On Theoretical Study of a Molecular Dimer System

In this paper, the vibronic structure of a dimer system is studied both theoretically and numerically. To construct adiabatic potential surfaces and electronic and vibrational wave functions for a dimer system, the adiabatic approximation is applied to two identical molecules, each of which has two electronic states with one vibrational mode. In this scheme, the excitonic splitting results not only from the electronic coupling of two molecules, but also from the vibronic coupling in each molecule. By using the resulting wavefunctions and the corresponding energies, the absorption and fluorescence spectra are studied. The effect of temperature on these spectra is also studied.     

Regioselectivity in multiple cycloadditions to fullerene C60: vibronic coupling density analysis

We theoretically investigate multiple cycloadditions of butadienes to fullerene C₆₀ with the concept of vibronic coupling density (VCD) as a reactivity index. Recently, we have clarified that C₆₀ bears ethylene moieties at its reactive sites as functional groups, on the basis of VCD. In this study, we calculate the VCDs of C₆₀ adducts with n butadienes (n=1, …, 5) and show that the ethylene moieties are conserved with almost the same distributions as those in C₆₀. The VCD analysis predicts that the hexakisadduct with a six-fold axis is finally obtained, which is consistent with experimental observations.     

Molecular design for high-spin molecules in view of vibronic couplings

A high-spin ground state is possible if a molecule has degenerate or pseudo-degenerate frontier orbitals. Since strong vibronic couplings, or electron-vibration interactions give rise to reduce the degeneracy or pseudo degeneracy, a lower-spin state is the ground state in such a molecule. Therefore small vibronic couplings are desirable for designing molecules with a high-spin ground state. Vibronic coupling constants of derivatives of m-phenylene diamine are evaluated. The calculated results are analyzed based on vibronic coupling density which enables us to control the vibronic coupling constants. Based on the vibronic coupling density analysis, we succeed in recovering the high-spin ground state from the closed-shell singlet ground state of a methoxy derivative of m-phenylene diamine by introducing an appropriate substituent.     

Enhanced two-photon absorption of novel four-branched chromophore via vibronic coupling

A novel four-branched chromophore TOZ-4 with starburst linker was synthesized and showed two-photon absorption cross-section (δ) as large as 5254 GM, which was principally resulted from vibronic coupling enhancement.     

The dynamical crystal field model of selective vibronic coupling in d-d spectra

Summary The dynamic crystal field operators, corresponding to the normal point-charge displacements in an octahedral complex are analyzed in detail. The strict equivalence of absolute versus relative coordinate treatments is established. The resulting formalism is applied to the intensity distribution in the vibronic side bands of the sharp line luminescence spectra ofd ³ complexes. Thereby special attention is given to the role of spin-orbit coupling and to the elastic properties of the molecular force field. Using the closure procedure, the relative intensities of the side bands may be expressed in terms of a single dynamic crystal field parameter. These expressions provide a simple rationalization of the observed vibronic selection rules, entirely within the framework of dynamic crystal field theory.     

Photodecay dynamics of octaethylporphine in the condensed phase explored via resonance Raman spectroscopy and density functional theory calculation

Resonance Raman spectra of free-base octaethylporphine (OEP) were obtained with 368.9 nm, 397.9 nm and 416.0 nm excitation wavelengths, and density functional calculations were done to help the elucidation of Soret (B(x) and B(y)-band) electronic transitions and the corresponding photo relaxation dynamics of OEP. The RRs indicate that the Franck-Condon region photo relaxation dynamics upon S(0)→S(8) electronic transition is predominantly along the totally symmetric C(m)C(α) stretch, the C(β)C(β) stretch, and simultaneously along the asymmetric δ(pyr deformation),γ(CH(2)) vibrational relaxation processes. The excited state structural dynamics of OEP determined from resonance Raman spectra show that the internal conversion between B(y) and B(x) electronic states occurs in tens of femtoseconds and the electronic relaxation dynamics were firstly interpreted with account of the time-dependent wave packet theory and Herzberg-Teller (vibronic coupling) contributions.     

Acetylene-insertion reactions into Pt(II)–H and Pt(II)–SiH3 bonds. An ab initio MO study and analysis based on the vibronic coupling model

Acetylene insertion into Pt(II)–H and Pt(II)SiH₃ bonds of PtH(SiH₃)(PH₃) was investigated using ab initio molecular orbital and M?ller-Plesset perturbation theory methods. The insertion into PtH was predicted to proceed with a smaller activation energy (E _a =12.8 kcal/mol) than that into PtSiH₃ (E _a =20.9 kcal/mol). The reaction energy (ΔE) of the insertion into PtH is 10 kcal/mol smaller than that for the insertion into PtSiH₃, which reflects differences in bond energies between CH and CSi and between PtH and PtSiH₃. A comparison with ethylene insertion revealed that the acetylene insertion occurs more easily, and the latter reaction is more exothermic. A simple vibronic coupling model combined with Toyozawa's interaction mode analysis was used to examine interesting differences in E _a and ΔE between insertions into PtH and PtSiH₃, and between acetylene and ethylene insertions. This analysis suggests that the factors determining E _a are the stiffness of the PtH and PtSiH₃ bonds and the vibronic coupling strength of acetylene and ethylene. Received: 13 August 1998 / Accepted: 2 September 1998 / Published online: 15 February 1999     

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.

