Formation and direct detection of non-conjugated triplet 1,2-biradical from β,γ-vinylarylketone

Article abstract of DOI:10.1071/CH15401

Laser flash photolysis of 2-methyl-1-phenylbut-3-en-1-one (1) conducted at irradiation wavelengths of 266 and 308nm results in the formation of triplet 1,2-biradical 2 that has max at 370 and 480nm. Biradical 2 is formed with a rate constant of 1.1×107s-1 and decays with a rate constant of 2.3×105s-1. Isoprene-quenching studies support the notion that biradical 2 is formed by energy transfer from the triplet-excited state of the ketone chromophore of 1. Density functional theory calculations were used to verify the characterization of triplet biradical 2 and validate the mechanism for its formation. Thus, it has been demonstrated that intramolecular sensitization of simple alkenes can be used to form triplet 1,2-biradicals with the two radical centres localized on the adjacent carbon atoms.

Full text of DOI:10.1071/CH15401

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2015, 68, 1707–1714
Full Paper
http://dx.doi.org/10.1071/CH15401
Formation and Direct Detection of Non-Conjugated Triplet
1,2-Biradical from b,g-Vinylarylketone
A
A
H. Dushanee M. Sriyarathne, Kosala R. S. Thenna-Hewa,
A
A B
,
Tianeka Scott, and Anna D. Gudmundsdottir
A
Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221, USA.
Corresponding author. Email: anna.gudmundsdottir@uc.edu
B
Laser flash photolysis of 2-methyl-1-phenylbut-3-en-1-one (1) conducted at irradiation wavelengths of 266 and 308 nm
results in the formation of triplet 1,2-biradical 2 that has lmax at 370 and 480 nm. Biradical 2 is formed with a rate constant
of 1.1 ꢀ 107 sꢁ1 and decays with a rate constant of 2.3 ꢀ 105 sꢁ1. Isoprene-quenching studies support the notion that
biradical 2 is formed by energy transfer from the triplet-excited state of the ketone chromophore of 1. Density functional
theory calculations were used to verify the characterization of triplet biradical 2 and validate the mechanism for its
formation. Thus, it has been demonstrated that intramolecular sensitization of simple alkenes can be used to form triplet
1,2-biradicals with the two radical centres localized on the adjacent carbon atoms.
Manuscript received: 4 July 2015.
Manuscript accepted: 1 September 2015.
Published online: 9 October 2015.
Introduction
surface of alkenes through the formation of 1,2-biradicals. As
the intersystem crossing from the singlet-excited states of simple
alkenes to their triplet-excited states is generally not efficient,
1,2-biradicals have only been studied sporadically as their for-
mation typically requires triplet sensitization.[9,10] The limited
knowledge of trans–cis isomerization through 1,2-biradicals
restricts their potential use in applications.
Recently, we reported that the a,b-vinylarylketones undergo
cis–trans isomerization upon irradiation. Excitation at shorter
wavelengths results in efficient isomerization on the singlet
surface.[11] In comparison, irradiation at longer wavelengths
consequently yields cis–trans isomerization on the triplet
surface through the formation of 1,2-biradicals (Scheme 1).
Irradiation at wavelengths above 300 nm selectively forms
the triplet-excited state of the arylketone chromophore, which
Nature elegantly converts light into molecular actions in various
biological systems. For example, light-activated trans–cis
isomerization in the retinal rods of the eye is an important
process in the vision of most vertebrates.[1] Similarly, light-
initiated isomerization of simple alkenes has been used in var-
ious applications such as the activation of photoswitches and
the initiation of photoreleasing of photoremovable protecting
group.[2–5] Detailed knowledge of the isomerization mechan-
isms is essential to develop photoswitches and photoremovable
protecting groups that can be tailored to specific applications.
Direct irradiation of simple alkenes results in fast cis–trans
isomerization on their singlet-excited surface, and the isomeriza-
tion is generally not affected by molecular oxygen.[6–8] Cis–trans
isomerization can, however, also take place on the triplet-excited
∗
3
O
O
O
Rearrangement
Ar
308 nm
Ar
Ar
Triplet arylketone
α,β-vinylarylketones
Triplet 1,2-biradical
ISC
266 nm
∗
O
O
1
O
Ar
Ar
Ar
ꢀ
Trans
Cis
Singlet-excited state
vinylketone
Ar: Ph and p-MeOPh
Scheme 1. Proposed mechanism for the cis–trans isomerization of a,b-vinylarylketones. ISC ¼ Intersystem
crossing.
