Theoretical Study On The Photoisomerization Mechanism Of Light Stimuli-responsive Materials

The diarylethene is a typical photoisomerization molecule, which is widely usedin molecular switches, signal transmission, data storage, electroluminescent devices,and other areas because of its good thermal stability and fatigue resistance. Whenadding the diarylethene molecule to other photosensitive molecules, such as aplatinum(II) terpyridyl complex, a fulgimide, and an azobenzene, the combinedsystem may bring some new optical and electrical properties. And in this paper, weinvestigated these different systems through theoretical calculations.1. Theoretical Investigation on Excited-State Cyclization Reactions ofPlatinum-Sensitized Dithienylethenes ComplexesThe ring-closing reaction occurs at the lowest singlet excited state commonlywhen the open-form dithienylethene is irradiated at about300nm. And connecting thedithienylethene and platinum-terpyridyl segment to be a complex through an ethynyllinker or an ethynyl-ether linker, a lower-energy light at425nm can also arouse thisring-closing reaction. Through the calculation of the energy levels at singlet andtriplet excited states, we propose the ring-closing process as follows. The lightabsorbed by the platinum-terpyridyl unit excites the molecule to a singlet excited state.Meanwhile, this electronic state of the molecule transfers to a lower-energy tripletexcited state through intersystem crossing. Then, the ring-closing reaction takes placein the triplet state. Moreover, different patterns of linkers bring about differentefficiency of the reaction. And a direct shared linker may facilitate the ring-closingprocess. In addition, the conjugated linker also causes the maximum wavelength ofthe complex red shift, because the energy gap between the involved frontier molecularorbitals becomes lower.2. Theoretical investigation on the spectroscopy prosperities of four isomers of anencoder molecule FGDTE A molecule consisting of one dithienylethene covalently linked to two fulgimidephotochromes has been designed as a4-to-2encoder. The combined molecule hasfour stable isomers, one may convert to another by particular irradiations. In thispaper, we calculated the Gibbs free energies of the four isomers and got the order oftheir stabilities. Then the mechanisms of ring-opening and ring-closing of themonomers were explored in detail and the reaction conditions for structuralconversion were also discussed. The corresponding absorption spectra of thecombined molecules were researched and the new characters in the designed systemswere predicted as well. Finally, we optimized their corresponding excited-statestructures and analyzed the emission properties.3. Theoretical investigation on remote-control photocycloreversion ofdithienylethene driven by azobenzene chromophoresWhen adding two azobenzene chromophores on a dithienylenthene molecule, theirradiation at450nm can effectively arouse the cycloreversion reaction of thering-closed dithienylenthene. We investigated the frontier molecular orbitals andabsorption properties of such series of molecules and inferred the mechanism of thering-opening reaction. The450nm light sensed by the side azobenzene groups canexcite the whole molecule to a high excited state, and through electronic transitionand energy transfer the active electron may centre on the dithienylenthene ring part,then the ring-opening happens. While for the ring-open form, the energy of the450nm light is not high enough to promote the electron to the exact molecular orbitaloccupying the central dithienylenthene ring, so it cannot cause the ring-closingreaction.