Synthesis, experimental and theoretical photophysical study of proton transfer based oxazoline fluorophores. Potential tailor made optical sensors for enantiomeric detection in solution

Article abstract of DOI:10.1016/j.dyepig.2019.02.047

A series of oxazoline based compounds was obtained via amidation formation from salicylic acid derivatives and benzyl-protected L-threonine with good yields (50–90%). These compounds present absorption in the UV region (～300 nm) and fluorescence emission

Full text of DOI:10.1016/j.dyepig.2019.02.047

Accepted Manuscript
Synthesis, experimental and theoretical photophysical study of proton transfer
based oxazoline fluorophores. Potential tailor made optical sensors for enantiomeric
detection in solution
Fabiano S. Santos, Gabriel M. Zanotto, Luis M.Z. Argomedo, Mariana P. Darbem,
Paulo F.B. Gonçalves, Helio A. Stefani, Fabiano S. Rodembusch
PII:
S0143-7208(19)30019-1
https://doi.org/10.1016/j.dyepig.2019.02.047
DYPI 7381
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 3 January 2019
Revised Date: 11 February 2019
Accepted Date: 25 February 2019
Please cite this article as: Santos FS, Zanotto GM, Argomedo LMZ, Darbem MP, Gonçalves PFB,
Stefani HA, Rodembusch FS, Synthesis, experimental and theoretical photophysical study of proton
transfer based oxazoline fluorophores. Potential tailor made optical sensors for enantiomeric detection in
solution, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.02.047.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis, experimental and theoretical photophysical study of
proton transfer based oxazoline fluorophores. Potential tailor
made optical sensors for enantiomeric detection in solution
a
b
c
Fabiano S. Santos, Gabriel M. Zanotto, Luis M. Z. Argomedo, Mariana P.
c
b
c,
Darbem, Paulo F. B. Gonçalves, Helio A. Stefani, * Fabiano S.
a,
Rodembusch *
a
Grupo de Pesquisa em Fotoquímica Orgânica Aplicada, Universidade Federal
do Rio Grande do Sul – Instituto de Química, Av. Bento Gonçalves 9500, CEP
9
1501-970 Porto Alegre, RS, Brazil. Fax: +55 51 3308 7304; Tel: +55 51 3308
204. E-mail: rodembusch@iq.ufrgs.br
7
b
Grupo de Química Teórica, Universidade Federal do Rio Grande do Sul –
Instituto de Química, Av. Bento Gonçalves 9500, CEP 91501-970, Porto Alegre,
RS, Brazil. E-mail: paulo@iq.ufrgs.br
c
Universidade de São Paulo, Faculdade de Ciências Farmacêuticas, Av. Prof.
Lineu Prestes 580 - Bloco 13 Superior, Cidade Universitária, CEP 05508-900,
São Paulo, SP, Brazil. E-mail: hstefani@usp.br
Keywords: oxazoline, proton transfer, optical sensor, enantiomeric sensing,
ESIPT
Abstract
A series of oxazoline based compounds was obtained via amidation formation
from salicylic acid derivatives and benzyl-protected L-threonine with good yields
(50-90%). These compounds present absorption in the UV region (~300 nm)
and fluorescence emission spectra show large Stokes shift emission (~10000
-1
cm ) in the blue-green region attributed to the ESIPT process. TD-DFT
calculations were carried out to investigate the process of proton transfer of the
structure of both tautomers (keto and enol) and corroborating the UV region
absorption and large Stokes shift. The oxazolines derivatives were investigated
as optical sensor for enantiomeric identification in solution. Preliminary results
indicate sensing for arabinose and tartaric acid, but not specificity. However, to
1

ACCEPTED MANUSCRIPT
the pair R-(-)-Mandelic and S-(+)-Mandelic acid, the oxazoline 4c showed a
enantiomeric seletivity identification, allowing to envisage these compounds as
potential photoactive building blocks to propose innovative optical sensors in
solution.
1
. Introduction
Oxazoline is a five-membered heterocyclic compound containing one oxygen
atom and one nitrogen atom. It was first characterized in 1889 and was named
according to the nomenclature of Hantzsch-Widman [1]. The oxazoline ring
itself has no applications, however, ring-containing compounds, present a wide
variety of uses, sucha as synthetic intermediates [2-4], protector groups [5-7],
binders [8], antioxidants [9-11], as well as chiral auxiliaries [12-14] in many
organic transformations. Moreover, the oxazoline moiety can also be found in a
number of compounds with applications in gasoline additives, protective films,
photographic materials, fluorescent molecular probes and agrochemicals [2-
4
,15-24]. This broad use can be justified by resistance to several nucleophiles,
free radicals, oxidizing agents and various acids, which facilitates their
production through different synthetic routes, which can lead to highly complex
compounds [15-21,25]. Oxazolines are central structures of many biologically
active molecules, which exhibit cytotoxic [15], anti-bacterial [16] anti-tumor [17],
antidepressant [19] and anti-Alzheimer activities [21]. In view of the structural
viability offered by the nucleus and the possibility of the extension of π-
conjugated systems, oxazolines are very promising building blocks due to their
coordination properties, in the preparation of fluorescent markers in the
detection of metallic elements, aiming at the identification of analytes of
biological, technological or environmental interest [2-4,15-21,26-28].
Thus, the synthesis of molecules which exhibit remarkable photophysical
properties, such as high photostability, solid state emission, almost absent inner
filter effect due to large Stokes' shift are of great interest [29-36].
In this context, the literature has presented several structures based on
flavonoid and benzazolic cores that present these properties due to a
phototautomerism in the excited state [37]. However, compounds based on
oxazolinic and chiral nuclei that exhibit these properties are poorly reported [38].
2

ACCEPTED MANUSCRIPT
The phototautomerism in the excited state, so-called excited state
intramolecular proton transfer (ESIPT) is a reversible photochemical process
that produces a tautomeric specie from conformer present in the ground state
(Figure 1) [39]. Generally, chemical structures which present intramolecular
hydrogen bond (IHB) between the H atom of the donor group and the acceptor
within a distance less than 2Å are able to proton transfer in the excited state
[
40]. The observed electronic redistribution in the excited state takes place
between a donor moiety, usually a phenol or aniline groups and an acceptor
group (imino/azonitrogen- or carbonyl-oxygen-containing ring skeleton), which
are more acidic and basic after radiation absorption, respectively [41].
Figure 1. ESIPT process for a generic oxazolinic core, where N and T are the
normal and tautomeric species, respectively and GSRPT is the ground state
retro proton transfer. The asterisk indicates the excited state.
2
2
. Experimental
.1. Materials and methods
UV-Vis absorption spectra in solution were performedon a Shimadzu UV-2450
-5
spectrophotometer at a concentration range of 10 M. Steady state fluorescence
spectra were taken using a Shimadzu spectrofluorometer RF-5301PC. The
maxima absorption wavelengths were used as the excitation wavelengths for
fluorescence measurements. Spectrum correction was performed to enable a
3

