Synthesis and properties of covalently linked di-p-benzihomoporphyrin-BODIPY conjugates

Two mono meso-functionalized [20]di-p-benzihomoporphyrins containing p-formylphenyl and p-iodophenyl groups at meso-position respectively were synthesized by condensing one equivalent of appropriate tetrapyrrane with one equivalent of p-formyl benzaldehyde/p-iodo benzaldehyde in CH2Cl2 under mild acid catalyzed conditions. The meso-formylphenyl and meso-iodophenyl functionalized di-p-benzihomoporphyrins were used to synthesize two covalently linked di-p-benzihomoporphyrins-BODIPY conjugates. The meso-formylphenyl-functionalized di-p-benzihomoporphyrin was converted to corresponding meso-dipyrrolyl substituted di-p-benzihomoporphyrin by treating with excess pyrrole under acid catalyzed conditions. In the next step, the meso-dipyrrolyl di-p-benzihomoporphyrin was subjected to oxidation followed by BF2 complexation to afford the directly linked di-p-benzihomoporphyrin-BODIPY conjugate. The meso-iodophenyl functionalized di-p-benzihomoporphyrin was coupled with ethynyl-functionalized BODIPY under mild Pd(0) coupling condition to synthesize diphenylethyne-bridged di-p-benzihomoporphyrin-BODIPY conjugate. The two conjugates were characterized by HR-MS, NMR, absorption, electrochemical, fluorescence and DFT studies. The spectral and electrochemical studies indicated that the two constituents, di-p-benzihomoporphyrin and BODIPY units in the conjugates interact weakly and retain their individual characteristic features. DFT studies indicated a possibility of charge transfer between di-p-benzihomoporphyrin and BODIPY units in conjugates.