Transient Kinetics and Quantum Yield Studies of Nanocrystalline α-Phenyl-Substituted Ketones: Sorting Out Reactions from Singlet and Triplet Excited States

Article abstract of DOI:10.1021/jacs.8b03247

Recent work has shown that diarylmethyl radicals generated by pulsed laser excitation in nanocrystalline (NC) suspensions of tetraarylacetones constitute a valuable probe for the detailed mechanistic analysis of the solid-state photodecarbonylation reaction. Using a combination of reaction quantum yields and laser flash photolysis in nanocrystalline suspensions of ketones with different substituents on one of the α-carbons, we are able to suggest with confidence that a significant fraction of the initial α-cleavage reaction takes place from the ketone singlet excited state, that the originally formed diarylmethyl-acyl radical pair loses CO in the crystal with time constants in the sub-nanosecond regime, and that the secondary bis(diarylmethyl) triplet radical pair has a lifetime limited by the rate of intersystem crossing of ca. 70 ns.

Full text of DOI:10.1021/jacs.8b03247

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Article
Transient Kinetics and Quantum Yield Studies of
Nanocrystalline #-Phenyl-Substituted Ketones: Sorting
Out Reactions from Singlet and Triplet Excited States
Jin H. Park, Tim S Chung, Vince M Hipwell, Edris A. Rivera, and Miguel A. Garcia-Garibay
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03247 • Publication Date (Web): 11 Jun 2018
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Journal of the American Chemical Society
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Transient Kinetics and Quantum Yield Studies of Nanocrystalline α-
Phenyl-Substituted Ketones: Sorting Out Reactions from Singlet
and Triplet Excited States
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Jin H. Park, Tim S. Chung, Vince M. Hipwell, Edris Rivera, and Miguel A. Garcia-Garibay*
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024-1569.
ABSTRACT: Recent work has shown that diarylmethyl radicals generated by pulsed laser excitation in nanocrystalline (NC) suspen-
sions of tetraarylacetones constitute a valuable probe for the detailed mechanistic analysis of the solid-state photodecarbonylation
reaction. Using a combination of reaction quantum yields and laser flash photolysis in nanocrystalline suspensions of ketones with
different substituents on one of the α-carbons we are able to suggest with confidence that a significant fraction of the initial α-cleavage
reaction takes place from the ketone singlet excited state, that the originally formed diarylmethyl-acyl radical pair loses CO in the crys-
tal with time constants in the sub-nanosecond regime, and that the secondary bis(diarylmethyl) triplet radical pair has a lifetime lim-
ited by the rate of intersystem crossing of ca. 70 ns.
9
carbons (the RSE of an α-phenyl group is 16.5 kcal/mol). In
Introduction
this study, we take advantage of asymmetric ketones that form
diphenylmethyl radicals to determine the generation and life-
times of the radical pairs using transmission laser flash photoly-
Solid-state photochemical reactions have some attractive fea-
tures, which often include high chemical yields, selectivity and
specificity, and the opportunity to reduce the amount of vola-
tile organic compounds in the production of complex chemi-
9
sis. Additionally, we measure the quantum yield of the reaction
to determine the efficiency of product formation. With that
information, we can estimate the extent of α-cleavage taking
place from the triplet (step 3) and singlet (step 3’) excited states
1
cals. However, the implementation of solid-state photochemi-
2
cal methods by the synthetic community is limited, partly due
3
1
to challenges in their experimental execution and scale up, and
due to limitations in mechanistic knowledge that make it chal-
lenging to generalize reactions in crystals. Fortunately, advances
in this area have been made possible by the use of aqueous
to form RP-1 and RP-1, respectively. The contribution from
each multiplicity is determined by assuming that the long-lived
triplet undergoes decarbonylation (step 6), while the singlet
1
RP-1 quickly undergoes recombination to the starting ketone
3,4
10
nanocrystalline suspensions. On one hand, nanocrystals sus-
(step 5). As in our previous study, we assign the transient
3
pended in water can be exposed to UV light in a step-flow reac-
tor such that solid-state photoreactions can be easily carried to
reported in this work as RP-2 and we propose that its lifetime
1
is determined by the rate of intersystem crossing to RP-2 (step
3
completion at multi-gram scales. On the other hand, nano-
7), which does not accumulate but goes on very rapidly to form
crystals with sizes smaller than the wavelength of light can be
seen as a state in transition between supramolecular entities
and bulk crystals, with strongly reduced scattering, dichroism
product 2 .
5
Scheme 1
and birefringence that enables their measurement by transmis-
sion spectroscopy methods, including laser flash photolysis.
