The development of new materials with applications to light-harvesting and transport for solar cells represent a major task to address the global challenge of renewable energy resources. Light-harvesting relies on the efficient absorption of photons followed by spatial transfer of the energy to where it can lead to transduction into other forms of energy. The spatial localization of the energy delimits its efficacy and it has been a point of intense research for synthetic light-harvesters. In particular, phenylene-ethynylene dendrimers combine π-electrons with unique architectures and serve as models for other organic light-harvesters. Understanding the complex electronic dynamics is a critical step for a broad range of technological applications. Their highly polarizable and π-electron manifold is responsible for many of their unique electronic and photophysical properties. Developing a clear picture of their photoexcitation dynamics is essential to understand exciton formation, evolution, and decay via non-adiabatic dynamics. Herein, we present a comparison of branched conjugated molecules with and without an energy gradient leading to different mechanisms for the redistribution of energy. The results of a combined theoretical-experimental study elucidating ultrafast electronic energy relaxation and redistribution in dendrimers. For this purpose, time-resolved spectroscopy and non-adiabatic excited-state molecular dynamics have been performed. The excellent agreement between theory and experiments validates our approach and reveals a consistent picture of ultrafast localization due to rapid spatial confinement within an individual branch followed by hopping between localized transition densities. For molecules without an energy gradient, the final ensemble describes the random distribution of self-trapped excitons on the different equivalent units with each branch retaining a similar probability of significant contribution of the transition density. In the presence of an energy gradient, the localized exciton undergoes incoherent energy transfer into a final sink. In both cases, the localization of the transition density precludes any energy transfer process.

Electronic energy redistribution in conjugated molecules and the effect of an energy gradient

253rd National Meeting (2017)

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