one-
photon excitation (ca. 540 nm). The second TPA emission band of
C1 from 500 to 700 nm decreases in TEA. The addition of aqueous
potassium carbonate into DMSO or DMF solution also causes the
diminishment of the second TPA emission band, which is recovered
500
300
C1
C4
C7
(a)
(b)
C1
C4
C7
400
200
100
0
300
200
100
0
400
500
600
700
400
500
600
700
Wavelength(nm)
Wavelength
Figure 4. TPA fluorescence emission spectra (a) and (b) of the partners C1, C4, and C7 in benzene and DMF respectively under 730 nm femtosecond laser irradiation, the
concentration is 5 ꢁ 10ꢀ4 mol/L.

Y. Gong et al. / Tetrahedron Letters 56 (2015) 236–242
241
(a)
(b)
(c)
Figure 5. Optimized geometries of enol (a), TS (b), and keto (c) forms of C1 by Gaussian 09 program package.
as aqueous HCl solution is further titrated (Fig. S13, Supplementary
materials).
dual emission spectra under one-photon and near-infrared
two-photon irradiation. Furthermore, ESIPT becomes predominant
decay pathway in the excited states of the target dyads as
irradiated by near-IR femtosecond laser. The results presented in
the work would be much beneficial for the development and
application of new near-infrared two-photon benzotriazole-based
organic chromophores with ESIPT.
The results suggest that the first emission band of C1 is TPA nor-
mal decay process, and the second emission band is TPA ESIPT pro-
cess. The same phenomena are also shown by C2 and C3. The
results suggest that C1, C2 and C3 are able to undergo ESIPT under
femtosecond near-IR laser irradiation, while C4–C9 can not show
these properties.
It is interesting that IE*/IN* ratio in TPA emission spectra of C1 is
much higher than that in one-photon emission in DMSO and DMF
(such as in DMF, for C1, 2.92, C2, 1.91, and C3, 2.42). Even in
methanol, C1 exhibits competitive ESIPT emission excited by
730 nm femtosecond laser as well (IE*/IN*, 1.18 in methanol,
Fig. S14 Supplementary materials). This shows that ESIPT emission
becomes the primary deactivation route of the excited states of C1
under near-IR femtosecond laser excitation.17 Compounds C2 and
C3 show similar experimental phenomena under near-IR laser
irradiation.
Acknowledgements
This work was financially supported by CSTC2012jjB50007 and
CSTC2010BB0216. H. Li is grateful to the Postdoctoral Science
Foundation of China for their encouraging supporting (Grants
22012T50762 & 2011M501388). L.Y. also thanks Graduate Student
Innovation Foundation of Chongqing University (CDJX11131146).
We greatly appreciate Key Laboratory of Photochemical
Conversion and Optoelectronic Materials, CAS for the help in the
laser detection.
It is accepted that intramolecular phototautomerization process
between enol and keto tautomers in the excited state of an organic
molecule may be finished in femtosecond time scale,20 which
matches well with near-IR femtosecond pulsed laser. Hence, ESIPT
becomes predominant pathway to deactivate the excited states of
C1 during femtosecond near-IR induced two-photon process.
It is further noticed that benzotriazoles derivatives S1 and S4 do
not emit TPA fluorescence emission under near-IR femtosecond
laser irradiation. This shows that S1 and S4 possess small TPA
ability, and coumarin part makes a major contribution to TPA of
the target dyads.
The transition state (TS) from enol to keto in the excited state of
the target molecules is further revealed by molecular geometry
optimization. The representative optimized geometries of enol,
TS, and keto forms of C1 in the excited state are shown in Figure 5,
which clearly suggests the C1 undergoes active hydrogen atom
transfer process from enol to keto in the excited state.
To be summarized, this Letter firstly reports some new
benzotriazole–coumarin dyads. It is demonstrated that internal
Supplementary data
Supplementary data (the synthesis of the molecules, NMR
spectra as well as optical spectra) associated with this article can
be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2014.11.074.
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