Unidirectional Energy Transfer in Conjugated Molecules: The Crucial Role of High-Frequency CtC Bonds

Excited-state nonadiabatic molecular dynamics is used to study energy transfer in dendrimer building blocks, between two-, three-, and four-ring linear polyphenylene ethynylene units linked by meta-substitutions. Upon excitation, dendrimers with these building blocks have been shown to undergo highly efficient and unidirectional energy transfer. The simulations start by initial vertical excitation to the S4, localized on the two-ring unit. We observe ultrafast directional S4 f S3 f S2 f S1 electronic energy transfer, corresponding to sequential two-ring f three-ring f four-ring transfer. The electronic energy transfer is concomitant with vibrational energy transfer through a dominant CtC stretching motion. Upon Snþ1f Snpopulation transfer, a rapid increase of the Snþ1-Sn energy gaps and decrease of the corresponding values for Sn-Sn-1 gaps are observed. As a consequence, the Snþ1 and Sn states become less coupled, while the Sn and Sn-1 become more coupled. This behavior guarantees the successful Snþ1 f Sn f Sn-1 unidirectional energy transfer associated with the efficient energy funneling in light-harvesting dendrimers. SECTION Electron Transport, Optical and Electronic Devices, Hard Matter T he development of newmaterials with applications to light-harvesting and transport for solar cells represent a major task to address the global challenge of renewable energy resources. Advances in organic synthesis can yield macromolecules with well-defined structures, and it has become possible to synthesize artificial light-harvesting dendritic macromolecules with built-in energy gradients. Such dendrimers are branched conjugated macromolecules with regular structures, allowing very efficient energy funneling through the molecular system. Dentritic macromolecules are arrays of coupled chromophores, with the energy of each unit depending on backbone structure and conformation (affected by nuclear dynamics). The relative strengths of the couplings between units control the exciton transport, which competes with deactivation. Pathways of intramolecular vibrational energy flow can be associated with the ultrafast electronic energy-transfer process. Previous results from a wide variety of symmetric and unsymmetric dendrimers has provided a qualitative picture of energy gradients, including their relation to dendrimer size, architecture, and energy transfer. Recent experiments reported by Kleiman et al. have shown that the coherent control of excited-state dynamics in dendritic macromolecules is possible. From a theoretical point of view, the complexity of the dendritic architectures makes it difficult to explore the interplay between nuclear motion and electronic couplings that guarantees the efficient energy funneling. Simple approaches such as the F€ orster framework for modeling energy-transfer rates frequently fail to reproduce experimental observables in such complexmacromolecules. Less complex systems, composed of model building blocks, are useful to investigate the process. In this paper, we study the ultrafast electronic and vibrational energy transfer in a building block of a well-known dendrimer (the nanostar). The energy transfer occurs between two-, three-, and four-ring linear polyphenylene ethynylene (PPE) units linked through meta-substitutions, as shown in Figure 1(inset). The meta-branching localizes excitons within each linear fragment, with relatively small leakage into the next section of the molecule. We use this system as a model to understand the successful directional energy transfer that takes place in more complex phenylethynylperylene-terminated dendrimers. In those systems, the light-harvesting action takes place by a highly Received Date: June 9, 2010 Accepted Date: August 27, 2010