The n-π transitions of the pyrazine molecule: II. Polarized spectrum in benzene crystal,and anharmonicity in the upper singlet state potential function

The polarized absorption spectra of pyrazine (h₄ and d₄) in single benzene crystals have been measured at high resolution at 2 K. Vibrational assignments in the ¹Ag → ¹B_3u (nπ^*) transition have been confirmed and considerably extended. The quartic potential component of hydrogen bending mode :Oa has been found to originate primarily from vibronic coupling with the nearby ¹B_2u (ππ^*). The coupling between in-plane 6a and out-of-plane 10a modes is described theoretically, and leads to further spectral assignments. Other out-of-plane modes 4 and 5 are identified and shown to have combination defects with 6a. A quartic component found for the out-of-plane ring bending mode 4 could not be explained by vibronic coupling.     

Spectra and emission lifetimes of H2CS(Ã 1A2)

Fluorescence spectra and lifetimes of single vibronic levels of the first excited singlet state of H₂CS have been measured under effusive flow conditions. Fluorescence lifetimes of the single vibronic levels decrease from 140±3 μs (0°) to 68 μs(3¹4¹5¹) with increasing excitation energies. The promoting vibrational modes for the non-radiative transition are considered to be the out-of-plane bending (ν₄) and the asymmetric rocking (ν₆) modes rather than the asymmetric stretching mode (ν₅).     

Discrimination of two low-energy conformers of octahydroanthracene in supersonic jet

The S₀ and S₁ states’ vibronic features of two low-energy conformers of octahydroanthracene have been investigated by laser induced fluorescence excitation, dispersed emission and UV–UV hole burning spectroscopy in a cold seeded jet. UV–UV hole burning spectra have been used to distinguish the two conformers and their S₁ state vibrational modes. In addition, semi-empirical and ab initio calculations have been performed to correlate the calculated S₀ frequencies of vibrational modes with the experimental results. It is found that the observed spectra correspond well with sym-twist (a) (C_2h) and antisym-twist (b) (D₂) forms.     

Nuclear Coordinate dependence of electronic transition moments in orbitally forbidden transitions: A new interpretation of deuterium isotope effects

The nuclear coordinate dependence of electronic transtion moments has been investigated for the purpose of finding new interpretations of deuterium isotope effects on spectral intensities and radiative decay rates in orbitally forbidden electronic transitions. By using “AO following nuclei” wavefunctions as the building block for the electronic wavefunction in the adiabatic BO vibronic wavefunction, the spin-free hamiltonian is diagonalized to generate eigenfunctions and eigen-energies. It is found that the electronic transtion moments based on these eigenfunctions show dependences upon the vibrational modes which are not directly involved in vibronic coupling. This leads to interpretations of the deuterium isotope effects in T₁ → S₀ radiative transitions of aromatic hydrocarbons and S₀ → S₁ absorption in pyrazine which are not based on the conventional Herzberg—Teller or non-BO coupling.     

Nonradiative decay processes in benzene

We investigate the behavior of single vibronic level nonradiative decay rates in benzene and benzene-d₆. The effects of excitation in a promoting mode which undergoes frequency and geometry changes in the S₁ relaxation (to T₁ or S₀) are considered in detail. Calculated relative nonradiative decay rates are compared with experimental values and are used to assign triplet state vibrational frequencies to the ν_s, ν₁₀ and ν₁₆ vibrations. This comparison also indicates that none of these modes, nor the modes ν₁ and ν6, are likely to be the dominant promoting modes for the S₁ → T₁ decay. Some simple expressions are given which provide good estimates of the vibronic state dependence of the non-radiative decay rates. In conjuction with experimental decay rate data, these estimates can aid in guiding spectral assignments of vibronic bands. Simple but general theoretical criteria are derived which are useful in determining those vibrations which are poor (or good) accepting modes. Our previous theory is generalized to consider absolute nonradiative decay rates. The results are used to suggest a possible mechanism for the “channel three” decay process observed by Callomon . Although the numerical applications presented here are to benzene electronic relaxation processes, the theoretical developments also apply to and the calcultions illustrate general features of nonradiative decay in the statistical limit.     

