Understanding excited-state structure in metal polypyridyl complexes using resonance Raman excitation profiles,time-resolved resonance Raman spectroscopy and density functional theory

This article discusses the use of Raman spectroscopy, in concert with density functional theory, as a strategy for understanding excited-state structure in metal polypyridyl complexes. The first sections of the article discuss how one can use resonance Raman spectra of the ground-state molecule to understand the resonant Franck-Condon excited state. The theories behind these analyses are based on the sum-over-states and time-dependent approaches; a brief introduction to each of these methods is given. The use of density functional theory and its use in the determination of normal modes of vibration and infrared and Raman band intensities are discussed, with reference to a number of recent papers. The application of these methods is illustrated through the analysis of a number of selected examples which exemplify the strategies used to extract data from probing the Franck-Condon region. These data include the displacements of the resonant excited state with respect to the electronic ground state, the reorganisation energies associated with photoexcitation, bond length changes with excitation and other electronic parameters. The use, and limitations, of these methods are discussed. The direct calculation of resonance Raman band intensities is introduced. The direct measurement of excited-state vibrational spectra through time-resolved methods is discussed in the latter section of the article; with particular regard to the use of transient resonance Raman and time-resolved resonance Raman techniques to probe structural changes in metal polypyridyl complexes.     

Ultrafast heme dynamics in ferrous versus ferric cytochrome c studied by time-resolved resonance Raman and transient absorption spectroscopy

Cytochrome c (Cyt c) is a heme protein involved in electron transfer and also in apoptosis. Its heme iron is bisaxially ligated to histidine and methionine side chains and both ferric and ferrous redox states are physiologically relevant, as well as a ligand exchange between internal residue and external diatomic molecule. The photodissociation of internal axial ligand was observed for several ferrous heme proteins including Cyt c, but no time-resolved studies have been reported on ferric Cyt c. To investigate how the oxidation state of the heme influences the primary photoprocesses, we performed a comprehensive comparative study on horse heart Cyt c by subpicosecond time-resolved resonance Raman and femtosecond transient absorption spectroscopy. We found that in ferric Cyt c, in contrast to ferrous Cyt c, the photodissociation of an internal ligand does not take place, and relaxation dynamics is dominated by vibrational cooling in the ground electronic state of the heme. The intermolecular vibrational energy transfer was found to proceed in a single phase with a temperature decay of approximately 7 ps in both ferric and ferrous Cyt c. For ferrous Cyt c, the instantaneous photodissociation of the methionine side chain from the heme iron is the dominant event, and its rebinding proceeds in two phases, with time constants of approximately 5 and approximately 16 ps. A mechanism of this process is discussed, and the difference in photoinduced coordination behavior between ferric and ferrous Cyt c is explained by an involvement of the excited electronic state coupled with conformational relaxation of the heme.     

Femtosecond transient absorption, Raman, and electrochemistry studies of tetrasulfonated copper phthalocyanine in water solutions

Ultrafast time-resolved electronic spectra of the primary events induced in the copper tetrasulfonated phthalocyanine Cu(tsPc)4-) in aqueous solution has been measured by femtosecond pump-probe transient absorption spectroscopy. The primary events initiated by the absorption of a photon occurring within the femtosecond time scale are discussed on the basis of the electron transfer mechanism between the adjacent phthalocyanine rings proposed recently in our laboratory. The femtosecond transient absorption results are compared with the low temperature emission spectra obtained with Raman spectroscopy and the voltammetric curves.     

Resonance Raman and density functional study of the excited state structural dynamics of 3-amino-2-cyclohexen-1-one in water and acetonitrile solvents

FT-Raman and/or FT-IR spectra of 3-amino-2-cyclohexen-1-one (ACyO) in solid state and/or in solvents of water and acetonitrile were obtained. Density functional theory calculations were done to help elucidate the vibrational band assignments. The A-band resonance Raman spectra of ACyO were acquired in water and acetonitrile solvents to examine the excited state structural dynamics and the state-mixing or curve-crossing tuned by solvents. A preliminary resonance Raman intensity analysis using the time-dependent wave-packet theory and simple model was done for ACyO in acetonitrile solvent. Resonance Raman spectroscopic probing of the excited state curve-crossing or state-mixing was proposed.     

