A series of four new statistical copolymers of 9,9-dihexylfluorene and 9-fluorenone with well-defined structures and a new fluorene−fluorenone−fluorene trimer model compound were synthesized and used to investigate the photophysics, origin of the green emission, and electroluminescence of this class of light-emitting materials. We show that the new oligofluorene trimer with a central fluorenone moiety is an excellent model of the green-emitting chromophore in polyfluorenes. From systematic studies of the steady-state photoluminescence (PL) and PL decay dynamics of solutions of the fluorenone-containing copolymers and oligomer and thin films of the copolymers, we show that the controversial 535-nm green emission band originates from the fluorenone defects in single-chain polyfluorenes and not from intermolecular aggregates or excimers. The green emission, centered at 535 nm, was observed in dilute toluene solutions of all fluorenone-containing copolymers and oligomer; it was long-lived with a single-expone...

Fluorenone-Containing Polyfluorenes and Oligofluorenes: Photophysics, Origin of the Green Emission and Efficient Green Electroluminescence†

Oligofluorenes (a trimer, pentamer, and heptamer) with one fluorenone unit in the center (OFnK: n=3, 5, or 7) were synthesized and used as models to understand the origin of the low-energy emission band in the photoluminescence and electroluminescence spectra of some polyfluorenes. All compounds form glasses with T(g) at 30 degrees C (OF3 K), 50 degrees C (OF5 K) and 57 degrees C (OF7 K). Oligomers OF5 K and OF7 K exhibit smectic liquid crystal phases that undergo transition to isotropic melts at 107 and 205 degrees C, respectively. Oligomer OF5 K could be obtained in form of single crystals. The X-ray structure analysis revealed the helical nature of the molecule and a helix reversal defect located at the central fluorenone unit. The packing pattern precludes formation of excimers. Electrochemical properties were investigated by cyclic voltammetry. The ionization potential (I(p)) and electron affinity (E(a)) were calculated from these data. Studies of the photophysical properties of OFnK in solution and thin film by steady-state and time-resolved fluorescence spectroscopic measurements suggest efficient funneling of excitation energy from the photoexcited fluorene segments to the low-energy fluorenone sites by both intra- and intermolecular hopping events whereby they give rise to green emission. Intermolecular energy transfer was investigated by using a model system composed of a highly defect free polyfluorene PF2/6 doped by OFnK. Forster-type energy transfer takes place from PF2/6 to OFnK. The energy-transfer efficiency increases predictably with increasing concentration of OFnK.

Monodisperse Oligofluorenes with Keto Defect as Models to Investigate the Origin of Green Emission From Polyfluorenes: Synthesis, Self-Assembly, and Photophysical Properties

Determination of the quadratic hyperpolarizability of trans-4-[4-(dimethylamino)styryl]pyridine and 5-dimethylamino-1,10-phenanthroline from solvatochromism of absorption and fluorescence spectra: a comparison with the electric-field-induced second-harmonic generation technique.

We have synthesized and characterized a new fluorene copolymer exhibiting bright yellow luminescence. In order to ensure a complete π-stacking of the active layer, a 9-fluorenone monomeric unit (FOne) has been used as comonomer in conjunction with the more classical 9,9-di-n-nonylfluorene unit. As expected with fluorene-based materials, when excited at 370 nm, the corresponding dilute copolymer solution photoluminescence spectra exhibit a main peak centered at 450 nm in the blue part of the visible spectrum. However, in the solid state, immediate structural reorganization of the layer occurs, leading to a red-shifted emission (bright yellow emission) centered at 550 nm. The origin of the emitted light has been attributed to excimers and/or aggregates based on short FOne segments and involves mainly exciton transfer between nonaggregated fluorene segments and aggregated ones. It is noteworthy that organic light-emitting devices based on these new materials exhibit no spectral evolution upon device operatio...

Fluorene–fluorenone copolymer: Stable and efficient yellow-emitting material for electroluminescent devices

Poly(9,9-dioctylfluorene-co-fluorenone) with 1% fluorenone, (PFO-F(1%)), was synthesized as a model compound to investigate the optical and electrical effects of fluorenone defects in poly(9,9-dioctylfluoren2,7-diyl), PFO. Photoluminescence (PL) and electroluminescence (EL) measurements demonstrate that PFO-F(1%) emits stable green light. PL and EL studies indicate that Forster energy transfer to and charge carrier trapping on fluorenone defects (with subsequent fluorenone emission) are responsible for the color degradation typically observed with the polyfluorenes. By utilization of "fluorenone defects" in PFO-fluorenone copolymers (PFO-F), white electrophosphorescent light-emitting diodes were fabricated. Polymer blends were spin-cast from solution containing PFO, PFO-F (1%), and tris[2,5-bis(9,9-dihexylfluoren-2-yl)pyridine-κ 2 NC 3 ]iridium-(III), Ir(HFP) 3 . The white emission turns on at approximately 5 V, with a luminance (L) of 6100 cd/m 2 at 17 V. The luminous efficiency is 3 cd/A at current density of 8.5 mA/cm 2 (L = 255 cd/m 2 ). The white light exhibits stable color coordinates and stable color temperature and has a high color rendering index.

White Light Electrophosphorescence from Polyfluorene-Based Light-Emitting Diodes: Utilization of Fluorenone Defects†

The electronic absorption and steady state fluorescence spectra of fluorenone in the regions 280–340 and 320–420 nm respectively have been obtained at room temperature for various concentrations in a series of non-polar and polar solvents. The effects of fluorenone concentration in various solvents in the range 10 −6 −10 −3 M on the spectral properties are discussed. In addition to the monomer fluorescence, we have also observed another fluorescence band at longer wavelength in the region 420–600 nm which has been ascribed to excimer formation. The monomer and excimer fluorescence lifetimes of fluorenone in typical solvents such as benzene and acetonitrile have been determined experimentally. By combining our results of steady state fluorescence and measurements of fluorescence lifetimes with previously reported studies of laser-induced fluorescence, the mechanism of excimer formation has been explained. The absence of excimer formation in hexane, cyclohexane and alcohols is discussed.

S. Arathi Rani

Papers

Solvent and concentration effects on the steady state fluorescence of fluorenone

Determination of the excited state dipole moment of fluorenone using the method of solvatochromism