Photophysical properties of fluorene-based copolymers synthesized by connecting twisted biphenyl units with fluorene via para- and meta-linkages

BACKGROUND: Wide bandgap semiconducting polymers are of great interest in the development of organic and polymeric emissive materials for display purposes since they can be used to generate light of all colors either by irradiation of luminescent dyes or by energy transfer to emissive dopants. The aim of the present work is to construct new fluorene-based semiconducting polymers with a wide bandgap. RESULTS: A novel polyfluorene derivative, poly[(9,9-dihexyl-2,7-fluorene)-alt-(5,7-dihydrodibenz[c,e]oxepin)], with a wide bandgap, was synthesized by connecting rigidly twisted biphenyl monomers with dihexylfluorene via para-linkages and it was compared with poly[(9,9-dihexyl-2,7-fluorene)-alt-(spirocyclohexane-1,6'dibenzo[d,f][1,3]dioxepin)], which has meta-linkages. Both polymers emit in the ultraviolet and blue regions. Electronic spectral absorption data and electrochemical measurements demonstrate that ca 40 degrees torsion angle of the biphenyl units induces an increase in the HOMO-LUMO gap of 0.18 eV, and that meta-linkage of the twisted segment in the polymer induces another increase of 0.24 eV compared to polydihexylfluorene. CONCLUSION: The new twisted biphenyl compounds are efficient segments to tune the bandgaps of conjugated polymers. The two fluorene-based copolymers have wide bandgaps and exhibit potential as host materials. (C) 2008 Society of Chemical Industry.