Viscosity dependence of the chemically induced electron-exchange chemiluminescence triggered from a bicyclic dioxetane

Article abstract of DOI:10.1021/ja000408w

The viscosity dependence for the excitation step of the chemically initiated electron-exchange luminescence (CIEEL) triggered from a novel bicyclic furan-annelated dioxetane has been studied. The observed small but mechanistically significant enhancement

Full text of DOI:10.1021/ja000408w

J. Am. Chem. Soc. 2000, 122, 8631-8634
8631
Viscosity Dependence of the Chemically Induced Electron-Exchange
Chemiluminescence Triggered from a Bicyclic Dioxetane
Waldemar Adam,† Masakatsu Matsumoto,‡ and Alexei V. Trofimov*,†,§
Contribution from Institut fu¨r Organische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany, Department of Material Science, Kanagawa UniVersity, Tsuchiya,
Hiratsuka, Kanagawa 259-12, Japan, and Institute of Biochemical Physics, United Institute of
Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow 117977, Russia
ReceiVed February 3, 2000. ReVised Manuscript ReceiVed May 22, 2000
Abstract: The viscosity dependence for the excitation step of the chemically initiated electron-exchange
luminescence (CIEEL) triggered from a novel bicyclic furan-annelated dioxetane has been studied. The observed
small but mechanistically significant enhancement of the CIEEL efficiency with increasing viscosity is
rationalized in terms of the free-volume model, in which the molecular bond-rotation process that leads to the
ground-state product is frictionally impeded and the chemiexcitation yield is higher.
Introduction
applications, utilize thermally persistent monocyclic spiroada-
mantyl-substituted dioxetanes with a properly protected phe-
nolate ion. Also the novel triggerable bicyclic dioxetanes12
possess remarkable thermal persistence and appreciable CIEEL
efficiency and thereby qualify for chemiluminescence bio-
assays.
Chemically initiated electron-exchange luminescence
(CIEEL),1 a chemiluminescence process derived from electron-
transfer chemistry,2,3 is a general phenomenon which was
originally discovered by Schuster for diphenoyl peroxide4 and
abundantly reported for R-peroxy lactones5 and appropriate
dioxetanes.6 The CIEEL generation may result from both inter-
and intramolecular electron transfer; the latter has been proposed
in the case of the firefly bioluminescence.7 The intramolecular
CIEEL is of particular interest for modern chemiluminescent
bioassays,8,9 in which CIEEL-active dioxetanes are used. The
most popular CIEEL systems,10,11 developed for clinical bioassay
A pertinent feature of the phenolate-ion-initiated CIEEL
process is the intramolecular electron transfer (ET) from the
phenolate moiety to the antibonding σ* orbital of the peroxide
bond, concomitant with O-O bond cleavage; however, a
detailed mechanism of excited-state generation in this chemi-
luminescent process still needs to be established.13 Recently,
we have provided a valuable experimental probe, namely, the
viscosity effect on the CIEEL efficiency, to distinguish between
the two main mechanistic alternatives for the CIEEL generation
of the monocyclic dioxetane,14 namely, direct chemiexcitation
by concerted dioxetane cleavage through charge transfer and a
stepwise electron-transfer process, in which chemiexcitation
arises from electron back-transfer (BET) between the primary
solvent-caged radical fragments as the key step. The basic
concept of the BET mechanism for the CIEEL process1,4,7 rests
on the observed chemiexcitation in the electron transfer for
chemically3 and electrochemically2 generated ion-radical pairs.
If the BET process operates in the CIEEL generation, its
efficiency should be subject to a solvent-cage effect; e.g., it
should obey a viscosity dependence. In our previous study on
the monocyclic dioxetane, we have found an increase of the
chemiluminescence efficiency on increasing viscosity,14 which
is consistent with the BET mechanism.
