Strongly Entangled Triplet Acyl-Alkyl Radical Pairs in Crystals of Photostable Diphenylmethyl Adamantyl Ketones

Article abstract of DOI:10.1021/jacs.1c03026

Radical pairs generated in crystalline solids by bond cleavage reactions of triplet ketones offer the unique opportunity to explore a frontier of spin dynamics where rigid radicals are highly entangled as the result of short inter-radical distances, large

Full text of DOI:10.1021/jacs.1c03026

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Strongly Entangled Triplet Acyl−Alkyl Radical Pairs in Crystals of
Photostable Diphenylmethyl Adamantyl Ketones
Jin H. Park,† Vince M. Hipwell,† Edris A. Rivera, and Miguel A. Garcia-Garibay*
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ABSTRACT: Radical pairs generated in crystalline solids by bond cleavage
reactions of triplet ketones offer the unique opportunity to explore a frontier of
spin dynamics where rigid radicals are highly entangled as the result of short inter-
radical distances, large singlet−triplet energy gaps (ΔEST), and limited spin−lattice
relaxation mechanisms. Here we report the pulsed laser generation and detection of
strongly entangled triplet acyl−alkyl radical pairs generated in nanocrystalline
suspensions of 1,1-diphenylmethyl 2-ketones with various 3-admantyl substituents.
The sought-after triplet acyl−alkyl radical pairs could be studied for the first time in
the solid state by taking advantage of the efficient triplet excited state α-cleavage
reactions of 1,1-diphenylmethyl ketones and the slow rate of CO loss from the acyl
radicals, which would have to generate highly unstable phenyl and primary alkyl
radicals or relatively unstable secondary and tertiary alkyl radicals. With the loss of
CO prevented, the lifetime of the triplet acyl−alkyl radical pair intermediates is
determined by intersystem crossing to the singlet radical pair state, which is followed by immediate bond formation to the ground
state starting ketone. Experimental results revealed biexponential kinetics with long-lived components that account for ca. 87−92%
of the transient population and lifetimes that extend to the range of 53−63 μs, the longest reported so far for this type of radical pair.
Structural information inferred from the starting ketone will make it possible to analyze the affects of proximity and orientation of
the singly occupied orbitals and potentially help set a path for the use of triplet radical pairs as qubits in quantum information
technologies.
electron−nuclear interactions ineffective. Under these con-
ditions, spin−orbit coupling is likely to operate even if the
geometry of the singly occupied orbitals is not ideal and the
closed-shell electronic states are likely to be much higher in
energy in a nonpolar medium with a very low dielectric
constant. Furthermore, short inter-radical distances and high
radical asymmetry may lead to large zero-field splitting (ZFS)
parameters such that equilibration between the spaced triplet
sublevels (Tx, Ty, and Tz) may be less likely. On the basis of
this analysis we expected a triplet acyl−alkyl radical pair (3RP-
1) to have significantly longer lifetimes in the solid state than
in solution. An appealing feature of our hypothesis is that long-
lived and well-characterized spin states are promising platforms
for quantum information science (QIS). In fact, molecular
triplets formed by thermal activation or photoinduced charge
transfer mechanisms have shown promise as spin qubit pairs
(SQPs).19−23 In contrast, strongly interacting transient qubits
INTRODUCTION
■
Reactions in crystals provide unique opportunities to explore
chemical kinetics and spin dynamics under conditions that are
not possible in any other media.1−10 For example, using
crystalline tetraarylacetones (1, R = CHAr2; Scheme 1), we
have shown that pulsed laser excitation of nanocrystals
suspended in water11 makes it possible to detect triplet radical
3
pairs RP-2 formed by the loss of CO and to follow their
subsequent decay to the corresponding photoproduct 2 (R =
CHAr2).12 The reaction starts by electronic excitation (step 1)
and is followed by intersystem crossing (step 2) to the triplet
excited state (31*), which goes on to cleave an α-bond (step 3)
to form a strongly correlated triplet acyl−alkyl radical pair
(3RP-1). When R = CHAr2, there is a rapid spin-conserving
loss of CO to form the relatively long lived triplet alkyl−alkyl
3
radical pair RP-2 (step 4), which goes on to the product 2
1
(step 7) through the singlet radical pair RP-2 via a rate-
limiting intersystem crossing step (step 6) shown to occur with
time constants between 40 and 90 μs depending on Ar.13,14
These rather long lifetimes are a strong indication that short
inter-radical distances enforced by molecular crystals and the
ensuing large singlet−triplet energy gaps (ΔEST) create an
entangled spin system with limited intersystem crossing
mechanisms.15−18 As indicated in Figure 1, large ΔEST values
render an intersystem crossing mechanism based on hyperfine
Received: April 8, 2021
Published: June 3, 2021
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© 2021 American Chemical Society
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significant difference in the g factors between the alkyl and acyl
radical partners and greater spin−orbit coupling (SOC) in the
carbonyl-containing 3RP-1 provide intersystem crossing
Scheme 1
3
features that are not available to RP-2. In addition, a critical
difference in the solid state in comparison to solution comes
from the short electron−electron distances and high rigidity of
crystals, which also limit potential spin−lattice relaxation
mechanisms.
