Electronic Excited State Specific IR Spectra for Phenylene Ethynylene Dendrimer Building Blocks

Abstract

Dendrimers are excellent candidates for applications in molecular devices and light harvesting where creating an energy gradient is crucial. Poly(phenylene ethynylene) (PPE) molecules are building blocks for dendrimers that also display the necessary characteristics for efficient energy transfer, including differential spatial localization associated with different excited states. In this work we calculated the ground state (S0) as well as the excited IR spectra for the S1 and S2 states of ortho- and meta- substituted PPE (o-PPE and m-PPE). To compute IR spectra, a conformational space exploration was performed using ground-state classical molecular dynamics followed by direct adiabatic and non-adiabatic excited state molecular dynamics. IR spectra were computed from the autocorrelation function of the dipole moment in each state. We identified a band at 2150 cm–1 that is characteristic of S1 in m-PPE. We show that in m-PPE, S1 and S2 have transition densities localized over different regions of the molecule, while in o-PPE the states are spread over the entire molecule. We find that the coupling between vibrations associated to the C≡C triple bonds plays an important role in the non-adiabatic electronic energy transfer. These results are a guide to the experimental characterization of the specific electronic excited states vibrations of these molecules.