Proton-induced fluorescence in modified quino[7,8-h]quinolines: dual sensing for protons and π-donors

The synthesis, as well as spectral, structural and photoluminescence properties of dipyrido[3,2-e:2 ',3 '-h]acenaphthene 5 and quinazolino[7,8-h]quinazolines 6 as representatives of the bidentate -N=/-N= superbases, are reported. These nitrogen bases being more rigid (5) or pi-extended (6) analogs of optically-mute quino[7,8-h]quinoline are both active in terms of fluorescence with quantum yields up to phi = 0.71-0.77. At the same time, their luminescence behavior is opposite to that of peri-NMe2/NMe2 naphthalene proton sponges and their hybrid NMe2/-NMe2 analogs. Although 5 and 6 exhibit visible region emission upon protonation, for the hybrid systems the fluorescence is manifested only for bases. The most remarkable observation is that the fluorescence of compound 5 can be switched on not only by means of organic or inorganic acids, but also through the formation of chelate complexes with such weak H-donors as water and primary alcohols. It was disclosed that water is present in the complex as a cluster comprising 8 interconnected H2O molecules. Overall, the studied compounds demonstrate a previously unobserved type of dual mode optical response, H-sensing (emission enhancement in 5 and 6 on protonation) and pi-sensing (emission quenching in 5H(+) and 6H(+) on coordination with pi-donors). This work seems to be an important contribution to areas such as chemosensorics, the creation of new ligands, hydrogen transfer and some other phenomena representing different types of supramolecular interactions.