A study of the solution photophysics of the isomeric aminobenzoic acids, N, N-dimethylaminobenzoic acids and their methyl esters

Abstract

The solution photophysics of the isomeric aminobenzoic acids and N,Ndimethylaminobenzoic acids and their methyl esters were studied under a variety of conditions of solvent, pH, concentration and temperature.
The most straightforward behaviour was obtained when the amino or N,N-dimethylamino substituent was in the meta- position relative to the carboxylic add or ester function. The amino compounds absorb with a peak between 300 and 325 nm depending on solvent, their fluorescence peaks lie between 350 and 410 rim. The corresponding N,N-dimethylamino compounds usually absorb and fluoresce some 20 - 30 rim to the red of these wavelengths. Both protonation and deprotonation of the ground state molecules takes place under weakly acidic conditions with pKi, values of approximately 3.0 and pK a values of approximately 5.0 being obtained. The observed variation in the fluorescence of the meta isomers with pH is largely caused by ground state equilibria. Förster cycle calculations suggest that the excited state pK a is little changed from the ground state whereas pKC is much smaller. This is in good agreement with the observed fluorescence data and the nature of the excited nglet state (usually some 10 -20 ns) than is seen for the other isomers. The fluorescence intensity of the meta- isomers was observed to rise with decreasing temperature, although this could possibly be attributed to absorbance changes.
The photophysics of the corresponding ortho compounds is complicated by steric and hydrogen-bonding interactions especially in the compounds containing an N,N-dimethylamino group. The increased interaction between the two substituent groups leads to an increase in extinction coefficient and fluorescence quantum yield and a decrease in fluorescence lifetime for 2-aminobenzoic acid and its methyl ester compared to the meta derivatives.
Spectral band positions, pK values and the effect of temperature on the fluorescence are similar in the two sets of compounds. The 2-N,Ndimethylamino compounds do not parallel the corresponding meta- derivatives due to steric hindrance and intra-molecular hydrogen bonding. These cause weak, blue shifted absorptions and low fluorescence quantum yields. There was no evidence found in this work for the presence of a zwitterionic species in the absorption properties of 2-N,N-dimethylaminobenzoic acid.
The photophysical properties of 4-aminobenzoic acid and its methyl ester are in many ways very similar to those of the corresponding orthosubstituted compounds. However the same certainly cannot be said for 4-N,Ndimethylaminobenzoic
add and its methyl ester. These two compounds exhibit anomalous fluorescence in the form of two fluorescence bands in some solvents. The appearance of this anomalous fluorescence is both concentration and solvent composition dependent. Various mechanisms to explain these observations were tested; excimer, exciplex, TICT state and ground state dimer
formation. The latter was found to be in best accord with the properties of these molecules, but it was clear that in binary solvent mixtures such as hexane / acetonitrile and acetonitrile / water other mechanisms could also be involved.