A series of hemicyanine derivatives are excellent fluorescent viscosity sensors in live cells and in imaging of living tissues due to their low quantum yields in solution but large fluorescence enhancements in viscous environments. Herein, three carbazole-based hemicyanine dyes with different heterocycles are studied. They have different background quantum yields, and hence different sensitivities to viscosity detection, large Stokes shifts, and high sensitivity. Better understanding of the structure–property relationships for viscosity sensitivity could benefit the design of improved dyes. Computational studies on these dyes reveal the mechanism of viscosity sensitivity of fluorescent molecular rotors and the nature of the difference in viscosity sensitivity of the three dyes. The results show that the greatly raised HOMO and greatly lowered LUMO in the S₁ state compared with the S₀ state are responsible for the large Stokes shift of the three dyes. The heterocyclic moieties have the primary influence on the LUMO levels of the three hemicyanine dyes. Rotation about the CC bond adjacent to the carbazole moiety of the three dyes drives the molecule toward a small energy gap between the ground state and the first excited state, which causes mainly nonradiative deactivation. The oscillator strengths in the lowest singlet excited state drop rapidly with increasing rotation between 0 and 95°, which leads to a dark state for these dyes when fully twisted at 95°. We draw a mechanistic picture at the molecular level to illustrate how these dyes work as viscosity-sensitive fluorescent probes. The activation barriers and energy gaps of CC bond rotation strongly depend on the choice of heterocycle, which plays a major role in reducing fluorescence quantum yield in the free state and provides high sensitivity to viscosity detection in viscous environments for the carbazole-based hemicyanine dyes.

The well-known rhodamine spiro-lactam framework offers an ideal model for the development of fluorescence-enhanced chemosensors through simple and convenient syntheses. Herein, we report a new tridentate PNO receptor, which was introduced into a rhodamine spiro-lactam system to develop Pd²⁺-chemosensor RPd4, that displayed significantly improved sensing properties for palladium. Compound RPd4 shows a very fast response time (about 5 s), high sensitivity (5 nM), and excellent specificity for Pd²⁺ ions over other PGE ions (Pt²⁺, Rh³⁺, and Ru³⁺). In addition, RPd4 displays quite different responses to different valence states of the Pd ions, that is, very fast response towards Pd²⁺ ions but slow response towards Pd⁰, which may provide us with a convenient method for the selective discrimination of Pd species in different valence states. According to proof-of-concept experiments, RPd4 has potential applications in Pd²⁺-analysis in drug compounds, water, soil, and leaf samples. Owing to its good reversibility, RPd4 can also be used as a sensor material for the selective detection and visual recovery of trace Pd²⁺ ions in environmental samples.

In the search for synthetic competitive catalysts that function with hydrogenase-like capability, a series of (Pyrrol-1-yl)phosphane-substituted diiron complexes [(μ-pdt)Fe₂(CO)₅L] [pdt = propanedithiolate, L = Ph₂PPyr (2), PPyr₃ (4); Pyr = pyrrolyl] and [(μ-pdt)Fe₂(CO)₄L₂] [L = Ph₂PPyr (3), PPyr₃ (5)] were prepared as functional models for the active site of Fe-only hydrogenase. The structures of these complexes were fully characterized by spectroscopy and X-ray crystallography. In the IR spectra the CO bands for complexes 2–5 are shifted to higher energy relative to those of complexes with “traditional” phosphane ligands, such as PPh₃, PMe₃, and PTA (1,3,5-triaza-7-phosphaadamantane), indicating that (pyrrol-1-yl)phosphanes are poor σ-donors and better π-acceptors. The electrochemical properties of complexes 2–5 were studied by cyclic voltammetry in CH₃CN in the absence and presence of the the weak acid HOAc. The reduction potentials of these complexes show an anodic shift relative to other phosphane-substituted derivatives. All of the complexes can catalyze proton reduction from HOAc to H₂ in CH₃CN at their respective Fe^IFe⁰ level. Complex 4 is the most effective electrocatalyst, which catalytically generates H₂ from HOAc at –1.66 V vs. Fc⁺/Fc with only ca. 0.2 V overpotential in CH₃CN.

Because palladium is widely used in various catalysts and converters, which results in a high level of contamination of water systems and the soil by residual palladium, there is an urgent need for Pd²⁺-sensitive and -selective probes. Based on the special affinity of Pd²⁺ to conjugated double-bond ligands, two fluorescence probes (RPd2 and RPd3) that contain conjugated allylidene-hydrazone ligands that link to colorless rhodamine-spirolactam have been developed. The results show that conjugated allylidene-hydrazones have a much better affinity toward Pd²⁺, and consequently provide the probes with more acute color change and fluorescence enhancement (≈170-fold), and better selectivity over other metal ions (especially platinum-group elements, or PGEs) than the unconjugated allyl-hydrazine. With richer electron density and a more suitable stereo effect in the allylidene-hydrazone group, RPd2 displays the best specificity toward Pd²⁺ and affords convenient detection by the naked eye. Its potential application for Pd²⁺-contaminated water and soil-sample analysis is revealed by proof-of-concept experiments.