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Three-dimensional (3D) and two-dimensional (2D) perovskite hybrid systems, known for their exceptional optoelectronic properties and stability, are revolutionizing optoelectronic materials research. However, fundamental physics of the 3D/2D interfaces and their dynamics remain poorly understood. We use fluorescence microspectroscopy to study the photoluminescence (PL) properties of 3D/2D nano-heterostructures of CsPbBr 3 /PEA 2 PbBr 4 formed by postgrowth self-assembly. The in situ PL spectra uncover the presence of new structural phases, quasi-2D PEA 2 Cs n -1 Pb n Br 3 n +1 layers of varying n , at the 3D/2D interface and demonstrate their reversible restructuring under light excitation at room temperature. The restructuring is a result of layer-by-layer cation diffusion at the epitaxial interfaces, manifested as reversible spectral shifts occurring on a time scale of seconds. Such dynamics ultimately leads to optimized distribution of the quasi-2D phases in the system for efficient energy transfer from the 2D to the 3D phases. Our findings provide new insights into controlling energy flow in 3D/2D perovskites for next-generation optoelectronic devices.

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.

The self-assembled state of molecules plays a pivotal role in determining how inherent molecular properties transform and give rise to supramolecular functionalities and has long attracted attention. However, understanding the influence of morphologies spanning the nano- to mesoscopic scales of supramolecular assemblies derived from identical intermolecular interactions has been notoriously challenging due to dynamic structural change and monomer exchange of assemblies in solution. In this study, we demonstrate that curved one-dimensional molecular assemblies (supramolecular polymers) of lengths of around 70-200 nm, originating from the same luminescent molecule, exhibit distinct photoluminescent properties when they form closed circular structures (toroids) versus when they possess chain termini in solution (random coils). By exploiting the difference in kinetic stability between the toroids and random coils, we developed a dialysis protocol to selectively purify the former. It was revealed that these terminus-free closed structures manifest higher energy and more efficient luminescence compared with their mixed state with random coils. Time-resolved fluorescence measurements unveiled that random coils, due to their dynamic structural fluctuation in solution, generate local defects throughout the main chain, leading to luminescence from lower energy levels. In mixtures of the two assemblies, luminescence was exclusively observed from such a lower energy level of random coils, a result attributed to energy transfer between the assemblies. This work emphasizes that for identical supramolecular assemblies, only averaged properties have traditionally been considered, but their structures at the nano- to mesoscopic scale are important especially if they have a certain degree of shape persistency even in solution.

Purpose: Cancer patients with advanced-stage disease have poor prognosis, typically having limited options for efficacious treatment, and genomics-based therapy guidance continues to benefit only a fraction of patients. Next-generation ex vivo approaches, such as cell mass-based response testing (MRT), offer an alternative precision medicine approach for a broader population of patients with cancer, but validation of clinical feasibility and potential impact remain necessary., Materials and Methods: We evaluated the clinical feasibility and accuracy of using live-cell MRT to predict patient drug sensitivity. Using a unified measurement workflow with a 48-hour result turnaround time, samples were subjected to MRT after treatment with a panel of drugs in vitro. After completion of therapeutic course, clinical response data were correlated with MRT-based predictions of outcome. Specimens were collected from 104 patients with solid (n = 69) and hematologic (n = 35) malignancies, using tissue formats including needle biopsies, malignant fluids, bone marrow aspirates, and blood samples. Of the 81 (78%) specimens qualified for MRT, 41 (51%) patients receiving physician-selected therapies had treatments matched to MRT., Results: MRT demonstrated high concordance with clinical responses with an odds ratio (OR) of 14.80 ( P = .0003 [95% CI, 2.83 to 102.9]). This performance held for both solid and hematologic malignances with ORs of 20.67 ( P = .0128 [95% CI, 1.45 to 1,375.57]) and 8.20 ( P = .045 [95% CI, 0.77 to 133.56]), respectively. Overall, these results had a predictive accuracy of 80% ( P = .0026 [95% CI, 65 to 91])., Conclusion: MRT showed highly significant correlation with clinical response to therapy. Routine clinical use is technically feasible and broadly applicable to a wide range of samples and malignancy types, supporting the need for future validation studies.

