有机添加物对7-胺基香豆素分子发光的增强效应

测定了在有卤代苯等存在时,一些7-胺基香豆素在不同溶剂中的吸收和发射谱及荧光寿命。所得结果表明,具有给电子能力的有机添加物可对非刚性7-胺基香豆素衍生物在极性溶剂中的荧光起增强作用。基于荧光衰变动力学的分析及添加物的荧光增强能力与其氧化电位的线性关系,提出荧光增强作用的机理是添加物通过分子间电荷转移作用使香豆素分子激发态不发光的TICT态向发光的ICT态转化所致。     

基于跃迁偶极矩取向特征研究乙醚溶液中分子的各向异性度

利用偏振荧光光谱和偏振激发光谱研究了乙醚溶液中荧光分子跃迁偶极距的取向特征.实验结果表明,在垂直线偏振光照射下,乙醚溶液发射出峰值位于305 nm的荧光谱,对应的最佳激励光波长为256 nm.由偏振激发光谱分析得到荧光体的吸收跃迁偶极矩和发射跃迁偶极矩间夹角α的变化规律,揭示了荧光去偏振过程：在粘性溶液中的荧光分子具有一定的偶极取向,α随激发光波长发生变化,当激发波长接近最佳激发光波长时,吸收跃迁偶极矩和发射跃迁偶极矩间趋于平行,荧光的退偏效果较弱,偏振度最大.研究结果能为分子空间取向特征的理论研究提供参考.     

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.     

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。     

2-(2’-羟基苯基)苯并咪唑的光谱特性

2-(2’-羟基苯基)苯并咪唑(简称HBI)是一类具有激发态质子转移效应的有机分子。观测了HBI分子在甲苯、甲苯与乙醇混合及乙醇三种溶液的吸收光谱,发现其吸收光谱相类似。用317 nm光激发,观测其荧光光谱,在甲苯溶液中只观测到一个荧光带,其峰值位于470 nm;在甲苯与乙醇的混合溶液及乙醇溶液的荧光光谱中,都观测到两个荧光带,其峰值分别位于370和450 nm。根据激发态质子转移理论可知,470 nm的荧光带是由激发态质子转移生成的互变异构体,即HBI酮式构型分子的发射,峰值位于370 nm的荧光带是HBI稀醇式构型分子的发射。由于乙醇具有较大的极性,可与HBI分子相互作用形成分子间氢键,即生成溶剂化物。当溶剂化物被激发,在激发态发生激发态质子转移生成两性离子,两性离子发射荧光回到基态。因此,在极性溶剂中HBI发射峰值位于450 nm的荧光带应归于两性离子的发射。当用532 nm强光激发HBI溶液时,发现HBI分子在非极性溶剂中无双光子效应,而在极性溶剂中却存在双光子效应,表明HBI的溶剂化物具有双光子效应。     

4-四氢喹啉酮的合成及发光性能

对喹吖啶酮结构改造合成了新型结构荧光体4-四氢喹啉酮,对其吸收光谱及荧光光谱进行了研究,发现该化合物的吸收峰及荧光发射峰随溶剂极性的增加而不断红移,表明荧光来源于分子内激发态电荷转移发光.该化合物在固态即显示荧光,表明经结构改造后,消除了喹吖啶酮类的固态荧光猝灭效应.4-四氢喹啉酮结构简单,易于合成和结构修饰.这对于寻找新的主体荧光发射材料具有重大意义.     