Journal compilation Ó CSIRO 2015
www.publish.csiro.au/journals/ajc

1708
H. D. M. Sriyarathne et al.
rearranges to form the subsequent 1,2-biradicals. The lifetimes
of these biradicals are of a few microseconds and they decay by
intersystem crossing to form the trans and cis isomers of their
precursors. Density functional theory (DFT) calculations show
that the radical centred on the a-carbon atom is stabilized by
conjugation with the ketone chromophore, whereas the radical
on the b-carbon atom is entirely localized on that carbon atom.
Caldwell et al. showed that 1,2-biradicals formed from
irradiating styrene derivatives results in 1,2-biradicals, and their
lifetimes are affected by their flexibility. Flexible acyclic
1,2-biradicals have lifetimes on the order of a few nanoseconds,
whereas the rigid cyclic derivatives have significantly longer
lifetimes (Chart 1).[12] Comparably, Garc´ıa-Expo´sito et al.
revealed that irradiation of 2-pentenoate ester derivatives results
in triplet 1,2-biradicals that have lifetimes of a few micro-
seconds.[13] The styrene biradicals are stabilized by conjugation
with the phenyl group, and, similarly, the pentenoate ester
biradicals are stabilized by conjugation with the adjacent car-
bonyl moiety. Nguyen et al. preformed ab initio calculation to
optimize the relaxed twisted triplet excited state of ethylene
or the triplet 1,2-ethylene biradical; it was between 62 and
66 kcal molꢁ1 above its ground state. Furthermore, the triplet
1,2-ethylene biradical has been experimentally estimated to be
58 ꢂ 3 kcal molꢁ1 above its ground state.[14,15]
O
O
Ar
CO2CH3
Chart 1. 1,2-Biradicals formed from styrene and pentenoate ester
derivatives.
O
Ph
1
Chart 2. Vinylketone 1.
0.1–0.2 μs
(a)
0.020
0.010
0
0.2–2.1 μs
2.1– 6.2 μs
6.2–17.0 μs
ꢁ0.010
350
350
350
400
400
400
450
500
550
550
550
600
Wavelength [nm]
In this paper, we report the photochemistry of b,g-vinyl-
arylketone with a built-in triplet sensitizer. Flash photolysis of
vinylketone 1 (Chart 2) using irradiation wavelengths of 266 and
308 nm results in the formation of its triplet-excited state
that undergoes energy transfer to form 1,2-biradical 2, which
has the radical centres localized on the b and g atoms. The triplet
biradical has lmax at 370 and 480 nm, a lifetime of 4.3 ms, and
decays by intersystem crossing to reform 1.
(b)
0.04
0.02
0
2
450
500
600
Wavelength [nm]
Results
(c)
0.10
0.05
0
T1K of 1
Laser Flash Photolysis
Laser flash photolysis[16–17] studies of 1 were performed to
identify the excited states and intermediates formed upon
excitation of 1. The argon-saturated acetonitrile laser flash
photolysis of 1 produced a transient absorption with lmax at
,370 and ,480 nm (Fig. 1). These absorption bands were
assigned to have contributions from both the first triplet-excited
state (T1K) of 1 and triplet biradical 2 based on their time-
dependent density functional theory (TD-DFT)-calculated
spectra (see Calculations below for details). The calculations
place the major electron transition for T1K of 1 in acetonitrile
at 301 ( f ¼ 0.0598), 306 ( f ¼ 0.0226), 357 ( f ¼ 0.0132), 402
( f ¼ 0.0659), and 407 ( f ¼ 0.0142) and for biradical 2 in aceto-
nitrile at 362 ( f ¼ 0.0160) and 466 ( f ¼ 0.0332) nm ( f denotes
calculated oscillator strength). The results fit well with the
experimentally observed spectra (Fig. 1). A similar transient
absorption was obtained in methanol, hexane, toluene, and
dichloromethane.
