Synthesis and properties of covalently linked thiaporphyrin–ferrocene conjugates

Four examples of ferrocene-thiaporphyrin conjugates in which the ferrocenyl group was covalently connected either directly at meso-position of thiaporphyrin or to meso-phenyl group of thiaporphyrin via ethyne bridge were prepared by coupling bromo- or iodo thiaporphyrin with α-ethynylferrocene under mild Pd(0) coupling conditions. NMR, absorption and electrochemical studies indicated that the thiaporphyrin and ferrocenyl units interact strongly in ethyne bridged porphyrin-ferrocene conjugates but the interaction is very weak in phenyl ethyne bridged porphyrin-ferrocene conjugates. The steady state fluorescence studies indicated that the fluorescence yields are reduced to 50% in phenyl ethyne conjugates but the fluorescence is completely quenched in ethyne bridged conjugates. The partial or complete quenching of porphyrin fluorescence in these conjugates is due to electron transfer from ferrocene unit to excited state of porphyrin sub-unit. Oxidation of ferrocene to ferrocenium ion with an oxidizing agent in ethyne bridged conjugates resulted in a recovery of porphyrin fluorescence.

Synthesis of four thiaporphyrin–ferrocene conjugates and their spectral, electrochemical and photophysical properties are described.Figure optionsDownload as PowerPoint slideResearch highlights
► Phenyl ethyne and ethyne bridged thiaporphyrin–ferrocene conjugates were synthesized under mild Pd(0) mediated coupling reaction conditions.
► The spectral and electrochemical properties indicated that porphyrin and ferrocene units interact very weakly in phenyl ethyne bridged conjugates but strongly in ethyne bridged conjugates.
► The fluorescence was strongly quenched in phenyl ethyne bridged conjugates and completely absent in ethyne bridged conjugates due to photo-induced electron transfer from ferrocene to porphyrin unit.
► Upon oxidation of ferrocene to ferrocenium ion, the fluorescence of porphyrin in ethyne bridged conjugates was restored to a significant extent suggesting that these conjugates can act as fluorescence on/off switches.