Photoinduced Changes of the Refractive Index in Substituted Fluorenyl-p-phenylene Copolymers

A series of substituted 9,90-dihexylfluorenylene- p-phenylene copolymers (DFPP) were synthesised via a Suzuki coupling reaction. The copolymers were chemically modified to introduce photoreactive functionalities. Thiocyanate groups (SCN) were attached onto a side chain of the DFPP copolymers. After exposure to UV light the SCN groups underwent photoisomerisation to the corresponding isothiocyanate groups (NCS). A significant change of the refractive index was observed after UV illumination. After this process the polymer was further modified by a gas phase reaction with propylamine to transform the photo-generated NCS groups to derivatives of thiourea. An additional variation of the refractive index, as well as an increase of the film thickness h(Dhþ15%), was observed after this reaction. All polymerisation products were characterised by means of gel permeation chromatography, thermoanalysis as well asNMR and IR spectroscopy. The polymers are thermally stable with decomposition temperatures of around 260 8C and glass transition temperatures in the range of 150–155 8C. The polymers display blue photoluminescence, which remains unchanged after UV modification and reaction with propylamine. A convenient way to produce index and relief patterns in films of conjugated polymers is demonstrated.