Design, synthesis and photophysical studies of styryl-based push-pull fluorophores with remarkable solvatofluorochromism

Article abstract of DOI:10.1039/c7nj03142d

A library of 20 styryl-based push-pull dyes derived from 6-amino substituted benzothiazoles were prepared by an efficient and practical synthetic route from low-cost starting materials. The dyes were firstly designed to present an effective anchoring site for subsequent conjugation. A series of aryl scaffolds, from substituted phenyl rings containing electron donating and withdrawing groups to polycyclic aromatic derivatives, were screened. The inductive effect of N-alkyl substituted benzothiazoles was also explored for three different arrangements. The investigation of the structure-photophysics relationship was performed by UV-vis absorption and steady-state fluorescence emission measurements in solution and by TD-DFT calculations. The dyes presented high brightness, absorption bands in the visible range (～370-453 nm) and large solvatofluorochromism comprising all the visible spectrum, as a consequence of the strong intramolecular charge transfer (ICT) nature of their excited state.

Full text of DOI:10.1039/c7nj03142d

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Design, Synthesis and Photophysical Studies of Styryl-Based Push-Pull Fluorophores with Remarkable
Solvatofluorochromism
†
,‡
†
†,
†,
Mauro Safir Filho , Sebastien Fiorucci , Anthony R. Martin * and Rachid Benhida *
†
Université Côte d'Azur, CNRS, Institut de Chimie de Nice UMR7272, 06108 Nice, France
CAPES Foundation, Ministry of Education of Brazil, Brasília DF 70040-020, Brazil
‡
Corresponding Authors
*
anmartin@unice.fr, benhida@unice.fr
Abstract Graphic
hν1
ICT
R
1
N
S
N
hν2
R
2
Ar
CF
3
O
CF
3
CF
3
Abstract
A library of 20 styryl-based push-pull dyes derived from 6-amino substituted benzothiazoles were prepared by an efficient and
practical synthetic route from low-cost starting materials. The dyes were firstly designed to present an effective anchoring site
for subsequent conjugation. A series of aryl scaffolds, from substituted phenyl rings containing electron donating and
withdrawing groups to polycyclic aromatic derivatives, were screened. The inductive effect of N-alkyl substituted
benzothiazoles was also explored for three different arrangements. The investigation of the structure-photophysics
relationship was performed by UV-vis absorption and steady-state fluorescence emission measurements in solution and by TD-
DFT calculations. The dyes presented high brightness, absorption bands in the visible range (~ 370 - 453 nm) and large
solvatofluorochromism comprising all the visible spectrum, as a consequence of the strong intramolecular charge transfer
(ICT) nature of their excited state.
Introduction
1
The design and development of powerful fluorescent tools is a continuously expanding field of research. Within this area, the
quest for evermore bright, robust, fine-tuned and sensitive environment-responsive dyes has attracted a lot of attention.
Environment-sensitive dyes based on push-pull systems are a class of molecules that are able to change their spectroscopic
2
properties in response to physical or chemical modifications in their microenvironment. Push-pull solvatofluorochromic dyes,
a known class of polarity-sensitive molecules, have the peculiar feature of displaying different emission maxima as a function
of the polarity of the surrounding media (i.e. solvent). Thus, sensitive push-pull dyes have been the subject of intense research
3
,4
5–7
8–10
due to their wide range of potential applications including molecular probes, fluorescent markers, bioimaging,
ion
1
1,12
13,14
15
16–18
sensing, organic photovoltaics,
organic light-emitting diodes (OLED) and photoresponsive materials
as a result of
1
9
their linear and non-linear optical properties. Structurally, these molecules feature an electron-donor moiety (D) and an
electron-acceptor (A) group linked together through a π-conjugated system (D-π-A). This D-π-A arrangement may enable,
upon photon irradiation, the formation of a photo-induced intramolecular charge transfer (ICT) excited state that generates
2
0,21
dipole moments that enhance the fluorophore sensitivity to environment polarity.
Meanwhile, the range of the emission
wavelength can be easily modulated by controlling the electronic properties of the donor and the acceptor groups.
In this context, styryl-based fluorophores constitute a common class of molecules exhibiting this D-π-A framework and
2
2
23–29
encompass a broad variety of applications. Nevertheless, while a range of diverse styryl-based dyes have been reported,
3
0,31
including derivatives of benzothiazoles,
studies focused on neutral, highly emissive and solvatofluorochromic styryl-
6
,32–34
6
benzothiazole scaffolds remain scarce in the literature.
In 2013, Herner et al. described a class of fluorogenic dyes
adaptable for direct bioconjugation via azido-alkyne cycloaddition albeit with relative reduced quantum yield. Ono and co-
3
2
workers reported two methylenemalononitrile-containing benzothiazole push-pull dyes with high affinity to probe αβ
3
3
amyloid aggregates in the human brain. Finally, Bashmakova et al. investigated the photophysical behavior of three new
solvatofluorochromic styryl-benzothiazole dyes by steady state, time-resolved fluorescence techniques and computational
calculations. There is still a need for a more comprehensive study devoted to the establishment of a robust structure-
photophysics relationship in this peculiar class of dyes. This may prove helpful in the design of new fluorescent tools harboring
optimized photophysical properties.
In line with the studies mentioned above that aim to access new fluorescent probes for practical applications, we report herein
the synthesis of 20 styryl-based push-pull dyes derived from 6-aminobenzothiazoles. We decided to focus our work on these
dyes where A is a σ-acceptor as no comprehensive structure-photophysics relationship were reported for this type of
framework. Moreover, the presence of the 6-amino function can be beneficial for further modifications (Table 1). The study

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and analysis of this family of dyes has enabled us to understand the structure–photophysics relationship. It is noteworthy that
3
5–38
heretofore only a small number of studies have been done to evaluate such relationships.
The synthesis of this family of
highly solvatofluorochromic dyes, starting from simple and inexpensive 2-methylbenzothiazole, has allowed us to probe the
influence of the three types of modifications of the central 2-styryl-6-aminobenzothiazole (Table 1): 1- the alkylation of the 6-
amino function (D); 2- decoration of the phenyl ring of the styryl moiety with various electron-withdrawing (EWG) and
electron-donating groups (EDG) (A); and 3- the replacement of this simple phenyl motif with larger frameworks (e.g. naphthyl,
anthryl, etc.). Finally, we present the spectroscopic characterization and DFT calculations for dyes with distinct photophysical
properties in terms of visible absorption and emission, brightness and Stokes shift. Altogether, we think that the detailed study
presented herein could be used as a database for the design of fluorescent tools, based on the styryl-6-aminobenzothiazole
motif, for specific applications.
Table 1. Chemical structure of the benzothiazole-based push-pull dyes.
Dye
D1b
D2a
R
1
R
2
Ar
Phenyl
Ethyl
H
H
H
4-methoxyphenyl
4-methoxyphenyl
4-methoxyphenyl
4-(trifluoromethyl)phenyl
4-bromophenyl
2-(trifluoromethyl)phenyl
2,3-(dimethoxy)phenyl
3,4,5-(trimethoxy)phenyl
2,4-bis(trifluoromethyl)phenyl
trans-4-methoxy-β-styryl
4-biphenyl
D2b
D2c
Ethyl
Ethyl
Ethyl
Ethyl
Ethyl
Ethyl
Ethyl
Ethyl
Ethyl
Ethyl
H
Ethyl
Ethyl
Ethyl
Ethyl
H
H
octyl
H
H
H
H
H
H
H
H
H
H
octyl
H
H
H
H
octyl
D3b
D4b
D5b
D6b
D7b
D8b
D9b
D10b
D11a
D11b
D11c
D12b
D13b
D14a
D14b
D14c
1-naphthyl
1-naphthyl
1-naphthyl
2-methoxy-1-naphthyl
9-anthryl
1-pyrenyl
1-pyrenyl
1-pyrenyl
Ethyl
Ethyl
Results and Discussion
Synthesis of Benzothiazole Precursors 3 and 5. The precursor 6-amino-2-methylbenzo[d]thiazole (3) was synthesized
starting from commercially available 2-methylbenzothiazole (1) in two steps in 57% overall yield (Scheme 1). Firstly, 2-
methylbenzothiazole was converted to the respective 2-methyl-6-nitrobenzo[d]thiazole (2) by nitration at 0 °C using a
stoichiometric amount of sodium nitrate in concentrated sulfuric acid as the nitrating mixture. The nitro derivative 2 was
39
obtained in good yield (74%) as light yellow needles after recrystallization from ethanol. Next, the treatment of a methanolic
solution of 2 at room temperature with zinc powder and ammonium formate furnished the respective amino derivative 3 (77
%
). Subsequent acylation of the amino group of 3 using 1.1 equivalents of acetyl chloride in dry DMF and in the presence of
DIPEA afforded the N-acetamido derivative 4 in excellent yield (96 %). The synthesis of the key intermediate 5, N-ethyl-2-
methylbenzo[d]thiazol-6-amine, was accomplished by reduction of 4 using lithium aluminum hydride at room temperature for
12 hours. Compound 5 was obtained in high yield (87 %) after purification by flash column chromatography. This route was
performed on a multigram scale to provide material for the subsequent steps.
a
Scheme 1. Synthesis of benzothiazole precursors
a
Reagents and conditions: (i) NaNO
DMF, 0 - 20 °C, 1 h, 96 %. (iv) LiAlH
3
, H
2 4 4
SO , 0°C, 2 h, 74 %. (ii) Zn, NH COOH, methanol, r.t., overnight, 77 %. (iii) AcCl, DIEA,
4
, THF, r.t., 12 h, 87 %.
Dye synthesis. The synthetic route towards dyes D1b-D14b involves the aldol condensation between various aromatic
aldehydes and the intermediate 5 that bears an activated methyl group on its 2-position (Scheme 2). The aromatic aldehydes

