Cooperative effect of anion-π and electrostatic interactions in NIR absorbing phenolate naphthalene diimide conjugates

A halochromic naphthalene diimide bearing an anionic phenolate substituent at the peripheral positions exhibits not only so far for the NDI dye family elusive NIR absorption close to 1000 nm, but also an unusual strong anion-π interaction enhanced by the cooperative effect of the simultaneous electrostatic interactions between the added counter cations and phenolate ions.