Dynamics of Interfacial Electron Transfer from Betanin to Nanocrystalline TiO2: The Pursuit of Two-Electron Injection

We report spectroelectrochemical and transient absorption spectroscopic studies of electron injection from the plant pigment betanin (Bt) to nanocrystalline TiO2. Spectroelectrochemical experiments and density functional theory (DFT) calculations are used to interpret transient absorption data in terms of excited state absorption of Bt and ground state absorption of oxidation intermediates and products. Comparison of the amplitudes of transient signals of Bt on TiO2 and on ZrO2, for which no electron injection takes place, reveals the signature of two-electron injection from electronically excited Bt to TiO2. Transient signals observed for Bt on TiO2 (in contrast to ZrO2) on the nanosecond time scale reveal the spectral signatures of photo-oxidation products of Bt absorbing in the red and the blue. These are assigned to a oneelectron oxidation product formed by recombination of injected electrons with the two-electron oxidation product. We conclude that whereas electron injection is a simultaneous two-electron process, recombination is a one-electron process. The formation of a semiquinone radical through recombination limits the efficiency and long-term stability of the Bt-based dye-sensitized solar cell. Strategies are suggested for enhancing photocurrents of dye-sensitized solar cells by harnessing the two-electron oxidation of organic dye sensitizers. ■ INTRODUCTION Although dye-sensitized solar cells offer a promising alternative to the present generation of photovoltaic devices, their economical and environmental advantages are offset by reliance on synthetic organic and metal-organic sensitizers. In the latter category, Ru-based dyes offer stability advantages owing to the reversible redox chemistry of the Ru center, but ruthenium is a rare and expensive metal. Consequently, there is much interest in replacing synthetic sensitizers with organic pigments derived from plants. One-electron oxidation of organic compounds, on the other hand, results in the formation of free radicals, which can pose stability problems for the solar cell. In the present work, we explore the question of harnessing the proton-coupled two-electron oxidation of a plant-based dye sensitizer, betanin, which we and others have previously used in TiO2-based dye-sensitized solar cells (DSSCs). Betanin (Bt, I of Figure 1) belongs to the betalain family of plant pigments, which comprises the purple betacyanins and yellow betaxanthins. These pigments are found in plants of the order Caryophyllales and play photoprotective and antioxidant roles. Betacyanins are Schiff base adducts of betalamic acid (II of Figure 1) with cyclo-DOPA derivatives such as cyclo-DOPA 5-O-glucoside in the case of Bt. The yellow betaxanthins, of which indicaxanthin (In, III of Figure 1) is an example, are Schiff base linkages of betalamic acid with amino acids or amines. Owing to their nutritional advantages as antioxidants and their applications as food colorants, the redox properties of betalain compounds have been of great interest to the food science community. The radical scavenging ability of betacyanins is correlated to the phenolic groups, and recently the molecular details of their oxidation have begun to emerge. In ref 15, three oxidations of Bt were detected at 0.62, 0.84, and 1.2 V NHE, whereas two oxidations of In were observed at 0.84 and 1.2 V. This indicates that the first oxidation of Bt involves the aromatic ring or the sugar group. An early study by Martıńez-Parra and Muñoz used horseradish peroxidase to oxidize betanin and found an oxidation intermediate with an absorption maximum similar to that of betanin (∼530 nm) and a yellow final oxidation product (λmax = 446 nm), which they assigned to betalamic acid. However, more recent studies assign λmax of betalamic acid to be about 420 nm. Wybraniec et al. have reported enzymatic and electrochemical oxidation of betanin and its aglycone betanidin (Bd). The electrochemical oxidation of Bd was found to proceed by a two-electron, one-proton process in the pH range of 3−5 and by a two-electron, two-proton process in the pH range of 6−8. Enzymatic oxidation of Bt was observed to lead Received: June 19, 2015 Revised: July 28, 2015 Published: July 30, 2015 Article