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were chosen to modulate the steric and electronic properties of the final dyes. For this family of compounds, either the parent
3
benzaldehyde, or benzaldehydes that are mono/poly substituted with EWG (-CF , Br) or EDG (-OMe), were used.
Cinnamaldehyde bearing p-OMe EDG was selected to investigate the influence of the length of the π-conjugated bridge.
Bulkier aromatic aldehydes, such as 1-naphthaldehyde, were also employed. Importantly, we included polycyclic aromatic
aldehydes able to promote excimer formation (9-anthryl and 1-pyrenyl radicals) in this study.
Scheme 2. Synthesis of dyes containing the N-ethyl-6-aminobenzothiazole core.
H
H
N
N
S
S
N
i
N
Ar
5
D1b-D14b
O
CF
3
D1b, 69%
Br
D2b, 78%
D3b, 48%
O
F
3
C
O
D4b, 82%
D5b, 74%
D6b, 79%
O
O
O
F
3
C
CF
3
O
D8b, 36%
D9b, 59%
D7b, 72%
O
D10b, 77%
D11b, 72%
D12b, 78%
D13b, 53%
D14b, 81%
a
Reagent and conditions: (i) aromatic aldehyde, KOH, DMF, r.t., sonication, 1 h.
The preparation of D1b-D14b was performed at room temperature with ultrasonic irradiation, in N,N-dimethylformamide
using a stoichiometric amount of potassium hydroxide. It is worth mentioning that attempts to use milder bases such as
tertiary amines or sodium acetate did not afford any product. On the contrary, potassium tert-butoxide behaved similarly to
hydroxide and proved efficient in the activation of the 2-methyl group of 5. Thus, using KOH as the base, all dyes (D1b-D14b)
were isolated in satisfactory yield (36-82 %) after purification by chromatography. Finally, to ensure the high purity required
for photophysical studies, D1b-D14b were further recrystallized from methanol (12-49 % yield).
Next, the completion of this family of dyes was pursued by synthesizing two more series in which the 6-amino function of
precursor 3 either remains as a primary amine or is converted to the tertiary N-ethyl-N-octyl-6-amino derivative. These two
series enable the evaluation of the inductive effect of the alkyl substituents on the donating ability of the amino group attached
to the benzothiazole scaffold (Scheme 3). Dyes bearing the primary amino group (D2a, D11a and D14a) were accessed in a
similar way to dyes D1b-D14b but starting from the benzothiazole precursor 3 instead of 5. For the second series, the tertiary
amino derivatives D2c, D11c and D14c were obtained in excellent yields (80-88 %) by alkylation of the corresponding dye
(D2b, D11b and D14b) using 1-iodooctane in the presence of potassium carbonate (Scheme 3). Interestingly, no byproducts
corresponding to the quaternarization of the endocyclic nitrogen in the thiazole ring was observed. Hence, this simple and
straightforward transformation could also be employed in the design and preparation of valuable spectroscopic tools; for
instance, this easy entry to dye modification could be used in the tailoring of membrane probes or for biolabeling experiments.
Scheme 3. Synthesis of dyes bearing a primary or tertiary 6-amino function.

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H
2
N
H
2
N
S
N
S
N
i
Ar
3
D2a, Ar = 4-methoxyphenyl, 59%
D11a, Ar = 1-naphthyl, 65%
D14a, Ar = 1-Pyrenyl, 46%
C
8 17
H
H
N
S
N
S
N
ii
N
Ar
Ar
D2b, Ar = 4-methoxyphenyl
D11b, Ar = 1-naphthyl
D14b, Ar = 1-Pyrenyl
D2c, Ar = 4-methoxyphenyl, 86%
D11c, Ar = 1-naphthyl, 88%
D14c, Ar = 1-Pyrenyl, 80%
a
Reagent and conditions: (i) aromatic aldehyde, KOH, DMF, r.t., sonication, 1 h. (ii) 1-iodooctane, K CO , acetonitrile, 80°C, 24 h.
2
3
Photophysical Studies
Substituted Phenyl Series: Dyes D1b-D9b.
The photophysical properties in various solvents for D1b-D9b are summarized in Table 2.
Table 2. Relevant photophysical data from UV-visible absorption and fluorescence emission of dyes D1b-D9b
Solvent Varied phenyl substituents (acceptor)/Fixed 6-ethylamino- (donor)
[a]
T
E (30)
λ,
Δλ
[b]
,
ST
c]
[
Φ
fl
D1b
D2b
D3b
D4b
D5b
D6b
D7b
D8b
D9b
3
ε
× 10
1
-
-1
25.4
48.3
27.1
36.8
34.4
34.3
41.1
38.1
62.4
(
M .cm )
λ
λ
ΔλST
abs
380
445
3843
0.81
385
467
4560
0.83
387
474
4742
0.73
385
486
5398
0.61
400
530
380
447
3944
0.58
386
464
4355
0.75
387
469
4517
0.79
384
394
463
389
456
3777
0.56
394
389
462
4062
0.78
394
379
449
4113
0.53
386
468
4539
0.65
388
384
450
3819
0.50
390
470
4364
0.83
390
474
4544
0.74
388
407
487
4036
0.80
414
401
473
3796
0.44
409
491
4083
0.57
409
498
4369
0.63
406
507
Cyclohexane
30.9)
em
(
3782
0.63
400
484
4338
0.67
398
497
5005
0.74
400
521
Φ
fl
abs
em
λ
λ
Toluene
33.9)
471
483
516
(
ΔλST
Φ
4149
0.66
395
482
4569
0.57
394
4677
0.64
396
499
5212
0.70
395
4775
0.83
415
fl
λabs
λ
ΔλST
Φ
λabs
λ
ΔλST
1,4-Dioxane
em
475
533
(
36.0)
4720
0.63
387
487
5306
0.62
404
531
5920
0.79
386
512
6375
0.45
393
5334
0.76
416
fl
Ethyl
acetate
em
476
501
518
484
5112
0.61
403
565
5033
0.74
399
5806
0.47
417
5420
0.56
409
546
6134
0.87
393
6011
0.68
415
6339
0.57
438
4906
0.48
421
(
38.1)
Φ
fl
abs
em
λ
λ
DMSO
45.1)
514
575
572
526
630
549
(
ΔλST
6132
0.67
384
5607
0.78
385
495
5772
0.64
390
509
5994
0.61
6589
0.78
398
549
6911
0.52
405
548
6443
0.52
6614
0.85
395
549
7101
0.59
401
547
6656
0.68
5802
0.76
390
507
5917
0.73
393
519
6177
0.53
6958
0.61
412
600
7605
0.55
422
591
6776
0.65
5538
0.68
405
Φ
fl
abs
em
λ
λ
MeCN
45.6)
510
525
528
(
ΔλST
6434
0.63
389
519
6439
0.57
6397
0.50
398
5751
0.46
408
536
5853
0.45
Φ
fl
abs
em
λ
λ
MeOH
55.4)
529
521
6251
0.52
(
ΔλST
Φ
6222
0.52
fl
2
0
-1
[
a] Reichardt's E (30) was selected to measure solvent polarity. [b] Stokes shifts (Δλ ) are reported in cm . [c] Fluorescence
T ST
4
quantum yields (Φfl) were measured using quinine sulfate in an aqueous 0.1 M HClO solution (λexc = 350 nm, Φfl = 0.59) or
4
0
fluorescein in 0.1 M NaOH aq. solution (λexc = 475 nm, Φfl = 0.89) as the references.
To evaluate the influence of the phenyl substituents on the spectroscopic behavior of the dyes, we varied these substituents
while keeping the donor group (ethylamino-) on the benzothiazole scaffold unchanged. Dye D1b, containing an unsubstituted
phenyl ring was chosen as the reference. All of the dyes D2b-D8b containing substituted phenyl rings absorbed more strongly
3
-1
-1
3
-1
-1
than D1b (27.1 - 48.3 × 10 M .cm vs. 25.4 × 10 M .cm , respectively), with D2b (p-OMe) presenting the highest hyperchromic

