Photoinduced non-adiabatic energy transfer pathways in dendrimer building blocks.

V M Freixas,D V Shalashilin,D V Makhov,S Tretiak,S Fernandez-Alberti,D Ondarse-Alvarez

doi:10.1063/1.5086680

Abstract

The efficiency of the intramolecular energy transfer in light harvesting dendrimers is determined by their well-defined architecture with high degree of order. After photoexcitation, through-space and through-bond energy transfer mechanisms can take place, involving vectorial exciton migration among different chromophores within dendrimer highly branched structures. Their inherent intramolecular energy gradient depends on how the multiple chromophoric units have been assembled, subject to their inter-connects, spatial distances, and orientations. Herein, we compare the photoinduced nonadiabatic molecular dynamics simulations performed on a set of different combinations of a chain of linked dendrimer building blocks composed of two-, three-, and four-ring linear polyphenylene chromophoric units. The calculations are performed with the recently developed ab initio multiple cloning-time dependent diabatic basis implementation of the Multiconfigurational Ehrenfest (MCE) approach. Despite differences in short time relaxation pathways and different initial exciton localization, at longer time scales, electronic relaxation rates and exciton final redistributions are very similar for all combinations. Unlike the systems composed of two building blocks, considered previously, for the larger 3 block systems here we observe that bifurcation of the wave function accounted by cloning is important. In all the systems considered in this work, at the time scale of few hundreds of femtoseconds, cloning enhances the electronic energy relaxation by ∼13% compared to that of the MCE method without cloning. Thus, accurate description of quantum effects is essential for understanding of the energy exchange in dendrimers both at short and long time scales.

Highlights

An efficient conversion of solar radiation into other usable forms of energy promises an unlimited clean energy source
We compare the photoinduced nonadiabatic molecular dynamics simulations performed on a set of different combinations of a chain of linked dendrimer building blocks composed of two, three, and four-ring linear polyphenylene chromophoric units
We compare photoinduced nonadiabatic molecular dynamics simulations performed on a set of different combinations of dendrimer building blocks composed of two, three, and four-ring linear poly(phenylene ethynylene) (PPE) chromophoric units linked by meta-substitutions, which we denote as 234PPE, 243PPE, and 324PPE

Summary

Introduction

An efficient conversion of solar radiation into other usable forms of energy promises an unlimited clean energy source Such conversion is achieved in living organisms by means of complex arrays of conjugated chromophores.[1,2] Synthesis of artificial organic materials[3,4,5] has shown a glimpse of the opportunity to mimic nature light-harvesting capabilities and high efficiency energy funnelling. Dendrimers are conjugated macromolecules with highly branched structures that can perform such a function. Those based on poly(phenylene ethynylene) (PPE) have both the collection and transport features present in photosynthetic systems.[6,7,8,9,10] Linear PPE chromophore units with different conjugation lengths, linked by meta-substitution at the branching phenylene nodes, constitute the building blocks of the well-studied perylene-terminated dendrimer known as the nanostar.[11,12,13] These meta-branching vertices localize excitons within each linear PPE unit, allowing the study of PPE dendrimers as an ensemble of independent chromophore units with a weak coupling between them.[8,11,12]

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Results

Conclusion