Communication of Bichromophore Emission upon Aggregation – Aroyl-S,N-ketene Acetals as Multifunctional Sensor Merocyanines

Article abstract of DOI:10.1002/chem.202102052

Aroyl-S,N-ketene acetal-based bichromophores can be readily synthesized in a consecutive three-component synthesis in good to excellent yields by condensation of aroyl chlorides and an N-(p-bromobenzyl) 2-methyl benzothiazolium salt followed by a Suzuki coupling, yielding a library of 31 bichromophoric fluorophores with substitution pattern-tunable emission properties. Varying both chromophores enables different communication pathways between the chromophores, exploiting aggregation-induced emission (AIE) and energy transfer (ET) properties, and thus, furnishing aggregation-based fluorescence switches. Possible applications range from fluorometric analysis of alcoholic beverages to pH sensors.

Full text of DOI:10.1002/chem.202102052

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Communication of Bichromophore Emission upon
Aggregation – Aroyl-S,N-ketene Acetals as Multifunctional
Sensor Merocyanines
Lukas Biesen,[a] Lars May,[a] Nithiya Nirmalananthan-Budau,[b] Katrin Hoffmann,[b]
Ute Resch-Genger,*[b] and Thomas J. J. Müller*[a]
Dedicated to Prof. Dr. Barry M. Trost on the occasion of his 80th birthday
Abstract: Aroyl-S,N-ketene acetal-based bichromophores can
be readily synthesized in a consecutive three-component
synthesis in good to excellent yields by condensation of aroyl
chlorides and an N-(p-bromobenzyl) 2-methyl benzothiazo-
lium salt followed by a Suzuki coupling, yielding a library of
31 bichromophoric fluorophores with substitution pattern-
tunable emission properties. Varying both chromophores
enables different communication pathways between the
chromophores, exploiting aggregation-induced emission (AIE)
and energy transfer (ET) properties, and thus, furnishing
aggregation-based fluorescence switches. Possible applica-
tions range from fluorometric analysis of alcoholic beverages
to pH sensors.
Introduction
increasingly popular concept behind such multichromophoric
reporters and switches is aggregation induced emission (AIE)[9]
or aggregation induced enhanced emission (AIEE).[10] These
phenomena exploit deactivation of nonradiative pathways by
restriction of intramolecular motions (RIM) or by restricted
access to conical intersections (RACI).[10b] For fluorophore
conjugates, which undergo partial or frustrated ET and also
respond to external stimuli, such as solvent polarity, pH (e.g.,
by excited state intra-molecular proton transfer (ESIPT)),[11] or
the presence of a specific target/analyte, multi-parametric
responses can be generated.[12–14]
Essential for the generation of novel functional organic dyes
and molecular photonic materials is the control of involved
communication pathways and intra- and intermolecular
interactions.[1] This can be elegantly achieved with multi-
chromophoric systems, consisting of covalently linked and
spatially (within the range of the Förster radius) separated, non-
conjugated fluorophores,[2] which interact by distance- and
orientation-dependent energy transfer (ET) processes.[3] Switch-
ing from individually emitting fluorophores in solution to
rigidized or aggregated systems in the solid-state, in aggregates
or incorporated into nanoarchitectures can enable the tuning of
the emission intensity and color, providing dual emission Results and Discussion
(aggregation induced dual emission, AIDE)[4] or white light
generation for applications in photonic[5] and sensor
technologies,[6] bioanalysis,[7] and medicinal diagnostics.[8] The
Synthesis
Recently, we presented a novel AIE-active chromophore system
with tunable solid-state emission utilizing aroyl-S,N-ketene
acetals.[15] Essential for the occurrence of AIE is N-benzyl
substitution. The promising photophysical and AIE properties of
these chromophores encouraged us to develop a novel highly
diversity-oriented, rapid modular one-pot synthesis[16] of N-
benzyl aroyl S,N-ketene acetal bichromophores, implementing
the p-bromo benzyl group for further functionalization and as
coupling site for other nonconjugated chromophores. Optimi-
zation of a model reaction (see Supporting Information, chapter
3) with tetrakis(triphenylphosphane)palladium(0) as a catalyst
and cesium carbonate as a base set the stage for concatenating
a condensation-Suzuki coupling sequence in the sense of a
consecutive three-component reaction in a one-pot fashion.
