Stable nonaromatic [20]dithiaporphyrin (2.1.1.1) macrocycles: Synthesis, structure, spectral, electrochemical, and metal ion sensing studies

Stable nonaromatic [20]dithiaporphyrin (2.1.1.1) macrocycles were synthesized in decent yields by condensing readily available butene-2,3-diyl-bisthiophene-2,5-diyl-bis(p-methoxyphenylmethanol) with different meso-aryl dipyrromethanes under mild acid-catalyzed conditions. The [20]dithiaporphyrin (2.1.1.1) macrocycles are the first members of the expanded porphyrin analogues of [18]-dithiaporphyrin (1.1.1.1) and consist of two pyrroles and two thiophenes connected through five meso-carbon bridges. The [20]dithiaporphyrin macrocycles were confirmed by mass spectroscopy, 1D and 2D NMR spectroscopy, and X-ray crystallography. The X-ray structure revealed that the macrocycle is highly distorted and that the two thiophene rings are completely out-of-plane from the "mean-plane" defined by the dipyrromethene moiety and the two meso-carbons. In the absorption spectrum, the macrocycles showed one strong band at ∼420 nm and one weak band at ∼720 nm. The electrochemical studies revealed that the macrocycles are stable under redox conditions. The metal sensing studies indicated that these macrocycles have the potential to sense specific metal ions such as Hg²⁺ ions. Two covalently linked dithiahomoporphyrin-fluorophore dyads were synthesized by coupling iodo-functionalized dithiahomoporphyrin with an ethynyl-functionalized fluorophore such as boron-dipyrromethene (BODIPY) and BF₂-smaragdyrin under mild Pd(0) coupling conditions. The potential of these dyads as a fluorescent sensor for Hg²⁺ was explored, and the studies indicated that both dyads can be used as fluorescent sensors. (Chemical Equation Presented).