用于监控过氧化氮的近红外荧光探针的光物理性质及PET机理

含有机硒的七甲川菁染料是基于光诱导电子转移(PET)的近红外(IR)荧光探针, 能在生理条件下高灵敏、高选择性地监控过氧化氮. 本文应用含时密度泛函理论(TD-DFT)计算方法研究其光物理性质和PET机理.结果表明, 在激发态, 荧光母体发生最高占有分子轨道(HOMO)到最低非占有分子轨道(LUMO)的电子跃迁, 识别基团上的HOMO轨道能级提高到荧光母体的单电子占据的HOMO轨道能级之上, 并向其转移一个电子, 使激发态电子回落过程受阻而导致荧光部分淬灭. 硒被氧化后, 识别基团上的HOMO轨道能级降低, PET过程被阻断, 荧光发射恢复. 研究进一步证明, PET效应来自于识别基团上苯胺N原子的p电子, 它的电子转移能力受到其对位苯硒基的氧化-还原状态的影响, 产生了荧光信号的“开-关”作用.     

8-羟基喹啉银(铂)金属配合物电子光谱与非线性光学性质

运用密度泛函理论B3LYP方法对8-羟基喹啉(银、铂)(AgQ、PtQ₂)金属配合物及其衍生物的非线性光学性质进行理论计算研究. 结果表明: 引入取代基使铂配合物的最强吸收波长产生较大红移. 最低能量跃迁吸收来自最高占据分子轨道(HOMO)到最低空分子轨道(LUMO)的d→π^*和π→π^*跃迁, 属于金属配体电荷转移(MLCT)与配体配体电荷转移(LLCT). 金属银和铂掺杂8-羟基喹啉使其三阶非线性光学系数γ值明显增大, 并且在配合物上引入―Ph, ―PhOCH₃, ―PhF₂, ―PhF₅基团将进一步增大γ值. 引入基团的供电子性越强, γ值增大的幅度越大, 引入基团的吸电子性越强, γ值增大的幅度越小.     

乙烯基噻吩共轭螺噁嗪化合物的密度泛函理论研究

采用密度泛函理论(DFT)方法, 在B3LYP/6-31G^* 水平上对乙烯基噻吩共轭螺噁嗪化合物 SO-SO3 的几何构型、电子结构、前线分子轨道等进行了理论研究, 计算结果表明: SO-SO3的开环过程会使得开环体的左右两个部分键长均等化, 导致共轭体系变大, 能隙明显减小; 乙烯基噻吩基团共轭接入螺噁嗪母体后, 导致体系的共轭作用变大, 在激发态下电子流动增强, 形成由乙烯基噻吩向萘并噁嗪的有效电荷转移与能量转移; 结合前线分子轨道成分分析乙烯基噻吩单元在最高占据分子轨道(HOMO)和最低未占据分子轨道(LUMO)中的轨道贡献率明显增加. 含时密度泛函理论(TD-DFT)计算的电子吸收光谱结果显示: 当接入的乙烯基噻吩单元达到2-3个时, 影响SO2和SO3开环的最低能量激发态变为第一激发单重态S₁, 并且均源自电子从HOMO至LUMO的跃迁且为π-π^*跃迁; 其最大吸收波长λ_max 达到466-540 nm, 且红移十分明显, 其对应开环体O-SO2与O-SO3的λ_max 达到605和647 nm.     

全氟聚醚润滑油添加剂抗氧抗腐蚀机理研究及分子设计——Ⅰ. 添加剂的作用机理和授受电子性质

采用量子化学方法,全优化计算了含N的全氟聚醚芳烃和N-取代全氟聚醚芳酰胺抗氧抗腐添加剂与全氟聚醚氧自由基(R_fO.)的结合能和与铁原子的吸附作用能.探讨了这些化合物的结构特征、作用机理、授受电子的性质和取代基效应.计算发现这些添加剂的HOMO均为带有杂原子孤对电子的π -分子轨道;添加剂的HOMO可与金属原子的LUMO发生相互作用,HOMO的电子转移到金属原子的LUMO上形成配位键和稳定的吸附态.添加剂的LUMO又可与R_fO.的SOMO相互作用,LUMO接受R_f O.的电子生成稳定的加成产物,添加剂具有授受电子的性质.研究还发现在全氟聚醚芳酰胺N原子上引入供电子基团或苯基后,可增加添加剂与R_fO.的结合能或与铁原子的化学吸附作用能,预示将增加抗氧抗腐性能.依据计算的结果推测前5个化合物的抗氧抗腐蚀效果有如下之序:化合物 Ⅲ＞Ⅱ＞Ⅰ＞Ⅳ＞Ⅴ.     

