Aggregation-Induced Fluorescence Upconversion of Pyrene under Low Fluence: In Solutions and Polymeric Nanoparticles.

In this study, aggregation-induced photon upconversion (iPUC) is demonstrated in the small polyaromatic molecule, pyrene. In binary-solvent mixtures, water, which induces the aggregation of polyaromatic molecules, assisted in triplet-triplet annihilation-based upconversion. No upconverted emission was observed in a dry solvent. Although upconverted emission in the absence of a triplet sensitizer was assigned to pyrene-aggregate-induced sensitization, the presence of a triplet sensitizer enhanced the upconversion efficiency. This experimental finding was further simulated to explore the possibility of iPUC in the condensed-phase polymer matrix. We studied 2-aminoethyl methacrylate hydrochloride-polystyrene copolymer nanoparticles embedded with the molecular upconversion system. The nanoparticle iPUC agreed with the proposition that water domains were present in polymer nanoparticles and helped aggregate pyrene in the host polymer. Despite the low systemic upconversion efficiency, this study provides a method for achieving fluorescence upconversion in relatively simple systems.

Active Far-Field Control of the Thermal Near-Field via Plasmon Hybridization.

The ability to control and manipulate temperature at nanoscale dimensions has the potential to impact applications including heat-assisted magnetic recording, photothermal therapies, and temperature-driven reactivity. One challenge with controlling temperature at nanometer dimensions is the need to mitigate heat diffusion, such that the temperature only changes in well-defined nanoscopic regions of the sample. Here we demonstrate the ability to use far-field laser excitation to actively shape the thermal near-field in individual gold nanorod heterodimers by resonantly pumping either the in-phase or out-of-phase hybridized dipole plasmon modes. Using single-particle photothermal heterodyne imaging, we demonstrate localization bias in the photothermal intensity due to preferential heating of one of the nanorods within the pair. Theoretical modeling and numerical simulation make explicit how the resulting photothermal images encode wavelength-dependent temperature biases between each nanorod within a heterodimer, demonstrating the ability to actively manage the thermal near-field by simply tuning the color of incident light.

Using Fluorescence Spectroscopy To Identify Milk from Grass-Fed Dairy Cows and To Monitor Its Photodegradation.

Owing to its high ω-3 fatty acid content, milk from grass-fed dairy cows is becoming increasingly more attractive to consumers. Consequently, it is important to identify the origins of such products and to measure their content, at least relative to some standard. To date, chromatography has been the most extensively used technique. Sample preparation and cost, however, often reduce its widespread applicability. Here, we report the effectiveness of fluorescence spectroscopy for such quantification by measuring the amount of chlorophyll metabolites in the sample. Their content is significantly higher for milk from grass-fed cows compared to milk from grain/silage-fed cows. It is 0.11-0.13 µM in milk samples from grass-fed cows, whereas in milk from cows fed grain/silage rations, the concentration was 0.01-0.04 µM. In various organic milk samples, the chlorophyll metabolite concentration was in the range of 0.07-0.09 µM. In addition, we explored the mechanisms of photodegradation of milk. Riboflavin and chlorophyll metabolites act as photosensitizers in milk for type-I and type-II reactions, respectively. It was also observed that the presence of high levels of chlorophyll metabolites can synergistically degrade riboflavin, contributing to the degradation of milk quality.

Photoinduced Trans-to-cis Phase Transition of Polycrystalline Azobenzene at Low Irradiance Occurs in the Solid State.

The ability to produce large-scale, reversible structural changes in a variety of materials by photoexcitation of a wide variety of azobenzene derivatives has been recognized for almost two decades. Because photoexcitation of trans-azobenzene produces the cis-isomer in solution, it has generally been inferred that the macroscopic structural changes occurring in materials are also initiated by a similar large-amplitude trans-to-cis isomerization. This work provides the first demonstration that a trans-to-cis photoisomerization occurs in polycrystalline azobenzene, and is consistent with the previously hypothesized nature of the trigger in the photoactuated mechanisms of the materials in question. It is also demonstrated that under low irradiance, trans-to-cis isomerization occurs in the solid (not via a pre-melted phase); and the presence of the cis-isomer thus lowers the melting point of the sample, providing a liquid phase. A variety of experimental techniques were employed, including X-ray diffraction measurements of polycrystalline azobenzene during exposure to laser irradiation and fluorescence measurements of the solid sample. A practical consequence of this work is that it establishes trans-azobenzene as an easily obtainable and well-defined control for monitoring photoinduced structural changes in X-ray diffraction experiments, using easily accessible laser wavelengths.

