Structure-Kinetics Correlations in Isostructural Crystals of α-(ortho-Tolyl)-acetophenones: Pinning Down Electronic Effects Using Laser-Flash Photolysis in the Solid State

Article abstract of DOI:10.1021/jacs.5b11657

Aqueous suspensions of nanocrystals in the 200-500 nm size range of isostructural α-(ortho-tolyl)-acetophenone (1a) and α-(ortho-tolyl)-para-methylacetophenone (1b) displayed good absorption characteristics for flash photolysis experiments in a flow system, with transient spectra and decay kinetics with a quality that is similar to that recorded in solution. In contrast to solution measurements, reactions in the solid state were characterized by a rate limiting hydrogen transfer reaction from the triplet excited state and a very short-lived biradical intermediate, which does not accumulate. Notably, the rate for δ-hydrogen atom transfer of 1a (2.7 × 10⁷ s^-1) in the crystalline phase is 18-fold larger than that of 1b (1.5 × 10⁶ s^-1). With nearly identical molecular and crystal structures, this decrease in the rate of δ-hydrogen abstraction can be assigned unambiguously to an electronic effect by the para-methyl group in 1b, which increases the contribution of the ³π,π? configuration relative to the reactive ³n,π? configuration in the lowest triplet excited state. These results highlight the potential of relating single crystal X-ray structural data with absolute kinetics from laser flash photolysis.

Full text of DOI:10.1021/jacs.5b11657

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Article
Structure–Kinetics Correlations in Isostructural Crystals of
#–(ortho-tolyl)-Acetophenones: Pinning Down Electronic
Effects Using Laser-Flash Photolysis in the Solid State
Anoklase J.-L. Ayitou, Kristen Flynn, Steffen Jockusch, Saeed I. Khan, and Miguel A. Garcia-Garibay
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b11657 • Publication Date (Web): 05 Feb 2016
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Journal of the American Chemical Society
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Structure–Kinetics Correlations in Isostructural Crystals of α–
ortho-tolyl)-Acetophenones: Pinning Down Electronic Effects Using
Laser-Flash Photolysis in the Solid State
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a
a
b
a
Anoklase J.-L. Ayitou, Kristen Flynn, Steffen Jockusch, Saeed I. Khan and Miguel A. Garcia-
Garibay *
a
a
Department of Chemistry and Biochemistry, University of California Los Angeles, Los Angeles, CA 90095, USA
b
Department of Chemistry, Columbia University, New York, NY 10027, USA
ABSTRACT: Aqueous suspensions of nanocrystals in the 200-500 nm size range of isostructural α-(ortho-tolyl)-acetophenone 1a
and α-(ortho-tolyl)-para-methylacetophenone 1b displayed good absorption characteristics for flash photolysis experiments in a
flow system, with transient spectra and decay kinetics with a quality that is similar to that recorded in solution. In contrast to solu-
tion measurements, reactions in the solid state were characterized by a rate limiting hydrogen transfer reaction from the triplet ex-
cited state and a very short lived biradical intermediate, which does not accumulate. Notably, the rate for δ-hydrogen atom transfer
7
-1
6
-1
of 1a (2.7x10 s ) in the crystalline phase is 18-fold larger than that of 1b (1.5x10 s ). With nearly identical molecular and crystal
structures, this decrease in the rate of δ-hydrogen abstraction can be assigned unambiguously to an electronic effect by the para-
3
3
methyl group in 1b, which increases the contribution of the π,π* configuration relative to the reactive n,π* configuration in the
lowest triplet excited state. These results highlight the potential of relating single crystal X-ray structural data with absolute kinet-
ics from laser flash photolysis.
(
X=Me) in 1b. We recognized that their isomorphism would
Introduction
provide us with an opportunity to quantify the electronic effect
of the para-methyl group, which would be an excellent way
to validate the broad potential of structure-kinetics correlation
based on single crystal diffraction and kinetic studies based on
suspended nanocrystals.