采用密度泛函理论研究二甲基胺-二苯甲酮的分子内电荷转移激发态氢键

利用(含时)密度泛函理论研究了二甲基胺-二苯甲酮(DMABP)及其氢键二聚物DMABP-MeOH的光物理性质和弛豫动力学过程. 结果表明,在非极性和非质子性溶剂中,DMABP分子的第一和第二激发态跃迁同时具有局域激发和分子内电荷转移的特征;在极性质子性溶剂中,分子间氢键C=O…H-O的形成增加了这两个最低激发态之间的能量差,使DMABP-MeOH的第一激发态具有较强极性的分子内电荷转移特性. 通过计算DMABP和DMABP-MeOH分子的激发态构型弛豫势能曲线研究了激发态动力学弛豫过程. 结果表明,通过扭     

外电场操控单分子的偶极取向极化特性研究

单分子偶极取向的有效操控对于提高单分子荧光收集效率及荧光共振能量转移等相关研究具有重要的意义.本文通过测量单分子荧光偏振特性的变化,研究了外部电场作用下极性单分子的偶极取向极化特性,实现了外电场对单分子偶极取向的有效操控.研究发现电场方向与单分子样品表面平行时,掺杂在聚甲基丙烯酸甲酯聚合物薄膜中的取向随机分布的极性单分子荧光偏振方向呈现出双峰统计分布规律,表明在溶剂挥发过程中外电场诱导极性单分子偶极取向进行了重新分布.     

溶剂化对有机给、受体分子热激活延迟荧光的影响及调控EI北大核心CSCD

具有热激活延迟荧光(Thermally activated delayed fluorescence,TADF)特性的有机给、受体(Donor-acceptor,D-A)分子体系通过反向系间窜越捕获三重态激子,可以将内量子效率的理论上限提高到100%,因而受到极大关注。通常,具有分子内电荷转移特性的D-A体系可以通过构建扭曲的分子构象来减小单、三重态之间的能差ΔES-T,以确保反向系间窜越快速发生。当分子被激发后,若激发态构象中D-A的二面角更接近90°时,ΔES-T会更小,延迟荧光也会增强。然而,快速的溶剂化过程常常会影响激发态构象、分子内电荷转移过程、延迟荧光发射,这使得研究TADF分子发光过程更富有挑战。本文综述了本课题组近期在溶剂化对D-A体系延迟荧光的影响及调控方面所取得的初步进展。结果显示,强极性溶剂会导致非辐射弛豫增加,不利于TADF发射;改变溶剂粘度会影响激发态构象弛豫,从而可以实现对TADF的增强或减弱的调控。这些结果有助于理解溶剂化效应与构象弛豫、TADF之间的关系,为TADF分子的设计与合成提供指导。     

溶剂对香豆素343染料光谱性质和最低激发态偶极矩的影响

通过对香豆素343(C343)在不同溶剂中的稳态吸收光谱、稳态荧光光谱和时间分辨荧光光谱的分析,研究了溶剂对C343的光谱性质的影响,并获得了光谱特性与溶剂极性之间的依赖关系. 吸收光谱峰值的红移随着溶剂极性的增加而发生较小的变化. 然而,荧光光谱的峰值对溶剂的极性却很敏感,并随着溶剂极性参数f(ε,n)的增加呈线性增长. 这是由于C343激发态电荷分布的变化导致了它在极性溶剂中第一激发单重态能级的变化. 用溶剂效应测量法和量子化学计算方法确定了C343最低激发态的偶极矩,这两方法所得的结果一致. C343在不同溶剂中的时间分辨荧光光谱研究表明荧光寿命随着溶剂极性的增加而增加,即从甲苯溶液的3.09 ns线性地增加到水溶液中4.45 ns;荧光寿命延长的根源可归因于C343与氢键给体溶剂之间的分子间氢键相互作用.     

Solvent effect on the steady-state fluorescence anisotropy of two-photon absorbing fluorene derivatives

The effect of solvent polarity on the fluorescence properties of several two-photon absorbing fluorene derivatives was investigated. A strong solvatochromic effect was observed for symmetrical fluorene compound, which can be explained by large changes in the quadrupolar moment under excitation. Limiting values of excitation anisotropy of the investigated fluorenes exhibited a dependence on solvent polarity, and the angles between the absorption and emission transition dipole moments decrease in polar solvents by more than a factor of two.     

