A series of fluorinated distyrylbenzene (DSB) derivatives were synthesized and studied in order to probe the effect of fluorine substitution on molecular properties and on the arrangement of molecules in the solid state. Reaction of 1,4-diiodobenzene with 4-fluorostyrene or pentafluorostyrene in the presence of Pd(OAc)2 gives trans-trans-bis(4-fluorostyryl)benzene (2F t ) and trans- trans-1,4-bis(pentafluorostyryl)benzene (10F t ), respectively. Bis(2,5-difluorostyryl)benzene (4F t ) was prepared by the reaction of 2,5-difluorobenzaldehyde with p-xylylenebis-(triphenylphosphonium bromide) in the presence of LiOEt in EtOH. Coupling of 1,4-dibromo-2,5-difluorobenzene with styrene, 4-fluorostyrene, or pentafluorostyrene using Pd(OAc)2 gives 1,4-bis(styryl)-2,5-difluorobenzene (2F c ), 1,4-bis(4-fluorostyryl)-2,5-difluorobenzene (2F c 2F t ), and 1,4-bis(pentafluorostyryl)-2,5-difluorobenzene (2F c 10F t ), respectively. Absorption spectroscopy shows that DSB, 2F t , 2F c , 2F c 2F t , 4F t , 10F t , and 2F c 10F t have a λmax at approximately 350 nm. Addition of dimethylaniline to hexane solutions results in exciplex emission with λmax ranging from 458 to 514 nm, depending on fluorine regiochemistry. Cyclic voltammetry shows that, as the fluorine load increases, the reduction of the DSB framework becomes more facile. It is also shown that the regiochemistry of substitution makes an impact on the inductive ability of fluorine to facilitate reduction. The lattice properties of 2F c , 2F c 2F t , 4F t , 10F t , and 2F c 10F t were determined by X-ray diffraction experiments. Two structural motifs emerge from these studies. One is the tendency of the DSB framework to stack cofacially and form vertical “columns” within the crystal. The second motif is the alignment of these “columns” to maximize C−H···F electrostatic registry.
The monomer 9-[(tert-butyldimethylsilyl)oxy][2.2]paracyclophan-1-ene (1) is polymerized by Mo(NAr)(CHCMe2Ph)(OCMe(CF3)2)2 (Ar = 2,6-diisopropylphenyl) in a living manner. Block copolymers containing poly(1) and polynorbornene (poly(NBE)) can be prepared and have narrow polydispersities. Treatment of poly(1) with NBu4F produces poly(9-hydroxy[2.2]paracyclophan-1-ene) which dehydrates to poly(p-phenylenevinylene) (PPV) under mild conditions. Block copolymers containing segments of size-specific and defect-free PPV with poly(NBE) of various dimensions can be prepared readily and are soluble in a variety of common organic solvents. These polymers serve as excellent candidates to measure the effect of chain length and frequency of interchain contacts on PPV's fluorescence quantum yield (Φf). Solution and solid-state quantum yield determinations, coupled to fluorescence lifetime measurements, reveal an acute drop in Φf as the degree of polymerization of the emissive species increases and as the average distance between PPV chains decreases. PPV x -block-poly(NBE) y prepared in solution consistently shows larger Φf than material prepared by solid-state dehydration. The poly(NBE) companion block also serves to protect PPV from atmospheric degradation.
Optoelectronic devices, such as light emitting diodes (LEDs), are an emerging technology where organic materials are finding a niche."] Initial reports of polymer-based electroluminescence used poly(p-phenylenevinylene) (PPV), albeit with inefficient quantum yield. ['] In recent years, new materials and optimization of devices, in particular choice of electrode13] and multiple-layer structures,[41 have raised the level of performance to quantum yields of up to 4 %. ['I Flexible, all-plastic as well as inverted cylindrical devices ['] exist, illustrating the drive and ingenuity for new device configurations. By taking advantage of the porous cavities in microfiltration membranes, LEDs have also been produced on the micrometer and submicrometer scale for potential use as light sources in scanning near-field optical microscopes.lxl Control over spatial arrangement and dimensions of conjugated materials will play an important role in diversifying their applications. In this communication we report a microlithographic process for patterning PPV and show preliminary results from pixel-like LED arrays prepared by this new technique. Standard photolithography used in the fabrication of integrated circuits['' is merged with the synthesis of conjugated emissive polymers to create well-defined microscalar-patterned LEDs.There is abundant precedent in the literature that indicates that the method of PPV synthesis influences LED performance. The choice of synthetic route also determines, to a great extent, the material's processability and therefore limits the types of applications. The paracyclophane route to PPV["'I has a number of distinct advantages over previous synthetic methods. '"] Because the polymerization is living, the average degree of polymerization can be controlled and optimized to maximize photoluminescence quantum PPV can also be incorporated into block copolymers to modify properties such as solubility and viscosity. Another important feature of the method are the mild conditions (thermally at 105 "C or at 50-60 "C in the presence of HC1) that convert precursor material to conjugated PPV. Scheme 1 illustrates how poly(9-hydroxy-[2.2]-paracyclophan-l-ene),-block-poly(norbornene), (poly1,-block-polyNBE,; subscripts refer to the average degree of polymerization) converts to PPVZ-block-poly-NBE,The acute sensitivity towards acid catalysis in the elimination step of poly(paracyc1ophenes) opens the door to applications in microlithography. To accomplish the elimination in discrete areas, we take advantage of photoacid generators. These molecules generate acids upon photolysis and have found applications in photoresist technology, where acid catalysis is responsible for structural changes at the molecular level. If this reaction is controlled within the irradiated areas then outstanding definition is possible.["] The overall photolithographic sequence using poly(paracyclophene) is illustrated in Scheme 2 for the preparation of PPV2,-block-polyNBE,. First, a uniform film containing poly1,-block-polyNBE, and th...
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