Using Pyrene Excimer Fluorescence To Probe the Interactions between Viscosity Index Improvers and Waxes Present in Automotive Oil

Probing the Interactions between Pour Point Depressants (PPDs), Viscosity Index Improvers (VIIs), and Wax in Octane Using Fluorescently Labeled PPDs

A poly(octadecyl methacrylate) sample fluorescently labeled with 6.7 mol% of pyrene (Py(6.7)-PC18MA) was used as a mimic of a pour point depressant (PPD) to investigate how Py(6.7)-PC18MA interacts with wax found in engine oils and ethylene-propylene (EP) copolymers used as mimics of viscosity index improvers (VIIs). The fluorescence spectra of Py(6.7)-PC18MA solutions in octane were acquired in octane at low and high concentrations of Py(6.7)-PC18MA and analyzed to obtain the molar fraction (finter) of pyrene labels, that formed excimer intermolecularly, a measure of the level of intermolecular interactions between Py(6.7)-PC18MA molecules in the solution. The finter-versus-T profile obtained for Py(6.7)-PC18MA alone in octane confirmed that Py(6.7)-PC18MA formed microcrystals at solution temperatures below 0 oC. The effect induced by the addition of wax and an amorphous (EP(AM)) and semicrystalline (EP(SM)) EP copolymer on the interactions experienced by Py(6.7)-PC18MA were characterized by monitoring finter as a function of temperature and comparing the different finter-versus-T plots obtained after the addition of the different components with the finter-versus-T plot obtained for Py(6.7)-PC18MA alone. These studies demonstrated that wax and EP(AM) increased the level of intermolecular interactions between the Py(6.7)-PC18MA molecules at all temperatures in octane. EP(SM) increased the interactions between Py(6.7)-PC18MA molecules at high temperature, where it was soluble in octane, but finter reverted to its value in the absence of EP(SM) at low temperatures, where EP(SM) had crystallized. These experiments illustrate how pyrene excimer fluorescence can be applied to probe the complex interactions taking place between the different components found in engine oils.

Isomerization-Induced Excimer Formation of Pyrene-Based Acylhydrazone Controlled by Light- and Solvent-Sensing Aromatic Analytes

Pyrene is a fluorescent polycyclic aromatic hydrocarbon, and it would be interesting to determine whether its C═N-based conjugate can be used for sensing of aromatic analytes at its supramolecular aggregated state. For this purpose, we have synthesized (E)-3,4,5-tris(dodecyloxy)-N'-(pyren-1-ylmethylene)benzohydrazide (Py@B) by alkylation, substitution, and the Schiff base reaction methodology. The E-isomer of Py@B (E-Py@B) exhibits a bright fluorescence due to excimer formation in nonaromatic solvents. Upon photoirradiation with λ = 254 nm, it exhibits E-Z isomerization across the C═N bond at a low concentration (10^-4 M), resulting in a quenched fluorescence intensity, and interestingly, upon photoirradiation with λ = 365 nm, the Z-isomer of Py@B returns to the E-isomer again, indicating that E-Z isomerization of Py@B is reversible in nature. The thick supramolecular aggregated morphology of E-Py@B changes to a flowery needlelike morphology after photoirradiation with λ = 254 nm. The UV-vis absorption band at 370 nm for 10^-4 M Py@B in methyl cyclohexane (MCH) is due to excimer formation for closer proximity of pyrene moieties present in E-Py@B and changes to the absorption peak at 344 nm for its Z-isomer formation. The fluorescence spectroscopy results also support the fact that the optimum concentration of the E-isomer of Py@B is 2 × 10^-4 M in MCH for excimer formation. From spectral results, it may be concluded that nonaromatic solvents assist in constructing the excimer, but aromatic solvents resist forming an excimer complex of E-Py@B. The fluorescent emission of E-Py@B in MCH is quickly quenched on addition of different aromatic analytes through both static and dynamic pathways. In the solid state, E-Py@B also senses aromatic vapors efficiently via fluorescence quenching. Absorbance spectra of a model molecule obtained using time-dependent density functional theory (TDDFT) calculations on a DFT-optimized structure indicate complex adduct formation between E-Py@B and aromatic analytes from the well-matched theoretical and experimental UV-vis spectra on addition of different analytes with E-Py@B.

Amphiphilic copolymer fluorescent probe for mitochondrial viscosity detection and its application in living cells

The mitochondrial viscosity measurement with the amphiphilic copolymer fluorescent probe (PP) has been successfully revealed for the first time. PP was synthesized, starting from a hydrophobic rhodamine derivative fluorophore and hydrophilic 2-hydroxyethyl acrylate (HEA) by radical polymerization, which could be used to detect mitochondrial viscosity specifically. The systematic investigation demonstrated that the fluorescence emission of PP with a deep red emission increased about 9-fold when the medium is changed from methanol to 99% glycerol, indicating high viscosity dependence. Moreover, PP could self-assemble into nanospheres with the particle size of about 140 nm in water and the nano-structure enabled PP to enter living cells quickly. Cytotoxicity test showed that the cells survival rate remained above 70% at 70 μg·mL^-1 of PP. Good biocompatibility and low cytotoxicity of PP are promising to provide a high contrast fluorescence imaging. Taken together, the results point the way to development of novel amphiphilic copolymer fluorescent probes-based the detection in solutions, physiology and pathology.

Spectroscopic and Theoretical Studies of Fluorescence Effects in 2-Methylamino-5-(2,4-dihydroxyphenyl)-1,3,4-thiadiazole Induced by Molecular Aggregation

The article presents the results of fluorescence analyses of 2-methylamino-5-(2,4-dihydroxyphenyl)-1,3,4-thiadiazole (MDFT) in an aqueous environment. MDFT dissolved in aqueous solutions with a pH value in the range from 1 to 4.5 yielded an interesting effect of two clearly separated fluorescence emissions. In turn, a single fluorescence was observed in MDFT dissolved in water solutions with a pH value from 4.5 to 12. As it was suggested in the previous investigations of other 1,3,4-thiadiazole compounds, these effects may be associated with conformational changes in the structure of the analysed molecule accompanied by aggregation effects. Crystallographic data showed that the effect of the two separated fluorescence emissions occurred in a conformation with the –OH group in the resorcyl ring bound on the side of the sulphur atom from the 1,3,4-thiadiazole ring. The hypothesis of aggregation as the mechanism involved in the change in the spectral properties at low pH is supported by the results of (Time-Dependent) Density Functional Theory calculations. The possibility of rapid analysis of conformational changes with the fluorescence spectroscopy technique may be rather important outcome obtained from the spectroscopic studies presented in this article. Additionally, the presented results seem to be highly important as they can be easily observed in solutions and biologically important samples.