A theoretical investigation of the low lying electronic structure of poly(p-phenylene vinylene)

TL;DR

This paper uses a theoretical model and advanced computational methods to analyze the low-lying electronic states of poly(p-phenylene vinylene), revealing excitonic levels, energies, and their implications for nonlinear optical properties.

Contribution

It applies the density matrix renormalization group method to a two-state model, providing detailed predictions of exciton energies and states in poly(p-phenylene vinylene).

Findings

Identifies Bu and Ag excitonic levels below the band threshold.

Calculates exciton energies and binding energies for finite and infinite chains.

Provides a consistent interpretation of nonlinear optical spectroscopic data.

Abstract

The two-state molecular orbital model of the one-dimensional phenyl-based semiconductors is applied to poly(p-phenylene vinylene). The energies of the low-lying excited states are calculated using the density matrix renormalization group method. Calculations of both the exciton size and the charge gap show that there are both Bu and Ag excitonic levels below the band threshold. The energy of the 1Bu exciton extrapolates to 2.60 eV in the limit of infinite polymers, while the energy of the 2Ag exciton extrapolates to 2.94 eV. The calculated binding energy of the 1Bu exciton is 0.9 eV for a 13 phenylene unit chain and 0.6 eV for an infinite polymer. This is expected to decrease due to solvation effects. The lowest triplet state is calculated to be at ca. 1.6 eV, with the triplet-triplet gap being ca. 1.6 eV. A comparison between theory, and two-photon absorption and electroabsorption is made, leading to a consistent picture of the essential states responsible for most of the third-order nonlinear optical properties. An interpretation of the experimental nonlinear optical spectroscopies suggests an energy difference of ca. 0.4 eV between the vertical energy and ca. 0.8 eV between the relaxed energy, of the 1Bu exciton and the band gap, respectively.