Interfacial Electron Transfer between the Photoexcited Porphyrin Molecule and TiO 2 Nanoparticles: Effect of Catecholate Binding G. Ramakrishna, † Sandeep Verma, † D. Amilan Jose, ‡ D. Krishna Kumar, ‡ Amitava Das,* ,‡ Dipak K. Palit, † and Hirendra N. Ghosh* ,† Radiation & Photo Chemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400085, India, and Central Salt and Marine Chemicals Research Institute, BhaVnagar 364002, Gujarat, India ReceiVed: September 16, 2005; In Final Form: January 30, 2006 Interfacial electron transfer (ET) dynamics of 5,10,15-trisphenyl-20-(3,4-dihydroxybenzene) porphyrin (TPP- cat) adsorbed on TiO 2 nanoparticles has been studied by femtosecond transient absorption spectroscopy in the visible and near-IR region exciting at 400 and 800 nm. TPP-cat molecule forms a charge transfer (CT) complex with TiO 2 nanoparticles through the catechol moiety with the formation of a five-membered ring. Optical absorption measurements have shown that the Q-band of TPP-cat interacts strongly with TiO 2 due to chelation; however, the Soret band is affected very little. Optical absorption measurements indicate that the catechol moiety also interacts with TiO 2 nanoparticles showing the characteristic band of pure catechol-TiO 2 charge transfer (CT) in the visible region. Electron injection has been confirmed by monitoring the cation radical, instant bleach, and injected electron in the conduction band of TiO 2 nanoparticles. Electron injection time has been measured to be <100 fs and recombination kinetics has been best fitted with a multiexponential function, where the majority of the injected electrons come back to the parent cation radical with a time constant of ∼800 fs for both excitation wavelengths. However, the reaction channel for the electron injection process has been found to be different for both wavelengths. Excitation at 800 nm, found to populate the CT state of the Q-band, and from the photoexcited CT state electron injection into the conduction band, takes place through diffusion. On the other hand, with excitation at 400 nm, a complicated reaction channel takes place. Excitation with 400 nm light excites both the CT band of Cat-TiO 2 and also the Soret band of TPP-cat. We have discussed the reaction path in the TPP-cat/TiO 2 system after exciting with both 400 and 800 nm laser light. We have also compared ET dynamics by exciting at both wavelengths. 1. Introduction The emergence of a dye-sensitized solar cell 1 with conversion efficiency as high as 10.4% from the sensitization of TiO 2 nanoparticles has triggered a massive research interest in the investigations of interfacial electron transfer (ET) from molec- ular adsorbates to wide band gap semiconductor nanoparticles. Efforts are underway in numerous fronts to design a highly efficient and cost-effective solar cell. In this context, new dye molecules have been tried for sensitizing the semiconductor nanoparticles and many more novel dye molecules are being synthesized with better molecular engineering. In this regard maximum effort has been focused on dye molecules based on metal polypyridine complexes and their analogues. 2-5 However, the development of organic sensitizers, which can exhibit performances similar to those of metal complexes, gained a lot of interest in the context of dye-sensitized solar cells. During the past few years, a number of organic dyes, such as phthalocyanines, 5,6 triphenylmethane, 7,8 xanthenes, 9-12 hemi- cyanines, 13,14 coumarins, 15-19 and porphyrins, 20-24 have been tested as sensitizers, but the light-to-electricity conversion efficiency is rather low compared to what has been shown by metal polypyridine complexes. 3-5 It is still unclear why these dyes have limited ability to inject electrons in the conduction band of n-type oxide semiconductors. 21,25 The ruthenium(II) polypyridyl and its various derivatives have so far been regarded as the best sensitizing dyes for solar energy conversion for their strong visible absorption bands, long-lived excited state, and excellent photochemical stability. However, these complexes show poor red/near-infrared light absorption. In view of the rich photochemical and electrochemical properties of porphyrins and their low cost of production and broad absorption in the visible and NIR region, many researchers have taken keen interest in elucidating the factors that limit their efficiency in solar cells based on dye-sensitized nanocrystalline TiO 2 . An induced-photon-to-current conversion efficiency (IPCE) value up to 80% has been demonstrated by Gra ¨tzel and co- workers, 26,27 Wamser and Cherian, 28 and Durrant et al. 21 with porphyrin sensitizers, while others such as Fungo et al. 29 and Koehorst et al. 30 reported lower performance with similar or identical dyes (maximum IPCE 12%). It is a widely known fact that to design a more efficient dye-sensitized solar cell, one has to meet at least two necessary conditions: (a) fast electron injection into the conduction band of the semiconductor and (b) slow charge recombination between the injected electron and the sibling dye cation. Considering the above, it is very important to study interfacial ET dynamics in porphyrin dye- sensitized TiO 2 nanoparticles in the fast and ultrafast time domain. Vast literature 31-34 is available on ET dynamics in the fast and ultrafast time domain on metal polypyridine complexe- sensitized TiO 2 nanoparticles. However, to date not many reports are available on porphyrin dye-sensitized TiO 2 nanoparticles, where ET dynamics is monitored in the femto-/picosecond time domain, except some reports by Tachibana et al. 21 and Yang et * To whom correspondence should be addressed. Fax: 00-91-22- 25505151 (H.N.G.). E-mail: hnghosh@magnum.barc.ernet.in (H.N.G.), amitava@csmcri.org (A.D.). † Bhabha Atomic Research Centre. ‡ Central Salt and Marine Chemicals Research Institute. 9012 J. Phys. Chem. B 2006, 110, 9012-9021 10.1021/jp0552630 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/14/2006