‘Remote’ Adiabatic Photoinduced Deprotonation and Aggregate Formation of Amphiphilic <i>N</i>-Alkyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium Chloride Salts

The absorption and emission properties of a series of amphiphilic <i>N</i>-alkyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride salts were investigated in solvents of different polarities and over a wide concentration range. For example, at 10^–5 M concentrations in tetrahydrofuran (THF), salts with at least one N–H bond exhibited broad, structureless emissions even though time-correlated single photon counting (TCSPC) experiments indicated negligible static or dynamic intermolecular interactions. Salts with a butylene spacer or lacking an N–H bond showed no discernible structureless emission; their emission spectra were dominated by the normal monomeric fluorescence of a pyrenyl group and the TCSPC histograms could be interpreted on the basis of intramolecular photophysics. The broad, structureless emission is attributed to an unprecedented, rapid, adiabatic proton-transfer to the medium, followed by the formation of an intramolecular <i>exciplex</i> consisting of amine and pyrenyl groups. The proposed mechanism involves excitation of a ground-state conformer of the salts in which the ammonium group sits over the pyrenyl ring due to electrostatic stabilization. At higher concentrations, with longer <i>N</i>-alkyl groups, or in selected solvents, electronic excitation of the salts led to dynamic and static <i>excimeric</i> emissions. For example, whereas the emission spectrum of 10^–3 M <i>N</i>-hexyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride in THF consisted of comparable amounts of monomeric and excimeric emission, the emission from 10^–5 M <i>N</i>-dodecyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride in 1:9 (v:v) ethanol/water solutions was dominated by excimeric emission, and discrete particles near micrometer size were discernible from confocal microscopy and dynamic light scattering experiments. Comparison of the static and dynamic emission characteristics of the particles and of the neat solid of <i>N</i>-dodecyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride indicate that molecular packing in the microparticles and in the single crystal are very similar if not the same. It is suggested that other examples of the adiabatic proton transfer found in the dilute concentration regime with the pyrenyl salts may be occurring in very different systems, such as in proteins where conformational constraints hold ammonium groups over aromatic rings of peptide units

‘Remote’ Adiabatic Photoinduced Deprotonation and Aggregate Formation of Amphiphilic <i>N</i>-Alkyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium Chloride Salts

The absorption and emission properties of a series of amphiphilic <i>N</i>-alkyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride salts were investigated in solvents of different polarities and over a wide concentration range. For example, at 10^–5 M concentrations in tetrahydrofuran (THF), salts with at least one N–H bond exhibited broad, structureless emissions even though time-correlated single photon counting (TCSPC) experiments indicated negligible static or dynamic intermolecular interactions. Salts with a butylene spacer or lacking an N–H bond showed no discernible structureless emission; their emission spectra were dominated by the normal monomeric fluorescence of a pyrenyl group and the TCSPC histograms could be interpreted on the basis of intramolecular photophysics. The broad, structureless emission is attributed to an unprecedented, rapid, adiabatic proton-transfer to the medium, followed by the formation of an intramolecular <i>exciplex</i> consisting of amine and pyrenyl groups. The proposed mechanism involves excitation of a ground-state conformer of the salts in which the ammonium group sits over the pyrenyl ring due to electrostatic stabilization. At higher concentrations, with longer <i>N</i>-alkyl groups, or in selected solvents, electronic excitation of the salts led to dynamic and static <i>excimeric</i> emissions. For example, whereas the emission spectrum of 10^–3 M <i>N</i>-hexyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride in THF consisted of comparable amounts of monomeric and excimeric emission, the emission from 10^–5 M <i>N</i>-dodecyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride in 1:9 (v:v) ethanol/water solutions was dominated by excimeric emission, and discrete particles near micrometer size were discernible from confocal microscopy and dynamic light scattering experiments. Comparison of the static and dynamic emission characteristics of the particles and of the neat solid of <i>N</i>-dodecyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride indicate that molecular packing in the microparticles and in the single crystal are very similar if not the same. It is suggested that other examples of the adiabatic proton transfer found in the dilute concentration regime with the pyrenyl salts may be occurring in very different systems, such as in proteins where conformational constraints hold ammonium groups over aromatic rings of peptide units

