N‐Boc‐Indolylbenzothiadiazole Derivatives: Efficient Full‐Color Solid‐State Fluorescence and Self‐Recovering Mechanochromic Luminescence

Abstract

Herein, the solid-state emission with good fluorescence quantum yields of N-Boc-indolylbenzothiadiazoles as a new class of fluorophores is described. Their solid-state emission covers the wide range of the visible spectrum and the emission color can be tuned easily by changing the substituents on the two heteroaromatic rings. Among these, 3-methylindolyl derivatives exhibit moreover autonomously self-recovering mechanochromic luminescence, whereby the original solid-state emission could be recovered spontaneously at room temperature after exposure to a mechanical stimulus. The emission color, as well as the recovery time for the color change could be tuned via the introduction of different substituents on the benzothiadiazole ring. We propose that the mechanism of the autonomously self-recovering mechanochromic luminescence of 3-methylindolylbenzothiadiazoles is based on a partial amorphization of the crystals upon exposure to the mechanical stimulus, followed by autonomous recovering in the form of recrystallization.

Indolylbenzothiadiazoles with varying substituents on the indole ring: a systematic study on the self-recovering mechanochromic luminescence

“…Following our previously reported procedure for the synthesis of N-Boc-indolylbenzothiadiazole 1a (R 1 ¼ Me), 11 3-alkylsubstituted 1b (R 1 ¼ Et) and 1c (R 1 ¼ i Pr) were obtained in 48% and 64% yield, respectively, from the Suzuki-Miyaura coupling between 4-bromo-2,1,3-benzothiadiazole and the corresponding N-Boc-3-alkylindole-2-boronic acid, which was prepared from the respective indole derivatives 2 (Scheme 1). Removal of the Boc group from 1a using triuoroacetic acid (TFA) in dichloromethane afforded 4-…”

“…Nevertheless, all derivatives exhibited mechanochromic luminescence, suggesting that their crystal structures are relatively labile compared to the welloverlapping stacking structures of N-Bocindolylbenzothiadiazoles, which do not contain a substituent at the 3-position (R 1 ¼ H) and hence not exhibit mechanochromic properties. 11 For 1e-g, PXRD analyses of the state following the mechanical stimulus were carried out, given that these derivatives exhibited relatively long recovery times. The intensity of the diffraction pattern of 1e signicantly decreased aer grinding ( Fig.…”

“…Recently, we have reported N-Boc-4-(3-methyl-1H-indol-2-yl)-2,1,3-benzothiadiazole (Boc ¼ tert-butoxycarbonyl) derivatives as a new class of non-AIE-type mechanochromic uorophores. 11 The uorophores exhibited autonomously self-recovering mechanochromic luminescence, and the emission color, as well as the recovery time for the color change could be tuned via the substituents on the benzothiadiazole ring. The presence of a methyl group at the 3-position of the indole ring was indispensable to attain mechano-responsive properties, while the presence of the Boc group is important to achieve good uorescence quantum yields in the solid state.…”

Two‐Step Mechanochromic Luminescence of N,N′‐Bis‐Boc‐3,3′‐di(pyren‐1‐yl)‐2,2′‐biindole

“…It is well known that ortho ‐phenylenes and their heteroaromatic analogues form flexible helical structures, in which intramolecular stacking occurs between aromatic rings 1 and 4 (Figure a) . Inspired by our recently developed mechanochromic fluorophores, N ‐ tert ‐butoxycarbonyl (Boc)‐2,3‐disubstituted indole derivatives, we designed N , N ′‐bis‐Boc‐3,3′‐diaryl‐2,2′‐biindole derivatives 1 a – c , as these should exhibit relatively flexible intramolecular stacking between the two aryl moieties at the 3‐ and 3′‐position (Figure b). Herein, we report the synthesis and emission properties of the diarylbiindole derivatives 1 a – c .…”

“…It is well known that ortho-phenylenesa nd their heteroaromatic analogues form flexible helical structures, in which intramolecular stacking occurs betweenaromatic rings 1and 4 (Figure 1a). [11] Inspired by our recently developed mechanochromic fluorophores, N-tert-butoxycarbonyl (Boc)-2,3-disubstituted indole derivatives, [12] we designed N,N'-bis-Boc-3,3'-diaryl-2,2'biindole derivatives 1a-c,a st hese should exhibit relatively flexible intramolecular stacking between the two aryl moieties at the 3-and 3'-position ( Figure 1b). Herein,w er eport the synthesis and emissionp roperties of the diarylbiindoled erivatives 1a-c.A mong these, 3,3'-dipyrenyl-substituted derivative 1c exhibited partially overlapping intramolecular stacking of pyrene rings in the crystalline state, which result in at wo-step mechanochromic luminescencet hat may adopt three different colors.…”