8
One of the long-standing and most promising objectives of
solid state photochemistry is to correlate kinetic parameters
with structural information from single crystal X-ray diffrac-
1
,2
tion.
However, due to challenges in the determination of
Scheme 1
absolute kinetics in the solid state, this aim has remained elu-
sive. In fact, optical analysis of species generated by pulsed-
laser excitation in large single crystals and bulk powders are
complicated by problems associated with their high optical
densities, intense light scattering, multi-photonic events (such
T-T annihilation), and interference by accumulated photo-
CH3
3O*
CH2
OH
3) H-abs
3K (1)
31,5-BR
X
X
1) hν
4) isc
3
2) isc
5) bond
products. Knowing that crystals smaller than the wavelength
CH3
O
of light can be suspended in water and behave as large supra-
molecular assemblies while retaining the structure of macro-
scopic crystals, we showed that a promising approach to
meet these challenges is based on the use of crystals in the
100-300 nm size range. With a strongly diminished scattering
OH
4
K
X
X
Indanol (2)
b) X = Me
5
a) X = H
and single excitations under low laser power, the resulting
nanocrystalline suspensions may be used in a flow cell so that
a fresh sample is exposed to every shot in a laser flash photol-
CH3
O
6
ysis experiment.
1c
Hoping to document the effects of substituents on a solid
state photoreaction, we explored the nanosecond spectroscopy
and kinetics of crystalline α-(ortho-tolyl)-acetophenones 1a
Result and Discussion
7
In order to document the spectral characteristics of the tri-
plet states, we also analyzed aqueous crystalline suspension of
α-(para-tolyl)-acetophenone 1c, which is unable to undergo
the Norrish type II reaction. Ketones 1a-1c were synthesized
and 1b (Scheme 1). These compounds are known to undergo
an intramolecular hydrogen transfer reaction from their triplet
3
3
excited states ( K*) to form 1,5-biradicals ( 1,5-BR), which
transform into the corresponding 2-hydroxy-indanols 2a and
by AlCl
3
-catalyzed Friedel-Crafts acylations using commer-
2
b in excellent chemical yields (Scheme 1). A notable feature
cially available arylacetyl chlorides and
of these ketones is that they have nearly identical confor-
mations (Figure 1) and differ only by a para-methyl group
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Journal of the American Chemical Society
Page 2 of 5
electronic influence of the para-methyl group (X=Me) in 1b
as compared to the aromatic hydrogen (X=H) in 1a.
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Nanocrystalline suspensions for the laser flash photolysis
1
2
studies were prepared via the reprecipitation method as de-
1
3
scribed previously. Dynamic light scattering (DLS) analyses
showed mean particle sizes for 1a and 1b to be in 200-500 nm
size range. While particles of 1c were consistently larger, we
were able to obtain reasonable transmission spectra. As illus-
trated for 1b in Figure 2, a comparison of the powder X-ray
diffraction patterns (PXRD) of nanocrystalline samples col-
lected by centrifugation and those obtained from powered
single crystals confirmed that they both belong to the same
polymorph (Figure 2). Similar results were obtained for 1a and
Figure 1. Molecular structures of ketones 1a and 1b illustrating
their high similarity, which includes a suitable disposition for a δ-
hydrogen transfer from the ortho-tolylmethyl group to the car-
bonyl oxygen.
9
benzene (1a and 1c) or toluene (1b). All three samples were
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purified by flash chromatography on silica gel and recrystal-
lized from methylene chloride/pentane (1/10, v/v). X-Ray
diffraction data obtained from single crystals of 1a and 1b
were solved in the space group P2 /c with remarkably similar
1
molecular (Figure 1) and packing structures (Table 1).
1
c.
CH3
O
40
Table 1. Key structural parameters from crystals of ke-
3
0
0
a
CH3
1
b
tones 1a and 1b.
