Ultrafast Excited-State Energy Migration Dynamics in an Efficient Light-Harvesting Antenna Polymer Based on Ru(II) and Os(II) Polypyridyl Complexes

• Published in: Journal of the American Chemical Society 2001-10, Vol.123 (42), p.10336-10347
• Main Authors: Fleming, Cavan N, Maxwell, Kimberly A, DeSimone, Joseph M, Meyer, Thomas J, Papanikolas, John M
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja016304i

A detailed study of the excited state energy migration dynamics that take place within an assembly of Ru(II) and Os(II) polypyridyl complexes linked together through a polymer backbone is presented. The energy migration process is initiated by the photoexcitation of the metal-to-ligand charge transfer (MLCT) transition in one of the Ru(II) complexes and terminated by energy transfer to a lower energy Os(II) trap. Energy transfer sensitization of Os(II) can occur in a single step if the excited state is formed adjacent to a trap, or after a series of hops between isoenergetic rutheniums prior to reaching a trap. The dynamics of the energy transfer process are followed by monitoring the growth of Os(II)* luminescence at 780 nm. The kinetics of the growth are complex and can be fit by a sum of two exponentials. This kinetic complexity arises both from the presence of a distribution of donor−acceptor distances and the variety of time scales by which Os(II)* can be formed. We have augmented the time-resolved experiments with Monte Carlo simulations, which provide insight into the polymer array's structure and at the same time form the basis of a molecular-level description of the energy migration dynamics. The simulations indicate that the most probable Ru*→Os energy transfer time is ∼400 ps while the time scale for Ru*→Ru hopping is approximately 1−4 ns. The time scale for Ru*→Ru hopping relative to its natural lifetime (1000 ns) suggests that this polymer system could be extended to considerably longer dimensions without an appreciable loss in its overall efficiency.