Pronounced solvent effect on the absorption spectra of the photochemically produced 2,4-dinitrobenzyl carbanion

The photochromism of 2,4-dinitrotoluene in aqueous solution has been attributed to formation of the 2,4-dinitrobenzyl carbanion (J. Chem. Phys. 39, 1218 (1963)). The spectrum of the colored transient is, however, significantly different from that of ground state carbanion when formed in aprotic solvents through thermal decarboxylation of potassium 2,4-dinitro-phenylacetate (J. Org. Chem. 49, 413 (1984)). In this paper absorption spectra obtained upon 248 nm laser photolysis are reported, for solutions in water, acetonitrile, and their mixtures. The spectrum in acetonitrile does closely resemble that of the carbanion formed by decarboxylation. It is suggested that the same ion is formed in water, and a solvent effect is responsible for the significantly different spectrum. Spectra in the mixed solvents suggest that there is an equilibrium between differently solvated forms of the anion, the extremes being an unspecifically solvated form with λmax 400 and 640 nm as in acetonitrile and a form in water that is hydrogen bonded through the nitro group with λmax 350 and 500 nm. Experiments with conductivity detection provide corroborative evidence for carbanion formation. In acetonitrile a non-conducting transient is produced as a precursor to the anion. This is suggested to be the conjugate acid of the aci-nitro anion, formed by intramolecular hydrogen abstraction in the excited nitrotoluene.