Chloroform Solution Research Articles

Reductive Coupling Synthesis of a Soluble Poly(9,10-anthrylene ethynylene)

A fully soluble poly(9,10-anthrylene ethynylene), poly[2,6-(2-octyldecyl)-9,10-anthrylene ethynylene] PAAE, with moderate degrees of polymerization Pn of ca. 10 is generated in a reductive, dehalogenative homocoupling scheme, starting from a 2,6-dialkylated 9,10-bis(dibromomethylene)-9,10-dihydroanthracene monomer and n-BuLi/CuCN as the reducing agent. PAAE shows surprisingly broad and unstructured absorption and photoluminescence emission bands with peaks at 506 nm and 611 nm, respectively, both in chloroform solution. The long absorption tail ranging into the 600–700 nm region and the large Stokes shift points to a high degree of geometrical disorder in the arrangement of the 9,10-anthrylene chromophores along the distorted polymer backbone. This disorder is borne out in the unusually strong wavelength dependence of fluorescence depolarisation, both with regards to the excitation and the emission wavelengths. Picosecond fluorescence depolarisation spectroscopy provides clear evidence for the presence of orthogonal transition dipole moments, presumably arising from the off-axis transition of the anthracene unit and the on-axis transition of the polymer backbone. Intramolecular energy relaxation then gives rise to the observed fluorescence depolarization dynamics.

Investigation of polyaniline doped with camphorsulfonic acid in chloroform solution as a hole transporting layer in PTB7: PCBM and perovskite-based solar cells

Polyaniline was chemically synthesized and protonated (doped) with CSA in chloroform with different PANI:CSA molar ratios, ranging from 6:1 to 1:4. Protonation level of the polymer was monitored by means of UV–Vis spectra of the solution and FTIR measurements of thin films. The results were correlated with the changes of specific conductivity in a function of PANI:CSA molar ratio in the films. A broad maximum of conductivity was found in the range from 1:1 to 1:3, i.e. for over-protonated polymer. The solutions with various PANI doping levels were used as hole transport layers (HTLs) in the polymer-fullerene (PTB7:PC70BM) and perovskite (MAPbI3) solar cells. The highest power conversion efficiency (PCE=3.8%) obtained for the organic cell with molar ratio PANI:CSA=1:1 was only slightly lower than that of the reference cell with PEDOT:PSS HTL (4%). The best perovskite cells achieved PCE of about 4% which was markedly lower that the efficiency of cells with Spiro-OMeTAD HTL. However, some of the perovskite cells with PANI:CSA revealed very high short current density, of about 20 mA cm−2 but relatively low open circuit photovoltage (0.42 V). The changes of the PCE in a function of PANI protonation level were discussed in terms of alignment of HOMO and LUMO levels of HTL, donor and acceptor layers in the solar cells as well as quality of interfacial layers.

Unveiling the Molecular Structure of Antimalarial Drugs Chloroquine and Hydroxychloroquine in Solution through Analysis of 1H NMR Chemical Shifts

Chloroquine (CQ) and hydroxychloroquine (HCQ) have been standard antimalarial drugs since the early 1950s, and very recently, the possibility of their use for the treatment of COVID-19 patients has been considered. To understand the drug mode of action at the submicroscopic level (atoms and molecules), molecular modeling studies with the aid of computational chemistry methods have been of great help. A fundamental step in such theoretical investigations is the knowledge of the predominant drug molecular structure in solution, which is the real environment for the interaction with biological targets. Our strategy to access this valuable information is to perform density functional theory (DFT) calculations of 1H NMR chemical shifts for several plausible molecular conformers and then find the best match with experimental NMR profile in solution (since it is extremely sensitive to conformational changes). Through this procedure, after optimizing 30 trial distinct molecular structures (ωB97x-D/6-31G(d,p)-PCM level of calculation), which may be considered representative conformations, we concluded that the global minimum (named M24), stabilized by an intramolecular N-H hydrogen bond, is not likely to be observed in water, chloroform, and dimethyl sulfoxide (DMSO) solution. Among fully optimized conformations (named M1 to M30, and MD1 and MD2), we found M12 (having no intramolecular H-bond) as the most probable structure of CQ and HCQ in water solution, which is a good approximate starting geometry in drug-receptor interaction simulations. On the other hand, the preferred CQ and HCQ structure in chloroform (and CQ in DMSO-d6) solution was assigned as M8, showing the solvent effects on conformational preferences. We believe that the analysis of 1H NMR data in solution can establish the connection between the macro level (experimental) and the sub-micro level (theoretical), which is not so apparent to us and appears to be more appropriate than the thermodynamic stability criterion in conformational analysis studies.

