Synthesis of Novel Push-Pull ?-substituted ZnII-Porphyrinates by a Microwave Assisted Sonogashira Coupling Approach

Inspired by the process of solar energy collection by photosynthetic cores of bacteria and plants, involving a porphyrinic centre as light harvesting chromophore, porphyrinic structures have been considered as interesting dyes so that some of them have been synthesized and investigated for applications in Dye Sensitized Solar Cells (DSSCs).1 Even though meso disubstituted push-pull ZnII-porphyrinates are featured by interesting light conversion efficiencies, their synthesis required multi-step procedures. Conversely, ?-substituted tetraaryl ZnII-porphyrinates, which involve a tetraryl porphyrinic core as starting material, are synthetically more attractive,2 since this core can be easily obtained by means of a one-pot reaction between pyrrole and the appropriate aryl aldehyde. The more sterically hindered architecture of the tetraryl porphyrinic core guarantees a decrease of ?-staking aggregation of the dye when adsorbed on TiO2 photoanode, and further promotes a superior passivation of the surface against charge recombination with the oxidized specie (I3-) of the electrolyte.3 Hence these evidences, together with the promising light conversion efficiencies for DSSCs, have encouraged us to develop a facile synthetic pathway to obtain in good yield a series of ?-substituted [tetrakis-(3,5-di-tert-butylphenyl)porphyrinates]ZnII.2,4 Here, we report the optimized conditions to functionalize the porphyrinic core in ?-pyrrolic position by using a light-induced bromination procedure. Subsequently a microwave enhanced Sonogashira coupling is exploited to circumvent the low reactivity of such brominated derivatives successfully providing both mono and, for the first time, disubstituted push-pull ZnII-porphyrinates bearing a variety of substituents.