TAIMAN (TAI), the only insect ortholog of mammalian Steroid Receptor Coactivators (SRCs), is a critical modulator of ecdysone and juvenile hormone (JH) signaling pathways, which govern insect development and reproduction. The modulatory effect is mediated by JH-dependent TAI's heterodimerization with JH receptor Methoprene-tolerant and association with the Ecdysone Receptor complex. Insect hormones regulate insect physiology and development in concert with abiotic cues, such as photo- and thermoperiod. Here we tested the effects of JH and ecdysone signaling on the circadian clock by a combination of microsurgical operations, application of hormones and hormone mimics, and gene knockdowns in the linden bug Pyrrhocoris apterus males. Silencing taiman by each of three non-overlapping double-strand RNA fragments dramatically slowed the free-running period (FRP) to 27-29 hours, contrasting to 24 hours in controls. To further corroborate TAIMAN's clock modulatory function in the insect circadian clock, we performed taiman knockdown in the cockroach Blattella germanica. Although Blattella and Pyrrhocoris lineages separated ~380 mya, B. germanica taiman silencing slowed the FRP by more than 2 hours, suggesting a conserved TAI clock function in (at least) some insect groups. Interestingly, the pace of the linden bug circadian clock was neither changed by blocking JH and ecdysone synthesis, by application of the hormones or their mimics nor by the knockdown of corresponding hormone receptors. Our results promote TAI as a new circadian clock modulator, a role described for the first time in insects. We speculate that TAI participation in the clock is congruent with the mammalian SRC-2 role in orchestrating metabolism and circadian rhythms, and that TAI/SRCs might be conserved components of the circadian clock in animals., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Smykal et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Photoluminescence blinking in individual semiconducting and perovskite quantum dots reflects reduced emission quantum yield and represents an obstacle towards quantum dot applications. One of the origins of blinking is the presence of surface structural defects that can function as charge traps. To reduce the defects the surface can be modified by, e.g. , covering with ligands that are more strongly bound to the surface. Here, we report exchange of ligands on the CsPbBr 3 perovskite nanocrystal surface and the effect of the exchange on photoluminescence blinking. Replacement of the oleic acid and oleylamine ligands which are used in the synthesis process with quaternary amine ligands leads to substantial increase of photoluminescence quantum yield. On single particle level this is reflected by significantly improved blinking characteristics. Statistical analysis using the probability density function shows that the ligand exchange leads to longer duration of ON-times and shorter OFF-times, as well as to the presence of a higher fraction of ON-time intervals. These characteristics are not affected by sample aging within three weeks. On the contrary, storage of the samples in solution for one-to-two weeks leads to further improvement of the ON-time interval fraction statistics.

We report the preparation and nanoscale photophysical characterization of mixed cation perovskite films of the composition MA 1- x FA x PbI 3 , with x = 0, 0.3 and 0.5. Films with x = 0.5 and 0.3 prepared in air using ethyl acetate as an antisolvent in a one-step spin-coating process are compositionally stable in ambient air for more than a year, in contrast to films prepared using a chlorobenzene antisolvent. The onset of degradation of the films near the film edges was monitored using in situ photoluminescence (PL) spectroscopy. The PL spectra of the degradation products are consistent with the PL spectra of 2D perovskite sheets of varying thicknesses. Morphologically, aging of the films brings about coalescing of the film grain structure into larger crystal grains. Furthermore, monitoring of the time traces of PL from individual nanoscale locations in the films (PL blinking) reveals that aging of the films does not change the extent of dynamic PL quenching or affect the observed long-range charge diffusion on the order of micrometers.

Conjugated polymer chains in compact conformations or in films exhibit spectral features that can be attributed to interactions between individual conjugated segments of the chain, including formation of aggregates or excimers. Here, we use atomic force microscopy (AFM) on single chains of the conjugated polymer polyfluorene (PFO) to control the intersegment interactions by mechanically unfolding the chain. Simultaneously with the force spectroscopy we monitor fluorescence from the single PFO chains using a fluorescence microscope. We found that mechanical stretching of the chain causes disappearance of the green emission band. This observation provides evidence that the green emission originates from an intrachain aggregated state on the self-folded chain, which is decoupled by the stretching. In addition, the stretching upon laser irradiation leads to the appearance of additional features in the force spectra, small force peaks in the initial stages of the unfolding. These features are attributed to a combination of excitonic and van der Waals coupling of a ground-state intrachain aggregate.

Functional precision medicine offers a promising complement to genomics-based cancer therapy guidance by testing drug efficacy directly on a patient's tumor cells. Here, we describe a workflow that utilizes single-cell mass measurements with inline brightfield imaging and machine-learning based image classification to broaden the clinical utility of such functional testing for cancer. Using these image-curated mass measurements, we characterize mass response signals for 60 different drugs with various mechanisms of action across twelve different cell types, demonstrating an improved ability to detect response for several slow acting drugs as compared with standard cell viability assays. Furthermore, we use this workflow to assess drug responses for various primary tumor specimen formats including blood, bone marrow, fine needle aspirates (FNA), and malignant fluids, all with reports generated within two days and with results consistent with patient clinical responses. The combination of high-resolution measurement, broad drug and malignancy applicability, and rapid return of results offered by this workflow suggests that it is well-suited to performing clinically relevant functional assessment of cancer drug response., (© 2022. The Author(s).)

Polyfluorene-based copolymers such as poly(9,9-dioctylfluorene)-alt-5% [bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine] (F8-5% BSP) are efficient blue-emitting polymers with various electronic phases: F8 blue-emitting glassy phase, F8 ordered more red-emitting β-phase, and F8/BSP charge transfer (CT) state. Polymer light-emitting device performance and color purity can be significantly improved by forming β-phase segments. However, the role of the β-phase on energy transfer (ET) among glassy F8, β-phase, and F8/BSP CT state is unclear. Herein, we identify dynamic molecular conformation-controlled ET from locally excited states to either the CT state or β-phase in light-emitting copolymers. By conducting single-molecule spectroscopy for single F8-5% BSP chains, we find inefficient intra-chain ET from glassy segments to the CT state, while efficient ET from the glassy to the β-phase. Spontaneous and reversible CT on-off emission is observed both in the presence and absence of the β-phase. The density functional theory calculations reveal the origin of the on-chain CT state and indicate this CT emission on-off switching behavior could be related to molecule torsional motion between BSP and F8 units. The population of the CT state by ET can be increased via through-space interaction between the F8 block and the BSP unit on a self-folded chain. Temperature-dependent single-molecule spectroscopy confirms such interaction showing a gradual increase in intensity of the CT emission with the temperature. Based on these observations, we propose the dynamic molecular motion-induced conformation change as the origin of the glassy-to-CT ET, and thermal energy may provide the activation for such a change to enhance the ET from glassy or β-phases to the CT state.

