Regulation Of Metastable-state Photoacid By Cyclodextrins

In recent years,metastable-state photoacid（mPAH）has received extensive attention from chemists and materials scientists.It can be excited by visible light and has excellent properties such as long metastable lifetimes,reversibly donate and recycle protons,weak dark acidity and strong photoacidity.In this work,UV-Vis absorption spectroscopy was used to study the stoichiometry and inclusion constants of the inclusion complexes between mPAH1 andβ-cyclodextrin（β-CD）,γ-CD,（2-hydroxypropyl）-β-cyclodextrin（HP-β-CD）or HP-γ-CD.It was reported that different CDs can make effects on kinetics of thermal relaxation reaction,ground-state acidity and metastable acidity,thermodynamics and kinetics of MC?SP isomerization,and hydrolytic stability of mPAH1.The inclusion interaction between CDs and mPAH1 and thermodynamics properties of inclusion complexes were also analyzed by theoretical calculations.1.The Benesi-Hildebrand method was used to characterize the inclusion of mPAH1withγ-CD,β-CD,HP-γ-CD and HP-β-CD at the molar ratio of mPAH1:CD=1:2,and the relevant inclusion constants were 2.99×10⁵,1.78×10⁵,1.20×10⁵,and 6.59×10⁴M^-2,respectively.Our analysis suggests thatγ-CD and HP-γ-CD are better thanβ-CD and HP-β-CD on the matching degree between cavity size of the host molecules and mPAH1,and the steric hindrance of 2-hydroxypropyl makes the stability of inclusion complexes reduce.2.At 298 K,the thermal relaxation reaction rate constant of mPAH1 is 8.21×10^-3s^-1,and the thermal relaxation reaction rate constants of inclusion complexes are 1.81×10^-3-3.06×10^-3s^-1,equaling 0.22 to 0.37 times of mPAH1.Kinetic parameters such as activation energy（E_a）,activation enthalpy（ΔH^≠）and activation entropy（ΔS^≠）were calculated by measuring the rate constants of thermal relaxation reactions at different temperatures.For mPAH1 and four inclusion complexes[mPAH1·（γ-CD）₂],[mPAH1·（HP-γ-CD）₂],[mPAH1·（β-CD）₂],and[mPAH1·（HP-β-CD）₂],their values of E_a,ΔH^≠andΔS^≠of the thermal relaxation reaction decreased sequentially,and theΔH^≠andΔS^≠of the four inclusion complexes showed a good linear relationship.Based on these results,we hypothesize that polarity of microenvironment where SP type of mPAH1 exists in the above substances decrease sequentially,such that the energy barrier,corresponding with the process of the configurational rotation of the cis-MC transition state where the C-O bond is broken,decreases sequentially.3.The ground-state acidic p K_a^GSand metastable acidic p K_a^MSof mPAH1 and the four inclusion complexes were determined,and the p K_a^GSand p K_a^MSof the four inclusion complexes were found to be 0.86-1.22 and 0.96-1.32 units lower than that of mPAH1,respectively.It shows that the ground state acidity and metastable acidity are enhanced after the formation of inclusion complexes.The enhanced metastable acidity of the inclusion complexes may make mPAH1 match with the basicity of weaker proton acceptors,thereby increasing the efficiency of the photoswitchable switch.4.The thermodynamics and kinetics of the MC?SP isomerization reaction of inclusion complexes were studied with mPAH1 as reference.After inclusion,the main driving force for the reaction is changed from the entropy increasement of mPAH1 to the enthalpy decreasement of the four inclusion complexes.At the same temperature,the rate constants of the isomerization reaction of the four inclusion complexes are significantly improved,and the activation energy is significantly reduced,which is attributed to the weak interaction between the host and guest molecules and the weakly polar microenvironment provided by the hydrophobic cavity of the host molecule.5.We also studied the hydrolytic stability of mPAH1,and found that HP-γ-CD could hardly inhibit the hydrolysis of mPAH1,andβ-CD,γ-CD and HP-β-CD could significantly inhibit the hydrolysis of mPAH1,which may be related to the degree of complete encapsulation of C=C bond deriving from mPAH1 by the hydrophobic cavity and whether CDs can effectively prevent the nucleophile OH^-from attacking C=C bond deriving from mPAH1.In addition,after the formation of the inclusion complexes,their reversibility adjusting the p H value of the solution controlled by visible light is also good.6.According to IGMH theory,we employed the Multiwfn 3.8 software to carry out theoretical calculations of the inclusion of mPAH1 and CDs.The simulation results corroborated the inclusion ratio and inclusion constant measured by spectroscopic method,and we get the inclusion site.Moreover,it also proved that the interaction between host and guest is the hydrogen bond.