2
nanocrystals X 5
1a
1b
o
10
m.p. [ C]
67–69
P21/c (4)
85–88
P21/c (4)
bulk powder
b
Space group (Z )
a [Å]
12.1826(10)
9.1323(7)
11.2711(11)
90, 114.959(5), 90
78.3, 124.5, 69.3
2.78
12.8879(11)
8.7706(7)
5
10
15
20
theta
25
30
35
40
2
b [Å]
Figure 2. X-Ray powder diffraction patterns of ketone 1c ob-
tained from bulk powder (bottom) and from nanocrystals collect-
ed by centrifugation of an aqueous suspension (top).
c [Å]
11.5969(10)
90, 109.124(6), 90
76.9, 123.6, 72.8
2.80
o
α, β, γ[ ]
o
c
η, θ, Δ [ ] (90, 180, 0)
c
Table 2. Laser flash photolysis-derived time constants (ns)
of α-(ortho-aryl)-acetophenones
d [Å] (2.5)
a
b
Diffraction data acquired at 100 K. Number of molecules per
c
unit cell. Please refer to Scheme 2 for the definition of these pa-
rameters. The values indicated in parentheses represent the “ide-
al” values according to Scheffer.
Ketone
τ
Triplet State (ns)
τ
1,5-Biradical (ns)
1
0
1
2
1
2
ACN
NC
37
ACN
NC
3
5
6
Knowing that intramolecular hydrogen atom transfer in tri-
plet ketones occurs from n,π* excited states, and based on
orbital and state correlation diagrams, Scheffer suggested that
the rate of reaction should depend on the overlap between the
1a
6
50 and 650
-
4
6
6
1
b
ca. 70 (50
660
-
-
5
and 309)
4
10
1c
ca. 308 (210
(88 and
1030)
N.A.
N.A.
transferring hydrogen atom and the n-orbital. He suggested
that an ideal ground state conformation would have angular
5
5
and 548)
o
o
o
1
2
3
(
η=90 , θ=180 , Δ=0 ), and distance (d=2.5 Å) parameters re-
Acetonitrile; Nanocrystalline suspension; Reference 18;
4
5
6
lated to the best C-H••O=C orbitals overlap, as indicated in
Scheme 2.
Single exponential fit. Double exponential fit. The triplet birad-
ical was not observable.
Nanocrystalline suspension prepared with a loading of 1.5
mg of ketone per 20 ml of water in the presence of submicellar
Scheme 2
d
-
3
H
C
H
(0.4 x 10 M) cetyltrimethylammonium bromide (CTAB) to
passivate the crystal surfaces resulted in clear suspensions
suitable for transmission UV–Vis spectroscopic measure-
ments. As shown in Figure 3 (and Figure S1), the spectrum
obtained with nanocrystalline samples of 1a-c are very similar
to those obtained in acetonitrile (ACN), with the former being
red shifted by ca. 10 nm. Prior to laser flash photolysis meas-
θ
O
η
C
O
Δ
n
o
η, θ, Δ [ ] Ideal: 90, 180, 0
d [Å] Ideal: 2.5 (Ref. 10)
1
A comparison of the geometric parameters reveals that the
relation between the carbonyl oxygen and the closest transfer-
rable δ-hydrogen in the case of 1a and 1b (Table 1) shows
that they are very similar, with angles η, Θ, and Δ that vary by
urements, and using a combination of H NMR and GC analy-
sis, we confirmed that the solid state photochemistry of 1a and
1
b with nanocrystals matches the results with bulk solids pre-
viously reported by Wagner et al., with the formation of inda-
nols 2a and 2b as the major photoproducts (ca. 90%) along
with some non-identified minor components. Ready to ex-
plore the detection of the excited triplet states and biradical
intermediates, we carried out transient absorption measure-
ments with samples having an optical density of 0.3 at 266
o
less than 3.5 , and C-H•••O distances, d, that vary by 0.2 Å.