Tailoring Nanoscale Morphology of Polymer:Fullerene Blends Using Electrostatic Field.

To tailor the nanomorphology in polymer/fullerene blends, we study the effect of electrostatic field (E-field) on the solidification of poly(3-hexylthiophene-2, 5-diyl) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) bulk heterojunction (BHJ). In addition to control; wet P3HT:PC60BM thin films were exposed to E-field of Van de Graaff (VDG) generator at three different directions-horizontal (H), tilted (T), and vertical (V)-relative to the plane of the substrate. Surface and bulk characterizations of the field-treated BHJs affirmed that fullerene molecules can easily penetrate the spaghetti-like P3HT and move up and down following the E-field. Using E-field treatment, we achieved favorable morphologies with efficient charge separation, transport, and collection. We improve; (1) the hole mobility values up to 19.4 × 10-4 ± 1.6 × 10-4 cm2 V-1 s-1 and (2) the power conversion efficiency (PCE) of conventional and inverted OPVs up to 2.58 ± 0.02% and 4.1 ± 0.40%, respectively. This E-field approach can serve as a new morphology-tuning technique, which is generally applicable to other polymer-fullerene systems.

PTOX Mediates Novel Pathways of Electron Transport in Etioplasts of Arabidopsis.

The immutans (im) variegation mutant of Arabidopsis defines the gene for PTOX (plastid terminal oxidase), a versatile plastoquinol oxidase in chloroplast membranes. In this report we used im to gain insight into the function of PTOX in etioplasts of dark-grown seedlings. We discovered that PTOX helps control the redox state of the plastoquinone (PQ) pool in these organelles, and that it plays an essential role in etioplast metabolism by participating in the desaturation reactions of carotenogenesis and in one or more redox pathways mediated by PGR5 (PROTON GRADIENT REGULATION 5) and NDH (NAD(P)H dehydrogenase), both of which are central players in cyclic electron transport. We propose that these elements couple PTOX with electron flow from NAD(P)H to oxygen, and by analogy to chlororespiration (in chloroplasts) and chromorespiration (in chromoplasts), we suggest that they define a respiratory process in etioplasts that we have termed "etiorespiration". We further show that the redox state of the PQ pool in etioplasts might control chlorophyll biosynthesis, perhaps by participating in mechanisms of retrograde (plastid-to-nucleus) signaling that coordinate biosynthetic and photoprotective activities required to poise the etioplast for light development. We conclude that PTOX is an important component of metabolism and redox sensing in etioplasts.

Fluorescence Spectroscopy of the Retina for the Screening of Bovine Spongiform Encephalopathy.

Transmissible spongiform encephalopathies (TSE) are progressive, neurodegenerative disorders, of which bovine spongiform encephalopathy (BSE) is of special concern because it is infectious and debilitating to humans. The possibility of using fluorescence spectroscopy to screen for BSE in cattle was explored. Fluorescence spectra from the retinas of experimentally infected BSE-positive cattle with clinical disease were compared with those from both sham-inoculated and non-inoculated BSE-negative cattle. The distinct intensity difference of about 4-10-fold between the spectra of the BSE-positive and the BSE-negative (sham-inoculated and non-inoculated) eyes suggests the basis for a means of developing a rapid, noninvasive examination of BSE in particular and TSEs in general.

Shape evolution and single particle luminescence of organometal halide perovskite nanocrystals.

