Unidirectional Energy Transfer in Conjugated Molecules: The Crucial Role of High-Frequency C≡C 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 S 4, localized on the two-ring unit. We observe ultrafast directional S 4 → S 3 → S 2 → S 1 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 S n +1 → S n population transfer, a rapid increase of the S n +1 −S n energy gaps and decrease of the corresponding values for S n −S n −1 gaps are observed. As a consequence, the S n +1 and S n states become less coupled, while the S n and S n −1 become more coupled. This behavior guarantees the successful S n +1 → S n → S n −1 unidirectional energy transfer associated with the efficient energy funneling in light-harvesting dendrimers.