R
1
Ph
R
1
Ph
5
R
R
2
3
H
6) k-CO
R
R
2
3
H
3
kα
3
1,
Ph
Ph
With that in mind, it is now possible to address subtle mecha-
nistic aspects of several solid-state photoreactions, including the
photo-induced decarbonylation of crystalline ketones with radi-
cal stabilizing substituents at the two α-positions, such as aryl-
3) if
R
1
Ph
H
CO
R
R
2
3
ca. 25%)
O
3RP-2
(
3
RP-1
3
'
) if
Ph
1
1
4) kisc(1)
(
,
1
O
ca. 75% kα
7) kisc(2)
*
)
6
,7
2) isc 3
R1
Ph
H
susbtituted ketones 1a-1c in Scheme 1 and Scheme 2.
The
11
1*
R
1
Ph
H
6') k-CO
*
R
R
2
3
R
R
2
3
reaction is expected to proceed by an initial α-cleavage of the
weakest C-C bond and ultimately results in the formation of
product 2, which maintains the stereochemical information of
Ph
Ph
CO
O
bond(1)
5) k
1RP-1
1
RP-2
8) kbond(2)
fast
the reactant by the stereospecific formation of a sterically con-
6,7
gested C-C bond.
As indicated in Scheme 1 the reaction
R1 Ph
R
1
Ph
R
R
2
H
Kinetic Model
Step 2 ≈ Steps 3, 3'
Step 6 >> Step 4
Step 5 >> Step 6'
Step 8 >> Step 7
R
2
H
8
starts by electronic excitation (step 1, Scheme 1), which results
in the sequential cleavage of the two α-bonds (steps 3 and 6) to
generate, respectively, radical pairs RP-1 and RP-2, which can
R
3
3
Ph
Ph
CO
O
1
2 ( Φ ≈ 0.25)
8
exist either in a singlet or a triple state. It has been reported
that reactions in crystals require the two bond-cleavage steps to
be exothermic and with α-substituents that provide radical sta-
bilization energies (RSE) greater than 11 kcal/mol, a require-
ment that is easily met in ketones with aryl groups on both α-
Results and Discussion
To carry out and document the kinetic scheme above, we se-
lected three ketones that would generate a common diphenyl-
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Page 2 of 7
methyl group in one α-carbon, and either a diphenylmethyl
1a), 1,1-diphenylethyl (2b) or triphenylmethyl (3c) group on
confirmed by measuring the UV-Vis absorption spectra for all
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
three samples, as shown for 1b in Figure 2. The spectral fea-
tures of the nanocrystalline suspension are a close match to
those observed in solution with an underlying scattering contri-
bution that increases exponentially as the wavelength gets
shorter. Relatively weak benzenoid transitions in the neighbor-
hood of 275 nm are accompanied by the carbonyl n,π* transi-
tion with a λmax ≈ 300 nm. The stability of suspended nanocrys-
tals was ascertained by following changes in the UV-Vis absorp-
tion as a function of time. Over the course of time, an increase
in baseline accompanied with a decrease in absorption was of-
ten observed as the result of aggregation. Therefore, measure-
ment had to be carried out with freshly prepared samples in
increments of 20 mL batches.
the other (Scheme 2). Ketones 1a-1c were obtained in reasona-
ble yields by simple substitution reactions with a suitable acyl
chloride and lithium carbanion couple, as illustrated in Scheme
2. All three ketones are crystalline solids with reasonably high
melting points (Table 1). A detailed description of the corre-
sponding synthetic procedures, characterization and analytical
results are included in the SI section.
Scheme 2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Li
Li
Ph
Ph
Ph
O
O
O
Ph
Ph
R
1
R
Ph
H
1
Ph
H
Cl
o
Cl
R
2
THF, 0 C
R
THF, 0 C
o
2
R
R
Ph
3
Ph
3
1
4
R1 R2 R3
Ph Ph
D
C
B
A
1
1
1
a
H
b
c
Ph Ph Me
Ph Ph Ph
Table 1. Characterizations and lifetimes of α-phenyl-
substituted acetones
Ketone
R
1
; R
2
;
m.p.
( C)
Crystal
size
(nm)
Biradical
Lifetimes,
Quantum
Yield of
NC
o
R
3
a
c
b
µs
5
10
15
20
25
30
35
1
a
Ph; Ph;
H
133-
134
140 ±40
150 ±60
1.5 (MeCN)
2.4, 70
(97%)
0.23
(0.06)
Angle (2θ)
d
b
1
b
Ph; Ph;
Me
101-
102
1.5 (MeCN)
0.25
(0.06)
Figure 1. Powder X-ray diffraction (PXRD) of 1b (A) bulk-powder,
B) nanocrystals, (C) as-formed photoproduct, (D) recrystallized
d
3.9, 70
(
b
(
92%)
photoproduct.