Organometallic halide perovskites CH3NH3PbX3 (X = I, Br, Cl) have quickly become one of the most promising semiconductors for solar cells, with photovoltaics made of these materials reaching power conversion efficiencies of near 20%. Improving our ability to harness the full potential of organometal halide perovskites will require more controllable syntheses that permit a detailed understanding of their fundamental chemistry and photophysics. In this manuscript, we systematically synthesize CH3NH3PbX3 (X = I, Br) nanocrystals with different morphologies (dots, rods, plates or sheets) by using different solvents and capping ligands. CH3NH3PbX3 nanowires and nanorods capped with octylammonium halides show relatively higher photoluminescence (PL) quantum yields and long PL lifetimes. CH3NH3PbI3 nanowires monitored at the single particle level show shape-correlated PL emission across whole particles, with little photobleaching observed and very few off periods. This work highlights the potential of low-dimensional organometal halide perovskite semiconductors in constructing new porous and nanostructured solar cell architectures, as well as in applying these materials to other fields such as light-emitting devices and single particle imaging and tracking.

Tryptophan and ATTO 590: mutual fluorescence quenching and exciplex formation.

Investigation of fluorescence quenching of probes, such as ATTO dyes, is becoming an increasingly important topic owing to the use of these dyes in super-resolution microscopies and in single-molecule studies. Photoinduced electron transfer is their most important nonradiative pathway. Because of the increasing frequency of the use of ATTO and related dyes to investigate biological systems, studies are presented for inter- and intramolecular quenching of ATTO 590 with tryptophan. In order to examine intramolecular quenching, an ATTO 590-tryptophan conjugate was synthesized. It was determined that tryptophan is efficiently quenching ATTO 590 fluorescence by excited-state charge transfer and two charge transfer complexes are forming. In addition, it was discovered that an exciplex (whose lifetime is 5.6 ns) can be formed between tryptophan and ATTO 590, and it is suggested that the possibility of such exciplex formation should be taken into account when protein fluorescence is monitored in a system tagged with ATTO dyes.

The number of accumulated photons and the quality of stimulated emission depletion lifetime images.

Time binning is used to increase the number of photon counts in the peak channel of stimulated emission depletion fluorescence lifetime decay curves to determine how it affects the resulting lifetime image. The fluorescence lifetime of the fluorophore, Alexa Fluor 594 phalloidin, bound to F-actin is probed in cultured S2 cells at a spatial resolution of ~40 nm. This corresponds to a 10-fold smaller probe volume compared to confocal imaging, and a reduced number of photons contributing to the signal. Pixel-by-pixel fluorescence lifetime measurements and error analysis show that an average of 40 ± 30 photon counts in the peak channel with a signal-to-noise ratio of 20 is enough to calculate a reliable fluorescence lifetime from a single exponential fluorescence decay. No heterogeneity in the actin cytoskeleton in different regions of the cultured cells was measured in the 40-400 nm spatial regime.

Plant hemoglobins may be maintained in functional form by reduced flavins in the nuclei, and confer differential tolerance to nitro-oxidative stress.

The heme of bacteria, plant and animal hemoglobins (Hbs) must be in the ferrous state to bind O(2) and other physiological ligands. Here we have characterized the full set of non-symbiotic (class 1 and 2) and 'truncated' (class 3) Hbs of Lotus japonicus. Class 1 Hbs are hexacoordinate, but class 2 and 3 Hbs are pentacoordinate. Three of the globins, Glb1-1, Glb2 and Glb3-1, are nodule-enhanced proteins. The O(2) affinity of Glb1-1 (50 pm) was the highest known for any Hb, and the protein may function as an O(2) scavenger. The five globins were reduced by free flavins, which transfer electrons from NAD(P)H to the heme iron under aerobic and anaerobic conditions. Class 1 Hbs were reduced at very fast rates by FAD, class 2 Hbs at slower rates by both FMN and FAD, and class 3 Hbs at intermediate rates by FMN. The members of the three globin classes were immunolocalized predominantly in the nuclei. Flavins were quantified in legume nodules and nuclei, and their concentrations were sufficient to maintain Hbs in their functional state. All Hbs, except Glb1-1, were expressed in a flavohemoglobin-deficient yeast mutant and found to confer tolerance to oxidative stress induced by methyl viologen, copper or low temperature, indicating an anti-oxidative role for the hemes. However, only Glb1-2 and Glb2 afforded protection against nitrosative stress induced by S-nitrosoglutathione. Because this compound is specifically involved in transnitrosylation reactions with thiol groups, our results suggest a contribution of the single cysteine residues of both proteins in the stress response.

