Photophysical and Electroluminescent Properties of Hyperbranched Polyfluorenes

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Abstract

Summary: Properties of a series of hyperbranched polyfluorenes were investigated. Increasing the benzene “crosspoint” density blue-shifts the absorption and PL since the crosspoints interrupt π-electron conjugation, decreasing conjugation length, and thus increasing chromophore band gap. The relative intensity of S10 → S00 transition in PL increases with crosspoint density increase, indicating that the energy and the energy distribution of vibronic transitions were controlled by hyperbranch structure. When the polymer solution is cooled down, the lattice vibration is reduced and the effective conjugation length (coherence length for electron wavefunction) is increased, thus causing bathochromic shift in absorption and PL. These polymers do not favor excimer formation in solution due to their remarkable rigidity and extra-large molecular size. With increase in crosspoint density, LED color changed from green to violet. In double-layer LEDs, PPV not only improves hole injection/transport but also contributes to device emission through its bulk emission and interfacial emission as a result of interfacial energy transfer. Higher electrical field favors interfacial energy transfer probably by facilitating hole–electron recombination and by encouraging the excitons formed in polyfluorene layer to migrate to the interface and to further diffuse into PPV layer. The polymer with more crosspoints shows higher EL efficiency. The crosspoints interrupt π-electron conjugation, isolate excitons and inhibit intrachain exciton annihilation. They can also increase rigidity of the macromolecular backbone, and the large spatial hindrance of these macromolecules makes the molecular packing very difficult, thus decreasing interchain exciton annihilation. All these structural features help to increase exciton lifetime and improve LED performance.

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Synthesis of the hyperbranched polyfluorenes.

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