Naphthalenothiophene imide-based polymer exhibiting over 17% efficiency

Summary Developing novel electron-deficient monomers is crucially important for the future development of non-fullerene organic solar cells. Here, we report a new electron-deficient monomer naphthalenothiophene imide (NTI) and NTI-based copolymers PNTB and PNTB-2T. The strong electron-withdrawing ability of NTI could effectively lower the HOMO level of the polymer that facilitates the tuning of the polymer’s physical properties while simultaneously maintaining the low-lying HOMO value. Polymer chains modification by introducing extra thiophenes results in the closed π−π stacking and ordered polymer chain packing of PNTB-2T and is proven to be an effective strategy to increase the device performance. Thus, solar cell devices for PNTB-2T:Y6 exhibit an efficiency of 16.72%, which is further enhanced to 17.35% by the third component PC71BM, whereas that for PNTB:Y6 is only 3.81%. Importantly, PNTB-2T-based devices exhibit excellent batch-to-batch reproducibility. These features make this family of polymer donors very promising and the design strategies will enlighten future development of high-performance polymer donors for organic solar cells (OSC).

Biopolymer Geotextiles Based on Mixtures of Polyhydroxybutyrate and Polylactic Acid

In this work, studies of the hydrolytic destruction of biodegradable nonwoven fibrous and film materials based on the mixtures of biobased polymers - polyhydroxybutyrate and polylactic acid, have been carried out. Fibers were obtained from a solution in chloroform by electrospinning. It was shown that materials with high content of polylactide had a low resistance to hydrolysis as compared to polyhydroxybutyrate. Nonwoven fibrous materials obtained by electrospinning had a higher water absorption and a high reactivity to hydrolysis, which significantly accelerated (by about 5 times) the biodegradation process under environmental conditions. The degree of hydrolytic destruction of fibrous materials based on polylactide and mixtures with its content of 50 wt.% or more is higher than that of samples enriched with polyhydroxybutyrate. The studied materials can be successfully used as nonwoven geotextiles for reinforcing the soil on the slopes of various types of roads being capable of accelerating the germination of grass cover.

“Small in size but mighty in force” – The first principle study of the impact of A/D units in A/D-phenyl-π-phenothiazine-π-dicyanovinyl systems on photophysical and optoelectronic properties

A set of N-octylphenothiazine malononitriles (P1–P4) containing (p-methoxyphenyl, p-trifluoromethylphenyl, 3,5-bis(trifluoromethyl)phenyl, or p-cyanophenyl) substituents connected via acetylene linker were designed and synthesized. The obtained structures, which exhibit A/D-π-D-A architecture, were characterized by NMR and mass spectroscopy, and elemental analyses. Photophysical, thermal, and electrical properties of these novel dyes P1–P4 were comprehensively studied and confronted with theoretical calculations. They exhibit high thermal stability of up to 306–369 °C. Compounds showed an intramolecular charge transfer (ICT) absorption band at about 480 nm and emission band at around 620 nm with quantum yields between 31 and 50% in chloroform solution. Comparison of the solution and thin film photoluminescence (PL) spectra of P1–P4 exhibit a significant red-shift of 40–70 nm in the thin film PL spectrum. Furthermore, the impact of the end-capped groups in P1–P4 on optical and electrochemical properties was investigated in detail with the support of TD/DFT calculations. The diodes based on P1–P4 with configuration ITO/PDOT:PSS/PBK:PBD:compound/Al were constructed and their electroluminescence (EL) spectra were registered. Considering the preliminary EL investigations, the beneficial effect of the presence of p-cyanophenyl as a substituent (P4) was demonstrated.

