Photochemistry of 1H-Benzotriazole in Aqueous Solution: A Photolatent Base

Flash photolysis of 1 H -benzotriazole ( 1 ) has shown that N−N bond fission occurs within a few picoseconds of excitation (248 nm) giving 6-diazo-2,4-cyclohexadien-1-imine ( 2 ), λ max = 430 nm. Fluorescence and intersystem crossing do not compete measurably, and photodenitrogenation is a minor side reaction (<1%). The rate constant for recyclization in the ground state, 2 → 1, is on the order of 2 × 10 9 s - 1 in aprotic solvents, and 3.2 × 10 7 s - 1 in aqueous solution. DFT calculations gave Δ 2 → 1 G ⧧ = 3.7 and Δ 2 → 1 G ° = −28.0 kcal mol - 1 for the activation and reaction free energies, respectively. Imine 2 is a moderately strong base, p K a = 12.7, that is protonated by water yielding 2-aminophenyl diazonium ion ( 2 + ), λ max = 405 nm. The rate constant for cyclization of the cation, 2 + → 1 + H +, is k = 3 s - 1 . The reversible photoreaction of 1 may be used to induce an upward pH jump in unbuffered water, or as a kinetic pH-indicator for the range of 6−12 in buffered solutions. An erroneous value for the acidity constant of protonated 1 is corrected to p K a ( 1 + ) = 0.42 ± 0.02 ( I = 1.0 M). Several derivatives of 1 were also investigated.