Intramolecular charge-transfer-induced decomposition promoted by an aprotic polar solvent for bicyclic dioxetanes bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety

Bicyclic dioxetanes 3a-d bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl group decomposed to give the corresponding keto esters 4a-d accompanied by the emission of bright light when simply dissolved in an aprotic polar solvent such as N-methylpyrrolidone (NMP) or DMF at 50-100 °C. This solvent-promoted decomposition (SPD) was effectively a chemiluminescence process caused by the hydrogen bonding of a phenolic OH with a solvent molecule(s). The characteristics of the chemiluminescence in SPD resembled those in base-induced decomposition (BID), which occurs through an oxidoaryl-substituted dioxetane 5 by an intramolecular charge-transfer-induced decomposition (CTID) mechanism. Both free energies of activation, ΔG(doubledagger)(SPD) and ΔG(doubledagger)(BID), increased in the order 3a < 3b < 3c < 3d, and were linearly correlated with each other. However, SPD showed features different from those of BID in terms of enthalpy of activation and entropy of activation. SPD had large negative values for ΔS(doubledagger) (ca. -71 J mol(-1) K(-1)) regardless of the substituent R at the 5-position for 3a-d, while the ΔS(doubledagger) values for BID changed from 0.5 to -22 J mol(-1) K(-1) as R became smaller. The enthalpy of activation ΔH(doubledagger) for SPD was 14-21 kJ mol(-1) smaller than that for BID.