* Correspondence should be addressed as follows: Telefax: +49 931
8884756; E-mail: adam@chemie.uni-wuerzburg.de; Internet: http://www-
organik.chemie.uni-wuerzburg.de.
† Universita¨t Wu¨rzburg.
‡ Kanagawa University.
§ Russian Academy of Sciences.
(1) Schuster, G. B.; Horn, K. A. Chemically Initiated Electron-Exchange
Luminescence. In Chemical and Biological Generation of Excited States;
Adam, W., Cilento, G., Eds.; Academic Press: London, 1982; pp 229-
247.
(2) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley
& Sons: New York, 1980; pp 621-629. (b) Faulkner, L. R. Int. ReV. Sci.:
Phys. Chem. Ser. 2 1976, 9, 213-263. (c) Hercules, D. M. Acc. Chem.
Res. 1969, 2, 301-307. (d) Kavarnos, G. J.; Turro, N. J. Chem. ReV., 1986,
86, 401-449.
(3) (a) Weller, A.; Zachariasse, K. J. Chem. Phys. 1967, 46, 4984-4985.
(b) Weller, A.; Zachariasse, K. Chem. Phys. Lett. 1971, 10, 197-200. (c)
Weller, A.; Zachariasse, K. Chem. Phys. Lett. 1971, 10, 424-427.
(4) Koo, J.-Y.; Schuster, G. B. J. Am. Chem. Soc. 1978, 100, 4496-
4503.
(5) Adam, W.; Cueto, O. J. Am. Chem. Soc. 1979, 101, 6511-6115.
(6) Adam, W.; Zinner, K.; Krebs, A.; Schmalstieg, H. Tetrahedron Lett.
1981, 22, 4567-4570.
Presently, we have investigated the viscosity dependence of
the CIEEL-active bicyclic dioxetane 112 (Scheme 1), for a
rigorous scrutiny of the CIEEL mechanism. The reasons for
(7) Koo, J.-Y.; Schmidt, S. P.; Schuster, G. B. Proc. Natl. Acad. Sci.
U.S.A. 1978, 75, 30-33.
(8) Adam, W.; Reinhardt, D.; Saha-Mo¨ller, C. R. Analyst 1996, 121,
1527-1531.
(11) (a) Schaap, A. P.; Chen, T.-S.; Handley, R. S.; DeSilva, R.; Giri,
B. P. Tetrahedron Lett. 1987, 28, 1155-1158. (b) Schaap, A. P.; Handley,
R. S.; Giri, B. P. Tetrahedron Lett. 1987, 28, 935-938.
(12) (a) Matsumoto, M.; Watanabe, N.; Kasuga, N. C.; Hamada, F.;
Tadokoro, K. Tetrahedron Lett. 1997, 38, 2863-2866. (b) Adam, W.;
Matsumoto, M.; Trofimov, A. V. J. Org. Chem. 2000, 65, 2078-2082.
(13) Wilson, T. Photochem. Photobiol. 1995, 62, 601-606.
(14) Adam, W.; Bronstein, I.; Trofimov, A. V.; Vasil’ev, R. F. J. Am.
Chem. Soc. 1999, 121, 958-961.
(9) Beck, S.; Ko¨ster, H. Anal. Chem. 1990, 62, 2258-2270.
(10) (a) Bronstein, I.; Edwards, B.; Voyta, J. C. J. Biolumin. Chemilumin.
1988, 2, 186. (b) Edwards, B.; Sparks, A.; Voyta, J. C.; Bronstein, I. J.
Biolumin. Chemilumin. 1990, 5, 1-4. (c) Bronstein, I.; Edwards, B.; Voyta,
J. C. J. Biolumin. Chemilumin. 1989, 4, 99-111. (d) Edwards, B.; Sparks,
A.; Voyta, J. C.; Strong, R.; Murphy, O.; Bronstein, I. J. Org. Chem. 1990,
55, 6225-6229.
