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We developed a technique allowing the direct observation of photoinduced charge-transfer states (CTSs)—the weakly coupled electron–hole pairs preceding the completely separated charges in organic photovoltaic (OPV) blends. Quadrature detection of the electron spin echo (ESE) signal enables the observation of an out-of-phase ESE signal of CTS. The out-of-phase Electron–Electron Double Resonance (ELDOR) allows measuring electron–hole distance distributions within CTS and its temporal evolution in the microsecond range. The technique was applied to OPV bulk heterojunction blends of different donor polymers, including the benchmark polymer P3HT and the high-performance polymer PCDTBT, with the fullerene PC61BM acceptor. The corresponding electron–hole distance distributions were obtained using the Tikhonov regularization. It was found that not only the dipolar interaction but also the exchange interaction contributes to the formation of the out-of-phase ELDOR signal. By varying the delay time after photoexcitation, we observed CTSs at different stages of charge separation. The initial distribution of the electron–hole distances for different blends correlates with their photoelectric conversion efficiency, with shorter average thermalization distances found for the blends of PC61BM with the less efficient regiorandom polymer P3HT. Spin-selective recombination of the CTS was unambiguously demonstrated for the blend of regioregular P3HT with PC61BM. It produces characteristic features in the out-of-phase ELDOR trace for small "dipolar" evolution times. These data allow us to estimate the CTS recombination rate for a certain distance between the electron and the hole within the CTS. The proposed method can be used to probe CTS in a variety of OPV active layer materials. [ABSTRACT FROM AUTHOR]

Photoinduced charge separation at donor–acceptor composites (active layer material of organic solar cells) is an important step of photoelectric energy conversion. It results in the formation of the interfacial charge-transfer state (CTS), which is a Coulombically bound electron-hole pair. We developed the mathematical procedure of direct quantification of the electron-hole distance on the basis of time-domain pulse electron paramagnetic resonance data, obtained in an electron spin echo (ESE) experiment. For an ensemble of CTSs characterized by a distribution of electron-hole distances, this procedure derives the average electron-hole distance without numerical simulation of the experimental data, which is a superposition of the oscillating functions, corresponding to CTSs with a certain electron-hole distance. This procedure was tested on model distance distributions, yielding very accurate results. The data for highly efficient organic photovoltaic composite PM6/Y6 were also analyzed; the average electron-hole distance within the CTS and its dependence on temperature were determined. This procedure can be useful for tracing small changes in CTS structure during optimization of the donor–acceptor composite morphology, which is tightly related to the photovoltaic efficiency of the composite. [ABSTRACT FROM AUTHOR]

Background/Objectives: Some specific anthraquinone derivatives (AQs) are known to be used widely as effective chemotherapeutic agents in the treatment of cancer. However, their fundamental shortcoming is the high rate of cardiotoxicity observed in treated patients, which is thought to be caused by the increase in production of reactive oxygen species (ROS) catalyzed by iron and copper. The development of improved AQs and other anticancer drugs with enhanced efficacy but reduced toxicity remains a high priority. The aim of this study was to evaluate the cytotoxic and ROS production effects of chelate iron and copper complexes of two novel AQs, namely 4-hydroxynaphto[2,3-h]cinnoline-7,12-dione (Q2) and 3-(hydroxymethyl)naphto[2,3-h]cinnoline-4,7,12(1H)-trione (Q3). Methods: The chelation ability of Q2 and Q3 was studied using NMR and UV–Vis spectroscopy. Cytotoxicity studies were carried out using the MTT assay. The influence of chelation on ROS production was studied using NMR spectroscopy in linoleic acid micelles. Results: It was found that only Q3 forms complexes with Fe(III) and Cu(II) ions, whereas Q2 does not demonstrate chelating properties. A cytotoxicity study revealed that Fe[Q3]3 significantly decreased the viability of lung cancer A549 cells, while Q3 and Cu[Q3]2 did not demonstrate cytotoxic properties in this cell line. Furthermore, the presence of Q3 lowered the rate of iron-induced lipid peroxidation in linoleic acid micelles. By contrast, Q2 did not influence the rate of lipid peroxidation, probably due to the absence of effective metal chelating ability. Conclusions: The high cytotoxic effects observed with the iron complex of Q3 against cancer cells in combination with a reduced rate of iron induced lipid peroxidation in the presence of Q3, make Q3 and its iron complex promising for further evaluation and use as chemotherapeutic agents in cancer. [ABSTRACT FROM AUTHOR]

