Interchromophoric Coupling in Oligo(p-phenylenevinylene)-Substituted Poly(propyleneimine) Dendrimers

Poly(propyleneimine) dendrimers functionalized with oligo(p-phenylenevinylene) (OPV) end groups based on distyrylbenzene show fluorescence properties that depend on dendrimer generation and thereby on the number of end groups. The compounds are investigated using site-selective and time-resolved fluorescence methods. A red shift of the low-temperature emission spectrum is observed with increasing dendrimer generation. This is accompanied by changes in the shape of the lowest absorption band of the dendrimers. Results are interpreted in terms of rapid migration of electronic excitation energy among the OPV units. We find that coupling between the electronic motion on the OPV groups has to be taken into account to explain the magnitude of the red shift in emission. This implies that interchromophoric interactions are sufficiently strong to induce delocalization of the excitation over more than one chromophoric group. Experimental data indicate that off-diagonal disorder is large; i.e., excited-state interchromophoric interaction energies are given by a broad statistical distribution. The off-diagonal disorder is interpreted in terms of the flexible nature of the dendrimer, leading to a broad distribution of distances between the chromophoric end groups. A delayed emission component is observed for the dendrimers, but this component is absent for the isolated OPV unit. The delayed emission is attributed to excimer and/or exciplex formation within the dendrimer.