DOI: 10.1002/chem.201001798 Efficient Charge Separation in TiO 2 Films Sensitized with Ruthenium(II)– Polypyridyl Complexes: Hole Stabilization by Ligand-Localized Charge- Transfer States Sandeep Verma, [a] Prasenjit Kar, [b] Amitava Das,* [b] and Hirendra N. Ghosh* [a] Introduction Dye-sensitized solar cells (DSSCs) have gained considerable attention for their cost effectiveness relative to conventional silicon solar cells. [1] Research into dye-sensitized solar cells has intensified over the past decade for improvement of the overall conversion efficiency (h) of 10.4 %, a value achieved earlier by Grätzel and co-workers [2] using a state-of-the-art N3-dye-sensitized TiO 2 DSSC. In this field, Ru II –polypyridyl complexes exhibiting a large optical response of metal-to- ligand-charge-transfer (MLCT) transitions in the long-wave- length (400–700 nm) region have served as the manifestation of high conversion efficiency. [3] The demonstration of inci- dent photon-to-electron conversion efficiency (IPCE) values as high as approximately 85 % and the long-term stability of Ru II –bipyridyl complexes has set the standard in the quest for a suitable photosensitizer adsorbate for TiO 2 semicon- ductor material. [4] In these complexes, extensive molecular modifications have been reported for further improvement of the molar extinction coefficient and redox levels in favor of efficient charge separation at the dye/TiO 2 interface. [5] From these studies it was found that an efficient electron in- jection from a photoexcited dye to the TiO 2 conduction band and a slow back-electron-transfer (BET) process is de- sired for a prospective dye/TiO 2 system. [6] In recent advance- ments, these norms can be realized by using multifunctional sensitizer wherein the bpy ancillary ligands of Ru complexes are substituted with a secondary electron-donor moiety. [7] For example, Durrant, Nazeerudin, and co-workers [8] have shown an improved charge-separation efficiency by employ- ing an aromatic amine that is p-conjugated with one of the ancillary bpy ligands. Recently, Chen et al. [9] have shown a Abstract: We have studied the interfa- cial electron-transfer dynamics on TiO 2 film sensitized with synthesized ruthe- nium(II)–polypyridyl complexes— [Ru II ACHTUNGTRENNUNG(bpy) 2 (L 1 )] (1) and [Ru II - ACHTUNGTRENNUNG(bpy)(L 1 )(L 2 )] (2), in which bpy = 2,2’- bipyridyl, L 1 = 4-[2-(4’-methyl-2,2’-bi- pyridinyl-4-yl)vinyl]benzene-1,2-diol, and L 2 = 4-(N,N-dimethylaminophen- yl)-2,2’-bipyridine—by using femtosec- ond transient absorption spectroscopy. The presence of electron-donor L 2 and electron-acceptor L 1 ligands in complex 2 introduces lower energetic ligand-to- ligand charge-transfer (LLCT) excited states in addition to metal-to-ligand (ML) CT manifolds of complex 2. On photoexcitation, a pulse-width-limited (< 100 fs) electron injection from pop- ulating LLCT and MLCT states are ob- served on account of strong catecholate binding on the TiO 2 surface. The hole is transferred directly or stepwise to the electron-donor ligand (L 2 ) as a con- sequence of electron injection from LLCT and MLCT states, respectively. This results an increased spatial charge separation between the hole residing at the electron-donor (L 2 ) ligand and the electron injected in TiO 2 nanoparticles (NPs). Thus, we observed a significant slow back-electron-transfer (BET) pro- cess in the 2/TiO 2 system relative to the 1/TiO 2 system. Our results suggest that Ru II –polypyridyl complexes com- prising LLCT states can be a better photosensitizer for improved electron injection yield and slow BET processes in comparison with Ru II –polypyridyl complexes comprising MLCT states only. Keywords: electron transfer · LLCT states · photochemistry · ruthenium · titanium dioxide [a] Dr. S. Verma, Dr. H.N. Ghosh Radiation & Photochemistry Division Bhabha Atomic Research Centre Mumbai, 400 085 (India) Fax: (+ 91) 22-25505151 E-mail: hnghosh@barc.gov.in [b] Dr. P. Kar, Dr. A. Das Central Salt & Marine Chemicals Research Institute Bhavnagar, 364002, Gujarat (India) E-mail : amitava@csmcri.org Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem201001798. Chem. Eur. J. 2011, 17, 1561 – 1568  2011 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 1561 FULL PAPER