Tuning anion binding properties of Bis(indolyl)methane Receptors: Effect of substitutions on optical responses.

Chromogenic probes based onoxidizedbis(indolyl)methanes have been synthesized with varying substituents (R = -Me [1], -OMe [2], -OH, [3]) on the central aryl ring. In addition to electronic influence, the involvement of substituents in ion-dipole and charge-assisted hydrogen bonding interactions significantly alters the solvatochromic response and pH-sensitive behavior. In polar aprotic solvents, like CHCN, a concentration-dependent stepwise color change was observed with F ions. In the case of2, a reversible hydrogen bonding interaction between the deprotonated probe and HF dimer might be responsible for that, while step-wise deprotonation caused by F ions could be the probable reason with3. Since the formation of HF is energetically unfavorable in a polar protic solvent, the response of 2 with F ions appears to be very different in EtOH medium. Interestingly, no such alteration in anion sensing behavior was noticed with3going from an aprotic to a protic solvent.