Extremely Long Lived Localized Singlet Diradicals in a Macrocyclic Structure: A Case Study on the Stretch Effect

Article abstract of DOI:10.1002/chem.201803076

Localized singlet diradicals have attracted much attention, not only in the field of bond-homolysis chemistry, but also in nonlinear optical materials. In this study, an extremely long lived localized singlet diradical was obtained by using a new molecular design strategy in which it is kinetically stabilized by means of a macrocycle that increases the molecular strain of the corresponding σ-bonded compound. Notably, the lifetime of this diradical (14 μs) is two orders of magnitude longer than that of a standard singlet diradical without a macrocyclic structure (≈0.2 μs) at 293 K. The species is persistent below a temperature of 100 K. In addition to the kinetic stabilization of the singlet diradical, the spontaneous oxidation of its corresponding ring-closed compound at 298 K produced oxygenated products under atmospheric conditions. Apparently, the “stretch effect” induced by the macrocyclic structure plays a crucial role in extending the lifetime of localized singlet diradicals and increasing the reactivity of their corresponding σ-bonded compounds.

Full text of DOI:10.1002/chem.201803076

A Journal of
Accepted Article
Title: Extremely Long-lived Localized Singlet Diradicals in a
Macrocyclic Structure: A Case Study of the Stretch Effect
Authors: Manabu Abe, Yuta Harada, Zhe Wang, Sayaka Hatano, and
Shunsuke Kumashiro
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Extremely Long-lived Localized Singlet Diradicals in a
Macrocyclic Structure: A Case Study of the Stretch Effect
Yuta Harada,[a] Zhe Wang,[a] Shunsuke Kumasiro,[a] Sayaka Hatano,[a] and Manabu Abe*,[a],[b]
Abstract: Localized singlet diradicals have attracted much attention
not only in the field of bond-homolysis chemistry, but also in non-
linear optical materials. In this study, an extremely long-lived
localized singlet diradical emerged using a new molecular design–a
macrocyclic structural effect–that is kinetically stabilized by
increasing the molecular strain of the corresponding σ-bonded
product. Notably, the lifetime of this diradical (14 μs) is two-orders of
magnitude longer than that of a standard singlet diradical without a
macrocyclic structure (~0.2 μs) at 293 K. The species is persistent at
below the temperature of 100 K. In addition to the kinetic
stabilization of the singlet diradical, the spontaneous oxidation of its
corresponding ring-closed compound at 298 K produced oxygenated
products under atmospheric conditions. Apparently, the “stretch
effect” induced by the macrocyclic structure plays a crucial role in
extending the lifetime of localized singlet diradicals and increasing
the reactivity of their corresponding σ-bonded products.
DR2a-f have been intensively investigated for over a decade
(Chart 1).[11] The lifetime in benzene at 293 K has been reported
to range from τ293 = 2 ns to ~5500 ns depending on the
substituents around the radical sites, which kinetically and/or
thermodynamically stabilize the localized singlet diradicals. A
unique nonlinear optical property has been also discovered for
species with diradicaloid character.[12]
A new kinetic stabilization effect, termed the “stretch
effect”,[13] on the reactivity of the localized singlet diradical S-
DR2g,h was designed and examined in the present study.
Notably, laser flash photolysis (LFP) experiments of the
precursor azoalkanes AZ2g,h (Scheme 1) revealed that the
lifetimes of S-DR2g,h are roughly two orders of magnitude
longer (τ293 = ~14 μs) than S-DR2e[14] (τ293 = ~0.2 μs). The
intermolecular reactivity of the extremely long-lived singlet
diradical S-DR2h was also investigated in this study.
MeO OMe
Y
Y
O
O
Introduction
X
X
λmax ~560-590 nm
S-DR2b: Y = H, τ293 = 320 ns
S-DR2c: Y = CN, τ293 = 625 ns
S-DR2d: Y = OMe, τ293 = 1050 ns
S-DR2a
τ293 = 2 ns
λmax ~508 nm
Open-shell molecules with unpaired electrons have attracted
considerable attention not only in the field of reactive
intermediate chemistry[1] but also in other chemical sciences
such as non-linear optical materials,[2] organic magnets,[3]
luminescent materials,[4] and biological studies.[5] The three
dimensional and electronic structures of such high-energy
molecules play a crucial role in their functionality. In general,
however, the structure and reactivity of such species are
experimentally elusive because of their short-lived character.