Polypyrrole Bilayers: Charge Trapping and Application for Amperometric Ions Sensing

Bilayers composed of polypyrrole: doped by perchlorate ions (PPy(ClO₄) – anion exchanging inner layer) and by dodecyl sulfate ions (PPy(DS) – cation exchanging outer layer) are very effective charge trapping systems that are usually not observed for other bilayers comprised of polypyrrole. Chronopotentiometric experiments carried out for oxidation and reduction showed that the trapping effect in the inner layer resulted from different ion exchange properties of the component polymers, leading to a low permeability of the reduced outer layer towards anions. Estimated diffusion coefficients of Cl^? anions in the oxidized and reduced PPy(DS) are in the range of 10^?9 and lower than 10^?10 cm² s^?1, respectively. The presence of the outer layer limiting the ion transfer was found to be beneficial to improve the signal resolution in amperometric mode of ion sensing within wide KCl concentration range, from 10^?5 M up to 3 M. The influence of experimental conditions (film thickness, response time) on optimization of this novel kind of polymeric bilayer ion sensors was studied.     

Vibrational redistribution effects in the time resolved emission of isolated pyrene molecules

Arguments are given that the appearance of a short component in the fluorescence decay of pyrene after excitation in a vibronic origin of the S₀ → S₁ transition, has to be ascribed to a redistribution of vibrational energy of the inducing mode over the other vibrational modes in the molecule.     

Molecular Dynamics Simulation of HIV-1 Integrase Dimer Complexed with Viral DNA

The biological function of HIV‐1 integrase (IN) is to integrate viral DNA into the host cell chromosome, and the specific binding of IN with viral DNA is a precondition for IN to function correctly. Beforehand, the binding mode of IN dimer (IN₂) with the 27 bp segment of viral DNA before 3′ processing (3′‐P) was obtained via a molecular docking method. Based on the binding mode, the aim of this article was to explore the changes of motive mode and correlative motion for the IN₂ and DNA systems after their binding through dynamical cross‐correlation map (DCCM) and principal component analysis (PCA). Finally, solvent effect during the association was analyzed briefly. The results show that there is a significantly increased positive correlation in the interface region between IN₂ and viral DNA, and some obvious motive mode changes of the two systems (IN₂ and DNA) were also observed after their binding. It was found that water molecules played an important role in the recognition between IN₂ and viral DNA through analyzing the water‐mediated hydrogen bonds.     

Excited state spectroscopy of para di-substituted benzenes in a supersonic beam using resonant two photon ionization

Excited state vibronic spectra of p-aminophenol, p-cresol, p-fluoroaniline, p-fluorophenol, hydroquinone and p-toluidine have been obtained using resonant two photon ionization supersonic beam mass spectrometry. Despite marked similarities in the spectra, notable differences exist and different para polyatomic substituents in the same molecule show vibronic evidence of their real molecular symmetry of C_2ν. Expansion of the ring is also noted upon excitation in all cases. Further, it is now evident that the assignment of some vibronic bands historically interpreted as sequence structure must be reconsidered since molecules like hydroquinone are mixtures of cis and trans and others have a vibronic structure arising from the polyatomic nature of the substituents (cƒ. CH₃).     

Studies on the Interaction of Dinitratobis（phen） Cadmium Complex with DNA

DNA-binding properties of the dinitratobis（phen） cadmium complex [Cd（phen）2（NO3）2] （where phen = 1,10-phenanthroline） have been investigated with absorption titration, fluorescence spectroscopy, viscosity measurement, molecular modeling and density functional theory （DFT） calculations. The results indictate DNA-binding mode of the complex to be weak groove binding rather than partial intercalative interaction expected of the extended planar aromatic phen ring. In addition, the DNA cleavage study was carried out by gel electrophoresis experiment. The results showed that the complex both hardly cleaves pBR322 DNA in the absence and present ascorbate. So it is suggested that the formation of cadmium complex can decrease cadmium toxicity to some extents.     

Coherent Motion Reveals Non‐Ergodic Nature of Internal Conversion between Excited States

We found that specific nuclear motion along low‐frequency modes is effective in coupling electronic states and that this motion prevail in some small molecules. Thus, in direct contradiction to what is expected based on the standard models, the internal conversion process can proceed faster for smaller molecules. Specifically, we focus on the S₂→S₁ internal conversion in cyclobutanone, cyclopentanone, and cyclohexanone. By means of time‐resolved mass spectrometry and photoelectron spectroscopy the relative rate of this transition is determined to be 13:2:1. Remarkably, we observe coherent nuclear motion on the S₂ surface in a ring‐puckering mode and motion along this mode in combination with symmetry considerations allow for a consistent explanation of the observed relative time‐scales not afforded by only considering the density of vibrational states or other aspects of the standard models.     

