Hurry Up and Wait: Managing the Inherent Mismatches in Time Scales in Natural and Artificial Photosynthetic Systems

Because single catalysts cannot selectively convert CO2 into a specific reduced product more complex than CO either electrochemically or photoelectrochemically, an alternative is to use multiple catalysts organized into a cascade, which offers a means of generating complex chemicals with improved specificity by providing a channel for the reduction of CO and similar intermediates. The rules for how to select catalysts for this purpose and design systems that have a high degree of chemical control using them are not known however. The efficiency and selectivity for individual catalysts may be optimized, but the resulting efficiency of an entire system will depend on how well the timings of all the chemical steps in the cascade as well as transport between the catalysts are managed. In this Perspective, we discuss these challenges and examine the ways in which control is exerted in natural photosynthetic systems which use cascaded reactions to convert CO2 from the air into sugars using sunlight as the sole source of energy. These cascades take place within complex architectural elements that ensure that the chemistry is as efficient as possible. Adaptations of the functions of these natural design elements to artificial systems may offer ways to attain the promise of artificial photosynthesis.