Reinvestigation of the synthesis of “covalent-assembly” type probes for fluoride ion detection. Identification of novel 7-(diethylamino)coumarins with aggregation-induced emission properties

Article abstract of DOI:10.1016/j.tetlet.2019.151279

An unprecedented C-3 functionalization of 4-(diethylamino)salicylaldehyde through a Friedel-Crafts type alkylation reaction has been discovered during the synthesis of “covalent-assembly”-based fluorescent probes for detection of fluoride ions. The resulting Friedel-Crafts adduct was successfully used for the preparation of two novel 8-substituted 7-(diethylamino)coumarin dyes. The photophysical study of these fluorophores has enabled us to highlight their remarkable aggregation-induced emission (AIE) properties characterized by a yellow-orange emission of aggregates in water. Therefore, 4-(tert-butyldimethylsilyloxy)benzyl substituent was identified as a novel AIE-active moiety which could be seen as a possible alternative to popular tetraphenylethylene (TPE).

Full text of DOI:10.1016/j.tetlet.2019.151279

Tetrahedron Letters xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Reinvestigation of the synthesis of ‘‘covalent-assembly” type probes for
fluoride ion detection. Identification of novel 7-(diethylamino)coumarins
with aggregation-induced emission properties
Valentin Quesneau a, Benoît Roubinet b, Pierre-Yves Renard b, Anthony Romieu a,
⇑
a ICMUB, UMR 6302, CNRS, Univ. Bourgogne Franche-Comté, 9, Avenue Alain Savary, 21078 Dijon Cedex, France
b Normandie Univ, UNIROUEN, INSA Rouen, CNRS, COBRA (UMR 6014), IRCOF, 1 Rue Tesnières, 76821 Mont-Saint-Aignan Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
An unprecedented C-3 functionalization of 4-(diethylamino)salicylaldehyde through a Friedel-Crafts type
alkylation reaction has been discovered during the synthesis of ‘‘covalent-assembly”-based fluorescent
probes for detection of fluoride ions. The resulting Friedel-Crafts adduct was successfully used for the
preparation of two novel 8-substituted 7-(diethylamino)coumarin dyes. The photophysical study of these
fluorophores has enabled us to highlight their remarkable aggregation-induced emission (AIE) properties
characterized by a yellow-orange emission of aggregates in water. Therefore, 4-(tert-butyldimethylsily-
loxy)benzyl substituent was identified as a novel AIE-active moiety which could be seen as a possible
alternative to popular tetraphenylethylene (TPE).
Received 3 September 2019
Revised 9 October 2019
Accepted 11 October 2019
Available online xxxx
Keywords:
Aggregation-induced emission (AIE)
Coumarin
Covalent-assembly
Fluorescent probe
Ó 2019 Elsevier Ltd. All rights reserved.
Introduction
area, pioneering works of the Swager (2003) [4] and Anslyn
(2005) groups [2a], on the fluorescent detection of fluoride anions
Among the various classes of organic-based fluorophores cur-
rently used for the construction of small molecule fluorescent
chemosensors/chemodosimeters, coumarin derivatives figure
prominently because of several valuable features including: (1)
convenient synthetic accessibility, (2) good structural flexibility,
(3) valuable spectral features (large Stokes shift and high fluores-
cence quantum yield), (4) easy modulation of their fluorescence
properties through the implementation of different photophysical
processes (e.g., PeT, ICT, FRET, . . .) and/or protection-deprotection
of an optically tunable amino or hydroxyl group introduced at
the C-7 position and (5) low toxicity [1]. Consistently seeking to
optimize the fluorogenic response (mainly, intensometric detec-
tion mode) arising from selective interaction/reaction of the cou-
marin-based probe with its supposed target (bio)analyte, a new
probe design principle namely the ‘‘covalent-assembly” approach
has recently emerged [2,3]. This cutting-edge strategy is based
on in situ formation of a fluorophore from a non-fluorescent com-
pound (also known as caged precursor) and through an effective
intramolecular reaction triggered/catalyzed by the species to be
detected. The fundamental feature of this approach is that it guar-
antees, in theory at least, a ‘‘turn-on” fluorimetric signal from a
zero background, hence an optimal detection sensitivity. In this
and divalent heavy metal cations respectively, are based on in situ
formation of 7-(diethylamino)coumarin (DEAC) derivatives
through lactonisation or Heck-type cyclization reactions. Since
these early remarkable achievements, more than 50 examples of
‘‘covalent-assembly” type probes for the detection of a wide range
of analytes including biothiols, enzymes, metal cations and ROS/
RNS, have already been published [5]. In all these cases, creation
of blue-green emitting 7-dialkylamino/7-hydroxy-(2-imino)-
coumarins or related fluorophores through lactonisation or Pinner
cyclization reactions, is the keystone of the sensing mechanism
(Fig. 1).
