958
J. Am. Chem. Soc. 1999, 121, 958-961
Solvent-Cage Effect (Viscosity Dependence) as a Diagnostic Probe
for the Mechanism of the Intramolecular Chemically Initiated
Electron-Exchange Luminescence (CIEEL) Triggered from a
Spiroadamantyl-Substituted Dioxetane
Waldemar Adam,† Irena Bronstein,‡ Alexei V. Trofimov,*,†,§ and Rostislav F. Vasil’ev§
Contribution from the Institute of Organic Chemistry, UniVersity of Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany, Tropix, Inc., 47 Wiggins AVenue, Bedford, Massachusetts 01730, and
Institute of Biochemical Physics, United Institute of Chemical Physics, Russian Academy of Sciences,
ul. Kosygina 4, Moscow 117977, Russia
ReceiVed August 20, 1998
Abstract: The excitation step of the intramolecular CIEEL generation in the triggered cleavage of
spiroadamantyl-substituted dioxetane has been studied. The electron back-transfer (BET) process versus the
direct chemiexcitation of the phenolate-anion emitter have been considered as mechanistic alternatives. The
observed solvent-cage effect on the CIEEL generation, manifested by the increase of the singlet chemiexcitation
yield at increased viscosity, provides evidence that the BET process operates in the intramolecular CIEEL
mechanism.
Introduction
have revealed remarkable thermal persistence and CIEEL
efficiency. The CIEEL of these dioxetanes may be generated
at will by treatment with an appropriate reagent (trigger) to
release the phenolate ion; the choice of the trigger depends on
the nature of the protective group. The rational design8 of more
effective CIEEL systems requires a detailed knowledge of the
mechanism for the intramolecular CIEEL process, the purpose
of our present study.
Recently we have reported model studies on the phenolate-
initiated intramolecular CIEEL process, in which we have
examined the reaction kinetics of the silyloxy-substituted
spiroadamantyl dioxetanes chemically triggered by fluoride
ions13 and of the phosphate-protected ones enzymatically
triggered by alkaline phosphatase14 and elucidated the solva-
tochromic effects on the CIEEL versus the photoexcited
fluorescence of the authentic CIEEL emitter.15 However, the
most fundamental question on the CIEEL mechanism remained
open. Scheme 1 illustrates the mechanistic alternatives for the
intramolecular CIEEL process of the spiroadamantyl-substituted
dioxetane 1. The CIEEL is generated by the cleavage of the
Chemically initiated electron-exchange luminescence (CIEEL),1
a phenomenon of light emission derived from electron-transfer
chemistry,2,3 constitutes a general chemiluminescent process.
This phenomenon was originally discovered by Schuster for
diphenoyl peroxide4 and in the meantime documented 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 bioassays8,9
developed for clinical applications, in which CIEEL-active
dioxetanes are used. The most efficient CIEEL systems10,11 for
the latter purpose utilize thermally persistent spiroadamantyl-
substituted dioxetanes with a properly protected phenolate ion.
Also diisopropyl-substituted derivatives synthesized recently12
† University of Wu¨rzburg.
‡ Tropix Inc.
§ 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.:
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(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.
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(5) Adam, W.; Cueto, O. J. Am. Chem. Soc. 1979, 101, 6511-6115.
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(7) Koo, J.-Y.; Schmidt, S. P.; Schuster, G. B. Proc. Natl. Acad. Sci.
U.S.A. 1978, 75, 30-33.
(14) Adam, W.; Bronstein, I.; Edwards, B.; Engel, T.; Reinhardt, D.;
Schneider, F. W.; Trofimov, A.; Vasil’ev, R. F. J. Am. Chem. Soc. 1996,
118, 10400-10407.
(8) Adam, W.; Reinhardt, D.; Saha-Mo¨ller, C. R. Analyst 1996, 121,
1527-1531.
(15) Adam, W.; Bronstein, I.; Trofimov, A. V. J. Phys. Chem. A 1998,
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(9) Beck, S.; Ko¨ster, H. Anal. Chem. 1990, 62, 2258-2270.
10.1021/ja982999k CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/22/1999