Chemiexcitation efficiency for the charge-transfer-induced chemiluminescent decomposition of 3-hydroxyphenyl-substituted dioxetanes in an aqueous system

Article abstract of DOI:10.1016/j.tetlet.2014.01.089

The decomposition of 3-oxyphenyl-3-methoxy-4-(2′-spiroadamantane)-1, 2-dioxetane (A) and 5-tert-butyl-4,4-dimethyl-1-(3-oxyphenyl)bicyclo[3.2.0] heptane (B) in NaOH/H₂O gives light in poor yield, which is several orders of magnitude lower than that in aprotic solvents. To understand the poor chemiluminescence efficiency in NaOH/H₂O, we investigated the behaviors of the authentic emitters, methyl 3-oxidobenzoate (C) and 2,2,4,4-tetramethyl-3-oxopentyl 3-oxidobenzoate (D). We found that D was weakly fluorescent though hydrolyzed in NaOH/H₂O, and estimated that the singlet-chemiexcitation efficiency Φ_S was 6.1 × 10 ^-3 for the decomposition of B in NaOH/H₂O. On the other hand, Φ_S for A could not be estimated, since C was hydrolyzed too rapidly to observe its fluorescence.

Full text of DOI:10.1016/j.tetlet.2014.01.089

Tetrahedron Letters 55 (2014) 1644–1647
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Chemiexcitation efficiency for the charge-transfer-induced
chemiluminescent decomposition of 3-hydroxyphenyl-substituted
dioxetanes in an aqueous system
⇑
Nobuko Watanabe , Azusa Oguri, Miho Horikoshi, Hikaru Takatsuka, Hisako K. Ijuin,
⇑
Masakatsu Matsumoto
Department of Chemistry, Kanagawa University, Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan
a r t i c l e i n f o
a b s t r a c t
The decomposition of 3-oxyphenyl-3-methoxy-4-(20-spiroadamantane)-1,2-dioxetane (A) and 5-tert-
butyl-4,4-dimethyl-1-(3-oxyphenyl)bicyclo[3.2.0]heptane (B) in NaOH/H2O gives light in poor yield,
which is several orders of magnitude lower than that in aprotic solvents. To understand the poor chemi-
luminescence efficiency in NaOH/H2O, we investigated the behaviors of the authentic emitters, methyl 3-
oxidobenzoate (C) and 2,2,4,4-tetramethyl-3-oxopentyl 3-oxidobenzoate (D). We found that D was
weakly fluorescent though hydrolyzed in NaOH/H2O, and estimated that the singlet-chemiexcitation effi-
ciency US was 6.1 ꢀ 10ꢁ3 for the decomposition of B in NaOH/H2O. On the other hand, US for A could not
be estimated, since C was hydrolyzed too rapidly to observe its fluorescence.
Article history:
Received 6 December 2013
Revised 14 January 2014
Accepted 22 January 2014
Available online 31 January 2014
Keywords:
Chemiluminescence
Dioxetane
Chemiexcitation efficiency
Aqueous system
Ó 2014 Elsevier Ltd. All rights reserved.
The intramolecular charge-transfer-induced decomposition
(CTID) of oxidophenyl-substituted dioxetanes has received
considerable attention due to interest in the mechanisms of
bioluminescence and chemiluminescence and because of possible
applications in modern biological and clinical analyses using
chemiluminescence.1–4 Typical examples are adamantylidene-
substituted dioxetanes 1 and bicyclic dioxetanes 2, which undergo
chemiluminescent CTID through unstable oxidophenyl-substituted
dioxetane 3 or 4 produced by deprotonation or deprotection
(Scheme 1).2,5,6 Although dioxetanes 1 and 2 both effectively emit
light in an aprotic polar medium, they give light in quite poor yield
in an aqueous medium: the chemiluminescence efficiency UCL in
H2O versus CH3CN was ca. 1/16,000 for 1, and ca. 1/10,000 for 2.
This significant defect has been considerably improved through
the addition of a fluorescer and/or a surfactant for practical use in
an aqueous system.7,8 However, it is still unclear whether the
markedly low UCL is mainly due to poor singlet-chemiexcitation
efficiency US and/or to poor fluorescence efficiency Ufl of the
emitter produced for CTID of 1 or 2 in H2O. Since UCL is given as
estimate US as well as U
fl, when the fluorescence spectrum of
the authentic emitter coincides with the chemiluminescence
spectrum.
For the CTID of 3 and 4, the emitters produced are methyl 3-
oxidobenzoate 5 and its 2,2,4,4-tetramethyl-3-oxopentyl analog
6, respectively. However, both the fluorescence spectrum of the
authentic emitter 5 and the chemiluminescence spectrum of 3
have been reported to be considerably different from each other
in NaOH/H2O, while they are similar in an aprotic polar solvent
such as DMSO or acetonitrile.9 A similar discrepancy has also been
reported between CTID emission from 4 and fluorescence of 6.10 It
has very recently been reported that 5 undergoes rapid hydrolysis
to give a dianion 15 (vide infra) of 3-hydroxybenzoic acid 9, which
fl
shows a strong fluorescence with k
= 412 nm in a basic aqueous
max
solution.11 This work prompted us to report our findings that may
lead to a better understanding of the markedly low
U
CL for CTID of
3 and 4 in an aqueous system.
Bicyclic dioxetane 10 bearing a 4-hydroxy-2-methylbenzoxazol-
6-yl group has been reported to show
U
CL that is considerably high-
US
ꢀ
Ufl for dioxetane-based chemiluminescence, it is important
er than that for 2 in a NaOH/H2O system.12 As in the case of 1 and 2,
the fluorescence spectrum of the spent reaction mixture does not
coincide with the chemiluminescence spectrum of 10 in NaOH/
H2O (Scheme 2). The authentic emitter 11 prepared by dissolving
for the estimation of US to characterize the emitter and to under-
stand its fluorescence properties. Thus, we can first reliably
2,2,4,4-tetramethyl-3-oxopentyl
4-hydroxy-2-methylbenzoxaz-
⇑
Corresponding authors. Tel./fax: +81 463594111; +81 463589684.
E-mail addresses: nwatanab@kanagawa-u.ac.jp (N. Watanabe), matsumo-chem@
ole-6-carboxylate (12) in NaOH/H2O showed fluorescence
fl
max
(k
= 413 nm), the spectrum of which resembled that of the spent
kanagawa-u.ac.jp (M. Matsumoto).
http://dx.doi.org/10.1016/j.tetlet.2014.01.089
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

