Sequential Energy and Electron Transfer in Polynuclear Complex Sensitized TiO 2 Nanoparticles Sandeep Verma, †,§ Prasenjit Kar, ‡,§ Tanmay Banerjee, ‡ Amitava Das,* ,‡,§ and Hirendra N. Ghosh* ,† † Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India ‡ Central Salt and Marine Chemicals Research Institute (CSIR), Bhavnagar 364002, India * S Supporting Information 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 Ru 2 ∧ 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 ∧ Os 1 , Ru 2 ∧ Ru 1 , and Os 1 complex sensitized TiO 2 nanoparticle (TiO 2 NP) systems. The intramolecular energy transfer causes sequential excitation of the core part of the Ru 2 ∧ Os 1 complex, which leads to multiexponential electron injection into TiO 2 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. SECTION: Physical Processes in Nanomaterials and Nanostructures T he light-transducing function of energy-efficient natural photosynthetic pigments involves long-range energy- and electron-transfer reactions. 1 The multicomponent light absorp- tion and sequential energy transfer improve the primary charge separation at the reaction center. 2,3 Besides this, the charge- separated (CS) states are stabilized by a multication center and secondary electron-donating groups. 2 Thus, various photo- physical processes are cohesively used in natural-light-harvest- ing complexes. Similar molecular function in biomimetic systems can be very useful in molecular charge storage devices. 4,5 It is feasible by polynuclear complexes that exhibit multicomplex light absorptions and energy transfer together. 6-9 In this regard, Ru(II)- and Os(II)-polypyridyl complexes have the advantage of tunable metal to ligand charge transfer (MLCT) states. 6 This allows light absorption in the visible and near-IR regions and also offers an optimum gradient for intramolecular energy transfer. Additionally, the use of π- acceptor multidentate bridging ligands allows electronic coupling between redox asymmetric M(+2)/M′(+3) ion pairs (M/M′ = Ru, Os, etc.). This facilitates intervalence electron transfer (i.e., d 6 ↔ d 5 exchange reaction), which can give a better charge stabilization of higher oxidation states. 7,10 Thus, multinuclear complexes offer novel intramolecular energy- and electron-transfer pathways that can be used in solar energy conversion, molecular switches, and sensors applications. 11,12 The complex as ligand/complex as metal strategy is well- implemented in polynuclear complex synthesis. 9,13 It introduces intramolecular energy relay depending on different metal ions, ligands, shapes, and sizes. 6,9,13 Numerous spectroelectrochem- ical studies in the past 6-10,13-16 show that the antenna function of polynuclear complexes arises from redox asymmetry of metal ion pairs and bridging ligands. However, not many studies describe the time dependence of energy transfer 17,18 in various sensitization processes. To understand this aspect, newly synthesized trinuclear Ru 2 ∧ Os 1 and Ru 2 ∧ Ru 1 complexes (Scheme 1 and Supporting Information (SI)) have been studied by transient absorption spectroscopy. The 2,3-bis(2′- pyridyl)pyrazine (dpp is represented as ∧ ) ligand is used in bridging Ru(II)/Ru(II) and Ru(II)/Os(II) ion pairs. 6,8,13 The redox asymmetry of the Ru(II) ∧ Os(II) ion pair gives rise to intramolecular energy transfer in the Ru 2 ∧ Os 1 complex, 6,14 which is very appealing in photosensitization applications. It is illustrated in the photosensitization study of the TiO 2 NP. For this purpose, a catechol moiety is attached to the core of the trinuclear complex, which allows surface immobilizations on TiO 2 NPs. Scheme 1 depicts the sensitizing ability of Ru 2 ∧ Os 1 and Os 1 complexes for TiO 2 NPs. Photophysical properties of mononuclear Ru 1 and Os 1 complexes were well-known previously. 19 The use of similar molecular fragments in the trinuclear complexes should help us to understand the photophysical changes that are brought about after electronic coupling through the bridging ligand. The role of redox asymmetry in directional energy transfer is explored by transient absorption studies of trinuclear Ru 2 ∧ Os 1 Received: April 30, 2012 Accepted: May 18, 2012 Published: May 19, 2012 Letter pubs.acs.org/JPCL © 2012 American Chemical Society 1543 dx.doi.org/10.1021/jz3005305 | J. Phys. Chem. Lett. 2012, 3, 1543-1548