Monodisperse Oligofluorenes Forming Glassy-Nematic Films for Polarized Blue Emission

Abstract

The first series of monodisperse glass-forming, nematic oligofluorenes was synthesized following a divergent−convergent approach. Both chain length and pendant structure were found to affect solid morphology and phase transition temperatures. With optimized structural parameters, a T g close to 150 °C and a T c beyond 375 °C were achieved. This material class is characterized by its ability to form monodomain glassy nematic films via spin-coating, thermal annealing at 10 to 20 °C above T g for 15−30 min, and cooling to room temperature without encountering crystallization. The absorption and emission dichroic ratios of 80−90-nm-thick films increase from 7 to 17 with an increasing molecular aspect ratio, while the photoluminescence quantum yield varies erratically from 43 to 60%. Superior stability of emissive color and of emission dichroic ratio and that against thermally activated crystallization were also demonstrated through prolonged heating of a film at 10 °C above T g under argon.

Strongly Polarized and Efficient Blue Organic Light‐Emitting Diodes Using Monodisperse Glassy Nematic Oligo(fluorene)s

“…[19] As a class of photonic materials, monodisperse conjugated oligomers are characterized by a relatively short and uniform chain length, solubility and chemical purity, and ease of processing [20] with a potential for spontaneous uniaxial alignment mediated by nematic mesomorphism. Few existing conjugated oligomers are capable of forming glassy nematic films for polarized light emission with the exception of properly functionalized oligo(fluorene)s. [21] The potential of this material class has also been substantially enhanced with elevated glass transition and clearing temperatures while avoiding spontaneous crystallization normally encountered with low molecular mass organic materials. Here, we report the first polarized OLEDs using monodisperse oligo(fluorene)s on a PEDOT/PSS conductive alignment layer.…”

“…[21] It was found that the oligomer's chain length, pendant structure, and molecular aspect ratio affect thermotropic properties, phase transition temperatures, glass-forming ability, and absorption and photoluminescence dichroic ratios. Three promising oligo(fluorene)s were selected for an exploration of polarized OLEDs, viz.…”

Room‐temperature processing of π‐conjugated oligomers into uniaxially oriented monodomain films on coumarin‐based photoalignment layers

“…Recall that thermal annealing of these two films on rubbed polyimide alignment layers produced essentially the same S value, 0.83 for OF 16 and 0.82 for OF2T 21 annealed at 160 and 115 C, respectively, despite the difference in their molecular structures. Thermal annealing of OF and OF2T films on Polymer 1 photoalignment layers at 185 and 155 C also FIGURE 4 S of 90-nm-thick OF2T films on 10-nm-thick Polymer 1 photoalignment layers with X ¼ 0.31 as a function of exposure time to chloroform vapor at specified P r values followed by vacuum drying at room temperature for 3 h. At P r ¼ 1.00 over 1 =2 and 1 min exposure, S ¼ 0.28 and 0.00 (dewetting), respectively.…”

“…The synthesis and properties of OF, OF2T, and Polymer 1 have been reported previously; 16,21,39 Polymer 1 was characterized to have a number average molecular weight of 8,960 g mol À1 and a polydispersity index of 4.6. The polymer analog of OF2T and PF2T with a number-average molecular weight of 36,200 g mol À1 and a polydispersity index of 3.1, was used as received from American Dye Source (Quebec, Canada); its thermotropic properties have been reported earlier.…”

“…Uniaxial orientation of conjugated backbones enables polarized light emission as energy-efficient backlights for liquid crystal displays, 8 anisotropic charge transport to suppress crosstalk in logic circuit and pixel switching elements, 9 and polarization-sensitive photodiodes for sensing applications. [10][11][12] Monodomain glassy-nematic films have been accomplished with poly(fluorene)s, 13,14 poly(fluoreneco-bithiophene)s, 15 and monodisperse oilgo(fluorene)s, [16][17][18][19][20] and oligo(fluorene-co-bithiophene)s 21 films through thermal annealing above their glass-transition temperatures on rubbed polyimide, PEDOT:PSS, and poly(p-phenylenevinylene) alignment layers with subsequent cooling to room temperature. To overcome the problems arising from rubbing, e.g., mechanical damage and the generation of dust and electrostatic charges, photoalignment induced by polarized UVirradiation has emerged as an attractive alternative.…”