In the specific case of such reaction-based chemodosimeters
devised for the selective sensing of fluoride ions whose detec-
tion/quantification in drinking or surface water is very important
for public health, the analyte-responsive caged precursor is a
O-protected
2-(dialkylamino)-4-hydroxycinnamonitrile
(or
cinnamate ester) derivative and the phenol is masked as a
tert-butyldimethylsilyl (TBDMS) or tert-butyldiphenylsilyl (TBDPS)
ether (Fig. 1). The direct linkage of this triggering unit to the phenol
moiety may negatively affect probe properties such as poor stabil-
ity and slow response time. By analogy with some colorimetric,
fluorescent or chemiluminescent probes recently published in the
literature [6], a self-immolative spacer can be used to circumvent
these possible issues. In this context, we wished to explore the
synthesis and fluorogenic reactivity (towards fluoride ions) of
⇑
Corresponding author.
E-mail address: anthony.romieu@u-bourgogne.fr (A. Romieu).
https://doi.org/10.1016/j.tetlet.2019.151279
0040-4039/Ó 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: V. Quesneau, B. Roubinet, P. Y. Renard et al., Reinvestigation of the synthesis of ‘‘covalent-assembly” type probes for fluoride ion
detection. Identification of novel 7-(diethylamino)coumarins with aggregation-induced emission properties, Tetrahedron Letters, https://doi.org/10.1016/
j.tetlet.2019.151279

2
V. Quesneau et al. / Tetrahedron Letters xxx (xxxx) xxx
Fig. 1. (Top) ‘‘Covalent-assembly” probe design principle applied to fluorogenic detection of (bio)analytes through in situ synthesis of 7-dialkylamino/7-hydroxy-2-
iminocoumarins or 7-dialkylamino/7-hydroxycoumarins; (bottom) examples of ‘‘covalent-assembly” type probes for F- detection reported in the literature (BZT = 2-
benzothiazolyl, TBDMS = tert-butyldimethylsilyl, TBDPS = tert-butyldiphenylsilyl) [4,5f,i,k,n]. aPlease note: fused julolidine, 4-dimethylaminophenyl and 1,4-diethylpiperazine
fragments are also frequently used in the design of such ‘‘caged” precursors, to red-shift spectral features of in situ formed coumarin.
next-generation ‘‘covalent-assembly” type probes that incorporate
a self-immolative para-hydroxybenzyl alcohol (PHBA) linker [5j,7].
During the course of this work, we have highlighted an unexpected
SEAr reaction leading to regioselective C-3 benzylation of
4-(diethylamino)salicylaldehyde. Since this type of building
blocks is frequently used as starting material for the synthesis
of coumarins, we next considered the Knoevenagel
condensation between this 2-hydroxybenzaldehyde derivative
and C-nucleophiles (i.e., malononitrile and benzothiazole-2-
acetonitrile) to produce novel DEAC-based fluorophores.
