Photophysical and Electroluminescent Properties of Hyperbranched Polyfluorenes

Abstract

Properties of a series of hyperbranched polyfluorenes were investigated. Increasing the benzene ''crosspoint'' density blue-shifts the absorption and PL since the crosspoints interrupt p-electron conjugation, decreasing conjugation length, and thus increasing chromophore band gap. The relative intensity of S 10 ! S 00 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 holeelectron 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 p-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.Synthesis of the hyperbranched polyfluorenes.

Photoluminescence of poly(thiophene) nanowires confined in porous anodic alumina membrane

“…Generally, Fö rster energy transfer could take place from PAA to embedded nanomaterials and induce an energy shift of optical transition [33]. In order to realize an efficient Fö rster energy transfer, two requirements have to be fulfilled [34]: (a) sufficient spectral overlap between the emission of the wider band-gap substance and the absorption of the narrower band-gap substance, and (b) a Fö rster interaction range from several to tens of nanometers, where the first one is the essential condition. In this study, the PL spectra of PAAs almost completely overlap with the UV-vis absorption spectrum of the PMP-Th thin film on ITO as shown in Supplementary data (Fig.…”

Photocrosslinkable hyperbranched polyfluorenes containing oxadiazole: synthesis, photophysics and electroluminescence

“…9,9‐Dioctylfluorene‐2,7‐bis‐(trimethylene boronate), 2,7‐dibromo‐9,9‐di‐(6‐(2‐(3‐oxetanyl)‐butoxyl)‐ hexyl)fluorene and 2‐(4‐bromophenyl)‐5‐(3,5‐dibro‐ mophenyl)‐1,3,4‐oxadiazole were synthesized according to the literature 22–25…”

“…The keto defect13–17 and excimers18, 19 have been used for green emission. As highly branched and globular molecular features could improve the light‐emitting efficiency and also make the materials amenable to the formation of good‐quality amorphous films,20, 21 there have been some reports of hyperbranched polyfluorenes with improved luminescent property stability 22–25…”

Facile synthesis of hyperbranched poly(p‐phenyleneethynylene‐alt‐m‐phenyleneethynylene) with narrow dispersity and high branching degree by sonogashira polymerization of an AB2 monomer

“…Moreover, the hyperbranched structure can enhance exciton confinement, suppress intrachain and interchain exciton annihilation, thereby, improve fluorescence quantum yields and electroluminescent efficiency significantly. 1 The global molecular shape can also induce efficient intramolecular energy transfer. [2][3][4] Hyperbranched conjugated polymers have attracted considerable attention as novel materials in the fields of light emitting, [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] solar cells, [24][25][26] fluorescent chemosensors, [27][28][29][30][31][32] and so on.…”

“…Dried under vacuum at room temperature, 1.08 g polymer was obtained. 1 3 , and 42 mg CuI were added to a two-necked flask. The flask was vacuumed and refilled with N 2 .…”