To prevent the chemical reaction and explore the lifetimes of
acyl−alkyl radical pairs, we selected the diphenylmethyl
adamantyl ketones in Scheme 1. A diphenylmethyl substituent
was chosen both to facilitate a fast and efficient α-cleavage
reaction and to take advantage of the diphenylmethyl radical as
a spectroscopic handle in pump−probe experiments carried
out at 266 nm with the fourth harmonic of a nanosecond
Nd:YAG laser. We selected adamantane as a building block in
ketones 1b−d to impart crystallinity and for the acyl radical
generated upon light excitation to have a primary (1b),
secondary (1c), or tertiary (1d) carbon at the α position.25
Acyl radicals with similar substituents in solution are known to
lose CO with relatively long time constants of ca. 5 ms and 70
and 1.9 μs (Table 1).26,27 The tertiary radical that would be
Table 1. Crystalline Ketones That Generate Transient
Acyl−Alkyl Radical Pairs
lifetime (μs) MeCN/
a
c
ketone
R
mp (°C)
solid
τ−CO (μs)
b
d
1a
1b
1c
1d
Ph
66.5−67.9
102.3−103.2
89.4−89.7
104.4−105
1.3/0.6, 1.5 (97)
∼1013
b
d
1-Adam
2-Adam
3-Adam
1.2/2.3, 53 (92)
1.2/3.5, 61 (87)
1.2/1.8, 63 (87)
∼5000
b
70
1.9
b
a
Measurements in solids were carried out in aqueous nanocrystalline
b
suspensions. Percentage of radical decay accounted for by the long-
lived component. Decarbonylation rates for analogous free acyl
c
radicals that generate phenyl, primary, secondary, and tertiary alkyl
d
radicals.26,27 Measurements made in the gas phase.
Figure 1. Energetics of an α-cleavage reaction in a triplet excited state
crystalline ketone leading to the formation of the triplet acyl−alkyl
radical pair 3RP-1 that is forced to remain within close to bond-
distance proximity. The high rigidity and short inter-radical distance
lead to a large ST energy gap and large zero-field splitting (ZFS) that
energetically separate the Tx, Ty, and Tz triplet sublevels, all of which
inhibit the mechanisms for intersystem crossing.
generated by decarbonylation of 1d would be at the
bridgehead position of the adamantyl group and therefore
would be less stable than typical tertiary alkyl radicals.28
Ketone 1a is expected to generate a benzoyl radical with a
completely different electronic structure that is essentially
stable toward the loss of CO, with an extrapolated time
constant of ca. 0.32 years at 298 K.27 While the rate of reaction
from 3RP-1 to 3RP-2 by the loss of CO is likely to be different
in crystals, in the absence of product formation we should be
able to determine the intersystem crossing kinetics of the spin-
resulting from photoinduced σ bond cleavage reactions
constitute an alternative platform that has never been explored.
With that in mind, useful photochemically responsive QIS
materials should be able to (1) generate strongly entangled and
spin-polarized triplet radical pairs upon the action of a short
laser pulse,24 (2) favor conditions where the triplet radical
pairs live sufficiently long for logic gate manipulation and
readout using microwaves and/or optical methods, and (3)
utilize a molecular unit that is reusable by virtue of the radical
pair re-forming the starting material so that the corresponding
qubits may be available multiple times upon demand. In this
paper we explore a strategy based on the selection of the R
group in Scheme 1 to prevent the formation of 3RP-2 in order
to avoid product formation and in the process explore the
lifetime of the triplet acyl−alkyl radical pairs 3RP-1. It is
known that strongly interacting triplet acyl−alkyl spin systems
measured in short-chain biradicals have lifetimes that are much
shorter than those of analogous dialkyl spin systems. A
3
entangled RP-1.
