‘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

Tuning Microstructures in Organogels: Gelation and Spectroscopic Properties of Mono- and Bis-cholesterol-Linked Diphenylbutadiene Derivatives

The gelation and photophysical properties of mono- and bis-cholesterol derivatives linked to diphenylbutadiene have been investigated. Scanning electron microscopy of xerogels of the monocholesterol derivatives indicated that these molecules self-assemble into 3D networks consisting of helically twisted fibers. In contrast, the morphology of xerogels of the bis-cholesterol derivatives indicated agglomerated spheres. In concentrated solutions (>10−4 M), the self-assembled superstructure of the monocholesterol derivatives consists of helically twisted fibers whereas that of the bis-cholesterol derivatives indicated clustered spheres. An investigation of spectroscopic properties suggests that the morphology of the superstructures formed in these systems may be correlated to the nature of the molecular aggregates formed. Absorption and emission spectral studies as a function of concentration and temperature suggested the formation of predominantly J-type aggregates in the monocholesterol and H-type aggregates in the bis-cholesterol derivatives. It is proposed that the slipped stack arrangement within the J aggregates of the monocholesterol derivatives resulted in the formation of helically twisted fibers, whereas the cofacial arrangement in the H aggregates of the bis-cholesterol derivatives could prevent such fiber formation, resulting in the formation of the agglomerated spheres

Tuning Microstructures in Organogels: Gelation and Spectroscopic Properties of Mono- and Bis-cholesterol-Linked Diphenylbutadiene Derivatives

The gelation and photophysical properties of mono- and bis-cholesterol derivatives linked to diphenylbutadiene have been investigated. Scanning electron microscopy of xerogels of the monocholesterol derivatives indicated that these molecules self-assemble into 3D networks consisting of helically twisted fibers. In contrast, the morphology of xerogels of the bis-cholesterol derivatives indicated agglomerated spheres. In concentrated solutions (>10−4 M), the self-assembled superstructure of the monocholesterol derivatives consists of helically twisted fibers whereas that of the bis-cholesterol derivatives indicated clustered spheres. An investigation of spectroscopic properties suggests that the morphology of the superstructures formed in these systems may be correlated to the nature of the molecular aggregates formed. Absorption and emission spectral studies as a function of concentration and temperature suggested the formation of predominantly J-type aggregates in the monocholesterol and H-type aggregates in the bis-cholesterol derivatives. It is proposed that the slipped stack arrangement within the J aggregates of the monocholesterol derivatives resulted in the formation of helically twisted fibers, whereas the cofacial arrangement in the H aggregates of the bis-cholesterol derivatives could prevent such fiber formation, resulting in the formation of the agglomerated spheres

Effects of Temperature and Alkyl Groups of Poly(Alkyl methacrylate)s on Inter- and Intramolecular Interactions of Excited Singlet States of Pyrenyl Guest Molecules

Temperature-induced changes in the static and dynamic characteristics of the fluorescence from pyrene and N,N-dimethyl-3-(pyren-1-yl)propan-1-amine (PyC3NMe2) have been used to determine the locations and mobilities of these probes in the anisotropic environments provided by films of 5 poly(alkyl methacrylate) (PAMA) polymers in which alkyl is ethyl, butyl, isobutyl, cyclohexyl, and hexadecyl. Whereas emission from pyrene reports on the polarity of the guest sites and the ability of molecules to diffuse translationally between sites, emission from PyC3NMe2 yields information about the fluidity and the shape of the guest sites. Data have been obtained from 20 to >400 K, a range that spans the onsets of several relaxation processes in the hosts. Those data indicate that the pyrenyl groups reside near to ester functionalities in most of the PAMAs, although the distance from them (and the main chains) depends upon the bulkiness of the alkyl groups. Among the most important conclusions derived from this research is that the rates of segmental relaxation phenomena near the probe moleculesand not free volume, as was concluded previously from fluorescence measurements in polyethylene filmsare the dominant contributors to the fluorescence changes. Of practical importance, changes in those rates have permitted the onset temperatures of many of the relaxation phenomena occurring in the vicinity of the probes to be located

Comparison of Dipolar, H‑Bonding, and Dispersive Interactions on Gelation Efficiency of Positional Isomers of Keto and Hydroxy Substituted Octadecanoic Acids

A systematic study of the importance of functional group position and type on the gelator efficiencies of structurally simple, low molecular-mass gelators is reported. Thus, the gelation abilities of a series of positional isomers of ketooctadecanoic acid (<i>n</i>-KSA) are compared in a wide range of liquids. The gelation abilities of the <i>n</i>-KSA as a function of <i>n</i>, the keto group position along the chain, are characterized by several structural, thermal, and rheological techniques and are compared with those of the corresponding hydroxyoctadecanoic acid isomers (<i>n</i>-HSA) and the parent molecule, octadecanoic acid (SA). Analyses of the gels according to the strengths of functional group interactions along the alkyl chain in terms of group position and type are made. The conclusions derived from the study indicate that gel stability is enhanced when the functional group is located relatively far from the carboxylic headgroup and when group–group interactions are stronger (i.e., hydrogen-bonding interactions are stronger in the <i>n</i>-HSA than dipole interactions in the <i>n</i>-KSA, which are stronger than the London dispersion interactions in SA). Co-crystals of the keto- and hydroxy-substituted octadecanoic acids are found to be less efficient gelators than even the ketooctadecanoic acids, due to molecular packing and limited group interactions within the gelator networks

Reversible Thermal and Photochemical Switching of Liquid Crystalline Phases and Luminescence in Diphenylbutadiene-Based Mesogenic Dimers

The synthesis and study of the photo- and thermoresponsive behavior of a series of novel asymmetric mesogenic dimers, consisting of a cholesterol moiety linked to a diphenylbutadiene chromophore via flexible alkyl chains are reported. These mesogenic dimers possess the combined glass forming properties of the cholesterol moiety and the photochromic and luminescent properties of the butadiene moiety. Photoinduced cis/trans isomerization of the butadiene chromophore in these materials could be utilized to bring about an isothermal phase transition from the smectic to the cholesteric state. By photochemically controlling the cis/trans isomer ratio, the pitch of the cholesteric could be continuously varied making it possible to tune the color of the film over the entire visible region, and the color images thus generated could be stabilized by converting them to N* glasses. These materials were also polymorphic, exhibiting two crystalline forms possessing distinctly different fluorescence properties. The ability to thermally switch these materials from one crystalline form to the other in a reversible manner also makes them useful for recording fluorescent images

Molecular Packing and Solid-State Fluorescence of Alkoxy-Cyano Substituted Diphenylbutadienes: Structure of the Luminescent Aggregates

A detailed study on the photophysical properties of a series of alkoxy substituted diphenylbutadienes in solution and in the solid state providing a molecular level understanding of the factors controlling their solid-state luminescence behavior is reported. Our studies provide clear evidence for exciton splitting in the solid state resulting in red-shifted emission for this class of materials. The role of the number of alkoxy substituents and the alkyl chain length in controlling the nature of the molecular packing and consequently their fluorescence properties has been elucidated. Whereas in the di- and tri-alkoxy substituted derivatives, the solid-state fluorescence was independent of the length of the alkyl chains, in the monoalkoxy substituted derivatives, increasing the length of the alkyl chain resulted in a visual change in fluorescence from green to blue. On the basis of the analysis of the molecular packing in the single crystals, this difference could be attributed to fluorescence arising from aggregates with an edge-to-face alignment in the molecules possessing short alkyl chains (methyl and butyl) to monomer fluorescence in the long alkyl chain containing derivatives