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 Å 3 is available close to the molecule, and that such free volume lasts for an average of 1.2 s.

A combination of conductive atomic force microscopy (AFM) and confocal fluorescence microscopy was used to measure photocurrents passing through single trimeric photosytem I (PSI) complexes located in the vicinity of single gold nanorods (AuNRs). Simultaneous excitation of PSI and of the AuNR longitudinal plasmon mode and detection of photocurrents from individual PSI in relation to the position of single AuNRs enable insight into plasmon-induced phenomena that are otherwise inaccessible in ensemble experiments. We have observed photocurrent enhancement by the localized plasmons by a factor of 2.9 on average, with maximum enhancement values of up to 8. Selective excitation of the longitudinal plasmon modes by the polarization of the excitation laser enables controllable switch-on of the photocurrent enhancement. The dependence of the extent of enhancement on the distance between PSI and AuNRs indicates that, apart from the enhancement of absorption, there is an additional enhancement mechanism affecting directly the electron transport process. The present study provides deeper insight into the molecular mechanisms of plasmon-enhanced photocurrents, not only in PSI but also potentially in other systems as well.

Poly(9,9-dioctylfluorene) (PFO) is one of the most important conjugated polymer materials, exhibiting outstanding photophysical and electrical properties. PFO is also known for a diversity of morphological phases determined by conformational states of the main chain. Our goal in this work is to address some of the key questions on formation and dynamics of one such conformation, the β-phase, by following in real time the evolution of fluorescence spectra of single PFO chains. The PFO is dispersed in a thin polystyrene film, and the spectra are monitored during the process of solvent vapor annealing with toluene. We confirm unambiguously that the PFO β-phase segments are formed on a true single-chain level at room temperature in the solvent-softened polystyrene. We further find that the formation of the β-phase is a dynamic and reversible process occurring on the order of seconds, leading to repeated spontaneous transitions between the glassy and β-phase segments during the annealing. Comparison of PFO with two largely different molecular weights ( M w ) shows that chains with lower M w form the β-phase segments much faster. For the high M w PFO chains, a detailed Franck-Condon analysis of the β-phase spectra shows a large distribution of the Huang-Rhys factor, S , and even dynamic changes of this factor occurring on a single chain. Such dynamics are likely a manifestation of changing coherence length of the exciton. Further, for the high M w PFO chains we observe an additional conformational state, a crystalline γ-phase. The γ-phase formation is also a spontaneous reversible process in the solvent-softened matrix. The phase can form from both the β-phase and the glassy phase, and the formation requires high M w to enable intersegment interactions in a self-folded chain.

The increased production of semiconductor nanomaterials such as heavy metal quantum dots and perovskites for applications such as in energy harvesting, optoelectronic devices, bioanalysis, phototherapy and consumer health products raises concerns regarding nanotoxicity. After disposal, these materials degrade upon interaction with the environment, such as rain and surface waters, soil and oxygen, and solar irradiation, leading to the release of heavy metal ions in the environment with exposure to aquatic and terrestrial animals and plants, and humans. Researchers are in the early stages of understanding the potential toxicity of such nanomaterials by quantifying the amount of heavy metal ions released due to environmental or biological transformation. Here, we evaluate the toxicity of environmentally transformed nanomaterials by considering PbS quantum dots as a model system. Using metal ion sensors and steady-state fluorescence spectroscopy, we quantify the amount of Pb2+ released by the photochemical etching of quantum dots. Furthermore, with the help of cytotoxicity and comet assays, and DNA gel electrophoresis, we evaluate the adverse effects of the released metal ions into the cultured lung epithelial (H1650), and neuronal (PC12) cells. These studies reveal higher levels of cell proliferation and DNA damage to PC12 cells, suggesting the neurotoxicity of lead due to not only the downregulation of glutathione, elevated levels of reactive oxygen and nitrogen species, and a calcium influx but also the proactivation of activator protein 1 that is correlated with protein kinase c. This research shows the significance of molecular biology studies on different cells and animals to critically understand the health and environmental costs of heavy metal-based engineered nanomaterials.

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.

Trivalent lanthanide complexes are an important class of luminescent material characterized by their strong absorption of light by the organic ligands and subsequent energy transfer to the lanthanide ion, realizing intense luminescence from the ion. With this mechanism of luminescence, the total quantum yield of a lanthanide complex is the product of the energy transfer efficiency from the ligand to the lanthanide ion and the "intrinsic" quantum yield of the lanthanide ion itself. The "absolute" method in measuring the quantum yield uses an integrating sphere, and this method can be used for measuring both the total and the intrinsic quantum yields. The presence of back energy transfer (the reverse process of energy transfer) adds complication to this by affecting both the dynamics of the excited state of the ligands and the lanthanide ion. Herein, we theoretically derive an equation that shows that in the presence of back energy transfer the intrinsic quantum yield may differ depending on whether it is determined from the measurement through excitation of the ligands or the lanthanide directly. The value measured by direct lanthanide excitation could decrease to 20% or less of the actual value when back energy transfer is prominent. Several previously reported Tb(iii) complexes are within the range to be cautious. This report shows that the "absolute" method for measuring the lanthanide ion-centered quantum yield may not be suitable in the presence of back energy transfer by principle. We also provide a possible workaround in the case that several approximations and assumptions can be made.