ACCEPTED MANUSCRIPT
true spectrum to be recorded by eliminating instrumental responses such as
wavelength characteristics of the monochromator or the detector using
Rhodamine B as a standard. The quantum yield of fluorescence (Φ ) was
FL
o
measured at 25 C using spectroscopic grade solvents within solutions applying
the optical dilute methodology and Quinine sulfate (Riedel) in H SO (1 N) as
2
4
the quantum yield standard (Φ =0.55) [42]. Time resolved fluorescence curves
FL
were performed in 1,4-dioxane and acetonitrile with an EasyLife V
spectrophotometer (Optical Building Blocks). All measurements were performed
at room temperature. The decay curves were analysed using EasyLife V
software (OBB). A nonlinear least square method was employed for the fit of the
2
decay to a sum of exponentials. The value of χ , respective residuals and the
autocorrelation function were used to determine the quality of the proposed fit.
-5
To the enantiomer interaction study, in a solution of 4a-d 10 M in acetonitrile
-3
different amounts of DMSO stock solutions 10 M of D-(-)-arabinose, L-(+)-
arabinose, D-Tartaric acid, L-Tartaric acid, R-Mandelic acid or S-Mandelic acid
were added. To this study a control experiment was performed using the achiral
carboxilic benzoic acid (BA), where the oxazolines were measured in absence
and in presence of BA at a molar ratio 1:1 sensor:analyte. The enantiomer and
dye work concentrations are in ESI. All photophysical characterization was
o
performed at 25 C.
2
.2. Synthesis
General procedure for the synthesis of (2S,3R)-benzyl 3-hydroxy-2-(2-
hydroxybenzamido)butanoate derivates (3a-d).
2
-hydroxybenzoic acid derivatives (1a-d) (1 mmol, 1 equiv) and benzyl
threonine (1 mmol, equiv), EDC·HCl (1-ethyl-3-(3-
1
dimethylaminopropyl)carbodiimide (1.3 mmol, 1.3 equiv), 1-HOBt (1-
hydroxybenzotriazole) (1.2 mmol, 1.2 equiv), N-methylmorpholine (1.5 mmol,
1
.5 equiv) were dissolved in 5 mL of CH Cl . The reaction mixture was stirred
2 2
for the required time at room temperature in ultrasonic bath. The completion of
the reaction was monitored by thin layer chromatography (TLC). Then the
reaction mixture was diluted with ethyl acetate and washed with saturated
solution of NH Cl (3 × 20 mL), the organic phase was collected, dried over
4
4

ACCEPTED MANUSCRIPT
MgSO , filtered and the solvent was removed under vacuum. The crude residue
4
was purified by flash chromatography on silica gel (eluting with ethyl
acetate/hexane 3:7).
(2S,3R)-benzyl 3-hydroxy-2-(2-hydroxybenzamido)butanoate (3a)
1
H NMR (300 MHz, CDCl ) δ 11.99 (s, 1H), 7.55 (d, J = 7.9 Hz, 1H), 7.39 (d, J =
3
8
2
.0 Hz, 6H), 7.02 (d, J = 8.3 Hz, 1H), 6.89 (t, J = 7.6 Hz, 1H), 5.35 – 5.16 (m,
13
H), 4.85 (d, J = 10.7 Hz, 1H), 4.50 (s, 1H), 1.30 (d, J = 6.5 Hz, 3H). C NMR
(
75 MHz, CDCl ) δ 170.65, 170.39, 161.58, 135.09, 134.65, 128.70, 128.23,
3
1
25.95, 118.85, 118.56, 113.84, 68.11, 67.62, 57.16, 20.17. HRMS-ESI: calcd.
+
for C H NO [M+H ]: 329,1263, found: 328,0800
18
19
5
(2S,3R)-Benzil 2-(4-azido-2-hidroxibenzamido)-3-hidroxibutanoato (3b)
1
RMN H (300 MHz, CDCl ) δ 11,69 (s, 1H); 7,36 (s, 5H); 7,26 (dd, J = 8,0, 3,5
3
Hz, 1H); 7,17 – 7,12 (m, 1H); 6,94 (dd, J = 9,1, 4,7 Hz, 1H); 5,24 (d, J = 5,4 Hz,
2
H); 4,81 (d, J = 8,7 Hz, 1H); 4,49 (d, J = 5,1 Hz, 1H); 1,27 (d, J = 6,4 Hz, 3H).
13
RMN C (75 MHz, CDCl -d) δ 170,62; 169,28; 160,56; 137,35; 128,74; 128,69;
3
1
28,58; 128,29; 120,46; 115,3; 110,44; 68,06; 67,81; 57,27; 20,23. HRMS-ESI:
+
calcd. for C H N O [M+H ]: 370,11277, found: 371,1760.
18
18
4
5
(2S,3R)-benzyl 3-hydroxy-2-(2-hydroxy-5-methoxybenzamido)butanoate (3c)
1
H NMR (300 MHz, CDCl ) δ 7.38 (s, 4H), 7.15 (d, J = 7.1 Hz, 1H), 7.06 (d, J =
3
8
1
.4 Hz, 2H), 6.95 (d, J = 8.6 Hz, 1H), 5.45 – 5.14 (m, 2H), 4.85 (d, J = 8.6 Hz,
13
H), 4.64 – 4.37 (m, 1H), 3.78 (s, 3H), 1.29 (d, J = 6.4 Hz, 3H). C NMR (75
MHz, CDCl ) δ 170.68, 170.09, 155.60, 151.93, 135.09, 128.69, 128.59,
3
1
28.23, 121.53, 119.26, 113.74, 110.30, 68.12, 67.64, 57.26, 56.11, 20.18.
+
HRMS-ESI: calcd. for C H NO [M+H ]: 359,1369, found: 360,1760.
19
21
6
(2S,3R)-benzyl 3-hydroxy-2-(2-hydroxy-6-methoxybenzamido)butanoate (3d)
1
H NMR (300 MHz, CDCl ) δ 9.15 (d, J = 6.9 Hz, 1H), 7.65 – 6.99 (m, 6H), 6.63
3
(d, J = 8.4 Hz, 1H), 6.41 (d, J = 8.3 Hz, 1H), 5.24 (s, 2H), 4.82 (d, J = 8.1 Hz,
13
1
H), 4.43 (s, 1H), 3.91 (d, J = 2.0 Hz, 3H), 1.27 (d, J = 6.4 Hz, 3H). C NMR
(
75 MHz, CDCl -d) δ 170.69, 164.41, 159.00, 133.75, 128.67, 128.52, 128.21,
3
5

