Ab initio optical absorption in conjugated polymers:: the role of dimensionality

The optical behavior of poly(para-phenylene vinylene) is explored through an ab initio scheme based on the Bethe-Salpeter equation, where the electron-hole interaction is included on top of a density functional theory calculation. Results for different solid-state packings are reviewed, demonstrating that the details of crystalline arrangement dramatically alter the optical properties and lead to a rich excitonic structure, where also charge-transfer states appear (electron and hole on different chains). Moreover, for a typical herringbone packing the excitonic state of the isolated molecule splits in two direct components (with electron and hole on the same chain), one for each non-translationally-invariant chain in the unit cell, and the optical inactivity of the lowest component can crucially quench the luminescence efficiency. Besides the far-field absorption spectra and the description of the excitonic states, a formalism to simulate the near-field spectra is presented that allows one to detect also excitonic states that are dipole-forbidden in the far-field spectra.