Studies aimed at extending the lifetime of such reactive species
is worthwhile to fully characterize their structure and reactivity,
which give rise to their unique properties. Several studies on the
isolation of highly energetic species such as carbenes,[6]
radicals,[7] and delocalized diradicals,[8] have been reported
using kinetic and thermodynamic stabilization.
n
DR1
E
E
E
E
1/τ
ArO OAr
E2E2
MeO OMe
Ar
Ar
X
X
τ293 = 5394 ns
n
S-DR2f
OMe
S-DR2e
τ293 = 209 ns
λmax ~593 nm
Ar =
λmax ~573 nm
OMe
Chart 1
MeO
OMe
MeO OMe
MeO OMe
Ar’ Ar’
Ar'
N
hν
(355 nm)
Ar’
Ar’
1/τ293
N
Localized diradicals are key intermediates in bond-
homolysis processes.[9] The isolation of singlet diradicals has
been reported in the four-membered heterocyclic systems
(E2E2).[10] The ground state spin-multiplicity of propane-1,3-diyl
diradicals DR1 and the direct observation of singlet diradicals S-
–N2
Ar'
AZ2g,h
S-DR2g,h
CP2g,h
DR2g: τ293 = 12.7 µs
DR2h: τ293 = 14.0 µs
R
R
[a]
Yuta Harada Zhe Wang Shunsuke Kumashiro, Sayaka Hatano,
Prof. Manabu Abe*
Department of Chemistry, Graduate School of Science, Hiroshima
University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-
8526, Japan
Ar’
Ar’
E-mail: mabe@hiroshima-u.ac.jp
[b]
Prof. M. Abe
g: R = H, h: R = C8H17
Hiroshima University Research Center for Photo-Drug-Delivery
Systems (HiU-P-DDS)
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
Scheme 1. This study: Macrocyclic effect on the reactivity of localized singlet
diradicals.
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Results and Discussion
8
21
22
29
Synthesis of azoalkane AZ2g,h. The synthesis of
azoalkanes AZ2g,h, the precursors of DR2g,h, were achieved
by the Hünig[15] and the inter- and intramolecular Sonogashira
cross-coupling[16] reactions as the key steps (Scheme S1). The
n,π* electronic transition of the azo chromophore was observed
at 359 nm (ε = 183 dm3 mol–1 cm–1) and 358 nm (ε = 171 dm3
mol–1 cm–1) in benzene for AZ2g and AZ2h, respectively (Figure
S1). The molar extinction coefficients (ε) at ~360 nm were
somewhat larger than that of a typical azo chromophore. For
example, the ε of AZ2b in benzene, which is the precursor of
DR2b, has been reported to be ~112 at 367 nm, indicating that
the ε values of AZ2g,h contain a contribution from the 1,3-
diacetylphenyl chromophore. A high-quality single crystal of
AZ2g was obtained by recrystallization, and the macrocyclic
structure of AZ2g was confirmed by single-crystal X-ray
crystallographic analysis (Figure 1). Table 1 summarizes the
C9–C13 distance in AZ2g, which becomes the radical center
after denitrogenation, and selected bond angles. Slightly bent
structures were observed in the four triple bonds: C10–C16–C25
= 175.68° (entry 3), C16–C25–C15 = 175.36° (entry 4), C18–
C21–C29 = 176.63°(entry 7), and C21–C29–C2 = 174.61°(entry
8). The experimentally obtained values were well reproduced by
optimizing the geometry of AZ2g at the B3LYP/6-31G(d)[17] level
of theory (Table 1). For instance, the experimentally obtained
and calculated C9–C13 bond distances were found to be very
similar (2.264 and 2.269 Å, respectively; Table 1, entry 1). The
slightly bent triple bonding ~175° was also well reproduced by
the calculations.
Macrocyclic effect on the molecular structures. The
singlet diradical S-DR2g and its ring-closed compound trans-
CP2g were also computed at (R,U)B3LYP/6-31G(d) (Table 1).
The distance between the radical centers at C9 and C13 was
calculated to be 2.389 Å in S-DR2g, which was longer than that
in AZ2g (entry 1). The C9–C6–C13 angle in the propane-1,3-diyl
moiety was larger than that of AZ2g (entry 2), which is in
consistent with the longer distance between C9 and C13 (entry
1). The triple bonds in S-DR2g were found to be more linear
than those in AZ2g (entries 3–10). For example, the C10–C16–
C25 and C16–C25–C15 angles in S-DR2g were 179.12° and
178.32°, respectively, whereas they were 175.20° and 174.47°
in AZ2g (entries 3,4). In contrast to the linear triple bonding in S-
DR2g, relatively distorted triple bonds were computed in the
ring-closed compound trans-CP2g (entries 3–10). For example,
the angles C5–C22–C8, C22–C8–C7, and C18–C21–C29 were
7
18
5
2
20
15
25
16
9
13
23
17
6
10
3
13
9
6
Figure 1. Top and side views of X-ray crystallographic structures of AZ2g.