ACCEPTED MANUSCRIPT
1
11.68, 103.71, 101.18, 68.15, 67.41, 57.50, 56.36, 20.14. HRMS-ESI: calcd.
+
for C H NO [M+H ]: 359,1369, found: 360,1730.
19
21
6
General procedure for the synthesis (4S,5S)-benzyl 2-(2-hydroxyphenyl)-5-
methyl-4,5-dihydrooxazole-4-carboxylate derivates (4a-d).
Thionyl chloride (30 mmol, 30 equiv) was added dropwise to the compound 3 (1
mmol, 1 equiv) dissolved in dichloromethane (10 mL) under nitrogen
atmosphere. The reaction mixture was stirred at room temperature for 24 h.
Then Na CO was added until the solution turned basic. The inorganic solid was
2
3
filtered off, and organic phase was concentrated under vacuum. The crude
product was purified by flash chromatography on silica gel (eluting with ethyl
acetate/hexane 1:9).
(4S,5S)-benzyl 2-(2-hydroxyphenyl)-5-methyl-4,5-dihydrooxazole-4-carboxylate
(4a)
1
H NMR (300 MHz, CDCl ) δ 11.85 (s, 1H), 7.68 (d, J = 6.5 Hz, 1H), 7.41 (s,
3
6
3
1
H), 7.06 (d, J = 7.9 Hz, 1H), 6.90 (s, 1H), 5.26 (s, 2H), 5.07 (s, 2H), 1.38 (s,
13
H). C NMR (75 MHz, CDCl ) δ 169.04, 167.61, 160.20, 135.15, 133.89,
3
28.64, 128.58, 128.32, 118.62, 116.96, 110.24, 70.31, 67.13, 15.99. HRMS-
+
ESI: calcd. for C H NO [M+H ]: 312.1236, found: 312.1018.
18
17
4
(4S,5S)-benzyl-2-(4-azido-2-hydroxyphenyl)-5-methyl-4,5-dihydrooxazole-4-
carboxylate (4b)
1
H NMR (300 MHz, CDCl ) δ 7.40 (d, J = 8.5 Hz, 1H), 7.29 (s, 5H), 6.37 (s, 1H),
3
6
.27 (d, J = 8.0 Hz, 1H), 5.14 (s, 2H), 4.91 (d, J = 2.6 Hz, 2H), 1.25 (d, J = 5.5
13
Hz, 3H). C NMR (75 MHz, CDCl ) δ 168.65, 166.59, 160.99, 159.80, 134.08,
3
1
28.89, 127.59, 106.18, 102.58, 102.04, 69.06, 66.20, 14.95. HRMS-ESI: calcd.
+
for C H N O [M+H ]: 353.1250, found: 353.1003.
18
16
4
4
(4S,5S)-benzyl 2-(2-hydroxy-5-methoxyphenyl)-5-methyl-4,5-dihydrooxazole-4-
carboxylate (4c)
1
H NMR (300 MHz, CDCl ) δ 7.29 (s, 5H), 7.05 (s, 1H), 6.90 (d, J = 11.3 Hz,
3
13
2
H), 5.30 – 5.08 (m, 2H), 4.95 (s, 2H), 3.69 (s, 3H), 1.27 (s, 3H). C NMR (75
MHz, CDCl ) δ 169.03, 167.46, 154.64, 151.83, 135.16, 128.59, 121.79,
3
6

ACCEPTED MANUSCRIPT
1
17.89, 111.12, 109.75, 77.46, 70.43, 67.13, 55.94, 16.02. HRMS-ESI: calcd.
+
for C H NO [M+H ]: 342.1341, found: 342.1099.
19
19
5
(4S,5S)-benzyl 2-(2-hydroxy-6-methoxyphenyl)-5-methyl-4,5-dihydrooxazole-4-
carboxylate (4d)
1
H NMR (300 MHz, MeOD-d ) δ 7.85 (d, J = 7.9 Hz, 1H), 7.42 – 7.27 (m, 6H),
4
6
.67 (d, J = 8.1 Hz, 1H), 5.22 (s, 3H), 3.97 (s, 2H), 3.31 (s, 3H), 1.24 (d, J = 5.8
13
Hz, 3H). C NMR (75 MHz, MeOD-d ) δ 171.83, 168.12, 162.03, 143.33,
4
1
37.16, 129.58, 129.30, 129.14, 126.84, 105.37, 103.57, 101.40, 68.42, 68.21,
+
6
8.03, 59.49, 57.32, 20.78. HRMS-ESI: calcd. for C H NO [M+H] : 342.1341,
19
19
5
found: 342.1182.
2
.4 Theoretical calculations
Theoretical calculations were performed using Gaussian 16 package [43].
Geometry optimizations of both enol and keto ground and excited states were
carried out using ωB97XD, a hybrid functional which provides empirical
dispersion and long-range correction [44]. The cc-pVDZ basis set [45] were
used to all the geometry optimizations, while the electronic transition and
energies calculations were carried out with jun-cc-pVTZ basis set [45]. This
functional and basis functions has been used in previous works and have
demonstrated good outcomes [46]. In order to account for solvent effects the
Polarizable Continuum Model (PCM) [47] was applied to all DFT and TDDFT
calculations for three different solvents: 1,4-dioxane, acetonitrile and
dimethylsulfoxide. All the structures and frontier orbitals images were
renderized using the graphical software CHEMCRAFT [48].
3
3
. Results and Discussion
.1 Synthesis
The synthesis of the oxazoline compounds was initiated via amidation formation
step [49], starting with the derivatives of the salicylic acid (1a-d) and benzyl-
protected L-threonine (2), which was obtained by refluxing L-threonine with
benzyl alcohol [50] as shown in Scheme 1.
7

ACCEPTED MANUSCRIPT
Scheme 1. Synthetic route for the 2-ariloxazolines (4a-d) derived from salicylic
acid derivatives (1a-d).
The amidation formation was achieved using non-protected salicylic acid
derivatives 1a-d, benzyl threonine, 1.3 eq of EDC (1-ethyl-3-(3-
dimethylaminopropyl)carbodiimide), 1.2 eq. of 1-HOBt (1-hydroxybenzotriazole),
and 1.5 eq. of base in dichloromethane as solvent, leading to the coupling
products 3a-d, respectively with good yields (50-90%). This synthetic protocol
was optimized employing an ultrasonic bath [51], which reduced significantly the
reaction time to about 1h, in comparison with 12h for “on bench” synthesis [52].
In the next step to obtain the corresponding oxazolines 4a-d, thionyl chloride
(30 mmol, 30 equiv) was added dropwise to the compounds 3a-d (1 mmol, 1
equiv) dissolved in CH Cl (10 mL) under nitrogen atmosphere. The reaction
2
2
mixture was stirred at room temperature for 24 h. Then Na CO was added until
2
3
the solution turned basic.
3
.2. Photophysical characterization
The absorption spectra of the oxazoline 4b is shown in Figure 2. The relevant
data calculated from UV-Vis absorption spectroscopy for all compounds are
presented in Table 1. It can be observed an absorption maxima located around
3
00-334 nm. It was observed a small solvatochromic effect (∆λabs~2 nm) in
these compound. It could be observed that in all studied solvents, the derivative
4
c present red shifted absorption in despite of the 4d, indicating a better
8

