Andrea Peluso

Dynamics of radiationless transitions in large molecular systems: A Franck–Condon-based method accounting for displacements and rotations of all the normal coordinates

An efficient method to study the dynamics of radiationless transition in large molecular systems is proposed. It is based on the use of the whole set of normal coordinates of vibration and allows for taking properly into account both the displacements and the mix of the normal modes upon transition between two electronic states. The Hamiltonian matrix elements are written in terms of generalized Franck–Condon integrals and are analytically evaluated by recursion formulas. Applications to the S2→S1 internal conversion in pyrazine and to long-range electron transfer between quinones in photosynthetic reaction centers are given.

The vibrational progressions of the N→V electronic transition of ethylene: A test case for the computation of Franck-Condon factors of highly flexible photoexcited molecules

The vibrational progressions of the N-->V electronic transition of ethylene--a test case for the computation of Franck-Condon factors between electronic states exhibiting very different equilibrium geometries--have been calculated by using both the Cartesian and the curvilinear internal coordinate representations of the normal modes of vibration. The comparison of the theoretical spectra with the experimental one shows that the Cartesian representation yields vibrational progressions which are not observed in the experimental spectrum, whereas the curvilinear one gives a very satisfying agreement, even in harmonic approximation.

Vibronic coupling in electronic transitions with significant Duschinsky effect

Conceptual and mathematical difficulties arise in the study of transitions between vibronic states belonging to crossing Born-Oppenheimer hypersurfaces with significantly different positions and shapes of the minimum-energy nuclear configurations (Duschinsky effect). This article presents an analytic procedure for the evaluation of the pertinent coupling integrals, adopting a normal coordinate reference system for the internal nuclear motion and harmonic approximation. The Duschinsky effect is properly considered. The procedure was applied to the dynamics of electron transfer in neutral mixed valence monoradicals, using the diabatic representation.

Generating Function Approach to the Calculation of Spectral Band Shapes of Free-Base Chlorin Including Duschinsky and Herzberg–Teller Effects

A generating function approach to the calculation of spectral band shapes including Duschinsky and Herzberg-Teller effects is proposed and applied to the computation of the free-base chlorin Q absorption bands, using molecular geometries and normal vibrations obtained by density functional theory computations. The results clearly show that non-Condon effects can significantly affect the relative intensities of the weakest Q(y) and, to a lesser extent, Q(x) bands. The proposed approach is extremely powerful and can be used in the cases where the molecular size makes the direct calculation of Franck-Condon integrals by recurrence formulas prohibitive.

The electron photodetachment spectrum of c-C4F8-: A test case for the computation of Franck-Condon factors of highly flexible molecules

The long (approximately 20 000 cm(-1)) vibrational progression observed in the electron photodetachment spectrum of c-C(4)F(8)(-) anion is analyzed in terms of the Franck-Condon factors between the ground electronic states of the anionic and neutral species. The observed spectrum is correctly reproduced and its vibronic structure is assigned in detail. The very simple structure of the spectrum, consisting of a sequence of evenly spaced peaks, at 355 cm(-1) each other, is assigned to a series of overlapping progressions associated with vibrational excitations of two totally symmetric modes rather than to a single mode progression, as originally proposed. The underlying continuum observed in the experimental spectrum is traced back to the excitations of a low frequency ring-puckering vibration, which, in the neutral species, exhibits a double minimum potential energy profile.

Franck–Condon factors in curvilinear coordinates: the photoelectron spectrum of ammonia

An approach to the calculation of Franck–Condon factors in curvilinear coordinates is outlined. The approach is based on curvilinear normal coordinates, which allows for an easy extension of Duschinsky’s transformation to the case of curvilinear coordinates, and on the power series expansion of the kinetic energy operator. Its usefulness in the case of molecules undergoing large displacements of their equilibrium nuclear configurations upon excitation is then demonstrated by an application to the vibrational structure of the photoelectron spectrum of ammonia, using an anharmonic potential only for the symmetric stretching and bending coordinates of the radical cation.

A series of compounds forming polar crystals and showing single-crystal-to-single-crystal transitions between polar phases

Condensation of 4-hydroxybenzohydrazide with a variety of non-chiral aliphatic ketones, methylethylketone, acetone, cyclohexanone and cyclobutanone, affords imine compounds crystallizing in polar space groups (Pna21 or Cc). For all the compounds synthesized, the crystal structure was determined by single crystal X-ray analysis and second harmonic generation experiments were performed on powder samples. The imine obtained using acetone shows solid state polymorphism with unusual behaviour; three different crystal phases have been identified (phases I, II and III), all having the same polar space group Pna21, and the transitions from I to II and from III to II are topotactic, i.e. single-crystal-to-single-crystal. During the irreversible transition from phase I to II, the polar axis undergoes a strong compression (about 15%) and single crystals of the parent phase are violently shattered into single crystal fragments of the new phase, while in the reversible transition from phase III to II, the polar axis expands (about 14%) and the integrity of single crystals is preserved.

Structure and Electronic Properties of Extended Chromophores for Applications in Second-Order Nonlinear Optics

A structural and theoretical analysis of new 2-[(4-phenylazo)phenyl] benzoxazole chromophores of potential interest in the field of second-order nonlinear optics is presented. Computations predict comparatively high hyperpolarizabilities for most of the compounds in their equilibrium nuclear configuration, with a significant dependence upon some conformational degrees of freedom and independence upon some others. This behavior is rationalized in the frame of the two-level model and sheds light on the conjugation pattern of these new chromophores.

Quantum Dynamics of Electron Transfer from Bacteriochlorophyll to Pheophytin in Bacterial Reaction Centers

We extend our previous works on the early electron-transfer steps in bacterial photosynthetic reaction centers to the dynamics of electron transfer from the bacteriochlorophyll anion to pheophytin. The approach employed here takes into account the whole set of normal coordinates of the acceptor and donor groups, in order to reliably account both for shifts and mixing of the normal coordinates and for changes in vibrational frequencies upon electron transfer. It is shown that intramolecular modes provide not only a discrete set of states strongly coupled to the initial state but also a quasi-continuum of weakly coupled states, which account for faster dephasing effects. Detuning effects are accounted for by averaging the computed probability over a small range of the electronic energy difference. The computed transition probability is of the same order of magnitude, a few picoseconds, as the observed one.

Photoelectron spectrum of ammonia, a test case for the calculation of Franck-Condon factors in molecules undergoing large geometrical displacements upon photoionization.

The vibrational structure of the photoelectron spectrum of ammonia, the simplest molecule undergoing a large displacement of its equilibrium geometry upon photoionization, is analyzed by evaluating the Franck-Condon integrals at the anharmonic level of approximation. It is shown that if the rectilinear Cartesian representation of normal modes is adopted Duschinskys transformation yields a too large displacement of the bond distance coordinate, with the appearance of several progressions which are not observed in the experimental spectrum. This apparent failure is completely corrected by the inclusion of anharmonic couplings between the principal active mode, the out of plane bending of the planar cation, and the totally symmetric stretching mode, leading to a satisfying reproduction of the observed spectrum and to a more convincing assignment of the weaker progression observed in the high-resolution spectrum.