RESULTS AND DISCUSSION
■
The synthesis of each ketone began with the reaction of
commercially available diphenylmethane with n-butyllithium in
tetrahydrofuran at 0 °C to form the corresponding lithium
carbanion.29 The appropriate acid chloride was added
dropwise after stirring for 30 min. Acid chlorides were
obtained by treatment of the corresponding acetic acid
derivative with oxalyl chloride.30 All four ketones were
obtained in isolated chemical yields varying between 16%
and 60% and were characterized by conventional spectroscopic
methods (Supporting Information). Colorless crystals of 1a−d
were obtained by slow evaporation from ethanol, and melting
points were shown to vary from ca. 66.5 °C in the case of 1a up
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to 104.4 °C in the case of 1d, which makes them suitable for
solid-state spectroscopic measurements at ambient temper-
ature.
A single crystal of 1d obtained from the ethanol
recrystallization was analyzed by X-ray diffraction and the
structure was solved in the monoclinic space group P21/c with
two molecules with very nearly identical conformations in the
asymmetric unit. The structure of one the two molecules in the
crystal shown in Figure 2 has normal bond lengths and bond
Figure 3. Powder X-ray diffractogram of 1-((3R,5R,7R)-adamantan-1-
yl)-2,2-diphenylethan-1-one (1d) in the bulk solid (bottom) and in
nanocrystalline suspensions (middle) and the simulated pattern from
the single crystal X-ray structure (top).
The strict solid-state photostability of crystalline ketones
1a−d was confirmed by extended exposure (up to 24 h) to UV
light from a 500 W Hanovia medium-pressure mercury lamp,
under conditions where complex reaction mixtures were
obtained in solution. Interestingly, α-adamantyl ketone 1b
displayed no formation of Norrish−Yang cyclization products
in the solid state, despite the presence of abstractable
adamantane γ-hydrogen atoms,32 highlighting a significant
kinetic preference for the α-cleavage reaction. Pump−probe
experiments in solution and in nanocrystalline suspension were
conducted with a single-pass flow system. Solution samples
were sparged with argon or nitrogen gas for 1 h in order to
remove dissolved oxygen. In the case of the nanocrystalline
suspensions, DLS data were used to verify that suspensions
remained stable for the duration of the pump−probe laser
experiments. As shown in Figure 4 for ketone 1c, and as
indicated in Table 1 for all four ketone samples, both solution
and nanocrystalline suspensions produced a transient with a
λmax value in the range of 320−340 nm that is consistent with
the expected diphenylmethyl radical. The spectrum obtained
with nanocrystalline suspensions (Figure 4, bottom) showed a
substantially reduced intensity below 320 nm as a result of a
large increase in light scattering and decreased transmittance at
the shorter wavelength region.
Similar solution transients were observed for all four
ketones, and decay measurements at 330 nm were shown to
occur with a time constant τ = 1.2−1.3 μs that is assigned to
free radicals formed by rapid separation of the original radical
pair followed by subsequent recombination. Analogous kinetic
measurements carried out with nanocrystalline suspensions at
λmax ≈ 330 nm could be fit using a double-exponential function
with a small (3−13%) population of a short-lived component
and a larger contribution (97−87%) from longer-lived triplet
radical pairs. Notably, the intersystem crossing kinetics from
the electronically different diphenylmethyl−benzoyl triplet
radical pair from 1a are substantially different from those
observed with the diphenylmethyl−acyl radical pairs from
adamantyl ketones 1b−d. While the solution and solid-state
kinetics of triplet radical pairs from ketone 1a are very similar,
with lifetimes that vary from 1.3 to 1.5 μs, the corresponding
values in the case of 1b−d change from 1.2 μs in solution to
short- and long-lived components of 2.3 and 53 μs for 1b, 3.5
and 61 μs for 1c, and 1.8 and 63 μs for 1d, respectively.
Figure 2. (top left) Molecular structure of 1d determined by a single-
crystal X-ray diffractions analysis, and (top right) line formula
representation of the triplet acyl−alkyl biradical highlighting the close
proximity between the unpaired electrons. (bottom) Space-filling
model of the packing structure of ketone 1d showing that significant
separation and rehybridization of the two radical centers are unlikely.
angles with a conformation characterized by having one of the
C−C bonds of the adamantane group nearly eclipsed with the
carbonyl group, as indicated by a dihedral angle of 1.56°, while
the α-C−H bond is oriented almost antiperiplanar to the C
O group, with a dihedral angle of −171.51°. As indicated in the
figure, the α-cleavage reaction is expected to occur at the
weakest C−C bond, between the carbonyl carbon and the
diphenylmethyl group carbon, which in the ground state has a
normal bond length of 1.54 Å. It is worth pointing out that the
dihedral angle between the plane of one of the aromatic rings
and the cleaving C−C bond vector is close to orthogonal (D =
105.7°), which is necessary for the alkyl radical to experience
benzylic stabilization.