ACCEPTED MANUSCRIPT
electron delocalization in solution (Table 1). The observed blueshift values for
the absorption maxima in the polar protic ethanol are probably due to specific
solvent-fluorophore interaction, such as hydrogen bonding,[53] that was also
observed in methanol and isopropyl alcohol (data not shown, see SI).
-5
Figure 2. UV-Vis absorption spectra in solution (10 M) of oxazoline 4b in
different organic solvents (left) and oxazolines 4a-d in chloroform (right). The
inset presents the substitution pattern of the aromatic ring for comparison.
In order to discuss the photophysics in the ground state regarding the different
contribution and position of the substituents, Figure 2b presents the UV-Vis
spectra from 4a-d in chloroform. It can be observed that changes in the
substituent affect the absorption maxima due to different electronic contributions
of these lateral groups. An absorption maxima between 302 nm and 334 nm
were observed. Despite compound 4b present a chromophoric group, this
compound showed the shortest absorption maximum, located at 302 nm. This
result probably indicates that the azido group does not contributes significantly
to the conjugation of this compound [54]. On the other hand, the presence and
position of the methoxy auxochrome seems to affect the absorption maxima
location of these compounds. A redshift absorption is observed for compounds
4
c and 4d (334 nm and 326 nm, respectively) in despite of 4a and 4b. In
9

ACCEPTED MANUSCRIPT
addition, compound 4c with the methoxy group at the 6-position presents the
longer absorption wavelength probably due to electronic interaction with the
hydroxyl group present in the para-position.
The UV-Vis absorption spectra also allow to calculate, using Eq. 1-3, the
0
e
oscillator strength (f ) and the radiative rate constant for emission (k ), which is
e
the probability of emission of photons per time unit, as well as the pure radiative
0
lifetime (τ ), applying the Strickler-Berg equation [55,56].
ꢀ
ꢁ
= 4.32ꢂ × 10ꢃꢄ ꢅ ꢆ(ꢇ
̅
) ꢈ ꢇꢉ , (1)
ꢋ
ꢃꢄ
ꢍ
ꢋ
ꢊ ≈ 2.88ꢌ10 ꢇ
̅
ꢅ ꢆ(ꢇ)ꢈꢇ,ꢂ(2)
̅
̅
ꢁ
0
0
τ = 1/k , (3)
e
where the integral is the area under the absorption curve, which corresponds to
a single electron oscillator and ꢇꢋ is the wavenumber corresponding to the
maximum wavelength of absorption. The obtained molar absorptivity coefficient
̅
4
-1
-1
7
-1
ε values (10 M ·cm ) and the calculated radiative rate constant (10 s )
1
indicate spin and symmetry allowed π→π* electronic transitions for 4a-d, with
2
6
-1
-1
0
e
5
9
-1
typical range values to ε (10 -10 M ·cm ) and k (10 -10 s ). The values of
the intrinsic property and independent of the electromagnetic field responsible
-3
for the excitation, the oscillator strength, (f ≈10 -1) corroborates with electronic
e
dipole-allowed transitions, as expected [57]. In addition, the calculated radiative
0
lifetime (τ ) values indicates that the oxazolines 4a-d populate the same excited
state.
1
0

ACCEPTED MANUSCRIPT
Table 1. Relevant photophysical data of the UV-Vis absorption spectroscopy,
where λabs is the absorption maxima (nm), ε is the molar absorptivity coefficient
4
-1
-1
0
e
(
10 M ·cm ), ꢀ is the calculated oscillator strength, k is the calculated
ꢁ
7
-1
0
radiative rate constant (10 s ) and τ is the calculated pure radiative lifetime (in
ns).
Dye
Solvent
CHCl3
EtOH
MeCN
,4-Dioxane
CHCl3
EtOH
MeCN
,4-Dioxane
CHCl3
EtOH
MeCN
,4-Dioxane
CHCl3
EtOH
MeCN
,4-Dioxane
λabs
306
304
304
306
302
300
300
301
334
332
332
334
326
324
326
324
ε
ꢀ
k°e
τ°
ꢁ
4.80
4.63
3.81
5.03
11.6
9.11
10.2
12.2
5.76
5.76
4.92
5.00
2.71
2.67
2.30
2.67
0.08
0.08
0.06
0.08
0.14
0.11
0.12
0.15
0.10
0.10
0.09
0.09
0.05
0.05
0.05
0.05
8.19
8.21
6.63
8.45
15.6
11.7
12.9
16.4
8.79
9.48
7.91
8.51
4.8
12.2
12.2
15.1
11.8
6.39
8.52
7.73
6.10
11.4
10.5
12.6
11.8
20.7
21.0
20.5
20.1
4a
4b
4c
4d
1
1
1
1
4.75
4.89
4.98
The normalized fluorescence emission spectra of oxazolines 4a-d are
presented in Figure 3. The emission curves were obtained exciting the
compounds at the absorption maxima wavelength and the relevant data from
fluorescence emission spectroscopy is summarised in Table 2.
1
1

ACCEPTED MANUSCRIPT
Figure 3. Steady-state normalized fluorescence emission spectra in solution
-5
(
10 M) of the oxazolines 4a-d.
It can be observed that these compounds do not present any clear tendency
based on the substituents or the solvent polarity. However, all studied
compounds present at least a longer wavelength emission, ascribed to the
ESIPT process, indicating that the solvents stabilize the enol conformer that
proton transfer in the excited state. In these compounds, after UV absorption, a
locally excited enol-cis conformer (N*) tautomerizes to an excited-state keto (T*)
specie, which decays to the ground state (T) emitting fluorescence at 442-510
-1
nm with a large Stokes shift (~10000 cm ). It can also be observed that
compounds 4a-c present an additional blueshifted emission band, ascribed to a
normal emission. In these compounds, after UV absorption, a non-ESIPT
conformer (N*) decays emitting fluorescence at 369-400 nm. The ESIPT
process requires the existence of an intramolecular hydrogen bond (IHB)
1
2

ACCEPTED MANUSCRIPT
between the H atom of the donor group and the acceptor within a distance less
than 2Å, as well as both photoacid and photobasic character [58,59]. In this
way, the observed normal emission for the oxazolines 4a-c indicates that a
conformational equilibirium is present in the ground-state, as already observed
in ESIPT compounds [60]. This equilibrium takes place between an enol
conformer, which undergoes proton transfer in the excited state, so-called enol-
cis, with additional conformers that do not favor intramolecular hydrogen (Figure
4
). Thus, the dual fluorescence emission for 4a-c can be related to a
conformational equilibrium in solution in the ground state, with an emission at
longer wavelengths ascribed to the excited keto tautomer, and the blue shifted
one due to conformational forms which presents a normal relaxation. These
results are interesting, because the observed dual fluorescence emission [37]
can allow for their potential application as optical sensors for polarity, polar
protic solvents either quiral identification. The total fluorescence quantum yield
are 1.6-13%, as expected for ESIPT compounds [37].
Figure 4. ESIPT (Enol-cis) and non-ESIPT conformers of the oxazoline studied
in this work.
1
3

