A red-emissive aminobenzopyrano-xanthene dye: Elucidation of fluorescence emission mechanisms in solution and in the aggregate state

We have designed and synthesized a new class of rhodamine dyes with an extended π-conjugated system and named them 3′,3′′- bis(oxospiroisobenzofuran)-3,7-bis(diethylamino)benzopyrano-xanthene (ABPX01) dyes. ABPX01 exhibits fluorescence emission in both dilute solution and the aggregate state, whereas conventional rhodamine dyes show aggregation-induced quenching (AIQ). The chemical species of ABPX01 in solution were determined by spectrophotometric measurements and density functional theory (DFT) calculations to study the relationship among chemical species, color, and fluorescence emission. ABPX01 has various forms: the spirolactone form (ABPX01⁰), which is colorless; and the monocationic form (ABPX01H⁺) and the dicationic form (ABPX01H₂²⁺), which are colored. By orienting a pair of spirolactone benzene moieties differently, the stereoisomers of trans- and cis-ABPX01⁰ were separated and their crystal structures determined. ABPX01H₂²⁺ was identified to be a red fluorescent species. Detailed spectroscopic and electron microscopic investigations led to the assumption that the ABPX01H₂²⁺ formed ion associates with Cl^- as counter anions in HCl aqueous solution, and the nano- and submicrometer-sized colloidal aggregates of ABPX01 hydrochloride exhibit fluorescence emission. To further verify the aggregation-induced emission enhancement (AIEE) mechanism, ABPX01 hydrochloride was synthesized and its fluorescence was similarly checked in the powder state. AIEE in ABPX01 might be attributed to the synergistic combination of the restriction of dye-dye interaction induced dimer formation by sterically hindered ion associates and carboxylic benzene moieties, and the structural rigidity and intermolecular arrangement of the xanthene moiety. We expect that the design strategy of ABPX dyes will be extended to the development of a wide variety of functional organic-dye-based fluorophores (ODFs) with suitable fluorescence-emission controlled mechanisms for many useful applications in new electroluminescent devices.