Amalia Velardo

Absorption Band Shapes of a Push–Pull Dye Approaching the Cyanine Limit: A Challenging Case for First Principle Calculations

The absorption band shapes of a solvent tunable donor-acceptor dye have been theoretically investigated by using Kubos generating function approach, with minimum energy geometries and normal coordinates computed at the DFT level of theory. The adopted computational procedure allows us to include in the computation of Franck-Condon factors the whole set of normal modes, without any limitation on excitation quanta, allowing for an almost quantitative reproduction of the absorption band shape when the equilibrium geometries of the ground and the excited states are well predicted by electronic computations. Noteworthy, the functionals that yield more accurate band shapes also provide good prediction of the moment variations upon excitation; because the latter quantities are rarely available, theoretical simulation of band shapes could be a powerful tool for choosing the most appropriate computational method for predictive purposes.

Single-Stranded DNA Oligonucleotides Retain Rise Coordinates Characteristic of Double Helices

The structures of single-stranded DNA oligonucleotides from dimeric to hexameric sequences have been thoroughly investigated. Computations performed at the density functional level of theory including dispersion forces and solvation show that single-stranded helices adopt conformations very close to crystallographic B-DNA, with rise coordinates amounting up to 3.3 Å. Previous results, suggesting that single strands should be shorter than double helices, largely originated from the incompleteness of the adopted basis set. Although sensible deviations with respect to standard B-DNA are predicted, computations indicate that sequences rich in stacked adenines are the most ordered ones, favoring the B-DNA pattern and inducing regular arrangements also on flanking nucleobases. Several structural properties of double helices rich in adenine are indeed already reflected by the corresponding single strands.

Hole Hopping Rates in Organic Semiconductors: A Second-Order Cumulant Approach

Second-order cumulant expansion of the time dependent reduced density matrix has been employed to evaluate hole hopping rates in pentacene, tetracene, picene, and rubrene homodimers. The cumulant expansion is a full quantum mechanical approach, which enables the use of the whole set of nuclear coordinates in computations and the inclusion of both the effects of the equilibrium position displacements and of normal mode mixing upon hole transfer. The time dependent populations predicted by cumulant approach are in good agreement with those obtained by numerical solution of time dependent Schrödinger equation, even for ultrafast processes, where the Fermi Golden Rule fails.