ACCEPTED MANUSCRIPT
Table 2. Relevant photophysical data of the fluorescence emission
spectroscopy, where λem is the emission maxima (nm) from the normal (N*) and
-1
tutomeric (T*) species, ∆λST is the Stokes' shift (cm ) and Φ is the total
F
fluorescence quantum yield (%).
Normal
∆λST
Tautomeric
*
Dye
Solvent
ΦF
λem
λem
457
-
∆λST
10798
-
CHCl3
EtOH
MeCN
,4-Dioxane
CHCl3
EtOH
MeCN
,4-Dioxane
CHCl3
EtOH
MeCN
,4-Dioxane
CHCl3
EtOH
MeCN
,4-Dioxane
-
-
4.4
6.9
12.9
4.0
4.7
2.0
4.0
1.6
2.3
1.6
2.4
1.2
13.5
6.2
11.2
7.5
400
391
-
7895
7319
-
4a
4b
4c
4d
-
-
1
1
1
1
462
458
442
452
460
507
500
498
510
452
447
452
448
11035
11279
10709
11209
11483
10216
10120
10040
10332
8551
8493
8551
8543
-
-
385
386
391
386
381
369
380
-
7359
7427
7647
4033
3874
5903
6688
-
-
-
-
-
-
-
Time-resolved fluorescence was also employed to better understand the
photophysics of these compounds (Figure 5). The relevant data are
summarised in Table 3. The residuals are presented in the ESI.
Figure 5. Fluorescence decays of oxazolines 4a-d in (a) 1,4-dioxane and (b)
-5
acetonitrile [ca. 10 M]. IRF=instrument response factor.
1
4

ACCEPTED MANUSCRIPT
The fluorescence decay profile of the oxazolines 4a-c could be fitted by two-
2
exponential decays both below 1 ns with good χ values (Table 3). The
individual time decays (τ and τ ), as well as the average lifetimes presented
1
2
similar values (τ < 1 ns) for these dyes. In addition, the solvent seems to
present an almost absent influence on the decay profiles. Compound 4d
presented a quite particular behaviour, where one exponential decay was
observed with a longer fluorescence lifetime (τ >3.4 ns) in both solvents. This
0
latter corroborates with the previously calculated pure radiative lifetime (τ )
presented in Table 1, indicating that somehow the substituent position plays a
fundamental role on the dynamics of these compounds. Since 4d this is the only
derivative with a quite large substituent at the R position (-OCH ), it is believed
3
3
that it is not only an electronic effect that is taking place in the excited state to
allow such steady-state and time resolved photophysical behaviour. Probably
sterically hindrance is also present to favor in this case, the enol-cis conformer,
able to ESIPT in the excited state.
Table 3. Time resolved fluorescence spectroscopy of the oxazolines 4a-c,
where τ is the experimental fluorescence lifetime (in ns), τav is the weighted
2
average fluorescence lifetime (in ns) and χ determines the quality of the
exponential fit. The numbers 1 and 2 are related to individual exponential
decays.
2
Compound
Solvent
MeCN
τ1 (ns)
τ2 (ns)
τav (ns)
χ
0.08 3 (96.59)
3.534 (3.41)
0.201 0.978
0.961 0.974
0.445 0.967
0.204 1.052
0.304 1.195
0.528 1.139
3.476 1.151
3.892 1.145
4a
4b
4c
4d
1
1
1
1
,4-Dioxane
MeCN
0.414 (83.05) 3.640 (16.95)
0.192 (87.66) 2.239 (12.34)
,4-Dioxane
MeCN
0.088 (95.72)
0.260 (96.91)
0.459 (98.48)
3.476 (100)
3.892 (100)
2.805 (4.28)
1.673 (3.09)
4.958 (1.52)
,4-Dioxane
MeCN
,4-Dioxane
3
.3. Enantiomers identification in solution
Based on the chiral structure, as well as their photophysical characteristics, the
oxazolines 4a-d were evaluated as optical sensors for enantioselective
1
5

ACCEPTED MANUSCRIPT
detection of the follow optically active compounds: D-(-)-arabinose/L-(+)-
arabinose, D-(-)-Tartaric acid/L-(+)-Tartaric acid and R-(-)-Mandelic acid/S-(+)-
Mandelic acid. These compounds were chosen due to their different
substituents, geometries and chemical groups. In this investigation, for all
studied oxazoline it could be observed that their fundamental state was not
differently affectec by one of the enantiomers of the pair, that is, quite similar
shape and position were observed for the UV-Vis spectra of the oxazolines in
presence of the enantiomers (data not shown, see SI). From the steady-state
fluorescence spectra (Figure 6), it can be observed that the excited state of the
oxazolines were notably affected by the presence of the enantiomers D-(-)-
arabinose/L-(+)-arabinose and D-(-)-Tartaric acid/L-(+)-Tartaric acid. The
oxazoline 4a presents a main emission band locatd around 400 nm, which
decreases its intensity and split into two emission bands in presence of the
enantiomers, located at ~460 nm and ~375 nm. The emission at longer
wavelentghs seems to keep constant during the enantiomer additon and the
blue shifted emission bands presents a decrease on its intensity. The oxazoline
4
b in absence of the enantiomers already presents a dual fluorescence
emission, which decreases upon addition of the enantiomers, indicating a
significative interaction in the excited state between enantiomer and
fluorophore, which can probably be related to the increase of the rigidity of the
fluorophore and thus increasing its fluorescence quantum yield. Similar results
were observed for the other oxazolines in presence of these enantiomers (see
SI). The observed similar response in presence of both enantiomers (D or L)
indicated that these enantiomers present significant interaction with the
respective oxazolines, but not selectively. However, in presence of the R-(-)-
Mandelic acid/S-(+)-Mandelic acid the oxazoline 4c presents interesting
changes on the emission spectra, indicating selective interaction between the
fluorophore and the respective enantiomer (Figure 7). The derivatives 4a-b and
4
d do not presented such photophysical behaviour (see SI).
1
6

ACCEPTED MANUSCRIPT
-5
Figure 6. Fluorescence emission titration of oxazoline 4a (10 M in DMSO) in
diferents amounts of (a) D-(-)-Tartaric acid and (b) L-(+)-Tartaric acid and
-5
oxazoline 4b (10 M in DMSO) in diferents amounts of (c) L-(+)-Arabinose and
d) D-(-)-Arabinose.
(
Despite the quite similar emission profile for 4c in presence of R-(-)-Mandelic
acid and S-(+)-Mandelic (λ and λ regions), it can be observed that at shorter
2
3
wavelenghts, defined as λ , interesting changes were observed. In presence of
1
the R-enantiomer, the fluorescence intensity below 400 nm changes slighlty if
compared to the same region in presence of the S-enantiomer. It can be
observed an increase of the fluorescence intensity upon addition of the S-
enantiomer. Since this emission region can be related to non-ESIPT
conformers, the observed increase on the emission instensity indicates that the
S-enantiomer interacts with these species allowing their deactivation in despite
of the enol-cis conformer, which would proton transfer in the excited state.
1
7