Reversible Switching of Tb(III) Emission by Sensitization from 2,3-Dihydroxynaphthalene in an Isothermally Reversible Ionic Liquid

A reversible room-temperature ionic liquid (ILO) was prepared by the addition of CO₂ to an equimolar mixture of hexylamidine (AD) and butylamine (AN). The ILO and AD/AN mixture were cycled repeatedly by alternating the passage of CO₂ and N₂ gases through the liquid. The ILO was utilized to sensitize very efficiently energy transfer to and emission by Tb­(III) ions when 2,3-dihydroxynaphthalene (DHN) was irradiated. The emission was nearly completely quenched in the AD/AN mixture. The process described here is unique in its use of CO₂ and N₂ to “switch on and off” the emission by a lanthanide ion, Tb­(III) in this case. In the corresponding amidinium dithiocarbamate ionic liquid (ILS), no appreciable Tb­(III) emission was found due to quenching of the excited singlet state of DHN by thio groups. The ILS was not reconverted to the AD/AN mixture upon adding N₂; N₂ bubbling did not result in the displacement of CS₂

Ion-Transport Properties of Polydimethylsiloxane-Based Ionomers with Amidinium or Imidazolinium Alkyldithiocarbamate Pendant Groups in Low Dielectric Solvents or as Neat Liquids

The ion transport properties of ionomers comprised of polydimethylsiloxanes with amidinium or imidazolinium attached side chains and alkyldithiocarbamate anions (where alkyl is hexyl or octadecyl) have been investigated in chloroform solutions principally and as neat liquids. The influence of modifying the molecular weights of the polydimethylsiloxanes, the frequency of their amidininium or imidazolinium side groups, and temperature on the conductivity have been explored. When a solvent more polar than chloroform, syn-tetrachloroethane, was employed, a large increase in the ionic conductivity was found despite there being an increase in the viscosity of the solution. At least in these two solvents, polarity is more important in determining the conductivity than the viscosity. When normalized for ion content, Walden plots of the ionomer solutions at different ionomer concentrations approached values found for 1 M aqueous KCl. As neat liquids, the amidinium and imidazolinium hexyldithiocarbamate ionomers exceeded the values associated with the “superionic” region of the Walden plot (i.e., above the conductivity values for 1 M aqueous KCl). As ion content and polymer molecular weight increased, larger decoupling between bulk viscosity and ionic conductivity was noted, probably as a result of changes in the dynamic fragility of the ionomers

Correlations of Properties and Structures at Different Length Scales of Hydro- and Organo-gels Based on <i>N</i>-Alkyl-(<i>R</i>)-12-Hydroxyoctadecylammonium Chlorides

The self-assembly and gelating ability of a set of <i>N</i>-alkyl-(<i>R</i>)-12-hydroxyoctadecylammonium chlorides (NCl-<i>n</i>, where <i>n</i> = 0–6, 18 is the length of the alkyl chain on nitrogen) are described. Several are found to be ambidextrous (gelating both water and a variety of organic liquids) and very efficient (needing less than ca. 0.5 wt % at room temperature). Structure–property correlations at different distance scales of the NCl-<i>n</i> in their hydro- and organo-gels and neat, solid states have been made using X-ray diffraction, neutron scattering, thermal, optical, cryo-SEM and rheological techniques. The self-assembled fibrillar networks consist of spherulitic objects with fibers whose diameters and degrees of twisting differ in the hydro- and organo-gels. Increasing <i>n</i> (and, thus, the molecular length) increases the width of the fibers in their hydrogels; an irregular, less pronounced trend between <i>n</i> and fiber width is observed in the corresponding toluene gels. Time-dependent, small angle neutron scattering data for the isothermal sol-to-gel transformation of sols of NCl-18/toluene to their gels, treated according to Avrami theory, indicate heterogeneous nucleation involving rodlike growth. Rheological studies of gels of NCl-3 in water and toluene confirm their viscoelastic nature and show that the hydrogel is mechanically stronger than the toluene gel. Models for the different molecular packing arrangements within the fibrillar gel networks of the hydro- and organogels have been inferred from X-ray diffraction. The variations in the fibrillar networks provide a comprehensive picture and detailed insights into why seemingly very similar NCl-<i>n</i> behave very differently during their self-assembly processes in water and organic liquids. It is shown that the NCl-<i>n</i> provide a versatile platform for interrogating fundamental questions regarding the links between molecular structure and one-dimensional self-aggregation, leading to gelation

