Water-tunable solvatochromic and nanoaggregate fluorescence: dual colour visualisation and quantification of trace water in tetrahydrofuran.

While investigating the unique optical properties of aminobenzopyranoxanthenes (ABPXs), organic fluorescent dyes with the fusion of two rhodamines, we have found that the spirolactone form of ABPXs exhibited solvatochromic fluorescence in organic solvents. Detailed spectrophotometric and theoretical analyses showed that the solvatochromic fluorescence of ABPXs originated from the photo-excited charge separation in solvents of different dipolarities. Further studies revealed that fluorescent nanoaggregates were also formed in highly concentrated solution. The intriguing dual fluorescence properties of ABPXs were tunable in response to the water content, and served as a new detection principle for naked-eye visualisation (above 0.5 wt%) and quantification (0.010-0.125 wt%) of water in tetrahydrofuran.

Fused-Fluoran Leuco Dyes with Large Color-Change Derived from Two-Step Equilibrium: iso-Aminobenzopyranoxanthenes.

Fluorans are popular leuco dyes that are used in various applications, such as carbonless-copy papers and thermal papers. Here, we describe unique leuco dyes in which two fluoran units are fused into a C2h structure (iso-aminobenzopyranoxanthenes; iso-ABPXs). iso-ABPXs exhibited a large two-step color change (colorless/pink and pink/blue-green) due to opening-closing of two spirolactone rings. The two-step equilibrium among the colorless, pink, and blue-green forms could be well controlled by adjusting acid concentration, solvent, and/or temperature.

Reversible near-infrared/blue mechanofluorochromism of aminobenzopyranoxanthene.

Mechanochromic organic molecules (MOMs) that exhibit a large difference of fluorescence wavelength between two states have important potential applications, but few such compounds are known. Here, we report a new MOM, cis-ABPX01(0), which shows switchable near-IR and blue fluorescence responses. Detailed spectrophotometric and single-crystal X-ray analyses revealed that the near-IR fluorescence is attributable to fluorescence from slip-stacked dimeric structures in crystals, while the blue fluorescence is attributable to fluorescence from the monomer. Switching between the two is achieved by dynamic structural interconversion between the two molecular packing arrangements in response to mechanical grinding and solvent vapor-fuming.

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 (ABPX01(0)), which is colorless; and the monocationic form (ABPX01H(+)) and the dicationic form (ABPX01H(2)(2+)), which are colored. By orienting a pair of spirolactone benzene moieties differently, the stereoisomers of trans- and cis-ABPX01(0) were separated and their crystal structures determined. ABPX01H(2)(2+) was identified to be a red fluorescent species. Detailed spectroscopic and electron microscopic investigations led to the assumption that the ABPX01H(2)(2+) 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.

Design and syntheses of highly emissive aminobenzopyrano-xanthene dyes in the visible and far-red regions.

New derivatives of aminobenzopyrano-xanthene (ABPX) dyes have been designed and synthesized with high fluorescence quantum yields in the visible and far-red regions. It was kinetically demonstrated that the structurally rigid conjugation of the xanthene moiety, which is closely related to the reduction of the nonradiative deactivation process, is an effective molecular design for the drastic enhancement of fluorescence emission efficiency.