ACCEPTED MANUSCRIPT
-5
Figure 7. Fluorescence emission titration of oxazoline 4c (10 M in DMSO) in
diferents Molar Ratios [Mandelic Acid]/[Oxazoline] of (left) R-(-)-Mandelic acid
and (right) S-(+)-Mandelic, where λ1-3 are the respective emission bands. The
inset presents the magnification of the λ emission band.
3
These changes in intensity can be envisaged and clearly compared from
fluorescence emission titration plots (Figure 8), where it can be observed that
R-(-)-Mandelic and S-(+)-Mandelic acids present quite different behaviour. At
the λ , the S-enantiomer presents a linear increase of the fluorescence intensity
1
in relation to the enantiomer/oxazoline molar ratio if compared to the R-
enantiomer. At the λ , both compounds present decrease on the intensity upon
2
addition of the respective enantiomer. In addition, at λ both compounds
3
present a constant emission intensity. The observed photophysical features,
although are not outstanding on-off or off-on fluorescence responses, which
significantly restricts the application of fluorophores for enantiomers
identification in solution are promising results. In addition, taking the control
experiment into account, where benzoic acid (BA) was used as analyte, the
almost absent changes on the UV-vis and fluorescence emission spectra of the
oxazolines 4a-d (see SI) even at a 1:1 (dye:BA) molar ratio corroborates with
the potential application of these dyes for enantiomeric identification in solution.
1
8

ACCEPTED MANUSCRIPT
It indicates that asymetrical fluorescent compounds even do not presenting
specific chemical groups with their analytes can be applied as optical sensors,
as recently reported in the literature [61].
-5
Figure 8. Ratio of the fluorescence emission intensities of oxazoline 4c (10 M
in DMSO) at diferents amounts of the enantiomers at (a) λ and (b) λ .
1
2
3
.4 Theoretical calculations
The 4a and 4d molecules were chosen for the TD-DFT study, since they are
simple and representative structures of the studied compounds requiring minor
computational effort. The computational protocol used indicates that for both
molecules, the enol tautomer is the most stable in the ground state in all studied
solvents as the keto tautomer showed to be less stable with respect to the enol
conformer for both 4a and 4d molecules. Thus, the energy is absorbed by the
enol conformer, as experimentally observed. Based on the enol and keto
energies, it was decided to evaluate the absorption wavelengths only for the
enol conformer, since it is the specie present in the ground state. It is worth
mentioning that a similar analysis was carried out for the enol conformer and
keto tautomer in the first excited state, resulting in the evaluation of the
emission wavelengths only for the keto conformer. As expected, the geometry
optimization for the enol tautomer in the first excited state led directly to the keto
tautomer, indicating no energy barrier separating both tautomers. Figure 9
1
9

ACCEPTED MANUSCRIPT
qualitatively illustrates the potential energy curves for the 4a molecule.
Compound 4d presents the same energy curve profile.
Figure 9. Potential energy curves of the ESIPT process for molecules 4a,
where ∆e is the difference in energy from the conformers in the ground state.
The ESIPT is a well-known process where two species are involved: the enol
conformer and the respective keto tautomer. In this sense, Figure 10 presents
the optimized geometries of the predominant conformer to oxazolines 4a and
4
d in dimethyl-sulfoxide in the ground and excited states. A similar behaviour
was observed in acetonitrile and 1,4-dioxane (see SI).
2
0

ACCEPTED MANUSCRIPT
Figure 10. Enol (S ) and keto (S ) optimized geometries for oxazolines 4a and
0
1
4
d, computed at the CAM-B3LYP/cc-pVDZ level in dimethylsulfoxide.
In Figure 11 it can be observed that the orbitals involved in the first electronic
transition of compounds 4a and 4d in dimethylsulfoxide, indicate π-type orbitals
for both oxazolines. Thus, it was assumed that π→π* transitions were occurring
for both structures. In the ground state, the molecular ortbitals involved in the
transition are almost exclusively localized at the phenolic ring. However, in the
excited state, the orbitals have displaced towards the oxazolin ring, which is in
agreement with the proton transfer process in these compounds. Once again, a
similar behaviour was observed in acetonitrile and 1,4-dioxane (see SI).
2
1

ACCEPTED MANUSCRIPT
Figure 11. HOMO and LUMO for the first electronic transition of compounds 4a
and 4d, computed at the CAM-B3LYP/jun-cc-pVTZ level in dimethylsulfoxide
(DMSO).
The calculated photophysical data are shown in Table 4. The CAM-B3LYP/jun-
cc-pVTZ absorption and emission wavelengths are in good agreement with the
experimental data. Following the Förster cycle presented for the ESIPT
process, the absorption wavelength were analyzed for the enol conformer and
the emission wavelength for the keto tautomer. It could be observed that the
overall deviation of the absorption and emission wavelengths, from the
theoretical to the experimental data, is acceptable for the used protocol.
Benchmark calculations shows that the chosen functional appropriate for
electronic transitions analysis [62,63]. In addition, regarding the solvent effect,
the calculations showed a small influence of solvent polarity on absorption and
emision energies, in agreement with the experimental results.
It can also be observed that the theoretical Stokes shifts (∆λ) present values
around 140 nm. Usually, large Stokes shifts are related to charge transfer (CT)
processes, which in turn, are originated by large electronic displacement from
the electron donor groups to acceptor groups. Charge transfer mechanism is
also characterized by the great difference of the dipole moment between the
ground and excited states, (∆µ). However, based on the data presented in
Table 4, the almost absent changes on the ∆µ indicates that the ESIPT process
2
2

ACCEPTED MANUSCRIPT
can be ascribed as the main responsible for the large Stokes shifts, as expected
[
64].
Table 4. Theoretical photophysical data for molecules 4a and 4d, where λabs is
the absorption maxima (nm) for the enol conformer, λem is the emission maxima
(nm) for the keto tautomer, f is the oscillator strength and µ is the dipole
moment in Debye. The subscript 0 and 1 indicates the ground and excited
states, respectively.
enol
keto
Dye
Solvent
∆λ
∆µ
λabs
f
abs
µ0
λem
f
abs
µ1
1
1
,4-Dioxane 276.83 0.19 4.69
417.44 0.19 3.87 140.61 0.82
413.43 0.27 6.20 139.88 -0.52
413.42 0.27 6.25 139.58 -0.54
419.41 0.12 3.52 138.29 1.53
413.46 0.19 5.44 133.51 0.68
413.42 0.19 5.48 133.18 0.66
4
a
d
MeCN
DMSO
273.55 0.18 5.68
273.84 0.18 5.71
,4-Dioxane 281.12 0.11 5.05
4
MeCN
DMSO
279.95 0.11 6.12
280.24 0.11 6.14
4
. Conclusions
In summary, here we report the synthesis of new fluorescent proton-transfer
dyes from non-trivial aromatic moieties. The oxazoline dyes were synthesized
via amidation formation step with salicylic acid derivatives and benzyl-protected
L-threonine with good yields. In a second step, thionyl chloride was used under
nitrogen atmosphere to afford the desired products. The dyes presented
absorption in the UV region and dual fluorescence emission located in the cyan-
green with very large Stokes shift. Despite the absence of basic or acidic
moieties in their chemical structure, the photophysical properties, as well as
their chirality were used to explore these compounds for optical sensing in
solution for enantioselective detection. For all studied oxazoline it could be
observed that the enantiomers did not affectec the ground state of the
oxazolines. On the other hand, the excited state of the oxazolines were notably
affected by the presence of the enantiomers D-(-)-arabinose/L-(+)-arabinose
and D-(-)-Tartaric acid/L-(+)-Tartaric acid. The observed similar response in
presence of both enantiomers indicated that these enantiomers present
2
3

