Molecular machines working at interfaces: physics, chemistry, evolution and nanoarchitectonics

As a post-nanotechnology concept, nanoarchitectonics combines nanotechnology with advanced materials science. Molecular machines made by assembling molecular units and their organizational bodies are also products of nanoarchitectonics. They can be regarded as the smallest functional materials. Originally, studies on molecular machines analyzed the average properties of objects dispersed in solution by spectroscopic methods. Researchers' playgrounds partially shifted to solid interfaces, because high-resolution observation of molecular machines is usually done on solid interfaces under high vacuum and cryogenic conditions. Additionally, to ensure the practical applicability of molecular machines, operation under ambient conditions is necessary. The latter conditions are met in dynamic interfacial environments such as the surface of water at room temperature. According to these backgrounds, this review summarizes the trends of molecular machines that continue to evolve under the concept of nanoarchitectonics in interfacial environments. Some recent examples of molecular machines in solution are briefly introduced first, which is followed by an overview of studies of molecular machines and similar supramolecular structures in various interfacial environments. The interfacial environments are classified into (i) solid interfaces, (ii) liquid interfaces, and (iii) various material and biological interfaces. Molecular machines are expanding their activities from the static environment of a solid interface to the more dynamic environment of a liquid interface. Molecular machines change their field of activity while maintaining their basic functions and induce the accumulation of individual molecular machines into macroscopic physical properties molecular machines through macroscopic mechanical motions can be employed to control molecular machines. Moreover, research on molecular machines is not limited to solid and liquid interfaces; interfaces with living organisms are also crucial.

Exploring the Depth-Dependent Microviscosity inside a Micelle Using Butterfly-Motion-Based Fluorescent Probes

The viscosity distribution of micellar interiors from the very center to the outer surface is dramatically varied, which has been distinguished in theoretical models, yet it remains highly challenging to quantify this issue experimentally. Herein, a series of fluorophore-substituted surfactants DPAC-Fn (n = 3, 5, 7, 9, 11, 13, and 15) are developed by functionalizing the different alkyl-trimethylammonium bromides with the butterfly motion-based viscosity sensor, N,N'-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC). The immersion depth of DPAC units of DPAC-Fn in cetrimonium bromide (C16TAB) micelles depends on the alkyl chain lengths n. From deep (n = 15) to shallow (n = 3), DPAC-Fn in C16TAB micelles exhibits efficient viscosity-sensitive dynamic multicolor emissions. With external standards for quantification, the viscosity distribution inside a C16TAB micelle with the size of ∼4 nm is changed seriously from high viscosity (∼190 Pa s) in the core center to low viscosity (∼1 Pa s) near the outer surface. This work provides a tailored approach for powerful micelle tools to explore the depth-dependent microviscosity of micellar interiors.

Environment-sensitive fluorescence of COT-fused perylene bisimide based on symmetry-breaking charge separation

Flexible and aromatic photofunctional system (FLAP) is composed of flapping rigid aromatic wings fused with a flexible 8π ring at the center such as cyclooctatetraene (COT). A series of FLAP have been actively studied for the interesting dynamic behaviors. Here, we synthesized a new flapping molecule bearing naphtho-perylenebisimide wings (NPBI-FLAP), in which two perylene units are arranged side by side. As a reference compound, we also prepared COT-fused NPBI (NPBI-COT) that contains only single perylene unit. In both compounds, inherent strong fluorescence of the NPBI moiety is almost quenched and the FL lifetime becomes much shortened in highly polar solvents (acetone and DMF). Through the analyses of environment-sensitive fluorescence, electrochemical reduction/oxidation, and femtosecond transient absorption, the fluorescence quenching behavior was attributed to rapid symmetry-breaking charge separation (SB-CS) for NPBI-FLAP and to intramolecular charge transfer (ICT) for NPBI-COT. Most of the excited species of these compounds decay with the bent geometry, which is in contrast with the excited-state planarization behavior of a previously reported COT-fused peryleneimides with the double-headed arrangement of the perylene moieties. These results indicate that changing the fusion manners between COT and other π skeletons offers new functional molecules with distinct dynamics.

Dense and Acidic Organelle-Targeted Visualization in Living Cells: Application of Viscosity-Responsive Fluorescence Utilizing Restricted Access to Minimum Energy Conical Intersection

Cell-imaging methods with functional fluorescent probes are an indispensable technique to evaluate physical parameters in cellular microenvironments. In particular, molecular rotors, which take advantage of the twisted intramolecular charge transfer (TICT) process, have helped evaluate microviscosity. However, the involvement of charge-separated species in the fluorescence process potentially limits the quantitative evaluation of viscosity. Herein, we developed viscosity-responsive fluorescent probes for cell imaging that are not dependent on the TICT process. We synthesized AnP₂-H and AnP₂-OEG, both of which contain 9,10-di(piperazinyl)anthracene, based on 9,10-bis(N,N-dialkylamino)anthracene that adopts a nonflat geometry at minimum energy conical intersection. AnP₂-H and AnP₂-OEG exhibited enhanced fluorescence as the viscosity increased, with sensitivities comparable to those of conventional molecular rotors. In living cell systems, AnP₂-OEG showed low cytotoxicity and, reflecting its viscosity-responsive property, allowed specific visualization of dense and acidic organelles such as lysosomes, secretory granules, and melanosomes under washout-free conditions. These results provide a new direction for developing functional fluorescent probes targeting dense organelles.

