Effect of Core Structure on Photophysical and Hydrodynamic Properties of Porphyrin Dendrimers

The photophysical and hydrodynamic properties of dendrimers (GnPZn and GnTPPH2) with zinc porphyrin (PZn) and tetraphenylporphyrin (TPP) cores are studied in tetrahydrofuran (THF) and dimethylformamide (DMF). UV−vis absorption spectra of GnPZn exhibit a small red shift of the Soret band upon increasing the generation as a result of interactions between the dendrons and the core. All fluorescence decays obtained from global analysis show a monoexponential profile. The intrinsic viscosity obtained for GnPZn from the hydrodynamic volume (Vh) passes through a maximum as a function of generation (G) in agreement with earlier experimental findings and calculations suggesting that the internal density profile of dendrimers decrease monotonically outward from the center of the molecule. Within the investigated range (G = 1−3), GnTPPH2 exhibits an approximately constant intrinsic viscosity due to the linear dependence between the hydrodynamic volume and the molecular weight. The differences observed between GnPZn and GnTPPH2 are correlated to structural differences in their cores. The additional phenyl group of the TPP in GnTPPH2 increases the distance between the branches and the porphyrin moiety compared to GnPZn, resulting in a more flexible structure. The enhanced flexibility allows the terminal groups to sample more conformational space and therefore decreases the volume of the dendrimer as compared to the theoretical fully extended structure where Vh ∝ G3. A comparison of the results obtained from analysis of fluorescence anisotropy decays with previously reported viscometry measurements shows a dependence of the structural collapse on the core size.