To further support the spectral assignment, the kinetic
analysis of the transient absorption was performed at 360 and
480 nm; the kinetic profiles at 360 and 480 nm were the same,
and thus the absorptions at these two wavelengths belong to
the same intermediates. At shorter time scales, the transient
absorption was formed with a rate constant of 1.12 ꢀ 107 sꢁ1
(t ¼ lifetime ¼ 89 ns, Fig. 2). At longer time scales, it decays
with a rate constant of 2.33 ꢀ 105 sꢁ1. We assign the faster rate
constant to the rate of formation of biradical 2 from T1K of 1, and
thus T1K of 1 has a lifetime of 89 ns. The slower rate constant is
450
500
600
Wavelength [nm]
Fig. 1. (a) Laser flash photolysis of 1 in argon-saturated acetonitrile.
TD-DFT-calculated spectra of (b) biradical 2 and (c) T1K of 1 in acetonitrile.
the decay rate constant of biradical 2 that has a lifetime of
,4.3 ms. Furthermore, the laser flash photolysis of 1 in oxygen-
saturated acetonitrile demonstrated that oxygen quenched all
the transient absorption, presumably as oxygen efficiently
quenched T1K of 1 and biradical 2.
Additional support for the assignments of the transient
absorption came from the isoprene-quenching studies (Fig. 3).
Increasing isoprene concentrations reduced the yields of the
transient absorption at 480 nm, but the rate of decay of the
transient absorption was not affected, thus conforming that
the absorption is due to biradical 2, which is not quenched by
isoprene, but its precursor, T1K of 1. We used the Stern–Volmer
Eqn 1 to obtain the Stern–Vomer plot displayed in Fig. 3.[18]
A0
¼ 1 þ kqt0½Qꢃ
ð1Þ
A
In the Stern–Volmer equation A0 is the absorption of biradi-
cal 2 without any quencher, A is the absorption of biradical 2 at

Triplet 1,2-Biradicals
1709
(a)
(a)
0.1–0.2 μs
0.2–2.1 μs
2.1–6.2 μs
6.2–17.0 μs
0.010
0.005
0
0.04
0.02
0
ꢁ0.005
ꢁ0.010
0
10
20
30
40
50 ꢂ 10ꢁ6
300
350
400
450
500
550
600
Time [s]
Wavelength [nm]
(b)
20
(b)
0.02
10
0
0.01
0
ꢁ10
ꢁ20 ꢂ 10ꢁ3
0
0.5
1.0
1.5
2.0
2.5 ꢂ 10ꢁ6
ꢁ0.01
Time [s]
0
10
20
30
40
50 ꢂ 10ꢁ6
Time [s]
Fig. 2. Kinetic traces at 480 nm at (a) longer and (b) shorter timescales.
(c)
0.05
0
3.0
2.0
ꢁ0.05
ꢁ0.10
ꢁ0.15
Intercept ꢃ 1.0
Slope ꢃ 1.2 ꢂ 104 Mꢁ1
1.0
0
0
0.5
1.0
1.5
2.0
2.5 ꢂ 10ꢁ6
0
0.2
0.4
0.6
0.8
1.0 ꢂ 10ꢁ4
Time [s]
Concentration of isoprene [M]
Fig. 4. Laser flash photolysis of 1 with 266 nm irradiation in argon-
saturated acetonitrile: (a) transient absorption spectrum and kinetic traces
at 480 nm at (b) longer and (c) shorter timescales.
Fig. 3. Stern–Volmer plot obtained upon quenching the absorption at
480 nm with isoprene.
The laser flash photolysis of 1 demonstrates that upon
irradiation, 1 forms a T1K that decays by energy transfer to form
triplet biradical 2. The photochemistry of 1 is not affected by the
irradiation wavelength, thus indicating that the ketone chromo-
phore in 1 absorbs more significantly than the vinyl moiety at
308 and 266 nm. Based on the laser flash photolysis, we propose
that the mechanism for forming biradical 2 is as displayed in
Scheme 2.
various isoprene concentrations, kq is the quenching rate con-
stant of the triplet state, t0 is the lifetime of T1K of 1 without any
isoprene, and [Q] is the concentration of the isoprene.