1
c
Ph; Ph;
Ph
180- 180 ± 60
181
1.3 (MeCN)
5.4, 70
(87%)
0.28
(0.04)
d
b
a
b
Measured by dynamic light scattering (DLS). Short and long-
lived components were observed in the solid state. The number in
parenthesis indicates the percentage contribution of the long-lived
c
component to the total decay. Quantum yields of product for-
mation were measured with optically dense nanocrystalline (NC)
suspensions at 302 nm using dicumyl ketone as a chemical acti-
d
nometer. Standard deviation from six independent measurements
with samples from two independent nanocrystal preparations.
The formation of nanocrystalline suspensions was accom-
plished by the re-precipitation, or solvent-shift method, original-
11
ly reported by Kasai et al. (SI section). The crystalline nature
of the suspended solids was confirmed by powder X-ray diffrac-
tion (PXRD) analysis of nanocrystals collected by centrifuga-
tion, as shown in Figure 1B for samples of 1b. One can see that
filtered nanocrystals have a PXRD that is nearly identical to
those recorded with samples obtained by grinding large single
crystals (Figure 1A), indicating that the crystal packing of the
bulk powder and the nanocrystals of 1b correspond to the same
Figure 2. (Solid red line) Normalized UV spectra of 1b in MeCN
solution (0.01 g/L) and, (dashed blue line) nanocrystalline suspen-
sion of 1b in cetyltrimethylammonium bromide (CTAB) surfactant
(0.025 g/L).
Steady state photochemical experiments were carried out in
MeCN solutions with samples of 1a, 1b and 1c using a medium
pressure Hg arc lamp equipped with a Pyrex filter (λ ≤ 290 nm).
12
polymorph.
It was expected that nanocrystals of ketones 1a-1c with an aver-
age size < 200 nm (table 1), which is smaller than the wave-
length of light used for their excitation and detection, would be
suitable for transmission spectroscopy measurements. This was
1
Product formation was established by H NMR analysis. While
symmetric ketone 1a gave signals corresponding to photoprod-
uct 2a, consistent with the combination of two identical free
radicals (shown in blue in Scheme 3), ketone 1b gave three
2
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radical-radical combination products 2a, 2b and 2d, as expected with kinetics measurements and displayed the required nano-
1
2
3
4
5
6
7
8
9
from the three different radical combination possibilities. An
additional bis-benzylic singlet in the H NMR spectrum ob-
crystalline stability.
1
As illustrated in Figure 3 with data obtained from a sample of
ketone 1b with an OD < 1.0 in MeCN solution, the transient
absorption spectrum of diphenylmethyl free radicals in solution
displays a maximum at λ≈330 nm. This result is in excellent
agreement with data reported in the literature, including a rela-
tively recent study from our group that included samples of 1a.
Similar observations recorded for ketone 1c are included in the
SI
served in the reaction of 1b corresponds to the formation of
diphenylmethane 5, which is accompanied by formation of 1,1,-
diphenyl ethylene 6 by the transfer of a hydrogen atom from
the methyl group of 1,1-diphenylethyl radical (shown in green)
to the diphenylmethyl radical (in blue). Irradiation of ketone
1
c generates the relatively persistent trityl radical (shown in
red), which is able to form stable pentaphenylethane 2c by
combination with diphenylmethyl radicals (in blue). Interest-
ingly, self-combination of trityl radicals is known to generate the
Gomberg hydrocarbon 2e, in a reversible process, which is ob-
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
10
end of pulse
0
.80 us
1.60 us
.40 us
8
6
4
2
0
1
served in the H NMR in the form of several vinylic hydrogens.
2
As expected, reactions carried out in the crystalline state result
in the formation of a single product from the combination of
3.20 us
4.00 us
4.80 us
13
the corresponding “geminate” radical pairs. Crystals of ke-
5.60 us
6
7
.40 us
.20 us
tones 1a, 1b and 1c, give rise, respectively, to products 2a, 2b
and 2c, enclosed in a rectangle in Scheme 3. It was shown that
these reactions occur from solid reactant to solid photoproduct
by comparing the PXRD of the “as formed” products collected
after the reaction with the ones from samples of the recrystal-
lized photoproducts. This is shown in Figure 1 in the case of
1b with the PXRD of a solid sample exposed to UV light (Fig-
ure 1C), which gave essentially the same pattern as the one
obtained by recrystallization of pure 2b (Figure 1D), indicating
that the solid-state photoreaction occurs by a reconstructive
300
320
340
360
380
400
Wavelength (nm)
Figure 3. Transient absorption spectra of 1b in MeCN solution
max = 330 nm).