In this Letter, in the course of our research on self-immolative
linker based second generation ‘‘covalent-assembly” probes for flu-
oride ions, we report our findings on the unusual reactivity of 4-
(diethylamino)salicylaldehyde towards 4-(tert-butyldimethylsily-
loxy)benzyl bromide. This unprecedented reaction led us to con-
sider the preparation and photophysical characterization of novel
DEAC-based fluorophores. A comparative study of their spectral
features with those of parent DEAC derivatives lacking a C-8 sub-
stituent, and conducted in different solvents (aq. buffers, DMSO
and EtOH), has put in evidence an expected added value for these
coumarin-based fluorophores, namely aggregation-induced emis-
sion properties.
used to connect salicylaldehyde and recognition moiety, step (1)
is often a 2-O-alkylation reaction performed with a benzyl bromide
derived from PHBA. To the best of our knowledge, this latter strat-
egy was never applied to the design of fluoride-sensitive caged pre-
cursors of coumarins and TBDMS or TBDPS moiety is always
directly attached to the phenol oxygen atom of salicylaldehyde
derivative (Fig. 1) [4,5f,i,k,n]. In order to fill this gap and to assess
the performances of ‘‘covalent-assembly” type probes bearing a
fluoride-sensitive self-immolative linker, we planned the synthesis
of fluorescent chemodosimeters 1 and 2 (Fig. 2). This entailed us,
first, to revisit the synthesis of alkylating agent namely 4-(tert-
butyldimethylsilyloxy)benzyl bromide (4-OTBDMS benzyl bro-
mide) 3, because the published procedures are tedious and time-
consuming.
Revisited synthesis of 4-OTBDMS benzyl bromide 3
Published synthetic procedures towards 4-OTBDMS benzyl bro-
mide 3 are based on a four-step reaction sequence (Scheme 1, top)
Results and discussion
The vast majority of coumarin-based ‘‘covalent-assembly” type
probes reported in the literature (see Fig. 1 for the general struc-
ture), are readily prepared via a well-established two-step process:
(1) protection of the phenol group of salicylaldehyde derivative
(mostly, 2-OH of 4-(diethylamino)salicylaldehyde or 2,4-dihydrox-
ybenzaldehyde) with the selected trigger-recognition unit and sub-
sequent (2) Knoevenagel condensation with malononitrile or
related C-nucleophiles. In cases where a self-immolative spacer is
Fig. 2. Self-immolative ‘‘covalent-assembly” type probes for F- detection and based
on the use of PHBA linker (BZT = 2-benzothiazolyl, TBDMS = tert-butyldimethylsi-
lyl), targeted in our project.
Please cite this article as: V. Quesneau, B. Roubinet, P. Y. Renard et al., Reinvestigation of the synthesis of ‘‘covalent-assembly” type probes for fluoride ion
detection. Identification of novel 7-(diethylamino)coumarins with aggregation-induced emission properties, Tetrahedron Letters, https://doi.org/10.1016/
j.tetlet.2019.151279

V. Quesneau et al. / Tetrahedron Letters xxx (xxxx) xxx
3
not univocal with the formation, in addition to the desired benzyl
ether 4, of an unknown product which was isolated by column
chromatography over silica gel in a 10% to 25% yield (depending
on the reaction attempt and age of benzylation reagent 3). The
same mixture of these two compounds was obtained when
K2CO3 was replaced by NaH (60% dispersion in mineral oil). The
unknown compound is less polar than 4 but has the same molec-
ular mass as revealed by the HPLC-MS analyses (see Figs. S6, S7,
S13 and S14). Thus, two hypothetical structures 5 and 6 resulting
either from quaternarization of the N,N-diethylanilino moiety (SN
reaction) or from Friedel-Crafts type alkylation of the electron-rich
benzene ring (SEAr reaction) can be theoretically put forward. Grat-
ifyingly, a comprehensive NMR study based on 1D and 2D experi-
ments (i.e., COSY and NOESY, see Figs. S11 and S12) confirmed both
the lack of aromatic proton H-3, the ortho coupling between pro-
tons H-5 and H-6 (J = 8.5 Hz) and the spatial proximity between
benzylic methylene protons of 4-OTBDMS benzyl moiety and
CH2-protons of diethylamino substituent. We may thus conclude
that unknown product formed during the O-alkylation process is
the Friedel-Crafts adduct 6. Such C-3-alkyl-substituted 4-(diethy-
lamino)salicylaldehyde derivatives have never been described
and their formation under mild conditions as these presented here,
is somewhat surprising even if the corresponding benzene ring is
electronically enriched by phenolic hydroxyl (deprotonated form
in the reaction mixture containing K2CO3) and diethylamino donat-
ing groups. Indeed, the sole example of Fridel-Crafts type C-2 ben-
zylation of a 1,3-diheterosubstitued benzene was reported by
Kumar et al. and it was the reaction between resorcinol (1,3-dihy-
droxybenzene) and benzyl chloride performed under refluxing
xylene (140 °C) for 8 h. In this case, a mixture of mono- (C-2), di-
(C-2/C-4) and tri-benzyl (C-2/C-4/C-6) adducts in the ratio 6:3:1
was obtained (combined yield 82%) [9]. The regioselectivity of Frie-
del-Crafts type alkylation leading to 6, is in agreement with the
Holleman rules since diethylamino and phenol groups are ortho/-
para directing and formyl is known to be meta directing group
[10]. To gain further information about the possible scope of this
reaction, we implemented this synthetic procedure with other
alkylating agents including 4-nitrobenzyl bromide and 4-(2,4-dini-
trophenyloxy)benzyl bromide but O-alkylation of 4-(diethylamino)
salicylaldehyde was overwhelmingly observed and only traces of
Friedel-Crafts product was detected. This result is consistent with
the synthesis of ‘‘covalent-assembly” type probes presented above
and for which this latter undesired reaction was never mentioned.