6-N,N-二甲基腺嘌呤激发态结构动力学的共振拉曼光谱

采用共振拉曼光谱技术和密度泛函理论方法研究了6-N,N-二甲基腺嘌呤(DMA)的A带和B带电子激发和Franck-Condon 区域结构动力学. π_H→π_L^*跃迁是A带吸收的主体, 其振子强度约占整个A带吸收的79%.由弥散轨道参与的n→Ryd 和π_H→Ryd 跃迁在B带跃迁中扮演重要角色, 其振子强度约占B带吸收的62%,而在A带吸收中占主导的π_H→π_L^*跃迁的振子强度在B带吸收中仅占33%. 嘌呤环变形伸缩+C8H/N9H面内弯曲振动ν₂₃和五元环变形伸缩+C8H弯曲振动ν₁₃的基频、泛频和合频占据了A带共振拉曼光谱强度的绝大部分, 说明¹π_Hπ_L^*激发态结构动力学主要沿嘌呤环的变形伸缩振动, N9H/C8H/C2H弯曲振动等反应坐标展开, 而ν₁₀, ν₂₉, ν₂₁, ν₂₆和ν₄₀的基频、泛频和合频占据了B带共振拉曼光谱强度的主体部分, 它们决定了B带激发态的结构动力学. A带共振拉曼光谱中ν₂₆和ν₁₂被认为与1nπ*/1ππ*势能面锥型交叉有关. B带共振拉曼光谱中ν₂₁的激活与¹ππ^*/¹πσ_N9H^*势能面锥型交叉相关.     

两种4-氨基安替比林席夫碱-Pt(Ⅱ)配合物电子光谱性质的理论研究

采用密度泛函理论(DFT),在PBE0/6-31+G(d)-LANL2DZ水平下,对两种含有不同取代基的4-氨基安替比林席夫碱-Pt(Ⅱ)配合物A和B的几何构型、前线分子轨道及其分布特征进行理论计算.在优化构型的基础上,用含时密度泛函理论(TD-DFT)在相同水平下对上述配合物进行电子吸收光谱研究.计算还考虑了二氯甲烷溶剂对电子结构和光谱性质的影响.结果表明,配合物A和B的最强吸收波长分别来自于HOMO→LUMO和HOMO-5→LUMO的跃迁,以上跃迁存在明显的分子内电荷转移的特征.此外,在4-氨基安替比林配体上引入强的给电子基团-N(CH3)2,配合物A的最大吸收波长相对于配合物B发生了红移现象.     

三茚基钐(Sm(C9H7)3)的电子结构和化学键

采用INDO方法研究了三茚基钐的电子结构和化学键。结果表明,钐原子的6s、6p和5d轨道与配位体轨道有不同程度的混合;HOMO和LUMO是由钐原子的5d轨道和配位体原子轨道构成的π型分子轨道;4f轨道是高度定域的,参与成键不超过2%。三茚基钐的化学键具有相当程度的共价性,茚基中碳原子上的净电荷分布不均匀。讨论了三茚基钐的四氢呋喃加合物中的Sm-C键。     

苯在Au(100)表面化学吸附的周期性密度泛函理论研究

采用局域密度泛函理论(LDA)的VWN方法, 结合周期平板模型, 在DNP基组下, 研究了苯分子在Au(100)面的吸附情况. 构型优化的结果表明, 苯分子在穴位吸附活性最高, 吸附能为－184.8～－184.3 kJ•mol^-1, 苯环发生扭曲, C—C键明显拉长, 出现了介于苯和1,4-环己二烯之间的船状构型, 船头的2个C原子从sp²杂化重新进行sp³杂化. 苯分子在桥位和顶位的吸附活性较低, 吸附能分别为－156.7～－145.3 kJ•mol^-1、－116.5～－117.0 kJ•mol^-1, 苯分子构型有稍微的改变. 轨道分析的结果还表明, 吸附之后苯分子的轨道简并度降低, 苯分子的LUMO轨道和邻近Au原子的d_z²轨道叠加比较好, 两个对位的C原子以双σ形式连接到表面邻近的Au原子上.     

