A family of asymmetric thiazolo [5,4-d]thiazole (TTz) fluorescent dye sensors has been developed, and their photophysical sensing properties are reported. The π-conjugated, TTz-bridged compounds are synthesized via a single-step, double condensation/ oxidation of dithiooxamide and two different aromatic aldehydes: one with strong electron-donating characteristics and one with strong electron-accepting characteristics. The four reported dyes include electron-donating moieties (N,N-dibutylaniline and N,N-diphenylaniline) matched with three different electron-accepting moieties (pyridine, benzoic acid, and carboxaldehyde). The asymmetric TTz derivatives exhibit strong solvatofluorochromism with Stokes shifts between 0.269 and 0.750 eV (2270 and 6050 cm −1 ) and transition dipole moments (Δμ = 13−18 D) that are among the highest reported for push−pull dyes. Fluorescence quantum yields are as high as 0.93 in nonpolar solvents, and the fluorescence lifetimes (τ F ) vary from 1.50 to 3.01 ns depending on the solvent polarity. In addition, thermofluorochromic studies and spectrophotometric acid titrations were performed and indicate the possibility of using these dyes as temperature and/or acid sensors. In vitro cell studies indicate good cell membrane localization, negligible cytotoxicity, promising voltage sensitivities, and photostabilities that are 4 times higher than comparable dyes. Their ease of synthesis and purification, remarkable photophysical properties, and chemically sensitive TTz π-bridge make these asymmetric dye derivatives attractive for environmental and biological sensing or similar molecular optoelectronic applications.

Photoinduced electron and energy transfer through preorganized chromophore, donor, and acceptor arrays are key to light-harvesting capabilities of photosynthetic plants and bacteria. Mimicking the design principles of natural photosystems, we constructed a new luminescent pillared paddle wheel metal− organic framework (MOF), Zn 2 (NDC) 2 (DPTTZ), featuring naphthalene dicarboxylate (NDC) struts that served as antenna chromophores and energy donors and N,N′-di(4-pyridyl)thiazolo- [5,4-d]thiazole (DPTTZ) pillars as complementary energy acceptors and light emitters. Highly ordered arrangement and good overlap between the emission and absorption spectra of these two complementary energy donor and acceptor units enabled ligand-to-ligand Forster resonance energy transfer, allowing the MOF to display exclusively DPTTZ-centric blue emission (410 nm) regardless of the excitation of either chromophore at different wavelengths. In the presence of Hg 2+ , a toxic heavy metal ion, the photoluminescence (PL) of Zn 2 (NDC) 2 (DPTTZ) MOF underwent significant red-shift to 450 nm followed by quenching, whereas other transition metal ions (Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , and Cd 2+ ) caused only fluorescence quenching but no shift. The free DPTTZ ligand also displayed similar, albeit less efficient, fluorescence changes, suggesting that the heavy atom effect and coordination of Hg 2+ and other transition metal ions with the DPTTZ ligands were responsible for the fluorescence changes in the MOF. When exposed to a mixture of different metal ions, including Hg 2+ , the MOF still displayed the Hg 2+ -specific fluorescence signal, demonstrating that it could detect Hg 2+ in the presence of other metal ions. The powder X-ray diffraction studies verified that the framework remained intact after being exposed to Hg 2+ and other transition metal ions, and its original PL spectrum was restored upon washing. These studies demonstrated the light-harvesting and Hg 2+ sensing capabilities of a new bichromophoric luminescent MOF featuring a seldom-used photoactive ligand, which will likely spark an explosion of TTZbased MOFs for various optoelectronic applications in near future.

Solvatofluorochromic molecules provide strikingly high fluorescent outputs to monitor a wide range of biological, environmental, or materials‐related sensing processes. Here, thiazolo[5,4‐d]thiazole (TTz) fluorophores equipped with simple alkylamino and nitrophenyl substituents for solid‐state, high‐performance chemo‐responsive sensing applications are reported. Nitroaromatic substituents are known to strongly quench dye fluorescence, however, the TTz core subtly modulates intramolecular charge transfer (ICT) enabling strong, locally excited‐state fluorescence in non‐polar conditions. In polar media, a planar ICT excited‐state shows near complete quenching, enabling a twisted excited‐state emission to be observed. These unique fluorescent properties (spectral shifts of 0.13 – 0.87 eV and large transition dipole moments Δµ = 20.4 – 21.3 D) are leveraged to develop highly sought‐after chemo‐responsive, organic vapor optical sensors. The sensors are developed by embedding the TTz fluorophores within a poly(styrene‐isoprene‐styrene) block copolymer to form fluorescent dye/polymer composites (ΦF = 70 – 97%). The composites respond reversibly to a comprehensive list of organic solvents and show low vapor concentration sensing (e.g., 0.04% solvent saturation vapor pressure of THF – 66 ppm). The composite films can distinguish between solvent vapors with near complete fluorescent quenching observed when exposed to their saturated solvent vapor pressures, making this an extremely promising material for optical chemo‐responsive sensing.

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