The search for a spin crossover transition in small sizedπ-conjugated molecules: a Monte Carlo study

Abstract

The spin crossover transition inπ-conjugated polymers is a complex phenomenon involving a balance betweenCoulomb interaction and collective lattice distortions. We explore sucha transition with a minimal electronic model comprising a Hubbard-U on-site repulsive potential and both electron–phonon and hyperfine interactions. The modelis then solved numerically for small molecules at finite temperature by Monte Carlo methods inthe search for the spin crossover. This is done at the mean field level in the Hubbard-U interaction at half filling. We demonstrate that for a certain region of the parameter spacethere is a spin crossover, where the system transits from a low-spin to a high-spin state asthe temperature increases. In close analogy to standard spin crossover in divalent magneticmolecules such a transition is entropy driven, with both the spin and the vibrationalcontributions to the entropy being relevant. Such a transition is practically unaffected bythe hyperfine interaction, which only plays a minor role in determining the electronicproperties.