DISCRIMINATION BETWEEN HYDROGEN-ATOM AND PROTON ABSTRACTION IN THE QUENCHING OF N,PI-ASTERISK SINGLET-EXCITED STATES BY PROTIC SOLVENTS

The fluorescence quenching of the azoalkane 2,3-diazabicyclo[2.2.2]oct -2-ene by protic solvents (methanol, methanol-OD, water, deuterium oxi de, acetic acid) has been examined. The pseudo-unimolecular quenching rate constants (k(q)) vary from 0.30-44x10(6) s(-1) and decrease upon deuteration of the solvent OH bonds, e.g., the isotope effect for meth anol/methanol-OD is ca. 8.5. This demonstrates that the hydroxylic hyd rogens are predominantly responsible for the fluorescence quenching. T he activation parameters were determined for methanol and methanol-OD. The activation enthalpies are unexpectedly low (Delta H double dagger = 1.8 kcal mol(-1) for methanol) and increase upon deuteration (Delta H double dagger = 3.0 kcal mol(-1) for methanol-OD), while the activa tion entropies remain the same (Delta S double dagger ca. 17.5 cal K-1 mol(-1)). This provides evidence for a fully classical isotope effect related to differences in zero-point vibrational energies. Tunneling appears to play no significant role. The quenching rate constants disp lay no trend with the acidity of the solvent (pK(a) values) but with t he hemolytic bond dissociation energies of the OH bends. This suggests the involvement of a hydrogen atom rather than a proton transfer. All important aspects (activation enthalpies, isotope effects, etc.) of t his novel quenching mechanism of protic solvents are reproduced by MCS CF quantum-chemical calculations with a complete active space of CAS ( 12,10) or CAS (8,7). Most importantly, the computed data indicate the occurrence of a conical intersection, i.e., a real surface crossing, w hich follows the transition;state and provides an efficient trigger fo r radiationless return to the ground-state energy surface (fluorescenc e quenching).