The Stern–Volmer plot of A0/A versus isoprene concentra-
.
tion can be fitted as a straight line with a slope of 1.2 ꢀ 104 Mꢁ1
By assuming diffusion-controlled quenching or kq is between 1
and 10 ꢀ 109 Mꢁ1sꢁ1, it can estimated that the lifetime of T1K of
1 is between 1200 and 120 ns, which fits reasonably with the
lifetime obtained directly from the rate of formation of biradical
2. We did not attempt to measure the effect of isoprene on the
rate of formation of biradical 2 as it is comparable to the time
resolution of the laser (,17 ns),[16] and thus, it is difficult to
obtain accurate data.
Calculations
The stationary points on the singlet and triplet surface of 1 were
calculated using Gaussian09 at the B3LYP level of theory and
with the 6–31þG(d) basis set[19–21] to better understand the
factors governing its photochemistry and the formation of triplet
biradical 2. In addition, we calculated the TD-DFT spectra for
the optimized structures of T1K of 1 and biradical 2 to aid with
their characterization using laser flash photolysis (Fig. 1b, c).
We used the TD-DFT calculations to locate the first singlet-
excited state (S1) and the first and second triplet-excited states
(T1K and T2K) of 1. The calculations revealed that S1K of 1 was
at 83 kcal molꢁ1 above its ground state (S0), T1K of 1 was at
72 kcal molꢁ1 above its S0, and T2K was only a few kcal molꢁ1
above the T1K of 1 i.e. at 76 kcal molꢁ1 (Fig. 5 and 6). The
optimized structure of T1K of 1 reveals that T1K of 1 is at
68 kcal molꢁ1 above its S0, considerably lower than the energy
obtained by the TD-DFT calculations. However, it should be
noted that DFT/B3LYP optimization of the triplet ketones with
(n,p*) configuration underestimates their energy as reported
earlier.[22–26] Analysis of the calculated bond lengths in the T1K
of 1 shows indeed that the C¼O bond is elongated in comparison
to the calculated C¼O bond length in its S0 (Fig. 5), as was
expected for triplet ketones with (n,p*) configuration. Further-
more, the Cb–Cg bond length in the T1K of 1 is similar to the one
in its S0, thus demonstrating that T1K of 1 is best described as
The laser flash photolysis of 1 in argon-saturated acetonitrile
was also performed at irradiation wavelength of 266 nm, result-
ing in a transient spectrum with lmax at 360 and 480 nm (Fig. 4).
This transient spectrum is similar to the one obtained using
irradiation wavelength of 308 nm. In addition, no negative
absorption was observed around 350 nm, and thus it was possi-
ble to obtain a transient absorption at shorter wavelengths.
Because irradiation with a 266-nm laser requires a lower con-
centration of 1, the extent of bleaching of the starting material
around 350 nm is smaller.
Kinetic analysis of the absorption at 360 and 480 nm reveals
that both wavelengths have the same kinetic profile. The rate
of forming the transient absorption is 1.38 ꢀ 107 sꢁ1 (t , 75 ns),
and it decays with a rate constant of 2.34 ꢀ 105 sꢁ1 (t , 4.3 ms).
We demonstrated that the transient absorption obtained by
laser flash photolysis of acetophenone is quenched with crotyl
chloride (see Supplementary Material). The triplet-excited
state of acetophenone is quenched by energy transfer to crotyl
chloride with a rate constant of 1.2 ꢀ 108 Mꢁ1 sꢁ1, thus, further
supporting that the energy transfer from triplet ketones to simple
alkenes is feasible.

1710
H. D. M. Sriyarathne et al.
∗
3
O
O
O
Energy transfer
308 nm
266 nm
ISC
Ph
Cis-1 and Trans-1
Ph
Ph
T1K of 1
1
2
Scheme 2. Proposed mechanism for forming triplet biradical 2 from 1.
S0 of 1
1.22
T1K of 1
1.3
Biradical 2
1.22
CꢃO
Ph–C
Cβ–Cγ
1.50
1.43
1.50
1.34
1.34
1.45
˚
Fig. 5. Optimized conformers of 1, T1K of 1, and biradical 2. The bond lengths are given in A.
