Photoisomerization of “Partially Embedded Dihydropyridazine” with a Helical Structure

Abstract

AbstractHerein, we report the synthesis of two “partially embedded fused‐dihydropyridazine N‐aryl aza[5]helicene derivatives” (PDHs) and the demonstration of their intrinsic photo‐triggered multi‐functional properties based on a Kekulé biradical structure. Introducing bulky electron‐withdrawing trifluoromethyl or pentafluoroethyl groups into the aza[5]helicene framework (PDH‐CF3 and −C2F5) gives PDH axial chirality based on the helicity of the P and M forms, even at room temperature. Upon photo‐irradiation of PDH‐CF3 in a frozen solution, an ESR signal from the triplet biradical with zero‐field splitting values, generated by N−N bond dissociation, was observed. However, when the irradiation was turned off, the ESR signal became silent, thus indicating the existence of two equilibria: between the biradical and quinoidal forms based on the Kekulé structure, and between N−N bond cleavage and recombination. The observed photo‐ and thermally induced behaviors indicate that T‐type photochromic molecules are involved in the photoisomerization mechanism involving the two equilibria. Inspired by the photoisomerization, chirality control of PDH by photoracemization was achieved. Multiple functionalities, such as T‐type photochromism, photo‐excitation‐mediated triplet biradical formation, and photoracemization, which are attributed to the “partially embedded dihydropyridazine” structure, are demonstrated.