Vibronic properties of para-polyphenylene ethynylenes: TD-DFT insights

The first singlet excited states of a series of para-polyphenylene ethynylenes (PPEs) are investigated using time-dependent density functional theory (TD-DFT). Vibronic absorption spectra are calculated and show excellent agreement with the experiments, thus validating the adequacy of TD-DFT for such systems. The vibronic structure is assigned to the excitation of a few typical stretching and bending modes. The significant discrepancy between the simulated vertical-transition energies and the experimental absorption maxima in PPEs is underlined and explained. The evolution of the spectroscopic properties and of the electronic structure with the chain length is discussed and characterized.