以苯基吡唑为主配体的Ir配合物电子结构和光谱性质的密度泛函理论研究

用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)的B3LYP方法对以苯基吡唑ppz为主配体的4种Ir配合物Ir(ppz)₃, Ir(ppz)₂(acac), Ir(ppz)₂(pic)和Ir(ppz)₂(dbm)的电子结构和光谱性质进行了理论研究. 计算结果表明, 辅助配体的改变对Ir配合物的最高占据轨道(HOMO)的影响不大, 但会显著的降低分子最低空轨道(LUMO)的能级, 从而调节Ir配合物的HOMO和LUMO间的能隙. 4种配合物对应的发射跃迁分别为Ir(ppz)₃:d(Ir)+π(ppz)→π*(ppz); Ir(ppz)₂(pic):d(Ir)+(ppz)→π*(pic); Ir(ppz)₂(acac), Ir(ppz)₂(dbm):d(Ir)+π(acacdbm)→π*(acacdbm). 金属配合物的发光颜色可以通过选择合适的辅助配体调节.     

1-甲基胸腺嘧啶A-带结构动力学

获取了1-甲基胸腺嘧啶(MT)涵盖紫外光谱中A带和B带吸收的共5 个激发波长的共振拉曼光谱, 并结合密度泛函理论方法研究了MT的电子激发和Franck-Condon 区域结构动力学. 在TD-B3LYP/6-311++G(d,p)计算水平下, A带和B带吸收被分别指认为π_H→π_L^*/π_H-2→π_L+2^*和π_H→π_L+2^→/π_H-2→π_L^*跃迁. 甲基参与嘧啶环的共轭使MT的A带最大吸收波长λ_max相对于胸腺嘧啶(T)发生明显红移, 并对Franck-Condon区域的动态结构产生一定影响. A带和B带共振拉曼光谱分别被指认为14 个振动模式和11 个振动模式的基频、泛频和组合频. C5＝C6伸缩+C6H12面内弯曲振动v₉, 环变形振动v₁₆和N3C2N1反对称伸缩+C4C5C10反对称伸缩振动v₁₈占据了A带共振拉曼光谱强度的绝大部分. 这表明¹π_Hπ_L^*激发态结构动力学主要沿这些反应坐标展开. 考察了溶剂对共振拉曼光谱的影响, 结果表明, C4＝O9伸缩+N3H11面内弯曲振动v₈的活性与溶剂性质有关, 其激发态位移量随溶剂性质的变化规律与胸腺嘧啶一致.     

Sensitive and selective PET-based diimidazole luminophore for Zn(II) ions: a structure-activity correlation

3-(3-Ethoxymethyl-1H-imidazol-2-yl)-3-(1-ethoxymethyl-1H-imidazol-2-yl)-3H-benzo[de]isochromen-1-one, 4, is a novel photoinduced electron transfer (PET) chemosensor that becomes fluorescent upon binding metal ions and shows a strong preference toward Zn(II) ions. The new bisimidazol PET sensor and its zinc complex were prepared and characterized in terms of their crystal structures, absorption and emission spectra, and orbital energy diagrams. Free 4 is a weakly luminescent species. On the basis of detailed DFT calculations, we suggest that the poor luminescence yield of free 4 originates from its orbital structure in which two pi-orbitals of the two imidazole rings, HOMO and HOMO-1, are situated between two pi-orbitals of the isochromene-one system, HOMO-2 and LUMO. The absorption and emission processes occur between the two pi-orbitals of the isochromene-one system, HOMO-2 and LUMO, and the two pi-imidazole orbitals serve as quenchers for the excited state of the molecule through nonradiative processes. Upon binding Zn(II) ions, 4 becomes a highly luminescent species having a luminescence maximum peaking at 375 nm (lambda(ex) = 329 nm). The significant 900-fold enhancement in luminescence upon binding of the Zn(II) ions is attributed to the stabilization of the pi-orbitals of the imidazole rings upon their engagement in new bonds with the zinc ion. The affinity of 4 to zinc ions in acetonitrile is found to be very high, Ka > 3 x 10(6) M(-1), while with other metals ions, the association constants are considerably weaker.     

Electronic and molecular structure of aminimides (1-acyl-2,2,2-trimethyldiazan-2-ium-1-ide). 1. Formaminimide (HCON- N+ Me3)

