The optical absorption, Raman scattering and photoluminescence of two phases of tetrahexyl-sexithiophene (4HT6) display properties coherently related to the different molecular conformations imposed by the chain packing. We analyse the temperature dependence of the optical properties of a sample in which the two phases coexist. In the photoluminescence the lower energy emission from the phase possessing planar molecules is thermally activated at room temperature, while the emission from the phase possessing less planar molecules becomes dominant at low temperature due to effective excitation confinement. The time resolved photoluminescence study indicates that exciton migration processes within the inhomogeneous density of states of the two phases sample introduce relevant non-radiative quenching effects.