Intrinsic Fluorescence of Triazine Dendrimers Provides a New Approach to Study Dendrimer Structure and Conformational Dynamics

We present basic spectroscopic studies of 5 triazine dendrimers ranging from generations one through nine, G1, G3, G5, G7, and G9, based on the intrinsic fluorescence of these molecules. The extinction spectra of each generation can be separated into two components; the absorption spectra from triazine chromophores and Rayleigh scattering by dendrimer particles. Rayleigh scattering into the UV spectral range is significant and may contribute more than 50% to the measured light attenuation (extinction) for larger dendrimer generations. Deviations from the Rayleigh model at long wavelengths (where the triazine chromophore does not absorb) are clear indications of dendrimer aggregation. These larger particles can be eliminated by dilution and sonication. Importantly, this model system represents a comprehensive case study where the intrinsic fluorescence of the dendrimer when combined with insights from molecular dynamics (MD) simulations can be utilized to probe molecular conformations and dynamics. Experimental results from fluorescence lifetimes, time-resolved anisotropies, and diffusional quenching indicate an increasingly compact core as size increases from G1 to G5. This trend is reversed for G7 and G9 generations, which present more extended, and porous structures, less dense cores, and a denser peripheries. Simulations corroborate this picture and better anchor intuition of the behavior of these molecules.