10.1021/ja000408w CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/26/2000

8632 J. Am. Chem. Soc., Vol. 122, No. 36, 2000
Adam et al.
Scheme 1
Figure 1. Viscosity dependence of the singlet chemiexcitation yield
(ΦS , 9) the fluoride-ion-triggered ([n-Bu4NF] ) 8.5 × 10-3 M) CIEEL
1
cleavage of the dioxetane 1 ([1] ) 6.5 × 10-9 M) and the fluorescence
quantum yield (Φfl, b) of the authentic methyl m-oxybenzoate ion ([4]
) 1.1 × 10-5 M); the error bars represent the range of four
measurements for each data point.
this choice are the following: (a) Contrary to the monocyclic
dioxetane,14 cage escape is not possible for bicyclic dioxetane
1; (b) the CIEEL emitters triggered from mono-14 and bicyclic
dioxetanes possess the same chromophore, i.e., the m-oxyben-
zoate ion moiety; (c) as we have recently established,12b no
exciplex is involved in both CIEEL processes (Scheme 1). The
relevant question is, however, whether the CIEEL intensity
triggered from 1 is subject to a viscosity effect, as we have
observed for the monocyclic dioxetane. Herein, we report our
results on the viscosity dependence of the CIEEL process for
the bicyclic furan-annelated dioxetane 1 and compare it with
that of the monocyclic spiroadamantyl-substituted dioxetane.14
The CIEEL intensity, iCIEEL ) ΦCIEELV, is a product of the
monocyclic spiroadamantyl-substituted dioxetane (ca. 2.6 times
for the 4-fold viscosity increase14) but mechanistically signifi-
cant. What molecular features are responsible for this small (ca.
40%) but definitive viscosity effect in the triggered CIEEL
process of bicyclic dioxetane 1? It certainly cannot be cage
escape in the BET step, as was previously established for the
monocyclic dioxetane.14
To rationalize the observed viscosity behavior for 1 (Scheme
1), it is helpful to consider the MOs of the CIEEL emitter.12b
The AM1-based calculations have revealed that the π f π*
excitation of this CIEEL emitter constitutes an electronic
transition between HOMO and LUMO+1. For the subsequent
discussion, it is important to note that the HOMO coefficients
are maximal on the phenolate ion functionality, while in the
LUMO+1 they are concentrated on the ester moiety12b (cf.
Supporting Information). For the BET process, this implies
that the electron transfer from the ketyl radical to the phenoxyl
group of the intermediate 3 should result in the ground-state
reaction rate V and the CIEEL yield, ΦCIEEL ) ΦS Φfl, in which
1
ΦS and Φfl are the singlet chemiexcitation yield and the
1
fluorescence efficiency of the CIEEL emitter. The required
experimental data for ΦS versus viscosity (η) were obtained
by measurement of the Φ1CIEEL and Φfl values as a function of
viscosity, as described previously.14 The viscosity was varied
by changing the amount of the viscous Ph2CH2 in benzene-
Ph2CH2 mixtures. This solvent combination was used originally
with success for the viscosity studies on triplet-triplet energy
transfer15 and autoxidation kinetics.16
product 4 (kBSET), while the electron transfer to the ester moiety
0
should generate the electronically excited 4*, i.e., chemiexci-
tation (kBSET). The reason for the latter is that the ester-ketyl/
phenoxyl1diradical species generated from 3 is cross-conjugated
and represents the first π,π*-excited state of the phenolate
Results and Discussion
The experimental results for the excitation efficiency (ΦS )
1
of dioxetane 1 and the fluorescence quantum yield (Φfl) of the
authentic CIEEL emitter 4 are shown in Figure 1. It is fortunate
that the fluorescence yield of the methyl m-oxybenzoate anion
(4), the CIEEL emitter, does not depend on the Ph2CH2
concentration, as manifested by the constant values Φfl ) 0.220
( 0.014 over the entire concentration range (Figure 1). The
same behavior was observed for the m-oxybenzoate anion
triggered from the monocyclic dioxetane.14
product 4. Consequently, the chemiexcitation yield (ΦS ), as
1
expressed by eq 3, is a result of the competition between these
excited-state (kSBET ) and ground-state (kBSET ) BET channels.