T2K (n,π∗) (76)
S1 (83)
(62)
T
1K (n,π∗) (68–72)
2
O
ISC
O
H
H
S0 of 1
S0 of 1
(0)
(0)
Fig. 6. Stationary points on the triplet energy surface of 1. Calculated energies are in kcal molꢁ1. The energies were
obtained from optimization of 1, T1K of 1, and biradical 2. TD-DFT calculations were used to obtain the energies of S1K
,
T1K, and T2K of 1 using the optimized structure of 1.
0.97
O
triplet acetophenones. The spin density calculation further sup-
ported this notion as the unpaired spin is localized on the carbonyl
oxygen atom and the phenyl group of T1K of 1 (Chart 3).
The optimization of triplet biradical 2 shows that it is located
at 62 kcal molꢁ1 above the S0 of 1, only a few kcal molꢁ1 below
0.04 0.87
O
0.08
0.20
0.22
˚
the T1K of 1. The calculated Cb–Cg bond length in 2 is 1.45 A
0.96
0.18 0.48
(Fig. 5), and thus it can be described as having single-bond
character. The dihedral angle of H–Cb–Cg–H in 2 is 928 to
reduce the overlap between the two adjacent radical centres.
Furthermore, the spin density calculation placed the unpaired
electrons on the Cb and Cg atoms, which have spin densities
of 0.87 and 0.96, respectively, demonstrating that 2 is a
1,2-biradical with the two radical centres localized on the Cb
and Cg atoms (Chart 3).
Chart 3. Calculated spin densities for T1K of 1 and triplet biradical 2.
O
H
H
20
10
0
Ph
H
The calculated rotational barrier for the biradical 2 around
its Cb–Cg bond is displayed in Fig. 7. The rotational calculations
further support that biradical 2 is the most stable, as the
two radical centres are almost perpendicular to each other.
The calculated rotational barriers around the Cb–Cg bond are
ꢁ50
0
50
100
150
200
250
significant at 16 and 15 kcal molꢁ1
.
Torsion angle of H–CꢃC–H [ꢄ]
The calculated stationary points on the triplet surface of 1 are
shown in Fig. 6. The calculations show that an energy transfer
from T1K of 1 to form biradical 2 is feasible as the biradical
2 is several kcal molꢁ1 lower in energy than the T1K of 1.
Fig. 7. Calculated rotational barrier for 2. Torsion angle is represented by
the blue chemical bonds in the structure displayed in the upper left corner
of the plot.

Triplet 1,2-Biradicals
1711
0.55
1.0 ꢂ 106
∗
O
1
0.20
O
Ph
0.5
0
0.22
Ph
O
3
T1K of 1
0.19
Calculated spin density
0.95
300
350
400
450
500
550
600
Wavelength [nm]
Scheme 3. Formation of biradical 3 from T1K of 1 and calculated spin
densities of biradical 3.
Fig. 9. Phosphorescence of 1 in ethanol at 77 K obtained upon 308-nm
excitation.
0.16
0.12
Table 1. Comparison of the measured and calculated energies
(kcal mol21) of T1K and T2K of 1
3
0.08
0.04
0
PhosphorescenceA
OptimizationB
TD-DFTC
300
350
400
450
500
550
600
T1
73
T1
68
T1
72
T2
76
Wavelength [nm]
AObtained from phosphorescence spectrum of 1; BObtained from optimi-
zation; and CObtained from TD-DFT calculations.
Fig. 8. TD-DFT-Calculated spectrum for biradical 3 in acetonitrile.
Furthermore, the calculations also support the isoprene
quenches T1K of 1 efficiently as the energy of the triplet-excited
state of isoprene is 60 kcal molꢁ1[27] and thus lower than for T1K
of 1. Furthermore, isoprene is not expected to quench biradical
2 efficiently due to their similar energies.
We also calculated the triplet biradical formed by addition
of T1K of 1 to the vinyl bond to form biradicals 3 (Scheme 3).
