The Theoretical Study Of Substituent And Charge Effects In The Conformational Transformation Process Of Molecular Machine Unit Spiropyran

Molecular machines have obtained extensive attention over recent decades.Spiropyrans are a class of typical photochromic compounds and a unit of molecular machine.Because of the good photallochromy,fast reading speed,lowly cost and highly sensitivity recording,spiropyrans were considered as photochromic materials and optical storage materials widely.Owing to the optical and thermal properties of spiropyran compound can be modified by adjusting the structure of the molecule,spiropyrans have become the focus of the recent study.Besides the photochromism isomerization,the impact of the electrochemical conditions on the spiropyran/merocyanine(SP/MC)isomerization process is also getting more and more attention.In a certain electrochemical cell,spiropyran reduced to form a radical anion and the SP/MC isomerization becomes spontaneous.Therefore,thermal reaction mechanisms for the neutral,cationic and anionic SP/MC isomerization with a series of electron-donating and electron-withdrawing groups in the 6?-position of the benzopyran moiety in ground state have been systematically investigated using the density functional method.Some notable changes in structure,activation energy barrier and reaction rate caused by charge changes of spiropyran have been found and discussed in detail.For anionic SP/MC isomerization,due to the electron density distribution changed,the activation energy barriers decreased and the reaction rate constants of every step increased greatly.The geometrical parameter,bond length alternation(BLA),which relates to the effect of substituents,has been employed to explain the changes in the reaction mechanism induced by different electron-donating and electron-withdrawing substituents.Our results explained the control method of the small unit of the molecular machine conformational transformation process deeply from the theoretical viewpoint.