气相水杨酸激发态分子内质子转移特性分析

采用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)研究了气相水杨酸(SA)分子的激发态氢键动力学过程。通过对水杨酸分子基态和激发态结构的优化,以及对其稳态吸收和发射光谱特性、前线分子轨道、红外振动光谱和势能曲线的计算分析,阐明水杨酸分子内质子转移可在激发态下自发地发生,导致其激发态可存在烯醇式和酮式两种异构体结构,并揭示了这种质子转移源于分子内电荷转移的激发态氢键的加强机制。     

吩噻嗪衍生物的分子内电荷转移激发态的特性

A series of N-bonded donor-acceptor derivatives of phenothiazine containing benzene (PHPZ), anisole (ANPZ), pyridine (PYPZ), naphthalene (NAPZ), acetophenone (PEPZ), and benzonitrile (BNPZ) as an electron acceptor was synthesized. Their photophysical properties were investigated in solvents of different polarities by absorption and emission techniques. These studies clearly reveals the existence of an intramolecular charge transfer (ICT) excited state in the latter four compounds. The solvent dependent Stokes shift values were analyzed by the modified Lippert-Mataga equation to obtain the excited state dipole moment values. The large excited state dipole moment suggests that the full electron transfer takes place in the A-D systems. The obtained values of redox potentials indicate that both subunits of all the A-D molecules studied interact very weakly in the ground states. The results obtained from the analysis of the CT fluorescence spectra confirm that the small conformational changes accompanying excited state charge transfer, the twist angle between the donor and acceptor moieties in the excited 1CT state seems to be similar to that in the ground state.     

Deflation techniques in quantum chemistry: Excited states from ground states

Applications of deflation techniques to the study of excited states of quantum systems are analyzed. It is demonstrated how these methods allow us to transform the excited state problem of one Hamiltonian, into the ground state problem of an auxiliary one. As an example, potential application in the density functional treatment of excited states is discussed. The inclusion of approximations in this scheme, such as the solution of the proposed model within a finite basis set is discussed. An extension of the Hartree–Fock (HF) method to excited states is presented. This new treatment includes previous self consistent field extensions to excited states and provides us with a way to obtain the HF extension to excited states of any ground state method. These results make the excited states of a system accessible through all ground state theoretical techniques. © 2013 Wiley Periodicals, Inc.     

Ground state and excited state dipole moments of 6,8-diphenylimidazo[1,2-α]pyrazine determined from solvatochromic shifts of absorption and fluorescence spectra

Electronic absorption and dual fluorescence spectra of 6,8-diphenylimidazo[1,2-α]pyrazine (68DIP) was recorded in various solvents with different polarity at room temperature. The ground state (μg) and the excited state (μg) dipole moments of 68DIP were estimated from solvatochromic shifts of absorption and fluorescence spectra as a function of the dielectric constant (?) and refractive index (n). The results show that the value of excited state dipole moment in SE: μeSE=2.8772 D and twisted intramolecular charge transfer (TICT) excited equilibrated state dipole moment value of μeLE=2.9744 D was found. The solvent dependent spectral shifts in absorption and fluorescence spectra were analyzed by the polarizability-polarity and Kamlet-Taft parameters.     

Rydberg spectra of trans-1,2-dibromoethylene

(2+1) resonance-enhanced multiphoton ionization spectra of jet-cooled trans-1,2-dibromoethylene are reported for the first time. The two-photon spectral region between 149.7 and 141.2 nm was examined. A 4p(z)<--pi Rydberg transition between 66,800 and 68,000 cm(-1) with A(g) excited state symmetry was analyzed, as well as two 4f<--pi Rydberg transitions with B(g) excited state symmetry and one 4f<--pi Rydberg transition with A(g) excited state symmetry between 68,000 and 70,800 cm(-1). All Rydberg transitions observed in this work belong to series that converge to the first ionization potential of the molecule. The short vibrational progressions observed involve two totally symmetric in-plane normal modes: C=C-H bending (nu(3)) and C=C-Br bending (nu(5)) with average excited state frequencies of 829 and 226 cm(-1), respectively.     

Ultrafast relaxation dynamics observed through time-resolved photoelectron angular distributions

Time-resolved photoelectron imaging of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion is presented. Photoelectron angular distributions (PADs) are qualitatively analyzed in terms of the simple s-p model that is based on symmetry arguments. The internal conversion dynamics from the first excited state (1(2)B(3u)) to the ground state ((2)B(2g)) may be observed through temporal changes in the PADs of the spectrally overlapping photoelectron features arising from photodetachment of the ground state and the excited state. A formulism for extracting the population dynamics from the β(2) anisotropy parameter of overlapping spectroscopic features is presented. This is used to extract the lifetime of the first excited state, which is in good agreement with that observed in the time-resolved photoelectron spectra.     

