Photochemical Synthesis of 7,12-Dioxa[8]helicene and Density Functional Theory Studies: Unravelling the One-Way Valve System Involving Steric Crowding and Aromatic Stability

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