核黄素在不同极性溶剂中的光谱特性研究

实验测得核黄素在水、二甲基亚砜(DMSO)和三氯甲烷三种不同极性溶剂中的稳态吸收光谱、荧光光谱和时间分辨荧光光谱,研究了溶剂对核黄素光谱性质的影响。实验结果表明,在不同极性的溶剂中,核黄素的吸收峰位置几乎不变,而荧光光谱峰值随着溶剂极性的增大而出现红移。这是由于溶质分子的电子激发及溶剂化效应引起的电子重新分布导致它在极性溶剂中第一激发单重态能级的变化。在时间分辨荧光光谱实验中,核黄素在水溶液中荧光寿命也高于在其他两种溶剂中,荧光寿命的延长可归因于核黄素与氢键对体溶剂之间的分子间氢键相互作用。应用Gaussian09软件,采用密度泛函理论和含时密度泛函理论,结合基于密度的溶剂化模型,对不同极性溶剂中的核黄素分子进行基态和激发态优化和计算。通过前线分子轨道分析,核黄素的受激跃迁属于苯环和含氮杂环上的π电子向苯环及C N,C O共轭双键的反键轨道π*的跃迁。分子偶极矩的计算结果表明,核黄素分子的第一激发态偶极矩大于基态偶极矩,偶极矩的增大,导致溶质与溶剂分子之间的相互作用的增大。而溶剂分子与溶质分子基态和激发态的相互作用程度不同,使得吸收峰和荧光峰出现不同变化情况。极性越大的溶剂越有利于对激发态的稳定作用,使激发态能量降低,相应的发射波长发生红移。最后,通过分子表面静电势和弱相互作用分析,在水溶剂中考虑氢键作用对核黄素分子光谱的影响。多聚体结构的理论吸收和发射峰值更接近实验结果,说明多聚体结构合理。水分子二聚体与核黄素形成的环状结构,有利于提高核黄素分子的刚性,有利于荧光的发射,减少非辐射跃迁的几率,荧光寿命延长。     

激发态下甘氨酸与水分子间氢键性质研究

在生物体中氨基酸通常以水作为溶剂,是形成细胞的重要成分.在该环境下,分子间氢键的产生会对氨基酸分子与水分子的结构和性质产生影响.为了研究其在基态和激发态下的性质,本文利用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)对甘氨酸分子和H₂O分子在基态和激发态下的分子间氢键的静电势、键长、自然键轨道(NBO)电荷、分子中的原子理论(AIM)分析、Wiberg键级b、红外(IR)光谱、空穴-电子轨道和基态与激发态之间的电子转移进行了理论研究.结果表明：分子间氢键的形成会导致分子结构的改变和红外光谱振动频率的移动.在激发态下,分子间氢键有不同程度的增强或减弱.该计算结果为氢键的形成和激发态下分子间氢键的研究提供理论依据.     

外电场下AgCl分子的结构与性质

采用密度泛函理论B3P86方法计算了AgCl基态分子在不同场强条件下的稳定结构及激发态性质。研究了外电场对AgCl基态分子键长、总能量、电子结构、谐振频率和红外光谱强度及前10个激发态的吸收谱、激发能、振子强度的影响。结果表明随着沿Ag-Cl分子轴向电场的逐渐增大, AgCl分子键长逐渐减小;总能量则逐渐增大;电偶极矩单调减小;HOMO能级升高,而LUMO能级、费米能级和能隙均减小。谐振频率随正向电场增加而增大,而红外谱强度则逐渐减小。随着电场的增强,由基态至第1～10激发态的波长增大,激发能减小。     

外电场下AgCl分子的结构与性质

采用密度泛函理论B3P86方法计算了Ag Cl基态分子在不同场强条件下的稳定结构及激发态性质.研究了外电场对Ag Cl基态分子键长、总能量、电子结构、谐振频率和红外光谱强度及前10个激发态的吸收谱、激发能、振子强度的影响.结果表明随着沿Ag-Cl分子轴向电场的逐渐增大,Ag Cl分子键长逐渐减小;总能量则逐渐增大;电偶极矩单调减小;HOMO能级升高,而LUMO能级、费米能级和能隙均减小.谐振频率随正向电场增加而增大,而红外谱强度则逐渐减小.随着电场的增强,由基态至第1~10激发态的波长增大,激发能减小.     

