Anomalous Shift in the Fluorescence Spectra of a High-Generation Dendrimer in Nonpolar Solvents

摘要

Charge-transfer (CT) states are characterized by broad, structureless absorption and emission spectra. The peak of the emission band is usually red-shifted (bathochromic shift) with increasing polarity of the medium. This phenomenon of solvatochromism. that is the shift of emission maximum with the change in polarity of the medium, has been used to probe the microenvironment of micelles, the extent of cure in epoxy resins, the mobility of polymer backbones, the morphology of poly(ethyleneglycol) matrices, and the polarity and viscosity of various biological and other heterogeneous systems. Frechet et al. attached a solvatochromic chromophore. 4-(N-benzyl-N-methylamino)-1-nitrobenzene, to the focal point of polyether dendritic macromolecules and investigated solvatochromism as a function of generation. They observed a dramatic change in the absorption maximum on going from third generation to fourth generation dendrimers in solvents of low polarity and correlated this transition with the transformation from an extended to a globular structure. Such a change in shape was consistent with their studies of intrinsic and melt viscosities of these materials. All of these observations and earlier molecular dynamics calculations from other groups indicatethat the higher generation dendrimers have compact, globular structure with a transition from an open extended form beyond a certain generation. We report here an anomalous spectral shift in the fluorescence maximum of a CT state in higher generation phenylacetylene dendrimers. In particular, large spectral shifts are observed in high generation monodendrons even in nonpolar hydrocarbon solvents in much the same way as those reported for other dendritic systems. We also show that this behavior is very dependent on dendrimer generation numbers. These findings suggest significant changes in the size and shape of these dendrimers beyond the fifth generation and the observations may have important implications for the design of dendritic molecules capableof undergoing long-range intramolecular electron transfer.