Synthesis of 8-heteroatom-substituted 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene dyes (BODIPY)

Thioketones, bis-(5-R-1H-pyrrol-2-yl)-methanethiones (R=H, Me, Et), 4 react with methyl iodide or isopropyl triflate to give the pyrrolium salts, which are treated with tertiary amine and boron trifluoride to produce the 8-(thiomethyl/thioisopropyl) 4,4-difluoro-3,5-di-R-4-bora-3a,4a-diaza-s-indacenes 6a-6c and 8a-8c. The reaction of the methyl thioether group of 6a and 6b with aniline gives the substitution products whose structure corresponds to formula 10. The structures of the thioethers 6a-6c and compound 10a were determined by X-ray diffraction. The thiomethyl groups in 6a-6c are close to be coplanar to the flat ring system, the strain due to the interaction of methyl with the hydrogen at C1 is released by shifting of the sulfur atom away from carbon C1 and opening of the angle C8-S-methyl. This coplanarity of the thiomethyl group with ring system agrees with the preference of the syn conformation of methyl vinyl thioether. In the structures of the aniline compound 10a the length of the nitrogen to C8 is close to that of NC double bond. Thioethers 6a-6c show high wavelength absorption at 523-530 nm and fluorescence with a Stokes shift of 12-24 nm and with a quantum yield of 0.15-0.37. In contrast the aniline substituted compounds 10a and 10b showed absorption at 410 and 430 nm, respectively, with no fluorescence. According to their spectral properties they are better described by structure 10 than 7.