ABSTRACT
The chemiluminescence (CL) mechanism of oxidophenyl-substituted 1,2-dioxetane was investigated by performing TD-DFT calculations on biradicals of three model compounds. We propose a novel mechanism of CL in which excitation of a dissociative intermediate by infrared radiation (IRE) of the surrounding solvent is considered. The excitation energies and oscillator strengths (f-values) were estimated for intermediates along the reaction coordinate (Rx). The difference in efficiencies of CL between syn- and anti-isomers of m-oxidophenyl-dioxetane is explained using the difference in potential curves of the singlet excited states (S) and the IRE mechanism. At the point where the biradical of the anti-isomer decomposes into two fragments, the interaction between the S and triplet (T) states is induced by a significant back electron transfer (BET) from the dioxetane group to the oxido-phenyl group and the S(1) excited state is stabilized and CL efficiency is enhanced. In the syn-isomer, the barrier in the S(1) potential curve to reach the final CL state is higher than for the anti-isomer, which reduces the efficiency. The poor CL yield for the p-isomer is ascribed to a much higher barrier in the potential curve of the S(1) state.
ABSTRACT
Thermally stable rotamers of bicyclic dioxetanes bearing 6-hydroxynaphthalen-1-yl (anti-5a and syn-5a), 3-hydroxynaphthalen-1-yl (anti-5b and syn-5b), and 5-hydroxy-2-methylphenyl groups (anti-5c and syn-5c) were synthesized. These dioxetanes underwent TBAF (tetrabutylammonium fluoride)-induced decomposition accompanied by the emission of light in DMSO and in acetonitrile at 25 °C. For all three pairs of rotamers, the chemiluminescence efficiency Φ(CL) for anti-5 was 8-19 times higher than that for syn-5, and the rate of CTID (charge-transfer-induced decomposition) for anti-5 was faster than that for syn-5. The chemiluminescence spectra of the rotamers for 5a and 5c, respectively, were different. This discrepancy in the chemiluminescence spectra between rotamers can presumably be attributed to the difference in the structures of de novo keto imide anti-14 and syn-14 in an excited state, which inherit the structures of the corresponding intermediary anionic dioxetanes anti-13 and syn-13. The important difference in chemiluminescence efficiency between anti-5 and syn-5 is discussed from the viewpoint of a chemiexcitation mechanism for CTID of oxidophenyl-substituted dioxetane.
