Résumé
Excitation-energy transfer in multichromophoric assemblies is often pictured in terms of excitonic models whereby the underlying diabatic states correlate with the ground (acceptor) and excited (donnor) electronic states of well-defined separated fragments. Poly(phenylene ethynylene) dendrimers also exhibit ultrafast unidirectional excitation-energy transfer; however, the definition of the fragments is no longer straightforward, as adjacent chromophores share a common meta-substituted phenylene ring, which cannot be viewed as a spectator bridge from an electronic perspective. Here, we show how a pseudofragmentation scheme can be used to define the relevant diabatic representation, provided the interacting sites are based on orbital subsets rather than atomic clusters. This is illustrated with the smallest meta-substituted oligomer, for which we characterised a conical intersection responsible of nonadiabatic internal conversion between the first two excited electronic states, consistent with a diabatic picture based on para-conjugated pseudofragments sharing a ring.