[a] L. Biesen, Dr. L. May, Prof. Dr. T. J. J. Müller
Institut für Organische Chemie und Makromolekulare Chemie
Heinrich-Heine-Universität Düsseldorf
Universitätsstraße 1, 40225 Düsseldorf (Germany)
E-mail: ThomasJJ.Mueller@hhu.de
[b] Dr. N. Nirmalananthan-Budau, Dr. K. Hoffmann, Dr. U. Resch-Genger
Division Biophotonics
Bundesanstalt für Materialforschung und -prüfung (BAM), Department 1
Richard-Willstätter-Straße 11, 12489 Berlin (Germany)
E-mail: ute.resch@bam.de
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202102052
© 2021 The Authors. Chemistry - A European Journal published by Wiley-
VCH GmbH. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial NoDerivs License, which permits use
and distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are made.
Employing acid chlorides
1 bearing electron-donating or
electron-withdrawing substituents or heterocyclic moieties (see
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Supporting Information, Table S7), N-(p-bromobenzyl) 2-methyl
benzothiazolium bromide (2)[15] and different boronic acid
esters 3 comprising of blue-emitting core structures, such as
triphenylamine, tri-phenylethene-dicarbazole, phenylcarbazole,
perylene, and tetraphenyl-ethene,[17] a library of 31 aroyl-S,N-
ketene acetal bichromophores 4 was synthesized in moderate
to excellent yields of 31–96% (Figure 1).
The absorption and emission properties of the novel
bichromophoric dyes 4 were studied in the solid state, in
solvent mixtures of different polarity, and incorporated into
polystyrene nanoparticles. In addition, two examples as poten-
tial sensor molecules were identified, utilizing dually emissive
dyes for protonation studies and the determination of the water
content of common alcoholic beverages.
Absorption and emission studies in the solid state
Previously, we showed that aroyl-S,N-ketene acetals account for
a rainbow-type tuning of solid-state emission color[15] by
variation of the electronic nature of the substituents in para-
position of the aroyl moiety.[15,18] In a similar fashion, the
bichromophores 4 can be readily distinguished by their solid-
state emission color ranging from blue to orange-red depend-
ing both on the substituents in para-position of the aroyl
moiety (R1) and the second chromophore (R2) (Figure 2).
Phenylene carbazole bichromophore 4r exhibits the shortest
wavelength emission maximum (λem =497 nm), while the para-
cyanophenyl triarylamine bichromophore 4h displays the most
red-shifted emission maximum (λem =600 nm) (Figure 2; for
details see Supporting Information, chapter 7).
Solid-state fluorescence quantum yields (Φf) were deter-
mined for selected para-cyano substituted bichromophores.
Figure 1. Consecutive three-component condensation-Suzuki coupling synthesis of aroyl-S,N-ketene acetal bichromophores 4 (all reactions were performed
on a 0.50 mmol scale: 1 (0.50 mmol), 2 (0.55 mmol), amine base (1.1 mmol) in 1,4-dioxane/ethanol 3:1 (4.0 mL) were stirred at room temp for 1 h and at
°
°
120 C for 20 h, then Pd(PPh3)4 (0.013 mmol), Cs2CO3 (1.50 mmol) and boronic acid ester 3 (1.0 mmol) were added and the reaction mixture stirred at 120 C for
4 h; yields are given after flash chromatography on silica gel; for examples 4a, 4l, 4p, 4v and 4z, diisopropylethylamine instead of triethylamine was used as
an amine base and the ethanol was added in the Suzuki-coupling step).
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Figure 2. Top, left: normalized solid state emission spectra obtained with a calibrated fluorometer; top, right: fluorescence lifetimes of selected aroyl-S,N-
ketene acetal bichromophores 4 (λexc =λabs,max at T=298 K); bottom: solid state fluorescence colors of selected aroyl-S,N-ketene acetal bichromophores 4
(λexc =365 nm) revealing emission color tuning by substitution pattern.
