Sequential Energy and Electron Transfer in Polynuclear Complex Sensitized TiO2 Nanoparticles

Polynuclear polypyridyl complexes exhibit a directional energy-transfer property that can improve their photosensitization activity. In the present work, the energy-transfer process is explored in a trinuclear Ru-2 boolean AND Os-1 complex using transient absorption spectroscopy. This study reveals an efficient excitation energy transfer from the terminal (Ru-II complex) to the core (Os-II complex) region in the ultrafast time domain (400 fs-40 ps). The excitation energy funnel is useful in improving the functionalized core activity. This is evidenced in an interfacial electron-transfer study of Ru-2 boolean AND Os-1, Ru-2 boolean AND Ru-1, and Os-1 complex sensitized TiO2 nanoparticle (TiO2 NP) systems. The intramolecular energy transfer causes sequential excitation of the core part of the Ru-2 boolean AND Os-1 complex, which leads to multiexponential electron injection into TiO2 NP. Besides this, the electronic coupling between the metal ion centers stabilizes the positive charge within the trinuclear complex, which results in a slow charge recombination reaction. This study shows that polynuclear complexes can be very useful for their panchromatic effects, unidirectional energy- and electron-transfer properties.