Interchain interaction and Davydov splitting in polythiophene crystals: An ab initio approach

The crystal-induced energy splitting of the lowest excitonic state in polymer crystals, the so-called Davydov splitting Δ, is calculated with a first-principles density-matrix scheme. We show that different crystalline arrangements lead to significant variations in Δ, from below to above the thermal energy kBT at room temperature, with relevant implications on the luminescence efficiency. This is one more piece of evidence supporting the fact that control of interchain interactions and solid-state packing is essential for the design of efficient optical devices.