Nanocrystalline suspensions of each ketone were prepared
by dropwise addition of acetonitrile stock solutions into
vortexing Millipore water containing cetrimonium bromide
(CTAB) surfactant at ca. 4% of the critical micelle
concentration.31 Slightly turbid suspensions obtained in this
manner were analyzed by dynamic light scattering, which
revealed particle size distributions centered around 100−300
nm for each derivative, as indicated in the Supporting
Information. Powder X-ray diffraction (PXRD) of each ketone
measured with bulk solids as well as centrifuged nanocrystal-
line suspensions confirmed the same crystal polymorph. The
corresponding PXRD patterns for 1d are shown in Figure 3 as
representative examples, and the data for the other derivatives
can be found in the Supporting Information.
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ketone (with a known solid-state quantum yield of Φ = 0.2),
which is completely converted to the corresponding photo-
product under the same experimental conditions. If our
assumption holds, the time constant for the decarbonylation
reaction (step 4, Scheme 1) must be at least 2 orders of
magnitude greater than that of the intersystem crossing step 5
in Scheme 1. Thus, a long-lived transient with a time constant
of 63 μs for the decay of the radical pair from 1d implies a
decarbonylation time constant of at least 6.3 ms. This lower-
bound value demonstrates that the crystalline solid state can
drastically lengthen the decarbonylation lifetime by more than
3 orders of magnitude.
The lifetime enhancement observed in the solid state for the
radical pairs featuring aliphatic acyl radicals (1b−d) is
consistent with reports of restricted radical pairs and biradicals
exhibiting longer lifetimes in the solid state than in
solution.39−43 The structural rigidity and least-motion
character of reactions in solids suggested by the topochemical
postulate44 suggest that it is reasonable to analyze the lifetime
enhancement in terms of structural parameters of the ketone
molecular and crystal structure.45,46 Thus, on the assumption
that the acyl radical resides in an sp2 orbital, the two singly
occupied orbitals would remain in the original orientation of
the σ bond after the α-cleavage step, coaligned and close to the
ketone α bond length distance of 1.54 Å. As mentioned before
and suggested in Figures 1 and 2, the proximity of the two
radical centers is expected to result in large zero-field splitting
and a large singlet−triplet energy gap. When we consider that
energy must be conserved when there is a change in spin
angular momentum, the large energy gap between singlet and
triplet states poses a challenge for the intersystem crossing
step. Furthermore, on consideration that the open-shell singlet
should be ideally poised for bond formation, it is reasonable to
expect that dephasing and equilibration between the T0 and S
radical pair states, potentially facilitated by the relatively
different g factors of the two radical centers, should be very
unlikely. An alternative mechanism based on coupled
electron−nuclear spin flips determined by hyperfine coupling
(hfc) interactions are known to be effective when the S-T gap
is relatively small, at relatively long inter-radical distances.
Spin-flipping mechanisms caused by the dynamic modulation
of magnetic fields through spin−lattice relaxation requires fast
molecular dynamics, such as spin-rotation interactions and
solvent motion,47,48 which are ineffective under the rigidity of
the crystal lattice. Finally, while it may be expected that the
most efficient ISC mechanism for a triplet radical pair in a
crystalline solid will be based on spin−orbit coupling (SOC), it
is anticipated that the collinear arrangement of the two singly
occupied orbitals will be unfavorable, as there is a limited
change in angular momentum mediated by electron exchange
and/or mixing of zwitterionic states.15−17 While there is much
work to be done to ascertain the initial polarization, spin
sublevel equilibration, and relative contributions of the
different ISC mechanisms, it is clear that the restrictions
imposed by the crystal lattice are able to maintain these spin
systems for much longer times in comparison to any other
medium. To the best of our knowledge, the long-lived
Figure 4. Transient absorption spectra of 1-((1R,3R,5R,7R)-
adamantan-2-yl)-2,2-diphenylethan-1-one (1c) in acetonitrile solution
(top) and in aqueous nanocrystalline suspension (bottom). The fitted
exponential decay curves in the insets were used to determine the
lifetimes of the transient absorption signals.
