science/technology
Dendritic rod emits blue light
emitting backbone. They showed that fluorescence of the backbone is most in tense when the backbone is encapsulated hemists in Japan have synthe — by third-generation dendritic wedges. The enhancement of fluorescence re sized a dendritic macromolecular εΦ Φι sults from the steric or shielding effect rod that harvests light over a wide of the dendritic wedges. As the group range of wavelengths and converts it to υ Ι points out, the ability of poly(phenyleneblue light with high efficiency. 1 The rod consists of a rigid poly(pheAο ethynylene) to fluoresce at a blue wave length is well known. However, the mol nyleneethynylene) conjugated backbone ecules tend to collide and aggregate. wrapped with a flexible poly (benzyl This tendency leads to collisional ether) dendritic envelope. The backbone quenching of the molecules' photoexcitcollects photons of visible light in the ed state and spoils their potential use as wavelength range of 300 to 450 nm, while a light emitter. the envelope collects photons of ultravio let light ranging from 220 to 300 nm and The dendritic envelopes prevent channels the energy to the backbone, these collisions. Aida calls this resis which then fluoresces blue at 454 nm. tance to collisional quenching the "en velope effect." The effect increases with The work was carried out at the Uni each successive generation of the den versity of Tokyo by chemistry professor dritic wedge. Takuzo Aida, graduate student Takafumi Sato, and research associate Dong-Lin Balzani points out that the coupling of Jiang [/. Am. Chem. Soc, 1 2 1 , 10658 the light-harvesting antenna effect of den Aida: effect Increases with each generation drimers with a shielding effect of the den(1999)]. drimer branches to increase the lumines The rod is highly efficient in terms of its quantum yield—that is, the percent chemistry professor at the University of cence intensity of a dendritic core has age of trapped photons converted into Bologna, Italy, the research is an impor been previously reported, for example, fluorescence. 'We have obtained a new tant step toward applications of den- for dendrimers with a ruthenium (II) bipyridyl core and peripheral naphthyl blue luminescent organic material that drimers in optoelectronic devices. has a fluorescence quantum yield as high Dendrimers are well-defined, highly units (C&EN, Nov. 1, page 27). as 100% over a wide range of concentra branched macromolecules constructed "The importance of the dendritic tions," Aida tells C&EN. "The efficiency around a central core. The level of rods prepared by Aida's group relies on of the energy transfer from the light- branching is defined by a generation the wavelength of the emission—that is, harvesting antennae of the dendritic en number. the blue light—and on the very high lu velope to the chromophore units is also The Japanese researchers used den minescence efficiency," he comments. about 100%." Unprotected poly(phenyleneethyndritic wedges with generation numbers According to Vincenzo Balzani, a 1 to 3 to construct envelopes for the light- ylene) not only suffers from collisional quenching but also has another disadvantage as a potential organ ic photoconductive material: Be Dendritic wedges shield luminescent backbone cause of its high conformational rigidity, it has limited solubility Pn and is difficult to process. The polymer's dendritic deriv atives, on the other hand, are sol uble in solvents such as tetrahym Gn drofuran (THF). For example, al though the second-generation dendritic derivative prepared by the Japanese team has a molecu :H, lar weight of 280,000, its solubility in THF is excellent, Aida notes. "We are now collaborating with G„ = physicists to explore the photofunctions of these dendritic macromolecular rods in the solid state," n=1 he says. "We are also interested in introducing a photo- or redoxactive functionality at one or both Note: η is generation number of dendritic wedge. terminals of the conjugated back n=2 bone with a view to preparing pho H c' toconductive molecular wires." n=3 Michael Freemantle
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38
DECEMBER 13,1999 C&EN
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