Abstract
Light-harvesting (LH) molecular antennae are known for their ability to absorb light in the UV-visible domain via their chromophores and to transfer the associated excitation energy down to an energy trap within the system to be used for different purposes (photocatalysis, enhanced light emission…).In particular, the strong LH properties of poly(phenylene ethynylene) (PPE) dendrimers have been attributed to a unidirectional excitation-energy gradient along a molecular tree from the shortest branches (leaves: the chromophores) down to the longest branches (trunk: the energy trap). However, setting up a comprehensive atomistic simulation protocol of excitation-energy transfer (EET) in such π-conjugated macromolecules remains challenging to date, especially because of the large number of degrees freedom and the presence of many conical intersections responsible for nontrivial internal conversion among the manifold of electronic excited states.The architectures of both primary chromophores and further units for EET are all based on the same chemical groups (benzenes and acetylenes), only with different substitution schemes and lengths. In this thesis, we have carried out various theoretical chemistry studies (regarding structure and dynamics) on several PPE building blocks so as to rationalize the nature and efficiency of the ultrafast dynamics EET process via a nonadiabatic chemical perspective.First, we analyzed the properties of the electronic excited states for both isolated PPE-oligomers and the first generations of PPE-dendrimers using linear-response time-dependent density functional theory. After demonstrating the locally-excited character of electronic excited states, we explored their associated potential energy surfaces (PESs) and characterized their critical points (minima, transition states, and minimum-energy conical intersections).Some particular attention was given to the optimization and characterization of the conical intersections, especially as regards the branching-space vectors and their decompositions on the basis of the normal modes of vibration of the studied molecules. Together with these characterizations, we parametrized vibronic-coupling Hamiltonian models consistent with a so-called diabatization by ansatz of the coupled electronic excited states. From them, we could run wavepacket quantum dynamics simulations based on the multi-configuration time-dependent Hartree (MCTDH) formalism so as to simulate EET from first principles.Our simulations confirmed the utmost importance of the acetylenic stretching modes for EET to occur. They also highlighted the relevance of the stretching and rock-bending quinoidal modes of the connecting nodes (twofold or threefold meta-substituted phenylenes) within the dendritic graph. Comparisons between symmetrical and asymmetrical units illustrated the role of the primary structure (symmetrical meta-substitution) of the chromophore for efficient light absorption and/or EET. In particular, we investigated the steady-state spectroscopy of the PPE-chromophore itself, and its unusual emission spectrum observed experimentally. Here, we faced the case of strongly nonadiabatically coupled electronic excited states in the Franck-Condon region, for which the Born-Oppenheimer approximation breaks down. We calculated the vibronic eigenstates in the excited-state manifold, having contributions within two bright electronic excited states, and evaluated the contributions to both absorption and emission spectra. The results of our modelling suggest that the observed emissive contribution is the one that has gone through a transfer from one electronic state to another.Finally, we explored bottom-up approaches for modelling the coupled PESs of larger PPE-dendrimer units and simulating the EET process occuring through them. Our strategy made use of the knowledge of the isolated branches and of how their communication can be viewed as a weak coupling from one branch to another.