Halide perovskites nanocrystals (NCs) are being explored as promising materials for optoelectronic applications, such as light-emitting devices or lasers. However, electroluminescence devices prepared from such NCs have long suffered from low efficiency and there has been no systematic study on the nanoscale origin of the poor efficiencies. Here, we use single-particle spectroscopy to compare electroluminescence and photoluminescence on the level of individual NCs of the perovskite CsPbBr 3 . The NCs form aggregates in a conducting matrix used as an emission layer in an electroluminescence device. In electroluminescence, only a small fraction of the NCs within the aggregate is emitting as a result of efficient charge migration, accumulation and selective recombination on larger NCs, leading to pronounced blinking and decreased efficiency. Under the condition of comparable excitation rates in both electroluminescence and photoluminescence, the intrinsic quantum yield in electroluminescence is on average 0.36 of that in photoluminescence.

It is known that the circadian clock in Drosophila can be sensitive to static magnetic fields (MFs). Man-made radiofrequency (RF) electromagnetic fields have been shown to have effects on animal orientation responses at remarkably weak intensities in the nanotesla range. Here, we tested if weak broadband RF fields also affect the circadian rhythm of the German cockroach ( Blatella germanica ). We observed that static MFs slow down the cockroach clock rhythm under dim UV light, consistent with results on the Drosophila circadian clock. Remarkably, 300 times weaker RF fields likewise slowed down the cockroach clock in a near-zero static magnetic field. This demonstrates that the internal clock of organisms can be sensitive to weak RF fields, consequently opening the possibility of an influence of man-made RF fields on many clock-dependent events in living systems.

Nanocrystals of formamidinium lead bromide perovskite (FAPbBr3) self-assemble into clusters in powder and film samples and provide a prolonged photoluminescence lifetime, which is attributed to the diffusion of charge carriers through interparticle states formed among nanocrystals. Interestingly, the photoluminescence lifetime decreases and the emission intensity increases for the clusters, which is with the increase in the intensity of excitation light. By doping the nanocrystal clusters with C60, we successfully harvested the photogenerated charge carriers. Nonetheless, at high intensities of excitation, the rate of radiative recombination becomes comparable to that of the electron transfer to C60. Thus, the optimum rate of electron transfer to C60 is accomplished by minimally exciting the self-assembled nanocrystals.

Persistent room-temperature phosphorescence (RTP) under ambient conditions is attracting attention due to its strong potential for applications in bioimaging, sensing, or optical recording. Molecular packing leading to a rigid crystalline structure that minimizes nonradiative pathways from triplet state is often investigated for efficient RTP. However, for complex conjugated systems a key strategy to suppress the nonradiative deactivation is not found yet. Here, the origin of small rates of a nonradiative decay process from triplet states of conjugated molecular crystals showing RTP is reported. Optical microscopy analysis showed that, despite a favorable molecular stacking, an aromatic crystal with strong RTP is characterized by small diffusion length and small values of the diffusion coefficient of triplet excitons. Quantum chemical calculations reveal a large overlap between the lowest unoccupied molecular orbitals but very small overlap between the highest occupied molecular orbitals (HOMOs). Inefficient electron exchange caused by the small overlap of HOMOs prevents triplet excitons from diffusing over long distances and consequently from quenching at defect sites inside the crystal or at the crystal surface. These results will allow design of comprehensive molecular structures to obtain molecular solids with more efficient RTP., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Organic-inorganic halide perovskites have emerged as promising materials for next-generation solar cells. In nanostructured form also, these materials are excellent candidates for optoelectronic applications such as lasers and light-emitting diodes for displays and lighting. While great progress has been achieved so far in optimizing the intrinsic photophysical properties of perovskite nanocrystals (NCs), in working optoelectronic devices, external factors, such as the effects of conducting environment and the applied electric field on exciton generation and photon emission, have been largely unexplored. Here, we use NCs of the all-inorganic perovskite CsPbBr 3 dispersed polyvinyl carbazole, a hole-conductor, and in poly(methyl methacrylate), an insulator, to examine the effects of applied electric field and conductivity of the matrix on the perovskite photophysics at the single-particle level. We found that the conducting environment causes a significant decrease of photoluminescence (PL) brightness of individual NCs due the appearance of intermediate-intensity emitting states with significantly shortened lifetime. Applied electric field has a similar effect and, in addition, causes a nonlinear spectral shift of the PL maxima, a combination of linear and quadratic Stark effects caused by environment-induced polarity and field-related polarizability. The environment and electric-field effects are explained by ionization of the NCs through hole transfer and emission of the resulting negatively charged excitons.

The nonstoichiometry of I-III-VI semiconductor nanoparticles, especially the ratio of group I to group III elements, has been utilized to control their physicochemical properties. We report the solution-phase synthesis of nonstoichiometric Ag-In-S and Ag-In-Ga-S nanoparticles and results of the investigation of their photoluminescence (PL) properties in relation to their chemical compositions. While stoichiometric AgInS 2 nanoparticles simply exhibited only a broad PL band originating from defect sites in the particles, a narrow band edge PL peak newly appeared with a decrease in the Ag fraction in the nonstoichiometric Ag-In-S nanoparticles. The relative PL intensity of this band edge emission with respect to the defect-site emission was optimal at a Ag/(Ag + In) value of ca. 0.4. The peak wavelength of the band edge emission was tunable from 610 to 500 nm by increased doping with Ga 3+ into Ag-In-S nanoparticles due to an increase of the energy gap. Furthermore, surface coating of Ga 3+ -doped Ag-In-S nanoparticles, that is, Ag-In-Ga-S nanoparticles, with a GaS x shell drastically and selectively suppressed the broad defect-site PL peak and, at the same time, led to an increase in the PL quantum yield (QY) of the band edge emission peak. The optimal PL QY was 28% for Ag-In-Ga-S@GaS x core-shell particles, with green band-edge emission at 530 nm and a full width at half-maximum of 181 meV (41 nm). The observed wavelength tunability of the band-edge PL peak will facilitate possible use of these toxic-element-free I-III-VI-based nanoparticles in a wide area of applications.

