Photochemical Electrocyclization of Poly(phenylacetylene)s: Unwinding Helices to Elucidate their 3D Structure in Solution
Francisco Rey‐Tarrío, Rafael Rodríguez, Emilio Quiñoá, Ricardo Riguera, Félix Freire
Abstract
Abstract Photochemical electrocyclization of poly(phenylacetylene)s (PPAs) is used for the structural elucidation of a polyene backbone. This method not only allows classification of PPAs in cis‐cisoidal (ω 1 <90°) or cis‐transoidal structures (ω 1 >90°), but also approximating ω 1 . A PPA solution is illuminated with visible light and monitoring the photochemical electrocyclization of the PPA helix by measuring the ECD spectra at different times. PPAs with a cis‐cisoidal structure show a reduction of the ECD signal of at least 50 % before 30 min of irradiation, while cis‐transoidal helices need much longer time because the transoidal bond must be isomerized. The different cis‐cisoidal and cis‐transoidal helices require different times to decrease their ECD signal by 50 % (t 1/2 ), depending on the degree of compression or stretching of the helix, establishing a relationship between the secondary structure adopted by PPA (ω 1 ) and the time required to lose the ECD vinylic signal by light irradiation.