Low-frequency vibrations specific for conformers of 1-aminoindan studied by UV-UV hole-burning spectroscopy

The UV-UV hole-burning spectra of the jet-cooled 1-aminoindan were measured for the first time. Complicated spectral features observed in the laser-induced fluorescence excitation spectrum due to two conformers, R and B, were firmly separated. On the basis of fluorescence measurements and B3LYP/cc-pVTZ calculations, low-frequency ring twisting and ring puckering modes were assigned. These modes are coupled in the S1 state due to the Duschinsky rotation. The Duschinsky matrix was calculated from the normal modes predicted by quantum chemical calculations. The coupling between the twisting and puckering modes for conformer B is stronger than that for conformer R. The twisting mode was observed at 0+99 cm(-1) in the S1 state for conformer B, while not for conformer R. The Franck-Condon activity of the twisting mode substantially differs between the two conformers. The transition to the twisting level for conformer B would be allowed by the Duschinsky rotation. The fluorescence lifetime of conformer vibronic levels was also measured and differed for each conformer.     

Nonadiabatic quantum dynamics based on a hierarchical electron-phonon model: exciton dissociation in semiconducting polymers

A hierarchical electron-phonon coupling model is applied to describe the ultrafast decay of a photogenerated exciton at a donor-acceptor polymer heterojunction, via a vibronic coupling mechanism by which a charge-localized interfacial state is created. Expanding upon an earlier Communication [H. Tamura et al., J. Chem. Phys. 126, 021103 (2007)], we present a quantum dynamical analysis based on a two-state linear vibronic coupling model, which accounts for a two-band phonon bath including high-frequency C[Double Bond]C stretch modes and low-frequency ring torsional modes. Building upon this model, an analysis in terms of a hierarchical chain of effective modes is carried out, whose construction is detailed in the present paper. Truncation of this chain at the order n (i.e., 3n+3 modes) conserves the Hamiltonian moments (cumulants) up to the (2n+3)rd order. The effective-mode analysis highlights (i) the dominance of the high-frequency modes in the coupling to the electronic subsystem and (ii) the key role of the low-frequency modes in the intramolecular vibrational redistribution process that is essential in mediating the decay to the charge-localized state. Due to this dynamical interplay, the effective-mode hierarchy has to be carried beyond the first order in order to obtain a qualitatively correct picture of the nonadiabatic process. A reduced model of the dynamics, including a Markovian closure of the hierarchy, is presented. Dynamical calculations were carried out using the multiconfiguration time-dependent Hartree method.     

Depolarization dispersion curves of resonance Raman fundamentals of metalloporphyrins and metallophthalocyanines subject to asymmetric perturbations

A model is presented that allows the investigation of depolarization dispersion curves of a_1g,a_2g,b_1g and b_2g resonance Raman fundamentals in the region of the Q state of metalloporphyrins and metallophthalocyanines. This dispersion results from electronic and/or vibronic perturbations of A_2g,B_1g and B_2g symmetry due to asymmetric substituents and/or metal ion-ring interaction acting on the porphyrin (phthalocyanine) ring. The electronic perturbations affect the electronic configuration interaction pattern between the four orbital components of the Q and B states, yielding thereby similar depolarization dispersion curves for all modes of a given symmetry, whereas the vibronic perturbations affect selectively the vibronic coupling matrix of a particular mode. Depolarization dispersion curves resulting from A_2g and B_1g perturbations are treated separately, and many helpful perturbational formulas are given for use in analyzing experimental data. Examples of depolarization dispersion curves and excitation profiles of fundamentals of a_1g, a_2g, b_1g and b_2g symmetry are presented. It is shown that strong depolarization dispersion observed in copper chelate of mesoporphyrin IX dimethyl ester for a_1g and a_2g fundamentals can be explained in terms of an A_2g electronic perturbation and a vibronic a_2g perturbation suffered by the a_1g(1131 cm^?1) fundamental. Similarly, the depolarization dispersion curves observed for fundamentals in cytochrome c and Pt-phthalocyanine are explained in terms of an electronic B_1g perturbation, together with selective b_1g vibronic perturbations acting on the 1310 cm^?1 a_2g fundamental in cytochrome c and the 482 cm^?1 b_2g fundamental in Pt-phthalocyanine. The agreement between the depolarization dispersion curves predicted by our model and experimental data is shown to be satisfactory.     