Transient resonance Raman and density functional theory investigation of the 4-acetamidophenylnitrenium ion

This paper reports a transient resonance Raman and density functional theory study of the 4-acetamidophenylnitrenium ion in a mostly aqueous solvent. The transient Raman bands combined with results from density functional theory calculations indicate that the spectrum should be assigned to the singlet state of the 4-acetamidophenylnitrenium ion. The 4-acetamidophenylnitrenium ion was found to have a substantial iminocyclohexadienyl character comparable to previously studied para-phenyl-substituted phenylnitrenium ions and noticeable charge on both the acetamido and nitrenium moieties. The structure and properties of the 4-acetamidophenylnitrenium ion are compared to those of other arylnitrenium ions. We briefly discuss the chemical reactivity and selectivity of the para-acetamido-substituted phenylnitrenium ions compared to para-phenyl- or para-alkoxy-substituted phenylnitrenium ions.     

Femtosecond broadband time-resolved fluorescence and transient absorption study of the intramolecular charge transfer state of methyl 4-dimethylaminobenzoate

A combined application of femtosecond broadband time-resolved fluorescence (fs-TRF), fluorescence anisotropy (fs-TRFA) and fs to microsecond (μs) transient absorption (TA) have been used to probe directly the dynamics, nature, formation and decay paths of the singlet intramolecular charge transfer ((1)ICT) state of methyl 4-dimethylaminobenzoate (1a) in acetonitrile. The result reveals explicit evidence for a common electronic origin (the L(a) nature) of the (1)ICT state and its precursor the locally excited ((1)LE) state to account jointly for the dual florescence known to this system. It also shows that the ICT reaction from the (1)LE to (1)ICT state occurs with time constant of ~0.8 ps and the (1)ICT state formed decays with a ~1.9 ns time constant leading mainly to a ππ* natured triplet state ((3)T(1)). The (3)T(1) then relaxes with a ~4 μs lifetime under deoxygenated condition resulting in full recovery of the ground state (S(0)). As a case study, this work contributes novel experimental data for improved understanding of the mechanism of ICT reaction; it also reveals a distinct deactivation pattern for this prototype para-amino substituted aromatic carbonyl compound in acetonitrile.     

Time-resolved resonance Raman and density functional theory study of hydrogen-bonding effects on the triplet state of p-methoxyacetophenone

Picosecond and nanosecond time-resolved resonance Raman spectroscopy combined with density functional theory calculations have been performed to characterize the structure, dynamics, and hydrogen-bonding effects on the triplet state of the phototrigger model compound p-methoxyacetophenone (MAP) in cyclohexane, MeCN, and 50% H2O/50% MeCN (v:v) mixed solvent. Analogous work has also been done to study the corresponding ground state properties. The ground and triplet states of MAP were both found to be associated strongly with the water solvent molecules in the 50% H2O/50% MeCN solvent system. A hydrogen-bond complex model involving one or two water molecules bonded with the oxygen atoms of the MAP carbonyl and methoxy moieties has been employed to explore the hydrogen-bond interactions and their influence on the geometric and electronic properties for the ground and triplet states of MAP. Among the various hydrogen-bond configurations examined, the carbonyl hydrogen-bond configuration involving one water molecule was calculated to lead to the most stable hydrogen-bond complex for both the ground and the triplet states with the strength of the hydrogen-bond interaction being stronger in the triplet state than the ground state. The increased carbonyl located hydrogen-bond strength in the triplet state results in substantial modification of both the electronic and the structural conformation so that the triplet of the hydrogen-bond complex can be considered as a distinct species from the free MAP triplet state. This provides a framework to interpret the differences observed in the TR3 spectral and triplet lifetime obtained in the neat MeCN solvent (attributed to the free MAP triplet state) and the 50% H2O/50% MeCN solvent (due to the triplet of the hydrogen-bond complex). Temporal evolution at early picosecond times indicates rapid ISC conversion, and subsequent relaxation of the excess energy of the initially formed energetic triplets occurs for both the free MAP and the hydrogen-bond complex. The triplet of the carbonyl hydrogen-bond complex appears to be generated directly from the corresponding ground state complex and it does not dissociate back to the free triplet state within the triplet state lifetime. We briefly discuss the influence of the carbonyl hydrogen-bond effect on the pi pi* triplet reactivity for MAP and closely related compounds.     

Transient resonance Raman and density functional theory investigation of the 2-fluorenylnitrenium ion.

