芳香型偶氮污染物紫外光谱量子力学指认及不同共轭态偶氮键电子激发研究

Quantum mechanics identification of aromatic azo contaminants′UV⁃Vis spectra and electron excitation characters of different conjugated azo bonds

偶氮化合物是重要的工业化学品,也是常见的环境污染物,其紫外激发特征及“分子结构-紫外吸收”的构效关系暂未完全明确.本研究借助量子化学计算,通过对波函数与激发态“电子-空穴”的分析,描述偶氮化合物紫外吸收光谱和激发态特征.结果表明:反式偶氮苯实验紫外光谱的第一主要吸收峰应指认为π→π^(∗)跃迁,对应其S_(0)→S_(2)激发,电子从最高占据轨道(HOMO)跃迁到最低未占据轨道(LUMO);它的第一激发过程(S_(0)→S_(1))振子强度很小(f=0.031),对光谱贡献可以忽略.反式偶氮苯吸收峰位置与顺式相比普遍红移,表明反式结构中偶氮键通过“桥接效应”形成全局共轭,降低了激发能,对偶氮化合物的紫外吸收产生显著影响;在水溶液中,钠离子对偶氮键的电荷分布存在显著扰动,因此,孤立阴离子模型计算得到的甲基橙光谱与实验存在较大误差.“推拉”电子官能团促使甲基橙分子高度极化,与钠离子和水分子作用较强,正确处理偶氮键附近的阳离子扰动是建立预测模型的关键.通过量子化学计算指认偶氮污染物紫外可见吸收光谱能为污染修复、环境工程技术开发、光学材料合成提供理论依据,具有重要意义.

Azo compounds are important industrial chemicals and common environmental pollutants.Their UV excitation characteristics as well as the structure⁃activity relationship between molecular geometry and UV absorption have not been fully elucidated.With quantum calculation,the UV absorption spectrum and excited⁃state characteristics of azo compounds were described via the analysis of wave function and the distribution of“electron⁃hole”in an excited state.The results show that the first main absorption peak in the experimental UV spectrum of the Trans⁃azobenzene should be referred to theπ→π∗type,corresponding to its S_(0)→S_(2) excitation,in which electron is exited from the highest occupied orbital(HOMO)to the lowest unoccupied orbital(LUMO).The oscillator strength of its S_(0)→S_(1) excitation,also its first theoretical excitation,is small and account negligibly for its experimental adsorption spectrum.Generally,the absorption peak of trans⁃azobenzene exhibits red⁃shift compared to that of the cis⁃,indicating that the azo bond in trans structure forms a conjugation through"bridging effect",which reduces the excitation energy and has a significant effect on the UV absorption of the azo compounds.However,in the aqueous solution,the charge distribution of azo bonds is significantly disturbed in the presence of sodium ion.Therefore,the spectrum calculated by the isolated anion model would not match the experimental results.The"push⁃pull"ionizable functional group makes the methyl orange molecule highly polarized,which can form strong interaction with sodium ions and water molecules.Thus,considering the cationic disturbance effect on the azo bond is important to establish reasonable model.The identification of UV⁃Vis absorption spectra of azo pollutants by quantum calculation would be significant to provide theoretical basis for pollutant remediation,development of environmental technologies and synthesis of optical materials.