Benchmark ab initio calculations on intermolecular structures and the exciton character of poly(p -phenylenevinylene) dimers

Benchmark ab initio calculations have been performed for poly(p-phenylenevinylene) (PPV) dimers, a paradigmatic material for studying excitation energy transfer mechanisms. Second-order Møller-Plesset perturbation theory was utilized with the scaled opposite spin approach (SOS-MP2) and correlation consistent basis sets to determine the geometric properties and interaction energies in the ground state. Vertical excitations and optimized structures for the S₁ state were computed using the SOS second-order algebraic diagrammatic construction method. For the ground state properties, extrapolation to the complete basis set (CBS) limit and correction for the basis set superposition error (BSSE) were performed. While all results computed with different basis sets and considering BSSE correction or not agreed at the CBS limit, a strong bias was observed either using augmented basis sets or BSSE corrections, proving that these approaches are not advisable for calculating intermolecular distances and interaction energies with smaller basis sets. The lower states for vertical excitations were largely local excitons where the hole/electron pair was confined to single chains. For higher excited states, interchain charge transfer (CT) states were also observed. Geometry optimization of the S₁ state led to significant reductions in the intermolecular distances and energetic stabilization, with Stokes shifts between 1.4 eV and 0.9 eV (with increasing chain length), and significant CT values between 0.5e and 0.4e.