HETEROCYCLES

Paper | Special issue | Vol. 79, No. 1, 2009, pp. 1019-1024
Received, 1st November, 2008, Accepted, 7th January, 2009, Published online, 8th January, 2009.
DOI: 10.3987/COM-08-S(D)79

■ An Energy Transfer Chemiluminescent Reaction of Lophine Peroxides

Masaru Kimura,* Kaoru Akaki, Yasunobu Mishima, Hiroyuki Araki, and Takeshi Fukai

No. 185, Qianshan Middle Road, Hi-Tech Zone, Anshan, Liaoning, China

Abstract

A linear correlation existed between the absorption energy of activators (Eo-o's) and the sensitized chemiluminescence (CL) yields (φSCL's) for the CL reaction of lophine peroxides (2a and 2b), whereas a linear correlation did not exist between their oxidation potentials (Eox's) and φSCL's. Based on the finding, an energy transfer mechanism is likely rather than an electron transfer mechanism like the CIEEL mechanism.

INTRODUCTION
The chemiluminescent (CL) reactions, in which the chemical energy was utilized to promote organic substances to an excited state, have attracted chemists. It is the most important question for understanding the CL reaction what is the chemical excitation mechanism. Since the chemically initiated electron exchange luminescence (CIEEL) mechanism was suggested by Schuster,1 the CL reaction of a dioxetane system has been believed to obey the CIEEL mechanism. We have been examined the CL reaction of a lophine system which is believed to involve a dioxetane intermediate, for which there is no proof.2,3,4 We thought that if the dioxetane intermediate involves in the CL reaction, chemical excitation may obey the CIEEL mechanism. On this line, we started to tested the effect of sensitizers (S`s) on the CL reaction of 2-p-X-phenyl-4-hydroperoxy-4,5- dipheny-4H-isoimidazoles (lophine peroxide) (2a) (X = H, Y = H) and (2b) [X = N(Me)2, Y = F].5,6 When bis-(phenylethynyl)naphthacene (BPEN), rubrene, bis- (phenylethynyl)anthracene (BPEA), and perylene were used as S`s, the quantum yields of the sensitized CL (φSCL) were increased as the absorption energy (E0-0) of S decreased.

EXPERIMENTAL
General procedures: The CL spectra were recorded on a HAMAMATSU PHOTONICS model C-2491 photonic multi-channel analyzer. The fluorescent spectra were recorded on a HITACHI MPF-4 fluorescent spectrophotometer. The UV spectra were measured with a HITACHI 228 spectrophotometer. Eox's for activators were measured by ALS/H CH Instrument Electrochemical Analyzer. Lophine peroxides 2a and 2b were prepared by the method of Kimura et al.5,6

RESULTS AND DISCUSSION
The oxidation of imidazoles (1a) and (1b) with 1O2 (generated by methylene blue photosensitization at -78 °C) gave the corresponding hydroperoxides 2a and 2b in good yields (Scheme 1).3,4 CL emissions were measured by means of a photodiode array which recorded integrated light yields in terms of the number of photons. The CL light emission was counted by PMA upon addition of 0.2 mL of the KOH solution into a solution (1 mL) of the hydroperoxides (10-3 M) in CH2Cl2. The CL of 2a and 2b were detected around 550 nm and 480 nm, respectively. After usual work up, the corresponding amidines (3a) and (3b) were isolated in 81% yield and ~100% yield, respectively (Scheme 1). Amidine 3a was nonfluorecent, while 3b was fluorescent, under the CL reaction conditions. The CL efficiency (φCL) was determined by comparing the total amount of CL light emitted with that of light from the 3-(2’-spiroadamantane)-4-methoxy-4-(3”-hydroxy) phenyl-1, 2-dioxetane standard (φCL = 0.25).7 The CL quantum efficiency for 2a was 7.2 × 10-6 and that for 2b was 8.4 × 10-4.
In the presence of an activator (1 × 10-3 M), φSCL's were observed both from 2a and from 2b. We used the fluorescent S`s, BPEN, rubrene, BPEA, and perylene to confirm whether φSCL obeys the CIEEL mechanism or not. The oxidation potential (Eox's), the absorption energies (Eo-o's), and φSCL's for S`s are summarized in Table 1. A linear correlation existed between the Eo-o's and φSCL's, whereas a linear correlation did not exist between the Eox's and φSCL's as illustrated in Fig. 1. The CL intensity from S`s increases as the extent of overlap between absorption o-o bands of activators and the CL band of lophine peroxide increases. This finding is consistent with an energy transfer mechanism rather than an electron transfer mechanism like CIEEL.
The energy cascade from lophine peroxides to S as an emitter is presented in Eqs. (1) ~ (4).

Assume the mechanism of Eqs. (1) ~ (4) where the rate of formation of the amidine* (AM*) is constant; k3 represents the sum of the rate constants for decay of AM* by all other processes except CL and sensitized CL by sensitizers, for which fluorescence efficiencies are as shown in Table 1.
The rate of change of [AM*] is given by

So that applying the steady state approximation gives

The sensitized CL quantum yield in the presence of S is

The reciprocal of the quantum yield for production of S* is given by Eq. (8)

A plot of 1/φSCL versus 1/ [BPEN] gives a straight line with a slope (k2 + k3)/ k4. For 2a and 2b, slopes are 3.54 and 0.0779, respectively. The more efficient 2b has a slower deactivation process, while the less efficient 2a does a faster deactivation process.

CONCLUSION
A linear correlation existed between the Eo-o's and φSCL's, whereas a linear correlation did not exist between the Eox's and φSCL's. Based on the finding, an energy transfer mechanism is likely rather than an electron transfer mechanism like the CIEEL mechanism. This finding means that the dioxetane intermediate does not involve in the chemi-excitation step. The more efficient 2b has a slower deactivation process, while the less efficient 2a does a faster deactivation process.

ACKNOWLEDGEMENT
The Ministry of Education, Science, Sports and Culture of Japan financially supported this study by a Science Research Grant-in-Aid (17029042). We thank the SC-NMR Laboratory of Okayama University for the 1H NMR spectral measurement and Dr Yi Xuan Liu for drawing Figures.

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