1
0
What role does the medium viscosity play in this BET
process? Scheme 2 reveals that the initial conformation of
the diradical 3 formed on dioxetane cleavage has the ketyl-
radical functionality in proximity to the ester carbonyl group.
This geometrical arrangement is predestined for electron transfer
The experimental data in Figure 1 disclose a regular enhance-
ment of the chemiexcitation yield (ΦS ) with an increase in the
viscosity. The observed enhancement (ca. 1.4 times) of ΦS for
a 4-fold viscosity increase is considerably smaller than for the
(kBSET ) to afford the electronically excited phenolate product
1
1
4*; subsequent emission (hνCIEEL) leads to its ground state 4.
Rotations about the methylene bonds in diradical 3 competes
1
with this chemiexcitation BET process (kBSET) in that it sepa-
(15) (a) Belyakov, V. A.; Vasil’ev, R. F.; Fedorova, G. F. Bull. Acad.
Sci. USSR, Phys. Ser. 1978, 42, 145-149. (b) Belyakov, V. A.; Vasil’ev,
R. F.; Fedorova, G. F. Spectroscopy Lett. 1978, 11, 549-561.
(16) Belyakov, V. A.; Vasil’ev, R. F.; Fedorova, G. F. Kinet. Catal. 1996,
37, 508-518.
1
rates spatially the ketyl-radical and ester-carbonyl functionalities
and thereby makes improbable the electron jump between them.
At the same time, during such conformational motion, the ketyl-

CIEEL Triggered from a NoVel Bicyclic Dioxetane
J. Am. Chem. Soc., Vol. 122, No. 36, 2000 8633
radical fragment approaches the phenoxyl group, which corre-
sponds to the conformation 3′ in Scheme 2. Electron transfer
(kBSET ) from the ketyl radical to the phenoxyl moiety in 3′
0
should generate preferably the ground-state phenolate ion
product 4 (cf. Scheme 2) since HOMO coefficients are maximal
on the phenolate group.12b Expectedly, the molecular rotation
in the 3 f 3′ conformational change (competitive to the
chemiexcitation BET, kSBET ) is retarded in a more viscous
1
medium and, consequently, the chemiluminescence yield should
increase because more of the conformation 3 than 3′ is
populated. This was observed experimentally (Figure 1)!
Although the viscosity effect is small (ca. 40%) over the narrow
viscosity change (ca. 4-fold) investigated here, for such mo-
lecular rotations the viscosity retardation is expected to be
small.17,18
The frictional model19,20 should apply for the observed
viscosity dependence since the available “free volume” in the
medium will determine how readily conformational changes
through bond rotation will take place and thereby influence the
chemiexcitation yield (ΦS ). The latter is given by eq 1, in which
1
Figure 2. Plot of the ratio (1 - ΦS )/ΦS as a function of viscosity
1
1
according to eq 5 in the fluoride-ion-triggered CIEEL cleavage of
dioxetane 1. The insert displays the double-logarithmic plot according
to eq 6.