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.

Diverse animal species perceive Earth's magnetism and use their magnetic sense to orientate and navigate. Even non-migrating insects such as fruit flies and cockroaches have been shown to exploit the flavoprotein Cryptochrome (Cry) as a likely magnetic direction sensor; however, the transduction mechanism remains unknown. In order to work as a system to steer insect flight or control locomotion, the magnetic sense must transmit the signal from the receptor cells to the brain at a similar speed to other sensory systems, presumably within hundreds of milliseconds or less. So far, no electrophysiological or behavioral study has tackled the problem of the transduction delay in case of Cry-mediated magnetoreception specifically. Here, using a novel aversive conditioning assay on an American cockroach, we show that magnetic transduction is executed within a sub-second time span. A series of inter-stimulus intervals between conditioned stimuli (magnetic North rotation) and unconditioned aversive stimuli (hot air flow) provides original evidence that Cry-mediated magnetic transduction is sufficiently rapid to mediate insect orientation.

The optical properties of metal-film-coupled nanoparticles (NPs) are highly sensitive to physical and optical interactions between the NPs and the spacer medium in the gap between the NP and metal film. Here, we investigate the physical and optical interactions between gold NPs (AuNPs) and semiconducting conjugated polymer thin-film spacers in a "sphere-on-plane" type metal-film-coupled NP system, and their influence on the plasmonic scattering of individual AuNPs. We choose two different conjugated polymers: one with an absorption spectrum that is resonant with the plasmonic modes of the AuNPs and another that is non-resonant. By correlating dark-field back-scattering optical images with topographic atomic force microscope images, we find that partial embedding of the AuNPs occurs in both conjugated polymers to different extents. This can lead to partial quenching of certain plasmonic scattering modes, which results in a change of the back-scattering colors from the AuNPs. Pronounced, red-shifted scattering is observed due to deep embedding of the AuNPs, particularly for thicker conjugated polymer spacers that have resonant absorption with the plasmonic modes of the AuNPs. Polarization-controlled defocused dark-field imaging is employed to visualize the emergence of horizontally-polarized scattering modes upon embedding of AuNPs into the conjugated polymer spacer. These results demonstrate the importance of nanoparticle-spacer physical interactions to the control of the color and polarization of coupled plasmonic modes in nanoparticle-film systems relevant.

Ring expansion metathesis polymerisation (REMP) has proven to be a viable approach to prepare high purity cyclic polymers. Macrocyclic polymers with a fully conjugated defect free backbone are of particular interest as these polymers have no end groups that can act as charge traps. In this work soluble macrocyclic poly( p -phenylenevinylene)s ( c PPV s) have been prepared directly via the REMP of substituted paracyclophanedienes. Single-molecule spectroscopy of the two topological forms of PPV i.e. , linear ( l PPV ) and cyclic ( c PPV ) revealed that l PPV exists in an extended conformation whereas the c PPV adopts a restricted ring-like conformation. Despite such large differences in the chain conformation, the spectral properties of the two compounds are unexpectedly very similar, and are dominated by torsional deformations in relatively short conjugated segments.

Up-conversion materials composed of donor and acceptor molecules which convert low energy photons into higher energy ones by triplet-triplet annihilation can improve the sensitivity of photocatalysts or the efficiency of solar cells. The use of crystalline materials can lead to a decrease in the up-conversion threshold intensity due to increased diffusion length L T of triplet excitons. Here, we demonstrate direct microscopic imaging of triplet exciton diffusion in polycrystalline films. The generation of high local density of triplet states is achieved by functionalizing alumina nanospheres with donor molecules and dispersing them in the acceptor films. Diffusion of the triplet excitons and up-converted emission are reflected in enlarged microscopic images of the nanoparticles. Analysis provides the average L T values of 491 nm for the acceptor of polycrystalline anthracene and 172 nm for polycrystalline 9,10-diphenylanthracene, and reveals large distributions of the diffusion characteristics. These average values are more than an order of magnitude larger than L T obtained by a conventional method, and highlight the need for accurate nanoscale characterization of the up-conversion materials.

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.

Background: Donor-specific antibodies (DSAs) after lung transplantation correlate with poor outcomes. The ideal treatment strategy for antibody-mediated rejection AMR is not defined. Our institution implemented an aggressive multimodality protocol for the treatment of suspected AMR., Methods: Lung transplant recipients with suspected AMR were treated with a standardized protocol of plasma exchange, steroids, bortezomib, rituximab, and intravenous immune globulin. Primary outcome was DSA clearance at 6 months in those alive. Secondary endpoints included preserved allograft function at 6 months, survival at 6 and 12 months and complications due to the protocol., Results: Sixteen lung transplant recipients with documented DSA and allograft dysfunction were included in the analysis. Of the 16 patients, 11 survived to 6 months. Three of those 11 patients (27%) cleared all DSAs within 6 months of the protocol. Four of the 11 patients (36%) had preserved allograft function at 6 months. Overall 12-month patient survival was 56%. Complications included thrombocytopenia (50%) and abdominal pain or gastrointestinal discomfort (18.7%)., Conclusions: This multimodality protocol resulted in clearance of DSAs and preserved lung function in a minority of lung transplant recipients with suspected AMR surviving to 6 months after therapy. There were significant side effects of the protocol., (© 2017 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