Theoretical study of singlet and triplet excitation energies in oligothiophenes

We have analyzed singlet and triplet excitation energies in oligothiophenes (up to five rings) using time-dependent density-functional theory (TD-DFT) with different exchange-correlation functionals and compared them with results from the approximate coupled-cluster singles and doubles model (CC2) and experimental data. The excitation energies have been calculated in geometries obtained by TD-DFT optimization of the lowest excited singlet state and in the ground-state geometries of the neutral and anionic systems. TD-DFT methods underestimate photoluminescence energies but the energy difference between singlet and triplet states shows trends with the chain-length similar to CC2. We find that the second triplet excited state is below the first singlet excited state for long oligomers in contrast with the previous assignment of Rentsch et al. (Phys.Chem. Chem. Phys. 1999, 1, 1707). Their photodetachment photoelectron spectroscopy measurements are better described by considering higher triplet excited states.     

Excited state dynamics and fragmentation channels of the protonated dipeptide H2N-Leu-Trp-COOH

The excited state dynamics of the isolated and protonated peptide H(2)N-Leu-Trp-COOH are analyzed by fs pump-probe spectroscopy. The peptides are brought into the gas phase by electrospray ionization, and fs pump-probe excitation is detected by fragment ion formation. The pump laser addressed the excited pipi* state of the indole chromophore of the amino acid tryptophan. The subsequent excited state dynamics agreed with a biexponential decay with time constants of 500 fs and 10 ps. This is considerably shorter than the lifetime of neutral tryptophan in solution and in proteins, but similar to isolated, protonated tryptophan. Several models are discussed to explain the experimental results but the detailed quenching mechanism remains unresolved.     

Study of acyl group migration by femtosecond transient absorption spectroscopy and computational chemistry

The primary photophysical and photochemical processes in the photochemistry of 1-acetoxy-2-methoxyanthraquinone (1a) were studied using femtosecond transient absorption spectroscopy. Excitation of 1a at 270 nm results in the population of a set of highly excited singlet states. Internal conversion to the lowest singlet npi* excited state, followed by an intramolecular vibrational energy redistribution (IVR) process, proceeds with a time constant of 150 +/- 90 fs. The 1npi* excited state undergoes very fast intersystem crossing (ISC, 11 +/- 1 ps) to form the lowest triplet pipi* excited state which contains excess vibrational energy. The vibrational cooling occurs somewhat faster (4 +/- 1 ps) than ISC. The primary photochemical process, migration of acetoxy group, proceeds on the triplet potential energy surface with a time constant of 220 +/- 30 ps. The transient absorption spectra of the lowest singlet and triplet excited states of 1a, as well as the triplet excited state of the product, 9-acetoxy-2-methoxy-1,10-anthraquinone (2a), were detected. The assignments of the transient absorption spectra were supported by time-dependent DFT calculations of the UV-vis spectra of the proposed intermediates. All of the stationary points for acyl group migration on the triplet and ground state singlet potential energy surfaces were localized, and the influence of the acyl group substitution on the rate constants of the photochemical and thermal processes was analyzed.     

Is the HCCS radical linear in the excited state?

The A (2)Pi-X (2)Pi 415 nm band system of the linear HCCS radical has been known since 1978, but the vibronic structure in this complex spectrum, which has both spin-orbit and Renner-Teller complications, has never been satisfactorily assigned, despite serious experimental and theoretical efforts. In a further attempt to understand the spectrum, we have studied the laser-induced fluorescence spectra of jet-cooled HCCS and DCCS, produced from thiophene precursors using the discharge jet technique. The 0(0) (0) bands of HCCS and DCCS have been rotationally analyzed, providing precise ground and excited state spin-orbit splittings. The energy levels of the v(')=0 (2)Pi(3/2) component of DCCS are found to be perturbed by a very low-lying (2)Sigma vibronic level, indicating that the HCC bending mode Renner-Teller effect is much larger than predicted by ab initio calculations with a linear excited state geometry. With this observation, the vibronic bands in the spectra of both isotopomers have been consistently assigned for the first time. Model calculations show that the large Renner-Teller effect and substantially different HCCS and DCCS excited state zero-point spin-orbit splittings can be explained with the assumption of a quasilinear excited state geometry.     

