ESBODYR

Many complex molecular systems absorb light in the UV spectral range, including those of paramount biological importance, like DNA bases or proteins. The excited states created by UV absorption are endowed with mechanisms of deactivation which are of major importance for the photochemical stability of these species. These often ultrafast processes indeed provide a rapid and efficient way to dissipate the electronic energy into vibration, thus avoiding structural damages which could affect their biological function. The objectives of this project are to characterize the excited states of bio-relevant systems and to establish their nonradiative relaxation mechanisms in order to document the basic physical phenomena controlling the lifetime of excited states. The originality of this project, which involves gas phase physicists, physical chemists and theoreticians, resides in its synergetic approach with a strong theory/experiment interplay. Focused on a series of flexible molecules of biological interest which exhibit various conformations, this project will highlight the link between electronic dynamics and structure. - From the experimental side, the IR/UV double resonance spectroscopy carried out with nanosecond lasers is routinely used to obtain crossed-checked information about structure (namely, the H-bonding pattern) and dynamics. However, it becomes useless in the case of conformers having ultrashort-lived excited states (lifetimes typically lower than 1 ps). Such undetectable short-lived conformers are “missing” in the nanosecond experiments. This case is unfortunately relatively frequent among these flexible molecules of biological interest, making difficult the comparison between experiment and modeling. The first experimental objective consists in deploying novel experimental procedures in order to detect and identify these species from their IR spectroscopy, in particular by making an extensive use of the conformer-sensitive photoelectron technique. Then, the ultimate goal is to record a conformer-selective dynamics. Besides, IR spectroscopy in helium droplets will also be developed during the project and applied to a series of flexible molecules, with the unique advantage of detecting the whole conformational population. - On the theoretical side, the challenges are two-fold: i) identify, in these complex molecules, the critical motions that cause the electronic relaxation and ii) describe simultaneously in a balanced and accurate way several electronic states of different nature. Therefore, this project proposes an innovative two-step approach. First, non-adiabatic dynamics simulations based on time-dependent density functional theory (NA-TDDFT) will provide hints about the pathways driving the deactivation. These pathways will then be reinvestigated, at a more accurate level of theory, using two families of methods, either with a single coupled cluster (CC) or a multireference wavefunction (MRCI) scheme. The conformer-selective experimental findings will be directly compared with the theoretical investigations, enabling an efficient feedback between the two approaches. The outcomes will be: i) the assignment of the photophysical processes in conceptually important molecules, ii) a validation of the excited states computations by comparison with both experimental data and theoretical results obtained with a very highly sophisticated quantum chemistry method belonging to the family of the multireference wavefunction methods. This interdisciplinary project should promote advances in the understanding of the excited state dynamics after light absorption, as well as technical developments in modeling the fate of the energy absorbed. These processes of energy conversion are of paramount importance with potential applications in many fields as diverse as photochemistry, biology or material sciences.