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We are indebted to E. Braaten for his crucial input toward the
derivation of the numerical prefactors in Eq. 1 and a critical
reading of the manuscript. We thank M. Robert de Saint Vincent
for contributions in the early stages of the project, M. Sohmen
for experimental assistance, and S. Kokkelmans, E. Cornell,
D. Papoular, F. Werner, I. Bouchoule, I. Chuang, and J. Dalibard
for helpful discussions. This work was supported by the
U.K. Engineering and Physical Sciences Research Council
(grant no. EP/N011759/1), the European Research Council
(QBox), the U.S. Army Research Office (ARO), and the U.S.
Air Force Office of Scientific Research (AFOSR). N.N. acknowledges
support from Trinity College, Cambridge; R.P.S. from the Royal
Society; R.L. from the European Union Marie Curie program
(grant no. MSCA-IF-2015 704832); and M. W.Z. from the AFOSR
Multidisciplinary University Research Initiative on exotic quantum
phases, U.S. NSF, and ARO.
Materials and Methods
Figs. S1 to S4
16 August 2016; accepted 22 December 2016
catalysis through electronically
Eric R. Welin,1 Chip Le,1 Daniela M. Arias-Rotondo,2
James K. McCusker,2 David W. C. MacMillan1*
Transition metal catalysis has traditionally relied on organometallic complexes that can
cycle through a series of ground-state oxidation levels to achieve a series of discrete
yet fundamental fragment-coupling steps. The viability of excited-state organometallic
catalysis via direct photoexcitation has been demonstrated. Although the utility of triplet
sensitization by energy transfer has long been known as a powerful activation mode in
organic photochemistry, it is surprising to recognize that photosensitization mechanisms to
access excited-state organometallic catalysts have lagged far behind. Here, we demonstrate
excited-state organometallic catalysis via such an activation pathway: Energy transfer from
an iridium sensitizer produces an excited-state nickel complex that couples aryl halides
with carboxylic acids. Detailed mechanistic studies confirm the role of photosensitization
via energy transfer.
Developments in transition metal cataly- sis over the past half-century (1, 2) have led to the invention of a vast number of molecular transformations, among which modern cross-coupling chemistry, enantio-
selective hydrogenation, and olefin metathesis
have all been recognized as preeminent bond-
forming reactions in organic chemistry. Note
that all of these organometallic technologies em-
ploy catalysts that function exclusively within a
range of ground-state oxidation levels to perform
the fundamental fragment-coupling steps. Given
the century-old history of organometallic excited-
state (i.e., non–ground state) complexes that can
be accessed via photoexcitation (3), it is remark-
able to consider that the utility of excited-state
metal catalysts remains largely unexploited in
the realm of organic bond–forming reactions
(Fig. 1). As a notable exception, the direct pho-
toexcitation of transition-metal catalysts (with-
out photosensitization) has been disclosed in
elegant studies by Fu and Peters, Nocera, and
Doyle and their colleagues. (4–7).
In the past 5 years, the field of metallaphoto-redox catalysis has undergone widespread growth,
in part, because of the high efficiency with which
second- and third-row transition metals produce
long-lived photoexcited states that can perform
single-electron transfer (SET) with common organometallic catalysts (8–13). At the same time,
these metallophotocatalysts have also found
widespread use as photosensitizers to facilitate
energy transfer with a diverse range of organic
substrates (14–17). Indeed, these studies stand
as a direct analogy to the abundance of organic
excited-state reactions that can be accessed via
organocatalytic photosensitization (e.g., with benzophenone), as popularized in part by Schenck,
Turro, and Hammond and colleagues in the
1950s and 1960s (18–20). Given all of the above,
it is surprising to consider that photoinduced
energy transfer from photocatalysts to transition
metals as a means to access organometallic ex-
cited states has not previously been exploited
as a general activation pathway for reaction
invention—especially in light of the historical
success of energy transfer mechanisms in organic
photochemistry (21–23) (Fig. 1). In this context,
it is important to note that Molander and Doyle
and co-workers have described the light-mediated
arylation of a-oxy C–H bonds, wherein dual-
catalysis photosensitization or direct transition
metal photoexcitation were proposed, respectively,
as the operative catalytic mechanism (7, 24).
We recognized that the application of photosensitization via energy transfer as a mechanism
to switch on or facilitate excited-state organometallic catalysis might offer many opportunities in the field of organic cross coupling (11–14).
Among many benefits, we anticipated that the
use of energy-transfer photosensitization to access organometallic excited states would separate the roles of the light-harvesting complex
from that of the transition-metal coupling catalyst. More specifically, protocols that operate
via the direct photoexcitation of the cross-coupling catalyst are reliant on the visible light
absorption cross section of the catalyst complex—
a photophysical characteristic that is intrinsically
disparate for each catalyst-substrate pair. In contrast, the use of a discrete photosensitization
catalyst would effectively bypass the absorption
cross-section requirement for the organometallic
catalyst, thereby enabling a major improvement
in tunability, efficiency, and, most important, generality in the application of excited-state organometallic catalysts for any given cross-coupling
Triplet sensitization is a long-established catalysis activation platform within the field of
organic photochemistry. It is founded on the
concept that organic substrates in their triplet
excited states can participate in a range of valuable and unique bond-forming reactions. However, most organic molecules cannot readily access
high-energy triplet states due to inefficient intersystem crossing from initially formed singlet excited states and/or vanishingly small absorption
cross sections for direct singlet-triplet photoexcitation. To overcome this issue, photosensitizer
catalysts are used that readily perform a light-harvesting role before a triplet-triplet energy
transfer step with organic substrates—a pathway
that allows efficient access to organic triplet
excited states while bypassing the issues of direct photoexcitation. Quenching of the well-studied chromophore [Ru(bpy)3]2+ (where bpy is
380 27 JANUARY 2017 • VOL 355 ISSUE 6323 sciencemag.org SCIENCE
1Merck Center for Catalysis at Princeton University, Princeton,
NJ 08544, USA. 2Department of Chemistry, Michigan State
University, East Lansing, MI 48824, USA.
*Corresponding author. Email: firstname.lastname@example.org (J.K.M.);
email@example.com (D. W.C.M.)