• quantum dynamics
• DNA
• computational chemistry
• quadratic vibronic coupling
• nonadiabatic couplings
• PES

This work illustrates the theoretical modeling and the results of quantum and mixed quantum-classical dynamical simulations of the ππ* -> nπ* internal conversion in the thymine molecule. The importance of this process in the fields of photochemistry and photobiology is related to the photostability of nucleic acids. In fact, nucleobases strongly absorb UV light, undergoing electronic transitions to excited reactive ππ* states, leading to potentially mutagenic effects. However, time-resolved experiments show that the ππ* state is depopulated in an ultrafast (< 500 fs) time scale, and the relevance, in pyrimidine nucleobases, of its deactivation, via an internal conversion to a nπ* state, closely lying in energy, is strongly debated in literature.

For the purposes of this work, a novel methodology for the quantum dynamical study of semirigid photoexcited nonadiabatic (i. e. with electronic states close in energy) chromophores is illustrated. As an original result, it is proven that, if the excited state Potential Energy Surfaces (PES) are described within the harmonic approximation, it is possible to find effective coordinates partitioned into blocks, defining a hierarchical sequence of Hamiltonians to compute quantum dynamics, where the members of the hierarchy depend on a growing number of coordinates. A procedure is presented to build up this sequence of Hamiltonians, that has the property that the larger the number of effective coordinates, the longer the time scale to which the dynamics is reproduced exactly.

This is a new approach, generally applicable to the study of the excited state dynamics of semirigid molecular systems, and improves a pre-existing hierarchical model for harmonic PESs, valid only in the limit where the excited state has the same frequencies and normal modes of the ground state (in practice, quite 'rigid' assumptions, even according to the chemical intuition).

The procedure is applied here to the simulation of the ππ*->nπ* excited state transfer in thymine, exploiting different levels of accuracy for constructing the quadratic PESs of the excited states involved in the dynamics, and discussing the differences between such different approaches. The results show that the transfer is effective (80%) and occurs in a time scale of <100 fs; therefore the nπ* state is involved in the dynamics from the very beginning. The nonadiabatic absorption spectra are also computed and compared with experimental measurements.

Moreover, the convergence of the predicted population transfer and the absorption spectra, with respect to the number of coordinates included is tested and proved, so that the theoretical methodology developed reveals to be very satisfactory.