     

Magnetic properties of d–d–d MV trimers with partial electron delocalization in the generalized vibronic model

The effects of magnetic exchange, double exchange and vibronic coupling on magnetic properties of the d¹–d¹–d² trimer with C_2V symmetry are discussed in the generalized vibronic model. The magnetic moments are calculated with the semiclassical adiabatic approach. The results show that the PKS vibronic coupling and the P vibronic coupling favor the localization and the delocalization of the ‘extra’ electron, respectively. The magnetic properties are determined by the interplay among magnetic exchange, double exchange and vibronic coupling.     

The calculation of electric dipole vibronic intensities in centrosymmetric coordination compounds using the crystal field-closure-ligand polarization model

Equations are derived for the evaluation of the contribution to the electric dipole vibronic intensity of d-d and f-f transitions in octahedral complex ions. Expressions for the evaluation of the crystal-field and ligand polarization terms and the cross term between are presented in symmetry adapted form and tables of the required coupling constants are given.     

The role of vibronic interactions on intramolecular and intermolecular electron transfer in π-conjugated oligomers

The importance of electron-vibrational coupling for intermolecular and intramolecular electron-transfer processes is discussed on the basis of first-principles correlated quantum-mechanical calculations and of a dynamic vibronic approach. The methodology is illustrated for examples selected from some of our recent work. In all instances, the theoretical results are thoroughly compared to experimental data.     

Vibronic coupling in naphthalene anion: vibronic coupling density analysis for totally symmetric vibrational modes

Vibronic coupling, or electron-phonon coupling, of naphthalene is calculated. A method of vibronic coupling density analysis, which has been proposed for the vibronic coupling of the Jahn-Teller active modes in a Jahn-Teller molecule, is extended for totally symmetric vibrational modes of a molecule including a non-Jahn-Teller molecule. Contrary to non-totally-symmetric modes, orbital relaxation upon a charge transfer plays a crucial role in the vibronic coupling calculation for the totally symmetric modes. The method is applied for the ground state of the naphthalene anion to compare with that of the benzene anion. The relationship between the vibronic coupling density and a nuclear Fukui function is also discussed.     

Long-Lived Coherence Originating from Electronic-Vibrational Couplings in Light-Harvesting Complexes

We theoretically investigate the evolutions of two-dimensional, third-order, nonlinear pho-ton echo rephasing spectra with population time by using an exact numerical path integral method. It is shown that for the same system, the coherence time and relaxation time of excitonic states are short, however, if the couplings of electronic and intra-pigment vibra-tional modes are considered, the coherence time and relaxation time of this vibronic states are greatly extended. It means that the couplings between electronic and vibrational modes play important roles in keeping long-lived coherence in light-harvesting complexes. Particularly, by using the method we can fix the transition path of the energy transfer in bio-molecular systems.     

Nonadiabatic quantum dynamics of Br2 in solid Ar: A four-dimensional study of the B to C state predissociation

Quantum dynamics simulations are performed for a diatomics-in-molecules based model of Br₂ in solid Ar which incorporates four nuclear degrees of freedom and four electronic states. The nuclear motions comprise two large amplitude coordinates describing the Br₂ bond distance and an effective symmetry-preserving matrix mode. Two symmetry-lowering harmonic modes are added in the spirit of linear vibronic coupling theory. Initiating the dynamics on the B state by means of an ultrafast laser pulse, nonadiabatic transitions to the two degenerate C states are monitored and the effect of vibrational preexcitation in the electronic ground state is investigated.     

