ARTICLES The Dynamics of Electronic Energy Transfer in Novel Multiporphyrin Functionalized Dendrimers: A Time-Resolved Fluorescence Anisotropy Study Edwin K. L.Yeow and Kenneth P. Ghiggino* Photophysics Laboratory, School of Chemistry, The UniVersity of Melbourne, ParkVille, Victoria 3052, Australia Joost N. H. Reek ² and Maxwell J. Crossley School of Chemistry, The UniVersity of Sydney, NSW 2006, Australia Anton W. Bosman, Albert P. H. J. Schenning, and E. W. Meijer Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands ReceiVed: September 1, 1999; In Final Form: December 15, 1999 The dynamics of electronic energy transfer (EET) for a series of spherical porphyrin arrays based on different generations of poly(propylene imine) dendrimers have been investigated using time-resolved fluorescence anisotropy measurements (TRAMS) in a glass environment. The first, third, and fifth generation dendrimers consisting of 4, 16, and 64 porphyrin chromophores, respectively, are investigated in this study. We observe a depolarization of the fluorescence in all three dendrimers as compared to the monoporphyrin model compound, indicating that EET takes place between the chromophores within the dendrimers. The experimental TRAMS results were compared to computationally simulated data obtained from the Pauli master equation. For the first generation dendrimer, we find the rate of energy transfer is well described by Fo ¨ rster theory. Anomalous behavior is observed in the third generation dendrimer where the limiting anisotropy value suggests that energy transfer is confined to only the porphyrins contained within a dendron. Interdendron porphyrin EET is thus unfavorable due to dendron segregation. In the fifth generation dendrimer, the TRAMS data is best explained by a model which includes independent and simultaneous rapid EET between porphyrins contained on the surface of the dendrimer sphere and slow EET between porphyrins in adventitious dendrons found probably either outside or inside of the sphere. 1. Introduction A special class of hyperbranched, three-dimensional molec- ular structures known as dendrimers has attracted considerable attention in recent years. 1-5 The controlled synthesis of den- drimers results in their size and architecture being both regular and well-defined. Such unique physical properties have given rise to several important applications for these novel macro- molecules. In particular, dendrimers are now recognized to have important roles in guest-host chemistry, 6 catalysis, 7 optical electronics, 8 analytical chemistry, 3 biology 9 and medicine. 10 Recently, many studies have been focused on using dendrim- ers to mimic the photosynthetic light-harvesting antennae system. 11-15 Kopelman and co-workers 11 have considered the kinetics of excitation energy trapping from an initially excited state on a phenylacetylene dendrimer to a trap located at its center. An energy gradient created along the linear phenylacety- lene chain was found to facilitate the funneling of the excitation energy. 11,12,16,17 Jiang and Aida 13 have also examined the cooperation of dendron subunits in aryl ether dendrimer porphyrins for energy transduction. More recently, we have synthesized and reported a series of novel spherical porphyrin arrays based on different generations of poly(propylene imine) dendrimers. 18 The morphologies of these molecules resemble that of the photosynthetic light harvesting antenna LH2 system and have thus created interest as artificial photosynthetic devices. In this paper, we investigate the dynamics of electronic energy transfer (EET) for our multiporphyrin funtionalized dendrimers, GnPm, where n is the generation number and m is the number of porphyrin end-groups (see Chart 1), using time-resolved fluorescence anisotropy measurements (TRAMS). This tech- nique not only yields information on the dynamics of energy transfer but also provides a tool to study the structural properties of the dendrimers. 19,20 TRAMS involves the measurement of the fluorescence intensity decay profiles through a polarizing analyzer element arranged in a parallel and perpendicular orientation with respect to the polarization of the excitation light. * Corresponding author. E-mail: k.ghiggino@chemistry.unimelb.edu.au. ² Present address: Institute of Molecular Chemistry, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV, Amsterdam, The Netherlands. 2596 J. Phys. Chem. B 2000, 104, 2596-2606 10.1021/jp993116u CCC: $19.00 © 2000 American Chemical Society Published on Web 03/04/2000