We study the effect of Zn on the photophysical properties of a family of group I-III-VI nanocrystals (NCs), namely in solid solutions of (AgIn) x Zn 2(1-x) S 2 (ZAIS). We focus on the comparison of the photoluminescence (PL) properties of ZAIS NCs of comparable sizes and different amounts of Zn. This approach helps us to decouple the effects of size and varying chemical composition of the NCs which both influence the PL properties. We show that in the presence of Zn new radiative centers are generated which improve the NC quality in terms of PL quantum yield. However, an amount of Zn beyond a particular limit places the radiative recombination centers close to each other, leading to undesired interactions among charge carriers and non-radiative transitions. Proximity between the energy levels of these radiative centers and the conduction band leads to non-radiative localized-delocalized transitions, as evidenced from temperature dependent absorption, PL and lifetime measurements.

Förster resonant energy transfer (FRET) is a nonradiative process by which the energy of light absorbed by a donor molecule is transferred to an acceptor molecule over a distance of several nanometers. FRET plays a crucial role in photosynthesis and nature-inspired artificial light-harvesting systems that are being explored for use in energy conversion applications. Localized plasmons of metal nanoparticles can potentially lead to a significant increase of FRET efficiency and effective donor-acceptor distance. Here, we prepare hybrid nanostructures composed of a gold nanorod and donor and acceptor molecules covalently attached to its surface, and study them on the level of a single nanoparticle by simultaneous dark-field scattering, fluorescence imaging and spectroscopy. The single-particle approach enables selective excitation of the longitudinal plasmon of the gold nanorod by polarization of the excitation light. The emission intensity of the acceptor molecules can be controllably and reversibly modulated over a wide range by the polarization angle, thus enabling a selective turn-on of the FRET process and control over the emission color of the hybrid nanostructure. Numerical simulations show that the interactions of the donor and acceptor molecules with the plasmon lead to an increase of the energy transfer efficiency by a factor of ∼65. These findings represent the concept of a novel colour switching approach and could pave the way for innovative applications in optoelectronics and nanophotonics.

Objectives: The optimal dose of rabbit antithymocyte globulin induction therapy in kidney transplant recipients with high immunologic risk lacks consensus. The purpose of this study was to evaluate the effect of using ideal body weight rather than total body weight for the weight-based dose calculations in this patient population., Materials and Methods: Data were retrospectively collected on 89 adult patients who received rabbit antithymocyte globulin induction therapy for high immunologic risk kidney transplant. Hospital protocol changed from the use of cumulative rabbit antithymocyte globulin doses of 7.5 mg/kg total body weight to 7.5 mg/kg ideal body weight in 2009. Patients were separated into 2 cohorts based on the amount of rabbit antithymocyte globulin (in mg/kg total body weight) received. Rate of biopsy-proven acute rejection, patient survival, and allograft function were evaluated at 90 days and 1 year after transplant. Cost of induction therapy was also evaluated., Results: Baseline demographics were predominantly similar between the 2 cohorts. No significant difference in maintenance immunosuppression was identified. Rates of biopsy-proven acute rejection at 90 days and 1 year were similar between ideal and total body weight cohorts (4.2% vs 0% at 90 days, P = .5; 8.7% vs 0% at 1 year, P = .13). Patient survival and allograft function were also similar. Median cost of rabbit antithymocyte globulin induction therapy per patient was lower in the ideal body weight cohort, but this difference was not statistically significant ($17 542 vs $19 934; P = .3)., Conclusions: Our results suggest that use of ideal body weight for dose calculations of rabbit antithymocyte globulin induction therapy in high immunologic risk kidney transplant recipients at 7.5 mg/kg results in low rates of acute rejection with a safety profile similar to that shown with a total body weight dosage. Use of ideal body weight for lower cumulative doses may still need further evaluation in this patient population.

Ternary I-III-VI semiconductor nanocrystals have been explored as non-toxic alternatives to II-VI semiconductors for optoelectronic and sensing applications, but large photoluminescence spectral width and moderate brightness restrict their practical use. Here, using single-particle photoluminescence spectroscopy on nanocrystals of (AgIn)xZn2(1-x)S2 we show that the photoluminescence band is inhomogeneously broadened and that size distribution is the dominant factor in the broadening. The residual homogeneous linewidth of individual nanocrystals reaches up to 75% of the ensemble spectral width. Single nanocrystals undergo spectral diffusion which also contributes to the inhomogeneous band. Excitation with two lasers with energies above and below the bandgap reveals coexistence of two emitting donor states within one particle. Spectral diffusion in such particles is due to temporal activation and deactivation of one such state. Filling of a trap state with a lower-energy laser enables optical modulation of photoluminescence intermittency (blinking) and leads to an almost two-fold increase in brightness.

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.