Table 1. Experimental and computed structural dataa for AZ2e,g,b DR2e,g,c
trans-CP2e,gb
MeO
OMe
Ar'
6
MeO
6
OMe
OMe
Ar’
6
13
13
Ar'
13
9
N
9
N
9
OMe
Ar'
Ar’
Ar'
trans-CP2
S-DR2
AZ2
=
=
g
Ar’
Ar’
e
AZ2eexp
AZ2gex
p
AZ2ecal
AZ2gcal
S-DR2ecal
S-DR2gcal
trans-CP2ecal
trans-CP2gcal
entry
2.255
2.264
91.73
92.51
–
175.68
–
175.36
–
176.44
–
175.38
–
176.63
–
174.61
–
2.269
2.269
91.92
91.75
–
175.20
–
174.47
–
175.82
–
176.58
–
177.02
–
177.10
–
2.389
2.389
103.93
103.78
–
179.12
–
178.32
–
176.25
–
174.84
–
174.87
–
176.34
–
1.547
1.583
61.57
63.15
–
1
2
C9–C13
C9–C6–C13
C10–C16–C25
C16–C25–C15
C5–C22–C8
C22–C8–C7
C18–C21–C29
C21–C29–C2
C20–C23–C17
C23–C17–C3
3
176.06
–
169.37
–
169.76
–
169.56
–
173.38
–
177.75
–
4
5
6
7
8
9
175.81
–
177.18
176.48
–
176.30
178.30
–
179.17
178.35
–
10
176.27
a Bond lengths in Å, angles in °. b and c Optimized at (R)B3LYP/6-31G(d) and
(U)B3LYP/6-31G(d), respectively.
directions in AZ2g (Figure 2). This macrocyclic effect was also
observed more significant in the computed ring-closed structure
trans-CP2, although the distance in S-DR2 was calculated to be
nearly identical for S-DR2e and S-DR2g (Table 1, entries 1,2).
The distance in CP2g (1.583 Å) was found to be longer than that
in CP2e (1.547 Å), indicating that the two carbons in CP2g are
pulled in opposite directions because of the macrocyclic ring
strain (Figure 2). This new kinetic stabilization effect induced by
the macrocyclic ring is expected to extend the lifetime of the
singlet diradical S-DR2g and the reactivity of the ring-closed
compound CP2g relative to S-DR2e and CP2e.
found to be 169.76°, 167.56°, and 173.38° in CP2g (entries 5–7).
To examine the macrocyclic effect more fully, the
structures of AZ2e, S-DR2e, and trans-CP2e, which have no
macrocyclic ring, were compared with those of AZ2g, S-DR2g,
and trans-CP2g (Table 1, entries 1,2). As expected, the distance
between the two azocarbons C9 and C13 in AZ2g (2.264 Å) was
slightly longer than that of the related azocarbons in AZ2e
(2.255 Å). The single-crystal X-ray structure of AZ2e is shown in
Figure S2. In addition to the effect on the carbon–carbon
distance, the C9–C6–C13 bond angle in AZ2e (91.73°) was
smaller than that in AZ2g (92.51°), indicating that the
macrocyclic ring system stretches C9 and C13 in opposite
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isomer appears to be possible for cis-CP2g at 298 K; indeed,
the activation enthalpy for the transition from S-DR2g to trans-
CP2g is computed to be 68.5 kJ mol–1, as observed for the
thermal isomerization from cis-CP2b to trans-CP2b.[9n] The
singlet ground states of DR2e and DR2g were also confirmed by
the computations as ΔEST = ES – ET = ~ –17 kJ mol–1 (Table 2).
)
a in CP2e and CP2g
MeO OMe
13
Table 2. Macrocyclic effect on ΔHrel (ΔEZPE
9
199
ΔHrel
stretch
stretch
* *
+68.5
C9–C13
distance
in pm
199
+50.3
stretch
effect
241
241
2e
2g
=
N
N
* *
+63.6
208
+17.5
+17.1
AZ2g
CP2g
ΔHrel
2e
2g
+44.2
207
Figure 2. Stretch effect induced by a macrocyclic system.
