Propagation of a four wave mixing through a molecular system interacting with thermal reservoir was carried out. From that, analytical expressions to the absorption coefficient at different frequencies were obtained. Surfaces to that property and to the refractive index were exhibited. Saturation effect due the field can be viewed as well as the attenuation of the optical properties when the molecule-solvent coupling increases. That permit us to define an effective relaxation time which includes a parameter that measures the molecule-solvent interaction.