The ability to perceive geomagnetic fields (GMFs) represents a fascinating biological phenomenon. Studies on transgenic flies have provided evidence that photosensitive Cryptochromes (Cry) are involved in the response to magnetic fields (MFs). However, none of the studies tackled the problem of whether the Cry-dependent magnetosensitivity is coupled to the sole MF presence or to the direction of MF vector. In this study, we used gene silencing and a directional MF to show that mammalian-like Cry2 is necessary for a genuine directional response to periodic rotations of the GMF vector in two insect species. Longer wavelengths of light required higher photon fluxes for a detectable behavioral response, and a sharp detection border was present in the cyan/green spectral region. Both observations are consistent with involvement of the FADox, FAD(•-) and FADH(-) redox forms of flavin. The response was lost upon covering the eyes, demonstrating that the signal is perceived in the eye region. Immunohistochemical staining detected Cry2 in the hemispherical layer of laminal glia cells underneath the retina. Together, these findings identified the eye-localized Cry2 as an indispensable component and a likely photoreceptor of the directional GMF response. Our study is thus a clear step forward in deciphering the in vivo effects of GMF and supports the interaction of underlying mechanism with the visual system.

The systematic synthesis of five 1-, 3-, 6-, and 8-tetrasubstituted asymmetric pyrenes with electron donor and acceptor moieties is presented, together with an examination of their photophysical properties. Pyrene derivative PA1, containing one formyl and three piperidyl groups, showed bright solvatochromic fluorescence from green (λem = 557 nm, ΦFL = 0.94 in hexane) to red (λem = 648 nm, ΦFL = 0.50 in methanol), suggesting potential applications for PA1 as an environmentally responsive probe. Although the synthesis of simple 1- and 3-disubstituted pyrene derivatives is considered difficult, PA13, with two formyl groups at the 1- and 3-positions and two piperidyl groups at the 6- and 8-positions, could be synthesized successfully. PA13 exhibited less pronounced solvatochromism, but displayed a narrow fluorescent band with high ΦFL in all solvents (ΦFL > 0.75). Moreover, its absorption band displayed an exceptional bathochromic shift compared to the other derivatives (e.g., λabs = 480 and 522 nm in ethanol for PA1 and PA13, respectively), suggesting that such modifications of pyrene may be quite important for the modulation of its energy gap. Additionally, all compounds exhibited exceptionally high photostability, which highlights the advantage of these new dyes and provides new insights on the design of photostable fluorophores.

A metal-free aromatic compound with a chiral carbon sandwiched between a donor moiety and an acceptor moiety was designed. Under thermally activated delayed fluorescence, the compound displayed a photoluminescence quantum yield of 26%, and showed circularly polarized luminescence with a dissymmetry factor of 10(-3).

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.

Purpose: The aim of this literary search was to chart the etymology of 32 selected human skeletal muscles, representative of all body regions., Methods: In researching this study, analysis of 15 influential Latin and German anatomical textbooks, dating from the sixteenth to the nineteenth century, was undertaken, as well as reference to four versions of the official Latin anatomical terminologies. Particular emphasis has been placed on the historical development of muscular nomenclature, and the subsequent division of these data into groups, defined by similarities in the evolution of their names into the modern form., Results: The first group represents examples of muscles whose names have not changed since their introduction by Vesalius (1543). The second group comprises muscles which earned their definitive names during the seventeenth and eighteenth century. The third group is defined by acceptance into common anatomical vernacular by the late nineteenth century, including those outlined in the first official Latin terminology (B.N.A.) of 1895. The final group is reserved for six extra-ocular muscles with a particularly poetic history, favoured and popularised by the anatomical giants of late Renaissance and 1,700 s., Conclusions: As this study will demonstrate, it is evident that up until introduction of the B.N.A. there was an extremely liberal approach to naming muscles, deserving great respect in the retrospective terminological studies if complete and relevant results are to be achieved. Without this knowledge of the vernacular of the ages past, modern researchers can find themselves 'reinventing the wheel' in looking for their answers.

In materials showing reverse saturable absorption (RSA), the optical absorbance increases as the power of the light incident on them increases. To date, RSA has only been observed when very intense light sources, such as short-pulse lasers, are used. Here, we show that hydroxyl steroidal matrices embedding properly designed aromatic molecules as acceptors and transition-metal complexes as donors exhibit high RSA on exposure to weak incoherent light at room temperature and in air. Accumulation by photosensitization of long-lived room-temperature triplet excitons in acceptors with a large triplet-triplet absorption coefficient allows a nonlinear increase in absorbance also under low-power irradiation conditions. As a consequence, continuous exposure to weak light significantly decreases the transmittance of thin films fabricated with these compounds. These optical limiting properties may be used to protect eyes and light sensors from exposure to intense radiation generated by incoherent sources and for other light-absorption applications that have not been realized with conventional RSA materials.

We report a microscopic study of circularly polarized luminescence (CPL) from R and S isomers of aggregates of the conjugated polymer F8BT, a copolymer containing fluorene and thiophene monomer units. Chirality in the aggregates is induced by addition of either (R)- or (S)-limonene during aggregation in solution. Solid-state samples are prepared either by drop-casting or spin-coating of the aggregate solution. CPL dissymmetry parameter g measured for individual aggregate microstructures shows a broad distribution of values ranging from positive to negative even for the same isomer. The difference in the centers of the g distribution between the R and S isomers reflects the intrinsic CPL from the chiral aggregates, and is found to depend on the size of the aggregate structures. On the other hand, the large widths of the distributions are ascribed to an optical effect arising from phase retardation along the optical path of the CPL inside the sample.