(
λ
14
phase transition.
section. An absorption decay measurement monitored at the
max over a 10 µs window revealed a single exponential process
λ
Scheme 3
with a lifetime of 1.5 µs (Figure 4 and Table 1). The results
observed with samples of 1a - 1c were virtually indistinguisha-
ble, as expected from the high similarity of the phenyl-
substituted radicals (SI and Table 1). The fact that the growth
of all observed radical absorption occurs within the laser pulse
indicates that the α-cleavage and decarbonylation steps 3 and 6
(
a)
Solution
Products
Ph
Ph
Ph
H
Ph
H
H
+
H
Key:
Ph
Ph
Crystal
Product
Ph
Ph
2
a
Ph
H
Me
H
Ph
H
Ph
Me
Ph
Ph
Me
Me
Me
(
b)
(
Scheme 1) in solution are completed within the 8 ns laser
Ph
Ph
CH
Ph
3
Ph
Ph
Ph
Ph
Ph
Ph
H
2a
2b
2d
pulse. This implies that we can only measure the kinetics of the
product-forming radical-radical reactions in MeCN solution.
+
Ph
Ph
Ph
H
H
2
H C
Ph
5
6
(c)
20
Ph
Ph
Ph
H
Me
H
Ph
H
Ph
H
Ph
Ph
Ph
Ph
Ph
H
Ph
Ph
Ph
+
Ph
15
Ph
Ph Ph
Ph
2
a
2c
2e
1
0
5
0
Transient absorption experiments were carried out with the
fourth harmonic of a Nd-YAG laser at λex=266 nm with a pulse
width of ca. 8 ns. Considering that the buildup of photoprod-
ucts can interfere with the observation of transient species, we
use a single pass flow system to make sure that fresh sample is
always available in the laser path. The need to maintain sus-
pended crystals in a reservoir before they are flown into the cell
requires suspensions to be stable for no less than ca. 10 min,
which is approximately the length of time that it takes to reach
the cell. Prior to the laser flash photolysis experiments we es-
tablished the stability of our suspensions by measuring their
UV spectra as a function of time. Fortunately all three ketones
0
2
4
6
8
10
Time (us)
Figure 4. Transient decay profile of 1b in MeCN solution (λmax
30 nm).
=
3
The results from transient absorption experiments carried out
in the solid state with nanocrystalline suspensions of 1a-1c re-
veal some similarities to the ones carried out in solution, with
1
a-1c exhibit a modest level of scattering that does not interfere
3
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Journal of the American Chemical Society
spectra that seem sharper and with a λmax ≈ 340 nm (Figure 5).
Page 4 of 7
absorbed by bulk solids. Incident photons can be easily lost
through reflection and scattering, which depend on geometric
and optical variables. To address this challenge, we have im-
plemented the use of nanocrystalline suspensions and submers-
ible light sources. Using relatively high sample loadings (which
tend to have a milky appearance) one can assure that, at least
ideally, every photon emitted by the immersed light source is
absorbed by the crystalline suspension (Scheme 4). Further-
more, one can take advantage of a mixed suspension with
measured optical densities, where one of the components is a
chemical actinometer with a reference quantum yield. A very
convenient reference for the purpose of our study is nanocrys-
talline dicumyl ketone (DCK), which is known to form di-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
As shown in Figure 6 with a sample of 1b, the lifetimes of the
transient detected in nanocrystalline suspensions were moni-
tored at their respective maxima over a time window of 160 µs,
which is an order of magnitude larger than the window needed
to record the corresponding decay in solution. Unlike meas-
urements made in MeCN solution, where a single exponential
was observed, the decay of the radical pair in the solid state
consisted of a very rapid component followed by a slower one.
Fitting with a double exponential function showed that the
integrated contribution of the fast components, with lifetimes
between 2.5 and 5.5 µs, is relatively small (ca. 3-13%, Table 1)
compared to that of the long-lived components, all of which
have a lifetime of 70 µs.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
b
cumene (DC) with a quantum yield of ΦDCK = 0.2 (Scheme 4).
4
end of pulse
6
4
2
0
8
.00 us
6.00 us
24.00 us
1
3
2
1
0
32.00 us
40.00 us
48.00 us
56.00 us
64.00 us
72.00 us
300
320
340 360
Wavelength (nm)
380
400
0
20
40
60
80
100 120 140 160
Time (us)
Figure 5. Transient spectra of 1b in a nanocrystalline suspension
max = 340 nm).
