Degeneracy, orientational disorder and chromophore size effects on Frenkel excitons in columnar mesophases

The properties of Frenkel excitons in columnar mesophases are analysed using numerical calculations with a particular emphasis on the off-diagonal disorder induced by rotation of the molecular disks around the column axis. The influence of degeneracy of the molecular electronic transitions and the effect of the chromophore size are illustrated by means of the extended dipole model. The localisation/delocalisation behaviour of the exciton states is characterised by calculation of the participation ratio. It is shown that the existence of two orthogonal dipolar transitions per chromophore makes the optical spectra less sensitive to chromophore rotation and maintains the eigenstates bearing the oscillator strength quite delocalised, even in the presence of complete orientational disorder.