Optoelectronic properties of polyfluorene, a blue light-emitting organic semiconductor, are often degraded by the presence of green emission that originates mainly from oxidation of the polymer. Here, we use single-molecule electroluminescence (EL) and photoluminescence (PL) spectroscopy on polyfluorene chains confined in vertical cylinders of a phase-separated block copolymer to spectrally resolve the green band and investigate in detail the photophysical processes responsible for its appearance. In both EL and PL, a substantial fraction of polyfluorene chains shows spectrally stable green emission which is ascribed to a keto defect. In addition, in EL, we observe a new type of vibrationally resolved spectra distributed over a wide range of frequencies and showing strong spectral dynamics. Based on quantum chemical calculations, this type is proposed to originate from charge-assisted formation and stabilization of ground-state aggregates. The results are expected to have broad implications in the fields of photophysics and material design of polyfluorene materials.

We prepare artificial aggregates that mimic the structure and function of natural chlorosomal light harvesting complexes of green photosynthetic bacteria. Gold nanorods functionalized with hydroxyl groups and immobilized on a substrate serve as cores for the growth of bacteriochlorophyll (BChl) aggregates from a buffer solution. The BChl pigments form large self-assembled aggregate particles with sizes more than twice that of natural chlorosomes. The size is controllable by the aggregation time. The aggregates are characterized on a single-particle level by atomic force microscopy, electron microscopy, and single-molecule spectroscopy. The absorption and fluorescence spectral properties which reflect the molecular level arrangement of the BChl aggregates closely resemble those of the natural chlorosomes of the photosynthetic bacterium Chlorobaculum tepidum. On the other hand, the results of linear dichroism and circular dichroism are different from those of the chlorosomes and indicate a different mesoscopic structure for the artificial aggregates. These results emphasize the structural role played by the baseplate pigment-protein complex in natural chlorosomes.

We report a new methodology for studying diffusion of individual polymer chains in a melt state, with special emphasis on the effect of chain topology. A perylene diimide fluorophore was incorporated into the linear and cyclic poly(THF)s, and real-time diffusion behavior of individual chains in a melt of linear poly(THF) was measured by means of a single-molecule fluorescence imaging technique. The combination of mean squared displacement (MSD) and cumulative distribution function (CDF) analysis demonstrated the broad distribution of diffusion coefficient of both the linear and cyclic polymer chains in the melt state. This indicates the presence of spatiotemporal heterogeneity of the polymer diffusion which occurs at much larger time and length scales than those expected from the current polymer physics theory. We further demonstrated that the cyclic chains showed marginally slower diffusion in comparison with the linear counterparts, to suggest the effective suppression of the translocation through the threading-entanglement with the linear matrix chains. This coincides with the higher activation energy for the diffusion of the cyclic chains than of the linear chains. These results suggest that the single-molecule imaging technique provides a powerful tool to analyze complicated polymer dynamics and contributes to the molecular level understanding of the chain interaction.

Hybrid materials composed of phase-separated block copolymer films and conjugated polymers of the phenylenevinylene family (PPV) are prepared. The PPV chains are embedded in vertical cylinders of nanometer diameter in the block-copolymer films. The cylinders span continuously the whole film thickness of 70 nm. Incorporation of the PPV chains into the one-dimensional cylinders leads to modified photoluminescence spectra and to large absorption anisotropy. The hybrid films show electroluminescence from the PPV chains in a simple light-emitting device at minute doping concentrations, and also exhibit a factor of 19 increase in electron transport efficiency along the single PPV chains., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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.

Perylenediimide (PDI) molecules are promising building blocks for photophysical studies of electronic interactions within multichromophore arrays. Such PDI arrays are important materials for fabrication of molecular nanodevices such as organic light-emitting diodes, organic semiconductors, and biosensors because of their high photostability, chemical and physical inertness, electron affinity, and high tinctorial strength over the entire visible spectrum. In this work, PDIs have been organized into linear (L3) and trefoil (T3) trimer molecules and investigated by single-molecule fluorescence microscopy to probe the relationship between molecular structures and interchromophoric electronic interactions. We found a broad distribution of coupling strengths in both L3 and T3 and hence strong/weak coupling between PDI units by monitoring spectral peak shifts in single-molecule fluorescence spectra upon sequential photobleaching of each constituent chromophore. In addition, we used a wide-field defocused imaging technique to resolve heterogeneities in molecular structures of L3 and T3 embedded in a PMMA polymer matrix. A systematic comparison between the two sets of experimental results allowed us to infer the correlation between intermolecular interactions and molecular structures. Our results show control of the PDI intermolecular interactions using suitable multichromophoric structures.

Single-molecule study of phenylenevinylene oligomers revealed distinct spectral forms due to different conjugation lengths which are determined by torsional defects. Large spectral jumps between different spectral forms were ascribed to torsional flips of a single phenylene ring. These spectral changes reflect the dynamic nature of electron delocalization in oligophenylenevinylenes and enable estimation of the phenylene torsional barriers.

We have studied relaxation processes in thin supported films of poly(methyl acrylate) at the temperature corresponding to 13 K above the glass transition by monitoring the reorientation of single perylenediimide molecules doped into the films. The axial position of the dye molecules across the thickness of the film was determined with a resolution of 12 nm by analyzing astigmatic fluorescence images. The average relaxation times of the rotating molecules do not depend on the overall thickness of the film between 20 and 110 nm. The relaxation times also do not show any dependence on the axial position within the films for the film thickness between 70 and 110 nm. In addition to the rotating molecules we observed a fraction of spatially diffusing molecules and completely immobile molecules. These molecules indicate the presence of thin (<5 nm) high-mobility surface layer and low-mobility layer at the interface with the substrate.