While the triarylamine and phenylene carbazol bichromophores solvents.[15] The tetraphenylethene 4z–4ae and triphenylethe-
4h and 4t exhibit only low Φf values of 0.01, the tetraphenyle- nyldicarbazole containing bichromophores 4l–4o do not
thene bichromophore 4ad has an increased Φf of 0.06, luminesce in any organic solvents but triphenylamine, phenyl-
presumably due to the solid-state emission of both carbazole, and perylene containing bichromophores 4p–4u
chromophores.[9a,15] Three model bichromophores 4h, 4t, and and 4v–4y emit blue light in solution (for details, see
4ad were selected as combinations of the red emissive p- Supporting Information, Table S5 and chapter 7). For selected
cyano-phenyl-substituted aroyl-S,N-ketene acetal component examples 4h and 4t, Φf and τ in ethanol reach values between
and a blue luminescent chromophore, representing comple- Φf =0.03 and 0.07 and τ =2.13 ns and 3.87 ns, respectively.
mentary emission partners. While compounds 4t and 4ad have Phenylcarbazole aroyl-S,N-ketene acetals 4p–4u reveal
lifetimes τ of 1.51 (λem =555 nm) and 2.34 ns (λem =566 nm) at unique wavelength-depending solution photophysics upon
exc =330 nm, para-cyanophenyl triarylamine bichromophore excitation. Excitation at λexc =380–400 nm leads to a weaker
a
λ
4h reveals a lifetime τ of 13.1 ns, which is astonishingly long for single emission maximum λem between 440 and 564 nm,
a red emitting fluorophore (λem =600 nm) (Figure 2).[19] This comparable to the emission maxima of the corresponding
long lifetime is tentatively ascribed to a complete ET from the aroyl-S,N-ketene acetal chromophores.[15] Excitation at 290 nm,
triphenylamine to the aroyl-S,N-ketenacetal. This assumption is thereby exciting the phenylcarbazol moiety (λem =380 nm),[20]
in line with the occurrence of a single emission maximum at two emission maxima are observed arising from the emission of
λem =600 nm upon excitation at 330 nm.
both chromophores. Consequently, a partial ET appears to be
operative for PhCarb aroyl-S,N-ketene acetal bichromophores
4p–4u (for details, see Supporting Information chapter 7).
Absorption and fluorescence in ethanol
The absorption maxima of the series of aroyl-S,N-ketene acetal Protonation in ethanol solutions
bichromophores 4 vary between 290 and 400 nm. While
maxima around 400 nm can be ascribed to the aroyl-S,N-ketene Triphenylamine and phenylene carbazol bichromophores fur-
acetal,[15] maxima covering a wavelength region from 290 to ther reveal a distinct halochromicity upon protonation with
330 nm can be unequivocally attributed to various blue trifluoroacetic acid. In the absorption spectra, the spectral
emitters. Indeed, the bichromophores’ absorption spectra position of the longest wavelength maxima remains un-
match with the sum of the constituting chromophores.
changed, but the intensity shifts hypochromically with increas-
The emission properties of the bichromophores 4 in ing proton concentration (Figure 3, bottom left). However, a
solution are mostly governed by the blue emitters. In general, significant effect on the emission properties is induced upon
aroyl-S,N-ketene acetals do not intensely fluoresce in organic protonation (Figure 3, bottom right). The fluorescence of
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Figure 3. Top: Protonation of aroyl-S,N-ketene acetals using the example of 4e; center, left and right: Kohn-Sham Molecular frontier orbitals of protonated
bichromophore 4e-H+ using the polarizable continuum model (PCM) with ethanol (B3LYP/6-311G**, isosurface value at 0.04 a.u,); center, middle: Visual
impression of 4e in ethanol with addition of trifluoroacetic acid, followed by triethylamine upon excitation with a UV-lamp (λexc =365 nm); bottom, left:
Absorption spectra of 4e in ethanol with increasing amount of trifluoroacetic acid (recorded at T=298 K, c(4e)=10À 5 m); bottom, right: Emission spectra of
4e in ethanol with increasing amount of trifluoroacetic acid (recorded at T=298 K, c(4e)=10À 7 m, λexc =404 nm).
protonated compound 4e is quenched, however, upon adding
triethylamine as a base the fluorescence is reinaugurated[21]
(Figure 3, for details of measurements of 4r, see Supporting
Information, Figure S27). Therefore, this bichromophore may be
used as a turn ON-OFF proton sensor by exploiting emission
quenching upon protonation.
acetal π-system as indicated by the LUMO coefficient densities
(Figure 3, center left). Moreover, the TD-DFT calculated absorp-
tion spectra of both 4e and 4e-H+ very well reproduce the
experimental data (for details, see Supporting Information,
chapter 8). An NMR study of the protonated species further
underlines the protonation of the pronounced methine carbon
atom (see Supporting Information, chapter 6).