T-DR2
161
–0.5
cis-CP2g
The bending effect (θ) of the triple bonds on the
destabilization energy was estimated by the angle-dependent
energy change of 1,2-diphenylethyne at the B3LYP/6-31G(d)
level of theory (Figure 3). The total electronic energy at 175°
S-DR2
0.0
0.0
158
–20.9
cis-CP2e
trans-CP2g
–13.7
233
239
was
2
kJ mol–1 higher than that at 180°. Energetic
160
trans-CP2e
–36.9
destabilizations of 8, 18, and 33 kJ mol–1 were computed for
angles of 170°, 165°, and 160°, respectively, for one
diphenylethyne moiety.
155
ΔHrel (ΔErel) /kJ mol–1
S-DR
T-DR
trans-CP
cis-CP
TStrans/ νi, b
TScis/ νi, b
44.18
63.58
35
0.00
17.5
–36.87
–13.74
(–9.67)
θ
θ
2e
2g
(66.18)
/ –329.33
68.54
(45.96)
/ –371.65
50.31
30
25
20
15
10
5
(0.00)
(17.2)
(–36.53)
0.00
17.1
–20.85
–0.53
(0.38)
1,2-diphenylethyne
(71.23)
/ –45.09
(53.07)
/ –36.32
(0.00)
(16.7)
(–19.89)
aOptimized at the (R,U)B3LYP/6-31G(d) level of theory. bThe values of νi are
imaginary frequencies computed at the same level of theory.
0
160 162 164 166 168 170 172 174 176 178 180
θ / ꢀ
UV-vis and EPR studies on Photolysis of AZ2e,g,h and
16. Transient absorption spectroscopic analyses were
Figure 3. Bending effect on the energetic destabilization of the 1,2-
diphenylethyne moiety.
conducted following the LFP of AZ2g (5 mg, 2.5 mM, Abs355
=
0.46) and AZ2h (6 mg, 2.4 mM, Abs355 = 0.48) in benzene with a
Nd-YAG laser (λexc = 355 nm, 5 ns pulse, 7 mJ) at 293 ± 0.1 K
(Figure 4). Under a N2 atmosphere, two transient species were
observed at ~480 and ~580 nm for AZ2g and decayed with first
order kinetics (Figure 4a). The lifetimes of each species were
determined as 2.1 ± 0.1 μs and 12.7 ± 1.1 μs using single
exponential kinetic fitting. Under air conditions, only the transient
species at ~580 nm was observed with a small band at ~460 nm
(Figure 4b) and also decayed with first-order kinetics, giving a
lifetime of 14.0 ± 1.1 μs and 13.8 ± 1.2 μs, respectively. The
nearly the same lifetime of the 580 and 460 nm clearly indicates
that the two bands are derived from the same species. This
lifetime is nearly identical under a N2 atmosphere, indicating that
the species at 480 nm is a triplet and that at 580 nm is a singlet.
The absorption maximum of localized singlet diradicals DR2
typically occur at ~500–600 nm (Chart 1).[11] The absorption
band corresponds to a π-to-π* electronic transition of the π-
single bonding system (C–π–C).[18] All the experimental
evidence suggests that the transient species at 580 nm is the
singlet diradical S-DR2g. The lifetime of S-DR2g (~14 μs at 293
To further investigate the macrocyclic effect on the
energetic destabilization of CP2g, the energy difference
between the singlet diradicals S-DR2g and CP2g was computed
at the B3LYP/6-31G(d) level of theory and compared with that of
S-DR2e and CP2e (Table 2). As expected by the stretch effect,
as induced by the strain of bending the triple bond within the
system (Table 1, Figure 3) the enthalpy differences for S-DR2g
and CP2g with macrocyclic structures (i.e., –20.9 (trans isomer)
and –0.5 (cis isomer)) were found to be much smaller than those
of S-DR2e and CP2e (i.e., –36.9 (trans isomer) and –13.7 (cis
isomer)). Consistent with the destabilization of the ring-closed
compound CP2g, the transition state energies from the singlet
diradical S-DR2g to trans- and cis-CP2g (68.5 and 50.3 kJ mol–1,
respectively) were larger than those for the reaction from DR2e
without the macrocyclic ring to trans- and cis-CP2e (63.6 and
44.2 kJ mol–1, respectively), indicating that S-DR2g is kinetically
stabilized by the macrocyclic ring. As previously found for S-
DR2b, formation of cis-CP2e and cis-CP2g was predicted to be
energetically more favorable than formation of the trans
isomers.9n Thermal isomerization from the cis- to the trans-
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K) was almost two-orders of magnitude longer than the parent
S-DR2e compound (209 ns at 293 K).[14] As expected by the
stretch effect induced by the macrocyclic system (Figure 2), S-
DR2g is highly kinetically stabilized as an extremely long-lived
localized singlet diradical.