Ultrafast electrocyclic ring opening of 7-dehydrocholesterol in solution: the influence of solvent on excited state dynamics

Broadband UV-visible femtosecond transient absorption spectroscopy and steady-state integrated fluorescence were used to study the excited state dynamics of 7-dehydrocholesterol (provitamin D(3), DHC) in solution following excitation at 266 nm. The major results from these experiments are: (1) The excited state absorption spectrum is broad and structureless spanning the visible from 400 to 800 nm. (2) The state responsible for the excited state absorption is the initially excited state. Fluorescence from this state has a quantum yield of ～2.5 × 10(-4) in room temperature solution. (3) The decay of the excited state absorption is biexponential, with a fast component of ～0.4-0.65 ps and a slow component 1.0-1.8 ps depending on the solvent. The spectral profiles of the two components are similar, with the fast component redshifted with respect to the slow component. The relative amplitudes of the fast and slow components are influenced by the solvent. These data are discussed in the context of sequential and parallel models for the excited state internal conversion from the optically excited 1(1)B state. Although both models are possible, the more likely explanation is fast bifurcation between two excited state geometries leading to parallel decay channels. The relative yield of each conformation is dependent on details of the potential energy surface. Models for the temperature dependence of the excited state decay yield an intrinsic activation barrier of ～2 kJ/mol for internal conversion and ring opening. This model for the excited state behavior of DHC suggests new experiments to further understand the photochemistry and perhaps control the excited state pathways with optical pulse shaping.     

High resolution spectroscopy for the A (2)A(1) state of CH(3)I(+) by mass-analyzed threshold ionization/photodissociation

Photodissociation of CH(3)I(+) in the ground vibronic state generated by mass-analyzed threshold ionization resulted in a superb spectrum for the first excited electronic state (A (2)A(1)) with hardly any spurious peak. Rotational structure in the spectrum could be resolved by using a single mode laser. This structure for one vibronic band, 2(1)3(1)6(1), was analyzed with the assumption of Hund's case (a) scheme both in the ground and excited electronic states.     

Excited state reactions of amphiphilic Zn(II) porphyrin incorporated into liposomes as models for biological electron transfer

The amphiphilic porphinato zinc(II) complex (ZnP) containing four substituted amphiphilic alkyl chains was embedded in the liposomes of l,2-dipalmitoyl-sn-glycero-3phosphocholine. The distance from the embedded ZnP to the outer phase was changed from 9 to 27 ? by changing the substituted alkyl chain length. The electron transfer from the excited Zn complex to methylviologen (MV²⁺) or benzoquinone (BQ) added in the outer aqueous phase was studied. At first, quenching reactions were analyzed based on dynamic and static reaction models of the excited state. For the MV²⁺ quencher, only the triplet excited state of the embedded ZnP reacted, and electron transfer occurred at a distance less than 12 ?. In BQ both the singlet and the triplet excited states reacted, and the reaction of the singlet state was a static one indicating that BQ is incorporated into the liposomes. The distribution of the BQ molecule in the quenching sphere of ZnP was presented based on calculations assuming a stepwise incorporation into the quenching sphere.     

New study of the stability and of the spectroscopy of the molecular anions NCO- and CNO-

Using highly correlated wave functions, the ground and the low lying excited states of the molecular NCO(-) and CNO(-) anions have been reinvestigated. The stability of the electronic ground state of the two isomers with respect to dissociation and to electron detachment has been checked along the isomerization pathway. The regions of stability of the excited electronic states have been analyzed and identified and it is shown that only the ground state is stable and the corresponding potential energy surface presents three equilibrium positions. The rovibronic spectroscopy of the X (1)Σ(+) state of both NCO(-) and CNO(-) isomers has been determined by a variational approach leading to remarkable agreement with experimental data.     

Spectroscopy of highly excited vibrational states of HCN in its ground electronic state

An experimental technique based on a scheme of vibrationally mediated photodissociation has been developed and applied to the spectroscopic study of highly excited vibrational states in HCN, with energies between 29,000 and 30,000 cm(-1). The technique consists of four sequential steps: in the first one, a high power laser is used to vibrationally excite the sample to an intermediate state, typically (0,0,4), the nu3 mode being approximately equivalent to the C-H stretching vibration. Then a second laser is used to search for transitions between this intermediate state and highly vibrationally excited states. When one of these transitions is found, HCN molecules are transferred to a highly excited vibrational state. Third, a ultraviolet laser photodissociates the highly excited molecules to produce H and CN radicals in its A 2Pi electronic state. Finally, a fourth laser (probe) detects the presence of the CN(A) photofragments by means of an A-->B-->X laser induced fluorescence scheme. The spectra obtained with this technique, consisting of several rotationally resolved vibrational bands, have been analyzed. The positions and rotational parameters of the states observed are presented and compared with the results of a state-of-the-art variational calculation.     

