Unique CT emission from aryl terpyridine nanoparticles in aqueous Medium: A combined effect of excited state hydrogen bonding and conformational planarization

Two conformationally flexible aryl terpyridine molecules (aryl: pyrene and anthracene) have been synthesized that can form nanoscopic aggregates in aqueous medium. Unlike organic medium, these bichromophoric molecules showed broad, red-shifted multiple emission bands when microinjected in water. Experimental evidences indicate this unique emission behavior from nanoparticles originating from only a single ground state species. The existence of multiple emission bands and the large bathochromic shifts suggesting planarization of excited state induced by aggregation of chromophores. At the same time, due to higher basicity of terpyridine nitrogens at excited state (photo-basicity), the molecules could be involved H-bonding with water molecules. This could further stabilize a unique internally decoupled charge transfer (CT) state, as evident by time-dependent fluorescence studies. Intermolecular CT promoted by aggregation and mediated by H-bonding of terpyridine to water is indeed a rare example. Though the aggregate formation was found to be insensitive over a large pH range, a unique three-way optical switch can be constructed using metal ions (Zn2+ ions) as additive.