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effect of this series. Additionally, all dyes presented absorption bands in the visible range (384 - 412 nm in acetonitrile), with ε
*
-1
values in agreement with π-π electronic transitions. Large Stokes shifts in polar solvents, ca. 5751 - 7605 cm in acetonitrile,
were also observed. The relatively weak solvent dependence in the absorption maxima of D1b-D9b indicates that the
0 1
stabilization energy for the ground S and the Franck-Condon state of S are quite similar. In contrast, they showed gradually
increased bathochromic shifts in their emission maxima when going from the least polar aprotic solvent to the most polar
solvent (Table 2, Figure 1). Using D1b as the reference, dyes containing para-substituted phenyl rings (D2b-D4b), presented
similar absorption spectra, whereas the emission maxima was slightly blue-shifted for dye D2b that bears an EDG (p-OMe),
3
and red-shifted for dyes containing EWGs (D3b, p-CF and D4b, p-Br), in polar solvents. Fluorescence quantum yields for dyes
D1b-D9b were in the 0.44 - 0.87 range for all the tested solvents; no strong dependence on the solvent polarity was observed.
In this series, dye D8b that features the 2,4-bis(trifluoromethyl)phenyl pattern exhibited the most accentuated solvent
sensitivity as indicated by the largest red-shift of its emission maxima (Figure 2). Additionally, the ortho or para positioning of
the -CF substituent (D3b and D5b) does not significantly affect either the absorption or emission maxima, while increasing
3
the number of OMe groups (dye D6b and D7b), regardless of their location, resulted in a moderate bathochromic shift in the
emission maxima in polar solvents, compared to D2b.
Figure 1. Absorption (dashed) and fluorescence emission (solid) spectra of dyes D1b, D2b, D3b and D8b in cyclohexane (black), ethyl
acetate (red), DMSO (green) and MeOH (blue).
The influence of the alkene bridge length between the benzothiazole and the phenyl ring on the spectroscopic properties was
evaluated by comparing dyes D2b and D9b; the latter bears an additional vinyl unit. The absorption maximum for D9b was
equally red-shifted, by about 20 nm, in all the tested solvents. This effect was accompanied with an increase in the molar
extinction coefficient ε (+29 %, compared to D2b), as a consequence of its higher degree of conjugation. Moreover, it is
4
1
interesting to note that the λabs redshift, related to the additional vinyl unit, is significantly lower compared to similar cationic
and neutral
4
2,43
polymethine dyes. In contrast with the absorption maximum of D9b, the fluorescence emission spectra showed
some degree of solvent dependency, especially for the most polar media. In fact, while the λem difference between D9b and
D2b is only 26 nm in cyclohexane, it reached 35 nm in DMSO.
The sensitivity of the dyes to solvent polarity was evaluated by plotting the Stokes shifts vs Reichardt's E
T
(30) scale (Figure 2
and Figure S2). This latter quantitative parameter accounts for the dielectric constant of the solvent and its hydrogen-bond
2
0
donor ability. Among the first series of dyes (D1b-D9b), D8b showed the highest sensitivity (highest slope, Figure 3). As a
general trend, more structured emission band were observed in cyclohexane. This may be due to some degree of contribution
4
4
from a locally excited (LE) state emission. However, the large positive solvatofluorochromism and the proportionally red-
shifted emission maxima for the dyes along the range of polarity strongly suggest the formation of a relaxed ICT state in
solvents of increasing polarity. Moreover, the fact that only a single and broad structureless emission band is observed for all
the dyes also supports the hypothesis of a fluorescence emission from an ICT state. In addition, the relatively intense
fluorescence emission, even in polar environments, hints that the geometry of the ICT excited state seems to be close to the
ground state and contributions associated to the formation of a non-coplanar TICT state may not be extensively involved.
Emissive CT states are known to be associated with molecular rigidity; however, the strength of the donor/acceptor groups are

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4
5
also important to modulate TICT formation. Examples of highly emissive ICT excited states in highly polar solvents are
4
6
known in the literature either in the case of rigid molecular scaffolds or non-rigid architectures including chalcone,
4
5
38
courmarin and fluorene dyes. Differences between the dipole moments (μ
was estimated by plotting the Stokes shifts as a function of the Lippert-Mataga orientation polarizability (Δf) parameter
Supporting Information, Figure S2 and Table S1). Values of 10.3, 9.9, 11.7 and 13.4 D, for μ - μ , were obtained for dyes D1b,
e g
- μ ) of the ground and excited states of the dyes
4
7,48
(
e
g
D2b, D3b and D8b, respectively. The increase in the excited state dipole moment is accomplished by a higher level of charge
separation between the donor (6-ethylamino) and the acceptor (aryl group). Hence, since the donor group remained
unchanged in this series, the largest charge separation, corresponding to the highest μ
that features the most electron-poor arene (2,4-bis-CF ) on the acceptor moiety.
e g
- μ difference, was observed for D8b
3
Figure 2. Dependence of the Stokes shift on the empirical polarity parameter E (30) for dyes D1b, D2b, D3b and D8b. Correlation
T
2
factors R are, respectively, 0.96, 0.97, 0.95and 0.93. The linear plots shown were obtained with all the tested solvents except
methanol.
To complete the spectroscopic study, the dyes were further investigated by DFT calculations at the MPW1PW91/6-311+G(d,p)
level of theory, using Gaussian 03. Optimized ground-state geometries were found to be close to planar for all dyes except for
those presenting ortho-substituents (D5b, D6b and D8b), where the phenyl ring appeared slightly twisted respectively to the
2
-vinylbenzothiazole motif (Figure S3). The calculations revealed that the less energetic electronic transitions occur between
the frontier molecular orbitals HOMO and LUMO. The highest occupied molecular orbital (HOMO) is partially localized on
the benzothiazole donor and around the vinyl bridge while the lowest unoccupied molecular orbital (LUMO) is spread across
the bridge and the phenyl ring (Figure 3). Compared with other dyes, the LUMO of D8b features a higher electronic density on
3
its acceptor motif (2,4-bis-(CF )). This strongly supports the large ICT character that was observed for this dye. Furthermore,
for D1b-D3b and D8b, time-dependent (TD)-DFT calculations in cyclohexane and DMSO yielded their corresponding
calculated absorption maxima associated to the first electronic transition S0 →S1. Compared with the experimental values, the
calculated absorption maxima followed the same trend and were only slightly red-shifted (see Supporting Information).
a)
LUMO
LUMO
b)
H
H
N
N
S
N
S
HOMO
HOMO
O
N
CF3
D2b
D8b
3
F C
Figure 3. Representation of HOMO and LUMO, for dyes D2b (a) and D8b(b), obtained from optimized ground state geometries.
Finally, to overcome the challenges in predicting the emission maxima and Stokes shifts by DFT calculations, the correlation
between λem and ΔλST (in wavelength) for dyes containing para-substituted phenyl rings was performed using the σ
p
Hammett
= 0), while dyes D2b, D3b and D4b were selected as
representative dyes containing EDG and EWG. Figure 4 depicts the plots of emission maxima and Stokes shift vs σ for five
different solvents. Excellent correlations for the λem were achieved for all solvents (R = 0.94 - 0.98) except for those of low
polarity (cyclohexane and toluene, see Supporting Information). In parallel, linear trend between Stokes shift and σ values
was obtained exclusively for solvents with moderate and high polarity (1,4-dioxane to methanol), with R values ranging from
.80 to 0.98. To verify our prediction model, the emission maxima and the Stokes shift for the reported dye D1b were
4
9
substituent parameter model. Dye D1b was selected as reference (σ
p
p
2
p
2
0
calculated for five solvents. The values were slightly shifted compared to the experimental ones (Table S2), with errors in an
acceptable range (± 3 nm for both λem and ΔλST).

Page 7 of 15
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Figure 4. Dependence of the emission maxima (up) and Stokes shift (down) of D1b, D2b, D3b and D4b on the Hammett
values (σ ) in methanol (cyano), acetonitrile (blue), dimethyl sulfoxide (green), ethyl acetate (red) and 1,4-dioxane (black).
Correlation factors R are, respectively, 0.94, 0.98, 0.98, 0.99 and 0.98 related to λ and 0.80, 0.98, 0.97, 0.97 and 0.92 related
p
2
em
to ΔλST
.
Polycyclic Aromatic Hydrocarbon Series
To get further insight into the effect of the acceptor moiety on the photophysical properties of the dyes, different polycyclic
aromatic hydrocarbons were evaluated using the same benzothiazole scaffold while keeping the 6-ethylamino function
unchanged. The corresponding data are presented in Table 3. Figure 5 depicts the UV-visible and fluorescence emission
spectra of dyes D10b-D14b in four representative solvents.
Table 3. Relevant photophysical data from UV-visible absorption and fluorescence emission of dyes D10b-D14b
Solvent
Varied polycyclic aromatic (acceptor)/Fixed 6-
ethylamino- (donor)
λ,
ΔλST
Φfl
,
D10b
D11b D12b
D13b
D14b
3
ε
× 10
-
1
-1
44.7
37.3
36.8
28.9
54.6
(
M .cm )
λ
λ
ΔλST
abs
391
464
4024
0.63
398
479
4249
0.77
399
487
4529
0.60
396
383
475
5057
0.37
392
386
478
4986
0.42
396
494
5009
0.30
397
400
557
7047
0.47
408
571
6996
0.48
407
581
7358
0.40
405
418
508
4238
0.68
428
Cyclohexane
30.9)
em
(
Φ
fl
abs
em
λ
λ
Toluene
33.9)
491
526
(
ΔλST
Φ
5143
0.47
395
4353
0.71
427
fl
λ
abs
1
,4-Dioxane
36.0)
λem
502
503
531
(
ΔλST
5396
0.45
393
5308
0.48
394
4586
0.73
425
Φ
fl
abs
em
λ
λ
Ethyl
acetate
507
519
513
600
8025
0.36
419
558
ΔλST
5528
0.48
413
6177
0.51
412
5887
0.47
414
5608
0.60
443
(
38.1)
Φ
fl
abs
em
λ
λ
DMSO
45.1)
557
571
559
674
621
(
ΔλST
6259
0.79
396
6758
0.82
393
548
7197
0.54
399
548
6814
0.70
6265
0.75
396
9029
0.36
404
639
9103
0.33
412
628
9139
0.31
6470
0.65
425
Φ
fl
λ
abs
MeCN
45.6)
λem
ΔλST
Φfl
531
537
591
(
6420
0.52
401
533
6176
0.55
6630
0.43
402
536
6218
0.48
6608
0.59
429
580
6068
0.63
λ
abs
MeOH
55.4)
λ
em
(
ΔλST
Φ
fl

