Dibenzofuran/Dibenzothiophene-Embedded Dithia-bis(calix)-sapphyrins

A series of first examples of dibenzofuran (DBF)/dibenzothiophene (DBT)-embedded dithia-bis(calix)-sapphyrins were synthesized by condensing 1 equiv of dibenzofuran/dibenzothiophene-based tripyrrane with 1 equiv of [2,2'-bithiophene]-5,5'-diylbis(aryl)methanol under mild acid-catalyzed conditions in CH₂Cl₂ followed by oxidation with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) and alumina column chromatographic purification afforded new dithia-bis(calix)-sapphyrins with two meso-sp³ carbons in 5-7% yields. The DBF/DBT-embedded dithia-bis(calix)-sapphyrins were characterized by HRMS (high-resolution mass spectrometry), ¹H and ¹³C NMR, ¹H-¹H COSY, ¹H-¹H NOESY, ¹H-¹³C HSQC, and ¹H-¹³C HMBC spectroscopy, absorption spectroscopy, cyclic voltammetry, and density functional theory (DFT) studies. The macrocycles showed one broad absorption band at ∼553 nm with a shoulder peak at the higher energy side along with a sharp intense band at ∼415 nm. However, the protonated dithia-bis(calix)-sapphyrins showed large bathochromic shifts in the absorption bands, indicating that the electronic properties of dithia-bis(calix)-sapphyrins were altered significantly upon protonation of dithia-bis(calix)-sapphyrins. The electrochemical study indicated that dithia-bis(calix)-sapphyrins are relatively easier to reduce but difficult to oxidize. The DFT studies revealed that macrocycles adopt a highly folded half-chair conformation due to the disruption of conjugation of the macrocycle because of the presence of two sp³ meso-carbons. The DFT studies also support the significant bathochromic shifts observed for protonated dithia-bis(calix)-sapphyrins macrocycles.