Ultrafast Excited-State Excitation Dynamics in a Quasi-Two-Dimensional Light-Harvesting Antenna Based on Ruthenium(II) and Palladium(II) Chromophores

• Published in: Chemistry : a European journal 2006-06, Vol.12 (19), p.5105-5115
• Main Authors: Dietzek, Benjamin, Kiefer, Wolfgang, Blumhoff, Jörg, Böttcher, Lars, Rau, Sven, Walther, Dirk, Uhlemann, Ute, Schmitt, Michael, Popp, Jürgen
• Format: Article
• Language: English
• Subjects: artificial light-harvesting antenna; electron transfer; femtochemistry; heterometallic complexes; time-resolved spectroscopy
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200501093

A detailed study on the excited‐state‐excitation migration taking place within the tetranuclear complex [{(tbbpy)2Ru(tmbi)}2{Pd(allyl)}2](PF6)2 (tbbpy = 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine and tmbi = 5,6,5′,6′‐tetramethyl‐2,2′‐bibenzimidazolate) is presented. The charge transfer is initiated by the photoexcitation into the lowest metal‐to‐ligand charge‐transfer (MLCT) band of one of the peripheral ruthenium(II) chromophores and terminates on the central structurally complex Pd2II(allyl)2 subunit. Thus, the system under investigation can be thought of as a functional model for the photosynthesis reaction center in plants. The kinetic steps involved in the overall process are inferred from femtosecond time‐resolved transient‐grating kinetics recorded at spectral positions within the regions of ground‐state bleach and transient absorption. The kinetics features a complex non‐exponential time behavior and can be fitted to a bi‐exponential rise (τ1≥200 fs, τ2≈1.5 ps) and a mono‐ or bi‐exponential decay, depending on the experimental situation. The data leads to the formulation of a model for the intramolecular excitation‐hopping ascribing intersystem crossing and subsequent cooling as the two fastest observed processes. Following these initial steps, charge transfer from the ruthenium to the central complex Pd2(allyl)2 moiety is observed with a characteristic time constant of 50 ps. A 220‐ps component that is observed in the ground‐state recovery only is attributed to excitation equilibration between the two identical Pd(allyl) chromophores. Excitation transfer in a novel light‐harvesting antenna based on RuII–allyl and PdII–allyl chromophores occurs upon photoexcitation. Ultrafast dynamic processes ranging from 2 to 220 ps were monitored by using coherent four‐wave‐mixing spectroscopy (see picture), which allowed a detailed model of the excited‐state chemistry of this quasi‐two‐dimensional inorganic light‐harvesting antenna to be formulated.