胭脂红和苋菜红分子结构与光谱性质的对比研究

Comparative Study on the Molecular Structures and Spectral Properties of Ponceau 4R and Amaranth

应用英国Edinburgh FLS920P稳态-瞬态荧光光谱仪,对互为同分异构体的胭脂红、苋菜红分子的吸收光谱和荧光光谱实验检测,得到二者的光谱特性参数,并进行对比。分别采用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)对这两种分子的基态和激发态结构进行优化,将所得基态分子构型进行频率计算,结果显示均无虚频存在,进而比较两分子构型在不同能态下的差异。在此基础上,应用TD-DFT并结合极化连续介质模型(PCM)在6-311++G(d,p)水平上分别计算二者的吸收光谱和荧光光谱,对两种分子的发光机制、荧光光谱特性差异与分子结构的关系进行了分析。结果表明,两种分子的基态结构均为非平面,它们中的两个萘环不共面,存在一定的夹角;苋菜红含有分子内氢键,且氢键所在萘环的平面性要优于胭脂红分子对应的萘环部分;激发态时二者各自的两个萘环均共平面。量子化学计算得到的光谱理论值与实验结果较为吻合,说明优化所得胭脂红与苋菜红的分子构型基本合理。与苋菜红相比,胭脂红右侧萘环结构的平面性稍差,分子从激发态跃迁回到基态经历了更多的振动和转动,损耗了更多的能量,导致用来产生荧光光子的能量减少,因此,胭脂红的荧光发射波长更长。本文首次得到胭脂红、苋菜红分子基态与激发态下的分子结构信息,并找出二者光谱特性差异的原因,结果可为研究同分异构体分子的光谱特性与分子结构的关系提供参考。

The Edinburgh FLS920Psteady-instantaneous fluorescence spectrometer was applied on the detection of the absorption and the emission spectra of ponceau 4Rand amaranth,which are isomers to each other.After that,the spectral parameters of them were compared.Then,the density functional theory（DFT）and time-dependent density functional theory（TD-DFT）were used on the optimization of ponceau 4Rand amaranth under the ground and excited state,respectively,in order to compare the differences in configurations of them under different states.On the base of the results above,the absorption and emission spectra of the two isomers were calculated with TD-DFT,and the polarized continuum model（PCM）was applied on the base of6-311＋＋G（d,p）.The fluorescence mechanism,the relationships between the properties of fluorescence spectra and the molecular geometry were all analyzed.The results shows that,the structures of the two molecules are non-planar,these two naphthalene rings are not co-planar,respectively,and there＇s hydrogen bond in amaranth.When the two isomers were on the ground state,the planarity of the naphthalene ring which exists the hydrogen bond mentioned above in amaranth is better than the corresponding part of ponceau 4R.The two isomers are nearly co-planar when they＇re on the excited state.The molecular structures of ponceau 4Rand amaranth optimized above are basically reasonable,for the quantum chemistry calculation spectral results are agree with the experiments.The planarity of the naphthalene rings on the right side in ponceau 4Ris worse than that in amaranth,the ponceau 4R molecule experienced more vibration and rotation from the excited to the ground state,lost more energy,which lead to the reduction of energy for emitting fluorescent photons.So ponceau 4Rhas longer fluorescence emission wavelength than amaranth.In this paper,the molecular structure information of ponceau 4Rand amaranth were obtained,and the differences of the spectral characteristics between them were found out.The results can provide references for the study of the relationship between the spectral properties and the configuration of isomers.