New Journal of Chemistry
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DOI: 10.1039/C7NJ03142D
See Table 2.
Figure 5. Absorption (dashed) and fluorescence emission (solid) spectra of dyes D10b-D14b in cyclohexane (black), ethyl acetate
red), DMSO (green) and MeOH (blue).
(
Similarly to the phenyl series (D1b-D9b), dye D1b was again selected as the reference. All dyes containing polycyclic aromatic
3
-1
-1
3
-1
-1
groups, D10b-D14b, showed stronger absorption than D1b (28.9 - 54.6 × 10 M .cm vs. 25.4× 10 M .cm , in dioxane). The
absorption maxima for compounds containing up to three fused phenyl rings (D11b-D13b) exhibited a moderate bathochromic
shift (9-20 nm in acetonitrile) compared with D1b. In contrast, replacing the phenyl ring with a bulky 1-pyrenyl core (D14b)
strongly red-shifted the maximum (~ 40 nm in all the tested solvents) as a result of its higher degree of conjugation.
Furthermore, this series, D10b-D14b, showed little dependency on the solvent polarity in their λabs, thus supporting the
hypothesis of a weak charge transfer character in their ground state. In contrast with their absorption maxima, the
fluorescence emissions were affected in a similar manner to the phenyl derivatives D1b-D9b. Indeed, a high degree of solvent
-
1
polarity dependency on λem was observed (∆λST up to 9103 cm in acetonitrile).
Dye D13b, containing the 9-anthryl scaffold, showed the broadest absorption band with the lowest ε value among the
polycyclic aromatic hydrocarbon derivatives. Additionally, the large shoulder around 350 nm may indicate a lack of planarity
and that the resulted absorption band could contain a high contribution from a twisted anthracene moiety. This hypothesis
was supported by the optimized DFT structure of D13b (Figure 6). It showed both a non-coplanar orientation of the 9-anthryl
and vinylbenzothiazole motifs as well as a high electronic density located on the anthryl framework in the frontier orbitals.
Compared with the other dyes of the same series, D13b presented distinct emission behavior with the lowest fluorescence
-
1
quantum yield in polar solvents. It displayed an incredible mega-Stokes shift in DMSO (ca. 9029 cm / 255 nm), in the same
38
range that was observed for highly solvatochromic fluorene dyes with a methylenemalononitrile acceptor. In addition, the
-
1
intriguing highly red-shifted and broad structureless emission band observed in cyclohexane (ΔλST = 7046 cm / 157 nm) may
suggests that a highly dipolar state is also formed in nonpolar environments. Based on photophysical studies concerning
50,51
derivatives of 4-(9-anthryl)-N,N'-dimethylaniline (ADMA) and analogues,
we suspect that the strong electron acceptor
character of the anthracene group in the excited state may favor an intramolecular electron transfer mechanism from the 6-N-
+
-
ethylamino group to the anthryl radical, thus yielding an highly dipolar structure (D •A •) with reduced fluorescence emission.
H
b) LUMO
N
S
N
D13b
a)
HOMO
Figure 6. a) Optimized DFT structure of D13b. b) Frontier orbitals obtained from optimized ground state geometry.

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As for the first series, a linear correlation between the Stokes shifts of dyes D10b-D12b and D14b and the solvent polarity
(30) - Figure 7) was evidenced in this second series, except for D13b in which the reduced slope does not correlate with the
photophysical data (see Supporting Information). This last observation may indicate that emission takes place from a distinct
and highly polar CT state (e.g. involving an electron transfer process). The calculated values of μ - μ for dyes D10b-D12b and
(E
T
e
g
D14b (11.7, 10.6, 10.2 and 12.4 D, respectively, Figure S2, Table s2) are in agreement with the spectroscopic observation and
support the strong charge transfer character for these dyes in the excited state. Finally, further investigations to highlight a
putative excimer emission for the pyrene-containing dye, D14b, was performed. Surprisingly, no strong evidence associated
with an excited state dimer emission was observed regardless of the broad range of concentrations or the different solvents
that were probed (Figure S5). Nevertheless, these observations do not rule out the possibilities of designing dyes derived from
D14b as fluorescent ratiometric monomer-excimer emitting probes.
T
Figure 7. Dependence of the Stokes shift on the empirical polarity parameter E (30) for dyes D10b-D12b and D14b. Correlation
2
factors R are, respectively, 0.93, 0.92 and 0.92. The linear plots shown were obtained with all the tested solvents except methanol.
Spectroscopic Impact of the Alkykation of the 6-Amino Functional Group
The last part of our study aimed to evaluate the effect of the alkyl radicals attached to the 6-amino group of the benzothiazole
ring on the spectroscopic properties of the dyes. For this purpose, dyes D2, D11 and D14 bearing either a primary (D2a, D11a
and D14a), a secondary (D2b, D11b and D14b) or a tertiary amine at the 6-position of the benzothiazole (D2c, D11c and D14c)
were compared. Figure 8 depicts the UV-Vis absorption and fluorescence emission spectra for dyes D2a-c, D11a-c, and D14a-c,
in cyclohexane and acetonitrile. Additional photophysical data are reported in the Table 4. As expected and in agreement with
their positive inductive effect, increasing the number of alkyl substituents on the 6-amino group resulted in a gradual red-shift
of both absorption and emission maxima (~ 10 to 15 nm per alkyl substituent). In addition, more structured emission bands in
cyclohexane are also observed for dyes containing dialkyl substituted amines. It may be a consequence of the increased
hydrophobicity and the loss of the hydrogen bond formation ability, thereby reducing the intermolecular dye-dye interactions.
The main spectroscopic properties (ε, Φ
D11a, bearing the primary amine, showed reduced fluorescence quantum yield and higher ε values, in comparison with their
analogues D2c and D11c, respectively In contrast, dyes D14a and D14c presented similar Φ values, especially in polar solvents,
f
, and Δλ) were moderately different among the dye families (a, b and c). Dyes D2a and
f
with a higher extinction coefficient for D14c. Linear trends between the Stokes shift and the solvent polarity parameters E (30)
T
and Δf were also obtained (see Supporting Information).The transition dipole moments were directly correlated with the
number of alkyl substituent on the amino group, with dyes bearing the tertiary amine presenting larger μ
e g
- μ values (Table S1,
Supporting Information). Finally, the attachment of a second alkyl substituent on the 6-amino position (D2c, D11cand D14c)
does not significantly reduce the brightness (ε.Φ ), when compared to the analogue containing the secondary N-ethylamine
f
EDG (D2b, D11b and D14b).
Table 4. Relevant photophysical data from UV-visible absorption and fluorescence emission of dyes containing primary (D2a, D11a
and D14a) and tertiary amines (D2c, D11c and D14c)
Solvent
Varied amine/Fixed aryl
λ,
ΔλST
Φfl
,
D2a
D2c D11a D11c D14a D14c
3
ε× 10
-1
-1
37.7
33.1
34.4
29.7
54.2
65.9
(
M .cm )
λ
λ
abs
368
433
395
459
370
461
403
466
410
488
438
527
Cyclohexane
30.9)
em
(
ΔλST 4079 3529 5335 3354 3898 3855
Φ
λ
λ
fl
abs
em
0.25 0.69 0.37
0.49
413
497
0.73
417
508
0.65
443
547
374
449
401
480
378
480
Toluene
33.9)
(
ΔλST 4466 4104 5621 4092 4295 4291
Φ
λ
λ
fl
abs
em
0.33
379
463
0.80 0.25
0.66
413
514
0.69
417
517
0.80
441
545
400
480
380
492
1
,4-Dioxane
36.0)
(
ΔλST 4786 4166 5990 4757 4638 4327
Φ
fl
0.40 0.76 0.28
0.45
410
531
0.61
416
539
0.54
439
569
λ
abs
374
466
396
487
382
507
Ethyl
acetate(38.1)
λem
ΔλST
5279 4718 6454 5558 5485 5204