One possible interpretation of this peculiar reactivity is that the
formation of benzyl-type carbocation (the reactive intermediate
required for SEAr reactions on salicylaldehyde derivative) is
favored only in the case 4-OTBDMS benzyl bromide due to the elec-
tron-donating ability of its para-substituent (i.e., +M effect of the
silyloxy group vs. -M effect of nitro and 2,4-dinitrophenyloxy moi-
eties). Also noteworthy and in agreement with our hypotheses, is
that when we assayed the reaction of 4-(diethylamino)salicylalde-
hyde with 4-OTBDMS benzyl alcohol under Mitsunobu conditions
(i.e., treatment with PPh3 and DIAD in Et2O) [11] the sole observed
product was O-alkylated product 4.
Scheme 1. (Top) Published synthesis of 4-OTBDMS benzyl bromide 3 [8]; (bottom)
shortened synthesis of 4-OTBDMS benzyl bromide 3 devised by us (DBU = 1,8-
diazabicyclo[5.4.0]undec-7-ene,
TFAA = trifluoroacetic anhydride).
TBDMS-Cl = tert-butyldimethylsilyl
chloride,
[8]. First, selective protection of the phenol of 4-hydroxybenzalde-
hyde with TBDMS-Cl, followed by NaBH4-mediated reduction of
the formyl moiety provided 4-OTBDMS benzylic alcohol. Next, in
order to provide a good leaving group, this alcohol was esterified
with trifluoroacetic anhydride (TFAA), and finally this trifluoroac-
etate underwent nucleophilic reaction with LiBr. This procedure
enables obtaining 4-OTBDMS benzyl bromide 3 with a satisfactory
40% yield, yet requires tedious purification steps. To shorten the
overall number of steps, we have explored direct and selective sily-
lation of PHBA (phenol vs. primary alcohol) with TBDMS-Cl in the
presence of a base (Scheme 1, bottom). Several organic and inor-
ganic bases (i.e., DBU, DIEA, Cs2CO3 and K2CO3) were tested and
DBU appeared to be the best one providing a satisfying 65% iso-
lated yield. Thereafter, the implementation of Appel reaction (i.e.,
treatment of alcohol with CBr4 and PPh3 in MeCN) enabled the
quantitative conversion of 4-OTBDMS benzylic alcohol into the tar-
geted benzyl bromide derivative 3. However, the poor stability of
this latter compound over silica gel prevents its chromatographic
isolation in a pure form and with a good yield (only 39% was
obtained). To overcome this issue, we used liquid-liquid extrac-
tions with heptane to recover pure 3 and with an acceptable yield
of 55%. Its structure was unambiguously confirmed by NMR anal-
yses and comparison with published spectroscopic data (see Sup-
plementary data and Figs. S1 and S2). If the overall yield is
slightly lower than the published procedure (36% vs. 40%), this
new procedure competes thanks to a lower number of steps and
easier purification processes.
