Photophysical properties of dansylated poly (propylene amine) dendrimers

Abstract

Dendrimers are branched macromolecules having a treelike structural shape. Their symmetrical compact structure gives them unique properties as compared to irregularly branched polymers. In this thesis four generations of dansylated poly(propylene amine) (POPAM) dendrimers and their host-guest complexes with xanthene dyes are studied in order to obtain new experimental information on photophysical properties of dendrimers. The internal dynamics of the dendrimers were investigated by time-resolved fluorescence anisotropy measurements at various temperatures. The time scales of the overall rotation and the local motions of the studied dendrimers were resolved and the volumes of the dendrimers were evaluated from the rotational correlation times. Surprisingly the volume of generation three and four dendrimers were observed to shrink when the temperature was increased. With the support of the results from molecular dynamics simulations the contraction was concluded to originate from the balance of intra- and intermolecular interactions. The study of host-guest complexes concentrated to the intermolecular excitation energy transfer process from a dendrimer host to a xanthene dye guest. Femtosecond transient absorption spectroscopy was used to obtain EET rates for various host-guest combinations. EET rates were found to be practically independent on the dendrimer generation and the guest dye. The insensitivity of the EET rates to involved molecules was concluded to originate from almost identical average donor-acceptor distances. Further, guest to guest EET was observed in complexes having multiple guests. New experimental information on dendrimers may help to develop novel dendrimer based applications in the future. Applications developed for artificial light harvesting or host-guest applications, e.g. drug delivery, may benefit from the studies presented in this thesis.