Solid‐state emissive biphenylene bridged bisaroyl‐S,N‐ketene acetals as distinct aggregation‐induced enhanced emitters and fluorometric probes

Biphenylene bridged bisaroyl‐S,N‐ketene acetals can be readily synthesized by a one‐pot Masuda borylation‐Suzuki arylation sequence, thus, yielding a library of 20 bisaroyl‐S,N‐ketene acetals with tunable solid‐state emission and aggregation‐induced enhanced emission characteristics depending on the para substituents in the starting material. Potential applications as fluorometric probe of alcoholic beverages are outlined.

protection, [15] detection of explosives, [16] or in the generation of white light emission via aggregation-induced dual emission. [17] While the literature concerning AIE is widely dominated by tetraphenylethene and its derivatives, [3a,18] we could contribute polar heterocyclic systems, such as indolone merocyanines [17,19] and quinoxalines [20] , to the field of AIErelated research. Our recent contributions are aroyl-S,Nketene acetals, low molecular weight merocyanines with tunable solid-state emission, and AIE-behavior, which is strongly dependent on the N-benzylic substituent. [21] As previously described, the aroyl-S,N-ketene acetals can be accessed via a straightforward base-mediated condensation of acid chlorides 1 and benzothiazolium salts 2. [21] The envisioned biphenylene bridged aroyl-S,N-ketene acetals 4 are directly accessible in 30-98% yields from the aroyl-S,N-ketene acetal monomers by using the well-established Masuda borylation-Suzuki arylation (MBSA) coupling [22] sequence (Scheme 1). Therefore, an isolation of the intermediary formed borylated aroyl-S,N-ketene acetal was not necessary and the bisaroyl-S,N-ketene acetals were directly obtained starting from the brominated starting material 3. Homo-and hetero bisaroyl-S,N-ketene acetals alike are accessed by this methodology. Both aroyl-S,N-ketene acetals are mostly connected through the N-benzyl substituent, but as shown, for examples, 4k, 4n, and 4o, a ligation via the aroyl moiety can be realized as well. Aroyl-S,N-ketene acetals can bear electron-donating substituents like dimethylamino, S C H E M E 1 One-pot MBSA (Masuda borylation-Suzuki arylation) synthesis of biphenylene bridged bisaroyl-S,N-ketene acetal 4 (All reactions were performed on a 0.50 mmol scale. Reaction yields [in %] are given after flash chromatography on silica gel. For experimental details, see Supporting Information, chpt. 3) methoxy, or tert-butyl groups and electron-withdrawing substituents like halides, trifluoromethyl, and cyano groups in para-position as well as heterocyclic aroyl moieties like thiophene (4l, 4s) and furan (4j, 4 m, 4p, 4t). It proved beneficial with respect to yields to start the reaction sequence with aroyl-S,N-ketene acetal bearing electron-donating substituents. Generally speaking, furan bearing as well as strongly electron donating substituted and chlorinated aroyl-S,N-ketene acetals gave higher yields than strongly electron withdrawing substituted aroyl-S,N-ketene acetals.
Using the dibrominated aroyl-S,N-ketene acetal 3d, trisaroyl-S,N-ketene acetal 5 can be easily synthesized by employing the same protocol with an increased amount of catalyst, which is necessary to ensure a full conversion of the starting material (Scheme 2).
Bisaroyl-S,N-ketene acetals 4 exhibit two absorption maxima at around 250 and 380-400 nm. Indeed, the absorption spectra match with those of the constituting aroyl-S,N-ketene acetal monomers 3. [21] In contrast to aroyl-S,N-ketene acetal monomers 3, which exhibit, with one exception, no luminescence in organic solvents, [21] the bisaroyl-S,N-ketene acetals 4 emit blue to green light in solution with emission maxima ranging between 470 and 510 nm and Stokes shifts between 3200 and 5800 cm -1 . The emission maximum depends on the substituent of the aroyl moiety, the emission maximum is bathochromically shifted with increasing electron donating nature of the substituent. While most bisaroyl-S,N-ketene acetals luminesce rather weakly (fluorescence quantum yields [Φ em = 0.01-0.05]), dimethylamino substituted bisaroyl-S,N-ketene acetals 4d and e fluoresce more intensively (Φ em = 0.15) as already observed for the respective dimethylamino substituted monomers. [21] Furthermore, after coupling at the aroyl moiety (4k, n, and o), an increase in luminescence intensity is discernable as well (Φ em = 0.09). This can be ascribed to the extended chromophoric system and the formation of a biphenylene bridge, respectively ( Figure 1).
All aroyl-S,N-ketene acetals show a characteristic AIE behavior, [21] which encouraged us to perform extended AIE studies with bisaroyl-S,N-ketene acetals 4 as well. Bisaroyl-S,N-ketene acetals 4 are easily soluble in most polar organic solvents like acetonitrile, THF, 1,4-dioxane, and alcohols, like ethanol, but insoluble in water. Samples of the bisaroyl-S,N-ketene acetals were dissolved in various solvent/water mixtures. Most distinct results were obtained in ethanol/water mixtures with water contents varying from 0% to 95%, which were consequently used for all AIE studies. The fact that we already observed luminescence in pure ethanol poses bisaroyl-S,N-ketene acetals 4 as AIEE chromophores. Exemplarily, the AIEE behavior of bisaroyl-S,N-ketene acetals will be discussed for dimethylamino-cyano derivative 4e and methoxy-cyano derivative 4 g. The AIE characteristics of trimer 5 are summarized in the Supporting Information (chpt. 6).
