A highly fluorescent anthracene-containing hybrid material exhibiting tunable blue-green emission based on the formation of an unusual 'IT-shaped" excimer

A series of flexible bis(9-anthryldiamine) ligands (L1-L3) linked with alkyl spacers of different chain length was synthesized and characterized, in order to investigate the coordination behavior of these diamine ligands with metal ions (Zn2+, etc.) based on fluorescence measurements. The results showed that, in the case of anthryldiamine ligands bearing two- or four-carbon links, the zinc ion induced a chelation-enhanced fluorescence (CHEF) effect in aqueous media, while a trace amount of water could selectively quench the blue emission of the Zn-II complex with a three-carbon-linked ligand (1). Meanwhile, the introduction of more water (concentration > 11 %) resulted in the formation of a new green luminescent species; the luminescence intensity was enhanced stepwise to a maximum with addition of approximately 30% water in THF solution. The peak position (centered at approximately 500 nm) and the life-time measurement (tau=19.59ns) indicated that the green luminescence was attributable to a novel edge-to-face dimeric conformation ("T-shaped" conformation) of anthracene, and not to the more common face-to-face dimeric conformation. Accordingly, H-1 NMR spectroscopic studies in nonaqueous or aqueous solution confirmed this T-shaped conformation, which is consistent with the results of single-crystal Xray structure analysis and solid-state photoluminescence studies.