Dendrimers with a Pentaphenylene Core: A Photophysical Study

Novel dendrimers G2PC and G4PC consisting of a p-pentaphenylene core (PC) appended in the para position with two second-generation (G2) or two fourth-generation (G4) sulfonimide branches and two n-octyl chains, as well as a model compound of the pentaphenylene core (G0PC), are prepared. The photophysical properties (absorption, emission, and excitation spectra; fluorescence decay lifetime; and fluorescence anisotropy spectra) of the three compounds are investigated under different experimental conditions (dichloromethane solution and solid state at 293 K, dichloromethane/methanol rigid matrix at 77 K). In the absorption spectra contributions from both the branches and the core can be clearly identified. The fluorescence spectra show only the characteristic fluorescence of the pentaphenylene unit with λmax around 410 nm in fluid solution and 420 nm in the solid state. In solution the fluorescence quantum yields are 0.78, 0.76, and 0.72 for G0PC, G2PC, and G4PC, respectively, and the fluorescence lifetime is about 0.7 ns in all cases. Energy transfer from the chromophoric groups of the dendrimer branches to the core does not occur. The three compounds show the same, high steady-state anisotropy value (0.35) in dilute rigid-matrix solution at 77 K. In dichloromethane at 293 K, the increasing anisotropy values along the series G0PC (0.17), G2PC (0.27), and G4PC (0.32), with increasing molecular volume of the three compounds, show that depolarization takes place by molecular rotation. In the solid state the anisotropy is very low (0.015, 0.017, and 0.035 for G0PC, G2PC, and G4PC, respectively), probably because of fast depolarization via energy migration.