Prev. Article Next Article Table of Contents

Article

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

Hong Wang, Clemens Burda, Gaby Persy, and Jakob Wirz*

Contribution from the Institut für Physikalische Chemie der Universität Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland

J. Am. Chem. Soc., 2000, 122 (24), pp 5849–5855

DOI: 10.1021/ja994464c

Publication Date (Web): May 31, 2000

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

Abstract

Flash photolysis of 1H-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 × 10⁹ s^-¹ in aprotic solvents, and 3.2 × 10⁷ s^-¹ in aqueous solution. DFT calculations gave Δ₂_→₁G^⧧ = 3.7 and Δ₂_→1G° = −28.0 kcal mol^-¹ for the activation and reaction free energies, respectively. Imine 2 is a moderately strong base, pK_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 pK_a(1⁺) = 0.42 ± 0.02 (I = 1.0 M). Several derivatives of 1 were also investigated.