外电场下BN分子的结构及性质

采用QCISD方法研究了BN基态分子在不同场强条件下的稳定构型及激发态性质。分析了外电场对BN分子键长、总能量、能级、谐振频率和红外谱强度以及对BN分子前10个激发态的激发能、振子强度等的影响。结果表明随着正向电场的逐渐增大, BN分子键长先减小后增大;分子总能量则先增大后减小;分子电偶极矩 增大;费米能级和能隙均减小。谐振频率及红外谱强度均随正向电场的增大而增大。由基态到第1激发态的波长减小,激发能增大,而基态至第2-10激发态的波长增大而激发能减小。     

外电场下BN分子的结构及性质

采用QCISD方法研究了BN基态分子在不同场强条件下的稳定构型及激发态性质。分析了外电场对BN分子键长、总能量、能级、谐振频率和红外谱强度以及对BN分子前10个激发态的激发能、振子强度等的影响。结果表明随着正向电场的逐渐增大, BN分子键长先减小后增大;分子总能量则先增大后减小;分子电偶极矩μ增大;费米能级和能隙均减小。谐振频率及红外谱强度均随正向电场的增大而增大。由基态到第1激发态的波长减小,激发能增大,而基态至第2－10激发态的波长增大而激发能减小。     

2''-脱氧鸟苷在水溶液中激发态氢键动力学的密度泛函理论研究

用密度泛函理论和含时密度泛函理论研究了2'-脱氧鸟苷在水溶液中形成的一水和二水复合物在基态和激发态的氢键性质．计算了两种复合物的稳定构型、红外光谱、前沿分子轨道以及密里根电荷等光物理参数．结果表明,两种水合物的分子内氢键在光激发下变化不一致,分子间氢键变化相似．一水复合物的分子内氢键N3···H4-O2 在激发态是加强的,二水复合物的是减弱的．两种水合物的分子间氢键N2-H2···O3和O3-H5···O2及O3···H7-O4和H5···O1-C1在激发态全是减弱的．两种复合物红外光谱的计算结果与实验值一致．二水复合物中分子间氢键O4···H1-N1在激发态断裂,形成了新的分子间氢键O4···H3-N2,这种变化可能与鸟嘌呤部分的结构弯曲有关．最后分析了一水和二水复合物分子的分子内电子转移特性．     

氢键对2-(N-甲基)氨基-5-硝基吡啶分子非线性光学性质的影响

本文在杂化密度泛函理论水平上研究了溶剂对2-(N-甲基)氨基-5-硝基吡啶分子非线性光学性质的影响.在溶剂中,构造了包括氢键作用的超分子体系,在优化结构的基础上分别研究了由极化连续模型模拟的溶剂与该分子的长程相互作用、溶剂与该分子的氢键相互作用以及溶剂与包括氢键作用的超分子体系整体的相互作用对分子的几何结构、非线性光学性质、紫外吸收光谱和电荷分布等特性的影响.结果表明,溶剂中分子电偶极矩、线性极化率和第一超极化率都增大,而溶剂与溶质分子通过氢键形成的超分子结构与单体有着明显区别.因此,氢键对分子结构和性质的影响较大,从而将明显的影响该类分子的非线性光学性质.     

吩噻嗪衍生物的分子内电荷转移激发态的特性

合成了一系列的N-10位取代的吩噻嗪给体受体化合物,这些受体包括苯、苯甲醚、吡啶、萘、苯乙酮和苯乙腈.研究了不同极性溶剂中这些化合物的分子内的光诱导电荷转移现象.稳态荧光的溶剂化效应和较大的Stokes位移清楚表明,仅仅后四种吩噻嗪衍生物的激发态存在着分子内的电荷转移,而苯和苯甲醚取代吩噻嗪因为受体部分氧化电势低,所以不具有这种特性.修正过的Lippert-Mataga公式被用来分析Stokes位移值,从而获得激发态偶极矩.较大的激发态偶极矩说明在激发态时这四种给体受体化合物体系内发生了完全的电子转移.氧化还原电势的数据表明基态时这四种衍生物给体受体部分的作用比较弱.分析荧光光谱获得的结果说明伴随着电荷转移这四种衍生物的激发态构型变化比较小,给体和受体间的扭转角在电荷转移后的激发态与在基态时相似.     

用杂化CIS-DFT方法研究外电场对乙烯激发态的影响

采用杂化CIS-DFT方法研究了外电场对乙烯分子基态和激发态性质的影响,结果表明外电场对分子几何结构,总能量,偶极矩,极化率,振子强度和激发能有显著影响.CIS-DFT的优点在于能确定外场下分子的对称性,给出正确的激发顺序以及分子轨道的电子组态,由此导出乙烯分子的激发态,结果与实验一致.首次研究了乙烯分子的外电场效应.与其他从头算方法相比,杂化CIS-DFT方法计算精确和效率相对较高,可用于研究大分子体系.     