(EPR) spectroscopic and phosphorescence analyses were
conducted under similar conditions to those used for the UV-vis
hν
(a)
360 nm
during photolysis
under dark after 3 min photolysis
AZ2g
MTHF
90 K
0.1
hν
355 nm
(a) under N2
S-DR2f
AZ2g
benzene
0.05
0.04
at 480 nm, τ293 = 2.1 µs
at 580 nm, τ293 = 12.7 µs
1 µs
5 µs
10 µs
30 µs
0.03
0.02
0.01
0.05
0.04
0.03
0.02
0.01
0
0.05
0
T-T
20
40 60
80 100
time /µs
400
450
500
550
600
650
wavelength /nm
ms = ±2
(b)
400
450
500
550
600
650
700
D/hc = 0.123 cm–1
E/hc = < 0.001 cm–1
wavelength /nm
-0.01
ms = ±1
x,y
hν
355 nm
z
(b) under air
2049
4676
AZ2g
0.06
1000
2000
3000
4000
x,y
5000
benzene
0.05
z
0.04
0.05
1 µs
5 µs
10 µs
30 µs
at 580 nm
τ293 = 14.0 µs
magnetic field /G
0.03
0.02
ms = ±1
0.04
0.03
0.02
0.01
0
0.01
1512
20
40 60
80 100
time /µs
(c)
4000000
3500000
3000000
2500000
2000000
1500000
1000000
500000
0
25000
20000
15000
10000
5000
0
400
450
500
550
600
650
700
(1)
(2)
wavelength /nm
-0.01
Figure 4. Transient absorption spectra for the LFP (293 K, benzene) of (a)
AZ2g (5 mg, 2.5 mM, Abs355 = 0.46) under N2 conditions. Insert: time profiles
of the 480 and 580 nm species. (b) AZ2g (5 mg, 2.5 mM, Abs355 = 0.46) under
atmospheric conditions. Insert: time profile of the 580 nm species.
-5000
-10000
To gain additional insight into the triplet species observed
at ~480 nm in the LFP experiment (Figure 4a), the UV-vis
absorption spectrum for the photolysis of AZ2g was recorded
under low-temperature glassy matrix conditions using a solution
of AZ2g in degassed 2-methyltetrahydrofuran (MTHF). As
shown in Figure 5a, two bands were observed at ~470 nm and
~580 nm during the photolysis of AZ2g in the MTHF-glassy
matrix at 90 K. After 3 min of photolysis, the ~470 nm species
decayed under dark conditions while the 580 nm species having
the low-intensity band at ~460 nm persisted. These phenomena
clearly indicate that the 470 nm species is an electronically
excited state that has a T-T absorption at ~470 nm, and the 580
nm species with 460 nm is a reactive intermediate (i.e., S-DR2g),
which is existent at low temperature.
365
415
465
515
565
615
665
715
765
-500000
Figure 5. (a) UV-vis spectroscopic analysis of the photolysis of AZ2g using a
Xe lamp (360 ± 10 nm) in a MTHF matrix at 90 K. (b) EPR spectroscopic
analysis during the photolysis of AZ2g in a MTHF matrix at 80 K. (c) (1)
Fluorescence spectrum of AZ2g (0.12 mM, λexc = 360 nm) in MTHF at 77 K;