Figure 6. Transient decay profile of 1b in the solid-state via nano-
crystalline suspensions at λmax = 340 nm.
(
λ
Considering that the radical stabilizing energy that results from
Scheme 4
9
two α-phenyl groups (RSE ≥ 20.5 kcal/mol) is significantly
8
larger than the values known to enable the reaction (RSE ≥11
Immersed
light source
kcal/mol), one may expect the α-cleavage reaction in ketones
1a-1c to be very fast, and quantum efficiency for the solid state
reactions to be high, especially if the reaction were to proceed
exclusively from the triplet excited state. However, an increase
in the radical stabilization of the α-substituents also increases
Me Me
Me
Me
Me
Me
Me
Ph
Me
hν
Ph
Ph
Ph
Nanocrystals
CO
O
Φ=0.2
DCK
DC
1
5
the rate of α-cleavage from the singlet excited state, so that
R
1
Ph
H
R1 Ph
hν
1
R2
formation of RP-1 (Step 3’ in Scheme 1) may compete with
R2
H
Optically dense
mixed nanocrystals
suspension
R
3
Ph Nanocrystals
Φ=??
R
3
Ph
intersystem crossing (Step 2). Also indicated in Scheme 1, one
CO
1
O
may expect that formation of singlet state RP-1 would result in
1
very rapid return to the ground state ketone (Step 5), so that
very little, if any, singlet state decarbonylation (Step 6’) should
be observed. Effectively, singlet state α-cleavage would consti-
tute an energy-wasting step that would reduce the quantum
Thus, knowing the quantum yield of reaction for DCK, ΦDCK
and measuring the moles of product formed (Mol-2), the num-
ber of Einstein (Mol-hν) coming from the light source can be
calculated. One can use optically matched suspensions of DCK
and 1, or consider that the number of photons absorbed by
DCK and 1 are proportional to their optical densities, ODDCK
yield of reaction. The quantum yield of product formation Φ
P
is defined as the number of moles of product molecules
formed, i.e., Mol-2, in relation to the number of moles, or Ein-
stein of photons absorbed by the reactant, Mol-hν,
1
and OD . From the ratio of quantum yields for 1 and DCK
(Eq. 2) one can reorganize to obtain Eq. 3, where optical densi-
ties and the amounts of product can be easily measured to have
all the information needed.
Φ
P
= Mol-2/Mol-hν
Eq. 1
Measuring quantum yields in the solid state is complicated by
the fact that it is not easy to determine the number of photons
Φ
1
/ ΦDCK = (Mol-2/OD
1
) / (Mol-DC/ ODDCK
)
Eq. 2
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tentatively interpret the presence of short and long-lived com-
1
2
3
4
5
6
7
8
9
Φ
1
= ( ΦDCK) (Mol-2) (ODDCK) / (OD
1
) (Mol-DC)
Eq. 3
ponents as the result of distinct triplet sublevel populations in
3
RP-2 with different intersystem crossing rates. It is known that
1
3
As a convenient source for these experiments we use a pen light
with a 2.9 W output at λ=302 nm that can be easily immersed
in a test tube with a 2.54 cm diameter and 10.5 cm length.
Separate optically dense suspensions of DCK and 1 with initial
loading values on the order of 0.05-0.1 mg/ml were prepared,
and their relative optical densities values measured with a UV-
Vis immersion probe. Optical density values were adjusted by
adding some of the surfactant solution that was originally used
for suspension preparation. After reading final OD values in
the range of A≈0.5-1.0 at λ=302 nm, the two suspensions were
mixed together, one half was left unreacted and the other half
exposed to the immersed UV pen light under continued stir-
ring. Mixed suspensions were prepared in duplicate for each of
the three ketones, and up to three independent photochemical
experiments were carried out with variable exposure times and
conversion values. Quantum yield values close to 0.25 in Table
intersystem crossing in excited ketones from 1* to 1* results in
a spin-polarized triplet excited states that can carry their polari-
1
6
zation into the triplet radical pair. In addition to starting with
different triplet sublevel (T , T , and T) populations, those
+
0
-
sublevels are expected to have intersystem crossing rates that
depend on their energetic proximity to the singlet pair (S) with
mechanisms that may involve differences in g-factors, electron-
nuclear hyperfine coupling (HFC), or spin-orbit coupling
(SOC). Studies in progress addressing the effects of different
magnetic nuclear isotopes and magnetic field effects may eluci-
date the mechanisms that account for a small fraction (3-13%)
of the triplet biradical to intersystem crossing with rate con-
stants that are up to 30 times faster than the rest of the triplet
population.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
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3
3
4
4
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4
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5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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9
0
1
2
3
4
5
6
7
8
9
0
Conclusions
1
represent an average of six independent experiments with
Using three crystalline α-polyaryl substituted acetones we have
taken advantage of aqueous nanocrystalline suspensions to
measure their quantum yields of reaction and the kinetics of
decay of their triplet bis(diarylmethyl) radical pairs. As with
most previous examples, product analysis revealed reactions in
crystals to be highly selective and easily carried out to comple-
tion. Quantum yields of reaction of the order of 0.25 are in-
terpreted as the result of having most (ca. 75%) of the original
α-cleavage reaction taking place from the singlet excited state,
standard deviations suggesting that the small differences meas-
ured are probably not significant. A more detailed description
of the sample preparation and data analysis is included in the
SI section.