DFT calculations (B3LYP/6-311G**, applying PCM with
ethanol as a solvent as implemented in Gaussian 09[22]) were
performed to elucidate the protonation site of 4e. For
simplifying the calculation, we first performed calculations of
the isolated aroyl-S,N-ketene acetal in the absence of the
second chromophore. These model calculations reveal that the
energetically most favored protonation site is the benzothia-
zole-α-methine carbon atom with ΔG=À 3.98 kcal/mol, com-
pared to protonation at the aroyl oxygen atom (B3LYP/6-
311G**, PCM ethanol; for details, see Supporting Information,
chapter 8). These model calculations can be transferred to
bichromophore 4e, likewise assuming a similar first protonation
at the methine carbon atom of the aroyl-S,N-ketene acetal.
This is in line with fluorescence quenching upon protona-
tion due to a disruption of conjugation of the aroyl-S,N-ketene
Aggregation induced emission (AIE) studies
All aroyl-S,N-ketene acetal chromophores show a characteristic
AIE behavior, which in conjunction with the observed solid-
state emission encouraged us to perform AIE studies with
bichromophores 4. Bichromophores 4 are soluble in common,
polar organic solvents such as acetonitrile, THF, 1,4-dioxane,
and ethanol, but insoluble in water. Hence, samples of the
bichromophores were diluted in different organic solvent/water
mixtures with water contents varying from 0 to 95%. As for
aroyl-S,N-ketene acetals, the most distinct results were obtained
in ethanol/water mixtures, which were subsequently used for all
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Figure 4. Top: Emission spectra of 4o (left) and 4y (right) in ethanol/water upon increasing water content (recorded at T=298 K, c=10À 7 m, λexc
(4o)=400 nm, λexc (4y)=442 nm; 2nd row: Emission spectra of 4ad in ethanol/water upon increasing water content (recorded at T=298 K, c=10À 7 m at λexc
=
400 nm (left) and Φf and τ of 4ad depending on the water fraction of the ethanol/water mixture (right); bottom: Visual impressions of the AIE features of 4ad
in ethanol/water mixtures of increasing water content upon excitation with a UV-lamp (λexc =365 nm).
further AIE studies and will representatively be discussed in
detail the para-cyano substituted examples of each class of
bichromophores.
a fluorescence increase by a factor of 20, the emission
enhancement upon aggregation is most pronounced for
bichromophore 4ad.
Triphenylethene dicarbazole and tetraphenylethene bichro-
mophores 4o and 4ad show a similar emission behavior upon
aggregation. At a low water fraction, the compounds fluoresce
very weakly (Φf <0.01). Upon increasing the water fraction,
compounds 4o and 4ad start to aggregate, thus resulting in an
enhanced Φf (0.07) and τ (2.06 ns).
Perylene aroyl-S,N-ketene acetal bichromophores 4v–4y
show aggregation-caused quenching (ACQ) behavior, which is
typical for perylene chromophores[23] as exemplified by com-
pound 4y. At low water fractions, bichromophore 4y fluoresces
intensively blue. Increasing the water fraction, inducing aggre-
gation, massively quenches the fluorescence. At a water fraction
above 70%, the typical emission bands of perylene chromo-
phores disappear indicating that the perylene subchromophore
Ascribing these effects to the blocking non-radiative decay
of the excited singlet state by RIM or RACI is plausible.[9,10b] With
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Figure 5. Emission spectra of 4h in ethanol/water upon increasing water content (recorded at T=298 K, c=10À 7 m, λexc (4h)=290 nm (left), λexc (4h)=400 nm
(right); bottom: Emission spectra of 4t in ethanol/water upon increasing water content (recorded at T=298 K, c=10À 7 m, λexc (4t)=290 nm (left), λexc
(4t)=400 nm (right).
is turned off, the emission bands are redshifted from 460 and
490 to 560 nm suggesting an excimer formation, simultaneously
encompassing a loss of fluorescence intensity (Figure 4, top
right) presenting perylene aroyl-S,N-ketene acetal bichromo-
phores as aggregation off-switchable luminogens.