(2) Phosphorescence spectrum of AZ2g (0.12 mM, λexc = 360 nm) in MTHF at
77 K.
absorption measurements (Figures 5b). During the photolysis of
AZ2g using a 365 nm LED, triplet EPR signals were observed at
1512, 2049, 2682, 3944, and 4676 G. The high-intensity signal
at 1512 G corresponds to the half-field transition (ms = ± 2) of
the allowed transitions (ms = ± 1) at 2049 (z), 2682 (x,y), 3944
(x,y), and 4676 (z), from which the zero-field splitting parameters
D/hc and E/hc were determined as 0.123 and < 0.001 cm–1,
respectively. An accurate E value was difficult to obtain owing to
the broad x and y signals. The large D value suggests that the
To confirm the spin multiplicity of the 470 nm and 580 nm
species, low-temperature electron paramagnetic resonance
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triplet state is not derived from the 1,3-diradical moiety but from
Product Analysis in Photolysis of AZ2e,h, and 16 and
Intermolecular Reactivity of CPe,h, and 17. Product analysis
of the photochemical reaction of AZ2e, AZ2h, and 16 was
conducted at 365 nm using a LED in a sealed degassed NMR
tube in a C6D6 solution of azoalkanes at 298 K. The photolysate
was directly analyzed by 1H and 13C NMR spectroscopy
(Scheme 2, Figures 6, S6, S7). The NMR analysis of the
photochemical denitrogenation of AZ2g was difficult due to its
low solubility. A clean photochemical denitrogenation AZ2e,
AZ2h, and 16, Φ–N2 ~0.9,[23] was observed to quantitatively
produce the corresponding ring-closed compounds trans-CP2e,
trans-CP2h, and trans-17, respectively (Scheme 2). The trans-
selective formation of CP2 was observed at ~298 K as final
a
benzene moiety. The typical D value for triplet 1,3-
diphenylpropane-1,3-diyl diradical species is known to be ~0.05
cm–1.[19] D values of ~0.12–0.15 cm–1 have been reported for the
triplet state of benzene derivatives such as benzene itself[20] and
1,3-dicyanobenzene.[21] The typical triplet signals disappeared
under dark conditions, indicating that they were derived from the
470 nm species. In fact, structured phosphorescence was
observed at λ0-0 = ~470 nm following the 360 nm excitation of
AZ2h (0.12 mM) in MTHF at 77 K together with fluorescence at
λ0-0 ~370 nm (Figure 5c). The concomitant generation of the
triplet state of the di-meta-substituted benzene moiety at ~470
nm with the singlet diradical DR2g at 580 nm is reasonable,
because the absorbance at ~360 nm occurs viz. the mixture of
azo and 1,3-diacetylphenyl chromophores (cf. the ε values for
AZ2g,h). After 5 min of irradiation at 365 nm using a LED, no
triplet EPR signals were observed under dark conditions,
although purple-colored 580 nm species were clearly observed
by UV-vis spectroscopy (Figure 5a). This observation provides
evidence that the 580 nm species is the singlet state of the
localized diradical S-DR2g.
A similar transient absorption spectrum was also observed
for the LFP study of AZ2h (R = C8H14, Figure S3) under air
conditions. The lifetime of the 580 nm species, DR2h, was
determined as 14.2 ± 0.8 μs at 293 K in benzene, which is
nearly identical to that of DR2g, indicating a negligible alkyl
chain effect on the lifetime. The temperature-dependent change
of the lifetime of S-DR2g,h (Figures S3,S4) gave activation
parameters (Ea, logA, ΔH‡, ΔS‡, ΔG293‡) for the ring-closing
process of the singlet diradicals S-DR2g,h to CP2g,h using
Eyring and Arrhenius plots at temperatures between 283 and
313 K (Table 3). The activation energy and enthalpy of the
decay process of S-DR2g,h were found to be ~20 kJ mol–1
higher than that for DR2e, indicating that the singlet diradicals
are kinetically stabilized by the macrocyclic ring. To understand
the substituent effect at the meta-positions in S-DR2g,h on the
lifetime, a LFP study on compound 16 with two arylacetyl
substituents at the meta-position, which is the byproduct in the
synthesis of AZ2h, was also performed (Scheme S1, Scheme 2,
Table 3). The lifetime of the singlet diradical S-DR16 was found
to be 620 ± 12 ns at 293 K (Figure S5), which is much shorter
than that of S-DR2g,h with the macrocyclic system (Table 3). As
judged by the large logA values of ~12, the decay of 580 nm
species is spin-allowed process to give the corresponding
singlet products (Scheme 2).[22] The activation energies of the
first-order decay process of DR2g,h were found to be ~52–55 kJ
mol–1, which are close to the computed values for the ring-
closing reaction to the cis-isomer of the ring-closing product, cis-
CP2g (Table 2). The negligible substituent effect of the meta-
substituents on the thermodynamic stabilization of the diradical
was realized by the similar absorption maxima of S-DR2e, S-
DR2g, and S-DR16, which were found to be 575 nm, 575 nm,
and 576 nm. Furthermore, the computed spin-density of 0.735 at
the benzylic position of the triplet state of DR2e was also similar
to that of 0.736 in T-DR2g at the UB3LYP/6-31G(d) level of
theory, indicating the negligible meta-substituent effect on the
thermodynamic stability of DR2.
products, as observed for S-DR2b[9n]
.