Based on the quantum yield and transient decay values ob-
tained in our experiments, we propose the kinetic relations
indicated in Scheme 1. We propose that the relatively low
quantum yields of reaction are the result of a considerable con-
tribution of unproductive singlet state α-cleavage. This implies
that tetraphenyl substitution accelerates the singlet state bond-
cleavage reaction such that it competes with intersystem cross-
ing from the singlet to the triplet excited state. In Scheme 1,
this implies that step 2 (isc) is of the same order of magnitude
as the α-cleavage steps 3 and 3’. It is reasonable to expect that
1
which results in singlet diarylmethyl-acyl radical pairs ( RP-1)
that undergo recombination to reform the starting material.
Transient species detected by laser flash photolysis experiments
had spectra consistent with the well-known diphenylmethyl
radical, suggesting that spin-spin interactions in the radical pair
do not perturb in a drastic manner the absorption spectrum of
the monoradical. We propose that the lifetime of the radical is
determined by intersystem crossing, which acts as the rate-
limiting step for product formation. The consistent observa-
tion of fast and slow components suggests that reaction and
intersystem crossing in the radical pair are likely to be triplet
spin-sublevel specific. Studies addressing this mechanistic hy-
pothesis are now in progress.
1
RP-1 should have no barrier for the two radicals to make a
sigma bond, such that step 5 should be faster than cleavage of
the CO group (step 6’). Barring a self-quenching process in the
excited state, which is unlikely considering that our results rep-
resent three different crystal structures, we propose that the
sequence,
step 1 —> step 3’ —> step 5
Experimental Section
accounts for 75% of the total number of photons absorbed.
This suggests that the remaining 25% of the total excitation
starts with intersystem crossing to the triplet excited state (step
2) and is followed by a very efficient α-cleavage reaction (step 3).
The lack of a two-phase growth in the transient signal suggest
that the loss of CO from RP-1 (Step 6) occurs within the ca. 8
ns of the laser pulse, which gives RP-2 as the only observable
transient within the observable time frame of our instrument.
As previously noted with aryl-substituted analogs of 1a, it is
interesting that all three ketones 1a, 1b and 1c, have a major
component with a lifetime of ca. 70 µs that accounts for 97%,
Synthesis: Ketones 1a-1c were prepared as shown in Scheme 2 by
formation of a lithiated diphenylmethane species followed by
an addition-elimination onto the required acyl chloride at 0°C
in THF. Experimental details and analytical data are reported
in the SI.
3
Laser Flash Photolysis (LFP): Measurements were carried out with
3
th
an Edinburgh Laser Flash Photolysis (LFP) system using the 4
harmonic of a brilliant B Nd:YAG laser from Quantel® gener-
ating 8 ns pulses with 36-40 mJ of energy at 266 nm. Samples
were introduced via a single-pass 1 cm quartz flow cell placed on
the cross-section of the laser and a pulsed Xenon arc lamp
9
2% and 87% of the total, respectively. The relative consisten-
(
450W) that was used as the probe. A detailed description of
the experimental setup has been described in detail in reference
1.
cy of this kinetic behavior supports our suggestion that it repre-
sents the rate of intersystem crossing from RP-2 to RP-2, with
the latter going on to product 2 with little or no barrier. We
3
1
1
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Quantum Yield Determination: Separate nanocrystalline suspen-
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sions of dicumyl ketones (DCK) and ketones 1a-1c were pre-
pared, respectively, by injecting 0.1 mL of 17 mg/mL MeCN
solution of DCK, and 0.2 mL of a 20 mg/mL MeCN solution
of ketone 1a-1c into test tubes each containing 6 mL of a
cetyltrimethylammonium bromide (CTAB) solution (164
mg/L) that was subject to a vortex. The transmittance (%T) of
each resulting suspension (DCK and ketones 1a-1c) was meas-
ured via an Ocean Optics spectrometer (DT-MINI-2-GS UV-
VIS-NIR LightSource and USB2000+ using the SpectraSuite
software package). Mixed suspensions were prepared by adding
1 (a) Cole, J. M.; Irie, M. CrystEngComm., 2016, 18, 7175; (b) Cohen,
M. D. Tetrahedron, 1987,43,1211-1224; (c) Ramamurthy, V.; Venkate-
san, K., Chem. Rev., 1987, 87, 433-481; (d) Hadjoudis, E.; Mavridis, I.
