DOI: 10.1002/cphc.200800752 Electron Transfer from Photoexcited TiO 2 to Chelating Alizarin Molecules: Reversible Photochromic Effect in Alizarin@TiO 2 under UV Irradiation Yesica Di Iorio, [a] Marta A. Brusa, [a] A. Feldhoff, [b] and María A. Grela* [a] 1. Introduction Photosensitization of large-band-gap semiconductors is an ex- tremely important phenomenon which is central to the devel- opment of electrochromic and artificial photosynthetic devi- ces. [1–4] Organic molecules containing catechol moieties form coloured charge-transfer complexes with TiO 2 particles in the nanosize regime. [5, 6] It was previously shown by us and others that excitation of the charge transfer bands is a useful strategy for extending the semiconductor response into the visible region. [6, 7] The strong interaction between the nanoparticles and the enediol molecules confers these systems with high stability to both UV and visible irradiation, in sharp contrast with the behaviour observed for the adsorbed species on mi- croparticulate systems. [8] However, simultaneous coupling of an acceptor to titanium dioxide nanoparticles modified with catechol derivatives can substantially improve the efficiency of charge separation and allow the design of efficient triad-type assemblies. For example, Rajh and co-workers have shown that simultaneous chelation of pyrroloquinoline quinone (PQQ) and dopamine (DA) to TiO 2 nanoparticles results in spatial separa- tion of the photogenerated charges. [9] In this system, PQQ acts as a sink for the electrons injected into the conduction band after visible excitation of dopamine. We also recently demon- strated that, similar to the DA@TiO 2 @PQQ system, addition of Cr VI to a sol of alizarin red (AR)-modified TiO 2 nanoparticles allows construction of the triad AR@TiO 2 @Cr VI . [7] Remarkably, the efficiency of charge separation by visible-light excitation in the presence of air increases from 10 4 to 0.3 for after attach- ment of Cr VI to AR@TiO 2 . Alizarin (A) and its soluble derivative AR have particular properties that make the coupled TiO 2 system different from those with other enediol ligands. For instance, its excited states lie at the border of the conduction band, a fact that ac- counts for a clear maximum in the visible spectrum. Another special feature of alizarin systems is that they have two redox- active quinonoid moieties, namely, the 9,10-dioxo and the 1,2-catechol-like fragments, which may respectively act as ac- ceptor and donor in electron-transfer processes. Here we examine the reduction of alizarin molecules cou- pled to TiO 2 nanoparticles under UV irradiation and discuss the significance of our results to the field of photocatalysis and in the development of photovoltaic cells and photoelectrochro- mic displays based on anthraquinonic dyes. [10, 11] Experimental Section Chemicals: All chemicals were of the highest purity available. 1,2-dihydroxy-9,10-anthracenedione (alizarin, A) was purchased from Sigma-Aldrich and used as received. Phenylglyoxylic acid (Sigma) was recrystallized from carbon tetrachloride (Merck). All solutions were prepared using ultrapure water (resistivity 18 MW cm). HClO 4 (Merck, p.a. 60%), was used to adjust the pH. Ti- tanium tetraisopropoxide (99.999 %, Aldrich), and 2-propanol (Merck) were used in the synthesis of amorphous colloidal TiO 2 . Preparation of Colloidal TiO 2 Nanoparticles: Colloidal TiO 2 was pre- pared by controlled acid hydrolysis of titanium tetraisopropoxide following standard procedures. [12] Briefly, TiACHTUNGTRENNUNG[OCHACHTUNGTRENNUNG(CH 3 ) 2 ] 4 (1 mL) was dissolved in 2-propanol (20 mL) and the solution slowly added to an aqueous solution of HClO 4 (200 mL, pH 1.5) maintained at 1 8C. The solution was continuously stirred in the dark, for 48–72 h to [a] Y. Di Iorio, Dr. M. A. Brusa, Dr. M. A. Grela Departamento de Química Facultad de Ciencias Exactas y Naturales Universidad Nacional de Mar del Plata B7602AYL Mar de Plata - Buenos Aires (Argentina) Fax: (+ 54) 223-4753150 E-mail : magrela@mdp.edu.ar [b] Dr. A. Feldhoff Institut für Physikalische Chemie und Elektrochemie Leibniz Universität Hannover Callinstraße 3-3, 30167 Hannover (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200800752. Reduction of alizarin molecules coupled to TiO 2 nanoparticles (A@TiO 2 ) occurs on UV irradiation in the presence of a sacrifi- cial electron donor. Evidence is presented that reduction is mediated by conduction-band electrons and yields a 1,2,9,10- tetrahydroxyanthracene species which remains coupled to the TiO 2 nanoparticles. The spectrum of the reduced complex dis- plays two overlapping broad bands centred at 480 and 650 nm which can harvest visible photons besides 900 nm, in agreement with theoretical predictions by TDDFT. The poten- tial relevance of the dual-redox behaviour of strongly TiO 2 cou- pled anthraquinone dyes in the field of photocatalysis and in connection with their utilization in the development of dye- sensitized TiO 2 solar cells is briefly discussed. ChemPhysChem 2009, 10, 1077 – 1083  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1077