ACCEPTED MANUSCRIPT
significant interaction with the respective oxazolines, but not selectively.
However, in presence of the R-(-)-Mandelic acid/S-(+)-Mandelic acid the
oxazoline 4c presents interesting changes on the emission spectra, indicating
selective interaction between the fluorophore and the respective enantiomer,
which can be usefull to keep studying such structures for such application. The
computational protocol applied to this study presented good agreement with the
experimental findings in absorption and emission wavelengths
Acknowledgements
This work was supported by CNPq (fellowship 306119/2014-5 to HAS),
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil
(
CAPES) - Finance Code 001, FAPERGS (17/2551-0000968-1) and FAPESP
grant # 2017/24821-4 and fellowship to MPD 2016/24396-9
(
5
. References
1] Gabriel S. Zur kenntniss des bromäthylamins. Ber Dtsch Chem Ges
889;22:1139-1154.
2] Gant TG, Meyers AI. The chemistry of 2-oxazolines (1985–present).
Tetrahedron 1994;50:2297-2360.
3] Reuman M, Meyers AI. The synthetic utility of oxazolines in aromatic
substitution. Tetrahedron 1985;41:837-860.
4] Frump JA. Oxazolines. Their preparation, reactions, and applications. Chem
Rev 1971;71:483-505.
5] Lee YJ, Lee J, Kim MJ, Jeong BS, Lee JH, Kim TS, Lee J, Ku JM, Jew SS,
[
1
[
[
[
[
Park HG. Highly enantioselective synthesis of (2S)-alpha-(hydroxymethyl)-
glutamic acid by the catalytic Michael addition of 2-naphthalen-1-yl-2-oxazoline-
4
-carboxylic acid tert-butyl ester. Org Lett 2005;7:3207-3209.
6] Saravanan P, Corey EJ. A short, stereocontrolled, and practical synthesis of
alpha-methylomuralide, a potent inhibitor of proteasome function. J Org Chem
003;68:2760-2764.
7] Greene TW, Wutz PGM. Protective groups in organic synthesis, 2nd ed.
New York: John Wiley & Sons; 1991.
8] Hargaden GC, Guiry PJ. Recent applications of oxazoline-containing ligands
in asymmetric catalysis. Chem Rev 2009;109:2505-2550.
[
2
[
[
2
4

ACCEPTED MANUSCRIPT
[
9] Padmaja A, Rajasekhar C, Durgamma S, Venkatesh BC, Padmavathi V.
Synthesis and antioxidant activity of pyrazolyl-oxazolines/thiazolines and
isoxazolyl-oxazolines/thiazolines. Med Chem Res 2013;23:1084-1098.
[
10] Djurendić E, Vujašković SD, Sakač M, Ajduković J, Gaković A, Kojić V,
Bogdanović G, Klisurić O, Gaši KP. Synthesis and biological evaluation of some
new 2-oxazoline and salicylic acid derivatives. Arkivoc 2011;2:83-102.
[
11] Padmavathi V, Mahesh K, Dinneswara Reddy G, Padmaja A. Synthesis
and bioassay of pyrrolyl oxazolines and thiazolines. Eur J Med Chem
010;45:3178-3183.
12] Khumsubdee S, Fan Y, Burgess K. A comparison between oxazoline-
imidazolinylidene, -imidazolylidine, -benzimidazolylidene hydrogenation
catalysts. J Org Chem 2013;78:9969-9974.
13] McKeon SC, Müller-Bunz H, Guiry PJ. Synthesis of thiazoline-oxazoline
ligands and their application in asymmetric catalysis. Eur J Org Chem
011;35:7107-115.
14] Sawada T, Nakada M. Asymmetric catalysis of intramolecular
2
[
[
2
[
cyclopropanation of 5-aryl-1-diazo-1-mesitylsulfonyl-5-hexen-2-ones. Adv Synth
Catal 2005;347:1527-532.
[
15] Tsuda M, Yamakawa M, Oka S, Tanaka Y, Hoshino Y, Mikami Y, Sato A,
Fujiwara H, Ohizumi Y, Kobayashi J. Brasilibactin A, a cytotoxic compound from
actinomycete Nocardia brasiliensis. J Nat Prod 2005;68:462-464.
[
16] Onishi HR, Pelak BA, Gerckens LS, Silver LL, Kahan FM, Chen M-H, et al.
Antibacterial agents that inhibit Lipid A biosynthesis. Science 1996;274:980-
82.
17] Li Q, Woods KW, Claiborne A, Gwaltney II SL, Barr KJ, Liu G, Gehrke L,
9
[
Credo RB, Hui YH, Lee J, Warner RB, Kovar P, Nukkala MA, Zielinski NA, Tahir
SK, Fitzgerald M, Kim KH, Marsh K, Frost D, Ng SC, Rosenberg S, Sham HL.
Synthesis and biological evaluation of 2-indolyloxazolines as a new class of
Tubulin polymerization inhibitors. Discovery of A-289099 as an orally active
antitumor agent. Bioorg Med Chem Lett 2002;12:465-469.
[
18] Tsukamoto M, Murooka K, Nakajima S, Abe S, Suzuki H, Hirano K, Kondo
H, Kojiri K, Suda H. BE-32030 A, B, C, D and E, New antitumor substances
produced by Nocardia sp. A32030. J Antibiot 1997;50:815-21.
2
5