M. Chem. Soc. Rev., 2004, 33, 579-588; (e) Ramamurthy, V.; Sivaguru, J.
Chem. Rev., 2016, 116, 645-647
2
(a) Atkinson, M.B.J.; Mariappan, S.V.S.; Bucar, D.K.; Baltrusaitis,
J.; Friscic, T.; Sinada, N.G.; MacGillivray, L.R., Proc. Nat. Acad. Sci.
(USA). 2011, 108, 10974-10979. (b) Natarajan, A.; Ng, D.; Yang, Z.;
Garcia-Garibay, M.A. Angew Chem. Int. Ed. 2007, 46, 6485-6487; (c)
Ng, D.; Yang, Z.; Garcia-Garibay. M.A. Org. Lett. 2004, 6, 645-647. (d)
Ellison, M.E.; Ng, D.; Dang, H.; Garcia-Garibay, M.A. Org. Lett. 2003,
5, 2531-2534.
0
1
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3
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2
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5
6
7
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9
0
1
2
3
4
5
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8
9
0
1
2
3
4
5
6
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9
0
5
.6 mL of the DCK suspension to a 5.6 mL suspension of each
of the three ketones, 1a-1c, each in a separate container. Two
aliquots of 5 mL were drawn and one was set aside as a control
and the other was irradiated by immersing a 2.9 W pen light at
3
(a) Hernandez-Linares, M.G.; Guerrero-Luna, G.; Perez-Estrada, S.;
3
02 nm in a glass tube with a 2.54 cm diameter and 10.5 cm
Ellison, M.; Ortin, M.; Garcia-Garibay, M.A., J. Am. Chem.
Soc., 2015, 137, 1679-1684; (b) Veerman, M.; Resendiz, M.J.E.; Gar-
cia-Garibay, M.A. Org. Lett., 2006, 8, 2615–2617
height containing a magnetic bar. This procedure was repeated
twice for irradiations performed for 3, 5, and 7 min. The irra-
1
diated and non-irradiated suspensions were analyzed by H
4
(a)
Leonard
R.
MacGillivray,
L.
R.,
J.
Org.
NMR measurements. To that end, samples were diluted with
Chem., 2008, 73 (9), 3311–3317; (b) Khan, M.; Enkelmann, V.;
Brunklaus, G., J. Org. Chem., 2009, 74 (6), 2261–2270; (c) Chaudhary,
A; Mohammad, A., Crystal Growth & Design, 2017, 17 (5), 2893–2910
5 (a) Chung, T.S.; Ayitou, A. J.-L.; Park, J. H.; Breslin, V. M.; Garcia-
Garibay, M., Phys. Chem. Lett., 2017, 8, 1845–1850; (b) Breslin, V. M.;
Garcia-Garibay, M. A. Crystal Growth and Design, 2017, 7, 637–642; (c)
Anoklase J.-L. Ayitou, A. J.-L.; Flynn, K.; Jockusch, S.; Khan, S.I.; Gar-
cia-Garibay, M.A., J. Am. Chem. Soc., 2016, 138, 2644-2648;
1
0 mL of D.I. water and extracted with (2 x 12 mL) diethyl
ether. The ether phase was dried over MgSO and the solvent
4
was removed via rotary evaporation. The dry samples were
treated with 0.2 mL of a 2.7 mg/mL solution of 2-methoxy-
benzophenone in CDCl
3
and then sufficiently filled with
1
CDCl
3
for H NMR at 500 MHz. The (OMe) singlet at
δ = 3.73 ppm from the 2-methoxy-benzophenone was normal-
ized to 1.00 and the methine protons of ketones 1a-1c and the
methyl groups (δ = 1.28 ppm) of DCK were integrated relative
to the (OMe) signal. The quantum yields (Φ1a-1c) were calcu-
lated using Eq. 3 where Mol-2a-2c and Mol-DC are the relative
integrations of 2a-2c and dicumene (DC 2), respectively. The
values OD1a-1c and ODDCK are the optical densities of 1a-1c and
DCK, respectively and θDC = 0.2. In addition to a summary
included in Table 1 we have included all the tabulated data in
the SI section.
