Letter pubs.acs.org/JPCL
Sequential Energy and Electron Transfer in Polynuclear Complex Sensitized TiO2 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 Ru2∧Os1 complex using transient absorption spectroscopy. This study reveals an efficient excitation energy transfer from the terminal (RuII complex) to the core (OsII 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. SECTION: Physical Processes in Nanomaterials and Nanostructures
T
ical studies in the past6−10,13−16 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 transfer17,18 in various sensitization processes. To understand this aspect, newly synthesized trinuclear Ru2∧Os1 and Ru2∧Ru1 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 Ru2∧Os1 complex,6,14 which is very appealing in photosensitization applications. It is illustrated in the photosensitization study of the TiO2 NP. For this purpose, a catechol moiety is attached to the core of the trinuclear complex, which allows surface immobilizations on TiO2 NPs. Scheme 1 depicts the sensitizing ability of Ru2∧Os1 and Os1 complexes for TiO2 NPs. Photophysical properties of mononuclear Ru1 and Os1 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 Ru2∧Os1
he light-transducing function of energy-efficient natural photosynthetic pigments involves long-range energy- and electron-transfer reactions.1 The multicomponent light absorption and sequential energy transfer improve the primary charge separation at the reaction center.2,3 Besides this, the chargeseparated (CS) states are stabilized by a multication center and secondary electron-donating groups.2 Thus, various photophysical processes are cohesively used in natural-light-harvesting 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 πacceptor 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., d6 ↔ d5 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 wellimplemented 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© 2012 American Chemical Society
Received: April 30, 2012 Accepted: May 18, 2012 Published: May 19, 2012 1543
dx.doi.org/10.1021/jz3005305 | J. Phys. Chem. Lett. 2012, 3, 1543−1548
The Journal of Physical Chemistry Letters
Letter
Scheme 1. Light-Harvesting Scheme of Trinuclear the Ru2∧Os1 and Mononuclear Os1 Complexes for Spectral Sensitization of TiO2 NPsa
a
The energy-transfer direction in the Ru2∧Os1 complex is shown with an arrow (→).
and Ru2∧Ru1 complexes and also the mononuclear Ru1 and Os1 complexes. In addition, this study provides fundamental information on how interfacial electron-transfer dynamics changes upon inclusion of an individual component (MII complex) into a larger assembly. Spectral Response. Figure 1a shows the optical absorption spectrum of the Ru2∧Os1 (∼17 μM) complex, which covers the
complex fragment. These assignments are supported by the absorption spectra of Os1, Ru1, and Ru2∧Ru1 complexes (Figure 1c−1e). A large spectral shift in the absorption spectrum of the Ru2∧Os1 complex (1MLCT band; 550−750 nm region) in comparison to that of the Ru2∧Ru1 complex represents redox asymmetry of heterometallic Ru/Os ion pairs (SI). The significant increase in absorption of the Ru2∧Os1 complex after addition of TiO2 NPs is due to strong catecholate binding on TiO2 NPs. Earlier studies with a catechol-functionalized Os1 complex/TiO2 system show that the strong binding causes charge-transfer (CT) interaction between the complex and TiO2 NP.20 The CT complex formation enhances the optical density of the dye/TiO2 system. It is beneficial in terms of direct interfacial charge separation.19 Therefore, the absorption study indicates an efficient spectral sensitization of the TiO2 NP. The BH plot (SI) analysis reveals efficient binding of Ru2∧Os1 and Os1 complexes (104−105 M−1) on the TiO2 surface. The binding constant of catecholate-functionalized trinuclear Ru2∧Os1 complex is comparable to that of phosphateor carboxylate-functionalized mononuclear complexes (∼104 M−1), which are commonly used in DSSC.21 This shows the suitability of the catecholate functionality for bulky polynuclear complexes. Energy Transfer. The intramolecular energy transfer is explored by femtosecond time-resolved transient absorption spectroscopy22 using 400 nm pump excitation and 480−1000 nm probe detection. Figure 2 shows the transient absorption spectrum of Os1 and Ru2∧Os1 complexes. The transient absorption spectrum of the Os1 complex is comprised of S0 → 1MLCT and S0 → 3MLCT (t2 g→π*bpy) photobleach bands in the 490−550 and 550−700 nm regions, respectively. Because 1 MLCT → 3MLCT crossover occurs on a
