Sequential energy and electron transfer in polynuclear complex sensitized TiO₂ nanoparticles

Abstract

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 Ru₂^∧Os₁ 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 Ru₂^∧Os₁, Ru₂^∧Ru₁, and Os₁ complex sensitized TiO₂ nanoparticle (TiO₂ NP) systems. The intramolecular energy transfer causes sequential excitation of the core part of the Ru₂^∧Os₁ 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.