6
(a) Kuzmanich, G.; Garcia-Garibay, M.A., J. Phys. Org.
Chem., 2011, 23, 376–381; (b) Shiraki, S.; Natarajan, A.; Garcia-
Garibay, M.A. Photochem. Photobiol. Sci., 2011, 10, 1480-1487; (c)
Resendiz, M.J.E.; Family, F.; Fuller, K.; Campos, L.M.; Khan, S.I.;
Lebedeva, N.V.; Forbes, M.D.E.; Garcia-Garibay, M.A., J. Am. Chem.
Soc. 2009, 131, 8425-8433; (e) Family, F.; Garcia-Garibay, M.A., J. Org.
Chem. 2009, 74, 2476-2480; (f) Resendiz, M.J.E.; Taing, J.; Garcia-
Garibay, M.A., Org. Lett. 2007, 9, 4351-4354
7
(a) Mondal, R.; Okhrimenko, A. N.; Shah, B. K.; Neckers, D. C., J.
Phys. Chem. B, 2008, 112 (1), 11–15; (b) Poloukhtine, A.; Popik, V. V.,
J. Org. Chem., 2003, 68 (20), 7833–7840; (c) Mondal, R.; Shah, B. K.;
Neckers, D. C., J. Am. Chem. Soc., 2006, 128 (30), 9612–9613
8
(a) Campos, L. M.; Dang, H.; Ng, D.; Yang, Z.; Martinez, H. L.;
,
ASSOCIATED CONTENT
Garcia-Garibay M. A. J. Org. Chem. 2002, 67, 3749-3754. (b) Garcia-
Garibay, M.A.; Campos, L. in The Handbook for Organic Photochem-
istry and Photobiology, W. Horspool, Ed., CRC Press, Boca Raton, Fl.
2003; (c) Shiraki, S.; Garcia-Garibay, M.A. in Handbook of Synthetic
Photochemistry, Albini, A., Ed., John Wiley, New York, 2010, pp 25-66.
1
13
Supporting Information. Spectroscopic H, C NMR and UV Vis
data for compounds 1a-1c and their synthetic intermediates, pow-
der X-ray diffraction, and dynamic light scattering results from the
nanocrystalline samples. This material is available free of charge via
the Internet at http://pubs.acs.org.
9
a) Cozens, F. L.; Garcia, H.; Gessner, F.; Scaiano, J. C., J. Phys.
Chem., 1994, 98 (34), 8494–8497; (b) Garcia, H.; Scaiano, J. C., Acc.
Chem. Res., 1999, 32 (9), 783–793; (c) Jockush, S.; Hirano, T.; Liu, Z.;
Turro, N. J., Phys. Chem. B, 2000, 104 (6), 1212–1216; (d) Kantor, S.
W.; Hauser, C. R., J. Am. Chem. Soc., 1950, 72 (7), 3290–3291; (e)
Hirano, T.; Li, W.; Abrams, L.; Krusic, P. J.; Ottaviani, F.; Turro, N. J.,
J. Org. Chem., 2000, 65 (5), 1319–1330
AUTHOR INFORMATION
Corresponding Author
*
Miguel A. Garcia-Garibay, mgg@chem.ucla.edu
1
0 Park, J.H.; Hughs, M.; Chung, T.S.; Ayitou, A. J.-L.; Breslin,
V.M.; Garcia-Garibay, M.A J. Am. Chem. Soc. 2017, 139, 13312–
3317.
1 (a) Kasai, H.; Nalwa, H. S.; Oikawa, H.; Okada, S.; Matsuda, H.;
Funding Sources
We thank the National Science Foundation for support through
grant CHE-1566041.
1
1
Minami, N.; Kuakuta, A.; Ono, K.; Mukoh, A.; Nakanishi, H. Jpn. J.
Appl. Phys. 1992, 31, 1132.
ACKNOWLEDGMENT
Support from the UCLA graduate division for JP in the form of a
Cota Robles fellowship is gratefully acknowledged.
1
2 (a) Desiraju, G. R., J. Am. Chem. Soc., 2013, 135 (27), 9952–9967;
b) Eleguero, J., Crystal Growth & Design, 2011, 11 (11), 4731–4738
3 (a) Turro, N. J.; Weed, G. C., J. Am. Chem. Soc., 1983, 105 (7),
(
1
1
861–1868; (b) Garcia-Garibay, M. A.; Zhang, Z.; Turro, N. J., J. Am.
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