Indeno[2,1-c]fluorene-based blue fluorescent oligomers and polymers: Synthesis, structure, photophysical and electroluminescence properties

An alkylated skeleton of indeno[2,1-c]fluorene, viewed as a positional isomerism of indeno[1,2-b]fluorene, was developed to construct several conjugated oligomers and polymers through a Pd-catalyzed Suzuki coupling reaction for organic light-emitting diodes. Single crystal analysis indicated that the structure of indeno[2,1-c]fluorene was twisted due to close H---H contacts in the crowded region of molecules. P1-P3 showed good solubility in common organic solvents as well as facile film forming properties. In comparison with linear n-alkyl-substituted oligo(indeno[1,2-b]fluorene)s and poly(indeno[1,2-b]fluorene)s (2,8-PIFs), the absorption and emission features of twisted n-alkyl-substituted oligo(indeno[2,1-c]fluorene)s and poly(indeno[2,1-c]fluorene) (P1) in dilute solution exhibited a correlation to the conjugation length and obviously blue-shifted due to the distorted backbones. All polymers emitted strong blue fluorescence with very narrow full widths at half-maximum (fwhm) (about 50 nm) in dilute solution and in thin film. Electroluminescent (EL) devices with the configuration of ITO/PEDOT:PSS/PVK/polymers/Ba/Al were fabricated to achieve blue EL emission with maximum brightness up to 2800 cd/m2 at a bias of 10 V. The EL devices using P2 as the active materials showed a maximum external quantum efficiency of 1.2% and a maximum luminous efficiency of 1.5 cd/A. Our investigations demonstrate that indeno[2,1-c]fluorene moiety might be regarded as a potential building skeleton to develop the effective blue and UV light-emitting materials.