Table 3. Lifetime (τ293) and Activation Parameters (Ea, logA, ΔH‡, ΔS‡,
ΔG293‡) of S-DR2e,g,h, and S-DR16.
b
c
c
Ea
ΔH‡
ΔS‡
‡ c
τ293
log(A
ΔG293
DR
/kJ
/kJ
/J K–1
mol–1
–21.5
±0.8
b
/μsa
/s–1
)
/kJ mol–1
34.2±0.8
44.6±0.9
44.7±0.4
37.2±0.8
mol–1
30.5
±0.4
55.4
±0.7
52.3
±0.4
31.2
±0.8
mol–1
28.0
±0.4
52.9
±0.7
49.7
±0.4
28.7
±0.8
0.209
±0.012
14.0
12.1
±0.1
14.7
±0.2
14.1
±0.1
11.7
±0.1
DR2e
DR2g
DR2h
28.4
±1.1
±3.2
14.2
17.1
±0.8
±1.2
0.620
±0.012
–28.9
±2.7
DR16
a
In benzene at 293 K; errors are standard derivation from five data points.
b,c
Determined by Arrhenius plots and Eyring plots, respectively, of the
lifetimes of singlet diradicals monitored at 580 nm at five temperatures
between 283 and 313 K.
Compounds trans-CP2e and trans-17 with no macrocyclic
ring were stable under air conditions and also on silica gel, as
confirmed by 1H NMR spectroscopy after isolation of trans-CP2e
and trans-17, see the experimental section in supporting
information. Interestingly, however, trans-CP2h with the
macrocyclic system was highly air sensitive even at 298 K and
gave the oxygenated products 18, 19, and 20 with isolated
yields of 25%, 38%, and 15%, respectively (Figure 6, Scheme 3).
The structures of 18–20 are quite similar to the oxygenated
products of compound 21 under air at benzene reflux
conditions,[24] and their formation suggests that the
endoperoxide 22 is the intermediate for product formation. The
high reactivity of trans-CP2h clearly indicates that the C9–C13 σ
bond is weaken by the stretched effect induced by the
macrocyclic system.
In-situ
NMR
analysis
of
the
photochemical
denitrogenation of AZ2h at 188 K. As shown in Table 2, the
gas-phase DFT calculations suggest that S-DR2g is nearly the
same in energy with cis-CP2g. The computational study clarified
that the cis-isomers cis-CP2g,h are the photo-primary product of
AZ2g,h. Low-temperature in situ NMR analysis at 188 K was
conducted for the reaction of AZ2h (3.4 mg, 9.5 mM) in d8-
toluene under N2 using a 355 nm YAG laser (Figure 7), which
was guided into the NMR cavity using a quartz rod.[9n],[25] At 188
K, thermally labile species (signals a-d) were observed with
trans-CP2h (signals e-l) and unreacted AZ2h (signals m-p).
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After 15 min under dark conditions, the signals a-d disappeared
with a concomitant increase of signals e-l (Figure 7), suggesting
that the signals a-d are derived from cis-CP2h and/or S-DR2h.
From the activation parameter found for the decay process of S-
DR2h (Table 3), the lifetime of S-DR2h should be ~1 s at 188 K;
thus, the signals a-d do not correspond to S-DR2h but to cis-
CP2h. These observations suggest that the DFT calculations
overestimate the destabilization energy of CP2g and/or the
stability of S-DR2g. The formation of cis-CP2h and its thermal
isomerization to trans-CP2h indicate that the primary
photoproduct from AZ2h included cis-CP2h. Consistent with the
experimental observations, the experimentally determined
activation energy of the S-DR2g decay process, Ea = 55.4 kJ
mol–1 and ΔH‡ = 52.9 kJ mol–1 (Table 3), was close to the
18
20
19 19
c
MeO
19
18
e
OMe
O2
298 K
68 h
Ar'
d
b
trans-CP2h
a
Ar'
d
MeO
e
OMe
365 nm
–N2
Ar'
N
c
b
AZ2h
N
a
Ar'
computed energy barrier of ΔEZPE = 53.1 kJ mol–1 and ΔH‡
50.3 kJ mol–1 involving S-DR2g and cis-CP2g (Table 2).
