Monofunctionalized 1,3,5,7-TetraarylazaBODIPYs and Their Application in the Synthesis of AzaBODIPY Based Conjugates.

A series of monofunctionalized 1,3,5,7-tetraarylazaBODIPYs containing functional groups such as p-hydroxymethyl phenyl, p-hydroxyphenyl, p-cyanophenyl, p-nitrophenyl, and p-formylphenyl groups at the 1-position of the azaBODIPY core were synthesized by mixed condensation of two different nitrochalcones in n-butanol in the presence of CH3COONH4 at reflux followed by complexation with BF3·OEt2. The mixed condensation of nitrochalcones resulted in the formation of three different dipyrromethenes, which was treated with BF3·OEt2 to afford the desired monofunctionalized tetraarylazaBODIPYs in 30-36% yields. To demonstrate the application of monofunctionalized tetraarylazaBODIPYs, we reacted monoformyl functionalized tetraarylazaBODIPY with excess pyrrole to afford mono-dipyrromethanyl-substituted tetraarylazaBODIPY, which was used as a key precursor to prepare novel covalently linked azaBODIPY-based conjugates. The mono-dipyrromethanyl azaBODIPY was in situ oxidized with 2,3-dichloro-5,6-dicyanobenzoquinone and either reacted with BF3·OEt2 to afford azaBODIPY-BODIPY conjugates or reacted with metal salt such as Pd(acac)2 to afford azaBODIPY-Pd(II)dipyrrin conjugates. Alternately, dipyrromethanyl-substituted azaBODIPY was condensed with dipyrromethane dicarbinol or 16-oxatripyrrane under mild acid catalyzed conditions followed by oxidation and chromatographic purification to afford azaBODIPY-porphyrin or azaBODIPY-oxasmaragdyrin conjugates, respectively. The photophysical studies on conjugates revealed that azaBODIPY is a good energy acceptor and invoked the possibility of energy transfer from the donor to acceptor in covalently linked conjugates.