Electron-Deficient Bipyrrole Boomerangs: Bright Fluorophores Obtained via Double C-H Bond Activation.

Pd -mediated annulative double C-H activation is shown to efficiently convert 1,n-dipyrrolylalkanes into extensively π-conjugated bipyrroles not accessible by conventional oxidative coupling protocols. This approach is applicable to both electron-rich and electron-deficient systems, and has been further developed into tandem processes involving further cyclization of substituents or oxygenation of pyrrolic α-positions. The new bipyrrole intermediates show enhanced fluorescence as well as tunable optical properties controlled by the alignment of chromophore subunits. Photophysical data, including femtosecond transient absorptions, reveal solvent-induced intramolecular charge transfer in their excited states, dependent on the polarity of the medium.