Excited-state hydrogen bonding dynamics of methyl isocyanide in methanol solvent: A DFT/TDDFT study

The time-dependent density functional theory (TDDFT) method was performed to investigate the hydrogenbonding dynamics of methyl cyanide (MeNC) as hydrogen bond acceptor in hydrogen donating methanol (MeOH) solvent. The ground-state geometry optimizations and electronic transition energies and corresponding oscillation strengths of the low-lying electronically excited states for the isolated MeNC and MeOH monomers, the hydrogen-bonded MeNC-MeOH dimer and MeNC-2MeOH trimer are calculated by the DFT and TDDFT methods, respectively. An intermolecular hydrogen bond N≡C…H-O is formed between MeNC and methanol molecule. According to Zhao’s rule on the excited-state hydrogen bonding dynamics, we find the intermolecular hydrogen bonds N≡C…H-O are strengthened in electronically excited states of the hydrogen-bonded MeNC-MeOH dimer and MeNC-2MeOH trimer, with the excitation energy of a related excited state being lowered and electronic spectral redshifts being induced. Furthermore, the hydrogen bond strengthening in the electronically excited state plays an important role on the photophysics and photochemistry of MeNC in solutions     

A theoretical forecast of the hydrogen bond changes in the electronic excited state for BN and its derivatives

The relationship between electronic spectral shifts and hydrogen-bonding dynamics in electronically excited states of the hydrogen-bonded complex is put forward. Hydrogen bond strengthening will induce a redshift of the corresponding electronic spectra, while hydrogen bond weakening will cause a blueshift. Time-dependent density function theory (TDDFT) was used to study the excitation energies in both singlet and triplet electronically excited states of Benzonitrile (BN), 4-aminobenzonitrile (ABN), and 4-dimethylaminobenzonitrile (DMABN) in methanol solvents. Only the intermolecular hydrogen bond C≡N...H-O was involved in our system. A fairly accurate forecast of the hydrogen bond changes in lowlying electronically excited states were presented in light of a very thorough consideration of their related electronic spectra. The deduction we used to depict the trend of the hydrogen bond changes in excited states could help others understand hydrogen-bonding dynamics more effectively.     

3发色团激发态分子内质子转移机理的研究

采用密度泛函(DFT)和含时密度泛函理论(TDDFT)方法对一种新合成的发色团(3)在非质子性溶剂DMSO中的激发态分子内质子转移机制进行了理论研究.基于3发色团的基态和激发态优化结构,计算得到了该发色团中与氢键相关的键长和键角的大小,以及与氢键相连接的O-H键红外振动光谱,发现分子内氢键在激发态下有增强的趋势.理论计算得到的吸收谱和荧光谱的峰值与实验测得的结果吻合得很好,证明了所采用的理论方法的正确性与合理性.最终,通过对该发色团的分子内电荷转移与电荷分布的分析,证实了激发态分子内质子转移发生的可能性,并说明了其转移过程的发生机制.     

3发色团激发态分子内质子转移机理的研究

采用密度泛函(DFT)和含时密度泛函理论(TDDFT)方法对一种新合成的发色团(3)在非质子性溶剂DMSO中的激发态分子内质子转移机制进行了理论研究.基于3发色团的基态和激发态优化结构, 计算得到了该发色团中与氢键相关的键长和键角的大小, 以及与氢键相连接的 O-H键红外振动光谱, 发现分子内氢键在激发态下有增强的趋势. 理论计算得到的吸收谱和荧光谱的峰值与实验测得的结果吻合得很好, 证明了所采用的理论方法的正确性与合理性. 最终, 通过对该发色团的分子内电荷转移与电荷分布的分析, 证实了激发态分子内质子转移发生的可能性, 并说明了其转移过程的发生机制.     

特丁基对苯二酚的光谱及密度泛函研究

特丁基对苯二酚是重要的食品抗氧化剂.理论上,基于密度泛函理论,采用B3LYP泛函及6-311G(d,p)基组在气相环境下优化分子的结构并进行频率计算.在此基础上,基于含时密度泛函理论,选用SMD(solvation model based on density)溶剂模型,利用B3LYP泛函并结合def2-TZVP基组计算分子在无水乙醇溶剂中的前50个激发态.再通过Multiwfn软件对红外光谱做振动分析并考察分子间相互作用对红外光谱的影响,对紫外光谱做分子轨道和电子空穴分析.实验上,通过KBr压片法,利用傅里叶红外变换光谱仪测定样品红外光谱.采用液相法,以乙醇为溶剂,利用紫外可见分光光度计测定样品紫外光谱.通过对比分析可知,理论光谱与实验光谱总体吻合较好.红外光谱各基团的特征吸收峰都较为明显且较好吻合,特丁基对苯二酚二聚体存在氢键作用,这使得O—H键的强度被削弱,导致吸收频率降低并在3670—3070 cm^-1处出现一个宽峰.紫外光谱主要由基态跃迁至第1,2,6,7激发态形成,最大吸收峰位于200 nm以下,为π→π^*和s→π^*跃迁形成,268.8 nm和221.4 nm处的吸收峰均为n→π^*和π→π^*跃迁形成.由电子空穴图可知,这4个主要激发均为电子局域激发.     