The optimized structure of biradical 3 is located 51 kcal molꢁ1
above S0 of 1, and the calculated spin density shows that
one of the radical centres is localized on the carbon atom in
the five-membered heterocyle, whereas the other radical centre
is conjugated with the phenyl group. The transition state for
forming biradical 3 from T1K of 1 is located at 9 kcal molꢁ1
above its optimized structure. The calculated TD-DFT spectrum
of 3 is displayed in Fig. 8, and it does not match with the
observed spectra in Figs 1 and 4, thus further supporting that
biradical 3 is not formed upon irradiating 1.
which showed that 1 does not form any photoproduct as
expected. Furthermore, the irradiation of 1 in oxygen-saturated
[D]chloroform also yielded no new products (Scheme 4),
thereby confirming that biradical 2 is not trapped with oxygen to
form new products.
We prepared 1 with a deuterium (d) substitution on the
b-position (Scheme 4). Before photolysis, the ratio of the
1
trans-1-d-to-cis-1-d was 47 % according to H NMR spectro-
scopy. After 5 h of irradiation through Pyrex filter, the ratio of
trans-1-to-cis-1 became 44 %. Thus, verifying that 1 undergoes
cis–trans isomerization upon irradiation.
Discussion
Laser flash photolysis demonstrated that irradiation of 1 leads
to the formation of triplet biradical 2, and the formation
of biradical 2 is independent of irradiation wavelength. The
photochemistry of 1 is not affected by the irradiation wavelength
as the ketone chromophore absorbs more significantly than
the alkene moiety at both 308 and 266 nm. In comparison, the
photochemistry of a,b-vinylketones 4 is wavelength dependent
because irradiation at a shorter wavelength selectively excited
the vinylketone chromophore that did not undergo intersystem
crossing to the triplet surface. In contrast, irradiating 4 at longer
wavelengths resulted in the acetophenone chromophore to
absorb light to form T1K of 4, which rearranged to biradical
5 (Scheme 5).
The energy transfer rate from T1K of 1 to form biradical 2 is
similar to the rate for T1K of 4 to rearrange to form biradical
5. Similarly, the lifetimes of triplet biradicals 2 and 5 are
comparable, although the calculated rotational barrier for the
cis–trans isomerization is considerably larger for biradical
2 than that for 5 i.e. ,16 and ,8 kcal molꢁ1, respectively.
However, because biradical 2 only needs to rotate into a
conformation that is aligned for intersystem crossing to the
singlet surface to form both cis-1 and trans-1, rather than
undergoing cis–trans isomerization on the triplet surface, it is
rational that the lifetimes of 2 and 5 are similar as they are both
limited by intersystem crossing.
Although biradical 3 is more stable than biradical 2, it is
not formed upon irradiating 1 because energy transfer must be
more efficient than T1K of 1 rearranging into biradical 3. The
transition state barrier for forming biradical 3 from T1K of 1 is
significant enough that it cannot compete with energy transfer to
form biradical 2.
Phosphorescence
The phosphorescence spectrum of 1 was obtained at 77 K in
frozen ethanol matrices (Fig. 9). The obtained phosphorescence
spectrum of 1 has resolved the emission bands typically
observed for ketones with (n,p*) configurations. Furthermore,
the first vibrational band at 391 nm was assigned to the (0,0)
band of the phosphorescence, which corresponds to the energy
of T1K of 1 i.e. 73 kcal molꢁ1. Therefore, the energy obtained
for T1K of 1 from the TD-DFT calculations is in excellent agree-
ment with the experimental value (Table 1).
Product Studies
Photolysis of 1 in argon-saturated [D]chloroform through a
Pyrex filter (.300 nm) at ambient temperature yielded no
new products. The photolysis of 1 was monitored by 1H NMR
spectroscopy and gas chromatography mass spectrometry,
There is not a significant difference in the properties of
biradicals 2 and 5. The resonance stabilization of the a-radical

1712
H. D. M. Sriyarathne et al.
O
O
O
O
hν
D
Ph
Trans-1-d
Scheme 4. Product studies of 1.
