Abstract
A series of α- and β-ethynyl-substituted BODIPY derivatives (3a, 4a, 5a, 5b, 6a, 6b) were synthesized by gold(I)-catalyzed direct C–H alkynylation reactions of dipyrromethane and BODIPY, respectively, with ethynylbenziodoxolone (EBX) in a regioselective manner. Depending on the position of the ethynyl substituent in the BODIPY skeleton, the photophysical properties of the resulting α- and β-substituted BODIPYs are notably altered. The lowest S0–S1 transition absorbance and fluorescence bands are both bathochromically shifted as the number of substituents increases, while the emission quantum yields of the β-ethynylated derivatives are significantly lower than those of α-ethynylated ones. The current method should be useful for fine-tuning of the photophysical properties of BODIPY dyes as well as for constructing BODIPY-based building cores for functional π-materials.
Highlights
Inspired by the works of Waser and co-workers showing the gold(I)-catalyzed C–H electrophilic alkynylation of various heterocycles with ethynylbenziodoxolone (EBX) as an activated ethynyl synthon [38,39,40,41,42], we investigated the synthesis of ethynyl-substituted BODIPY derivatives 3–6 by gold(I)-catalyzed direct C–H functionalization (Figure 1c)
By taking advantage of the reactivity of β-(2 and 6)-positions of BODIPY (1), which are susceptible to electrophilic reactions, β,β'-diethynyl-substituted BODIPYs 5 and 6 were prepared regioselectively, through the C–H alkynylation of 1 without any directing groups
The yield of the β,β'diethynylated product 6b was improved to be 19%. These results indicate that electron-rich substrates facilitate the direct electrophilic alkynylation of the BODIPY core
Summary
Boron-dipyrromethene (BODIPY, 1) and its derivatives are representative families of fluorophores that have been widely used in applications for bioimaging [1,2,3,4,5,6], photodynamic therapy [7,8,9,10,11,12], photocatalysis [13,14,15,16], optics [17,18,19,20], and so forth. Inspired by the works of Waser and co-workers showing the gold(I)-catalyzed C–H electrophilic alkynylation of various heterocycles (e.g., pyrroles, indoles, etc.) with ethynylbenziodoxolone (EBX) as an activated ethynyl synthon [38,39,40,41,42], we investigated the synthesis of ethynyl-substituted BODIPY derivatives 3–6 by gold(I)-catalyzed direct C–H functionalization (Figure 1c). By taking advantage of the reactivity of β-(2 and 6)-positions of BODIPY (1), which are susceptible to electrophilic reactions, β,β'-diethynyl-substituted BODIPYs 5 and 6 were prepared regioselectively, through the C–H alkynylation of 1 without any directing groups.
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