For bisaroyl-S,N-ketene acetal 4 g, a typical AIEE behavior is observed. At low water fractions up to 40%, nearly no luminescence is detected (Φ em < 0.01). Further increasing the water fraction leads to aggregation of the dye, thus resulting in an enhanced Φ em (0.09). Blocking of nonradiative decay channels of the excited singlet state by aggregationcaused RIM [7] or RACI [8] rationalizes the observed AIEE. Raising the water fraction above 80%, the fluorescence intensity decreases due to precipitation of the dye. Alongside the 10-fold increase in fluorescence, the emission maximum is bathochromically shifted from 454 to 500 nm upon aggregation (Figure 2, right). A similar behavior can be observed for trisaroyl-S,N-ketene acetals 4 5 (see Supporting Information, Figure S24.) For dimethylamino-substituted derivatives 4d and 4e and aroyl-connected bisaroyl-S,N-ketene acetals (4k, n, and o), the fluorescence is quenched upon aggregation, and S C H E M E 2 One-pot MBSA synthesis of trisaroyl-S,N-ketene acetal 5 (reaction was performed on a 0.25 mmol scale. Reaction yield [in %] is given after flash chromatography on silica gel. For experimental details, see Supporting Information, chpt. 3, p. S60) F I G U R E 1 Top: Normalized absorbance and fluorescence spectra in ethanol obtained with a calibrated fluorometer (c(4) abs = 10 -5 M, c(4) em = 10 -7 M, λ exc = λ abs,max at T = 298 K); center: normalized solid-state emission spectra obtained with a calibrated fluorometer (λ exc = λ abs,max at T = 298 K); bottom: solid-state fluorescence colors of selected bisaroyl-S,N-ketene acetal derivatives 4 (λ exc = 365 nm) revealing emission color tuning by substitution pattern concomitantly, Φ of 4e is decreased from 0.15 to 0.05 in juxtaposition with a bathochromic shift of emission maximum from 460 to 565 nm (Figure 2, left). A similar behavior was observed for dimethylamino substituted single aroyl-S,N-ketene acetal, [20] the strongly electron-donating characteristic of the dimethylamino group is responsible for the occurrence of the strong luminescence, which is diminished and drastically bathochromically shifted upon aggregation.
The discernable emission color change of 4f upon aggregation (Figure 3, bottom), which is ascribed to the combination of the strong electron donating methoxy substituted, and the electron donating fluorine substituted aroyl-S,N-ketene acetal unit, which exhibited independently different colors upon aggregation, [21] encouraged us to establish a new naked eye analytics for the determination of the water content of different alcoholic beverages. Organic dyes used as high-end analytics for whiskey and other hard liquors by developing assays and chemical tongues have been reported. [23] In comparison, our sensor generates only a rough, yet correct output. Bisaroyl-S,N-ketene acetal 4f was chosen due to its diverse color spectrum in aggregated form. We analyzed and identified different colorless alcoholic beverages with a specified alcohol content, which resulted in emission spectra of varying intensity and shape (Figure 3). Comparing previous AIEE studies of 4f (Supporting Information, Figure S9) with emission in selected drinks, the water content can be determined although it has to be clarified that it is not possible to distinguish beverages with a water content between 0% and 40 % (Figure 3, middle). Only in the case of hazelnut liquor, the optically determined water fraction overestimates the water content of the hazelnut liquor as declared by the manufacturer (Figure 3, middle). The bisaroyl-S,N-ketene acetal 4f is well suited for the beverage analysis due to its clear emission color distinctness upon aggregation.
In summary, a library of 20 novel biphenylene bridged bisaroyl-S,N-ketene acetals and one trimer were synthesized in moderate to excellent yields by MBSA sequence starting from aroyl-S,N-ketene acetal monomers. As known for the monomers, the substitution pattern enables full control of the solid-state emission color and the AIE. In contrast to the monomers, weak blue to green luminescence of bisaroyl-S,N-ketene acetals in ethanol occurs as well, with F I G U R E 3 Top: Graphical illustration of alcohol content analysis; Center, left: Emission spectra of 4f in different alcohols (recorded at T = 298 K, c(4h) = 10 7 M, λ exc = 378 nm); Center, right: Comparison between prognosed and determined water fraction; Bottom: Visualization of 4f in ethanol/water mixtures with increasing water content (in %) (λ exc = 365 nm) intensities that strongly depend on the connectivity and the substituent pattern. The aggregation-induced change of emission color is well suited for applications in analyte screening as shown by the determination of the water fraction of various alcoholic beverages based on emission color of dissolved dye. Our future work will be directed to develop π-conjugation extended aroyl-S,N-ketene acetals as well as bridged aroyl-S,N-ketene acetals with tunable AIEE characteristics via one-pot methodologies.

A C K N O W L E D G M E N T S
Financial support by Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft (Mu 1088/9-1) is gratefully acknowledged.

C O N F L I C T O F I N T E R E S T
We thereby declare that this manuscript has not been published elsewhere and there are no conflicts of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are openly available in the Supporting Information at the end of this document.