With this alkylating reagent in hand, we next examined its reac-
tivity towards 4-(diethylamino)salicylaldehyde with the aim of
rapidly synthesizing self-immolative ‘‘covalent-assembly” type
probes 1 and 2.
To check the reactivity of the salicylaldehyde part of the mole-
cule 6 and to access to novel 7-(diethylamino)coumarins, we have
then explored its condensation with malononitrile and benzothia-
zole-2-acetonitrile.
2-O-Alkylation of 4-(diethylamino)salicylaldehyde with 4-OTBDMS
benzyl bromide 3
By analogy with the self-immolative caged precursors of 7-(di-
alkylamino)-(2-imino)coumarins already published, especially
those bearing either a 4-(pinacolboronate)benzyl [5p] or a 4-
nitrobenzyl moiety [5am], reactive towards RNS/ROS and nitrore-
ductase (NTR) enzymes respectively, phenol alkylation of 4-
(diethylamino)salicylaldehyde was achieved with 1 equiv. of 3, in
the presence of K2CO3 (2 equiv.) as base, in MeCN and at room tem-
perature (Scheme 2). Surprisingly, this reaction was found to be
Synthesis of 8-substituted-7-(diethylamino)coumarins 7 and 8
The synthesis of both coumarins 7 and 8, through Knoevenagel
condensation reaction, was achieved under conventional condi-
tions: treatment with C-nucleophile (1 equiv.), in the presence of
piperidine (1 equiv.) and anhydrous Na2SO4 (2 equiv.), in EtOH at
room temperature (Scheme 3). Purification by column chromatog-
Please cite this article as: V. Quesneau, B. Roubinet, P. Y. Renard et al., Reinvestigation of the synthesis of ‘‘covalent-assembly” type probes for fluoride ion
detection. Identification of novel 7-(diethylamino)coumarins with aggregation-induced emission properties, Tetrahedron Letters, https://doi.org/10.1016/
j.tetlet.2019.151279

4
V. Quesneau et al. / Tetrahedron Letters xxx (xxxx) xxx
Scheme 2. (Top) Published syntheses of NTR- and peroxynitrite-responsive self-immolative ‘‘covalent-assembly” type probes based on O-alkylation of 4-(diethylamino)
salicylaldehyde with 4-nitrobenzyl bromide or 4-(pinacolboronate)benzyl bromide and subsequent Knoevenagel condensation reaction (TBAOH = tetrabutylammonium
hydroxide) [5p,am]; (bottom) alkylation of 4-(diethylamino)salicylaldehyde with 4-OTBDMS benzyl bromide 3 leading to unexpected Friedel-Crafts alkylation product 6.
two-step Knoevenagel-cyclization sequence actually leads to 2-
iminocoumarin derivatives) occurred during the purification
process.
In order to assess the effect of 4-OTBDMS benzyl moiety on pho-
tophysical properties of theses novel 7-(diethylamino)coumarin
derivatives, we have also prepared the parent compounds 9 and
10 lacking this C-8 substituent (Scheme 4). 3-Cyano-7-(diethy-
lamino)coumarin 9 was prepared from 4-(diethylamino)salicy-
laldehyde and ethyl cyanoacetate, according to
a literature
procedure [12]. As for the Knoevenagel condensation with 2-(2-
benzothiazolyl)acetic acid ethyl ester, reaction time was dramati-
cally shortened using microwave irradiation (20 min at 160 °C vs.
6 h under refluxing MeOH [13]), and purification by column chro-
matography provided 10 with a good 87% yield. For these four
compounds, all spectroscopic data, especially IR, NMR and mass
spectrometry, were in agreement with the structures assigned
(see Supplementary data).
Scheme 3. Synthesis of 8-substituted 7-(diethylamino)coumarin dyes 7 and 8 (CC
(SiO2) = column chromatography over silica gel).
raphy over silica gel provided the 7-(diethylamino)coumarins 7
and 8 in a satisfying 60% yield even if an additional purification
by trituration with MeOH was required to recover 7 in a pure form.
It is important to note that the hydrolysis of imine moiety (this
Photophysical characterization of DEAC-based fluorophores
The photophysical properties of synthesized 7-(diethylamino)-
coumarins 7–10 were evaluated in DMSO (solvent in which these
Scheme 4. Synthesis of 7-(diethylamino)coumarin dyes 9 and 10 (MW = microwave irradiation).
