Poly{[9-(4-hexyloxyphenyl)-9-methoxyfluorene-2,7-diyl]alt-co-[1,4-(2,5-bisdodecyloxy)phenylenevinylene]} [P(PV-FOMe)] was synthesized and its optical property changes upon exposure to various Lewis acids were studied. The fluorescence emission of the polymer solution was completely quenched by Lewis acid such as BF3OEt2, AlCl3, SiCl4, TiCl4, and SnCl4. The fluorescence emission was recovered when methanol was added. The PL on/off switching behavior could be repeatedly observed by alternatively adding Lewis acid and methanol in sequence. The fluorescence quenching behavior followed the Stern-Volmer relationship with a K-sv value of 1.2 x 10(4) M-1 for a quencher BF3OEt2. From the H-1 NMR study on a model compound, we found that the PL on/off switching behavior of P(PV-FOMe) is related to the reversible bonding and cleavage of the methoxy group to/from the fluorene unit. While the morphology of a spin-coated P(PV-FOMe)-only film has some defects, defect-free coating was obtained by blending the polymer with host PVK at a level of the P(PV-FOMe) composition below 40 wt%. Among the LEDs composed of P(PV-FOMe):PVK emissive layer, the EL performance was optimal for a blend where P(PV-FOMe)-composition was 10 wt%. The maximum power efficiency and brightness were 1.33 cd/A and 1200 cd/m(2) in the device configuration of ITO/[P(PV-FOMe):PVK = 10/90]/Alq(3)/Al. (c) 2006 Elsevier B.V. All rights reserved.