Acidochromicity of bisarylethynylbenzenes: hydroxy versus dialkylamino substituents.

This contribution investigates the electronic difference of dibutylamino/dibutylammonium and phenol/phenolate groups in four simple distyrylbenzenes and bisarylethynylbenzenes. The compounds and their protonated and deprotonated species, respectively, were investigated by UV-vis and emission spectroscopies. While the anilinium and phenol compounds displayed similar spectroscopic properties, this was less so the case for the comparison of the dialkylanilines with the corresponding phenolates. In this case, the hydrogen bonding ability of the phenolates distorted the results as the hydrogen donating/accepting properties of the investigated solvents have a disproportional influence on the electronic properties of the phenolates when compared to the dialkylamines. If one uses acetonitrile as solvent, these effects disappear, as acetonitrile is neither a good hydrogen bond acceptor nor a hydrogen bond donor. The results are in line with the para-Hammet constant for OH (sigma = -0.37) being significantly smaller than that for NMe(2) substituents (sigma = -0.83) and reinforces the notion that the lone pairs in these phenols are not readily available for interaction with the pi-system, as they are perhaps energetically too low lying. However, in the case of the phenolates, the lone pairs do interact significantly.