The influence of vibronic interactions on the chiroptical spectra of dissymmetric pseudo tetragonal metal complexes

The influence of vibronic interactions on the chiroptical spectra associated with a threesome of nearly degenerate electronic excited states in a dissymmetric molecular system is examined on a formal theoretical model. The model considers two vibrational modes to be effective in promoting pseudo Jahn-Teller (PJT) type interactions between the three closely spaced electronic excited states. Formal expressions are developed for the rotatory strengths of individual vibronic levels derived from the coupled electronic states. Two mode (vibrational)-three state (electronic) vibronic Hamiltonians are constructed (basis set size, 63–108, depending upon interaction parameters used) and diagonalized for a large number of different parameter sets representative of various vibronic coupling strengths, electronic energy level spacings, oscillator (vibrational mode) frequencies, and electronic rotatory strengths. Diagonalization of these vibronic Hamiltonians yields vibronic wave functions and energies which are then used to calculate rotatory strength spectra for the model system. The calculated results demonstrate the profound influence which vibronic interactions of the PJT type may have on the sign patterns and intensity distributions within the rotatory strength spectrum associated with a set of nearly degenerate electronic states. The implication of these results for the interpretation of circular dichroism spectra of chiral transition metal complexes with pseudo tetragonal symmetry are discussed.     

Emission spectroscopy and excited-state dynamics of chromyl-chloride vapor

A tunable dye laser has been used to excite single vibronic features in the low-pressure vapor of CrO₂Cl₂. The fluorescence spectrum, fluorescence excitation spectrum and time-resolved fluorescence decay are discussed. It is shown that the active ν′₄ and ν″₄ modes are the same frequency in the gas phase, thus collapsing the sequence congestion normally observed in gas-phase spectra. This degeneracy makes impossible the excitation of single vibronic levels. It is shown that the fluorescence lifetime of the excited state in all except the vibrationally cold level is severely shortened by unimolecular radiationless decay. This radiationless rate is strongly dependent upon the partitioning of energy into various excited-state modes. The radiative lifetime of the vibrationally cold excited state is (1.34 ± 0.08) μs and the apparent bimolecular quenching rate is (5.9 ± 0.2) × 10^?10 cm³/molecules. No evidence of emission from the lowest-energy excited electronic state recently reported by Spoliti [J. Mol. Spectrosc. 52 (1973) 146] is observed.     

Vibronisches Spektralverhalten von Molekülen: XIV. Zum Einfluß der vibronischen Kopplung auf die S0-S1-Absorption und Fluoreszenz von ausgewählten 1,3-Diketonato-bor-komplexen im Rahmen der Herzberg-Teller-Näherung

Summary Based upon completely-optimized S₀ and S₁ molecular geometries the vibrational structures of S₀-S₁ absorption and fluorescence transitions of selected 1,3-diketonato boron complexes being differently substituted, are calculated within the Herzberg-Teller approach taking into account vibronic coupling contributions. In dependence on substituted diketone as well as on the co-ligand, the influence of vibronic coupling and the consequences of intensity borrowing on the spectral behaviour in absorption and fluorescence are found to be quite different for the studied boron complexes. Consequently, for some complexes their spectroscopic properties may be interpreted exclusively by means of the Herzberg-Teller approach. An analysis of the relevant vibrational modes is given.

     

Vibronic activity in the two-photon spectrum of pyrene

A CNDO/S Cl calculation of the two-photon vibronic activity of some b_1u and b_2u vibrational modes of pyrene is presented. The two-photon amplitude tensor is discussed in terms of vibronic coupling coefficients calculated by means of the orbital-following method. The results are in good agreement with the experimental data taken from a recently observed two-photon spectrum of pyrene in a Shpolskii matrix. The role of the ground-state coupling with the first excited B_1u state has been investigated and is shown to be responsible for most of the vibronically induced two-photon intensity of the spectrum. The calculations also show that the 1310 cm^?1 (b_1u; observed ground-state value) mode is associated with the largest vibronic coupling coefficients and the strongest two-photon amplitude tensor and therefore must be correlated with the most intense B_1u X b_1u false origin ≈ 1496 cm^?1 from the pure (0-0) line.