Fluorescence upconversion and transient absorption techniques are used to explain the source of the intense red/near-infrared emission of crystalline 4-dimethylamino-2'-hydroxychalcone. We found that the initially excited enol form undergoes tautomerization in 3 ps to form the keto tautomer. The latter is stable in the ground state as a consequence of J-type aggregation in the crystal packing and is manifested in an absorption peak at 550 nm that spectrally overlaps with the short-lived enol emission, leading to self-reabsorption and adding a factor to the complete depletion of the enol emission. Relaxation of the keto tautomer takes place in the form of intense fluorescence (600-750 nm) with 1.7 ns lifetime. The different spectroscopy in solution is due to vibrational cooling (300 fs), followed by solvation dynamics (5 ps in methanol) and twisting of the hydroxyphenyl ring (16 ps), before relaxation of the enol tautomer in the form of weak green fluorescence with 350 ps lifetime.

Water-driven self-assembly of lipids displays a variety of liquid crystalline phases that are crucial for membrane functions. Herein, we characterize the temperature-induced phase transitions in two compositions of an aqueous self-assembly system of the octyl β-D-glucoside (βGlcOC(8)) system, using steady-state and time-resolved fluorescence measurements. The phase transitions hexagonal ↔ micellar and cubic ↔ lamellar were investigated using tryptophan (Trp) and two of its ester derivatives (Trp-C(4) and Trp-C(8)) to probe the polar headgroup region and pyrene to probe the hydrophobic tail region. The polarity of the headgroup region was estimated to be close to that of simple alcohols (methanol and ethanol) for all phases. The pyrene fluorescence indicates that the pyrene molecules are dispersed among the tails of the hydrophobic region, yet remain in close proximity to the polar head groups. Comparing the present results with our previously reported one for βMaltoOC(12), increasing the tail length of the hexagonal phase from C(8) to C(12) leads to less interaction with pyrene, which is attributed to the more random and wobbling motion of the longer alkyl tail. We measured a reduction (more hydrophobic) in the ratio of the vibronic peak intensities of pyrene (I(1)/I(3)) for the lamellar phase compared to that of the cubic phase. The higher polarity in the cubic phase can be correlated to the nature of its interface, which curves toward the bulk water. This geometry also explains the slight reduction in polarity of the headgroup region compared to the other phases. Upon the addition of Trp-C(8), the fluorescence lifetime of pyrene is reduced by 28% in the lamellar and cubic phases, whereas the I(1)/I(3) value is only slightly reduced. The results reflect the dominant role of dynamic interaction mechanism between the C(8) chain of Trp-C(8) and pyrene. This mechanism may be important for these two phases since they participate in the process of membrane fusion. Both lipid compositions show completely reversible temperature-induced phase transitions, reflecting the thermodynamic equilibrium structures of their mesophases. Probing both regions of the different lipid phases reveals a large degree of heterogeneity and flexibility of the lipid self-assembly. These properties are crucial for carrying out different biological functions such as the ability to accommodate various molecular sizes.

The effect of chain branching on glycolipid thermotropic and lyotropic phases was investigated for a series of synthetic β-D-glucosides derived from Guerbet alcohols, whose total hydrocarbon chain length ranged from C 8 to C 24 . The compounds, which can be viewed as isosteric mimics for glycoglycerolipids, were synthesised in high purity and their liquid crystalline phases were studied using optical polarising microscopy (OPM), and small-angle X-ray diffraction. When dry, the shortest compound (total C 8 ) exhibits a monotropic L α phase while longer ones (C 16 and C 20 ) adopt inverse hexagonal H II phases. The C 24 compound forms an ordered lamellar phase at room temperature, but exhibits a metastable H II phase upon cooling. Curiously the intermediate chain length homologue (C 12 ) adopts an isotropic inverse micellar (L 2 ) phase in the dry state over the range of temperatures studied. Upon hydration, the C 8 compound dissolves, and the C 12 compound forms a fluid lamellar L α phase. The C 16 Guerbet glucoside (i.e. β-Glc-C 10 C 6 ) exhibits an inverse bicontinuous cubic phase of space group Ia3d in excess water, never previously observed in branched-chain lipids, and very seldom observed in excess water. The C 20 compound remains in the H II phase upon hydrating, with the lattice parameter swelling substantially.

Characteristic structures and properties of nanochannels are thought to play important roles in biological activity and chemical and physical processes at the interfaces. In this work, we characterized the polar nanochannels of the inverse bicontinuous cubic phase of the 2-hexyl-decyl-β-Dglucopyranoside/water system, which has a gyroid Ia3d space group symmetry, by introducing fluorescent probes. The fluorescence signals of tryptophan (Trp) and two of its ester derivatives (Trp-C 4 and Trp-C 8 ) show a local basic environment inside the nanochannels that is equivalent to an aqueous solution of pH ≥ 10.0. This was confirmed by measuring the fluorescence spectra of aqueous tryptophan in different pH solutions. The basic effect is attributed to the restricted motion of water in the nanochannels (diameter of 2.3 nm) in which water molecules are situated very close to the OH groups of the sugar units. The very small channels also force the tryptophan moiety to be very close to the sugar units and the water molecules in which the local environment is mostly basic. We characterized the hydrophobic region of the lipid by measuring the fluorescence change of pyrene upon incorporation in the tail region. A larger hydrophobicity was reflected in the measured small ratio of the vibronic peak intensities of pyrene (I 1 /I 3 ). This is attributed to the compact interaction of the double alkyl chains of the lipid with the pyrene molecules. When adding Trp-C 8 to the lipid assembly, there was no change in the I 1 /I 3 ratio nor in the pyrene lifetimes which implies that there is no interaction between the pyrene and the C 8 -chain. This observation indicates that the pyrene molecules are well-shielded inside the tail region. The measured two lifetime components for each tryptophan and pyrene point to the presence of a degree of heterogeneity and flexibility in the lipid self-assembly. These properties are crucial for carrying out different biological functions such as the ability to accommodate various molecular sizes. The current results suggest the structural importance of very narrow polar nanochannels of the lipidic cubic phase which may facilitate the transfer of highly polar substances, such as ions, to the cells.