Analysis of state-specific vibrations coupled to the unidirectional energy transfer in conjugated dendrimers

The nonadiabatic excited-state molecular dynamics (NA-ESMD) method and excited-state instantaneous normal modes (ES-INMs) analyses have been applied to describe the state-specific vibrations that participate in the unidirectional energy transfer between the coupled chromophores in a branched dendrimeric molecule. Our molecule is composed of two-, three-, and four-ring linear poly(phenyleneethynylene) (PPE) units linked through meta-substitutions. After an initial laser excitation, an ultrafast sequential S3 → S 2 → S1 electronic energy transfer from the shortest to longest segment takes place. During each Sn → Sn-1 (n = 3, 2) transition, ES-INM(Sn) and ES-INM(Sn-1) analyses have been performed on Sn and Sn-1 states, respectively. Our results reveal a unique vibrational mode localized on the Sn state that significantly matches with the corresponding nonadiabatic coupling vector dn,(n-1). This mode also corresponds to the highest frequency ES-INM(Sn) and it is seen mainly during the electronic transitions. Furthermore, its absence as a unique ES-INM(S n-1) reveals that state-specific vibrations play the main role in the efficiency of the unidirectional Sn → Sn-1 electronic and vibrational energy funneling in light-harvesting dendrimers. © 2012 American Chemical Society.