Solar Synthesis: Prospects
in Visible Light Photocatalysis
Danielle M. Schultz and Tehshik P. Yoon*
Background: Interest in photochemical synthesis has been motivated in part by the realization that
sunlight is effectively an inexhaustible energy source. Chemists have also long recognized distinctive
patterns of reactivity that are uniquely accessible via photochemical activation. However, most simple
organic molecules absorb only ultraviolet (UV) light and cannot be activated by the visible wavelengths that comprise most of the solar energy that reaches Earth’s surface. Consequently, organic
photochemistry has generally required the use of UV light sources.
Advances: Over the past several years, there has been a resurgence of interest in synthetic photochemistry, based on the recognition that the transition metal chromophores that have been so productively exploited in the design of technologies for solar energy conversion can also convert visible
light energy into useful chemical potential for synthetic purposes. Visible light enables productive
photoreactions of compounds possessing weak bonds that are sensitive toward UV photodegradation.
Furthermore, visible light photoreactions can be conducted by using essentially any source of white
light, including sunlight, which obviates the need for specialized UV photoreactors. This feature has
expanded the accessibility of photochemical reactions to a broader range of synthetic organic chemists. A variety of reaction types have now been shown to be amenable to visible light photocatalysis via
photoinduced electron transfer to or from the transition metal chromophore, as well as energy-transfer
processes. The predictable reactivity of the intermediates generated and the tolerance of the reaction
conditions to a wide range of functional groups have enabled the application of these reactions to the
synthesis of increasingly complex target molecules.
Outlook: This general strategy for the use of visible light in organic synthesis is already being adopted
by a growing community of synthetic chemists. Much of the current research in this emerging area is
geared toward the discovery of photochemical solutions for increasingly ambitious synthetic goals.
Visible light photocatalysis is also attracting the attention of researchers in chemical biology, materials
science, and drug discovery, who recognize that these reactions offer opportunities for innovation in
areas beyond traditional organic synthesis. The long-term goals of this emerging area are to continue
to improve efficiency and synthetic utility and to realize the long-standing goal of performing chemical synthesis using the sun.
Mechanisms of Visible Light Photocatalysis
Photogeneration of Organic Radicals
Photocatalytic Activation of Amines
Photogenerated Radical Ions
Conclusions and Outlook
C. K. Prier, D. A. Rankic, D. W. C. Macmillan,
Visible light photoredox catalysis with transition
metal complexes: Applications in organic
synthesis. Chem. Rev. 113, 5322–5363 (2013).
J. M. R. Narayanam, C. R. J. Stephenson, Visible
light photoredox catalysis: Applications in organic
synthesis. Chem. Soc. Rev. 40, 102–113 (2011).
T. P. Yoon, M. A. Ischay, J. Du, Visible light photocatalysis as a greener approach to photochemical
synthesis. Nat. Chem. 2, 527–532 (2010).
JACS Beta podcast: http://pubs.acs.org/JACSbeta/
RELATED ITEMS IN SCIENCE
M. T. Pirnot, D. A. Rankic, D. B. C. Martin, D.
W. C. MacMillan, Photoredox activation for the
direct β-arylation of ketones and aldehydes.
Science 339, 1593–1596 (2013).
D. A. Nicewicz, D. W. C. MacMillan, Merging
photoredox catalysis with organocatalysis:
The direct asymmetric alkylation of aldehydes.
Science 322, 77–80 (2008).
A. McNally, C. K. Prier, D. W. C. MacMillan,
Discovery of an α-amino C–H arylation reaction
using the strategy of accelerated serendipity.
Science 334, 1114–1117 (2011).
Ru(bpy)33+ 452 nm MLCT Ru(bpy)3+
Ru*(bpy)32+ D A
energy transfer (path iii)
•Strong visible absorption ( max = 452 nm)
•Efficient generation of Ru*(bpy)32+ ( ex 1)
•Long excited state lifetime ( = 0.9 µs)
•Stability under photolytic conditions
Visible light photocatalysis. (A) Transition metal photocatalysts, such as Ru(bpy)32+, readily absorb visible
light to access reactive excited states. (B) Photoexcited Ru*(bpy)32+ can act as an electron shuttle, interacting
with sacrificial electron donors D (path i) or acceptors A (path ii) to yield either a strongly reducing or oxidizing
catalyst toward organic substrates S. Ru*(bpy)32+ can also directly transfer energy to an organic substrate to yield
electronically excited species (path iii). bpy, 2,2'-bipyridine; MLCT, metal-to-ligand charge transfer.
READ THE FULL ARTICLE ONLINE
Cite this article as D. M. Schultz, T. P. Yoon,
Science 343, 1239176 (2014).
www.sciencemag.org SCIENCE VOL 343 28 FEBRUARY 2014 985
Department of Chemistry, University of Wisconsin–Madison, 1101 University Avenue, Madison, WI 53706, USA.
*Corresponding author. E-mail: firstname.lastname@example.org