Please cite this article as: V. Quesneau, B. Roubinet, P. Y. Renard et al., Reinvestigation of the synthesis of ‘‘covalent-assembly” type probes for fluoride ion
detection. Identification of novel 7-(diethylamino)coumarins with aggregation-induced emission properties, Tetrahedron Letters, https://doi.org/10.1016/
j.tetlet.2019.151279

V. Quesneau et al. / Tetrahedron Letters xxx (xxxx) xxx
5
compounds are perfectly soluble and used for the preparation of
their stock solutions), in EtOH and in phosphate-buffered saline
(PBS, pH 7.5) which mimics physiological conditions. Fluorophores
7–10 were found to be perfectly soluble in PBS only at low
absorption band, quite probably because the formation of aggre-
gates especially in the case of derivative 8 which bears the
hydrophobic benzothiazolyl C-3 substituent (e.g.,
Dk1/2max values:
89–93 nm for 8 vs. 64–65 nm for its C-8 unsubstituted counterpart
10). The lack of a significant blue-green fluorescence emission
upon excitation at 410 nm (UF = 1–2%), confirms the preponder-
ance of non-emissive species in solution and a possible radiation
less de-excitation pathway caused by rotational motions of
4-OTBDMS benzyl moiety. The spectral signature in aq. buffer
(PBS) is somewhat different because 7 and 8 exhibit a wide emis-
sion band ranging from 500 to 700 nm (centered at ca. 560 nm)
which is consistent with the behavior described by an aggrega-
tion-induced emission (AIE) effect [14].
concentrations (less than 5 lM). At higher concentrations, light
scattering by suspension was clearly observed on UV–vis absorp-
tion spectra (Fig. 3C and Figs. S21, S22, S32, S33, S36 and S39).
All results are compiled in Table 1 (see Fig. 3 for the absorption/
fluorescence spectra of 8 and Supplementary data for the absorp-
tion/fluorescence spectra of compounds 7, 9 and 10). In polar
organic solvents (DMSO and EtOH), the C-8 substitution of DEAC
scaffold caused the broadening and dramatic blue-shift of the
To confirm this property, the fluorescence behavior of 7 and 8 in
water/EtOH mixtures was studied. As displayed in Fig. 4, in pure
EtOH, 8 shows weak fluorescence centered at 500 nm. When water
fraction varies from 10% to 50%, the emission marginally decreases
but without alterations in the shape of emission curve. This is
probably related to the aggregation-caused quenching (ACQ)
effect. When the fraction of water is greater than 50%, the emission
maximum peak red shifts to 560 nm along with a fluorescence
intensity increase evidently.
As water is a poor solvent of 8, the addition of water would
induce the formation of aggregates, that hinders the intramolecu-
lar rotational motions of 4-OTBDMS benzyl moiety, thus blocking
the non-radiative relaxation pathways and boosting the yellow-
orange emission (i.e., restriction of intramolecular rotations (RIR)
mechanism) [14]. Moreover, the fluorescence quantum yield of 8
in the solid state was measured to be 7% for an emission maximum
centered at 534 nm (see Supplementary data for experimental
details related to the determination of solid state absolute fluores-
cence quantum yields and Figs. S41–S44). The same behavior was
observed with the less hydrophobic 3-cyano derivative 7 but the
formation of strongly emissive aggregates was occurred at a higher
fraction of water in EtOH (>70%) (Fig. S45). As expected, this AIE
effect was found to be negligible in the case of parent C-8 unsub-
stituted 7-(diethylamino)coumarins 9 and 10 (Figs. 4 and S45).