Photophysics of Pyrenyl-Functionalized Poly(isobutylene-<i>alt</i>-maleic anhydride) and Poly(isobutylene-<i>alt</i>-maleic <i>N</i>‑alkylimide). Influence of Solvent, Degree of Substitution, and Temperature

A series of polymers derived from poly­(isobutylene-<i>alt</i>-maleic anhydride) (PIMA) with ca. 1% to >90% of the anhydride units randomly substituted with 1-pyrenyl­methylimido groups (Py-PIMA) has been synthesized and characterized. The remaining anhydride units in the Py-PIMA with 10% pyrenyl substitution have been converted to <i>N</i>-hexyl and <i>N-</i>decyl imides. The photophysical properties of these polymers, as obtained from steady-state fluorescence intensities, time-correlated single photon counting experiments, and time-resolved emission spectra, have provided insights into the dependence of the polymer conformations and their labilities on the degree of pyrenyl substitution, the nature of the appended alkyl chains, temperature, and solvent properties according to the Flory interaction coefficient and Hansen solubility parameters. None of the solvent characteristics, alone, can account for all aspects of the polymer behavior. The interactions of the medium with the polymer chains and the excited singlet states of the pyrenyl units must be considered somewhat differently. The results also indicate that PIMA is a potentially very useful and versatile platform for other investigations of polymer chain and probe dynamics

l‑Carnosine-Derived Fmoc-Tripeptides Forming pH-Sensitive and Proteolytically Stable Supramolecular Hydrogels

A series of β-amino acid containing tripeptides has been designed and synthesized in order to develop oligopeptide-based, thermoreversible, pH-sensitive, and proteolytically stable hydrogels. The Fmoc [<i>N</i>-(fluorenyl-9-methoxycarbonyl)]-protected tripeptides were found to produce hydrogels in both pH 7 and 2 buffers at a very low concentration (<0.2% w/v). It has been shown that the Fmoc group plays an important role in the gelation process. Also a dependence of gelation ability on hydrophobicity of the side chain of the Fmoc-protected α-amino acid was observed. The effect of the addition of inorganic salts on the gelation process was investigated as well. Spectroscopic studies indicated formation of J-aggregates through π–π stacking interactions between Fmoc groups in solution as well as in the gel state. In the gel phase, these self-assembling tripeptides form long interconnected nanofibrils leading to the formation of 3-dimensional network structure. The hydrogels were characterized by various techniques, including field emission electron microscopy, transmission electron microscopy, atomic force microscopy, rheology, Fourier transform IR, circular dichroism (CD), and wide-angle X-ray diffraction (WAXD) spectroscopy. The CD studies and WAXD analyses show an antiparallel β-sheet structure in the gel state. l-Phenylalanine and l-tyrosine containing tripeptides formed helical aggregates with handedness opposite to those containing l-valine and l-leucine residues. The mechanical stability of the hydrogels was found to depend on the hydrophobicity of the side chain of the tripeptide as well as on the pH of the solution. Also, the tripeptides exhibit in vitro proteolytic stability against proteinase K enzyme

Oscillatory Rheology and Surface Water Wave Effects on Crude Oil and Corn Oil Gels with (<i>R</i>)‑12-Hydroxystearic Acid as Gelator

The mechanical properties of films of gels composed of a crude oil or corn oil and (<i>R</i>)-12-hydroxystearic acid are described using oscillatory rheology and water-surface waves. The integrity of these gel films are contrasted with those of neat oil films subjected to the same types of mechanical testing. The oil-based gels are thixotropic, and we quantify their post-recovery yield. A simple model is proposed to describe the loss of integrity when the gels are subjected to high amplitude surface waves. Our technique provides a novel method for quantifying the role of dynamic surface perturbations on the mechanics of viscoelastic films, providing an illustrative model system for testing coagulants and dispersants designed to mitigate oil spills