N. Watanabe et al. / Tetrahedron Letters 55 (2014) 1644–1647
1645
CT
O
O
O
O
MeO
Light
MeO
MeO
O
O
YO
O
O
Y
Y
3
5
1a-c
CT
Light
O
O
O
O
O
O
t-Bu
t-Bu
t-Bu
O
O
O
YO
O
O
4
6
2a-c
O
O
O
O
HO
HO
t-Bu
OH
OMe
a: Y = H
O
HO
b: Y = t-Bu(Me)2Si-
c: Na2O2P(O)-
9
7
8
Scheme 1. Base-induced chemiluminescent decomposition of 3-oxyphenyl-substituted 1,2-dioxetanes.
Me
Me
Me
O
O
O
B:
HB
O
O
O
O
O
O
N
N
N
H
t-Bu
t-Bu
t-Bu
O
O
O
HO
O
HO
Light
11
12
10
NaOH/ H2O
Me
Me
O
O
N
H
N
CO2H
CO2
HO
O
13
species X = 14
Scheme 2. Base-induced chemiluminescent decomposition of bicyclic dioxetane 10 bearing a 4-hydroxy-2-methylbenzoxazol-6-yl group.
reaction mixture when it was irradiated with light of kex = 313 nm.
fl
max
Furthermore, fluorescence with k
= 413 nm increased gradually
as time passed. On the other hand, when the solution of 11 was irra-
diated with light of kex = 370 nm, it showed weak fluorescence with
fl
max
k
= 469 nm, the spectrum of which coincided with that of chemi-
luminescence from 10 (Fig. 1).
Thus, we first attempted to carefully investigate the time-
course of absorption and the fluorescence spectra for freshly
prepared 11 in NaOH/H2O at 25 °C.13 Figure 2 shows that the
absorption at kabs = 337 nm decreased while the absorption at
k
abs = 320 nm increased over time. On the other hand, Figure 3
shows fluorescence spectra (kex = 370 nm) in which a peak at
fl
max
fl
max
k
= 469 nm decreased while a peak at k
= 413 nm increased
over time. Irradiation of the same sample with light of kex = 313 nm
fl
max
gave only fluorescence with k
at 413 nm that increased over
time, as shown in Figure 4.
These results strongly suggested that 11 with an absorption
abs
fl
maximum k
at 337 nm and a fluorescence maximum k
at
max
max
abs
469 nm changed gradually into a species X with k
at 320 nm
max
Figure 1. (A) Chemiluminescence spectrum of 10, (B) fluorescence spectrum of
authentic keto ester 11 (kex = 370 nm), and (C) fluorescence spectrum of 11
(kex = 313 nm) in NaOH/H2O.
fl
and k
at 413 nm in NaOH/H2O: species X was presumed to be
max
a dianion 14 of 4-hydroxy-2-methylbenzoxazole-6-carboxylic acid
13 based on the findings in a previous report (Scheme 2).11 Thus,
we subjected keto ester 12 to hydrolysis with NaOH in aqueous
EtOH to effectively give authentic 13: for the hydrolysis of 12 in
NaOH/H2O as a control experiment, the isolation of pure 12 was
The above results prompted us to reinvestigate the absorption
and fluorescence spectra of 6, as the authentic emitter for CTID
of 2, prepared from ester 8 in NaOH/H2O. When a freshly prepared
solution of 6 was irradiated with light of kex = 370 nm, it showed
quite hazardous. As expected, the authentic 13 showed strong fluo-
fl
max
rescence with k
at 413 nm when it was dissolved in NaOH/H2O.