Indeed, the shape and position of the fluorescence emission spec-
trum of 3-cyano derivative 9 are similar whatever the solvent used
for the spectral measurements. The fluorescence quantum yield is
also in the same order of magnitude. For its part, the 3-(2-benzoth-
iazolyl) derivative 10 emits intense green light both in DMSO and
in EtOH (UF = 67% and 75% respectively), but its dissolution in PBS
causes a large bathochromic shift in the emission color with an
almost complete quenching of its fluorescence (UF = 2.5%). These
features are consistent with an ACQ effect instead of AIE character-
istics displayed by this more hydrophobic derivative. To clarify
which rotors of 4-OTBDMS benzyl moiety are responsible for the
AIE effect, we have finally synthesized (i.e., aq. TFA deprotection
of TBDMS ether of 8) and studied photophysical properties of the
8-(4-hydroxybenzyl) derivative 11. This compound displays the
same spectral features and AIE behavior than that for 8 (Figs. 4
and S39 and S40), even if the fluorescence emission efficiency of
aggregates in water is lower (UF 2% vs. 7% for 8). These results
may indicate that the AIE activity of 7 and 8 is mainly ascribed
to rotational motions of the benzyl moiety and not those of silyl
group.
To the best of our knowledge, few works have already described
the facile functionalization of 2H-chromen-2-one scaffold with
AIE-active moieties to generate coumarin-based AIE fluorophores
with long-wavelength emission. Worth mentioning in this context
are 7-(diethylamino)coumarin or 4-methylumbelliferone Schiff
base derivatives bearing an AIE-active substituent connected to
C-3 or C-8 position via an imine linkage [16]. Some of them have
been used as fluorescent probes for effective detection of biothiols
(cysteine) or heavy metal cations such as Hg(II) [16a,b].
Fig. 3. Normalized absorption (blue), excitation (Em. 525 or 600 nm, slit 5 nm,
green) and emission (Ex. 410 nm, slit 5 nm, red) spectra of coumarin 8 in DMSO (A),
EtOH (B) and PBS (C) at 25 °C. Please note: the recorded absorption spectrum of 8 in
PBS is due both to light scattering (by aggregates) and absorption.
Please cite this article as: V. Quesneau, B. Roubinet, P. Y. Renard et al., Reinvestigation of the synthesis of ‘‘covalent-assembly” type probes for fluoride ion
detection. Identification of novel 7-(diethylamino)coumarins with aggregation-induced emission properties, Tetrahedron Letters, https://doi.org/10.1016/
j.tetlet.2019.151279

6
V. Quesneau et al. / Tetrahedron Letters xxx (xxxx) xxx
Table 1
Photophysical properties of DEAC-based fluorophores studied in this work, determined at 25 °C.
Cmpda
Solvent
kmax Abs (nm)
kmax Em (nm)
D
k1/2max (nm)
e
(MÀ1 cmÀ1
)
UF (%)b
7
7
7
8
8
8
9
9
9
9
10
10
10
PBS, pH 7.5
DMSO
EtOH
PBS, pH 7.5
DMSO
EtOH
PBS, pH 7.5
PBS + 5% EtOH, pH 7.5
DMSO
EtOH
PBS, pH 7.5
DMSO
419
411
405
423
414
405
352, 431
352, 431
430
561
468
467
556
506
502
479
479
478
467
557
516
503
90
88
76
112
93
89
17 700
16 300
16 400
20 550
23 800
22 900
12 200, 6 000
10 700, 7 100
46 600
2
–
–
7
1.5
2
–
1.5
3.5
3
2.5
67
75
c
c
d
c
–
–
d
49
49
423
463, 498
468
46 900
17 200, 18 000
56 400
d
–
65
64
EtOH
459
54 100
a
Stock solutions (1.0 mg/mL) of fluorophores were prepared in DMSO.
Determined using coumarin 153 as a standard (UF = 38% in EtOH, Ex. at 410 nm) except for 7 in PBS, Ru(bpy)3Cl2 was used in this case (UF = 2.8% in water saturated with
b
air, Ex. at 410 nm) [15].
c
Very low or no fluorescence.