=
Figure 6. In situ NMR (400 MHz) analysis of the photochemical
denitrogenation of AZ2h (10.6 mg, 30 mM) in a sealed NMR tube in d6-
benzene under degassing conditions with a LED lamp (365 nm) at 298 K. (a)
1HNMR spectrum of AZ2h before irradiation. (b) 1HNMR spectrum of trans-
CP2h after 15 min of irradiation. (c) 1HNMR spectrum of trans-CP2h after 67 h
under atmospheric conditions at 298 K.
MeO
NOE
OMe
MeO
OMe
Ph
hν
(365 nm)
Ph
N
H
N
–N2
Ph
Ph
OMe
Ar’
at 298 K
MeO OMe
AZ2e
trans-CP2e
O2
Ar’
Ar’
O
trans-CP2h
O
OMe
Ar’
DR2h
MeO
MeO
NOE
OMe
Ar'
OMe
Ar'
hν
(365 nm)
Ar'
N
H
Ar'
22
+
OMe
OMe
N
O
–N2
Ar'
trans-CP2h
Ar’
O
Ar’
Ar'
O
OMe
Ar'
AZ2h
MeO
Ar’
MeO
Ar’
CO2Me
O
O
cis-CP2h
detected at 188 K
Ar'
OMe
H
H
NOE
MeO
NOE
OMe
Ar”
OMe
NOE
18 (25%)
20 (15%)
19 (38%)
hν
(365 nm)
Ar”
N
H
N
Ar”
–N2
Ar”
trans-17
at 298 K
16
EtO
OEt
Ph
C8H17
=
Ar’
Ar’
Ph
21
Ar” =
Scheme 3. Intermolecular reaction of DR2h with molecular oxygen (O2).
Scheme 2. Product analysis of azoalkanes AZ2e, AZ2h, and 16 in the
photochemical denitrogenation in benzene at 298 K.
ΔT
hν
–N2
AZ2h
cis-CP2h
trans-CP2h
f
k
j
trans-CP2g at 188 K
h
i
l
g
(d)
(c)
(b)
(a)
e
f
15min
dark
60min
d
c
b
a
365 nm
o
p
m
n
AZ2h
6.0
5.5
4.0
3.5
3.0
2.5
2.0
4.5 5.0
chemical shift (δ) /ppm
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Figure 7. In situ NMR (400 MHz, 2–6 ppm) analysis of the photochemical
denitrogenation of (a) AZ2h (3.4 mg, 9.5 mM) in d8-toluene under N2
conditions with a YAG laser (355 nm, 30 mJ) at 188 K; (b) after 60 min; (c)
after 15 min under dark; (d) trans-CP2h at 188 K in d8-toluene.
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Conclusions
In the present study, a new kinetic stabilization (stretch)
effect, which is induced by the presence of a macrocyclic ring,
has been examined in the context of affecting the reactivity of
localized singlet diradicals and their corresponding ring-closed
compounds. Extremely long-lived singlet diradicals S-DR2g,h
(τ293 = ~14 μs) emerged in this study. The experimental and
computational studies clarified that the distorted triple bonding
system in the ring-closed compounds CP2g,h is central to
activating the stretch effect, which kinetically stabilizes the
intermediary singlet diradicals DR2g,h. The high reactivity of
CP2h, which was induced by the stretch effect, was confirmed
by the spontaneous intermolecular reaction with molecular
oxygen at 298 K to give the oxidation products 18–20. The
observed stretch effect may spread into other bond activation
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chemistry[26]
such
as
radical
generation
reactions,
denitrogenations, and photochromism.
Experimental Section
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Full experimental methods including detailed synthetic procedure and
characterization data, NMR, MS, UV-vis, X-ray crystallographic details,
and DFT calculations (PDF) are available in the supporting information.
Acknowledgements
NMR and MS measurements were performed at N-BARD,
Hiroshima University. M.A. gratefully acknowledges financial
support by JSPS KAKENHI (Grant No. JP17H03022).
Keywords: localized singlet diradical • kinetic stabilization •
stretch effect • macrocyclic effect • oxidation
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Entry for the Table of Contents (Please choose one layout)
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A new kinetic stabilization (stretch)
effect, which is induced by the
presence of a macrocyclic ring, has
been examined in the context of
affecting the reactivity of localized
single diradicals. Extremely long-
lived singlet diradicals S-DR2g,h
(τ293 = ~14 μs) emerged in this
study.
Yuta Harada,[a] Zhe Wang,[a] Shunsuke
Kumashiro,[a] Sayaka Hatano,[a] and
Manabu Abe*,[a],[b]
Page No. – Page No.
Title
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