2-溴噻吩和3-溴噻吩在267 nm的C-Br键解离机理

利用离子速度影像技术, 研究了2-溴噻吩和3-溴噻吩两种同分异构体在267 nm激光作用下的C—Br键解离机理, 获得了光解产物Br(2P3/2)和Br*(2P1/2)的能量和角度分布, 分析了两异构分子在267 nm 的C—Br键解离通道. 对于2-溴噻吩和3-溴噻吩, 产物Br来源于三个通道: (i) 从单重激发态系间窜跃到排斥的三重激发态的快速预解离; (ii)单重激发态内转化到高振动基态的热解离; (iii) 母体分子多光子电离后的解离. 2-溴噻吩的产物Br*具有类似的产生机制; 但对于3-溴噻吩, 从激发态内转换到高振动基态发生热解离成为产物Br*的主导通道, 而来自激发三重态的快速预解离通道则几乎消失. 定量地给出了各个通道的相对贡献、能量分配及各向异性分布信息. 实验发现, 随着溴原子在噻吩上取代位置远离硫原子, 来自通道(i)和(ii)产物之间的比例明显减小, 相应的各向异性分布有变弱趋势.     

Dynamics of solvent-mediated electron localization in electronically excited hexacyanoferrate(III)

We have used polarization-resolved UV pump-mid-IR probe spectroscopy to investigate the dynamics of electron hole localization for excited-state ligand-to-metal charge-transfer (LMCT) excitation in Fe(CN)(6)(3-). The initially generated LMCT excited state has a single CN-stretch absorption band with no anisotropy. This provides strong evidence that this initial excited state preserves the octahedral symmetry of the electronic ground state by delocalizing the ligand hole in the LMCT excited state on all six cyanide ligands. This delocalized LMCT excited state decays to a second excited state with two CN-stretch absorption bands. We attribute both peaks to a single excited state because the formation time for both peaks matches the decay time for the delocalized LMCT excited state. The presence of two CN-stretch absorption bands demonstrates that this secondary excited state has lower symmetry. This observation, in conjunction with the solvent-dependent time constant for the formation of the secondary excited state, leads us to conclude that the secondary excited state corresponds to a LMCT state with a localized ligand hole.     

A physiochemical study of azo dyes: DFT based ESIPT process

Azo linked dye derivatives were synthesized and characterized by NMR, mass and elemental analysis. An excited state intramolecular proton transfer (ESIPT) in hydroxy Schiff base has been analyzed, and found that two distinct ground state isomers of I and II are responsible for the observed dual emission. DFT calculation on energy, dipole moment, charge distribution of the rotamers in the ground and excited states support the ESIPT process. PES calculation indicates that the energy barrier for the interconversion of two rotamers is too high in the excited state than the ground state. By varying the addition of base concentration to hydroxy Schiff base, two isobestic points were found which confirm the equilibrium among the trans enol form, anion and the cis enol form. Fluorescence quenching with metal ions reveal that hydroxy Schiff base can be used as a new fluorescence sensor to detect the Cu(2+) ion.     

有机分子三重激发态的调控与应用

对近期有机分子三重激发态调控的研究进展进行了总结评述。控制分子的三重激发态性质,可以制备多种具有新颖性质的分子,如用于可激活光动力治疗(PDT)的光敏剂、磷光分子探针与生物标识试剂,以及可控的三重态湮灭上转换等。但目前对三重态控制方面的研究相对较少,其中的规律也很不明确。近期有文献陆续报道了使用超分子方法和共价修饰法进行的三重态调控,利用的光物理过程有单重态能量转移、三重态能量转移、电子转移等等。现有研究结果表明,三重态的调控规律与单重态的调控规律有所不同,例如:发色团的单重激发态(荧光)往往可以被光诱导电子转移(PET)所猝灭,但是在多个例子中已发现,相同发色团的三重态并不能被PET所猝灭。本文总结的研究结果及所作的分析,将对该领域的分子结构设计及后续研究起到一定的促进作用。