  We report on the origin of energy-shifts in organic polariton condensates.
The localised nature of Frenkel excitons in molecular semiconductors precludes
interparticle Coulomb exchange interactions -the latter being the dominant
mechanism for blueshifts in inorganic semiconductor microcavities that bear
Wannier-Mott excitons. We examine the contribution of optically induced change
of the intracavity non-linear refractive index, gain induced frequency-pulling
and quenching of the Rabi splitting, as well as the role of polariton-exciton
and polariton-polariton scattering in the energy-shift of the polariton mode at
condensation threshold in strongly coupled molecular dye microcavities. We
conclude that blueshifts in organic polariton condensates arise from the
interplay of the saturation of molecular optical transitions and intermolecular
energy migration. Our model predicts the commonly observed step-wise increase
of both the emission energy and degree of linear polarisation at polariton
condensation threshold.
