三元甲醇辅助质子转移的动力学旁证:硝基联苯酚激发三重态的瞬态吸收研究

Kinetic Evidence for Methanol Trimer Assisted Proton Transfer:Transient Absorption Study on Excited Triplet State of 4-Hydroxy-4′-nitrobiphenyl

羟基芳烃化合物是一种典型“光酸”,在催化合成和生物过程中具有广泛应用、由于激发单重态寿命较短,大多数具有中等激发态酸度的羟基芳烃化合物无法在非水溶剂(如醇)中解离.硝基联苯酚中的硝基可增加激发单重态和三重态间的自旋轨道耦合,通过超快的系间窜越产生最低激发三重态,且具有较高量子产率.本文利用瞬态吸收光谱和动力学分析发现:尽管三重态硝基联苯酚具有中等酸性,但由于其寿命较长,能够与甲醇发生质子转移反应,三重态硝基联苯酚连.续与三个甲醇分子反应形成氢键复合物是质子转移的决速步骤.这--发现表明,具有较长激发态寿命的光酸可以通过具有足够碱性的醇类低聚物诱导其解离.

Hydroxyaromatic compounds have a wide range of applications in catalytic synthesis and biological processes due to their enhanced acidity upon photo-excitation.Most hydroxyaromatic compounds with a medium excited state acidity are unable to deprotonate in non-aqueous solvents such as alcohol due to their short-lived excited singlet states.The nitro group in 4-hydroxy-4'-nitrobiphenyl(NO2-Bp-OH)increases the spin-orbit coupling between excited singlet states and the triplet state,resulting in ultrafast intersystem crossing and the formation of the long-lived lowest excited triplet state(T1)with a high yield.Using transient absorption spectroscopy and kinetic analysis,we discover that,despite its moderate acidity,the T1 state of NO2-Bp-OH(3NO2-Bp-OH)is able to transfer proton to methanol.Following the formation of the hydrogen-boned complex between 3NO2-Bp-OH and three methanol molecules in a consecutive process,proton transfer occurs very fast.This finding suggests that the long lifetime of the photoacid excited state allows for the formation of alcohol oligomer with sufficient basicity to induce photoacid deprotonation.