Bekah Bowers
Chemistry - 429
Photoredox catalysis has presented a bright future when it comes to organic syntheses
like CO
2
reduction and C-C bond creation.
1
The primary focus in transition metal
photoredox catalysis has been on 2
nd
- and 3
rd
-transition metals, i.e. Ru
2+
and Ir
3+
due to
extensive study of their photophysical properties and their long-lived metal-to-ligand
charge transfer (MLCT) states.
2
While Ru and Ir have provided great insight into the
power of photoredox catalysis, problems arise with scalability due to their low earth
abundance. With iron being the most abundant transition metal on earth,
3
our group
has shifted focus on mechanistic studies of Fe
2+
polypyridyls to optimize reactivity, since
they are isoelectronic to the well-studied 2
nd
and 3
rd
row counterparts. In order to
implement earth abundant photocatalysts in organic syntheses, properties relating to
their photocatalytic activity, like excited-state oxidation potential, must be understand
before application in new reactions. However, due to iron’s inability to emit light (lack of
radiative coupling between the
1
A
1
and
5
T
2
), we cannot directly measure the zero-point
energy (E
0,0
) of our Fe(II) chromophores using steady-state emission; therefore, one
possible approach is to perform transient absorption (TA) quenching studies to survey
the excited state oxidation potentials of our complexes.
3
My research utilizes
nanosecond TA to study the ground state recovery lifetimes of various Fe(II)
chromophores upon visible light excitation in the presence of a quencher (single-
electron acceptor). Quenchers with increasing reduction potentials are utilized so that
we can predict the excited-state oxidation potential range of each Fe(II) photocatalysts.
If the reduction potential of the quencher is higher than the excited-state oxidation
potential of our catalyst, then the ground state recovery lifetime will not change.
However, if the reduction potential of the quencher is lower than the oxidation potential
of the Fe(II) chromophore, then the lifetime of the
5
T
2
excited-state will decrease upon
one-electron transfer to the quencher. With this information, we can further apply our
photocatalysts to organic transformations within this oxidation potential.
References:
1. Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light Photoredox Catalysis with
Transition Metal Complexes: Applications in Organic Synthesis. Chem. Rev. 2013, 113,
5322-5363.
2. Arias-Rotondo, D. M.; McCusker, J. K. The Photophysics of Photoredox Catalysis: A
Roadmap for Catalyst Design. Chem. Soc. Rev. 2016, 45, 5803-5820.
3. Woodhouse, M. D.; McCusker, J. K. Mechanistic Origin of Photoredox Catalysis Involving
Iron(II) Polypyridyl Chromophores. J. Am. Chem. Soc. 2020, 142, 16229-16233.