New Journal of Chemistry
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DOI: 10.1039/C7NJ03142D
Φfl
0.29 0.66 0.38
0.69
425
584
0.55
432
608
0.58
453
630
λ
abs
388
501
410
525
401
559
DMSO
45.1)
λ
em
(
ΔλST
5813 5342 7048 6406 6700 6202
Φ
fl
abs
em
0.57 0.80 0.73
0.76
413
566
0.76
414
566
0.72
440
610
λ
λ
371
400
515
380
527
MeCN
45.6)
479
(
ΔλST 6077 5582 7340 6545 6486 6333
Φ
λ
λ
fl
abs
em
0.41
371
496
0.63
405
521
0.38
381
532
0.63
417
559
0.49
417
567
0.59
443
590
MeOH
54.4)
(
ΔλST 6793 5497 7449 6091 6344 5624
0.30 0.61 0.25 0.64 0.47 0.43
Φ
fl
See Table 2.
Figure 8. Absorption (dashed lines) and fluorescence emission (solid) spectra of dyes D2a-c (a and d), D11a-c (b and e) and D14a-c (c
and f) in cyclohexane (left) and acetonitrile (right).
Conclusions
A series of 20 styryl-based solvatofluorochromic dyes was efficiently prepared from 6-amino-2-methylbenzothiazole. The dyes
were synthesized in satisfactory overall yield, using a simple and practical synthetic route starting from commercially available
and inexpensive materials. The ease of N-alkylation renders this position ideal as a prospective anchoring point for further
(
bio)conjugation. Spectroscopic studies coupled with DFT calculations have enabled the understanding of the relationship
between structure and the photophysical properties of the presented dyes in terms of the electronic properties of the donor
first series D1b-D9b) and inductive effects associated with multiple 6-aminoalkylation. The dyes proved to be highly emissive
(
in polar and nonpolar solvents. They are characterized by the formation of an intramolecular charge transfer excited states as
corroborated by their large positive solvatochromic emission bands. High Stokes shifts, up to 192 and 255 nm, could be
accessed for the phenyl and the polycyclic aromatic series, respectively. This highlights that the commonly used π-acceptors
groups can be efficiently replaced by purely σ-acceptor to broadly tune the emission wavelength towards the red region.
Finally, for dyes containing para-substituted phenyl rings, the perfect correlation between Hammett parameter (σ ) and Stokes
P
shifts and λem provides a complementary approach to anticipate photophysical properties of other members of this family of
fluorescent probes. Using this correlation and the subtle changes brought by the 6-amino alkylation, specific fluorescent tools
that absorb and emit in a particular spectral range can be easily designed. Our laboratories have expertise in the sensing of
5
2-54
DNA/(bio)molecules interactions,
hence we envision to use of these finely-tunable dyes for this purpose.
Experimental Section
All solvents for absorption and fluorescence experiments were of spectroscopic grade. Absorption spectra were recorded on a
Cary 4 spectrophotometer (Varian) using quartz cells of 1 cm path length. Fluorescence spectra were recorded on FluoroMax
4.0 spectrofluorometer (Jobin Yvon, Horiba). The wavelengths corresponding to the absorption maxima were used as the
excitation wavelengths. Stock solutions of the solvatofluorochromic dyes were prepared using 1,4-dioxane. The samples used
for spectroscopic measurements contained ≈ 0.1% v/v of the stock solvent. UV-vis absorption spectra were recorded using a
-
5
-6
dye concentration of ca. 10 M. Fluorescence emission spectra were recorded using a dye concentration of ca. 10 M. All
chemical reagents were obtained from commercial sources (Aldrich, Acros, Alfa Aesar) and were used as supplied. The
reactions were monitored by thin-layer chromatography (TLC, Merck silica gel 60 F254 plates) and visualized both by UV
radiation (254 & 365 nm) and by spraying with a relevant staining agent followed by a subsequent warming with a heat gun.
1
13
19
Column chromatography was performed with flash silica gel (40–63 mm). All NMR spectra ( H, C, F) were recorded on
1
13
1
19
1
Bruker Advance Spectrometers (200 or 400 MHz). H NMR (200 and 400 MHz), C{ H} NMR (50 and 101 MHz), and F{ H} (188
and 377 MHz) spectra were obtained with samples dissolved in CDCl or DMSO-d , with the solvent residual signals as internal
references: 7.26 ppm for CHCl and 2.50 ppm for DMSO-d for H NMR experiments, and 77.16 ppm for CDCl and 39.52 ppm
3
6
1
3
6
3
1
3
55
1
13
for DMSO-d for C NMR experiments. Chemical shifts (δ) are given in ppm to the nearest 0.01 ( H) or 0.1 ppm ( C). The
6
coupling constants (J) are given in Hertz (Hz). The signals are reported as follows: chemical shift, multiplicity (s = singlet, d =

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doublet, t = triplet, m = multiplet, dd = doublet of doublets, br = broad), coupling constants (J) and integration. Dye syntheses
were performed in a Branson 5510 ultrasound bath. Low resolution mass spectra (MS) were recorded on an Esquire 3000 Plus
apparatus with ESI in both positive and negative mode. High Resolution Mass Spectra (HRMS) were recorded on a
ThermoFisher Q Exactive (ESIMS) at a resolution of 140000 at m/z 200. Density functional theory (DFT) and time-dependent
56
DFT (TD-DFT) calculations were performed using the Gaussian03 software at the MPW1PW91/6-311+G(d,p) level of theory for
all atoms without any symmetry restrictions. Optimized structures of the ground state were checked via a calculation of
vibrational frequencies, ensuring that no imaginary frequencies were present. The vertical excitation energies are determined
using the conventional TD-DFT procedure on the ground state geometries. The effects of solvents were taken into account by
the polarizable continuum model (PCM). Supplementary data associated with this article include: photophysical
characterizations, copies of NMR spectra - see Supporting Information for more details.
Benzothiazole precursors 2-5.
2-methyl-6-nitrobenzo[d]thiazole (2). In a 250 mL round-bottom flask were added 29.8 g (200 mmol) of 2-methylbenzothiazole
and 100 mL of concentrated sulfuric acid. The mixture was cooled to 0°C and a solution of 17.0 g (200 mmol) of sodium nitrate
in 100 mL of concentrated sulfuric acid was slowly added while keeping the temperature of the reaction media below 5 °C.
