Summary: 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 photochemical reactions.

      The objectives of this project are to characterize the excited states of bio-relevant systems and to establish their nonradiative relaxation mechanisms. The originality of this project, which involves gas phase physicists, physical chemists and theoreticians, resides in its synergetic approach with 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 10 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 (CC) or a multiconfigurational (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.

      Scientific and technical program: This interdisciplinary project rests on a synergetic experimental/theoretical approach aiming at investigating the excited states spectroscopy and dynamics of flexible bio-relevant systems. It is organized according to two inter-dependent tasks, an experimental one and a theoretical one, which will progress in parallel.

• Task 1 aims first at providing conformation-selective dynamics information about the excited states of model biomolecules. It is divided into two spectroscopic subtasks and one more centred on the dynamical properties. The first one (1.1) is focussed onto experimental ways to provide evidence about the undetectable short-lived, so-called “missing”, conformers in nanosecond experiments. This task will rely on two spectroscopic strategies: the first one will take advantage of the femtosecond diagnostic, including a photoelectron detection scheme, to provide the IR spectroscopy of short-lived species. The second will make use of the He-droplet spectroscopy, which does not require UV absorption to reveal the conformational populations. The assignment of the long-lived conformers will be carried out during a preliminary Subtask 1.0 and using the standard IR/UV double resonance spectroscopy, in association with quantum chemistry ground state calculations. The second spectroscopic Subtask 1.2 will be devoted to molecular species of interest devoid of UV chromophore: the spectroscopic techniques previously introduced seemed to us quite promising as potentially also applicable to these systems. These Subtasks are strongly interconnected with the theoretical Subtask 2.1. Finally, the last experimental Subtask 1.3 which is strongly interconnected with the theoretical Subtask 2.2 will be devoted to the ultimate goal of the project: to collect conformer-selective data on the excited state dynamics of biomolecular models, in order to directly compare with the theoretical analysis of the PES and in fine determine the critical motions responsible for the relaxation mechanisms.
  
  
• Task 2 aims first at, characterizing theoretically the first excited states of bio-relevant systems and, then, investigating their non-radiative relaxation mechanisms through an efficient modeling of their PES. Two innovative theoretical approaches will be developed, which both feature a combination of two families of quantum chemistry methods differing by their accuracy level. The Subtask 2.1 concerns the determination of the vertical and adiabatic transitions energies using the accurate quantum chemistry CC2 approach. This is crucial for a careful assignment of the experimental UV spectra. In the Subtask 2.2, the electronic dynamics of excited states are investigated by, first, an exploration of the PES using nonadiabatic time dependent density functional theory dynamics simulations, and then pursued by an efficient modeling of these surfaces at the more accurate CC2 level. In these theoretical Subtasks, the results obtained at the CC2 level will be refined by energetic calculations of some significant points of the excited states PES using a multiconfiguration scheme. Finally, the last theoretical Subtask 2.3 will be devoted to the assessment of new functionals in the TDDFT framework, in order to tackle very large systems such as peptides with 3 or more residues, which are out of reach of the ab initio methods used in the previous Subtasks.  This Task is strongly interconnected with the spectroscopy experiments performed in Subtasks 1.1 and 1.2 and with the conformer-selective dynamics experiments at the nanosecond, picosecond and femtosecond time scales developed in the Subtask 1.3.

      Confrontation between measurements on excited states, documenting both their dynamics through their lifetime and their nature through their photoelectron fingerprint, with the theoretical pathways suggested by a sophisticated multilevel quantum chemistry strategy will be crucial for an in-depth understanding of the electronic dynamics of systems of biological importance.