Theoretical investigation on excited-state cyclization reactions of platinum-sensitized dithienylethene complexes

The ring-closing reaction commonly occurs at the lowest singlet excited state when the open-form dithienylethene is irradiated at about 300 nm. A lower-energy light, at 425 nm, can also elicit this ring-closing reaction when there is a connection between the dithienylethene and platinum-terpyridyl segment to form a complex through an ethynyl linker or an ethynyl-ether linker. Through the calculation of the energy levels, we propose the ring-closing process as follows. The light absorbed by the platinum-terpyridyl unit excites the molecule to a singlet excited state. Meanwhile, this electronic state of the molecule transfers to the lowest triplet excited state through intersystem crossing and internal conversion. When energy is asborbed from the environment, this state goes up to a higher triplet state around the dithienylethene part, where the ring-closing reaction takes place. Moreover, different patterns of linkers bring about different efficiency of the reaction, and a direct shared linker may facilitate the ring-closing process. In addition, the conjugated linker also causes the maximum wavelength of the complex to red shift because the energy gap between the involved frontier molecular orbitals becomes lower.