We report a transient resonance Raman spectrum for the 2-fluorenylnitrenium ion obtained after photolysis of 2-azidofluorene. The 10 experimental Raman band frequencies of the transient spectrum show very good agreement with the computed frequencies from BPW91/cc-PVDZ density functional theory calculations for the 2-fluorenylnitrenium ion. Our results confirm the assignment of the approximately 460 nm transient absorption band formed after photolysis of 2-azidofluorene in water/acetonitrile or water solution to the singlet ground electronic state 2-fluorenylnitrenium ion. Our study indicates the 2-fluorenylnitrenium has a large degree of iminocyclohexadienyl cation character with significant delocalization of the charge over both phenyl rings of the fluorene moiety. We compare our results for the 2-fluoreneylnitrenium ion to those previously reported for several other arylnitrenium ions.     

Reaction of the 4-biphenylnitrenium ion with 4-biphenyl azide to produce a 4,4'-azobisbiphenyl stable product: a time-resolved resonance Raman and density functional theory study

A time-resolved resonance Raman (TR(3)) and density functional theory (DFT) study of the reaction of the 4-biphenylnitrenium ion with 4-biphenyl azide in a mixed aqueous solution is reported. The reaction of the 4-biphenylnitrenium ion with its unphotolyzed precursor 4-biphenyl azide in a mixed aqueous solution generates a 4,4'-azobisbiphenyl stable product via an intermediate species. With the aid of DFT calculations for likely transient species, this intermediate was tentatively assigned to a 4,4'-azobisbiphenyl cation. The DFT calculations predict this reaction can take place via two pathways that compete with one another to produce the trans and cis 4,4'-azobisbiphenyl product. The observation of the 4,4'-azobisbiphenyl cation intermediate demonstrates that the reaction of the arylnitrenium ion with its aryl azide to produce a stable azo product occurs via a stepwise mechanism.     

Surface-enhanced Raman spectroscopy and density functional theory study on 4,4'-bipyridine molecule

The molecular geometry and vibrational frequencies of 4,4'-bipyridine (BPE) in the ground state were calculated using density functional theory (DFT) methods (B3LYP) with 6-31++G(d,p) basis set. The optimized geometric bond lengths and bond angles are obtained by DFT employing the hybrid of Beckes non-local three parameter exchange and correlation functional and Lee-Yang-Parr correlation functional. Fourier transform infrared (FT-IR), Fourier transform Raman (FT-Raman) and near-infrared surface-enhanced Raman scattering (NIR-SERS) spectra of BPE on the silver foil substrate have been recorded. All FT-IR, FT-Raman and NIR-SERS band were assigned on the basis of the B3LYP/6-31++G(d,p) method. The vibrational frequencies obtained by DFT(3LYP) are in good agreement with observed results. The NIR-SERS of BPE excited by 1064nm laser line is little difference with that excited by visible laser line. This phenomenon is result to the increase of the contribution of CHEM enhancement effect. Surface selection rules derived from the electromagnetic enhancement model were employed to infer the orientations of BPE on the silver foil substrate surface. Some vibrational frequency which are sensitive to the planar or non-planar structure of BPE, and to the dihedral angle were concluded.     

Femtosecond time-resolved absorption spectroscopy of astaxanthin in solution and in alpha-crustacyanin

Steady-state absorption and femtosecond time-resolved spectroscopic studies have been carried out on astaxanthin dissolved in CS2, methanol, and acetonitrile, and in purified alpha-crustacyanin. The spectra of the S0 --> S2 and S1 --> S(n) transitions were found to be similarly dependent on solvent environment. The dynamics of the excited-state decay processes were analyzed with both single wavelength and global fitting procedures. In solution, the S1 lifetime of astaxanthin was found to be approximately 5 ps and independent of solvent. In alpha-crustacyanin, the lifetime was noticeably shorter at approximately 1.8 ps. Both fitting procedures led to the conclusion that the lifetime of the S2 state was either comparable to or shorter than the instrument response time. The data support the idea that dimerization of astaxanthin in alpha-crustacyanin is the primary molecular basis for the bathochromic shift of the S0 --> S2 and S1 --> S(n) transitions. Planarization of the astaxanthin molecule, which leads to a longer effective pi-electron conjugated chain and a lower S1 energy, accounts for the shorter tau1 in the protein.     

