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Contact mechanics, spanning nanometer to tectonic scales, faces long-standing challenges arising from multiscale random roughness, which hinders experimental validation of theories. Understanding multi-asperity rough contacts is vital for addressing catastrophic consequences of these contacts failing such as earthquakes and for diverse technological applications. To visualize such contacts, we introduce a super-resolution microscopy method utilizing spontaneous millisecond ON/OFF fluorescence blinking of contact-sensitive molecular rotor molecules immobilized on a glass coverslip. This technique achieves ∼55 nm lateral imaging resolution for rough poly(methyl methacrylate) and glass spheres on glass contacts. For soft polymer spheres due to large plastic deformation, the resolution improvement does not significantly affect the area of real contact. However, for hard glass spheres, the real contact area is found to be 2.4 times smaller than that found by diffraction-limited imaging. This study highlights, through direct visualization, the impact of material stiffness on the nanoscale structure within the area of real contact.

We explored the possibility of nanoscale mechanical manipulation and control of photophysical properties of conjugated polymer nanoparticles. We carried out a simultaneous atomic force microscopy (AFM) and fluorescence microspectroscopy study on single nanoparticles of the conjugated polymer poly(9,9-dioctylfluorene). The nanoparticles are prepared by a reprecipitation method and have an average height of 27 nm, and their emission is dominated by the well-ordered β-phase conformation. Fluorescence polarization anisotropy and numerical simulations show that each particle contains at least three partly oriented straight β-phase segments surrounded by amorphous glass-phase polyfluorene chains. In the simultaneous experiments, an AFM tip was used to apply external force on a single nanoparticle, and a confocal fluorescence microscope was used to monitor in real time the resulting changes in the fluorescence intensity and spectra. In a nitrogen atmosphere, weak to moderate force of up to 1 μN acts mainly on the glass-phase polyfluorene chains by forming quenchers that cause an efficient and reversible fluorescence decrease, whereas the β-phase segments stay unaffected. A higher force of 5 μN, on the contrary, breaks the β-phase segments into multiple glass-phase segments, causing a net increase in fluorescence intensity. Under ambient air conditions, even a moderate force of 1 μN strongly accelerates the degradation of the nanoparticle by preferably photobleaching the β-phase and partially transforming it into the glass phase. These results will contribute to the fundamental knowledge on the relationship between photophysical and structural properties of polyfluorene nanostructures, and will also provide important feedback for potential applications of such nanostructures in flexible optoelectronic devices.

A peculiarity of cyanobacterial Photosystem I (PSI) is the presence of so-called red chlorophylls absorbing at wavelengths longer than the reaction center P700. The origin and function of these chlorophylls have been debated in literature, but so far no consensus has been reached on either question. Here, we use plasmon-enhanced single-particle fluorescence spectroscopy to elucidate the origin of both short- and long-wavelength emitting species in monomeric PSI from Thermosynechococcus elongatus at room temperature. Polarized fluorescence spectra of single PSI complexes reveal a phase shift in the modulation of the short-wavelength (687 nm) and long-wavelength (717 nm) peaks. Numerical simulations show that this phase shift reflects a spatial angle of 15° between the transition dipole moments of the two forms. Quantum chemical calculations, together with reported X-ray structural and spectroscopic data, were used to assign the chlorophyll a monomer A3 as a candidate for the short-wavelength emitter and the B31-B32 chlorophyll dimer as a candidate for the long-wavelength emitter.

Large reverse saturable absorption (RSA) at the sunlight power level was observed from a host-guest amorphous material composed of d 12 -coronene as a guest and β-estradiol as a rigid amorphous host. The large highly symmetric two-dimensional (2D) aromatic structure of d 12 -coronene doped into the rigid host causes ultralong triplet lifetime as well as high triplet yield, serving efficient accumulation of the triplet excitons of d 12 -coronene under weak excitation light. The high absorption coefficient of the triplet state (σ 34 ) compared with that of the ground state (σ 01 ) for d 12 -coronene is also caused by the highly symmetric 2D aromatic structure. The efficient accumulation of the triplet excitons and high σ 34 - σ 01 led to an onset of the RSA characteristics under irradiance below 10 -1 mW cm -2 . A film of the amorphous material showed a large decrease of transmittance from 66% at 0.02 mW cm -2 to 24% at 50 mW cm -2 for continuous light at 405 nm.

Circularly polarized room-temperature phosphorescence (RTP) with persistent emission characteristics was observed from metal-free chiral binaphthyl structures. Enantiomers of the binaphthyl compounds doped into an amorphous hydroxylated steroid matrix produced blue fluorescence and yellow persistent RTP in air. The lifetime and quantum yield of the yellow persistent RTP were 0.67 s and 2.3%, respectively. The dissymmetry factors of circular dichroism (CD) in the first absorption band, circularly polarized fluorescence (CPF), and circularly polarized persistent RTP were |1.1 × 10(-3)|, |4.5 × 10(-4)|, and |2.3 × 10(-3)|, respectively. A comparison between the experimental data and calculations by time-dependent density functional theory for transient CD spectra confirmed that the binaphthyl conformations in the lowest singlet excited state (S1) and the lowest triplet state (T1) were different. The large difference in the dissymmetry factors for the CPF and the circularly polarized persistent RTP was likely caused by this conformational change between S1 and T1.

Anisotropic mass diffusion in liquid crystals (LCs) is important from the point of both basic LC physics and their applications in optoelectronic devices. We use super-resolution fluorescence microscopy with astigmatic imaging to track 3D diffusion of quantum dots (QDs) in an ordered nematic LC. The method allowed us to evaluate the diffusion coefficients independently along the three spatial axes as well as to determine the absolute position of the QD with respect to the cell wall. We found variations of the diffusion coefficient along the different directions across the cell thickness and explained these as being due to changes of a tilt angle of the LC director. Close to the surface, the diffusion is slowed down due to the confinement effect of the cell wall. Overall, the QD diffusion is much slower than expected for a corresponding particle size. This phenomenon is suggested to originate from reorientation of the LC director in the vicinity of the particle.

We report results on circular dichroism (CD) measured on single immobilized chlorosomes of a triple mutant of green sulfur bacterium Chlorobaculum tepidum . The CD signal is measured by monitoring chlorosomal bacteriochlorphyll c fluorescence excited by alternate left and right circularly polarized laser light with a fixed wavelength of 733 nm. The excitation wavelength is close to a maximum of the negative CD signal of a bulk solution of the same chlorosomes. The average CD dissymmetry parameter obtained from an ensemble of individual chlorosomes was gs = -0.025, with an intrinsic standard deviation (due to variations between individual chlorosomes) of 0.006. The dissymmetry value is about 2.5 times larger than that obtained at the same wavelength in the bulk solution. The difference can be satisfactorily explained by taking into account the orientation factor in the single-chlorosome experiments. The observed distribution of the dissymmetry parameter reflects the well-ordered nature of the mutant chlorosomes.