Ph
Cis-1-d
Ph
Ph
Ar/O2
D
1
1
145 mJ).[16] A stock solution of 1 in spectroscopic grade ace-
tonitrile was prepared such that the solutions displayed
absorption between 0.3 and 0.6 at 308-nm excitation wave-
length. In a typical experiment, the stock solution (,1 mL) was
placed in a 10 mm ꢀ 10 mm wide and 48 mm long, quartz
cuvette and was purged with argon or oxygen for 5 min. The rate
constants were obtained by fitting on average three-to-five
kinetic traces.
O
O
hν
308 nm
O
Ph
Ph
Ph
4
5
T1K of 4
Scheme 5. Proposed mechanism for the cis–trans isomerization of 4.
centre in biradical 5 does not affect its reactivity significantly in
comparison to 2.
Phosphorescence
Finally, we compared the reactivity of triplet biradical 2
with triplet 1,2-biradical 7, which is formed by intramolecular
sensitization of vinylazide 6 (Scheme 6).[17] The major differ-
ence between biradical 7 and biradicals 2 and 5 is that biradical
7 decays by extruding a nitrogen molecule to form triplet
vinylnitrene 8 rather than undergoing intersystem crossing to
reform vinylazides 6. Triplet biradical 7 has a lifetime of 2.4 ms
at ambient temperature in acetonitrile, which is on the same
order as the lifetimes observed for biradicals 2 and 5. Thus, the
only significant difference between among 7, 2 and 5 is that it
was possible to measure the rate of reaction biradicals 7 and
5 with oxygen, whereas for biradical 2, the transient absorption
is fully quenched in oxygen. It can be theorized that triplet
biradicals 7 and 5 react slower with oxygen than biradical
2 because they have an electron-withdrawing azido and car-
bonyl groups, respectively. Several research scientists have
studied the reactivity of oxygen with alkyl- and hydroxyalkyl
radicals in the gas phase and have shown that there is a
correlation between the ionization potentials of these radicals
and their rate of reaction with oxygen.[28–30] However, it is
possible that T1K of 2 more efficiently quenched with oxygen
than T1K of 6.
A 5 mM ethanol solution of 2-methyl-1-phenylbut-3-en-1-one
(1) was prepared, and its phosphorescence spectra was obtained
at 77 K (Horiba instruments Inc., Edison, NJ, USA; Horiba
Jobin-Yvon fluorolog with 5 nm as the emission monochro-
mator bandwidth). The solutions were irradiated at 308 and
266 nm, and the emission spectra were recorded in the range of
300–800 nm.
Preparations of Starting Materials
Synthesis of 2-methyl-1-phenylbut-3-en-1-ol
2-Methyl-1-phenylbut-3-en-1-ol was synthesized following
a modified procedure based on the method used by Imai and
Nishida.[39] Benzaldehyde (5.3 g, 50 mmol) was dissolved in
dimethylformamide (100 mL). To the magnetically stirred solu-
tion, crotyl chloride (5.0 g, 55 mmol, 1.1 equiv.) and SnCl2ꢄH2O
(17.0 g, 75 mmol, 1.5 equiv.) were added. NaI (11.2 g, 75 mmol,
1.5 equiv.) was added slowly over a period of 10 min, and a mild
exothermic reaction was noted at the early stage. After stirring
for 20 h at room temperature, 30 % aqueous NH4F (100 mL)
and diethyl ether (Et2O; 200 mL) were added. The mixture was
stirred for an additional 30 min. The ether layer was extracted
and washed twice with water (100 mL) and saturated aqueous
NaCl solution (100 mL). The ether layer was dried over anhy-
drous MgSO4 and evaporated under reduced pressure to get a
clear oil of 2-methyl-1-phenylbut-3-en-1-ol (7.5 g, 46 mmol,
Conclusion
We have demonstrated that triplet sensitization of simple
alkenes results in formation of 1,2-biradical, which has a life-
time of a few microseconds and intersystem crosses to reform its
ground state. Thus, the intramolecular sensitization of uncon-
jugated alkenes is an efficient method of forming 1,2-biradicals,
and it can potentially be used for cis–trans isomerization of
simple alkenes in applications.
