Tuning of Aromaticity in Meso-Thienyl 22-Oxabenzonorroles

Meso-thienyl 22-oxabenzonorroles containing one to three thienyl groups at the meso-positions were synthesized over sequence of four simple steps in 11%-15% yields using readily available oxabenzobilanes as the key precursors. Interestingly, in meso-monothienyl 22-oxabenzonorrole and meso-trithienyl 22-oxabenzonorrole, the chloro group was present at the inner core carbon of indole moiety of respective 22-oxabenzonorrole macrocycle whereas chloro was not present at the inner core carbon of meso-(bis)thienyl 22-oxabenzonorrole. The X-ray crystallography revealed that the meso-(bis)thienyl 22-oxabenzonorrole was almost planar like previously reported meso-tritolyl 22-oxabenzonorrole whereas the meso-monothienyl 22-oxabenzonorrole was nonplanar and the chloro substituted indole ring was deviated by an angle of 52.02° from the mean plane. NMR, NICS, and AICD studies indicated a possibility of tuning the aromaticity in the meso-thienyl 22-oxabenzonorroles depending on the presence of chloro group on inner core carbon of the macrocycles. The meso-monothienyl 22-oxabenzonorrole and meso-trithienyl 22-oxabenzonorrole were relatively less aromatic than meso-(bis)thienyl 22-oxabenzonorrole. The absorption spectra of meso-thienyl 22-oxabenzonorroles displayed sharp and well-defined absorption bands spanning over the visible region and the electrochemical studies indicated the electron rich nature of the macrocycles. DFT and TD-DFT studies were in agreement with the experimental observations.