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

Abstract

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 → S3 → S2 → S1 electronic energy transfer, corresponding to sequential two-ring → three-ring → four-ring transfer. The electronic energy transfer is concomitant with vibrational energy transfer through a dominant C≡C stretching motion. Upon Sn+1 → Sn population transfer, a rapid increase of the Sn+1−Sn energy gaps and decrease of the corresponding values for Sn−Sn−1 gaps are observed. As a consequence, the Sn+1 and Sn states become less coupled, while the Sn and Sn−1 become more coupled. This behavior guarantees the successful Sn+1 → ...