We have investigated the optical properties of a -conjugated poly(arylene-ethynylene) co-polymer, the poly-{[(2,5-dioctyloxy)-1,4-phenylene acetylene]-co-2,6-pyridylene acetylene}, in liquid solutions and blended films with nonconjugated polymers. The red-shifted emission from excimers strongly influences the photoluminescence of solid-state samples but it can be suppressed by using in-vacuum thermal ageing or photoirradiation. Experimental observations suggest this quenching is due to structural reorganization processes that still occur in blends within phase-separated regions of pure co-polymer. Significantly, the suppression of the excimer band does not affect the exciton blue emission in blends, which could help improve color purity for OLEDs and display applications.