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Local microviscosity monitoring in living cells is a powerful tool to determine their healthy status in either a specific organelle or in the cytosol. Here is presented the rational design of a new family of self-calibrating dual-microviscosity probes as a strategy to improve the probe response at low viscosity ranges. We found one of the probes is useful to determine low viscosity variations in living cells where subtle stiffening of the rotor group in the probe can increase its viscosity sensitivity. Further X-ray structure analysis, fluorescence anisotropic measurements, and quantum chemical calculations of the electronic excitation by means of a natural transition orbital (NTO) analysis confirmed the observed response. The present work demonstrates that small viscosity variations (0.01–0.1 cP) in cells require improved analytical sensitivity to be properly monitored.

2-Phenyl-3-hydroxy-4(1H)-quinolone possessing dual fluorescence due to excited-state intramolecular proton transfer (ESIPT) forms stable complex with phenylboronic acid with blue shifted emission maximum in micellar medium of a cationic surfactant even though the compound lacks required for complexation with boronic acids cis -diol structure. No complexation is observed in the presence of neutral or anionic surfactants. Titrations of this complex with polyols including sugars and nucleotides at pH 8 displace free quinolone showing ratiometric response, which allows determination of polyols with detection limits 0.05–1 mM and unusually wide linear dynamic ranges. Another ESIPT dye 2-(2′-hydroxyphenyl)−1H-benzimidazole also lacking cis -diol structure forms equally stable complex with phenylboronic acid and allows ratiometric determination of polyols with similar characteristics. The results of this study demonstrate that blocking ESIPT of signaling molecule by complexation of the receptor with the proton donor group eliminates the low energy emission from tautomeric form but strongly enhances the high energy emission typical for “normal” form of signaling molecule creating a possibility of ratiometric sensing.

The coordination properties and acid–base behavior of 2-[{(1 S ,2 R )-1-hydroxy-1-phenylpropan-2-ylimino}methyl]-4-bromophenol Schiff base probe ( L1 ) were characterized by UV–Vis and fluorescence titrations in water. The dissociation constants for the ligand account for a keto–enamine tautomer at pH 7. Detailed complexation studies were carried out, observing the formation of stable 1:1 complex for Zn 2+ , where a “turn-on” fluorescence effect is obtained. More importantly, no Cu 2+ interference is observed, which is a typical problem for Zn 2+ probes, this is awarded to the keto–enamine tautomeric form of the probe L1 according to UV–Vis and X-ray structure data. Also, the Zn 2+ vs . Cd 2+ ion discrimination for L1 is proved. Finally, TD-DFT theoretical calculations were conducted in order to stablish the detection mechanism of the probe.

Aza-flavonols are compounds containing the 3-hydroxy-4(1H)-quinolone moiety, a unique scaffold with applications expanding from fluorescent probes and bioimaging to potential metal-related pharmaceutical drugs. Compositions and stability constants of metal complexes of an aza-flavonol 1-methyl-2-phenyl-3-hydroxy-4(1H)-quinolone (LH) in aqueous micellar medium of hexadecyltrimethylammonium bromide were established by fluorescence and spectrophotometric titrations of LH in the presence of metal ions of different type at variable pH. Strong binding with quenching of ligand fluorescence was observed for Cu2+, Fe2+/3+ and Pb2+ cations. The complexation with Zn2+ induced strong fluorescence enhancement in ZnL+ complex and smaller enhancement in ZnL2 complex. Al3+, Ga3+ and Me2Sn2+ cations induced strong fluorescence enhancement in 1:1 ML complexes, but quenching in ML2 complexes. Together with the relevant literature data these results indicate that the quenching effect in 1:2 complexes along with strong fluorescence enhancement in respective 1:1 complexes is a general phenomenon for 3-hydroxypyranone and 3-hydroxypyridinone ligands. The quantum mechanical Natural Transition Orbital analysis of ground and excited states of LH, AlL2+ and AlL2+ species reveals that the fluorescence of AlL2+ complex is quenched by an internal electron transfer process, which is absent in the 1:1 AlL2+ complex. The results emphasize strong dependence of the coordination induced fluorescence changes on the complex composition.

A simple (R)-(−)-2-phenylglycinol functionalized Schiff base exhibited a dual response to Zn 2+ , Cu 2+ and Fe 2+ ions in water:methanol, (90:10 v/v). The compound displays an “off–on” fluorescent effect with Zn 2+ ions. The emission response decreased with Cu 2+ forming a Cu 2+ /Zn 2+ specie. The fluorescence signal is restored upon addition of sulfide anion (S 2 − ) using the Cu 2+ displacement approach , forming again the Zinc complex. Additionally, the sensor exhibits a second channel (colorimetric) response with Fe 2+ even in the presence of competing Fe 3+ ions. The pH profiles allowed to determine the p K a values and the pH-dependent change in the sensor recognition process. In addition, TD-DFT quantum chemical calculations were implemented to analyze the sensing mechanism by means of Natural Transition Orbital (NTO) analysis. Flow cytometry experiments demonstrated that the sensor successfully detects either exogenous or endogenous (natural) Zn 2+ ions in Jurkat cells with high sensitivity.

A simple ( R )-(−)-2-phenylglycinol functionalized Schiff base L1 and its characterization as a fluorescent–colorimetric sensor for Hg 2+ ion are described. The UV–vis and fluorescence analysis in methanol and aqueous solution show complex formation between L1 and Hg 2+ ion with a micromolar association constant. Competition experiments performed for the acetate salts of Hg 2+ , Zn 2+ , Co 2+ , Pb 2+ , Cd 2+ , Mn 2+ , Cu 2+ , Ni 2+ , and Ba 2+ revealed that compound L1 exhibits high selectivity toward Hg 2+ displaying a color change easily detectable by naked-eye and a turn-off fluorescent effect due to a chelation-enhanced quenching (CHEQ) mechanism. Moreover, addition of EDTA to L1 –Hg 2+ recovers the fluorescence and color offering receptor L1 as a reversible sensor for real-time applications.

The synthesis, chemical characterization and optical studies of three novel fluorene derivatives is reported. These compounds comprise a D–A–D architecture with fluorene moieties as donor groups and fluorenone or benzothiadiazole derivatives as acceptor groups. Theoretical analysis confirmed the existence and the nature of two principal electronic transitions (π → π* and intramolecular charge transfer). Spectroscopic studies in solution revealed that the intramolecular charge transfer character, and in turn the two-photon activity i.e., the fluorescence induced by two-photon absorption, is strongly affected by solvent polarity. The influence of specific solvent–solute interactions over emission properties was also studied through Lippert–Mataga plot. Evaluation of the two-photon absorption cross-sections, gave a maximum value of 105 GM (1 GM = 10−50 cm4 s) in toluene and a minimum value of 23 GM in THF solutions at 750 nm for the fluorenone derivative, a molecule with poor intramolecular charge transfer character and thus weak two-photon absorption; however the benzothiadiazole derivative, with stronger intramolecular charge transfer transition, produced a maximum value of 1000 GM in THF and a minimum value of 236 GM in methanol. Fluorescence quantum efficiency of these compounds was also affected by the medium, with fluorescence quenching in protic solvents such as methanol due to specific solvent interactions (i.e., hydrogen-bonding). Nevertheless, in a more polar medium such as water, nanoaggregates synthesized from the benzothiadiazole derivative exhibited good two-photon activity, i.e., ∼500 GM and fluorescence quantum efficiency of 0.83. Furthermore, these nanoaggregates exhibited more resistance against photodegradation processes than any of the organic solutions tested.