Antenna molecule drives solar hydrogen generation

Abstract

It would be convenient if gigantic reservoirs of molecular hydrogen H 2(g) existed in the earth's crust that could be safely tapped as an energy source. Instead, most hydrogen found near the earth's surface is bound up in water as protons, H + (aq) that must be reduced before utilization as a fuel (1). When the electrons for proton reduction come from the oxidation of water to O 2(g) , this redox chemistry is commonly referred to as "water splitting": 2H 2 O ðaqÞ → 2H 2ðgÞ + O 2ðgÞ ðΔG°= 4.92eVÞ.[1]As shown in Eq. 1, water splitting is thermodynamically uphill and hence provides a means for storing energy in chemical bonds. The reverse reaction produces water and energy. The identification of an inexpensive scalable process for water splitting has been a "holy grail" of science for decades that could one day enable a "hydrogen economy" (2). Electrolysis accomplishes water splitting, but practical utility requires an inexpensive source of electrical power that ideally would not involve greenhouse gas formation from fossil fuel combustion. Electricity generated from wind or photovoltaic panels could be used for this purpose, but this does not yet appear to be cost-effective. Approaches that integrate solar energy harvesting and catalysis are particularly attractive as they afford the real possibility for inexpensive production of H 2(g) . In 1972, Fujishima and Honda (3) reported sustained water splitting from a relatively simple photoelectrochemical cell based on TiO 2 and Pt electrodes separated by a membrane (Fig. 1). The drawback was that sunlight absorption was limited to the UV region, comprising less than 3% of the solar spectrum. Since that time, alternative semiconductor materials have been identified that effectively harvest sunlight and split water more efficiently (4, 5); however, the associated costs are not economically competitive with today's fossil fuels. In a step toward simple low-cost approaches to solar hydrogen generation, Zheng et al. (6) report sustained H 2(g) generation over periods of 12 d with ∼40,000 turnovers by a dye molecule anchored to TiO 2 /Pt particles in an aqueous suspension. The turnover numbers are unprecedented for solar fuel production and may enable practically useful molecular approaches to solar hydrogen production.Light absorption by "dyads" drives the newly discovered hydrogen generation (6). The particular dyad that proved to be the most efficient is shown in Fig. 1 and referred to herein as the "Zheng dyad." It is composed of an organic dye molecule (Bodipy) covalently linked to a transition metal complex (PtN 2 S 2 ). The transition metal complex absorbs light across much of the visible region, but only weakly. In contrast, the organic Bodipy dye has a sharp intense absorption band in the green region with an extinction coefficient that is about two orders of magnitude larger. When linked together in the dyad, the optical and redox properties were well modeled as sum of the two individual parts, indicative of weak electronic coupling. Nevertheless, rapid energy tra...

Supramolecular Control of Singlet Oxygen Generation

“…Dyads are molecular constructs designed to perform specific functions by integrating two independently active units. The independent chemical units are integrated to produce a desired chemical function, such as an antenna chromophore that captures energy (photon) and efficiently transfers it (ET) to a metal complex capable of splitting water (2H 2 O → 2H 2 + O 2 ) [ 69 ]. Examples of dyads with C 60 as one of the dyad components have been constructed as it improves triplet lifetimes of the sensitizing unit through increased ISC rates resulting from energy or electron transfer [ 66 , 70 , 71 , 72 ].…”