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.