Efficient Light-Harvesting Framework Uses Photosynthetic Bacterium-Based Design

Scientists from Japan have developed a hybrid biomimetic light-harvesting framework which involves many light-absorbing molecules, spread out over a large surface area, focusing their harvested energy onto a single reaction centre, thereby achieving high light-harvesting efficiency.

Traditional light-harvesters, generally silicone-based, have mostly had one single complex molecule carry out several tasks – the same complex absorbs light, separates charges to generate current flow, and conducts the charge carriers to current-collecting centres. Such multifunctional complexes are costly to make. Also, the entire absorption-conduction process takes only nanoseconds to complete, whilst the time difference between two photons hitting one molecule is about 0.1 seconds due to sunlight being a dilute energy source. This means that the multifunctional molecules sit idly most of the time, making such designs inefficient and uneconomical.

Yamamoto et al’s design, inspired by the photosynthetic mechanism of purple bacteria, contains 3 species of specialized molecular subunits – biphenyl-bridged mesoporous organosilica (Bp-PMO) which function as light-absorbers, rhenium (I) pentamer units which function as light-concentrators, and a ruthenium (II) trisdiimine core which function as the reaction centre. The 5 Rh (I) oligomers, covalently bonded to the Ru (II) core, form a metal complex, which is adsorbed onto the  surface of the 440-unit Bp-PMO structure. The Bp-PMO’s electronic structure causes it to preferentially absorb high-energy light while the Rh (I) and Ru (II) absorb low- and lower-energy light respectively, establishing an energy gradient that allows the large array of Bp-PMOs to “funnel” harvested photonic energy through the Rh (I) pentamer to the single Ru (II) reaction centre. This molecular specialization effectively reduces the number of molecular building blocks needed to harvest the same amount of light.

The authors have indicated that this framework may be incorporated into photocatalytic or photovoltaic designs in the future.

Source: Y Yamamoto et al, Chem. Sci., 2013, DOI: 10.1039/c3sc51959g