Unusual solvatochromic behaviour of the 4-hydroxy-7-nitrobenzofurazan conjugated anion in protic solvents

Abstract

The absorption spectrum of 4-hydroxy-7-nitrobenzofurazan (NBDOH) and its conjugate anion has been recorded in 23 solvents. The data has been analysed according to the Taft and Kamlet treatment and compared with that of some parent compounds: 4-methoxy-, 4-propylamino- and 4-diethylamine-7-nitrobenzofurazan. The phenolate anion NBDO^– has been shown to exhibit the solvatochromic behaviour characteristic of this NBD series in aprotic media, although in protic media hydrogen bonding had a drastic effect on the UV–VIS spectrum. Such a sensitivity to erotogenic solvents was not encountered for other phenoxide anions such as picrate. This difference of behaviour is discussed on the basis of electrostatic potentials obtained by MNDO calculations. Evidence is given of strong negative charges located on the C(5) and C(7) atoms of NBDO^–, indicating that strong hydrogen bonding may take place with solvent molecules. However, this stabilization of the anion by hydrogen bonding does not seem to influence the dissociation equilibrium, the acidity of NBDOH being perfectly in line with that of other nitrophenols whatever the solvent.