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 $S_2{\mathrm{\to S}}_1$ 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 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.

In this work, we investigate the mode of interaction of a family of fluorescent zinc complexes with HS^– and H₂S. Different experiments, performed by diverse spectroscopic techniques, provide evidence that HS^– binds the zinc center of all the complexes under investigation. Treatment with neutral H₂S exhibits a markedly different reactivity which indicates selectivity for HS^– over H₂S of the systems under investigation. Striking color changes, visible to the naked eye, occur when treating the systems with HS^– or by an H₂S flow. Accordingly, also the fluorescence is modulated by the presence of HS^–, with the possible formation of multiple adducts. The results highlight the potential of the devised systems to be implemented as HS^–/H₂S colorimetric and fluorescent sensors. Bioimaging experiments indicate the potential of using this class of compounds as probes for the detection of H₂S in living cells.

The calculation of radiationless transition rates and of their temperature dependence from first principles is addressed by combining reliable electronic computations of the normal modes of the two electronic states with Kubo's generating function approach for the evaluation of the Franck–Condon weighted density of states. The whole sets of normal modes of the involved cofactors have been employed, taking into account the effects of nuclear equilibrium position displacements, of vibrational frequency changes, and of mixing of the normal modes. Application to the case of the elementary electron transfer step between bacteriopheophytin and ubiquinone cofactors of bacterial photosynthetic reaction centers yields a temperature dependence of the electron transfer rates in very good agreement with the experimental data.