FULL PAPER DOI: 10.1002/ejoc.201300962 Asymmetric Tribranched Dyes: An Intramolecular Cosensitization Approach for Dye-Sensitized Solar Cells Valentina Leandri, [a] Riccardo Ruffo, [a] Vanira Trifiletti, [a] and Alessandro Abbotto* [a] Keywords: Solar cells / Dyes/Pigments / Sensitizers / Conjugation / Multianchoring A new multidonor and multianchoring asymmetric tri- branched organic photosensitizer (i.e., TB-PT) for dye-sensi- tized solar cells containing three different (D–π–D, D–π 1 –A, and D–π 2 –A; D = electron-rich moiety, A = electron-poor moi- ety, π = conjugated bridge) polar branches, two donor cores, and two acceptor/anchoring groups to TiO 2 was investigated Introduction In the last decades, dye-sensitized solar cells (DSSCs) [1] have become very attractive because of their easy and cheap manufacturing procedures combined with over 12% record efficiencies. [2] Metal-free organic photosensitizers, providing several advantages over conventional metal complexes, typi- cally own a linear dipolar D–π–A structure, in which the electron-rich (D) and the electron-poor (A) moieties are connected through a conjugated (π) bridge. [3] Unfortu- nately, these compounds present a single, often narrow, ab- sorption band in the visible region, related to intramolecu- lar charge transfer, which limits sunlight harvesting. Cosen- sitization, that is, the use of two or more dyes having com- plementary absorption spectra, is an attractive strategy to reach a panchromatic absorption and to enhance light har- vesting, [3,4] as in the case of the recent record efficiency. [2] However, conventional intermolecular cosensitization suf- fers from a number of drawbacks: (1) The concentration of each cosensitizing dye on the TiO 2 surface is about half of that in the absence of cosensitization, which thus negates the beneficial effects of improved optical properties. (2) Well-separated complementary absorption bands of the two dyes implies the presence of a region in the cosensitized spectrum in which the absorption intensity is very low. [5] (3) Intermolecular charge- and energy-transfer processes decrease cell performances as a result of intimate mixing of the two dyes. [4] We here propose an original approach consisting of the use of a multibranched dye in which three π-conjugated [a] Department of Materials Science and Milano-Bicocca Solar Energy Research Center – MIB-Solar, University of Milano- Bicocca, INSTM Unit, Via Cozzi 53, 20125 Milano, Italy E-mail: alessandro.abbotto@unimib.it Homepage: http://www.mater.unimib.it/utenti/abbotto Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejoc.201300962. Eur. J. Org. Chem. 2013, 6793–6801 © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 6793 and compared with the corresponding symmetric dyes. TB- PT combines the advantages arising from its π-extended tri- branched architecture and the intramolecular cosensitization approach, which results in enhanced tailored optical and energetic properties and, eventually, photovoltaic perform- ances. branches possess complementary absorption properties (Figure 1, middle). Even if the dye is constituted by a single π-conjugated framework, the net result is equivalent to an intramolecular cosensitization effect. This architecture is the evolution of our previous design based on an A–π 1 –D– π–D–π 1 –A tribranched structural motif (Figure 1, left) in which the presence of two donor and two acceptor/anchor- ing [6] fragments allowed enhanced optical and stability properties. [7] To further improve the optical properties through an intramolecular cosensitization approach, we have now modified the design by introducing two different D–π–A side arms (D–π 1 –A and D–π 2 –A) with complemen- tary absorption properties. The resulting dye can be re- garded, in terms of intramolecular cosensitization, as a multichromophore structure that is derived from the combi- nation of two linear D–π–A dyes linked through donor end- capping moieties by a conjugated bridge. We stress that our approach differs from the other reports of intramolecular cosensitization in that the two D–π–A arms are connected through saturated nonconjugated links, which thus limits π- structure extension. [8] Figure 1. General design strategy for the multibranched dyes: sym- metric tribranched (left), new geometry (middle), monobranched (right). The new asymmetric dye TB-PT (see Figure 2) was de- signed on the basis of the most common building blocks for DSSC sensitizers, that is, D = triarylamino, A = 2-cy- anoacrylic acid, and π 1 and π 2 = benzene and thiophene.