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Ultrafast dynamics in polycyclic aromatic hydrocarbons:

July 18, 2019

the key case of conical intersections at higher excited states and their role in the photophysics of phenanthrene monomer

The main outcomes of our study are twofold. On the one hand phenanthrenes cannot be described with a simple two-level model, as it would be expected for an apparently simple ππ* conjugated system. On the other hand, we prove the presence of a long-lasting higher-lying excited state, which might, for instance, question the validity of Kasha’s rule for phenanthrene-based supramolecular systems.

The relevance of these results goes well beyond the photophysics of phenanthrene monomers. Indeed phenanthrene is archetypal for the wide class of polycyclic aromatic hydrocarbons and these results could help for a better comprehension of the photochemistry of such an important class of molecules. In the field of supramolecular chemistry and molecular devices, phenanthrene has become a very popular building block for the design of efficient multichromophoric light harvesting systems. In such packed structures with strong inter-chromophoric coupling, the need to take into account also higher exited states with ps lifetimes could have a dramatic impact on understanding the photo-induced functional process and could suggest new concepts to exploit it.

(Above) A low-dimensional representation of the phenanthrene monomer PES as a function of two generalized linking the Franck-Condon region ((ξ,χ) = (0,0)) and the minima of the lower, dark exited state SA ((ξ,χ) = (1,0)) and the higher bright excited state SB (( ξ,χ) = (1,1)). An intersection region can be identified at χ = 0.6. The absorption and emission processes (the dark blue arrow at 320 nm and the light blue one at 420 nm, respectively) are also shown. Two possible relaxation paths are sketched for visual purposes.

(Right) The time-evolution of the population of the two lowest-lying singlet excited states of phenanthrene as obtained from the average of the non adiabatic molecular dynamics trajectories, the dark line is obtained by fitting the population of the SA state to a biexponential model.

Reference: M. Nazari, C. D. Bösch, A. Rondi, A. Francés Monerris, M. Marazzi, E. Lognon, M. Gazzetto, S. M. Langenegger, R. Häner, T. Feurer, A. Monari, A. Cannizzo, “Ultrafast dynamics in polycyclic aromatic hydrocarbons: the key case of conical intersections in higher excited states and their role in the photophysics of phenanthrene monomer”, Phys. Chem. Chem. Phys. 21 (2019) 16981. (DOI: 10.1039/c9cp03147b)
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