几个荧光蛋白发色团双光子吸收性质的理论研究

实验测得的荧光蛋白的单、双光子吸收光谱在低频和高频区域都表现出明显不同的特征。为了揭示这些不同点的起源和研究荧光蛋白的构–效关系,我们详细研究了三种荧光蛋白发色团(一种增强型蓝绿色荧光蛋白的中性发色团和两种红色荧光蛋白的阴离子发色团)的单、双光子吸收特性,分别计算了纯的和振动分辨的电子谱。计算结果表明：光谱线形与计算采用的交换相关密度泛函及谱截面计算所采用的近似关系密切;如果在计算光谱截面时,我们利用长程修正的交换相关泛函CAM-B3LYP来计算几何和电子结构参数,然后把Franck-Condon (FC)效应和包含Herzberg-Teller (HT)效果的电-声耦合效应都考虑进去,理论计算的光谱与实验测定的光谱可以很好地符合;对于两种离子态的发色团,HT电-声耦合效应使得对应于基态到第一激发态跃迁的双光子吸收最强峰相对于单光子吸收的最强峰发生了蓝移,但HT电-声耦合效应对高频的双光子吸收谱没有太大的影响;分子内电荷转移是导致高频区的双光子吸收明显强于单光子吸收的主要原因。     

Effect of magnetic exchange,double exchange,vibronic coupling,and asymmetry on magnetic properties in d2–d3 mixed‐valence dimers

The effect of magnetic exchange, double exchange, vibronic coupling, and asymmetry on magnetic properties of d²–d³ systems is discussed. The temperature‐dependent magnetic moment was calculated with the semiclassical adiabatic approach. The results show that the vibronic coupling from the out‐of‐phase breathing vibration on the metal sites (Piepho, Krausz, and Schatz [PKS] model) and the vibronic coupling from the stretching vibration between the metal sites (P model) favor the localization and delocalization of the “extra” electron in mixed‐valence dimers, respectively. The magnetic properties are determined by the interplay among magnetic exchange, double exchange, and vibronic coupling. The results obtained by analyzing d²–d³ systems can be generalized to other full delocalized dinuclear mixed valence systems with a unique transferable electron. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Substituent effects on the vibronic coupling for the phenoxyl/phenol self-exchange reaction

The impact of substituents on the vibronic coupling for the phenoxyl/phenol self-exchange reaction, which occurs by a proton-coupled electron transfer mechanism, is investigated. The vibronic couplings are calculated with a grid-based nonadiabatic method and a nuclear-electronic orbital nonorthogonal configuration interaction method. The quantitative agreement between these two methods for the unsubstituted phenoxyl/phenol system and the qualitative agreement in the predicted trends for the substituted phenoxyl/phenol systems provides a level of validation for both methods. Analysis of the results indicates that electron-donating groups enhance the vibronic coupling, while electron-withdrawing groups attenuate the vibronic coupling. Thus, if all other aspects of the reaction are the same, then electron-donating groups will increase the rate, while electron-withdrawing groups will decrease the rate. Correlations between the vibronic coupling and physical properties of the phenol are also analyzed. Negative Hammett constants correspond to higher vibronic couplings, while positive Hammett constants correspond to similar or slightly lower vibronic couplings relative to the unsubstituted phenoxyl/phenol system. In addition, lower bond dissociation enthalpies, ionization potentials, and redox potentials, as well as higher pKa values, tend to correspond to higher vibronic couplings relative to the unsubstituted phenoxyl/phenol system. The observed trends enable the prediction of the impact of general substituents on the vibronic coupling, and hence the rate, for the phenoxyl/phenol self-exchange reaction. The fundamental physical insights obtained from these studies are applicable to other proton-coupled electron transfer systems.     

Vibronic spectra of the perylene crystal

The theory of vibronic coupling in the perylene-type molecular crystals containing four molecules in a unit cell is presented. The vibronic absorption spectra computed theoretically for the perylene crystal are in good agreement with the experimental ones in both the frequency and intensity distributions of two different crystal polarizations.     

Analysis of the vibronic fine structure in circularly polarized emission spectra from chiral molecular aggregates

Using a Frenkel-exciton model, the degree of circular polarization of the luminescence (g(lum)) from one-dimensional, helical aggregates of chromophoric molecules is investigated theoretically. The coupling between the electronic excitation and a local, intramolecular vibrational mode is taken into account. Analytical expressions for the fluorescence band shape and g(lum) are presented for the case of strong and weak electronic coupling between the chromophoric units. Results are compared to those from numerical calculations obtained using the three particle approximation. g(lum) for the 0-0 vibronic band is found to be independent of the relative strength of electronic coupling between chromophores and excitation-vibration coupling. It depends solely on the number of coherently coupled molecules. In contrast, for the higher vibronic transitions[g(lum)] decreases with decreasing strength of the electronic coupling. In the limit of strong electronic coupling, [g(lum)] is almost constant throughout the series of vibronic transitions but for weak coupling [g(lum)] becomes vanishingly small for all vibronic transitions except for the 0-0 transition. The results are interpreted in terms of dynamic localization of the excitation during the zero point vibrational motion in the excited state of the aggregate. It is concluded that circular polarization measurements provide an independent way to determine the coherence size and bandwidth of the lowest exciton state for chiral aggregates.