Solvatochromic Sensitivity of BODIPY Probes: A New Tool for Selecting Fluorophores and Polarity Mapping

This research work is devoted to collecting a high-quality dataset of BODIPYs in a series of 10-30 solvents. In total, 115 individual compounds in 71 solvents are represented by 1698 arrays of the spectral and photophysical properties of the fluorophore. Each dye for a series of solvents is characterized by a calculated value of solvatochromic sensitivity according to a semiempirical approach applied to a series of solvents. The whole dataset is classified into 6 and 24 clusters of solvatochromic sensitivity, from high negative to high positive solvatochromism. The results of the analysis are visualized by the polarity mapping plots depicting, in terms of wavenumbers, the absorption versus emission, stokes shift versus - (absorption maxima + emission maxima), and quantum yield versus stokes shift. An analysis of the clusters combining several dyes in an individual series of solvents shows that dyes of a high solvatochromic sensitivity demonstrate regular behaviour of the corresponding plots suitable for polarity and viscosity mapping. The fluorophores collected in this study represent a high quality dataset of pattern dyes for analytical and bioanalytical applications. The developed tools could be applied for the analysis of the applicability domain of the fluorescent sensors.

Probing a microviscosity change at the nematic-isotropic liquid crystal phase transition by a ratiometric flapping fluorophore

Understanding the microviscosity of soft condensed matter is important to clarify the mechanisms of chemical, physical or biological events occurring at the nanoscale. Here, we report that flapping fluorophores (FLAP) can serve as microviscosity probes capable of detecting small changes. By the ratiometric fluorescence analysis, one of the FLAP probes detects a macroscopic viscosity change of a few cP, occurring at the thermal phase transition of a nematic liquid crystal. We discuss the impact of the chemical structure on the detection capability, and the orientation of the FLAP molecules in the ground and excited states. This work contributes to experimentally providing a molecular picture of liquid crystals, which are often viewed as a continuum.

Dynamic Polymer Free Volume Monitored by Single-Molecule Spectroscopy of a Dual Fluorescent Flapping Dopant

Single-molecule spectroscopy (SMS) of a dual fluorescent flapping molecular probe (N-FLAP) enabled real-time nanoscale monitoring of local free volume dynamics in polystyrenes. The SMS study was realized by structural improvement of a previously reported flapping molecule by nitrogen substitution, leading to increased brightness (22 times) of the probe. In a polystyrene thin film at the temperature of 5 K above the glass transition, the spectra of a single N-FLAP molecule undergo frequent jumps between short- and long-wavelength forms, the latter one indicating planarization of the molecule in the excited state. The observed spectral jumps were statistically analyzed to reveal the dynamics of the molecular environment. The analysis together with MD and QM/MM calculations show that the excited-state planarization of the flapping probe occurs only when sufficiently large polymer free volume of more than, at least, 280 ų is available close to the molecule, and that such free volume lasts for an average of 1.2 s.

Polarity studies of single polyelectrolyte layers in polyelectrolyte multilayers probed by steady state and life time doxorubicin fluorescence

HYPOTHESIS

Polarity in polyelectrolyte multilayers (PEMs) may vary from the inner to the top layers of the film as the charge compensation of the layers is more effective inside the PEMs than in outer layers. Doxorubicin hydrochloride (DX) is used here to sense polarity at the single polyelectrolyte level inside PEMS.

EXPERIMENTAL

DX is complexed electrostatically to a polyanion, either polystyrene sulfonate (PSS) or polyacrylic acid (PAA) and assembled at selected positions in a multilayer of the polyanion and polyallylamine hydrochloride (PAH) as polycation. Local polarity in the layer domain is evaluated through changes in the intensity ratio of the first to second band of spectra of DX (I₁/I₂ ratio) by steady state fluorescence, and by Lifetime fluorescence.

FINDINGS

PAH/PSS multilayers, show a polarity similar to water with DX/PSS as top layer, decreasing to I₁/I₂ ratios similar to organic solvents as the number of polyelectrolyte layers assembled on top increases. For PAH/PAA multilayers, polarity values reflect a more polar environment than water when DX/PAA is the top layer, remaining unaltered by the assembly of polyelectrolyte layers on top. Results show that different polar environments may be present in a PEM when considering polarity at the single layer level.