Introduction 10429 2. Historical Contextualization 10431 3. Synthesis of Pyridinium N-Phenolate Dyes 10433 4. Solvatochromic Properties of Pyridinium N-Phenolate Dyes 10435 4.1. X-ray Crystallographic Studies of the Molecular Structures of Pyridinium N-Phenolate Dyes 10437 4.2. Quantum Chemical Calculations Involving Pyridinium N-Phenolate Dyes 10438 4.3. The E T (30) and E T N Solvent Polarity Scale 10440 4.4. Multiparametric Approaches to the Analysis of the E T (30) and E T N Scale 10441 5. Secondary Solvatochromic Pyridinium N-Phenolate Dyes 10442 6. Investigation of the Physical Properties of Room-Temperature Ionic Liquids 10443 7. Thermosolvatochromism of Pyridinium N-Phenolate Dyes 10450 8. Halochromism of Pyridinium N-Phenolate Dyes 10451 9. Piezosolvatochromism of Pyridinium N-Phenolate Dyes 10452 10. Pyridinium N-Phenolate Dyes in the Investigation of Solvent Mixtures 10453 11. Application of Pyridinium N-Phenolate Dyes in the Construction of Chromogenic Chemosensors 10455 11.1. Chirosolvatochromism of Pyridinium N-Phenolate Dyes 10456 11.2. Chromoionophores 10456 11.3. Chromogenic Chemosensors for Anionic Species 10457 11.4. Chromogenic Chemosensors for Neutral Analytes 10458 12. Pyridinium N-Phenolate Dyes as Probes To Measure the Polarity of Solid Surfaces 10459 13. Pyridinium N-Phenolate Dyes in the Study of Microheterogeneous Systems 10460 13.1. Investigation of Surfactants in Aqueous and Organic Media 10462 13.2. Pyridinium N-Phenolate Dyes in the Investigation of Cyclodextrins in Solution 10463 13.

4-(Nitrostyryl)phenols 2a-9a were synthesized, and by deprotonation in solution, the solvatochromic phenolates 2b-9b were formed. Their absorption bands in the vis region of the spectra are due to π-π* electronic transitions, of an intramolecular charge-transfer nature, from the electron-donor phenolate toward the electron-acceptor nitroarene moiety. The frontier molecular orbitals and natural bond orbitals were analyzed for the protonated and deprotonated forms. The calculated geometries are in agreement with X-ray structures observed for 4a, 6a, and 8a. The HOMO-LUMO energy gaps suggest that, after their deprotonation, an increase in the electron delocalization is observed. In the protonated compounds, the HOMO is primarily localized over the phenol ring and the C═C bridge. After deprotonation, it extends toward the entire molecule, including the NO2 groups. The solvatochromism of each dye was studied in 28 organic solvents, and it was found that all compounds exhibit a reversal in solvatochromism, which is interpreted in terms of the ability of the media to stabilize their electronic ground and excited states to different extents. The Catalán multiparameter equation is used in the interpretation of the solvatochromic data, revealing that the most important contribution to the solute/solvent interaction is the hydrogen-bond donor acidity of the solvent.

The electrospinning technique allows the production of micro- and nanofibers, which can be used to obtain membranes with high surface area and high porosity. These properties are of importance with regard to the use of nanomaterials in the design of optical detection devices. In this Article, electrospun blends comprising poly­(ethylene oxide) [PEO] and sodium alginate [SA], with and without the adsorption of a fluorescent dye (4-[4-(dimethylamino)­styryl]-1-methypyridinium iodide, [DSMI]), were prepared and characterized. PEO/SA/DSMI nanofibers presented higher fluorescence emission intensity and higher absolute quantum yield compared to DSMI in solution. However, DSMI was leached into the solution during the nanofiber cross-linking process. Thus, in order to avoid this leaching, a xerogel [XSB30] was modified with 4-[4-(dimethylamino)­styryl]­pyridine [DMASP] to generate covalently anchored dye units [XSB30-DMASP]. The resulting novel material was then electrospun with PEO/SA. Cross-linking of the electrospun hybrid PEO/SA/XSB30-DMASP nanofibers produced a material exhibiting an increase in both fluorescence emission and absolute quantum yield. Cellulose acetate [CA] was used for comparison because of its solubility in acetone, a less polar solvent that leads to a better distribution of the xerogel. These electrospun systems associated with fluorescent dyes have the potential to be applied in the design of logical gates and chemical sensors.

A 4-(nitrostyryl)phenolate was synthesized and its use in pure solvents revealed a reversion in solvatochromism. Solutions of a dye in binary solvent mixtures, using as components the solvents in the region of the occurrence of the reversion, provided the first case in the literature of reverse solvatochromism in a binary mixture.

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