Photochemical Synthesis\nof 7,12-Dioxa[8]helicene and\nDensity Functional Theory Studies: Unravelling the One-Way Valve System\nInvolving Steric Crowding and Aromatic Stability

<i>C</i><sub>2</sub> symmetric chiral 7,12-dioxa[8]helicenes\nwere synthesized through a series of photochemical <i>E</i>–<i>Z</i> isomerization, electrocyclic reaction,\nand oxidation steps in a stepwise sequential manner at both the ends\nof 2,9-di((<i>E</i>)-styryl)naphtho[2,1-b:7,8-b′]difuran.\nThe chemical transformations complemented with density functional\ntheory (DFT) studies delineate some fundamental concepts, exhibiting\ncounter current effects, namely, destabilization caused by increasing\nsteric crowding and stabilization caused by aromatic units on the\noverall transformation. The calculated energy profile diagram unravels\nthe formation of photoinduced intermediate species with increasing\nfree energies for the <i>E</i>–<i>Z</i> isomerization and the electrocyclic reactions; the reverse processes\nfor the said steps are prevented by a specific barrier-less oxidation\nstep forming aromatic rings, presenting a one-way valve situation.\nThe steric crowding-related increase in free energy and its counterbalancing\nby aromaticity have been illustrated for the helicene system using\nDFT studies. HOMA analysis shows that each individual ring in 7,12-dioxa[8]helicene\nexhibits a strong aromatic character, supporting the Fries model empirically.\nInterestingly, despite the nonplanar and sterically crowded geometry,\n7,12-dioxa[8]helicene displayed a large HOMO–LUMO gap, typical\nof aromatic compounds.