We present the relationship between the spatial arrangement and the photophys. properties of fluorescent polymers in thin films with controlled structures. Eight surfactant poly(p-phenyleneethynylene)s were designed and studied. These detailed studies of the behavior of the polymers at the air-water interface, and of the photophys. properties of their transferred LB films, revealed key structure-property relationships. Some of the polymers displayed $π$-aggregates that are characteristic of an edge-on structure at the air-water interface. Monolayer LB films of these polymers showed greatly reduced quantum yields relative to soln. values. Other polymers exhibited a highly emissive face-on structure at the air-water interface, and did not form $π$-aggregates. The combination of pressure-area isotherms and the surface pressure dependent in situ UV-vis spectra of the polymers at the air-water interface revealed different behavioral details. In addn., the UV-vis spectra, fluorescence spectra, and quantum yields of the LB films provide design principles for making highly emissive films. [on SciFinder(R)]