Base-induced chemiluminescent decomposition of bicyclic dioxetanes bearing a (benzothiazol-2-yl)-3-hydroxyphenyl group: a radiationless pathway leading to marked decline of chemiluminescence efficiency.

Charge-transfer-induced decomposition (CTID) of bicyclic dioxetanes 1b-d bearing a 3-hydroxylphenyl moiety substituted with a benzothiazol-2-yl group at the 2-, 6-, or 5-position was investigated, and their chemiluminescence properties were compared to each other, based on those for a 4-benzothiazolyl analogue 1a. Dioxetanes 1c and 1d underwent CTID to give the corresponding oxido anions of keto esters 8c or 8d in the singlet excited state with high efficiencies similarly to the case of 1a. On the other hand, 1b showed chemiluminescence with quite low efficiency, though it gave exclusively keto ester 2b. The marked decline of chemiluminescence efficiency for 1b was attributed to 1b mainly being decomposed to 8b through a radiationless pathway, in which intramolecular nucleophilic attack of nitrogen in the benzothiazolyl group to dioxetane O-O took place to give cyclic intermediate cis-11.

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.

Thermodynamic aspects of thermal decomposition and charge-transfer-induced chemiluminescent decomposition for bicyclic dioxetanes bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety.

Uncatalyzed thermal decomposition (TD) and charge-transfer-induced decomposition (CTID) of dioxetanes were investigated to determine their thermodynamic characteristics. The dioxetanes examined were a series of 1-[4-(benzothiazol-2-yl)-3-hydroxyphenyl]-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptanes, 1a-d, bearing an alkyl substituent R, e.g., methyl, ethyl, isopropyl, or tert-butyl, at the 5-position and a parent dioxetane 1e with R = H. X-ray single crystallographic analysis was achieved for 1a-d. Both free energies of activation, DeltaG(double dagger)(TD) and DeltaG(double dagger)(CTID), increased in the order 1a < or = 1b < 1c < 1d. The free energy difference DeltaDeltaG(double dagger) = DeltaG(double dagger)(TD) - DeltaG(double dagger)(CTID) was ca. 27 kJ mol(-1) regardless of the substituent R. However, the use of Taft's dual-substituent parameter suggested that CTID was more sensitive to the polarity of the substituent R than TD. The entropy term for CTID, DeltaS(double dagger)(CTID), decreased from zero to a large negative value in the order of tert-butyl, isopropyl, ethyl, and methyl, whereas DeltaS(double dagger)(TD) did not show a similar tendency.

Synthesis of thermally stable acylamino-substituted bicyclic dioxetanes and their base-induced chemiluminescent decomposition.

Hydroxyaryl-substituted dioxetanes 2-4 fused with a pyrrolidine ring were selectively synthesized by singlet oxygenation of the corresponding dihydropyrroles 5-7. These N-acylamino-substituted bicyclic dioxetanes were quite stable thermally, and 2a and 2b were estimated to possess half-lives of 32 and 34 y at 25 degrees C. When treated with TBAF (tetrabutylammonium fluoride) in DMSO, these dioxetanes underwent charge-transfer-induced decomposition (CTID) to emit yellow-orange light. The chemiluminescence efficiencies Phi(CL) for dioxetanes 2 ranged from 10(-6) to 10(-2). On the other hand, dioxetane 4 bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety showed chemiluminescent decomposition with high efficiency (Phi(CL) = 0.15) comparable to its oxy-analogue 26. Prominent characteristics for CTID of the present dioxetanes were that the N-acylamino-group influenced the color of chemiluminescence as well as the rate of decomposition k(CTID), and furthermore an N-acyl substituent could decisively affect the singlet-chemiexcitation efficiency, as observed for the case of 2b.

Marked difference in fragmentation between collision-induced excitation and chemi-excitation of keto esters produced from dioxetanes bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety in negative-mode matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry.

A dioxetane bearing a hydroxyphenyl group produces an unstable oxidoaryl anion by deprotonation which rapidly decomposes with accompanying emission of light effectively by the intramolecular charge-transfer-induced decomposition (CTID) mechanism. Although several mechanisms have been proposed to explain chemi-excitation in CTID, strong experimental evidence is still lacking. In the course of our investigation to clarify the chemi-excitation process, negative-mode matrix-assisted laser desorption/ionization time-of-flight collision-induced dissociation tandem mass spectrometry (MALDI-TOF-CID-MS/MS) was used to investigate the decomposition of bicyclic dioxetanes bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety 1 and their related keto esters 2 in a gas phase. Dioxetanes 1 decomposed to give 2 in an electronically excited state, which underwent alpha-cleavage of a ketone moiety, while authentic 2 (ground state) hardly showed fragmentation in MS. On the other hand, 2 displayed fragment ions in CID-MS/MS, though the fragmentation pattern was significantly different between 2a (R = tert-butyl) and 2b-2d (R = isopropyl, ethyl and methyl, respectively): 2a exhibited mainly alpha-cleavage of a ketone moiety, while 2b-2d showed beta-cleavage of the aromatic ester moiety. The marked difference in fragmentation between 2 (electronically excited state) produced directly from 1 and authentic 2 under CID was most likely due to the difference in the excitation processes: chemi-excitation of 2 by CTID of 1 versus vibrational excitation of 2 induced by collision in MS/MS.