We were able to confirm that the short-lived component
observed with nanocrystalline suspensions does not originate
from a small amount of the sample in solution. The observed
transient and the decay kinetics were not affected by oxygen,
which would have trapped free radicals in solution.
Furthermore, a 1H NMR analysis of recovered samples showed
only the starting material. It is possible that the short-lived
component may arise from molecules in less rigid sites either at
the surface or at defect sites, but we cannot discount that
different triplet radical pair sublevels formed by state-selective
intersystem crossing in the starting ketone may have access to
different intersystem crossing mechanisms.33−38
The first key observation from the data shown in Table 1 is
the remarkable difference between the phenylacyl (benzoyl)
radical pair in 1a and the alkyl−acyl radical pairs in 1b−d. The
second most notable observation pertains to the unprece-
dented long lifetime for an acyl-containing radical pair state.
We will center our attention on the long-lived ca. 50−60 μs
components for ketones 1b−d, which account for the majority
of the transients formed, and compare them with solution and
gas-phase decarbonylation lifetimes drawn from the literature
for analogous acyl radicals26,27 (Table 1). An important
conclusion from ketone 1d is that the rate of decarbonylation
in the solid state is much slower than that in solution. This can
be shown by comparing the decarbonylation rate of 1.9 μs in
solution with a time constant for intersystem crossing of 63 μs
in the crystalline state. Given that no photoproducts are
observed by 1H NMR, we take an upper bound of 1%
decarbonylation to estimate a lower bound for the time
constant of this bond-cleavage reaction. This analysis assumes
3
component of the RP-1 state of ketones 1b−d (53−63 μs)
has the longest lifetimes for an acyl−alkyl triplet biradical or
radical pair reported to date, with previous lifetimes reaching
no more than ca. 5.5 μs.29,49−54 Further studies based on
magnetic resonance methods such as CIDEP will yield
3
that each molecule in the sample accesses the RP-1 state at
least once, which we believe to be a conservative estimate on
the basis of a comparison with an analogous sample of dicumyl
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significantly more detailed information on the nature and
properties of these interesting qubit pairs.
Edris A. Rivera − Department of Chemistry and Biochemistry,
University of California, Los Angeles, California 90024-
1569, United States
CONCLUSION
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c03026
■
Using four ketones with a diphenylmethyl group on one side
and either phenyl or alkyl groups derived from adamantane on
the other, the lifetimes of acyl−alkyl radical pairs in the
crystalline solid state were measured for the first time. It was
shown that the crystal lattice environment substantially
enhances the lifetime of the triplet radical pairs, as they
remain entangled by over 1 order of magnitude in comparison
to analogous biradicals in solution. On the assumption that the
formation of a σ bond occurs as soon as the singlet radical pairs
are formed, the long triplet radical pair lifetimes highlight the
importance of molecular and solvent dynamics on the rate and
efficiency of ISC mechanisms. While these results are
extremely interesting and promising within the context of
quantum information systems, it will be essential to explore the
complementary magnetic detection and analysis of the triplet
radical pairs24 to confirm the multiplicity of the optical signal
carriers and their polarization and spin dynamics, including the
unlikely possibility of reversible triplet−singlet dephasing
mediated by the different g factors of the acyl and alkyl radical
centers. These studies will take advantage of magnetic
resonance methods, including chemically induced dynamic
nuclear polarization (CIDEP).
Author Contributions
†J.H.P. and V.M.H. contributed equally.
Funding
This work was supported by NSF grant CHE-1855342
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support from the UCLA Graduate Division for J.H.P and
E.A.R. with Eugene V. Cota-Robles Fellowships and for V.M.H
with a dissertation year fellowship is gratefully acknowledged.
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The Supporting Information is available free of charge at
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suspensions by DLS and PXRD, 1H NMR photoproduct
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Accession Codes
CCDC 2071808 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Miguel A. Garcia-Garibay − Department of Chemistry and
Biochemistry, University of California, Los Angeles,
California 90024-1569, United States; orcid.org/0000-
0002-6268-1943; Email: mgg@chem.ucla.edu
Authors
Jin H. Park − Department of Chemistry and Biochemistry,
University of California, Los Angeles, California 90024-1569,
United States
Vince M. Hipwell − Department of Chemistry and
Biochemistry, University of California, Los Angeles,
California 90024-1569, United States
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