Water-Stable Aromatic Dianions

Organic anions are gen­er­ally accep­ted to be highly unstable in water, and dian­ions even more so. Nonetheless, two-electron reduc­tion of perylene diim­ide (PDI) res­ults in the dian­ion PDI2–;that is remark­ably stable in water, even for months, provided oxy­gen is excluded from the sys­tem. This remark­able sys­tem was invest­ig­ated using DFT and TDDFT. Part of the explan­a­tion behind the unex­pec­ted sta­bil­ity of PDI2– is that the sys­tem become more aro­matic upon reduc­tion. In PDI, there are two inde­pend­ent aro­matic sys­tems, each com­pris­ing of a naph­thalene unit, while the cent­ral ring is slightly anti-aromatic. However, in PDI2– the enitre sys­tems par­ti­cip­ates in the aro­matic sys­tem, includ­ing the cent­ral and diim­ide rings. This is clearly demon­strated by the AICD plot for PDI2– shown below. (In AICD plots. clock­wise arrows indic­ate aro­mati­city, which coun­ter­clock­wise arrows indic­ate anti­aro­mati­city; for more details on this method, see Herges’s review in Chemical Reviews.)