Solid-State Emissive Aroyl-S,N-Ketene Acetals with Tunable Aggregation-Induced Emission Characteristics

Article abstract of DOI:10.1002/anie.201916396

N-Benzyl aroyl-S,N-ketene acetals can be readily synthesized by condensation of aroyl chlorides and N-benzyl 2-methyl benzothiazolium salts in good to excellent yields, yielding a library of 35 chromophores with bright solid-state emission and aggregation

Full text of DOI:10.1002/anie.201916396

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Accepted Article
Title: Solid State Emissive Aroyl-S,N-Ketene Acetals With Tunable
Aggregation-Induced Emission Characteristics
Authors: Lukas Biesen, Nithiya Nirmalananthan-Budau, Katrin
Hoffmann, Ute Resch-Genger, and Thomas J. J. Müller
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10.1002/anie.201916396
Angewandte Chemie International Edition
COMMUNICATION
Solid State Emissive Aroyl-S,N-Ketene Acetals With Tunable
Aggregation-Induced Emission Characteristics
Lukas Biesen,[a] Nithiya Nirmalananthan-Budau,[b,c] Katrin Hoffmann,[b] Ute Resch-Genger,[b] and
Thomas J. J. Müller*[a]
Abstract: N-Benzyl aroyl-S,N-ketene acetals can be readily
synthesized by condensation of aroyl chlorides and N-benzyl 2-
methyl benzothiazolium salts in good to excellent yields, yielding a
library of 35 chromophores with bright solid state emission and
aggregation induced emission characteristics. Varying the
substituent from electron-donating to electron-withdrawing enables
the tuning of the solid state emission color from deep blue to red.
tetraphenylethene (TPE) and its derivatives[6c] and there are also
some reports of anthracene or carbazole based structures.[1c]
In recent years we could demonstrate that also quite polar
heterocyclic systems such as indolone merocyanines and
quinoxalines motifs[8,9,10] can be AIE active.[6d,11] We therefore
became interested in molecules with pronounced charge
transfer (CT) character, which could finally provide access to
blue emissive AIE chromophores. We reasoned that the aroyl
S,N-ketene acetals could be interesting candidates with AIE
characteristics tunable by variation of the substitution pattern.
While aroyl-S,N-ketene acetals are known as building blocks for
enlarged heterocycles[12] and benzothiazole motifs have been
previously utilized in AIE systems,[13] N-benzyl aroyl-S,N-ketene
acetals have not been yet studied with respect to their synthesis,
photophysical properties and AIE behavior. Only the
trifluoromethyl-substituted aroyl-S,N-ketene acetals 3z and
3aawere synthesized before and used as acid precursors and
radical initiators, but have not been spectroscopically studied.[14]
Here, we report the highly diverse, rapid modular synthesis of N-
benzyl aroyl S,N-ketene acetals and their emission
characteristics, with special emphasis dedicated to their AIE
behavior.
Many otherwise promising fluorophores with attractive
photophysical properties like cyanine or xanthene dyes suffer
from dye-dye interactions resulting in aggregation-caused
quenching (ACQ), and commonly the emission of most
chromophores is significantly quenched in the solid state.[1] In
2001, Tang observed the opposite phenomenon of ACQ for the
dye 1-methyl-1,2,3,4,5-pentaphenylsilole and introduced the
term aggregation-induced emission (AIE). AIE dyes are typically
non- or barely emissive in dilute solution but show a strong
emission upon aggregation.[2] Although the AIE mechanism is
not yet completely understood, the occurrence of emission upon
dye aggregation is commonly rationalized in terms of
restriction of intramolecular motions (RIM).[3]
a
Since Tang’s discovery AIE-active compounds gained
considerable interest due to their enormous application
potential.[4] For instance, AIE-active chromophores have been
employed as reporters for mitochondria-targeted cancer therapy,
cancer ablation[5] and as nanoprobes for tumor-targeted
bioimaging or as biosensors.[6] Fluorophores showing AIE are
promising candidates for novel signal enhancement strategies in
assays or improved nanoparticle (NP)-based bioimaging
approaches as such dyes can enable higher dye loading
densities than conventional ACQ dyes.[6c-d] Besides bioanalytical
applications[1c] AIE chromophores have also been successfully
applied in optoelectronic devices as well.[7] For example, for
applications such as organic light-emitting diodes (OLEDs),
there is a continuous interest in luminophores revealing solid
state emission.[4c-d] Many published AIE systems utilize
Aroyl-S,N-ketene acetals 3 were synthesized as a library of 35
compounds in a straightforward condensation by base mediated
addition-elimination of benzothiazolium salts
2 and acid
chlorides 1 in moderate to excellent yields (Scheme 1, Table 1).