Time-resolved resonance Raman and density functional theory study of the deprotonation reaction of the triplet state of p-hydroxyacetophenone in water solution

[reaction; see text] Picosecond and nanosecond time-resolved resonance Raman (TR(3)) spectroscopy was employed to investigate the deprotonation/ionization reaction of p-hydroxyacetophenone (HA) after ultraviolet photolysis in water solution. The TR(3) spectra in conjunction with density functional theory (DFT) calculations were used to characterize the structure and dynamics of the excited-state HA deprotonation to form HA anions in near neutral water solvent. DFT calculations based on a solute-solvent intermolecular H-bonded complex model containing up to three water molecules were used to evaluate the H-bond interactions and their influence on the deprotonation reaction and the structures of the intermediates. The deprotonation reaction was found to occur on the triplet manifold with a planar H-bonded HA triplet complex as the precursor species. The HA triplet species is generated within several picoseconds and then decays with a approximately 10 ns time constant to produce the HA triplet anion species after 267 nm photolysis of HA in water solution. The triplet anion species was observed to decay with a time constant of about 90 ns into the ground-state anion species that was found to have a lifetime of about 200 ns. The DFT calculations on the H-bonded complexes of the anion triplet and ground-states species suggest that these anion species are H-bonded complexes with planar quinonoidal structures containing two water molecules H-bonded, respectively, with oxygen lone pairs of the carbonyl and deprotonated hydroxyl moieties. A deactivation scheme of the photoexcited HA in regard to the deprotonation reaction in neutral water solutions was proposed. With the above dynamic and structural information available, we briefly discuss the possible implications of the model HA photochemistry in water solutions for the photodeprotection reactions of related p-HP phototrigger compounds in aqueous solutions.     

The effect of oxidation on the structure of styryl-substituted sexithiophenes: a resonance Raman spectroscopy and density functional theory study

The structures and vibrational properties of a series of styryl-substituted sexithiophenes and their charged species have been examined using resonance Raman spectroscopy in conjunction with density functional theory calculations. The calculated geometries of the radical cations and dications indicate that the quinoidal charged defects are more strongly localized in the center of the thiophene backbone than is observed in other sexithiophenes. This defect confinement, induced by the positions of the styryl substituents, is particularly evident in the dication species. However, the defect confinement weakens when alkoxy groups are added onto the phenyl rings by causing the extension of the charged defect into the styryl groups. The Raman spectra of the neutral styryl sexithiophenes are dominated by intense thiophene symmetrical stretching modes in both the measured and predicted spectra. Oxidation generates radical cations and dications, both of which can be observed in the solution state resonance Raman spectra. Unlike other sexithiophenes, which generally show a downshift of the intense thiophene stretching mode from the radical cation to the dication, a small upshift is observed for the styryl-substituted sexithiophenes. The theoretical spectra predict an insignificant change during this transition and the eigenvector for this mode reveals that it is localized over the same area occupied by the confined defect. In contrast, the solid state resonance Raman spectra of electrochemically oxidized films reveal evidence of solely radical cations and there is an appreciable downshift of the intense thiophene stretching mode compared with the corresponding mode in the solution spectra. This implies that the increase in the effective conjugation length from the solution to the solid state is greater for the radical cations than for the neutral species. It therefore appears that the radical cations form pi stacks in the solid film and the resulting intermolecular interactions effectively allow a further extension of the electron delocalization.     

The study of resonance Raman scattering spectrum on the surface of Cu nanoparticles with ultraviolet excitation and density functional theory

Cu colloid was prepared by oxidation-reduction; it was relatively steady in fixed conditions, with size about 10-30 nm. The Raman spectrum of p-hydroxybenzoic acid (PHBA) in Cu colloid solution with the ultraviolet (UV) excitation at 325 nm, was obtained, even it is usually difficult to obtain Raman signals in Ag or Au in the UV region. It was found that the Raman signal intensities result from the resonance enhanced of surface plasmon resonance of Cu nanoparticles excited at 325 nm. The adsorption behavior of PHBA on the Cu nanoparticles was studied by combining with density functional theory (DFT); it was found that the calculated Raman frequencies were in good agreement with experimental value. So one can conclude that the simplified model is probably reasonable to describe some resonance Raman experiments.     

烯丙醇与臭氧反应机理的密度泛函理论研究

应用密度泛函理论的MPW1K,BHandHLYP和MPWB1K方法,结合6-31+G(d,p)基组优化了烯丙醇与臭氧反应势能面上各驻点的几何构型,通过同一水平的振动频率分析确认了中间体和过渡态.反应路径上的驻点都在HL理论水平下进行单点能量校正,并进行了MPW1K/6-31+G(d,p)水平下的零点振动能校正(ZPE).对反应机理的详尽分析表明臭氧抽取烯丙醇羟基基团中H的通道的反应势垒比臭氧加合烯丙醇双键基团通道的反应势垒高,臭氧与烯丙醇双键加合生成臭氧化物为最可几反应路径.在加合反应历程中,氢迁移通道需经过氢迁移和离解等复杂过程,最终要产生少量的OH自由基,与烃烯类臭氧化反应产生大量OH自由基的结果相反.