Water-soluble naphthalene diimides: synthesis, optical properties, and colorimetric detection of biogenic amines†
A large number of water-soluble naphthalene diimides (NDIs) bearing an electron-withdrawing chloro and electron-donating amino-substituents with varied electronic character at the core were synthesized by a two-step synthetic approach comprising imidization of 2,6-dichloronaphthalene dianhydride with L-glutamic acid or 2-dimethylaminoethylamine and subsequent nucleophilic substitution of a chlorine atom with primary amines. These new donor–acceptor NDI dyes show absorption and emission maxima beyond 500 nm and fluorescence quantum yields up to 39% in aqueous phosphate buffer solution at acidic and neutral pH values. Our studies revealed a significant effect of core amino-substituents on the optical properties of these NDIs. With increasing electron-withdrawing character of the residues of amino-substituents the absorption and emission maxima are more hypsochromically shifted and the fluorescence quantum yields are markedly increased with the highest value of 39% for NDIs containing a strongly electron withdrawing trifluroethyl group in the amino-substituent. Furthermore, the high reactivity of the core-dichlorinated NDI dye could be utilized for the optical detection of primary amines and biogenic diamines putrescine and cadaverine.