Enhancement of Triplet-Sensitized Upconversion in Rigid Polymers \u3ci\u3evia\u3c/i\u3e Singlet Exciton Sink Approach

To increase the practical usefulness of solid-state sensitized upconversion (UC) materials as components of solar energy harvesting systems, it is important to identify and suppress loss mechanisms, and increase the UC quantum yield (ΦUC). Here we focus on a benchmark UC system consisting of the emitter 9,10-diphenylanthracene (DPA) and the sensitizer platinum octaethylporphyrin (PtOEP) in a rigid poly(methyl methacrylate) (PMMA) matrix, and show that one of the major losses originates from Förster resonant energy transfer (FRET) from DPA back to PtOEP. Even though DPA emission lies within the PtOEP transparency window, the quantitative assessment of singlet exciton diffusion for samples with a high DPA content evidences that long-range FRET results in effective exciton trapping by PtOEP. A dramatic factor-of-6 reduction of the DPA emission quantum yield occurs even at PtOEP concentrations as low as 0.05 wt%. To alleviate this problem, we demonstrate a new concept based on the introduction of highly emissive sink sites to trap the singlet excitons produced upon annihilation prior to their quenching by the sensitizer. For DPA/PtOEP blends in PMMA, 1,6-bis-[2,5-di(dodecyloxyphenyl)ethynyl]pyrene is shown to be a useful sink, which results in 1.5-fold increase of the ΦUC. A maximum ΦUC of 2.7% was achieved, which is among the highest reported values for rigid sensitized UC polymers

Diverse regimes of mode intensity correlation in nanofiber random lasers through nanoparticle doping

Random lasers are based on disordered materials with optical gain. These devices can exhibit either intensity or resonant feedback, relying on diffusive or interference behaviour of light, respectively, which leads to either coupling or independent operation of lasing modes. We study for the first time these regimes in complex, solid-state nanostructured materials. The number of lasing modes and their intensity correlation features are found to be tailorable in random lasers made of light-emitting, electrospun polymer fibers upon nanoparticle doping. By material engineering, directional waveguiding along the length of fibers is found to be relevant to enhance mode correlation in both intensity feedback and resonant feedback random lasing. The here reported findings can be used to establish new design rules for tuning the emission of nano-lasers and correlation properties by means of the compositional and morphological properties of complex nanostructured materials.Comment: 30 pages, 10 figure

Blue and Deep‐Blue‐Emitting Organic Lasers with Top‐Layer Distributed Feedback Resonators

All‐solution processed surface‐emitting organic distributed feedback lasers are attractive devices for low‐cost applications. Here, lasers emitting in the spectral region between 375 and 475 nm, in which both active material and resonator (1D relief gratings) are based on solution‐processable polymer films, are reported. Ten different organic compounds dispersed in polystyrene are used as active layers of the prepared devices. They include various carbon‐bridged oligo(p‐phenylenevinylene) (COPVn, with n = 1,2) derivatives and two terfluorene compounds. The synthesis and complete optical and amplified spontaneous emission properties of one of the COPV1 compounds, COPV1(Me)‐t‐Bu, designed for deep‐blue emission, are also included. The feasibility of the resonator fabrication, performed by holographic lithography with a dichromated gelatine photoresist over the active film, is successfully demonstrated for all devices. Remarkably, no resolution limitations are found even for the lowest grating period (235 nm) required for the fabrication of the laser based on COPV1(Me)‐t‐Bu. It is also demonstrated that the rectangular grating profile with duty cycle 0.75:0.25 (hill:valley) is very convenient to optimize the resonator efficacy.The Spanish team acknowledges support from the Spanish Government (MINECO) and the European Community (FEDER) through Grant No. MAT2015-66586-R. H.T. and E.N. thank the financial support from MEXT (JP19H05716 for H.T. and JP19H0549 for E.N.)

The Role of Triplet Exciton Diffusion in Light-Upconverting Polymer Glasses

Light upconversion (UC) via triplet-triplet annihilation (TTA) by using noncoherent photoexcitation at subsolar irradiance power densities is extremely attractive, particularly for enhanced solar energy harvesting. Unfortunately, practical TTA-UC application is hampered by low UC efficiency of upconverting polymer glasses, which is commonly attributed to poor exciton diffusion of the triplet excitons across emitter molecules. The present study addresses this issue by systematically evaluating triplet exciton diffusion coefficients and diffusion lengths (LD) in a UC model system based on platinum-octaethylporphyrin-sensitized poly(methyl methacrylate)/diphenylanthracene (emitter) films as a function of emitter concentration (15-40 wt %). For this evaluation time resolved photoluminescence bulk-quenching technique followed by Stern Volmer-type quenching analysis of experimental data was employed. The key finding is that although increasing emitter concentration in the disordered PMMA/DPA/PtOEP films improves triplet exciton diffusion, and thus LD, this does not result in enhanced UC quantum yield. Conversely, improved LD accompanied by the accelerated decay of UC intensity on millisecond time scale degrades TTA-UC performance at high emitter loadings (\u3e25 wt %) and suggests that diffusion-enhanced nonradiative decay of triplet excitons is the major limiting factor

Identification of flavonoids in the flowers and leaves of Monarda didyma L

Monarda didyma L. is a herbaceous perennial with the square stems and opposite leaves characteristic of herbs in the Labiate family. Monarda didyma L. is often used as a domestic medicine, being particularly useful in the treatment of digestive disorders. The leaves and flowering stems are antihelmintic, carminative, diuretic, expectorant, febrifuge, rubefacient and stimulant. Monarda didyma L. has twenty-six compounds in the essential oil, of which thymol, with smaller amounts of para-cymene, d-limonene, carvacrol, linalool, and hydrothymoquinone. A new method was used for the determining of the amount of separate flavonols by high-performance liquid chromatography. The flavonoids (rutin, hyperoside, quercitrin, luteolin, quercetin) were found in leaves and flowers of beebalm. The quantity of flavonoids was higher in flowers than in leaves of Monarda didyma L

Study of diversity of anthocyanin composition in bilberry (Vaccinium myrtillus L.) fruits

Qualitative and quantitative composition of anthocyanins in bilberry (Vaccinium myrtillus L.) fruits was assayed. The aim of our study was to evaluate total anthocyanin content and their composition in bilberries collected from various regions and at different time. For the quantification of total anthocyanins in frozen fruits, the spectrophotometrical assay was performed. The highest amount of anthocyanins in bilberry fruits, collected in Lithuania, was found in samples from Šilutė (0.399%), the lowest one – from Valkininkai region (0.264%), but higher amounts of anthocyanins were found in the samples collected in Russia (Archangelsk region) and Sweden (Stockholm region). High-performance liquid chromatography was applied for qualitative evaluation of individual anthocyanins in the different material. Quantification of anthocyanidin content was performed after acidic hydrolysis of anthocyanin glycosides. Chromatographic analysis has shown that there are no differences in qualitative composition of anthocyanidins. In all samples, cyanidin was found in the highest quantities (mean amount 0.053 µg/mL). Delphinidin and petunidin was found in quantities 2.5 fold lower than cyanidin, and malvidin and peonidin were found in the smallest quantities. Only in the blueberries collected in Sweden, malvidin was found in the highest amount. It was 1.5 fold higher than amounts of petunidin and delphinidin