A photo-crosslinkable polymer, poly[2,7(9,9-dioctylfluorene)-co-2,7-(9-hexyl-9-(2-acrylate ethyl)-9H-fluorene)] (P3), was synthesized and the photo-crosslinkable acrylate groups were introduced into the side-chains of the polyfluorene derivative after its polymerization. This method avoids the possible crosslinkage of the crosslinkable groups on the monomers during polymerization in the traditional synthesis route by the polymerization of the monomers with the crosslinkable side-chains. The soluble and processable polymer P3 could be crosslinked via the acrylate groups in its side-chains upon exposure to UV light in nitrogen atmosphere. The crosslinking was confirmed by IR spectroscopy: the IR peak of C=C bond at 1635 cm(-1) decreased and that of the vinyl C-H bond at 742 cm(-1) disappeared after the UV exposure. The absorption spectra of P3 remain unchanged after crosslinking, but a longer wavelength emission at 517 nm appeared in the photoluminescent and electroluminescent spectra of the crosslinked P3, Which Could be attributed to the formation of keto defects during the photo crosslinking. (c) 2006 Wiley Periodicals, Inc.