Scheme 1. Synthesis of aroyl-S,N-ketene acetals 3.
The condensations were performed in a mixture of 1,4-dioxane
and ethanol with triethylamine as a base, except for the
compounds 3a and 3b (Table 1, entries 1 and 2), where
diisopropylethylamine in 1,4-dioxane was used. Acid chloride 2
[a]
[b]
[c]
M. Sc. L. Biesen, Prof. Dr. T. J. J. Müller
Institut für Organische Chemie und Makromolekulare Chemie,
Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-
40225 Düsseldorf, Germany
E-mail: ThomasJJ.Mueller@hhu.de
Dr. N. Nirmalananthan-Budau, Dr. K. Hoffmann, Dr. U. Resch-
Genger
Division Biophotonics, Bundesanstalt fꢀr Materialforschung und -
prꢀfung (BAM), Department 1, Richard-Willstꢁtter-Straße 11, D-
12489 Berlin, Germany
Dr. N. Nirmalananthan-Budau, Institut für Chemie und Biochemie,
Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany
can
bear
electron-donating
and
electron-withdrawing
substituents in ortho-, meta-, and para-position. In addition,
heterocyclic acid chlorides like thiophenoyl, furoyl and pyridinoyl
chlorides are also applicable in this protocol (for details, see SI
Table S4). For preparing our spectroscopic reference
chromophores 3 besides N-benzyl 2-methylthiazolium bromides
2a and 2b, 2,3-dimethylthiazolium iodide (2c) was also
successfully transformed.
The absorption maxima of the investigated series of aroyl-S,N-
ketene acetals 3 cover a narrow wavelength region and vary
between 378 and 413 nm (see SI, Table S2 and
Figure S1). TD-DFT calculations (B3LYP/6-31G**, applying
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201916396
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COMMUNICATION
PCM on 17 selected chromophores 3 with ethanol as the solvent
as implemented in Gaussian 09[15]) reveal a good agreement of
the longest wavelength absorption maxima of the calculated and
experimentally obtained spectra of the dye solution (see SI,
Table S9). Essentially the absorption maxima can be assigned
to HOMO-LUMO transitions with considerable CT character,
typical for merocyanines. Therefore, for further rational design of
this type of chromophore quantum chemical calculations can be
favorably applied as a tool for property prediction.
Aroyl-S,N-ketene acetals 3 neither luminesce in dilute ethanol
solutions nor in other organic solvents except for the
dimethylamino-substituted derivatives 3a and 3b. However, all
aroyl-S,N-ketene acetals 3 are emissive in the solid state.
Moreover, their emission color varies considerably depending on
the electronic nature of the substituents in para-position of the
aroyl moiety. Consequently, they can be a readily distinguished
by their emission color (Figure 1, top).
