SPIN CHEMISTRY
Readout of spin quantum beats in a charge-separated
radical pair by pump-push spectroscopy
David Mims^1 , Jonathan Herpich^1 , Nikita N. Lukzen^2 , Ulrich E. Steiner^3 , Christoph Lambert1,4
Spin quantum beats prove the quantum nature of reactions involving radical pairs, the key species of
spin chemistry. However, such quantum beats remain hidden to transient absorption–based optical
observation because the spin hardly affects the absorption properties of the radical pairs. We succeed in
demonstrating such quantum beats in the photoinduced charge-separated state (CSS) of an electron
donor–acceptor dyad by using two laser pulses—one for pumping the sample and another one, with
variable delay, for further exciting the CSS to a higher electronic state, wherein ultrafast recombination
to distinct, optically detectable products of singlet or triplet multiplicity occurs. This represents a spin
quantum measurement of the spin state of the CSS at the time instant of the second (push) pulse.
C
hemical reactions of radical pairs (RPs)
are exceptional in that they can exhibit
pronounced nonclassical kinetics, char-
acterized by quantum beats of inter-
mediate populations, as opposed to
monotonic processes for elementary chemical
reactions in classical kinetics. Such behavior of
RPs is intimately related to what is known as
spin chemistry ( 1 , 2 ), a field dealing with the
interrelation between electron spin motion
and chemical reactivity—a phenomenon that
has been accounted for by the so-called radical
pair mechanism (RPM). The RPM has been
suggested independently by Kaptein and
Oosterhoff ( 3 )andbyCloss( 4 ) when explain-
ing the occurrence of unusual nuclear spin
polarization during certain chemical reactions
involving RP intermediates by chemically in-
duced dynamic nuclear polarization (CIDNP).
Spin chemistry is the physical basis for many
magnetic field–sensitive chemical reactions,
including biophysical processes ( 5 )—in par-
ticular, magnetoreception in migratory birds
( 6 ), where the RPM is supposed to function
as a molecular mechanism for the magnetic
compass sensor in the avian retina ( 7 , 8 ),
representing a prominent example of quan-
tum biology ( 9 ).
In RPs, the two electron spins may be
aligned antiparallel to result in a spin singlet,
with a total spin of zero, or parallel—i.e., a spin
tripletwithatotalspinof1,forwhichthree
distinct spatial orientations are possible. Ac-
cording to the Wigner-Witmer rules ( 10 , 11 ),
electron spin is conserved during chemical
reactions of the RP. Thus, RPs with singlet
spin can only react to singlet products, and
RPs with triplet spin can only react to triplet
products. This rule represents the key prin-
ciple of the RPM. Notably, the spin selectivity of
a reaction has also been interpreted in terms
of a quantum measurement of the spin state
( 12 , 13 ). As long as a RP stays together, either
because of a solvent cage or by virtue of a
chemical link ( 14 ), the electron spin state can
change as a result of magnetic interactions—e.g.,
the hyperfine interaction with magnetic nuclei
and/or the Zeeman interaction with external
magnetic fields. The latter allows for direct
control of the reaction behavior of the RP.
Because the change of electron spin under
magnetic interactions is a genuine quantum
process, it can exhibit quantum oscillations
that, by virtue of the spin selectivity rule,
may be transmitted to the chemical reaction
kinetics. The first studies on the observation
of quantum oscillations in the recombination
of RPs came from the field of radiolumines-
cence ( 15 – 17 ). In radioluminescence, the primary
ionizing processes occur with the solvent, and
charge and spin are transferred to fluorescent
solvent molecules in secondary events. How-
ever, in most studies of spin chemistry, the RPs
of interest are directly produced by photo-
chemical reactions, such as photoinduced
electron transfer or photocleavage of bonds.
In such cases, experimental demonstrations
of quantum oscillations in the kinetics of
RPs are rather rare. To our knowledge, there is
only one study ( 18 ) on time-resolved transient
absorption (TA) experiments that shows the
intrinsic kinetic quantum oscillations in RPs.
This is because in many experiments, only the
yields into the various reaction channels are
recorded, and therefore spin oscillations are
wiped out in a time-integrated type of observ-
able. On the other hand, in time-resolved ex-
periments of RP recombination kinetics, the
spin motion is hidden because the recombi-
nation kinetics is usually too slow to follow the
spin motion. There are some examples show-
ingthatspinmotioninRPscanbeaffectedin
a time-resolved manner by applying resonant
microwave pulses [reaction yield–detected mag-netic resonance (RYDMR)] ( 19 – 21 )orbypulsed
switching of steady magnetic fields. Another
technique, whereby quantum beats in RPs
have been shown by Kotheet al.( 22 ), is time-
resolved electron paramagnetic resonance
(EPR). With time-resolved EPR, the com-
bined effect of an external magnetic field
and resonant microwave radiation turns ini-
tial singlet-triplet coherences into oscillating
magnetization.
In this paper, we report a method to follow
the intrinsic, unperturbed motion of the RP
spin system. The proposed method can be
characterized as an optical readout technique
with a pump pulse for the photochemical
creation of a charge-separated state (CSS)
type of RP and a second, so-called push pulse
( 23 ) of variable delay probing the spin state.
However, in contrast to the familiar pump-
probe method, where the probe pulse is of
much weaker intensity than the pump pulse
and does not appreciably change the state
populations of the system, the second pulse
in our method excites a considerable fraction
of the populations to a higher excited state.
Thus, a fast quantum measurement of the spin
state takes place by virtually prompt irreversible
recombination into the singlet or triplet product
channel or by nonradiative deactivation to the
initial CSS-RP state. The method exploits the
fact that the rate of charge recombination (CR)
is drastically enhanced by electronic excitation
of the CSS. There have been several examples
of this observation in the literature ( 24 – 27 );
however, they have been restricted to a single
push pulse at a fixed delay time. Furthermore,
spin aspects have only been considered in a
paper from the Wasielewski group ( 28 ), where
the second laser pulse induced charge transport
toanearbyacceptor,andtheconservationof
zero quantum coherence was proved by EPR.
In our method, the delay time of the push
pulse is systematically varied, and therefore
information on the intrinsic spin evolution
can be collected in a quasi-continuous manner
as a function of time. Thus, by investigating
the photoinduced CSS state of a molecular
dyad TAA-An-PDI—consisting of a triarylamine
electron donor (TAA) and a perylene diimide
acceptor (PDI) linked by a dihydroanthra-
cene (An) bridge—we succeeded in directly
tracking the quantum oscillations between
singlet and triplet spin states by an optical
absorption method.Pump-push TA experiments
In a laser flash TA experiment, excitation of the
TAA-An-PDI dyad (Fig. 1A) in anisole solution
at 18,800 cm−^1 with 7-ns laser pulses populated
the PDI singlet state, which induced charge
separation to yield the CSS. In the CSS, the
PDI was reduced, and the TAA was oxidized
(TAA.+-An-PDI.−) (Fig. 1C), as proved by the
characteristic excited state absorption (ESA)1470 17 DECEMBER 2021•VOL 374 ISSUE 6574 science.orgSCIENCE
(^1) Institute of Organic Chemistry, University of Würzburg,
97074 Würzburg, Germany.^2 International Tomography
Center and Novosibirsk State Universit, Novosibirsk 630090,
Russia.^3 Department of Chemistry, University of Konstanz,
78464 Konstanz, Germany.^4 Center for Nanosystems
Chemistry, University of Würzburg, 97074 Würzburg,
Germany.
*Corresponding author. Email: [email protected]
(U.E.S.); [email protected] (C.L.)
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