After completion of the addition, the reaction was further stirred for 2 hours at 0°C before pouring the mixture into 1L of
crushed ice. The precipitate formed was filtered off, and washed with cold water and ethanol. It was subsequently
1
recrystallized from methanol to afford 28.7 g of light yellow solid. Yield: 74 %. H NMR (200 MHz, CDCl
2
3
) δ (ppm) 8.73 (d, J =
3
) δ(ppm) 173.4, 157.2,
1
3
1
.3 Hz, 1H), 8.29 (dd, J = 9.0, 2.3 Hz, 1H), 7.99 (d, J = 9.0 Hz, 1H), 2.89 (s, 3H). C{ H} NMR (50 MHz, CDCl
1
44.8, 136.1, 122.7, 121.6, 118.1, 20.8.
6-amino-2-methylbenzo[d]thiazole (3). To a solution of 19.4 g (100 mmol) of 2 in 200 mL of methanol, were successively added
6.5 g of zinc powder (100 mmol) and 25.4 g (400 mmol) of ammonium formate. The reaction was stirred overnight at room
temperature. Then, the crude mixture was filtered over celite, the filtrate was diluted with ethyl acetate and extracted with
water (3 × 200 mL). The organic phase was concentrated in vacuo and the crude residue was purified by flash chromatography
on silica gel, using a 3/7 (v/v) mixture of cyclohexane/ethyl acetate as the eluent, to afford 12.6 g of compound 3 as a grey
1
powder. Yield: 77 %. H NMR (200 MHz, CDCl
1
3
) δ (ppm) 7.68 (d, J = 8.6 Hz, 1H), 7.02 (d, J = 2.1 Hz, 1H), 6.76 (dd, J = 8.6, 2.3 Hz,
) δ (ppm) 162.7, 146.8, 144.1, 137.3, 122.7, 115.2, 105.9, 19.9.
1
3
1
H), 3.80 (br s, 2H), 2.73 (s, 3H). C{ H} NMR (50 MHz, CDCl
3
N-(2-methylbenzo[d]thiazol-6-yl)acetamide (4). In a 100 ml round-bottom flask, 4.9 g (30 mmol) of 3, 6.1 g (60 mmol) of
triethylamine and 0.3 g (3 mmol) of DMAP were dissolved in 30 mL of anhydrous DMF. The mixture was cooled to 0°C in an
ice bath before the dropwise addition of acetyl chloride (2.35 mL, 33 mmol). After completion of the addition, the ice bath was
removed and the mixture was stirred at r.t. for 1 hour. The solvent was removed under reduced pressure and the residue was
dissolved in 100 mL of ethyl acetate, washed with water (3×100 mL) and brine (100 mL). The organic layer was dried over
magnesium sulfate, filtered and the volatiles were removed in vacuo. Finally, the crude residue was purified by flash
chromatography on silica gel, using cyclohexane/ethyl acetate (6/4, v/v) as the eluent, to afford 5,92 g of pure compound 4 as
1
light yellow solid. Yield: 96 %. Mp: 139-140 °C. H NMR (400 MHz, CDCl ) δ (ppm) 8.39 (s, 1H), 8.08 (s, 1H), 7.82 (d, J = 8.7 Hz,
3
1
3
1
1
H), 7.27 (d, J = 8.7 Hz, 1H), 2.81 (s, 3H), 2.21 (s, 3H). C{ H} NMR (50 MHz, CDCl
3
) δ (ppm) 168.9, 166.6, 150.1, 136.6, 135.1, 122.3,
OS 207.0587; Found 207.0588.
-ethylamino-2-methylbenzo[d]thiazole (5). A 250 mL round-bottom flask was charged with 5.0 g (24 mmol) of 4, and 100 mL of
dry THF, under an inert atmosphere (Ar). To this mixture were slowly added 3.1 g (4 equiv.) of LiAlH and the reaction was
subsequently stirred overnight. The unreacted LiAlH was quenched via the dropwise addition of acetone. The crude was
+
1
6
18.8, 112.6, 24.6, 20.1. HRMS (ESI) m/z: [M + H] Calcd for
10 11 2
C H N
4
4
neutralized with 1 M HCl and filtered over celite. The filtrate was concentrated to dryness, dissolved in 150 mL of ethyl acetate
and washed with brine (3×150 mL). The organic layer was dried over magnesium sulfate, filtered and the volatiles were
removed in vacuo. The crude was purified by flash chromatography on silica gel, using cyclohexane/ethyl acetate (8/2, v/v) as
1
the eluent, to provide 4.0 g of precursor 5 as a grey solid. Yield: 87%. Mp: 107-108 °C. H NMR (200 MHz, CDCl
3
) δ (ppm) 7.68
(
d, J = 8.7 Hz, 1H), 6.90 (d, J = 2.3 Hz, 1H), 6.69 (dd, J = 8.7, 2.4 Hz, 1H), 3.69 (br s, 1H), 3.15 (q, J = 7.1 Hz, 2H), 2.72 (s, 3H), 1.25
1
3
1
(t, J = 7.1 Hz, 3H). C{ H} NMR (50 MHz, CDCl ) δ (ppm) 161.7, 146.3, 145.8, 137.7, 122.6, 113.9, 102.2, 38.8, 19.8, 14.8. HRMS (ESI)
3
+
m/z: [M + H] Calcd for C H N S 193.0794; Found 193.0796.
1
0
13
2
General procedure for the synthesis of dyes D2a, D11a and D14a. 3 (1 equiv.) in DMF (0.2 M), potassium hydroxide (1
equiv.), and the aldehyde (1,3 equiv.) were subjected to ultrasonic irradiation for 1 hour at room temperature. The solvent was
removed in vacuo, and the residue suspended in water. After acidification to pH 1 using concentrated HCl, the aqueous layer
was extracted with ethyl acetate to remove the aldehyde in excess. The water layer was neutralized with NaOH (1M) and
extracted with ethyl acetate. The organic layer was dried over magnesium sulfate, filtered and the volatiles were removed in
vacuo. The crude was purified by flash column chromatography on silica gel using cyclohexane/ethyl acetate mixtures as the
eluents. Finally, recrystallization from methanol afforded the desired dyes as bright solids.
1
Dye D2a.The crude product was purified using cyclohexane/ethyl acetate (8/2, v/v) as the eluent. Yield 59 %. Mp: 182-183 °C. H
NMR (400 MHz, CDCl ) δ (ppm) 7.74 (d, J = 8.6 Hz, 1H), 7.49 (d, J = 8.7 Hz, 2H), 7.32 (d, J = 16.2 Hz, 1H), 7.22 (d, J = 16.2 Hz,
3
1
3
1
1
(
H), 7.08 (d, J = 2.2 Hz, 1H), 6.92 (d, J = 8.7 Hz, 2H), 6.80 (dd, J = 8.6, 2.3 Hz, 1H), 3.84 (s, 5H). C{ H} NMR (101 MHz, CDCl
3
) δ
ppm) 163.6, 160.6, 147.5, 144.7, 136.2, 135.8, 128.7, 128.6, 123.4, 120.5, 115.7, 114.5, 105.8, 55.5. HRMS (ESI) m/z: [M + H] Calcd for
OS 283.0899; Found 283.0900.
Dye D11a. The crude product was purified using cyclohexane/ethyl acetate (8/2, v/v) as the eluent. Yield 65 %. Mp: 194-195 °C.
+
16 15 2
C H N
1
H NMR (400 MHz, CDCl
H), 7.61 - 7.50 (m, 3H), 7.44 (d, J = 16.0 Hz, 1H), 7.12 (s, 1H), 6.84 (d, J = 8.6 Hz, 1H), 3.88 (br s, 2H). C{ H} NMR (101 MHz,
CDCl ) δ (ppm) 163.1, 147.5, 145.0, 136.5, 133.9, 133.2, 132.9, 131.4, 129.5, 128.9, 126.7, 126.2, 125.8, 125.2, 124.4, 123.8, 123.6, 115.9, 105.8.
HRMS (ESI) m/z: [M + H] Calcd for
Dye D14a.The crude product was purified using cyclohexane/ethyl acetate (7/3, v/v) as the eluent. Yield 46 %. Mp: 225 °C
3
) δ (ppm) 8.25 (d, J = 8.4 Hz, 1H), 8.19 (d, J = 15.9 Hz, 1H), 7.88 (t, J = 8.8 Hz, 2H), 7.81 (t, J = 9.1 Hz,
1
3
1
2
3
+
19 15 2
C H N S 303.0950; Found 303.0951.
1
(
decomp.). H NMR (400 MHz, DMSO) δ (ppm) 8.66 (d, J = 9.4 Hz, 1H), 8.61 (d, J = 8.3 Hz, 1H), 8.48 (d, J = 15.9 Hz, 1H), 8.40 –