1
93 %). nmax (neat)/cmꢁ1 3439br (OH). The H NMR and IR
spectra matched those reported in the literature.[40] dH (CDCl3,
400 MHz) 0.88 (3H, d, J 8.0), 1.02 (2H, d, J 8.0), 2.17 (1H, br s),
2.51–2.41 (1H, m), 2.66–2.56 (1H, m), 4.37 (1H, d, J 8.0), 4.61
(1H, d, J 4.5), 5.08–5.03 (2H, m), 5.17–5.23 (2H, m), 5.72–5.86
(2H, m), 7.39–7.24 (10H, m).
Experimental
Calculations
Synthesis of 2-methyl-1-phenylbut-3-en-1-one (1)
The procedure reported by Ranaweera et al.[11] was followed
with some minor modifications. The Jones reagent was prepared
by dissolving CrO3 (10 g) in H2SO4 (10 mL) and water (30 mL).
A stirred slurry of 2-methyl-1-phenylbut-3-en-1-ol (7.0 g,
43 mmol) in acetone (100 mL) at 08C was added until the color
of Jones reagent changed to orange yellowish. The solution was
filtered, the solvent was removed under reduced pressure, and
the residue was dissolved in Et2O (200 mL). The ether layer was
washed with water (100 mL) and saturated aqueous NaHCO3
and dried over MgSO4. Removal of the solvent under vacuum
All geometries were optimized using Gaussian09 at B3LYP
level of theory and 6–31Gþ(d) basis set.[19,20,31] The UV
absorption spectra were calculated using the TD-DFT.[31–34]
The self-consistent reaction field method with the integral
equation formalism polarization continuum model with
acetonitrile as solvent were used to calculate the effect of
solvation.[35–38]
Laser Flash Photolysis
The laser flash photolysis was performed with an Excimer
laser (308 nm, 17 ns, 105 mJ) and Nd:YAG laser (266 nm, 2 ns,
produced
a crude 2-methyl-1-phenylbut-3-en-1-one. The

Triplet 1,2-Biradicals
1713
Energy
transfer
O
O
O
O
ꢁN2
N
N3
N3
hν
N3
Ph
Ph
Ph
Ph
T1K of 6
6
7
8
Scheme 6. Formation of triplet vinylnitrene through 1,2-biradical 6.
residue was purified on a silica column eluted with ethyl acetate/
hexane (3 : 7) to yield 1 as a clear oil (5.8 g, 36 mmol, 83 %). nmax
(neat)/cmꢁ1 3060, 2978, 2933, 1684 (C¼O), 1596, 1448, 1215,
962, 702. The 1H NMR, 13C NMR, and IR spectra matched the
those reported in the literature.[40] dH (CDCl3, 400 MHz) 1.35
(3H, d, J 6.8), 4.19 (1H, dq, J 6.8, 7.6), 5.15 (1H, d, J 10.2), 5.20
(1H, d, J 17.3), 6.01 (1H, ddd, J 7.6, 10.2, 17.3), 7.49 (2H, m),
7.58–7.54 (1H, m), 7.99 (2H, d, J 7.0). dC (CDCl3, 100 MHz)
201.1, 138.2, 136.3, 132.9, 128.5, 128.4, 116.5, 45.5, 17.0. m/z
(electron impact) 160 (Mþ).
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Photolysis of 2-methyl-1-phenylbut-3-en-1-one (1)
Product Studies of 1 in Argon or Oxygen-Saturated CDCl3
[18] O. Stern, M. Volmer, Phys. Z. 1919, 20, 183.
A solution of 1 (15 mg, 93 mmol) in CDCl3 (2 mL) was
purged with argon or oxygen for 5 min and photolyzed through a
Pyrex filter at ambient temperature. The 1H NMR spectrum of
the irradiated solution was recorded every 30 min during the 3-h
irradiation period, and no significant changes were observed in
the 1H NMR spectra.
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H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov,
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Supplementary Material
Cartesian coordinates and energies of 1 and NMR and IR spectra
1
of 1; synthesis and H NMR spectra of 1-d are available from
the Journal’s website.
Acknowledgements
¨
S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz,
This work was supported by National Science Foundation (CHE-1057481)
and the Ohio Supercomputer Center. HDMS acknowledges Doctoral
Enhancement fellowship from the Department of Chemistry, University of
Cincinnati.
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