Three small molecules consisting of thieno‐fused naphthalimide units connected via aromatic linkers were synthesized to study the effect of fused thiophene rings on the properties of naphthalimide‐based molecules. A protocol for the synthesis of thienonaphthalimide‐based molecules without prior preparation and use of thienonaphthalimide precursors and organoelement compounds (organolithium, organotin, or organoboron compounds) was developed. The protocol is based on double thienannulation of bis‐alkyne substrates with sodium sulfide. The thin‐film morphology, photophysical and electrochemical properties of the obtained small molecules were studied. Single‐layer non‐doped organic light‐emitting diodes based on fluorene‐bridged thienonaphthalimide dimer showed yellow emission with luminance of 2200 cd/m2, and current efficiency of 0.35 cd/A. Combination of transient electroluminescence and photo‐CELIV methods allowed to estimate electron and hole mobility in the synthesized compounds; both are in order of 10−6 cm2V−1 s−1. [ABSTRACT FROM AUTHOR]

Fluorinated multi-walled carbon nanotubes are implemented as the third component of the active layer of organic solar cells. The addition of approximately 1% weight fraction of these nanotubes into PCDTBT/PC60BM polymer/fullerene active layer leads to a noticeable increase in the main photovoltaic parameters of the solar cells, including their stability. Presumably, the origins of this influence are optimizing the donor/acceptor composite morphology and reinforcing its structure with fluorinated multi-walled carbon nanotubes. This hypothesis is supported by the dramatic influence of the fluorinated multi-walled carbon nanotube additive on the kinetics of the decay of the EPR signal of light-induced charges in the PCDTBT/PC60BM composite. [ABSTRACT FROM AUTHOR]

The overall manganese oxidization states of the oxygen-evolving complex (OEC) in the [S.sub.0] and [S.sub.2] states are determined for methanol containing spinach photosystem II (PSII) membrane fragments. The analysis has shown that the [Mn.sub.4][O.sub.x]Ca cluster has taken part in the unique water splitting chemistry through the structural changes, of which one can be located within the cluster.

The study on a new type of electron spin-echo envelope modulation(ESSEM), which was observed in a three pulse stimulation echo experiment on nitroxides in a molecular glass is presented. The results attributed that a sudden change of the EPR resonance frequency was due to reorientation of the methyl groups of the nitroxide during the T interval (a fixed interval between second and third pulses).

Research was conducted to examine the solvent effect on the rate of beta-scission of the tert-butoxyl radical. The laser flash photolysis method was used to obtain the transient absorption spectrum of the radical in the UV region. The Arrhenius parameters of the rate constant for beta-scission k(beta) were determined while the rate constants for beta-scission and self-termination reactions of tert-butoxyl radicals were calculated in five solvents.

Spin-lattice relaxation times T1 of photoexcited triplets ³C70 in glassy decalin were obtained from electron spin echo inversion recovery dependences. In the range 30-100 K, the temperature dependence of T1 was fitted by the Arrhenius law with an activation energy of 172 cm-1. This indicates that the dominant relaxation process of ³C70 is described by an Orbach-Aminov mechanism involving the higher triplet state t2 which lies 172 cm-1 above the lowest triplet state T1. Chemical modification of C70 fullerene not only decreases the intrinsic triplet lifetime by about ten times but also increases T1 by several orders of magnitude. The reason for this is the presence of a low-lying excited triplet state in ³C70 and its absence in triplet C70 derivatives. The presence of the higher triplet state in C70 is in good agreement with the previous results from phosphorescence spectroscopy. [ABSTRACT FROM AUTHOR]

Various aspects of surface enhanced Raman scattering amplification in gases and liquids have been investigated utilizing active Raman enhancing surfaces and waveguide resonators. We demonstrate that surface enhanced Raman scattering amplification in gases is not as selective as in liquids. Thus, we develop a portable express analyzer of gas based on a surface-enhanced waveguide resonator. Its Raman scattering gain coefficient is compared quantitatively with that of a bulk Kagome Raman resonator. We estimate the Raman scattering gain coefficient in a hollow metal-plated tube due to multiple wall reflections of the pumping and scattered light beams. We consider Raman scattering amplification in a mirror tube, given an enhancing Raman surface as opposed to a simple focusing of the beam in a gas medium. For in-field gas monitoring online, the surface-enhanced Raman scattering waveguide resonator was combined with a portable spectrometer. The express gas analyzer with an active surface enhanced Raman scattering waveguide resonator proves superior to similar conventional technology in measurement speed and stability. We discuss an example of the practical application of the proposed system. [ABSTRACT FROM AUTHOR]