Within the dye series studied, para-fluoro substituted derivative
3o reveals the shortest wavelength emission maximum em
(em = 442 nm) while para-nitro substituted compound 3ad
shows the most red-shifted emission maximum (em = 630 nm)
(for further details, see SI, Table S2 and Figure S2). The results
of the fluorescence quantum yield (f) and fluorescence lifetime
() measurements are displayed in the right panel of Figure 1. In
the solid state, dye 3ag shows the highest f of 0.14, but the
shortest  of 0.411 ns of this series, while dye 3d reveals the
lowest f of 0.04 and shows nevertheless a relatively long  of
2.27 ns. These features suggest that the rotational motions are
restricted in the solid state. The sometimes inconsistent trends
of f and  point to different processes determining the
photophysics of these dyes. The f values of 3a and 3b which
are also emissive in solution were determined to f = 0.07 in
ethanol for both dyes relative to coumarin 343 (f = 0.63 in
ethanol).[16] The dimethylamino substituted derivatives 3a and 3b
display a weak positive solvatochromism (for further details, see
SI, Tables S2 and S6, Figures S2-6 and S51-52).
The absence of fluorescence in solution in conjunction with the
observation of strong solid state emission encouraged us to
perform AIE studies with our dyes. Aroyl-S,N-ketene acetals 3
are soluble in common, polar organic solvents such as
acetonitrile, THF and ethanol, but they are insoluble in water.
Samples of aroyl-S,N-ketene acetals were hence diluted in
different mixtures of organic solvents and water with ratios
varying from 0 to 99 %. The most intriguing results were
obtained with ethanol/water mixtures which are subsequently
discussed in detail.
Upon increasing the water content, the solubility of the
hydrophobic dyes are significantly reduced favoring the
formation of dye aggregates. Up to a water fraction of 70% the
aroyl-S,N-ketene acetals did at maximum fluoresce very weakly
(f < 0.01). When increasing the water content above 80%,
compounds 3 started to aggregate, resulting in a considerable
enhancement in f and in an increase in  (see Figure 2). These
effects are ascribed to the blocking of non-radiative pathways
depopulating the excited singlet state by an aggregation-induced
RIM.[15] For most dyes the fluorescence intensity decreased
again at water fractions above 90% as the compounds
precipitated. The hydrodynamic diameters of the resulting dye
aggregates were determined to be between 160 and 425 nm for
the dyes 3d, 3j, 3s, 3ac, and 3ag with dynamic light scattering.
This confirms that aroyl-S,N-ketene acetals represent a novel
class of AIE chromophores with substitution pattern control of
the AIE behavior (Figure 2). However, while each derivative
emits in the solid state, irrespective of the N-substituent of the
benzothiazole moiety, this substituent affects obviously the AIE
behavior. For example, the N-methyl derivative 3k does not
fluoresce upon aggregation at the dye concentration used for all
dyes in our AIE studies. This demonstrates that the benzylic
Table 1. Condensation synthesis of aroyl-S,N-ketene acetals 3.[a]
Entry
1
R1
R2
Product (yield)[b]
4-Me2NC6H4
4-Me2NC6H4
4-MeOC6H4
4-MeOC6H4
4-tBuC6H4
4-tBuC6H4
4-MeC6H4
4-MeC6H4
Ph
4-BrC6H4CH2
PhCH2
3a (68%)
3b (72%)
3c (69%)
3d (52%)
3e (48%)
2
4-BrC6H4CH2
PhCH2
3
4
4-BrC6H4CH2
PhCH2
5
6
3f (65%)
3g (80%)
3h (65%)
3i (58%)
4-BrC6H4CH2
PhCH2
7
8
4-BrC6H4CH2
PhCH2
9
Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
3j (72%)
Ph
Me
3k (94%)
3l (22%)
trans-4-FC6H4CH=CH