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8
1
.31 (m, 4H), 8.23 (d, J = 1.7 Hz, 2H), 8.12 (t, J = 7.6 Hz, 1H), 7.81 (d, J = 15.9 Hz, 1H), 7.70 (d, J = 8.7 Hz, 1H), 7.12 (d, J = 2.1 Hz,
1
3
1
H), 6.83 (dd, J = 8.7, 2.1 Hz, 1H), 5.58 (d, 2H). C{ H} NMR (101 MHz, DMSO) δ (ppm) 159.9, 147.7, 145.2, 136.2, 131.1, 130.9, 130.4,
1
30.3, 129.5, 128.2, 128.1, 127.8, 127.4, 126.5, 125.8, 125.5, 125.4, 124.9, 124.2, 123.9, 123.8, 123.1, 122.6, 115.1, 103.6. HRMS (ESI) m/z: [M
+
+
H] Calcd for
C
25
H
17
N
2
S
377.1107; Found 377.1113.
General procedure for the synthesis of dyes D1b-D14b. Precursor 5 (1 equiv.) in DMF (0.2 M), potassium hydroxide (1
equiv.) and aldehyde (1,3 equiv.) were subjected to ultrasonic irradiation for 1 hour at room temperature. The solvent was
removed in vacuo, and the crude was purified by flash column chromatography on silica gel using cyclohexane/ethyl acetate
mixtures as the eluents. Finally, recrystallization from methanol afforded the desired dyes as bright solids.
Dye D1b. The crude product was purified using cyclohexane/ethyl acetate (8/2, v/v) as the eluent. Yield 69 %. Mp: 119-120 °C.
1
H NMR (200 MHz, CDCl
3
) δ (ppm) 7.75 (d, J = 8.8 Hz, 1H), 7.55 (dd, J = 8.0, 1.5 Hz, 2H), 7.45 – 7.30 (m, 5H), 6.95 (d, J = 2.3 Hz,
1
3
1
1
H), 6.74 (dd, J = 8.8, 2.4 Hz, 1H), 3.68 (br s, 1H), 3.21 (q, J = 7.1 Hz, 2H), 1.30 (t, J = 7.1 Hz, 3H). C{ H} NMR (50 MHz, CDCl ) δ
3
+
(
ppm) 162.2, 146.9, 146.4, 136.8, 135.9, 135.5, 129.0, 127.2, 123.5, 122.7, 114.7, 101.9, 38.8, 14.9. HRMS (ESI) m/z: [M + H] Calcd for
281.1107; Found 281.1108.
Dye D2b. The crude product was purified using cyclohexane/ethyl acetate (75/25, v/v) as the eluent. Yield 78 %. Mp: 134-135 °C.
17 17 2
C H N S
1
H NMR (400 MHz, CDCl
3
) δ (ppm) 7.73 (d, J = 8.8 Hz, 1H), 7.48 (d, J = 8.7 Hz, 2H), 7.30 (d, J = 16.2 Hz, 1H), 7.22 (d, J = 16.2 Hz,
1
7
H), 6.94 (d, J = 2.3 Hz, 1H), 6.91 (d, J = 8.8 Hz, 2H), 6.73 (dd, J = 8.8, 2.3 Hz, 1H), 3.83 (s, 4H), 3.20 (q, J = 7.1 Hz, 2H), 1.29 (t, J =
1
3
1
.1 Hz, 3H). C{ H} NMR (101 MHz, CDCl ) δ (ppm) 162.6, 160.4, 146.8, 146.5, 136.6, 135.2, 128.7, 128.6, 123.3, 120.6, 114.5, 114.4,
3
+
102.0, 55.5, 38.8, 14.9. HRMS (ESI) m/z: [M + H] Calcd for C H N OS 311.1213; Found 311.1213.
18 19 2
Dye D3b. The crude product was purified using cyclohexane/ethyl acetate (8/2, v/v) as the eluent. Yield 48 %. Mp: 140-141 °C.
1
H NMR (200 MHz, CDCl
d, J = 2.3 Hz, 1H), 6.77 (dd, J = 8.8, 2.3 Hz, 1H), 3.85 (br s, 1H), 3.23 (q, J = 7.1 Hz, 2H), 1.31 (t, J = 7.1 Hz, 3H). C{ H} NMR (50
MHz, CDCl ) δ (ppm) 161.2, 147.0, 146.5, 139.4, 137.1, 133.4, 130.4 (q, J = 32.6 Hz), 127.3, 125.9 (q, J = 3.8 Hz), 125.0, 123.8, 115.0, 101.9,
3
) δ (ppm) 7.76 (d, J = 8.8 Hz, 1H), 7.63 (s, 4H), 7.44 (d, J = 16.3 Hz, 1H), 7.32 (d, J = 16.3 Hz, 1H), 6.96
1
3
1
(
3
9
1
1
+
3
3
8.9, 14.8. F{ H} NMR (188 MHz, CDCl ) δ (ppm) -62.6. HRMS (ESI) m/z: [M + H] Calcd for C H N F S 349.0981; Found
18 16 2 3
3
49.0983.
Dye D4b. The crude product was purified using cyclohexane/ethyl acetate (8/2, v/v) as the eluent. Yield 82 %. Mp: 178-179 °C.
1
3
H NMR (400 MHz, CDCl ) δ (ppm) 7.75 (d, J = 8.8 Hz, 1H), 7.51 (d, J = 7.2 Hz, 2H), 7.40 (d, J = 7.2 Hz, 2H), 7.33 (d, J = 16.1 Hz,
1
H), 7.26 (d, J = 16.2 Hz, 1H), 6.96 (s, 1H), 6.75 (d, J = 8.8 Hz, 1H), 3.96 (br s, 1H), 3.22 (q, J = 7.0 Hz, 2H), 1.31 (t, J = 7.0 Hz, 3H).
1
3
1
C{ H} NMR (101 MHz, CDCl ) δ (ppm) 161.7, 146.9, 146.5, 136.9, 134.9, 133.9, 132.2, 128.6, 123.6, 123.4, 122.9, 114.8, 101.9, 38.9, 14.9.
3
+
HRMS (ESI) m/z: [M + H] Calcd for C H N BrS 359.0212; Found 359.0219.
1
7
16
2
Dye D5b. The crude product was purified using cyclohexane/ethyl acetate (8/2, v/v) as the eluent. Yield 74 %. Mp: 129-130 °C.
1
H NMR (400 MHz, CDCl
3
) δ (ppm) 7.79 (d, J = 7.9 Hz, 1H), 7.77 (d, J = 8.8 Hz, 1H), 7.70 – 7.64 (m, 2H), 7.57 (t, J = 7.7 Hz, 1H),
7
7
.41 (t, J = 7.7 Hz, 1H), 7.36 (d, J = 16.0 Hz, 1H), 6.96 (d, J = 2.3 Hz, 1H), 6.76 (dd, J = 8.8, 2.4 Hz, 1H), 3.99 (br s, 1H), 3.22 (q, J =
1
3
1
3
.1 Hz, 2H), 1.31 (t, J = 7.1 Hz, 3H). C{ H} NMR (101 MHz, CDCl ) δ (ppm) 161.5, 147.1, 146.6, 137.1, 134.9, 132.2, 130.8 (q, J = 1.9 Hz),
1
9
1
128.5, 128.1 (q, J = 30.0 Hz), 127.3, 126.7, 126.3 (q, J = 5.7 Hz), 124.4 (d, J = 273.9 Hz), 123.8, 114.9, 102.0, 38.9, 14.8. F{ H} NMR (188
+
MHz, CDCl ) δ (ppm) -59.0. HRMS (ESI) m/z: [M + H] Calcd for C H N F S 349.0981; Found 349.0983.
3
18 16
2 3
1
Dye D6b. The crude product was purified using cyclohexane/ethyl acetate (75/25, v/v) as the eluent. Yield 79 %. 69-70 °C. H
NMR (200 MHz, CDCl ) δ (ppm) 7.74 (d, J = 8.8 Hz, 1H), 7.64 (d, J = 16.5 Hz, 1H), 7.41 (d, J = 16.4 Hz, 1H), 7.23 (dd, J = 7.9, 1.3
Hz, 1H), 7.07 (t, J = 8.0 Hz, 1H), 6.94 (d, J = 2.2 Hz, 1H), 6.88 (dd, J = 8.1, 1.4 Hz, 1H), 6.73 (dd, J = 8.8, 2.3 Hz, 1H), 3.89 (s, 3H),
3
1
3
1
3
1
.88 (s, 3H), 3.21 (q, J = 7.1 Hz, 2H), 1.29 (t, J = 7.1 Hz, 3H). C{ H} NMR (101 MHz, CDCl ) δ (ppm) 162.8, 153.2, 147.6, 146.9, 146.4,
3
+
36.8, 130.2, 130.1, 124.4, 124.2, 123.5, 118.6, 114.5, 112.7, 101.9, 61.4, 55.9, 38.8, 14.8. HRMS (ESI) m/z: [M + H] Calcd for
341.1318; Found 341.1321.
Dye D7b. The crude product was purified using cyclohexane/ethyl acetate (7/3, v/v) as the eluent. Yield 72 %. Mp: 50-51 °C. H
NMR (200 MHz, CDCl ) δ (ppm) 7.65 (d, J = 8.8 Hz, 1H), 7.23 (d, J = 16.2 Hz, 1H), 7.14 (d, J = 16.3 Hz, 1H), 6.87 (d, J = 2.2 Hz, 1H),
.70 – 6.63 (m, 3H), 3.82 (s, 6H), 3.80 (s, 3H), 3.13 (q, J = 7.1 Hz, 2H), 1.22 (t, J = 7.1 Hz, 3H). C{ H} NMR (50 MHz, CDCl
ppm) 161.9, 153.5, 146.9, 146.3, 139.0, 136.7, 135.2, 131.6, 123.4, 122.2, 114.6, 104.2, 101.9, 61.1, 56.2, 38.8, 14.8. HRMS (ESI) m/z: [M +
19 21 2 2
C H N O S
1
3
1
3
1
6
(
3
) δ
+
H] Calcd for C H N O S 371.1424; Found 371.1431.
2
0
23
2
3
Dye D8b. The crude product was purified using cyclohexane/ethyl acetate (85/15, v/v) as the eluent. Yield 36 %. Mp: 122-123 °C.
1
3
H NMR (400 MHz, CDCl ) δ (ppm) 7.93 (s, 1H), 7.88 (d, J = 8.2 Hz, 1H), 7.83 – 7.73 (m, 2H), 7.62 (d, J = 16.1 Hz, 1H), 7.41 (d, J =
1
6.0 Hz, 1H), 6.93 (d, J = 1.9 Hz, 1H), 6.75 (dd, J = 8.8, 2.1 Hz, 1H), 3.89 (br s, 1H), 3.21 (q, J = 7.1 Hz, 2H), 1.30 (t, J = 7.1 Hz, 3H).
1
3
1
C{ H} NMR (101 MHz, CDCl
3
) δ (ppm) 160.4, 147.5, 146.4, 138.6, 137.4, 130.2 (q, J = 33.6 Hz), 129.0 (q, J = 3.4 Hz), 128.9, 128.7,
1
9
1
128.4 (q, J = 30.8 Hz), 127.8, 124.1, 123.5 (q, J = 274.3 Hz), 123.6 - 123.4 (m), 123.3 (q, J = 272.3 Hz), 115.1, 101.5, 38.7, 14.8. F{ H}
+
NMR (377 MHz, CDCl ) δ (ppm) -59.7, -62.9. HRMS (ESI) m/z: [M + H] Calcd for C H N F S 417.0855; Found 417.0856.
3
19 15
2 6
Dye D9b. The crude product was purified using cyclohexane/ethyl acetate (8/2, v/v) as the eluent. Yield 59 %. Mp: 148-149 °C.
1
H NMR (400 MHz, CDCl
m, 5H), 6.78 – 6.72 (m, 2H), 3.83 (s, 3H), 3.79 (br s, 1H), 3.22 (q, J = 6.7 Hz, 2H), 1.30 (t, J = 6.7 Hz, 3H). C{ H} NMR (101 MHz,
CDCl ) δ (ppm) 162.3, 160.0, 146.8, 146.6, 136.8, 136.4, 136.1, 129.7, 128.3, 125.9, 125.0, 123.3, 114.6, 114.4, 101.9, 55.5, 38.8, 14.9. HRMS
3
) δ (ppm) 7.72 (d, J = 8.7 Hz, 1H), 7.42 (d, J = 7.3 Hz, 2H), 7.14 (dd, J = 15.0, 10.4 Hz, 1H), 6.94 – 6.82
1
3
1
(
3
+
(
ESI) m/z: [M + H] Calcd for C H N OS 337.1369; Found 337.1370.
20 21 2
Dye D10b. The crude product was purified using cyclohexane/ethyl acetate (8/2, v/v) as the eluent. Yield 77 %. Mp: 194-195 °C.
1
H NMR (400 MHz, CDCl
m, 3H), 6.97 (s, 1H), 6.75 (d, J = 8.8 Hz, 1H), 3.81 (br s, 1H), 3.23 (q, J = 6.9 Hz, 2H), 1.31 (t, J = 7.0 Hz, 3H). C{ H} NMR (101
MHz, CDCl ) δ (ppm) 162.2, 147.0, 146.5, 141.7, 140.5, 136.9, 135.0, 134.9, 129.0, 127.7, 127.7, 127.6, 127.1, 123.5, 122.7, 114.7, 101.9, 38.8,
4.9. HRMS (ESI) m/z: [M + H] Calcd for
Dye D11b. The crude product was purified using cyclohexane/ethyl acetate (8/2, v/v) as the eluent. Yield 72 %. Mp: 116-117 °C.
3
) δ (ppm) 7.76 (d, J = 8.8 Hz, 1H), 7.68 – 7.59 (m, 6H), 7.63 (s, 6H), 7.46 (t, J = 7.3 Hz, 2H), 7.42 – 7.33
1
3
1
(
3
+
1
23 21 2
C H N S 357.1420; Found 357.1421.
1
H NMR (400 MHz, CDCl
3
) δ (ppm) 8.26 (d, J = 8.1 Hz, 1H), 8.16 (d, J = 15.9 Hz, 1H), 7.84 (m, 4H), 7.55 (m, 3H), 7.44 (d, J = 15.8
1
3
1
Hz, 1H), 6.99 (s, 1H), 6.77 (d, J = 8.7 Hz, 1H), 3.89 (br s, 1H), 3.24 (q, J = 6.6 Hz, 1H), 1.32 (t, J = 6.9 Hz, 1H). C{ H} NMR (101