Influence of Anions and Alkyl Chain Lengths of <i>N</i>‑Alkyl‑<i>n</i>‑(<i>R</i>)‑12-Hydroxyoctadecyl Ammonium Salts on Their Hydrogels and Organogels

The self-assembly and gelating characteristics of a set of <i>N</i>-alkyl-(<i>R</i>)-12-hydroxyoctadecylammonium salts (<b><i>n</i>-HOA-X</b>, where <i><b>n</b></i> = 0–6, 18 is the length of the alkyl chain on nitrogen, <b>X</b> = Cl, <i><b>n</b></i> = 3, and <b>X</b> = Br, NO₃, and BF₄) are described. Solid–solid phase transitions were observed for powders of <b><i>n</i>-HOA-Cl</b>, and orthorhombic-type crystal packing arrangements and lattice spacings were calculated from X-ray diffractograms at 22 °C. The diffractogram of <b>3-HOA-Br</b> indicates the presence of more than one morph at room temperature, and that of <b>3-HOA-I</b> corresponds to a lamellar packing arrangement. Differences in the molecular packing arrangements of <b>3-HOA-X</b> are reflected in their gelation abilities. The melting temperatures (<i>T</i>_gel) of the hydrogels of <b>3-HOA-Br</b> are higher than those of <b>3-HOA-Cl</b> at the same concentrations, and <b>3-HOA-I</b> failed to gelate any of the investigated liquids. <b>3-HOA-NO</b>_<b>3</b> gelated only water and CCl₄ and <b>3-HOA-BF</b>_<b>4</b> formed only hydrogels. Plots of changes in conductivities of the <b>3-HOA-X</b> salts (where <b>X</b> = Cl, Br, NO₃ and BF₄) as a function of temperature were used to calculate the critical aggregation concentrations (CGCs). Because the CGCs from the ‘falling drop’ method are nearly the same as those from the conductivity measurements, aggregation, nucleation, and gelation must occur within a very narrow <b>3-HOA-X</b> concentration range. <i>T</i>_gel values of 2 wt % <b>3-HOA-Cl</b> hydrogels (prepared by fast cooling of the sol phase) increased upon adding KCl up to 0.1 M. The effects can be attributed principally to the chloride anion rather than its cation partners. The properties of the hydrogels of <b>3-HOA-X</b> do not follow the Hofmeister ranking rule. The variations in the counterions afford detailed insight into the behavior of <b>3-HOA-X</b> in their neat solids and assemblies in gels as well as the processes accompanying gel formation in water and organic liquids

New Insights into an Old Problem. Fluorescence Quenching of Sterically-Graded Pyrenes by Tertiary Aliphatic Amines

Although the quenching of singlet-excited states of aromatic molecules by amines has been studied for several decades, important aspects of the mechanism(s) remain nebulous. To address some of the unknowns, steric, and electronic factors associated with the quenching of the singlet-excited states of three electronically related aromatic molecules, pyrene, 1,3,6,8-tetraphenylpyrene (TPPy), and 1,3,6,8-tetrakis­(4-methoxy-2,6-dimethylphenyl)­pyrene (PyOMe), by a wide range of tertiary aliphatic amines have been assessed quantitatively. Correlations among the steric and electronic properties of the amines and the pyrenes (e.g., sizes, shapes, conformational labilities, excitation energies, and oxidation or reduction potentials) have been used in conjunction with the steady-state and dynamic fluorescence quenching data and DFT calculations on the ground and excited state complexes to make quantitative assessments of the steric and electronic factors controlling the quenching processes. PyOMe is a rather rigid bowl-like molecule that, in its electronic ground state, does not make stable complexes with amines in solution. TPPy has a shallower bowl-like shape that is much more flexible. Experiments conducted with a crystalline ground-state complex of an amine and PyOMe demonstrate (as assumed in many other studies but not shown conclusively heretofore) that the geometry needed for quenching the excited singlet state of PyOMe must place the lone-pair of electrons of the amines over the π-system of the pyrenyl group. Furthermore, there is a significant dependence on the shape and size of the amine on its ability to quench PyOMe, but not on the less conformationally constrained TPPy. The conclusions obtained from these studies are clearly applicable to a wide variety of other systems in which fluorescence from an aromatic moiety is being quenched, and they provide insights into how weak host–guest pairs interact