Two acceptor small molecules consisting of 2‐(11‐oxoanthra[2,1‐b]thiophen‐6‐ylidene)dipropanedinitrile frameworks connected by a bis(thiophen‐2‐ylethynyl)arene bridge were synthesized by the two‐step sequence of Sonogashira cross‐coupling and Knoevenagel condensation in order to investigate their performance as non‐fullerene acceptors in organic solar cells. Optical, electrochemical and photovoltaic properties of new 2,2′‐[2,2′‐(5,5′‐(arenediyl)bis(ethyne‐2,1‐diyl)bis(thien‐5,2‐diyl))bis(11‐oxoanthra[2,1‐b]thiophen‐6‐ylidene)]dipropanedinitriles and previously studied analogs of 2,2′‐[2,2′‐arenediylbis(11‐oxoanthra[1,2‐b]thiophene‐6‐ylidene)]dipropanedinitriles were compared to assess the effect of introducing ethynylthiophene units between the arene linker and the 2‐(11‐oxoanthrathiophen‐6‐ylidene)propanedinitrile units. Despite the proper energy level alignment and morphology, composites of the newly synthesized compounds with the PBDB‐T donor polymer demonstrate a moderate PCE of 0.54 %. The small value of the short circuit current as compared to composites with fullerene acceptors is probably due to the low electron mobility in the newly synthesized compounds. The low electron mobility, in turn, is probably cause by the LUMO localization at the edge units of the acceptor‐type molecules, in contrast to the almost uniform LUMO delocalization over the whole π‐conjugated system for high‐performing non‐fullerene acceptors such as ITIC. [ABSTRACT FROM AUTHOR]

Continuous-wave X-band electron paramagnetic resonance (EPR) of fullerene C70 molecules excited to a triplet state by continuous light illumination was studied in molecular glasses of o-terphenyl and cis/trans-decaline and in the glassy polymers polymethylmethacrylate (PMMA) and polystyrene (PS). Above ∼100 K, a distinct narrowing of EPR lineshape of the triplet was observed, which was very similar for all systems studied. EPR lineshape was simulated reasonably well within a framework of a simple model of random jumps, which implies that the C70 molecule performs isotropic orientational motion by sudden jumps of arbitrary angles. In simulations, a single correlation time τc was used, varying in the range of 10-7-10-8 s. Near and below 100 K electron spin echo (ESE) signals were also obtained which were found to decay exponentially. Correlation times τc obtained from simulation of the EPR spectra in the slow-motion limit (τc close to 10-7 s) turned out to be in good agreement with the phase memory times TM of the ESE decay, which additionally supports the employed simple model. The observed motional effects provide evidence that the nanostructure of the solid glassy media of different origins is soft enough to allow a large asymmetric C70 molecule to reorient rapidly. Except for the EPR spectra of the triplet, in the center of the spectra, a small admixture of a narrow line was also observed; its possible nature is briefly discussed. [ABSTRACT FROM AUTHOR]

Electron spin echo (ESE) was applied to study transversal spin relaxation of photoexcited triplet state of fullerene C70 molecules in glassy o-terphenyl and cis-/trans-decalin matrices (glass transition temperatures of 243 and 137 K, respectively). The relaxation rate T2-1 was found to increase sharply above 110 K in o-terphenyl and above 100 K in decalin. It is suggested that this increase arises from interaction of 3C70 pseudorotation with fast molecular librations in the matrix. Both these types of motion involve atomic vibrations and are uniaxial in their nature, the known literature data on Raman light scattering and others indicate that molecular librations may be thermally activated in glasses just near 100 K. The increase in T2-1 near 100 K is not observed for photoexcited triplet state of fullerene C60, for which pseudorotation is not uniaxial. As the fullerene molecule has a size much larger than that for glass solvent molecules, it is likely that molecular librations in the matrix are of collective nature. [ABSTRACT FROM AUTHOR]