Parity chain and parity chain breaking in the two-level cavity quantum electrodynamics system

We investigate the transitions between energy levels and parity symmetry in an effective two-level polar molecule system strongly coupled with a quantized harmonic oscillator. By the dressed-state perturbation theory,the transition diagrams between the dressed-state energy levels are presented clearly and show that the odd(even) parity symmetry is broken by the permanent dipole moment(PDM) of the polar molecules. By the analytical and numerical methods, we find that when the coupling strength and the PDM increase, the more frequency components are induced by the counter-rotating terms and PDM.     

Estimation of Ground-State and Singlet Excited-State Dipole Moments of Substituted Schiff Bases Containing Oxazolidin-2-one Moiety through Solvatochromic Methods

Absorption and fluorescence studies on novel Schiff bases (E)-4-(4-(4-nitro benzylideneamino)benzyl)oxazolidin-2-one (NBOA) and (E)-4-(4-(4-chlorobenzylidene amino)benzyl)oxazolidin-2-one (CBOA) were recorded in a series of twelve solvents upon increasing polarity at room temperature. Large Stokes shift indicates bathochromic fluorescence band for both the molecules. The photoluminescence properties of Schiff bases containing electron withdrawing and donating substituents were analyzed. Intramolecular charge transfer behavior can be studied based on the influence of different substituents in Schiff bases. Changes in position and intensity of absorption and fluorescence spectra are responsible for the stabilization of singlet excited-states of Schiff base molecules with different substituents, in polar solvents. This is attributed to the Intramolecular charge transfer (ICT) mechanism. In case of electron donating (?Cl) substituent, ICT contributes largely to positive solvatochromism when compared to electron withdrawing (?NO₂) substituent. Ground-state and singlet excited-state dipole moments of NBOA and CBOA were calculated experimentally using solvent polarity function approaches given by Lippert–Mataga, Bakhshiev, Kawskii-Chamma-Viallet and Reichardt. Due to considerable π- electron density redistribution, singlet excited-state dipole moment was found to be greater than ground-state dipole moment. Ground-state dipole moment value which was determined by quantum chemical method was used to estimate excited-state dipole moment using solvatochromic correlations. Kamlet-Abboud-Taft and Catalan multiple linear regression approaches were used to study non-specific solute-solvent interaction and hydrogen bonding interactions in detail. Optimized geometry and HOMO-LUMO energies of NBOA and CBOA have been determined by DFT and TD-DFT/PCM (B3LYP/6-311G (d, p)). Mulliken charges and molecular electrostatic potential have also been evaluated from DFT calculations.     

Collisional control of ground state polar molecules and universal dipolar scattering

We explore the impact of the short-range interaction on the scattering of ground state polar molecules and study the transition from a weak to strong dipolar scattering over an experimentally reasonable range of energies and electric field values. In the strong dipolar limit, the scattering scales with respect to a dimensionless quantity defined by mass, induced dipole moment, and collision energy. The scaling has implications for all quantum mechanical dipolar scattering. Furthermore the universal scattering regime will readily be achieved with polar molecules at ultracold temperatures.     

Time-resolved fluorescence relaxation of 3-methyllumiflavin in polar solution

We have studied the fluorescent properties of a well-defined model flavin compound (3-methyllumiflavin) in a relatively polar solvent like propylene glycol or ethanol. Inhomogeneous spectral broadening effects were directly time-resolved by detection at the extreme blue and red edges of the fluorescence band of 3-methyllumiflavin using excitation in the main absorption band. At the high-energy side of the emission band a rapid decay component (tens of picoseconds) was resolved indicative for the disappearance of the initially prepared, nonequilibrium state with a characteristic dipolar relaxation time. At the low-energy side the rise of a solvent relaxed fluorescent species could be time-resolved. The wavelength-dependent effects on the dipolar relaxation were abolished when excitation was at the low-energy side of the absorption band. The experimental decays of the flavin solvate at different energies of fluorescence and excitation are presented as they represent an easy diagnosis for energy dependent solvation dynamics. Wavelength dependent rotation of 3-methyllumiflavin, examined by fluorescence anisotropy decay, turned out to be absent for 3-methyllumiflavin in propylene glycol between 263 and 293 K, probably because of the small change in dipole moment upon flavin excitation.     