Considering that these small differences fall within the range
of molecular vibrational amplitudes captured by thermal ellip-
11
soids at ambient temperatures, one could expect that differ-
ences in reactivity between 1a and 1b could be related to the
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Journal of the American Chemical Society
nm. The fourth harmonic of a Nd-YAG laser was used as an
ACN
1
.2
1.0
.8
0.6
.4
0.2
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excitation source. In order to avoid problems associated with
the accumulation of photoproducts, all measurements were
carried out in a flow system with the nanocrystalline suspen-
sion and solution samples pumped at a rate of 2–3 mL/min.
Ketone 1c, with no intramolecular H-abstraction pathway
available in its structure was used as a test sample to deter-
mine the spectrum and kinetic characteristics of the acetophe-
none triplet in the solid state. The end-of-pulse spectrum ob-
tained in acetonitrile (Figure 4A) is consistent with the one
reported in the literature with a λmax at 340 nm and a spectral
Nanocrystalline
suspension
A
0
3
H C
O
0
1c
3
00
350
400
450
500
0
1
2
3
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14
width covering from ca. 290 to 400 nm. The spectrum ob-
tained in the nanocrystalline suspension is an excellent match,
but shows a red shift of ca. 10 nm. Furthermore, while the
spectrum in solution is effectively quenched by molecular
oxygen, the one obtained in the nanocrystalline suspension is
not affected, as previously reported for nanocrystals of triplet
Wavelength (nm)
1
1
.2
.0
3
H C
B
O
0.8
0.6
0.4
0.2
0.0
ACN
1c
15
benzophenones.
Nanocrystalline
suspension
ACN
1
.2
Nanocrystalline
suspension
1.0
0
0
0
0
0
.8
.6
.4
.2
.0
CH3
O
0
2
4
6
Time (µs)
1a
Figure 4. A) Triplet spectra of α-(para-tolyl)-acetophenone 1c in
ACN and as a nanocrystalline suspension. B) Triplet decay kinet-
ics detected at λ = 340 nm with residual corresponding to double
exponential functions. (A
=548 ns for acetonitrile, and A
and τ =1030 ns for nanocrystals)
1
=0.69 and τ
1
=210 ns and A
2
=0.31 and
=0.22
2
20
240
260
280
300
320
τ
2
1
=0.79 and τ
1
=88 ns and A
2
Wavelength (nm)
2
Figure 3. A) Transmission UV-Vis spectra of 1a in ACN and in
an aqueous nanocrystalline suspension.
1.2
ACN
A
Nanocrystalline
suspension
Shown in Figure 4B, the triplet decay of 1c recorded in ace-
tonitrile at 340 nm could be fit to a single exponential function
with a lifetime of 308 ns, as expected for a triplet state that
1
0
0
0
.0
.8
.6
.4
CH3
O
undergoes an α-cleavage reaction in a relatively low quantum
16,17
yield.
By contrast, the triplet decay in the nanocrystalline
1
a
suspension displayed significant heterogeneity, as it is com-
18
monly observed for kinetic measurements in solids. The data
was fit to a double exponential function with a short-lived
component of 88 ns and a long-lived component or 1.03 µs.
Knowing that the triplet excited state of ketone 1a under-
goes a hydrogen transfer reaction with a time constant of ca. 6
0.2
300
350
400
450
500
Wavelength (nm)
19
ns in solution, which is shorter than the time resolution of
our experimental setup, we assigned the end-of-pulse spectrum
1
.2
CH
3
B
(
blue open squares) in Figure 5A to the corresponding triplet
1.0
0.8
O
3
ACN
1
,5-biradical ( 1,5-BR). As expected for a chromophore that
has benzyl and benzylketyl radicals, the spectrum has a λmax at
20 nm and decays with some heterogeneity with lifetimes of
Nanocrystalline
suspension
1a
0
.6
3
ca. 50 ns and 650 ns (Figure 5B ACN). The 50 ns component
is associated with the time needed for the triplet biradical to
intersystem cross to the singlet state, which is expected to
form the expected photoproduct, rapidly. The observation of a
long-lived component with τ = 650 ns is in good agreement
with reports by Scaiano and Wagner of a transient species
0.4
0.2
0.0
0
1
2
3
Time (µs)
20,21
formed by secondary excitation of the triplet 1,5-biradical.
