Perylene-terminated monodendrons 1−7 and phenyl-terminated reference monodendrons 8−14 have been synthesized, and the intramolecular energy transfer has been studied using steady-state as well as time-resolved fluorescence spectroscopy. In the series 2−7, the light-harvesting ability of these compounds increases with increasing generation due to the increase in molar extinction coefficient. However, the efficiency of the energy transfer decreases with increasing generation in this series. With increasing generation, the photoluminescence intensity from the perylene core still increases and the expected level-off in the photoluminescence intensity has not been reached in this series of compounds. Dendrimer 1 is unique in that the energy transfer in this molecule occurs at a very fast rate. The rate constant for energy transfer in 1 is at least 2 orders of magnitude larger than in 2−7. In contrast to monodendrons 2−7, 1 possesses a variable monomer type at each generation that creates an energy funnel. The ultrafast energy transfer in this system is best explained by the presence of this energy gradient.
Communications separating the PPV-rich parts and the pyridine-rich segments. This will be explained in more detail elsewhere.The significant red emission at high voltages is probably associated with the higher efficiency for light emission in pyridine-rich segments. As demonstrated by OnodaI7] the EL efficiency for light emission from PPyV is rather high (7 N 0.5 YO ) and it is further enhanced due to the presence of the junctionsr7 in PPV/PPyV. At the present time, we cannot exclude alternative explanations associated with the formation of exciton or impurity bands in the x-x* forbidden gap of PPV upon substitution of the pyridine moieties in the copolymer. ExperimentalThe 2,6-pyridyl-dimethylene-bis-(tetramethylene sulfonium chloride) monomer was prepared by adding tetrahydrothiophene (THT) (15 ml) to 2,6bis(chloromethy1)-pyridine (5 g) in methanol (150 ml). The reaction mixture was kept at 50°C for 4 days and was then concentrated to gelation. The hissulfonium salt was precipitated by addition of ice cold acetone (250 ml) and was isolated. The 1,4-phenylenedimethylene-bis-(tetramethylene sulfonium chloride) was prepared according to the literature procedure. The copolymerization was typically carried out as follows. A mixture of 20 YO (molar) of 2,6pyridyl-dimethylene-bis-(tetramethylene sulfonium chloride) (1.49 g) and 1,4phenylene dimethylene-bis-(tetramethylene sulfonium chloride) (5.96 g) in water (200 ml) was prepared. Polymerization was initiated under nitrogen at 0-2°C by addition of a stoichiometric quantity of 1 N NaOH in water (21.2 ml) under constant vigorous stirring for 1 h. The mixture was neutralized with 1 N HCl(9.4 ml) corresponding to 54.2 % conversion of the monomers to the copolyelectrolyte. The copolyelectrolyte solution was homogenized and dialyzed (Mw cut-off 12000) at 4°C for ten days.Films for the various measurements were prepared by casting the dialyzed copolyelectrolyte on relevant substrates by spin coating. The polyelectrolyte was then converted to the final copolymer, co(2,6PyV-PV) by heating in vacuum, 10-6torr, at 280°C for 12 h. Film thicknesses were determined by X-ray reflectivity at grazing angle using a 12 kW Rigaku rotating anode and found to be approximately 1000 A. Absorption spectra were measured on a HP 8452 diode array spectrometer. PL spectra were recorded on Perkin Elmer Luminescence Spectrometer LS 50. EL spectra were recorded using an Oriel monochromator and the light intensity was quantified with a photo-multiplier (products from Research Inc. model no. R955).
Continuous, slow addition of AB 2 monomer (3,5-diiodophenylacetylene) to a solution of a multifunctional core results in high molecular weight hyperbranched phenylacetylene polymers with narrow polydispersites. Control over the molecular weight (Mw ) 8-90 kDa) was achieved by varying the monomer:core ratio from 17.5 to 560. At low ratios (<140, Mw ) 49 kDa), monomodal molecular weight distributions were observed by SEC. A bimodal distribution was observed at higher ratios. Using an azobenzene functionalized core, in combination with SEC photodiode array detection, it was found that the core was not uniformly distributed over the entire bimodal distribution but rather predominantly found in the high molecular weight part of the distribution. The polydispersities of the hyperbranched polymers were found to decrease with increasing degree of polymerization and with increasing core functionality. Our experimental results are in good qualitative agreement with computer simulations reported by Frey et al. and the theoretical work of Mu ¨ller.
A new scheme for the synthesis of phenylacetylene dendritic macromolecules is described which greatly facilitates the large-scale production of high molecular weight monodendrons. Simply by inverting the monomer protecting group scheme from B2AP to A2BP (where A = ArC=CH; B = Arl; Ap and Bp are protected versions of these groups), we show that the repetitive synthesis can be propagated through at least one higher generation on reaction scales 2 orders of magnitude greater than previously possible. Using this new scheme, we have prepared gram quantities of phenylacetylene monodendrons through generation four (I-Mbs-U-BuJm), in high yields. Possible reasons for the improvements are discussed. We furthermore show that the new route is amenable to a solid-phase convergent dendrimer synthesis which involves tethering the focal point monomer to an insoluble support. Preparation of phenylacetylene monodendrons by the solid-phase method is demonstrated through generation four, yielding monodendron products identical to those synthesized by solution methods. However, at generation four, coupling reactions using polymer supports can only be driven to completion with light loading of the focal point monomer.The solid-phase convergent method offers several advantages, especially in the synthesis of early generation monodendrons. \
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