The light absorber or sensitizer is one of the most important components of dye-sensitized solar cells (DSCs). Adequate engineering of this material allows DSCs to achieve a fine balance among higher solar energy-to-electricity conversion efficiency, lower manufacturing costs, and better long-term stability. The most efficient DSCs to date are fabricated with transition metal based sensitizers. [1] For example, Grätzel and co-workers recently demonstrated that a Zn II porphyrin with donor-acceptor substituent shows a remarkable power conversion efficiency of h % 13 % under illumination with standard AM 1.5G simulated sunlight. [2] Furthermore, many Ru II sensitizers were also known to attain efficiencies greater than 10 %, [3] long before the discovery of the above Zn II dye. Besides these successes, a few quaterpyridine Ru II sensitizers showed notable absorption in the far-red to near-infrared (NIR) region, [4] with the potential to harvest lower energy protons needed for higher current density.With a view to harvesting lower energy photons, Os IIbased sensitizers seem to be an excellent option for expanding the spectral response well into the NIR region. [5] First, Os II polypyridine complexes tend to show lower energy metal-toligand charge-transfer (MLCT) transition, as a consequence of the lower oxidation potential compared to their Ru II counterparts. [6] In addition, larger spin-orbit coupling for the heavier Os II cation, in theory, induces nontrivial absorption of the 3 MLCT states extended to even lower energy. Thus, appropriately designed Os II sensitizers should display a much broader absorption profile and faster electron injection from both nonthermalized 1 MLCT and thermalized 3 MLCT excited states. [7] We expect that such a photophysical property should be important to both the DSC community and groups whose interests are in developing sensitizers for water splitting with dye-sensitized oxide semiconductors. [8] In this study, the design of Os II sensitizers conceptually takes advantage of our previously reported Ru II sensitizer TF-1, which contains 4,4',4''-tricarboxy-2,2':6,2''-terpyridine (H 3 tctpy) and dianionic 2,6-bis(1,2-pyrazol-5-yl)pyridine chelating ligands (Scheme 1). [9] This Ru II -based sensitizer showed panchromatic absorption extending to 830 nm and an oxidation potential of 0.94 V versus the normal hydrogen electrode (NHE) that ensures efficient regeneration of the oxidized sensitizers. However, if the identical architecture were adopted, the oxidation potential of the corresponding Os II sensitizer is predicted to be much less positive. [6,10] This hurdle can be circumvented by replacing pyrazolate with triazolate with aim of decreasing the electron density at the central Os II ion. This hypothesis is supported by the prior preparation of a relevant triazolate-based Ru II sensitizer, namely, TF-5 (see Scheme 1). The oxidation potential of TF-5 is shifted to 1.19 V (vs. NHE), which is 0.25 V higher in energy than that of TF-1.Encouraged by this preliminary result, we focused ...
