Synthesis and Electrochemical Characterization of Acetylacetone (acac) and Ethyl Acetate (EA) Appended β-Trisubstituted Push–Pull Porphyrins: Formation of Electronically Communicating Porphyrin Dimers

Abstract

Two new families of “push–pull” tetraphenylporphyrins with one acetylacetone (acac) or ethyl acetate (EA) moiety at a β-pyrrole position of the macrocycle and two Br or Ph substituents at the antipodal β-positions were synthesized and structurally, spectroscopically, and electrochemically characterized. The examined porphyrins are represented as MTPP(R)2acac and MTPP(R)2EA (where R = Br or Ph and M = H2, Co, Ni, Cu, or Zn). NiTPP(Br)2acac exhibits an extremely nonplanar conformation (Δ24 = 0.44 Å, ΔCβ = 0.82 Å), while H2TPP(Br)2EA and ZnTPP(Ph)2EA exhibit a quasi-planar conformation. All of the synthesized acac-appended porphyrins show a keto–enol tautomerism in solution, which results in formation of hydrogen bonded dimers as evidenced by 1H NMR and mass spectrometry. Dimers were also detected under the electrochemical conditions for the dibromo derivatives but not the diphenyl substituted porphyrins. A facile stepwise and reversible electrogeneration of the electronically communicating porphyrin dimers is observed for MTPP(Br)2acac where M = CuII, NiII, or ZnII.