Novel fluorophores for single-molecule imaging

Nonlinear optical chromophores based on dicyanodihydrofuran acceptors paired with amine donors have been found to exhibit sufficiently large fluorescence quantum yields and stability to enable single-molecule detection in polymeric hosts. To illustrate the breadth of this class, six fluorophores are presented, spanning the emission range from 505 to 646 nm. In contrast to conventional single-molecule fluorophores, the new molecules feature sensitivity to local rigidity, large ground-state dipole moments, and large polarizability anisotropies, properties that can be used to design new reporter experiments at the single-molecule level.     

The structural basis of transitions between highly ordered smectic phases in semifluorinated alkanes

The semifluorinated alkanes F(CF₂)₁₀(CH₂)₉H and F(CF₂)₁₀(CH₂)₁₁H both undergo a reversible transition between tilted smectic phases above room temperature as demonstrated previously for F(CF₂)₁₀(CH₂)₁₀H. The transformation on cooling is achieved in two stages: an increase in molecular tilt relative to the layer normal, followed by a change in the molecular interdigitation within each layer. The extent of complete transformation is sensitive to temperature and time, due to strains occasioned by the accompanying volume change, and because the second stage requires significant molecular translations. Characterization of the transformation is possible by transmitted light microscopy, supplemented by X-ray diffraction.     

Tetranaphthyleno[5,6‐bcd:11,12‐b′c′d′:17,18‐b′′c′′d′′:23,24‐b′′′c′′′d′′′]tetrafuran

The title compound, C₄₀H₁₆O₄ or [C₁₀H₄O]₄, is a planar tetrameric cyclooligomer which crystallizes in the monoclinic space group P2₁/n. The compound is located on an inversion center with the asymmetric unit consisting of half of the molecule. The compound displays an interesting packing structure, where the cyclooligomer displays both layered packing with respect to nearest neighbors and a rotation of adjacent planar rings that results in additional interactions. The geometric parameters of the compound agree well with those of comparable cyclooligomers, while the packing reveals some similarities and differences.     

Diffusion of lipid-like single-molecule fluorophores in the cell membrane

The dicyanomethylenedihydrofuran (DCDHF) class of single-molecule fluorophores contains an amine donor and a dicyanomethylenedihydrofuran acceptor linked by a conjugated unit (benzene, naphthalene, or styrene). Molecules in this class have a number of useful properties in addition to those usually required for single-molecule studies (such as high fluorescence quantum yield and photostability), including second-order optical nonlinearity, large ground-state dipole moment, and sensitivity to local environment. Moreover, most DCDHF molecules have amphiphilic structures, with a polar dicyanomethylenedihydrofuran headgroup and nonpolar hydrocarbon tails on the amine or furan ring, and can be used as fluorescent lipid analogues for live cell imaging. Here we demonstrate that individual molecules of several different DCDHF lipid analogues can be observed diffusing in the plasma membrane of Chinese hamster ovary cells. The photophysical and diffusive behaviors of the DCDHF lipid analogues in membranes are described and are found to be competitive with the well-known lipid probe N-(6-tetramethylrhodaminethiocarbamoyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine.     

Investigation of two-photon absorption properties in branched alkene and alkyne chromophores

Novel alkene and alkyne branched structures have been synthesized, and their two-photon absorption (2PA) properties are reported. This series of alkene and alkyne trimer systems tests the mechanistic approach for enhancing the 2PA process which is usually dictated by the pi-bridging, delocalization length, and corresponding charge transfer on the 2PA cross sections. The results suggest that alkene branched systems have higher 2PA cross sections. While steady-state absorption and emission measurements were not successful in predicting the observed trend of 2PA cross sections, time-resolved measurements have explained the trends observed. It was found that, upon photoexcitation, there is an ultrafast charge localization to an intramolecular charge-transfer (ICT) state, followed by the presence of a solvent and conformationally relaxed ICT state in these branched systems.     

Low loss liquid crystals for infrared applications

Seven fluorinated and chlorinated terphenyl compounds intended for mid-wave infrared (MWIR) applications are synthesised and two eutectic mixtures formulated, and their physical properties evaluated. In addition to low absorption, some desirable properties for MWIR applications include wide nematic range, high birefringence, large dielectric anisotropy and low viscosity. The fluorinated terphenyl mixture exhibits a relatively low absorption in the vicinities of λ ~ 3 μm, but a fairly strong overtone appears in the 4–5 μm region. To suppress these overtone absorptions, short-chain chlorinated terphenyl compounds are proven to be useful.