The electronic structure and geometries of (Z)- and (E)-H-CON- N+(CH3)3 have been examined at two levels of theory: B3LYP (basis sets 6-311+G(d,p), 6-311++G(d,p), and 6-311G(3df,3pd)) and MP2(full)/6-311++G(d,p). The (Z) conformation about the C(O)-N(-) bond is thermodynamically preferred over the (E) configuration. Natural bond orbital calculation locates one lone pair of the N- in the HOMO, which is the p(z) natural hybrid orbital (perpendicular to the O=CN- N+ plane). The second lone pair (of lower energy) of N- occupies the HOMO-3, which is the natural hybrid orbital sp(1.12) (sp(1.01) for the (E) conformation, sp(1.74) in the rotational transition state). The carbonyl pi bond is the HOMO-2. The charge-transfer ability of the negative nitrogen in H-CON- N+ (CH3)3 is more powerful than that of the neutral amidic nitrogen in dimethylformamide. The following facts convincingly sustain this view: (1) the higher rotational barrier (stronger C-N(-) bond) in the case of H-CON- N+ (CH3)3, (2) natural resonance theory analysis predicts almost equal weights for the (Z)-H-C(=O)N- N+ (CH3)3 and the (Z)-H-C(O-)=NN+ (CH3)3 canonical resonance structures whereas the weight of the HCON(CH3)2 structure is almost twice as large as that of HC(O-)=N+ (CH3)2, and (3) the second-order perturbation stabilization, as a result of the donor (N-)/acceptor (carbonyl) interaction, is 101.3 kcal/mol for H-CON- N+ (CH3)3 and only 64.4 kcal/mol for dimethylformamide.     

Highly selective fluorescent OFF-ON thiol probes based on dyads of BODIPY and potent intramolecular electron sink 2,4-dinitrobenzenesulfonyl subunits

Two highly selective OFF-ON green emitting fluorescent thiol probes (1 and 2) with intense absorption in the visible spectrum (molar extinction coefficient ε is up to 73?800 M(-1) cm(-1) at 509 nm) based on dyads of BODIPY (as electron donor of the photo-induced electron transfer, i.e.PET) and 2,4-dinitrobenzenesulfonyl (DNBS) (as electron acceptor of the PET process) were devised. The single crystal structures of the two probes were determined. The distance between the electron donor (BODIPY fluorophore) and the electron acceptor (DNBS) of probe 2 is larger than that of probe 1, as a result the contrast ratio (or the PET efficiency) of probe 2 is smaller than that of probe 1. However, fluorescence OFF-ON switching effects were observed for both probe 1 and probe 2 in the presence of cysteine (the emission enhancement is 300-fold for probe 1 and 54-fold for probe 2). The fluorescence OFF-ON sensing mechanism is rationalized by DFT/TDDFT calculations. We demonstrated with DFT calculations that DNBS is ca. 0.76 eV more potent to accept electrons than the maleimide moiety. The probes were used for fluorescent imaging of cellular thiols.     

The characterization of NMR shielding in monocyclic phosphines

The phenyl-substituted saturated monocyclic phosphines, PhP(CH₂)_n, n = 2–5, show an interesting variation in their phosphorus NMR shieldings. The shielding does not vary uniformly with ring size, but rather the smallest ring (n = 2) has the highest shielding while the next smallest (n = 3) has the lowest shielding. Hartree–Fock calculations in the gauge-including atomic orbital (GIAO) approach on the related hydrogen derivatives have reproduced this trend in shielding and allow a qualitative understanding of the experimental observations. With respect to the relatively unstrained n = 4,5 ring systems, the unusual behavior of the n = 2 and 3 molecules can be understood in terms of the differences in the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) gaps and the p-character of the phosphorus lone pair. The HOMO/LUMO gap is largest for phosphirane (n = 2) but smallest in phosphetane (n = 3). The hybrid character of the lone pair in phosphirane (n = 2) is almost sp while that for phosphetane (n = 3) is essentially sp². © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8: 451–457, 1997     

Influence of laser pulse parameters on dynamical processes during azobenzene photoisomerization

When a molecule is subjected to a short intense laser pulse, the ensuing dynamical processes depend qualitatively on the pulse parameters, including duration, frequency, and fluence. Here we report studies of cis to trans photoisomerization of azobenzene following femtosecond-scale laser pulses which are relatively short (10 fs) or long (100 fs) and which have a central frequency matched to either the first excited state (S1, or HOMO to LUMO in a molecular orbital picture) or the second (S2, or HOMO-1 to LUMO). The results presented here demonstrate that photoisomerization involves a rather intricate sequence of connected steps, with the nuclear and electronic degrees of freedom inextricably coupled. One important feature is the de-excitation required for the molecule to achieve its new ground-state after isomerization. If the primary excitation is to S1, then we find that only a single HOMO/LUMO avoided crossing is required and that this crossing occurs halfway along a rotational pathway involving the central CNNC dihedral angle. If the primary excitation is to S2, then the same HOMO/LUMO avoided crossing is observed, but it must be preceded by another avoided crossing that permits transfer of holes from the HOMO-1 to the HOMO, so that the HOMO is then able to accept electrons from the LUMO. We find that this earlier crossing can occur in either of two geometries, one near the cis configuration and the other near the trans. The fact that S2 (pi pi*) isomerization requires two steps may be related to the fact that isomerization yields are smaller for this (UV) excitation than for the S1 (n pi*, visible-light) excitation.     