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DOI: 10.1039/C7NJ03142D
MHz, CDCl
14.7, 102.0, 38.9, 14.9. HRMS (ESI) m/z: [M + H] Calcd for
Dye D12b. The crude product was purified using cyclohexane/ethyl acetate (8/2, v/v) as the eluent. Yield 78 %. 74-75 °C. H
NMR (400 MHz, CDCl ) δ (ppm) 8.27 (d, J = 8.6 Hz, 1H), 7.96 (d, J = 16.4 Hz, 1H), 7.84 - 7.76 (m, 3H), 7.69 (d, J = 16.4 Hz, 1H),
.53 (t, J = 7.7 Hz, 1H), 7.39 (t, J = 7.4 Hz, 1H), 7.32 (d, J = 9.0 Hz, 1H), 6.99 (s, 1H), 6.76 (d, J = 8.7 Hz, 1H), 4.03 (s, 3H), 3.91 (br s,
3
) δ (ppm) 162.2, 147.0, 146.5, 136.9, 133.9, 133.4, 132.3, 131.3, 129.4, 128.9, 126.6, 126.2, 125.8, 125.3, 124.3, 123.7, 123.6,
+
1
21 19 2
C H N S 331.1263; Found 331.1264.
1
3
7
1
3
1
1H), 3.23 (q, J = 7.1 Hz, 2H), 1.32 (t, J = 7.1 Hz, 3H). C{ H} NMR (101 MHz, CDCl ) δ (ppm) 163.8, 156.0, 146.8, 146.6, 136.7, 132.6,
3
+
130.4, 129.3, 129.1, 128.7, 128.4, 127.2, 123.9, 123.7, 123.5, 118.2, 114.5, 113.0, 102.2, 56.4, 38.9, 14.9. HRMS (ESI) m/z: [M + H] Calcd
for
22 21 2
C H N OS 361.1369; Found 361.1369.
Dye D13b. The crude product was purified using cyclohexane/ethyl acetate (75/25, v/v) as the eluent. Yield 53 %. Mp: 180-181
1
°
C. H NMR (400 MHz, CDCl
3
) δ (ppm) 8.45 (s, 1H), 8.42 – 8.34 (m, 2H), 8.27 (d, J = 16.4 Hz, 1H), 8.04 - 8.02 (m, 2H), 7.82 (d, J =
8
.8 Hz, 1H), 7.53 – 7.48 (m, 4H), 7.29 (d, J = 16.4 Hz, 1H), 7.05 (d, J = 2.1 Hz, 1H), 6.81 (dd, J = 8.8, 2.2 Hz, 1H), 4.17 (br s, 1H), 3.27
1
3
1
(q, J = 7.1 Hz, 2H), 1.34 (t, J = 7.1 Hz, 3H). C{ H} NMR (101 MHz, CDCl ) δ (ppm) 161.9, 146.8, 146.7, 136.9, 132.2, 131.6, 131.3, 130.8,
3
+
129.7, 128.9, 127.7, 126.2, 125.8, 125.5, 123.8, 114.9, 102.4, 39.1, 14.9. HRMS (ESI) m/z: [M + H] Calcd for C H N S 381.1420; Found
25 21 2
3
81.1422.
Dye D14b. The crude product was purified using cyclohexane/ethyl acetate (7/3, v/v) as the eluent. Yield 81 %. Mp: 179-180 °C.
1
H NMR (400 MHz, CDCl
3
) δ (ppm) 8.51 (d, J = 9.3 Hz, 1H), 8.45 (d, J = 15.9 Hz, 1H), 8.34 (d, J = 8.1 Hz, 1H), 8.21 – 8.16 (m, 4H),
8
.09 – 8.00 (m, 3H), 7.81 (d, J = 8.7 Hz, 1H), 7.62 (d, J = 15.9 Hz, 1H), 7.01 (d, J = 2.2 Hz, 1H), 6.79 (dd, J = 8.7, 2.3 Hz, 1H), 4.00 (br
1
3
1
s, 1H), 3.25 (q, J = 7.1 Hz, 2H), 1.33 (t, J = 7.1 Hz, 3H). C{ H} NMR (101 MHz, CDCl
3
) δ (ppm) 162.5, 146.9, 146.8, 137.0, 132.2, 131.9,
1
31.6, 131.0, 130.2, 129.0, 128.3, 128.1, 127.7, 126.3, 125.8, 125.6, 125.4, 125.2, 125.1, 125.0, 123.8, 123.6, 122.9, 114.8, 102.2, 38.9, 14.9.
+
HRMS (ESI) m/z: [M + H] Calcd for
27 21 2
C H N S 405.1420; Found 405.1420.
General procedure for the synthesis of dyes D2c, D11c and D14c. 6-ethylamino substituted dye D2b, D11b or D14b (1
equiv.) in acetonitrile (0.2 M), 1-iodooctane (3 equiv.) and potassium carbonate (3 equiv.) were heated to reflux during 24
hours in a sealed vial. The solvent was removed under reduced pressure and the crude was purified by flash column
chromatography on silica gel, using cyclohexane/ethyl acetate mixtures as the eluents. Recrystallization from methanol
afforded the desired dyes as bright solids.
Dye D2c. The crude product was purified using cyclohexane/ethyl acetate (95/5, v/v) as the eluent. Yield 86 %. Mp: 56-57 °C.
1
H NMR (400 MHz, CDCl
3
) δ (ppm) 7.74 (d, J = 9.1 Hz, 1H), 7.47 (d, J = 8.7 Hz, 2H), 7.27 (d, J = 16.2 Hz, 1H), 7.20 (d, J = 16.2 Hz,
1
7
H), 6.96 (d, J = 2.4 Hz, 1H), 6.90 (d, J = 8.7 Hz, 2H), 6.82 (dd, J = 9.1, 2.5 Hz, 1H), 3.82 (s, 3H), 3.41 (q, J = 7.0 Hz, 2H), 3.28 (t, J =
1
3
1
.8 Hz, 2H), 1.64 – 1.58 (m, 2H), 1.33 – 1.24 (m, 10H), 1.18 (t, J = 7.0 Hz, 3H), 0.88 (t, J = 6.8 Hz, 3H). C{ H} NMR (101 MHz,
) δ (ppm) 162.1, 160.4, 146.6, 145.1, 136.9, 134.9, 128.9, 128.6, 123.1, 120.8, 114.5, 112.7, 102.1, 55.5, 51.1, 45.6, 31.9, 29.6, 29.5, 27.7,
CDCl
7.3, 22.8, 14.2, 12.5. HRMS (ESI) m/z: [M + H] Calcd for
Dye D11c. The crude product was purified using cyclohexane/ethyl acetate (9/1, v/v) as the eluent. Yield 88 %. Mp: 44-45 °C. H
NMR (200 MHz, CDCl ) δ (ppm) 7.75 (d, J = 8.8 Hz, 1H), 7.55 (dd, J = 8.0, 1.5 Hz, 2H), 7.45 – 7.30 (m, 5H), 6.95 (d, J = 2.3 Hz,
3
+
2
26 35 2
C H N OS 423.2465; Found 423.2465.
1
3
1
3
1
1H), 6.74 (dd, J = 8.8, 2.4 Hz, 1H), 3.68 (s, 1H), 3.21 (q, J = 7.1 Hz, 2H), 1.30 (t, J = 7.1 Hz, 3H). C{ H} NMR (50 MHz, CDCl ) δ
3
+
(
ppm) 162.2, 146.9, 146.4, 136.8, 135.9, 135.5, 129.0, 127.2, 123.5, 122.7, 114.7, 101.9, 38.8, 14.9. HRMS (ESI) m/z: [M + H] Calcd for
443.2515; Found 443.2516.
Dye D14c. The crude product was purified using cyclohexane/ethyl acetate (9/1, v/v) as the eluent. Yield 80 %. Mp: 130-131 °C.
29 35 2
C H N S
1
H NMR (400 MHz, CDCl
.08 – 7.99 (m, 3H), 7.85 (d, J = 9.0 Hz, 1H), 7.62 (d, J = 15.9 Hz, 1H), 7.04 (d, J = 1.9 Hz, 1H), 6.89 (dd, J = 9.0, 2.1 Hz, 1H), 3.45 (q,
J = 6.9 Hz, 2H), 3.40 – 3.28 (t, J = 7.8 Hz), 1.73 - 1.59 (m, 2H), 1.37 – 1.32 (m, 10H), 1.23 (t, J = 7.0 Hz, 3H), 0.91 (t, J = 5.9 Hz, 3H).
3
) δ (ppm) 8.50 (d, J = 9.3 Hz, 1H), 8.41 (d, J = 15.9 Hz, 1H), 8.33 (d, J = 8.1 Hz, 1H), 8.20 - 8.14 (m, 4H),
8
1
3
1
C{ H} NMR (101 MHz, CDCl
3
) δ (ppm) 161.8, 146.9, 145.3, 137.3, 131., 131.7, 131.6, 131.0, 130.2, 128.9, 128.2, 127.9, 127.6, 126.3, 125.8,
1
25.5, 125.4, 125.2, 125.2, 125.0, 123.8, 123.5, 122.9, 112.9, 102.0, 51.1, 45.6, 31.9, 29.7, 29.5, 27.7, 27.4, 22.8, 14.2, 12.6. HRMS (ESI) m/z:
+
[
M + H] Calcd for
35 37 2
C H N S 517.2672; Found 517.2674.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgments
This work was supported by CNRS, University of Nice Sophia Antipolis and CAPES (fellowship to MSF, process number
99999.0011495/2015-01)
Associated Content
Supporting Information
1
19
13
Figures and tables, copies of H, F and C NMR and HRMS spectra of all reported compounds, spectra from the photophysical
study and additional data related to DFT calculation. This material is available free of charge via the Internet at
http://pubs.acs.org.
References
1
A. S. Klymchenko, Acc. Chem. Res., 2017, 50, 366–375.
2
M. Dell’Acqua, L. Ronda, R. Piano, S. Pellegrino, F. Clerici, E. Rossi, A. Mozzarelli, M. L. Gelmi and G. Abbiati, J.
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