ACCEPTED MANUSCRIPT
[
19] Vizi ES. Compounds acting on alpha1- and alpha2- adrenoceptors:
agonists and antagonists. Med Res Rev 1986;6:431-449.
20] Celanire S, Talaga P, Leurs R, Denonne F, Timmerman H, Lebon F.
[
Compounds comprising an oxazoline or thiazoline moiety, processes for making
them, and their uses. US patent WO 2006103057 A1, 2006 Oct 05.
[
21] Gross JL, Robichaud AJ, Mazzacani A, Williams MJ. Aminoalkylazole
derivatives as histamine-3 antagonists. US patent US 0023707, 2009 Jan 22.
22] Hoogenboom R. Poly(2-oxazoline)s: a polymer class with numerous
potential applications. Angew Chem Int Ed 2009;48:7978-7994.
23] Thijis L, Hanneke ML, Kuringen HPC, Put JPW, Schubert US,
[
[
Hoogenboom R. Temperature induced solubility transitions of various poly(2-
oxazoline)s in ethanol-water solvent mixtures. Polymers 2010;2:188-199.
[
24] Li Y, Li C, Zheng Y, Wei X, Ma Q, Wei P, Liu Y, Qin Y, Yang N, Sun Y, Ling
Y, Yang X, Wang Q. Design, synthesis, acaricidal activity, and mechanism of
oxazoline derivatives containing an oxime ether moiety. J Agric Food Chem
2
014;62:3064-3072.
25] Meyers AI, Mihelich ED. The synthetic utility of 2-oxazolines. Angew Chem
Int Ed 1976;15:270-281.
26] Wu WH, Dong JJ, Wang X, Li J, Sui SH, Chen GY, Liu JW, Zhang M.
[
[
Fluorogenic and chromogenic probe for rapid detection of a nerve agent
simulant DCP. Analyst 2012;137:3224-3226.
[
27] Dong XY, Si CD, Fan Y, Hu DC, Yao XQ, Yang YX, Liu JC. Effect of N-
donor ligands and metal ions on the coordination polymers based on a semirigid
carboxylic acid ligand: structures analysis, magnetic properties, and
photoluminescence. Cryst Growth Des 2016;16:2062-2073.
[
28] Chatterjee A, Santra M, Won N, Kim S, Kim JK, Kim SB, Ahn KH. Selective
fluorogenic and chromogenic probe for detection of silver ions and silver
nanoparticles in aqueous media. J Am Chem Soc 2009;131:2040-2041.
[
29] Kwon JE, Park SY. Advanced organic optoelectronic materials: harnessing
excited-state intramolecular proton transfer (ESIPT) process. Adv Mater
011;23:3615-3642.
30] Zhao JZ, Ji SM, Chen YH, Guo HM, Yang P. Excited state intramolecular
proton transfer (ESIPT): from principal photophysics to the development of new
2
[
2
6

ACCEPTED MANUSCRIPT
chromophores and applications in fluorescent molecular probes and
luminescent materials. Phys Chem Chem Phys 2012;14:8803-8817.
[
31] Lochbrunner S, Wurzer AJ, Riedle E. Microscopic mechanism of ultrafast
excited-state intramolecular proton transfer: A 30-fs study of 2-(2'-
hydroxyphenyl)benzothiazole. J Phys Chem A 2003;107:10580-10590.
[
32] Abou-Zied OK. Revealing the ionization ability of binding site I of human
serum albumin using 2-(2'-hydroxyphenyl)benzoxazole as a pH sensitive probe.
Phys Chem Chem Phys 2012;14:2832-2839.
[
33] Formosinho SJ, Arnaut LG. Excited-state proton transfer reactions II.
Intramolecular reactions. J Photochem Photobiol A Chem 1993;75:21-48.
34] Paul BK, Guchhait N. Photophysics of a proton transfer phototautomer
[
within biological confinement of a protein: Spectroscopic and molecular docking
studies. J Lumin 2014;153:430-438.
[
35] Simkovitch R, Shomer S, Gepshtein R, Shabat D, Huppert D. Excited-state
proton transfer from quinone-cyanine 9 to protic polar-solvent mixtures. J Phys
Chem A 2014;118:1832-1840.
[
36] Pal SK, Zewail AH. Dynamics of water in biological recognition. Chem Rev
004;104:2099-2123.
37] Santos FS, Ramasamy E, Ramamurthy V, Rodembusch FS. Excited state
2
[
behavior of benzoxazole derivatives in a confined environment afforded by a
water soluble octaacid capsule. J Photochem Photobiol A Chem 2016;317:175-
1
85.
38] Barbeiro CS, Vasconcelos SNS, Zukerman-Schpector J, Caracelli I,
Pimenta DC, Rodrigues ACB, Fernandes AS, Correra TC, Bastos EL, Stefani
HA. Chlorinated 2-hydroxynaphthalenoxazolines: synthesis, reaction
mechanism and fluorescence properties. ChemistrySelect 2016;1:5647-5652.
39] Suzuki N, Fukazawa A, Nagura K, Saito S, Kitoh-Nishioka H, Yokogawa D,
[
[
Irle S, Yamaguchi S. A strap strategy for construction of an excited-state
intramolecular proton transfer (ESIPT) system with dual fluorescence. Angew
Chem Int Ed 2014;53:8231-8235.
[
40] Das K, Sarkar N, Ghosh AK, Majumdar D, Nath DN, Bhattacharyya K.
Excited-state intramolecular proton transfer in 2-(2-
hydroxyphenyl)benzimidazole and -benzoxazole: effect of rotamerism and
2
7

ACCEPTED MANUSCRIPT
hydrogen bonding. J Phys Chem 1994;98:9126-9132 and references cited
therein.
[
41] Duarte LGTA, Germino JC, Braga CA, Barboza CA, Atvars TDZ, Santos
FS, Rodembusch FS. Photoacidity as a tool to rationalize excited state
intramolecular proton transfer reactivity in flavonols. Photochem Photobiol Sci
2
018;17:231-238.
[
42] Cheknlyuk A, Fadeev V, Georgiev G, Kalkanjiev T, Nickolov Z.
Determination of fluorescence quantum yields using a spontaneous Raman
scattering line of the solvent as internal standard. Spectrosc Lett 2006;15:355-
6
5.
[
43] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman
JR, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato,
A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P.
Hratchian. Gaussian Inc. 2016.
[
44] Chai JD, Head-Gordon M. Long-range corrected hybrid density functionals
with damped atom–atom dispersion corrections. Phys Chem Chem Phys
008;10:6615-6620.
45] Dunning Jr. TH. Gaussian basis sets for use in correlated molecular
calculations. I. The atoms boron through neon and hydrogen. J Chem Phys
989;90:1007-1023.
46] Coelho FL, Braga CA, Zanotto GM, Gil ES, Campo LF, Gonçalves PFB,
2
[
1
[
Rodembusch FS, Santos FS. Low pH optical sensor based on benzothiazole
azo dyes. Sens Actuators B 2018;259:514-525.
[
47] M. Cossi, V. Barone, R. Cammi, J. Tomasi. Ab initio study of solvated
molecules: a new implementation of the polarizable continuum model. Chem
Phys Lett 1996; 255:327-335.
[
[
48] Andrienko, GA. https://www.chemcraftprog.com
49] Husain SN, Gentile B, Sauers RR, Eichholz A. Synthesis of photoaffinity
labeling derivatives of d-glucose and d-galactose. Carbohyd Res 1983;118:57-
3.
50] Hanessian S, Vakiti RR, Dorich S, Banerjee S, Lecomte F, DelValle JR,
6
[
Zhang J, Deschênes-Simard B. Total synthesis of pactamycin. Angew Chem Int
Ed 2011;50:3497-3500.
2
8