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.