1646
N. Watanabe et al. / Tetrahedron Letters 55 (2014) 1644–1647
Figure 2. Time-course of absorption spectra for keto ester 11 in NaOH/H2O.
Figure 5. Time-course of fluorescence spectra for ketoester 6 (kex = 313 nm) in
NaOH/H2O.
Figure 3. Time-course of fluorescence spectra for keto ester 11 (kex = 370 nm) in
Figure 6. Time-course of absorption spectra for ketoester 6 in NaOH/H2O.
NaOH/H2O.
O
O
O
O
O
OR
NaOH
15
5: R = Me
6: R =
O
t-Bu
Scheme 3. Rapid hydrolysis of an emitter, 3-oxidobenzoate, in NaOH/H2O.
spectra of the solution of 6,
a
peak initially observed at
abs
k
= 329 nm decreased and finally disappeared, while a peak at
max
abs
k
= 313 nm appeared and increased with time (Fig. 6). These re-
max
sults showed that the time-courses of absorption and fluorescence
spectra for the solution of 6 resembled those for 11 in a NaOH/H2O
system. Thus, we can see that the authentic emitter 6 showed very
fl
max
CL
max
weak fluorescence with k
= 466 nm, which coincided with k
of 2, though 6 rapidly underwent hydrolysis to give dianion 15 of
m-hydroxybenzoic acid (Scheme 3).
Figure 4. Time-course of fluorescence spectra for keto ester 11 (kex = 313 nm) in
NaOH/H2O.
As has very recently been reported,11 a solution of authentic
emitter 5 prepared from methyl m-hydroxybenzoate 7 in NaOH/
H2O showed time-dependent changes in fluorescence and absorp-
fl
max
weak fluorescence with k
= 466 nm, which rapidly disappeared.
On the other hand, when the solution of 6 was irradiated with light
of kex = 313 nm, it showed strong fluorescence with kfml ax = 413 nm,
the peak of which increased with time (Fig. 5). In the absorption
tion which resembled the case of 6, as illustrated in Figures 7 and 8.
fl
However, fluorescence with
k
= 465-470 nm, which corre-
max
CL
sponded to the k
of 2, was not observed. These results showed
max

N. Watanabe et al. / Tetrahedron Letters 55 (2014) 1644–1647
1647
0.33, which was two orders of magnitude higher than those for an-
ions of esters 6 and 11. Furthermore, based on
fl for 6 and for 11
U
shown above, we estimated the singlet chemiexcitation efficiency
US = 6.1 ꢀ 10ꢁ3 for CTID of 2a (4) and 1.9 ꢀ 10ꢁ2 for 10 in NaOH/
H2O. Thus, we can understand that the markedly low
U
CL is attrib-
uted to poor
U
fl of the emitter 6 as well as poor singlet-chemiexci-
tation efficiency US for CTID of 4 in an aqueous system.14,15 This
conclusion is presumably applicable to the case of 1, though the
Ufl of 3 could not be estimated.
Acknowledgments
The authors gratefully acknowledge financial assistance in the
form of Grants-in-aid (Nos. 21550052 and 22550046) for Scientific
Research from the Ministry of Education, Culture, Sports, Science,
and Technology, Japan.
Supplementary data
Figure 7. Time-course of fluorescence spectra for methyl ester 5 (kex = 313 nm) in
NaOH/H2O.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.01.
089.
References and notes
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Figure 8. Time-course of absorption spectra for methyl ester 5 in NaOH/H2O.
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that ester 5 was hydrolyzed far more rapidly than 6 so that fluores-
cence of 5 could hardly be observed.
Next, we attempted to estimate the rates of hydrolysis for 5, 6
and 11, by monitoring the time-course of fluorescence intensity
fl
max
at k
= 413 nm due to dianion 15 or 14. The hydrolysis proceeded
according to pseudo-first order kinetics in the present investiga-
tion where a large excess of NaOH (0.1 M vs 2 ꢀ 10ꢁ4 M of a sub-
strate) in H2O was used. Thus, we found that methyl ester 5 was
hydrolyzed 30ꢁ40 times faster than 6 and 11: the rate constants
estimated k/sꢁ1 were 1.1 ꢀ 10ꢁ3 for 5, 3.1 ꢀ 10ꢁ4 for 6, and
4.3 ꢀ 10ꢁ4 for 11.
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13. All electronic spectral data shown here were obtained by using a 2 ꢀ 10ꢁ4
solution of a substrate at 25 °C.
M
14. Both
U
fl of the emitter 6 and US for the CTID of 4 have been reported far higher
Finally, we estimated the values of Ufl for mono-anions of 3-
oxidobenzoates to be 1.8 ꢀ 10ꢁ3 for 6 and 3.9 ꢀ 10ꢁ3 for 11 in
NaOH/H2O after analyses of the absorption and fluorescence spec-
tra of freshly prepared authentic emitters. On the other hand, Ufl
for dianion of 3-hydroxybenzoic acid 15 was estimated to be
in a TBAF/acetonitrile system than in an aqueous system:10,15 Ufl of 6 = 0.24
and US for the CTID of 4 = 0.46.
15. Watanabe, N.; Sano, Y.; Suzuki, H.; Tanimura, M.; Ijuin, H. K.; Matsumoto, M. J.
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