First-principles theoretical investigation of the electronic couplings in single crystals of phenanthroline-based organic semiconductors

The electronic couplings between adjacent molecules in the phenanthroline-based bathocuproine (BCP) and bathophenanthroline (Bphen) crystals have been studied using density functional theory on model dimers. Within the frame of the "two-state model" of charge-transfer theory, a generalized definition of the "effective transfer integral" is proposed. This definition addresses the issue arising when the lowest unoccupied molecular orbital (LUMO) [highest occupied molecular orbital (HOMO)] and LUMO+1 (HOMO-1) of the single molecules both have significant contributions to the dimer LUMO (HOMO) level. Charge-transfer integrals based on the new definition are compared with those from previous models; significant differences are found. The authors' results indicate that, within a simple Marcus theory approach, the charge-transport parameters of the BCP and Bphen crystals are expected to be similar.     

Ab initio calculations on the electronic structure of the divalent lead-water complex

We studied the electronic structure of the Pb (2+)-4H 2O system. Analysis of the complex orbital evidenced no mixing between the 6s lone pair orbital of the lead and the 6p orbital components. Moreover, we found that the HOMO is widely described by the mixture of the 6p components with the 7s valence orbital of the lead. This orbital shows an important elliptical electron charge density around the lead ion and opposite the direction of the short lead-water bonds. From these results, we demonstrated that the hemidirected conformation of the Pb (2+)-4H 2O system could be easily explained by the shape of the electron charge density distribution of the HOMO rather than by the stereochemically active character of the 6s (2) lone pair of lead electrons.     

Optical Sensing of Cesium Using 1,3-Alternate Calix[4]-mono- and Di(anthrylmethyl)aza-crown-6

We have synthesized two derivatives of alkylanthracene covalently bonded to 1,3-alternate calix[4]aza-crown-6 at the nitrogen position to study the effect of alkali metal ion complexation on the emission properties of anthracene fluorophore. The mono- and dianthryl-substituted probes are weakly fluorescent because their emission is partially quenched by photoinduced electron transfer (PET) from the nitrogen lone pair to the excited singlet state of anthracene. Upon complexation of alkali metal ions (e.g. K⁺, Cs⁺) by the crown moiety, the nitrogen lone pair can no longer participate in the PET process causing an enhancement in the emission of anthracene fluorophore (fluorescent turn on). The maximum fluorescence enhancement observed upon complexation of cesium ions by mono- and dianthryl-substituted calix[4]aza-crown-6 relative to the uncomplexed form was 8.5- and 11.6-fold, respectively.     

Photophysical and theoretical investigations of oligo(p-phenyleneethynylene)s: effect of alkoxy substitution and alkyne-aryl bond rotations

The unique photophysical, conformational, and electronic properties of two model phenyleneethynylene-based rigid rod molecular systems, possessing dialkoxy substitutions, are reported in comparison with an unsubstituted system. Twisting of the phenyl rings along the carbon-carbon triple bond is almost frictionless in these systems giving rise to planar as well as several twisted ground-state conformations, and this results in broad structureless absorption in the spectral region of 250-450 nm. In the case of 1,4-bis(phenylethynyl)benzene, a broad absorption band was observed due to the HOMO-LUMO transition, whereas dialkoxy-substituted compounds possess two well-separated bands. Dialkoxy substitution in the 2,5-position of the phenyl ring in phenyleneethynylenes alters its central arene pi-orbitals through the resonance interaction with oxygen lone pairs resulting in similar orbital features for HOMO and HOMO-1/HOMO-2. Electronic transition from the low-lying HOMO-1/HOMO-2 orbital to LUMO results in the high-energy band, and the red-shifted band originates from the HOMO-LUMO transition. The first excited-state transition energies at different dihedral angles, calculated by the TDDFT method, indicate that the orthogonal conformation has the highest excitation energy with an energy difference of 15 kcal/mol higher than the low-lying planar conformation. The emission of these compounds originates preferentially from the more relaxed planar conformation resulting in well-defined vibronic features. The fluorescence spectral profile and lifetimes were found to be independent of excitation wavelengths, confirming the existence of a single emitting species.