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Light-harvesting and intramolecular energy funneling are fundamental processes in natural photosynthesis. A comprehensive knowledge of the main structural, dynamic, and optical properties that regulate the efficiency of such processes can be deciphered through the study of artificial light-harvesting antennas, capable of mimicking natural systems. Dendrimers are some of the most explored artificial light-harvesting molecules. However, they have to be well-defined and highly branched conjugated structures, creating intramolecular energy gradients that guarantee efficient and unidirectional energy transfer. Herein, we explore the contributions of the different mechanisms responsible for the highly efficient energy funneling in a large, complex poly(phenylene–ethynylene) dendrimer, whose architecture was particularly designed to conduct the initially absorbed photons toward a spatially localized energy sink away from its surface, avoiding its quenching by the environment. For this purpose, the nonradiative photoinduced energy relaxation and redistribution are simulated by using nonadiabatic excited state molecular dynamics. In this way, the two possible direct and indirect pathways for exciton migrations, previously reported by time-resolved spectroscopy, are defined. Our results stimulate future developments of new synthetic dendrimers for applications in molecular-based photonic devices in which an enhancement in the photoemission efficiency can be predicted by changes in the detailed balance between the different intramolecular energy transfer pathways. 

Photoexcitation of multichromophoric light harvesting molecules induces a number of intramolecular electronic energy relaxation and redistribution pathways that can ultimately lead to ultrafast exciton self-trapping on a single chromophore unit. We investigate the photoinduced processes that take place on a phenylene-ethynylene dendrimer, consisting of nine equivalent linear chromophore units or branches. meta -Substituted links between branches break the conjugation giving rise to weak couplings between them and to localized excitations. Our nonadiabatic excited-state molecular dynamics simulations reveal that the ultrafast internal conversion process to the lowest excited state is accompanied by an inner → outer inter-branch migration of the exciton due to the entropic bias associated with energetically equivalent conjugated segments. The electronic energy redistribution among chromophore units occurs through several possible pathways in which through-bond transport and through-space exciton hopping mechanisms can be distinguished. Besides, triple bond excitations coincide with the localization of the electronic transition densities, suggesting that the intramolecular energy redistribution is a concerted electronic and vibrational energy transfer process.