Direct detection and reactivity of the short-lived phenyloxenium ion.

Photolysis of protonated phenylhydroxylamine was studied using product analysis, trapping experiments, and laser flash photolysis experiments (UV-vis and TR(3) detection) ranging from the femtosecond to the microsecond time scale. We find that the excited state of the photoprecursor is followed by two species: a longer-lived transient (150 ns) that we assign to the phenoxy radical and a shorter-lived (3-20 ns) transient that we assign to the singlet phenyloxenium ion. Product studies from photolysis of this precursor show rearranged protonated o-/p-aminophenols and solvent water adducts (catechol, hydroquinone) and ammonium ion. The former products can be largely ascribed to radical recombination or ion recombination, while the latter are ascribed to solvent water addition to the phenyloxenium ion. The phenyloxenium ion is apparently too short-lived under these conditions to be trapped by external nucleophiles other than solvent, giving only trace amounts of o-/p-chloro adducts upon addition of chloride trap. Product studies upon thermolysis of this precursor give the same products as those generated from photolysis, with the difference being that the ortho adducts (o-aminophenol, hydroquinone) are formed in a higher ratio in comparison to the photolysis products.

Unusually large coupling constants in diradicals obtained from excitation of mixed radical centers: a theoretical study on potential photomagnets.

Three sets of heterosubstituted, interconvertible, cyclophanediene (CPD), and dihydropyrenes (DDPs) and one such set involving dinitrilepyrenes were examined by UB3LYP broken-symmetry methodology with 6-311++g(d,p) bases. Nitronyl nitroxide and oxoverdazyl (with both N and C terminals) are monoradical centers, whereas CPD and DDP moieties serve as couplers. The photoexcited CPD converts to DDP. The calculated exchange coupling constant (J) for o-VER(N)-DDP-NN is surprisingly high, 6412 cm(-1), and much larger than 28.9 cm(-1) for the CPD species, but the unsubstituted DDP is known to transform readily into pyrene, with the loss of reversibility. Nevertheless, o-VER(N)-(15,16-dinitrile)DDP-NN also has a large J value, 589.4 cm(-1). The corresponding CPD species has J = 53.3 cm(-1). We predict that the latter CPD and DDP diradicals are potential molecules to synthesize photomagnetic materials. The o-VER(N)-DDP-NN can also be an excellent photomagnetic switch at a considerably low temperature.

Very strongly ferromagnetically coupled diradicals from mixed radical centers. II. Nitronyl nitroxide coupled to tetrathiafulvalene via spacers.

We predict large and positive intramolecular magnetic exchange coupling constants (J) for coupled diradicals constructed from nitronyl nitroxide and tetrathiafulvalene monoradical moieties. These diradicals have the general formula TTF-coupler-NN, where the couplers are mostly aromatic systems. Unrestricted density functional methodology (UB3LYP) has been used to optimize the molecular geometries of the triplet diradicals using the 6-311 g(d,p) basis set. This has been followed by single-point UB3LYP calculations for triplet and broken symmetry (BS) states using 6-311++g(3df,3pd) basis and the optimized triplet geometries. We find that the species comprising of ethylene (geminal coupling) and pyridine as couplers have singlet ground states whereas the other species have triplet ground states. These findings are in support of the spin alternation rule. The largest J value we predict is 648.6 cm(-1) for the molecule with the spacer pyrrole. We also determine the percent weightings of triplet and singlet components in the BS state, estimate the diradical nature, and calculate the relative weights of different singlet and triplet component functions in the BS solution in each case.