trans-4-FC6H4CH=CH
4-FC6H4
4-BrC6H4CH2
PhCH2
3m (31%)
3n (69%)
3o (85%)
3p (65%)
3q (49%)
3r (83%)
3s (88%)
3t (27%)
3u (27%)
3v (55%)
3w (88%)
3x (59%)
3y (65%)
3z (72%)
3aa (50%)
3ab (51%)
3ac (66%)
3ad (44%)
3ae (73%)
3af (72%)
3ag (87%)
3ah (77%)
3ai (95%)
4-BrC6H4CH2
PhCH2
4-FC6H4
3-FC6H4
4-BrC6H4CH2
PhCH2
2-FC6H4
4-ClC6H4
4-BrC6H4CH2
PhCH2
4-ClC6H4
6-Cl-pyridine-3-yl
6-Cl-pyridine-3-yl
4-BrC6H4
4-BrC6H4CH2
PhCH2
4-BrC6H4CH2
PhCH2
4-BrC6H4
4-IC6H4
4-BrC6H4CH2
PhCH2
4-IC6H4
4-F3CC6H4
4-F3CC6H4
4-NCC6H4
4-NCC6H4
4-O2NC6H4
4-O2NC6H4
thiophen-2-yl
thiophen-2-yl
furan-2-yl
4-BrC6H4CH2
PhCH2
4-BrC6H4CH2
PhCH2
4-BrC6H4CH2
PhCH2
4-BrC6H4CH2
PhCH2
4-BrC6H4CH2
furan-2-yl
PhCH2
[a]All reactions were performed on 1.0 mmol scale: 1 (1.0 mmol), 2 (1.1 mmol),
amine base (2.2 mmol) in 1,4-dioxanes/ethanol 5:2 (7.0 mL) were stirred at
room temp for 1 h and at 120 °C for 23 h. [b]Yields after flash chromatography
on silica gel.
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10.1002/anie.201916396
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COMMUNICATION
substituent of the aroyl-S,N-ketene acetals 3 is responsible for
the occurrence of AIE characteristics (Figure 2). The most
significant aggregation-induced increase in fluorescence
intensity was found for para-methyl substituted dye 3g and the
para-fluoro substituted chromophore 3o that both reveal a 20-
fold fluorescence enhancement upon dye aggregation.
suggests polarity-induced quenching with increasing CT
character of the dyes (see SI, chpt. 6).
Figure 3 Top: visual impression of the AIE features of 3ac in ethanol/water
mixtures of increasing water content upon excitation with
a UV-lamp
(exc = 365 nm); bottom, left: emission spectra of 3ac obtained at different
water fractions of the ethanol/water solvent mixtures; bottom, right:
corresponding f data (recorded at T = 298 K,c(3ac) = 10-7 M, exc = abs,max).
Figure 1 Top: color of the solid state fluorescence of selected aroyl-S,N-
ketene acetals 3 (exc = 365 nm, UV lamp) revealing emission color tuning by
substitution pattern; bottom, left: normalized solid state emission spectra
obtained with a calibrated fluorometer; bottom, right: absolutely measured
fluorescence quantum yields and fluorescence lifetime of selected aroyl-S,N-
ketene acetals 3 (exc = abs,max at T = 298 K).
The intense solid state fluorescence of the dyes 3 together with
their AIE behavior encouraged us to study their fluorescence
properties when encapsulated in an unpolar solid matrix like
polystyrene to underline the potential of AIE emitters for staining
applications. For this purpose, dyes 3d, 3j, 3s, 3ac, and 3ag
were incorporated into
8 m-sized carboxy-functionalized
polystyrene particles (PSP) from Kisker using a straightforward
swelling procedure from Behnke et al.[17] This encapsulation is
expected to sterically restrict or at least reduce intramolecular
rotations of the PSP-entrapped dye molecules. Additionally, a
high encapsulation concentration of the dyes (6 mM) was chosen
to encourage dye−dye interactions in PSP. Microscopic studies
of single dye-stained particles confirm the homogeneous dye
staining of the PSP and a slight change in particle shape (Figure
4, right). As shown in Figure 4 (left panel), the fluorescence
excitation and emission spectra of dye 3ac in PSP are
hypsochromically shifted compared to the corresponding spectra
of the dye in ethanol and dye aggregates in ethanol−water
mixtures. This is ascribed to the reduced polarity of the dye
molecules faced in the apolar polymer matrix.