Investigating the AIE properties of triphenylamine and
phenylene carbazole bichromophores reveals a different behav-
ior paradigmatically shown by bichromophores 4h and 4t.
Depending on the excitation wavelength λexc, the emission
spectra differ. Upon exciting at 290 nm, the emission of the
blue emitting chromophore can be harvested as well. For
bichromophore 4h, the emission band of the triphenylamine
moiety at 477 nm (λexc =290 nm) is completely quenched upon
aggregation (see Supporting Information, Figure S49). Changing
the blue emitting chromophore to phenylene carbazole, the
phenylene carbazole emission band appears at 375 nm and is
progressively attenuated upon further increase of the water
content inducing aggregation. Φf and τ are both reduced upon
aggregation (see Supporting Information, Figure S49). At a
water fraction above 65% a second emission band occurs
repetitively at 525 nm in each spectrum, which can be assigned
to the aroyl-S,N-ketene acetal chromophore. This rare dual
emission shows an equal intensity for both bands, thus,
resulting in a mixed emission color suggesting an emerging ET
(see Supporting Information, Figure S49). The dual fluorescence
can be coined to the AIE characteristics of both chromophores
in juxtaposition with solvent-induced changes of the spectral
overlap of the emission of phenylenecarbazole acting as a
donor and the aroyl-S,N-ketene acetal acting as an acceptor
affecting the energy transfer efficiency. A partial (frustrated)
energy transfer[4a,11a,24] from the phenylenecarbazole donor to
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Figure 6. Top: Graphical illustration of alcohol content analysis; Middle, left: Emission spectra of 4h in different alcohols (recorded at T=298 K, c(4h)=10À 7 m,
λexc =400 nm); Middle, right: Comparison between and determined water fraction; Bottom: Visual impression of 4h in ethanol/water mixtures with increasing
water content (λexc =365 nm).
Figure 7. Normalized fluorescence excitation and emission spectra (λexc =401 nm) of dispersed 8 μm-sized PSP loaded with dye 4ad (left) and CLSM image of
the dye-loaded PSP (right).
the aroyl-S,N-ketene acetal acceptor may occur in the aggregate
by both intra- and intermolecular energy transfer.
Upon selectively exciting the aroyl-S,N-ketene acetal part of
the bichromophores at 400 nm a different behavior is obtained.
For bichromophore 4t, a typical AIE behavior ascribed to the
aroyl-S,N-ketene acetal moiety can be observed. At water
fractions above 50%, the emission intensity increases by
enforcing ongoing aggregation by raising the water fraction.
The NCPh-TriPA bichromophore 4h fluoresces weakly, but upon
aggregation τ is more than doubled from 3.50 to 8.11 ns
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(Supporting Information Figure S49). At a water content of
20%, only the emission band of the triphenylamine was
detected. A second pronounced maximum is detectable at 558–
564 nm above a water content of 60%, giving rise to an
ethanol-water mixtures, and in the solid state. Simultaneously, τ
is increased to 3.73 ns.
aggregation-induced dual emission (AIDE) due to the contrast- Conclusion
ing AIE behavior of triphenylamine and aroyl-S,N-ketene acetal.
This intertwining AIDE behavior can be exploited for polarity
In summary, a diverse library of emissive bichromophores was
sensing (Figure 5, top right).[9a,25]
obtained with moderate to excellent yields by a concise
consecutive three-component condensation-Suzuki synthesis of
N-benzyl aroyl S,N-ketene acetal bichromophores in a one-pot
fashion. Spectroscopic studies of these differently substituted
bichromophores in the solid state, in organic solvents, upon
aggregation, and upon encapsulation into polystyrene particles
reveal substitution pattern control of the solid-state emission
color and the aggregation-induced emission (AIE). Variation of
the blue emitting chromophore allows elaboration of different
communication pathways between the ligated chromophores,
such as complete or partial energy transfer leading to a dual
emission or an aggregation-induced switching of the
fluorescence. The AIE chromicity of the dyes is well suited for
analyte screening and sensing as demonstrated by the analysis
of the water content of alcoholic beverages based on the extent
of the AIE color shifts. Our results underline that the rational
design of novel bichromophoric systems for multifunctional
chromophore applications requires a detailed understanding of
(partial) energy transfer in aggregates. This work is currently
addressed by tailored sets of aroyl-S,N-ketene acetals.
