Reinvestigation of photochemical debromination of 9,10-dibromoanthracene and 9-bromoanthracene by amine (triethylamine or N,N-dimethylaniline) studied by nanosecond laser photolysis in acetonitrile at room temperature

Abstract

Abstract From measurements of the fluorescence and triplet—triplet absorption spectra of bromoanthracenes (XA; the 9, 10-dibromo and 9-bromo compounds) in acetonitrile—amine (triethylamine or N,N -dimethylaniline) at room temperature, the lowest excited singlet states [ 1 XA(S 1 )] of XA are found to be quenched by amine with rate constants of the order for that of a diffusion-controlled reaction. Although the lowest excited triplet states [ 3 XA(T 1 )] of XA are also quenched by amine, the triplet quenching rate constants (4.9 × 10 4 -1.1 × 10 5 dm 3 mol −1 s −1 ) are much smaller than those (1.6 × 10 9 –6.2 × 10 9 dm 3 mol −1 s −1 ) obtained with azulene and ferrocene. Since the decrement in reactant absorptions during steady-state photolysis is not affected upon addition of azulene and ferrocene (1 × 10 −4 mol dm −3 ), the appearance of absorption spectra due to the bromoanthracene radical anions (XA .− ) within the duration of nanosecond pulse excitation and their decay rate constants of 2.4 × 10 5 -1.8 × 10 6 s −1 indicate that the intermediates for debromination are XA .− produced by a diffusion-controlled reaction of 1 XA(S 1 ) with ground-state amine yielding the singlet exciplexes [ 1 (XA—amine)*] followed by the decomposition into XA .− and the amine radical cations. Although 1 (XA—amine)* intersystem cross to the triplet exciplexes [ 3 (XA—amine)*] giving rise to an additional formation of 3 XA(T 1 ), no evidence supporting the participation of 3 (XA—amine)* in the debromination as proposed by Soloveichik et al., High Energy Chem., 23 (1989) 281, translated from Khim. Vys. Energii, 23 (1989) 351 is obtained.