Figure 2 Emission intensity (a,b), f (c) and fluorescence lifetime (d) of
selected aroyl-S,N-ketene acetals 3 as a function of the water fraction of the
ethanol/water mixtures. The measurements were performed at T = 298 K with
a
dye concentration c(3) = 10-7 M, the fluorescence was excited at the
respective absorption maximum exc = max
)
.
To highlight the fluorescence properties of this class of AIE
chromophores, exemplarily the emission spectra and visual
evolution of the fluorescence are shown in Figure 3 for
compound 3ac in ethanol/water mixtures with steadily increasing
water content. At a water content of 89% compound 3ac starts
to become emissive. The maximum fluorescence is observed at
a water content of 99%. Furthermore, f increased to 0.06 and
to 1.4 ns (for further details, see SI, chpt. 6).
Interestingly, the dimethylamino-substituted dyes 3a and 3b
represent exceptions from the typical AIE behavior of aroyl-S,N-
ketene acetals as they fluoresce in ethanol solution and exhibit
ACQ upon aggregation. The continuous red-shift from 461 to
500 nm in emission introduced by the increasing water content
Figure
4 Top: Normalized fluorescence excitation and emission spectra
(exc = 404 nm) of dispersed 8 μm-sized PSP loaded with dye 3ac (left) and
CLSM image of the dye-loaded PSP (right).
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COMMUNICATION
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Moreover, PSP encapsulation leads to a significantly higher f
of 0.35 exceeding the f values observed for this dye in ethanol,
ethanol−water mixtures, and in the solid state. This trend is also
reflected by a prolonged  of 2.15 ns. The fluorescence features
of dyes 3d, 3j, 3s, and 3ag shown in the SI (see Figure S54 and
Table S8) reveal similar trends.
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In summary, a library of 35 compounds of a new class of aroyl-
S,N-ketene acetals with tunable AIE characteristics could be
readily synthesized in good to excellent yields and the
spectroscopic properties of these dyes were studied in solution,
in the solid state and under conditions introducing dye
aggregation. These results revealed substitution pattern control
of the AIE effects. While the R1 substituent is essentially
responsible for the color of the chromophores, N-benzyl
substitution of the benzothiazolidene donor is most crucial for
the occurrence of AIE effects. These novel AIE chromophores
are well suited for implementation in optoelectronic devices.
Future work currently performed is directed to develop diversity
oriented one-pot syntheses for accessing -conjugation
extended aroyl-S,N-ketene acetals with tunable AIE
characteristics.
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Financial support by Deutsche Forschungsgemeinschaft (Mu
1088/9-1), and the Fonds der Chemischen Industrie is gratefully
acknowledged. The authors cordially thank M.Sc. Lars May
(Heinrich-Heine Universität Düsseldorf) for the support with DFT
calculations. M.Sc. Nithiya Nirmalananathan-Budau would like to
thank Christopher Kläber for technical assistance and
acknowledges financial support from the Bundesanstalt fꢀr
Materialforschung und -prꢀfung (BAM) within the framework of
the BAM funding programme “Menschen, Ideen” (MI, type III
project).
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Keywords: aggregation-induced emission • aroyl-S,N-ketene
acetals
• fluorescence • solid state emission • tuneable
photophysical properties
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10.1002/anie.201916396
Angewandte Chemie International Edition
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A diverse library of 35 N-Benzyl aroyl
S,N-ketene acetals with tunable color
of the solid state emission and
aggregation induced emission
characteristics is readily synthesized
by condensation of aroyl chlorides and
N-benzyl 2-methyl benzothiazolium
salts.
Lukas Biesen, Nithiya Nirmalananthan-
Budau, Katrin Hoffmann, Ute Resch-
Genger, Thomas J. J. Müller*
Page No. – Page No.
Solid State Emissive Aroyl-S,N-
Ketene Acetals With Tuneable
Aggregation-Induced Emission
Characteristics
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This article is protected by copyright. All rights reserved.