kBSET
1
ΦS )
(1)
kBSET + kBSET
1
0
1
Scheme 2
η ) A exp(V0/Vf)
(2)
(3)
kBSET ) k0 exp(-RV0/Vf)
0
kBSET ) k0(A/η)R
(4)
(5)
0
kBSET 1 - ΦS
k0
kBSET
0
1
)
)
(A/η)R
kBSET
S1
Φ
1
1
1 - ΦS
ln
1 ) const - Rlnη
(6)
ΦS
1
was first applied to rationalize the viscosity dependence for the
photoisomerization of stilbenes.20 Substitution of eq 2 into eq
3 gives eq 4 for the viscosity dependence of the rate constant
the rate constant kBSET needs to be expressed as a function of
solvent viscosity (η)0. According to the free-volume approach,19a
molecular motion (rotations, translations) in a liquid medium
is possible when sufficient free volume (Vf) is available, i.e.,
when the Vf per molecule is larger than some “critical” value
V0. The fluidity (η-1) is proportional to the probability factor
[exp(-V0/Vf)] for the motion of molecules in the liquid medium,
and, hence, the free-volume dependence of the viscosity may
be expressed by eq 2, in which A is a proportionality factor. In
contrast to the translational motion of a molecule, rotations about
bonds involve only a portion of the molecule and, thus, only a
fraction RV0 (R < 1) of V0 is required. Therefore, the rate
kBSET . Finally, the combination of eqs 1 and 4 leads to eq 5; its
0
double-logarithmic form (eq 6) predicts a linear dependence of
ΦS on η.
1
The experimental viscosity dependence of the chemiexcitation
yield is given in Figure 2, and the good linear fit of all the data
points in the double-logarithmic plot according to eq 6 (R2 )
0.985, cf. insert in Figure 2) manifests that the “free-volume”
model is well-obeyed. The slope of the double-logarithmic plot
provides R ) 0.35, which matches well the values (ca. 0.1-
0.6) obtained for a number of viscosity-controlled rearrangement
processes.19,20 The fact that R < 1 means that the motion of
only a fraction of the molecule is subject to the viscosity effect,
which in the present case corresponds to the bond rotations
required to bring the ketyl-radical fragment in close vicinity to
the phenoxyl-radical site (Scheme 2).
constant kBSET for the ground-state BET process is given by eq
0
3 (in which k0 is a preexponential factor), an expression which
(17) (a) Fo¨rster, T.; Hoffman, G. Z. Phys. Chem. (Munich) 1971, 75,
63-76. (b) Tredwell, C. J.; Osborne, A. D. J. Chem. Soc., Faraday Trans.
2 1980, 76, 1627-1637.
(18) (a) Velsko, S. P.; Fleming G. R. J. Chem. Phys. 1982, 76, 3553-
3562. (b) Velsko, S. P.; Waldeck, D. H.; Fleming G. R. J. Chem. Phys.
1983, 78, 249-258. (c) Keery, K. M.; Fleming, G. R. Chem. Phys. Lett.
1982, 93, 322-326. (d) Bagchi, B.; Oxtoby, D. J. Chem. Phys. 1983, 78,
2735-2741.
(19) (a) Doolitle, A. K. J. Appl. Phys. 1951, 22, 1471-1475. (b) Cohen,
M. J. Chem. Phys. 1959, 31, 1164-1168.
(20) Gegiou, D.; Muszkat, K. A.; Fisher, E. J. Am. Chem. Soc. 1968,
90, 12-18.
The present study demonstrates that the viscosity effect on
the CIEEL generation triggered from the bicyclic furan-
annelated dioxetane 1 is significantly lower than that observed
from the monocyclic spiroadamantane-substituted dioxetane.14
For the diradical derived from the bicyclic dioxetane, the
conformational changes are retarded by the more viscous

8634 J. Am. Chem. Soc., Vol. 122, No. 36, 2000
Adam et al.
medium; for the radical pair generated from the monocyclic
dioxetane, the more viscous medium slows down cage escape;
expectedly, the latter should display a more pronounced viscosity
effect in the CIEEL-triggered process.
ated. A.V.T. thanks also the Russian Foundation for Basic
Research (Grant No. 99-03-32121) for financial support.
Supporting Information Available: Text giving the Ex-
perimental Section . This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. Generous funding by the Deutsche For-
schungsgemeinschaft (Sonderforschungsbereich 347: “Selektive
Reaktionen Metall-aktivierter Moleku¨le”) is gratefully appreci-
JA000408W