Figure 5. A) Triplet absorption spectra (end-of-pulse) of α-
(ortho-tolyl)-acetophenone 1a in ACN (λmax = 320 nm) and in
nanocrystalline suspension (λmax = 340 nm). B) Triplet decay
kinetics detected at the respective λmax
.
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Journal of the American Chemical Society
Page 4 of 5
Notably, the spectrum obtained in nanocrystals of 1a (filled on the rate of reaction that is about half as large in solution as
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
red circles in Figure 5A) has the distinctive characteristics of
the triplet ketone excited state with λmax at 350 nm. The transi-
ent decays with a time constant of 37 ns (Figure 5B, nanocrys-
tals), consistent with a lifetime limited by the hydrogen trans-
fer reaction and a 1,5-biradical that rapidly converts to prod-
uct, without being able to accumulate. These observations
indicate that the time constant for hydrogen transfer in crystals
of 1a is increased by a factor of ca. 6 (37 ns) as compared with
the time constant in solution (6 ns). These results are con-
sistent with a favorable solid state hydrogen abstraction geom-
etry (Figure 1) and would imply an accelerated rate of inter-
system crossing for the 1,5-biradical in the solid state, which is
in agreement with suggestions that biradicals with orthogonal
p-orbitals facilitate intersystem crossing by a spin–orbit cou-
compared to the one in the solid state.
Conclusion
We have confirmed that ketone crystals in the 200-500 nm
size range form stable, free-flowing aqueous suspension,
which make it possible to do transmission spectroscopy in the
UV-Visible range. Taking advantage of laser flash photolysis
with nanocrystalline isostructural samples of two α-(ortho-
tolyl)acetophe-nones, we were able to determine the rates of
0
1
2
3
4
5
6
7
8
9
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1
2
3
4
5
6
7
8
9
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2
3
4
5
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7
8
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-1
hydrogen atom transfer that vary from 2.7x10 s for the par-
6
-1
ent acetophenone chromophore to 1.5x10 s for the aceto-
phenone with a para-methyl group. A 18-fold rate difference
between the two compounds can be associated with the elec-
tronic effect of the methyl group, which alters the configura-
tion of the triplet excite state. These results bode well for
structure-kinetic correlations based on single crystal X-ray
diffraction and laser flash photolysis data.
22
pling mechanism.
The similar solid state structural parameters for the ortho-
tolyl ketones 1a and 1b suggest that they should have analo-
gous chemical behavior. However, flash photolysis of para-
methyl-substituted ketone 1b carried in ACN solution resulted
in detection of a transient with a spectrum that corresponds to
the triplet excited state (Figure S3), which has a lifetime of ca.
ASSOCIATED CONTENT
7
0 ns. This is consistent with a relatively strong electronic
Supporting Information. Synthesis and characterization of ke-
tones 1a-1c. Dynamic light scattering (DLS) data, UV–Vis ab-
sorption and laser flash photolysis absorption and kinetics meas-
urements. This material is available free of charge via the Internet
at http://pubs.acs.org.
perturbation by the electron donating para-methyl group,
which is known to render triplet arylketones less reactive as a
result of an increased contribution of the π, π* state mixed
with the reactive n,π* state configuration.
3
3
AUTHOR INFORMATION
1.2
CH3
O
Corresponding Author
1
0
.0
.8
ACN
mgg@chem.ucla.edu
Nanocrystalline
suspension
1b
CH3
ACKNOWLEDGMENT
0.6
We thank NSF Grant CHE-1266405 for support and the UCOP,
UCLA Chancellor, and UNCF-Merck for postdoctoral support to
AJA. KF was an Amgen summer intern at UCLA.
