Evidence of preferential pi-stacking: a study of intermolecular and intramolecular charge transfer complexes

Nine combinations of pi-electron donors and acceptors were examined by UV-vis, fluorescence and (1)H-NMR spectroscopy to identify pi-stacked charge transfer complexes in macromolecular and supramolecular constructs. The high association constant of pyrene and naphthalene diimide suggests a preferentially pi-stacking pair rationalized by frontier orbital congruence

Highly chromic, proton-responsive phenyl pyrimidones

Aryl pyrimidones are pharmacologically relevant compounds whose optical properties have only been partially explored. We report the synthesis and optical characterization of a series of aryl- and diaryl-2(1H)-pyrimidones. The electronic transitions of these chromophores are modulated by the extent of conjugation between the pendant phenyl ring and the pyrimidone core as well as the presence of electron-donating auxochromes. Monoprotonation of the pyrimidone ring results in large hyperchromic and bathochromic shifts as well as switching of fluorescence making these phenyl pyrimidones of interest as sensory materials

Hierarchical self-assembly of donor-acceptor-substituted butadiene amphiphiles into photoresponsive vesicles and gels

It all gels well: Amphiphilic donor-acceptor-substituted butadienes undergo spontaneous concentration-dependent self-assembly into highly fluorescent vesicles and gels. The vesicles and gels exhibit light-induced destruction and self-repair as a result of the photochromic nature of the butadiene chromophore

Solid state optical properties of 4-alkoxy-pyridine butadiene derivatives: reversible thermal switching of luminescence

The synthesis and optical properties of a series of alkoxyphenyl-pyridyl butadiene derivatives in solution and in the solid state are described. All the derivatives were practically nonfluorescent in solution but showed good fluorescence in the solid-state. The role of molecular packing in controlling the solid-state fluorescence was investigated by studying the X-ray crystal structure of these molecules. One of the derivatives, 4-((1E,3E)-4-(4-butoxyphenyl)buta-1,3-dienyl)pyridine exhibited polymorphism, with the different polymorphs exhibiting visually distinguishable fluorescence. In the natural state it existed as a polymorph exhibiting blue fluorescence, while it's cooled melt emitted yellow light. The difference could be attributed to a transformation in the molecular packing of the material from a herringbone to a brickstone arrangement, resulting in a change from monomer to J-type aggregate fluorescence. The polymorph exhibiting yellow fluorescence was fairly stable (>6 months) but could be converted back to the original form by keeping the film at 110 °C for a short period of time (~8-10 min) before slowly cooling to room temperature. The thermally induced changes in fluorescence behavior were clearly reproducible over several cycles, indicating the utility of this material for thermal imaging applications

Correlation between solid-state photophysical properties and molecular packing in a series of indane-1,3-dione containing butadiene derivatives

The solid-state photophysical and photochromic properties and the molecular packing in single crystals of a series of donor-acceptor-substituted butadiene derivatives with alkoxy groups as donor and indane-1,3-dione as acceptor are reported. These materials showed significant enhancement and red-shift in fluorescence in the solid state compared to that in solution. The single crystal analysis of these derivatives indicated that these effects could be attributed to both improved intramolecular charge transfer due to planarization of the molecules and to intermolecular exciton coupling between adjacent molecules leading to aggregate fluorescence. The character of the aggregate formed (H- or J-type) and extent of aggregation were strongly dependent on the length of the alkyl substituent, and these differences could be correlated to variations in the molecular packing observed in their single crystals. Some of the derivatives could be super cooled to a metastable glassy state with significantly different optical properties. Transformation from crystalline to a highly stable glassy form could also be induced by light, making these materials useful for recording optical images

Indane-1,3-dione and cholesterol containing butadiene derivatives: photoresponsive liquid crystalline glasses for imaging applications

Synthesis, thermo-optical and glass forming properties of a series of photoresponsive luminescent mesogenic materials containing a butadiene moiety linked to a cholesterol group (CBIN, CBIN8 and CBIN12) as well as colour imaging devices based on them are described. These molecules exhibited relatively high fluorescence quantum efficiency in the solid and liquid crystalline states. The cholesteric phase was observed to be the major phase in these molecules and the pitch of cholesteric helix was sensitive to external stimuli such as temperature and light, making it possible to tune their iridescent colour in the visible region. The colour reflected by the cholesteric films could be stabilized by converting them to the glassy state by sudden cooling from their cholesteric phase to ~0 °C. Selective exposure of the material in its liquid crystalline phase to light resulted in trans-cis photoisomerization of the butadiene chromophore. Consequently the pitch of the irradiated portion changed depending upon the intensity of the illuminating light. The coloured images thus formed could be stored for long periods by converting the films to glasses by sudden cooling. The images stored in the glassy state were stable over long periods of time (>one year)

Aggregation and photoresponsive behavior of azobenzene-oligomethylene-glucopyranoside bolaamphiphiles

The self-aggregation behavior of a series of photoresponsive bolaamphiphiles consisting of a central azobenzene moiety linked to terminal sugar groups via oligomethylene spacers in water/DMSO mixed solvents are reported. Changes in the absorption spectra in these solvent mixtures indicated the formation of tightly packed H-aggregates. The ease of formation and the stability of the aggregates were observed to increase with increase in the length of the oligomethylene spacers. Microscopic studies indicated the formation of vesicles or fibers depending upon the length of the oligomethylene spacers. Disruption of the aggregates induced by photoisomerization of the azobenzene unit and their reformation via the subsequent thermal cis to trans isomerization was also investigated

Fluorescent reporters of monoamine transporter distribution and function

Serotonin is a monoamine serving as a chemical messenger in diverse brain regions, as well as in blood and various other organs. We synthesized six ethylamine functionalized fluorophores as fluorescent probes for serotonin. The one with best spectral properties and aqueous solubility, 6-amino-2-(2-aminoethyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione, was studied in detail both in vivo and in vitro. It was shown to act as a ligand for serotonin transporter (SERT) without acute cerebral or cardiovascular toxicity or adverse effects. Fluorescent serotonin analogs can be used for direct visualization of SERT distribution and activity in live tissue

Supergelation via Purely Aromatic π–π Driven Self-Assembly of Pseudodiscotic Oxadiazole Mesogens

A series of highly luminescent oxadiazole-based stilbene molecules (OXD4, OXD8, OXD10, and OXD12) exhibiting interesting enantiotropic liquid crystalline and gelation properties have been synthesized and characterized. The molecules possessing longer alkyl substituents, OXD10 and OXD12, possess a pseudodisc shape and are capable of behaving as supergelators in nonpolar solvents, forming self-standing gels with very high thermal and mechanical stability. Notably the self-assembly of these molecules, which do not possess any hydrogen-bonding motifs normally observed in most reported supergelators, is driven purely by π-stacking interactions of the constituent molecules. The <i>d</i>-spacing ratios estimated from XRD analysis of OXD derivatives possessing longer alkyl chains show that the molecules are arranged in a columnar fashion in the mesogens and the self-assembled nanofibers formed in the gelation process