Determination of the water content of alcoholic beverages
Inspired by the emission color change upon aggregation
(Figure 6, bottom), we tried to exploit this visual emission effect
in solutions with different ethanol-water ratios for naked eye
analytics. Anslyn[26] and Bunz[27] already used organic dyes for
high-end discriminations of complex analytes, such as whiskey
and other hard liquors by developing assays and chemical
tongues. Here, we implement a rudimentary fluorescence-based
tool for the naked-eye estimation of the water content of
different alcoholic beverages. The NCPhÀ TriPA bichromophore
4h was used for analyzing different colorless alcoholic
beverages with a specified alcohol content. As shown in
Figure 6, the emission spectra of 4h in the different beverages
vary in intensity and shape. By comparing the emission of 4h in
selected drinks to the results of previous AIE studies in ethanol-
water mixtures (Figure 5 top), the water content of each drink
could be visualized (Figure 6 top). Only Japanese rice wine,
sake, could not be examined due to its native fluorescence. As
summarized in Figure 6 and Table S8 (Supporting Information), Acknowledgements
only in case of the vodka sample, the optically determined
water fraction overestimates the real water content.
The authors cordially thank the Fonds der Chemischen Industrie
and the Deutsche Forschungsgemeinschaft (Mu 1088/9-1) for
financial support. Open access funding enabled and organized
by Projekt DEAL.
Encapsulation of a bichromophore in polystyrene
Encapsulation of aroyl-S,N-ketene acetals in an nonpolar solid
matrix like polystyrene led to a fluorescence enhancement due Conflict of Interest
to steric restriction or attenuation of intramolecular rotations of
the matrix-entrapped dye molecules.[15] To exemplify the
influence of bead encapsulation on the emission of the
bichromophores tetraphenylethene bichromophore 4ad was
chosen due to its intense solid-state fluorescence and
pronounced AIE behavior. The results of encapsulating 4t and
4ad into 8 μm-sized carboxy-functionalized polystyrene par-
ticles (PSP) are detailed in the Supporting Information.[28] We
chose a high encapsulation concentration of 4ad (6 mm) to
enhance the probability of dye-dye interactions in PSP. As
confirmed by CLSM studies of single dye-stained particles the
dye is homogeneously dispersed (Figure 7 right). The
fluorescence excitation and emission spectra (Figure 7 left) of
4ad@PSP reveal a hypsochromic shift of the respective maxima
in comparison to the corresponding spectra in ethanol and
ethanol-water mixtures, respectively, caused by the unpolar
polystyrene matrix. Additionally, Φf is enhanced to 15% upon
PSP encapsulation, exceeding Φf found for 4ad in ethanol,
The authors declare no conflict of interest.
Keywords: aggregation-induced emission
acetals · bichromophores · energy transfer · fluorescence · one-
pot synthesis
·
aroyl-S,N-ketene
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Manuscript received: June 10, 2021
Accepted manuscript online: June 25, 2021
Version of record online: ■■■, ■■■■
Chem. Eur. J. 2021, 27, 1–10
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FULL PAPER
Multifunctional shining tool: A library
of 31 aroyl-S,N-ketene acetal solid-
state emissive bichromophores, syn-
thesized by a consecutive three-
component reaction (3CR), exhibits
aggregation induced emission (AIE),
acidochromicity, as well as dual
emission and energy transfer in ag-
gregates suitable for alcoholic
L. Biesen, Dr. L. May, Dr. N. Nirmalanan-
than-Budau, Dr. K. Hoffmann, Dr. U.
Resch-Genger*, Prof. Dr. T. J. J. Müller*
1 – 10
Communication of Bichromophore
Emission upon Aggregation – Aroyl-
S,N-ketene Acetals as Multifunctional
Sensor Merocyanines
beverage analysis and encapsulation
in micrometer sized polystyrene
particles with enhanced fluorescence
quantum yield.