0
0
.4
.2
0.0
REFERENCES
0
.0
0.5
1.0
1.5
2.0
2.5
Time (µs)
Figure 6. Triplet absorption spectra (end-of-pulse) of α-(ortho-
tolyl)-para-methylacetophenone 1b in ACN (λmax = 340 nm) and
in nanocrystalline suspension (λmax = 360 nm). B) Triplet decay
1
. (a) Schmidt, G. M. Solid State Photochemistry; Verlag Chemie:
kinetics detected at the respective λmax
.
New York, 1976. (b) Hollingsworth, M.D.; McBride, J.M. Adv. Pho-
tochem., 1990, 15, 279. (c) Scheffer, J. R.; Garcia-Garibay, M.;
Nalamasu, O. Org. Photochem. 1987, 8, 249. (d) Ramamurthy, V.;
Venkatesan, K. Chem. Rev. 1987, 87, 433.
Measurements carried out in nanocrystalline samples also
revealed a transient consistent with the triplet excited state
ketone (Figure S3), which has a much longer hydrogen trans-
fer time constant of ca. 0.66 µs. Taken these results together,
one can deduce that hydrogen transfer is the rate-limiting step
2
. An approach based on laser excitation of single crystal and X-ray
diffraction detection to follow changes in real time also gaining mo-
mentum: (a) Techert, S. Crystallography Rev. 2006, 12, 25. (b) Ma-
kal, A.; Benedict,J. B.; Trzop, E.; Sokolow,J.; Fournier,B.; Chen, Y.;
Kalinowski, J. A.; Graber, T.; Henning, R.; Coppens, P. J. Phys.
Chem. 2012, 116, 3359. (c) Coppens, P. and S.-L. Zheng, Supramo-
lecular Photochemistry: Controlling Photochemical Processes, V.
Ramamurthy and Y. Inoue Eds., John Wiley & Sons, Hoboken, NJ,
USA (2011).
3
for triplet decay in both solids, and that the 1,5-BR must be
able to intersystem cross and form the product with rates that
are significantly greater than those observed in solution.
Based on the structural similarity between 1a and 1b, one can
assign the ca. 18-fold difference in H-transfer rates in the solid
state to the effect of the para-methyl group on the electronic
configuration of the triplet state. It is interesting to point out
that the rate of γ−hydrogen abstraction for the corresponding
valerophenones in non-polar benzene are reported to be
3
. (a) Wilkinson, F.; Willsher, C.J. Appl. Spectr. 1984, 38, 897. (b)
Wilkinson, F. J. Chem. Soc., Faraday Trans. 2, 1986, 82, 2073.
4 (a) Kasai, H.; Nalwa, H. S.; Oikawa, H.; Okada, S.; Matsuda, H.;
Minami, N.; Kakuta, A.; Ono, K.; Mukoh, A.; Nakanishi, H., Jpn. J.
Appl. Phys. 1992, 31, L1132. (b) Baba, K.; Kasai, H.; Okada, S.;
Oikawa, H.; Nakanishi, H. Opt. Mat. 2002, 21, 591-594.
7
-1
-7 -1
1
2.5x10 s for the unsubstituted compound and 1.8x10 s
8
for the structure with the para-methyl group. These corre-
spond to a 6.9-fold difference, suggesting an electronic effect
5. Chin, K. K.; Natarajan, A.; Campos, L. M.; Johansson, E.; Shep-
herd, H. and Garcia-Garibay, M. A. Chem. Comm., 2007, 41, 4266.
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polar solvents. We could not detect any product formation in acetoni-
trile.
. (a) Zand, A.; Park, B.-S.; Wagner, P.J. J. Org. Chem., 1997, 62,
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0
1
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9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
. See Supporting information S8
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8
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