Not determined due to the presence of two maxima.
d
Fig. 4. Demonstration of AIE properties of 8-substituted 3-(2-benzothiazolyl)-7-(diethylamino)coumarin 8. (A, left) Fluorescence emission (integrated between 425 and
800 nm) of 8, 10 and 11 vs. water fraction in EtOH/water mixtures (concentration = 1.0
lM, Ex. at 410 nm, Em. 425–800 nm, Ex./Em. slit = 5 nm for 8 and 11 and 2 nm for 10);
(A, right) schematic representation of intramolecular rotations in coumarins 8 and 11; (B) Photographs of 8 (15
lM) in PB/EtOH (fraction aq. buffer vol%) mixtures taken
under 365 nm UV illumination (PB = phosphate buffer, 100 mM, pH 7.6). Please note: for the recording of Em. spectra, water was used instead of PB to avoid interferences causing
by partial precipitation of phosphate salts in EtOH as observed in Fig. 4B. Suspension observed in quartz cells (Fig. 4B) was due to phosphate salts precipitation at low fractions of aq.
buffer, and to partial precipitation of coumarin 8 at high fractions of aq. buffer.
two novel 7-(diethylamino)coumarins through conventional Kno-
evenagel reaction. A comprehensive photophysical study of these
fluorophores led both to the discovery of their AIE properties and
the identification of 4-OTBDMS benzyl as a novel AIE-active moi-
ety. The conversion of conventional 7-(diethylamino)coumarin
dyes to AIE fluorophores is thus realized by a simple transforma-
tion (Friedel-Crafts alkylation), avoiding laborious and more com-
plex synthetic operations sometimes associated with the
incorporation of popular AIE-active moieties (e.g., tetraphenylethy-
lene, TPE) onto fluorescent scaffolds [14,17]. We are confident that
this strategy and others related could be applied to the introduc-
Conclusion
In summary, we have reported our findings related to the
unprecedented C-3 functionalization of 4-(diethylamino)salicy-
laldehyde through a Friedel-Crafts alkylation reaction conducted
under mild conditions and with electron-rich 4-OTBDMS benzyl
bromide 3. This unusual reaction is not applicable to a wide range
of para-substituted benzyl bromide derivatives, especially those
currently employed for the synthesis of self-immolative fluores-
cent probes for (bio)sensing purposes. However, the availability
of unusual salicylaldehyde derivative 6 has enabled us to prepare
Please cite this article as: V. Quesneau, B. Roubinet, P. Y. Renard et al., Reinvestigation of the synthesis of ‘‘covalent-assembly” type probes for fluoride ion
detection. Identification of novel 7-(diethylamino)coumarins with aggregation-induced emission properties, Tetrahedron Letters, https://doi.org/10.1016/
j.tetlet.2019.151279

V. Quesneau et al. / Tetrahedron Letters xxx (xxxx) xxx
7
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The authors declare that they have no known competing finan-
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Acknowledgments
This work is supported by the CNRS, Université de Bourgogne
and Conseil Régional de Bourgogne through the ‘‘Plan d’Actions
Régional pour l’Innovation (PARI) and the ”Fonds Européen de
Développement Régional (FEDER)‘‘ programs. B. R. gratefully
acknowledges the ”Région-Haute Normandie‘‘ for his Ph. D. grant
(2012-2015). Financial support from Agence Nationale de la
Recherche (ANR, AAPG 2018, DetectOP_BChE, ANR-18-CE39-
0014), especially for the post-doc fellowship of V. Q. is also greatly
acknowledged, as well as Labex SynOrg (ANR-11-LABX-0029). The
`
authors thank the ”Plateforme d’Analyse Chimique et de Synthese
´
´
Moleculaire de l’Universite de Bourgogne‘‘ (PACSMUB, http://
www.wpcm.fr) for access to spectroscopy instrumentation and Iris
Biotech company for the generous gift of some chemical reagents
used in this work. The authors also thank Marcel Soustelle (Univer-
sity of Burgundy, ICMUB, UMR CNRS 6302) for elemental analyses,
Prof. Ewen Bodio (University of Burgundy, ICMUB, UMR CNRS
6302, OCS team) for access to SAFAS Flx-Xenius XC spectrofluo-
rimeter (equipped with a BaSO4 integrating sphere), Dr. Jean-
Franck Bussotti / M. Olivier Chaudon (SAFAS Monaco company)
for their precious advice regarding the determination of absolute
fluorescence quantum yields, and Drs. Kévin Renault and Ibai E.
Valverde (ICMUB, UMR CNRS 6302) for helpful discussions and rel-
evant comments on this manuscript before publication.
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