Time-dependent density functional theory study on electronic excited states of the hydrogen-bonded solute-solvent phenol-(H2O)n (n=3-5) clusters

The solute-solvent interactions of hydrogen-bonded phenol-(H₂O)_n (n=3-5) clusters in electronic excited states were investigated by means of the time-dependent density functional theory (TDDFT) method. The geometric structures and IR spectra in ground state, S₁ state, and T₁ state of the clusters, were calculated using the density functional theory (DFT) and TDDFT methods. Only the ring form isomer, the most stable one of the cluster, was considered in this study. Four, five and six intermolecular hydrogen bonds were formed in phenol-(H₂O)₃, phenol-(H₂O)₄, and phenol-(H₂O)₅ clusters, respectively. Based on the analysis of IR spectra, it is revealed that the “window region” between unshifted and shifted absorption bands in both S₁ and T₁ state becomes broader compared with that in ground state for the corresponding clusters. Furthermore, two interesting phenomenon were observed: (1) with the anticlockwise order of the ring formed by the intermolecular hydrogen bonds in the H-bonded phenol-(H₂O)_n (n=3-5) clusters, the strengths of the intermolecular hydrogen bonds decrease in all the S₀, S₁ and T₁ states; (2) upon electronic excitation, the smaller the distance between phenol and water is, the larger the change of intermolecular hydrogen bonds strength is. Moreover, the intermolecular hydrogen bond (phenolic OH is the H donor) is strengthened in excited state compared with that in ground state. But the intermolecular hydrogen bond (phenolic OH is the H acceptor) is weakened in excited state.     

Fluorescence quenching as a method of identification of the character of photoreactions in excited states of molecules

We consider the steady-state monochromatic excitation of a luminophore that has fluorescent products. We consider how the fluorescence of singlet states is affected by different means of quenching highlying excited states such as quenching with impurities, temperature quenching, and shortening the lifetime with induced transitions in the light field. We show that the use of different methods of fluorescence quenching opens new possibilities for studying photoreactions that proceed via excited singlet states. Our consideration concerns a wide range of primary photoprocesses, such as electron density redistribution (internal electron transfer) in the excited state, protolytic reactions, intramolecular proton transfer (phototautomerization), hydrogen bond formation, intermolecular relaxation of molecules in solutions, and formation of excimers and exciplexes. The obtained relations have been used to analyze experimental fluorescence spectra of solutions of some organic compounds, including derivatives of 3-hydroxyflavone and 3-aminophthalimide, subjected to quenching by different methods.     

Time‐dependent density functional theory study on the excited‐state hydrogen bonding strengthening of photoexcited 4‐amino‐1,8‐naphthalimide in hydrogen‐donating solvents

The time‐dependent density functional theory (TDDFT) method was performed to investigate the excited‐state hydrogen bonding dynamics of 4‐amino‐1,8‐naphthalimide (4ANI) as hydrogen bond acceptor in hydrogen donating methanol (MeOH) solvent. The ground‐state geometry optimizations, electronic transition energies and corresponding oscillation strengths of the low‐lying electronically excited states for the isolated 4ANi and hydrogen‐bonded 4ANi‐(MeOH)_1,4 complexes were calculated by the DFT and TDDFT methods, respectively. We demonstrated that the intermolecular hydrogen bond C═O···H–O and N–H···O–H in the hydrogen‐bonded 4ANi‐(MeOH)_1,4 is strengthened in the electronically excited state, because the electronic excitation energies of the hydrogen‐bonded complex are correspondingly decreased compared with that of the isolated 4ANi. The calculated results are consistent with the mechanism of the hydrogen bond strengthening in the electronically excited state, while contrast with mechanism of hydrogen bond cleavage. Furthermore, we believe that the transient hydrogen bond strengthening behavior in electronically excited state of fluorescent dye in hydrogen‐donating solvents exists in many other systems in solution. Copyright © 2013 John Wiley & Sons, Ltd.     

Spectroscopic and proton-acceptor properties of biologically active molecules of benzoic acid and its derivatives

We have measured the absorption, fluorescence, fluorescence excitation spectra, fluorescence quantum yields, and IR Fourier-transform spectra of aromatic acids in solutions. We have considered spectroscopic particular features of benzoic acid, its hydroxylated and methoxylated derivatives. Using quantumchemistry methods, we have calculated and interpreted electronic spectra. Data of calculations have been compared with experimental results. We have ascertained the main channels and mechanisms of photophysical relaxation processes in the molecules under study. Proton-acceptor properties of molecules of aromatic acids have been evaluated. We have analyzed in detail mechanisms by which hydrogen bonds are formed, intramolecular charge redistribution is realized, and the electron density of oxygen atoms changes. Spectroscopic indications of participation of oxygen-containing groups in intramolecular interactions and in intermolecular interactions with the formation of cyclic dimers have been revealed.