Raffaele Borrelli

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.

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.

Computational investigation of the photoinduced homolytic dissociation of water in the pyridine–water complex

The photochemistry of the hydrogen-bonded pyridine-water complex has been investigated with ab initio computational methods. Vertical excitation energies, excited-state reaction paths for proton transfer as well as structures and energies of conical intersections and reaction barriers have been determined with multi-configuration self-consistent-field and multi-reference perturbation methods, as well as with single-reference coupled-cluster and propagator methods. In the pyridine-water complex, the energies of two charge-separated excited states of (1)nπ* and (1)ππ* character are connected to the energies of the locally excited (1)nπ* and (1)ππ* states of the pyridine chromophore via a low energy barrier. The charge-separated excited states are strongly stabilized by the transfer of a proton from water to pyridine. The energies of the resulting biradical states intersect the potential-energy surface of the closed-shell ground state as a function of the proton-transfer coordinate. The resulting radical pair may dissociate to yield pyridinium and hydroxyl radicals. The photochemistry of the hypervalent pyridinium radical has been explored with the same computational methods. It has been found that a low-lying dissociative (2)πσ* excited state exists in pyridinium, which can lead to the photodetachment of the hydrogen atom. Overall, the H2O molecule can thus be dissociated into H˙ and OH˙ radicals by the absorption of two ultraviolet photons. The relevance of these results for solar water splitting and solar carbon dioxide reduction is discussed.

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.

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.

Panchromatic symmetrical squaraines: a step forward in the molecular engineering of low cost blue-greenish sensitizers for dye-sensitized solar cells.

Two novel symmetrical blue squaraine sensitizers were synthesized, which exhibit panchromatic light harvesting and a record efficiency over 6% with Jsc exceeding 14 mA cm(-2), and Voc over 620 mV under 1 sun. Their color, low cost, easiness of synthesis, and relatively high photo- and thermal stability open up the way for commercial applications.

Quantum dynamics of electron-transfer reactions: photoinduced intermolecular electron transfer in a porphyrin–quinone complex

Photoinduced electron transfer (ET) between a magnesium-porphyrin and benzoquinone in a model molecular complex is investigated employing ab initio multi-configuration electronic-structure calculations combined with quantum dynamical methods. The microscopic parameters controlling the electron-transfer process are obtained using a first-principles diabatization procedure. A model Hamiltonian which includes both linear and quadratic vibronic couplings of all nuclear degrees of freedom of the system is constructed. Quantum dynamical simulations of the ET process are performed employing the multi-layer multi-configuration time-dependent Hartree method. A detailed analysis of the ET dynamics for models of increasing complexity reveals that the dynamics is strongly influenced by resonances associated with vibronically active nuclear modes, leading to significant deviations from the results of classical ET theory. The comparison with results obtained with the simplified spin-boson model reveals the effects related to the Duschinsky rotation of normal modes.