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

Abstract

Anion-π interactions attract much interest in theory but remain difficult to be seen at work. Naphthalene diimides are ideal to study anion-π interactions because their quadrupole moment is exceptionally positive. A known strategy to increase π-acidity is the introduction of the electron-withdrawing groups into naphthalene diimides through covalent bonding. Here we reported that such π-acidity could be modulated by a non-covalent approach. The incorporation of an anionic phenolate substituent into the naphthalene diimide core at the peripheral positions not only yielded an unprecedented NIR absorption band close to 1000 nm that was assigned to the pronounced intramolecular charge transfer from strong electron-donating oxide anion to electron-poor naphthalene diimide unit, but also strengthened the interplay between the added anions and naphthalene diimide acceptors due to the cooperative effect of the simultaneous electrostatic interactions between the counter cations and phenolate ions.