Further Solvatochromic, Thermochromic, and Theoretical Studies on Nile Red

Experimental steady-state solvatochromic and thermochromic studies of Nile Red absorption and emission in nine dipolar solvents are reported, as well as theoretical modeling results concerning the ground and excited states of Nile Red in solution. Solvatochromic absorption and emission data analyzed according to conventional methods yield ground- and excited-state dipole moments of 8.9 ± 0.5 and 14.4 ± 0.5 D. Application of this conventional model to thermochromic shift data gave dipole moments of 8.4 ± 1.0 and 13.4 ± 1.0 D. The thermochromic data were also analyzed using a novel solute monopole–solvent dipole model; this model did not appear to reproduce trends in the thermochromic shift data as well as the conventional model. Results of semiempirical ZINDO/S calculations on Nile Red combined with an Onsager model for solvation were also used to examine the energetics of the excited states of Nile Red in solution. These calculations suggest the presence of a TICT state in Nile Red comparable in energy to an emitting excited state at high solvent polarity. Conventional models for solvatochromic and thermochromic response, however, appear to explain the experimental results independently of any emission from this TICT state in the present solvents.     

Structure and dielectric properties of polar fluids with extended dipoles: results from numerical simulations

The strengths and shortcomings of the point dipole model for polar fluids of spherical molecules are illustrated by considering the physically more relevant case of extended dipoles formed by two opposite charges ±?q separated by a distance d (dipole moment μ=qd). Extensive molecular dynamics simulations on a high-density dipolar fluid are used to analyse the dependence of the pair structure, dielectric constant ε and dynamics as a function of the ratio d/σ (σ is the molecular diameter), for a fixed dipole moment μ. The point dipole model is found to agree well with the extended dipole model up to d/σ ? 0.3. Beyond that ratio, ε shows a non-trivial variation with d/σ. When d/σ > 0.6, a transition is observed towards a hexagonal columnar phase; the corresponding value of the dipole moment is found to be substantially lower than the value of the point dipole required to drive a similar transition.     

Dielectric alignment of a non-polar molecule substituting a polar molecule in a lattice of molecular dipoles

The dielectric alignment of a dilute non-polar component in a polar solvent may be determined by NMR spectroscopy. In this paper a series expansion of the alignment up to second order terms in the dipolar interaction is presented for a non-polar molecule replacing a single polar molecule in a rigid lattice of molecular dipoles. Assuming isotropic polarizabilities for all molecules Van Vleck earlier applied the lattice model in a theory of the dielectric constant of a dilute solution.In the present calculation the polarizabilities of the non-polar molecule and the dipoles are assumed to be anisotropic and isotropic, respectively. The anisotropy of the non-polar molecule is relevant, since the value of the alignment is zero for the isotropic case. Different results are obtained for cubic lattices and for a lattice points are uniformly distributed.In absence of a rigid dipole moment of the solvent molecules the latter formula may be compared with that implicity deduced by Buckingham for the Kerr effect. In the limit of strong dipoles the orientation imposed by the rigid dipole moments appears to be the major contribution to the alignment of the non-polar molecule.     

Critical temperature of weakly interacting dipolar condensates

We calculate perturbatively the effect of a dipolar interaction upon the Bose-Einstein condensation temperature. This dipolar shift depends on the angle between the symmetry axes of the trap and the aligned atomic dipole moments, and is extremal for parallel or orthogonal orientations, respectively. The difference of both critical temperatures exhibits most clearly the dipole-dipole interaction and can be enhanced by increasing both the number of atoms and the anisotropy of the trap. Applying our results to chromium atoms, which have a large magnetic dipole moment, shows that this dipolar shift of the critical temperature could be measured in the ongoing Stuttgart experiment.