Buchner Reaction/Azirine Modification Approach Toward Cycloheptatriene Containing Nitrogen Heterocyclic Scaffolds.

The reaction conditions for the Buchner reaction of 2-(diazoacetyl)-2H-azirines with benzene leading to 2-(cyclohepta-2,4,6-trien-1-yl)-2H-azirines have been found. The azirine fragment of these compounds was subsequently used to prepare 5-(cyclohepta-2,4,6-trien-1-yl)isoxazoles and α-(cyclohepta-2,4,6-triene-1-ylcarbonyl)-1H-pyrroles. NMR and DFT calculations showed that the cycloheptatriene-norcaradiene valence isomerization in the corresponding moieties of synthesized azirines, isoxazoles, and pyrroles is very fast at room temperature. Cycloheptatriene valence isomer predominates in azirines and pyrroles, while 5-(cycloheptatrienyl)isoxazoles exist in a pure cycloheptatriene form at 25 °C in chloroform solution. In accordance with the X-ray analysis, two of the synthesized pyrroles crystallized as norcaradiene isomers from a solution of equilibrium mixture of valence isomers. Reductive isoxazole ring opening using the Mo(CO)6/H2O reductive system afforded (Z)-3-amino-3-aryl-1-(cyclohepta-2,4,6-trien-1-yl)prop-2-en-1-ones, which are pure cycloheptatriene isomers. α-(Cycloheptatrieneylcarbonyl)-1H-pyrroles were transformed into the α-(benzylcarbonyl)-1H-pyrroles under cooperative TFA/Rh2(TFA)4 catalysis.

Analysis of the Impact on the Mechanical Properties of Modification of Oligohydroxyethers in Organic Solvent Solution with Rubbers.

This paper proposes and presents the chemical modification of linear hydroxyethers (LHE) with different molecular weights (380, 640, and 1830 g/mol) with the addition of three types of rubbers (polysulfide rubber (PSR), polychloroprene rubber (PCR), and styrene-butadiene rubber (SBR)). The main purpose of choosing this type of modification and the materials used was the possibility to use it in industrial settings. The modification process was conducted for a very wide range of modifier additions (rubber) per 100 g LHE. The materials obtained in the study were subjected to strength tests in order to determine the effect of the modification on functional properties. Mechanical properties of the modified materials were improved after the application of the modifier (rubber) to polyhydroxyether (up to certain modifier content). The most favorable changes in the tested materials were registered in the modification of LHE-1830 with PSR. In the case of LHE-380 and LHE-640 modified in cyclohexanol (CH) and chloroform (CF) solutions, an increase in the values of the tested properties was also obtained, but to a lesser extent than for LHE-1830. The largest changes were registered for LHE-1830 with PSR in CH solution: from 12.1 to 15.3 MPa for compressive strength tests, from 0.8 to 1.5 MPa for tensile testing, from 0.8 to 14.7 MPa for shear strength, and from 1% to 6.5% for the maximum elongation. The analysis of the available literature showed that the modification proposed by the authors has not yet been presented in any previous scientific paper.

Enhanced Hydrophobicity of Nanofibrillated Cellulose Through Surface Modification Using Cetyltrimethylammonium Chloride Derived from Palmityl Alcohol

This study aims to provide an approach for increasing the hydrophobicity of nanofibrillated cellulose in oil palm empty fruit bunches (NFC-OPEFBs) through surface modification using a sustainable cationic surfactant, i.e., cetyltrimethylammonium chloride derived from palmityl alcohol (CTAC-PA). The NFC-OPEFB was prepared by sulfuric acid treatment. The hydrophobicity of NFC-OPEFB was increased following modification using CTAC-PA, indicated by an increase in the water contact angle value. The dispersibility of the modified NFC-OPEFB (modNFC) in chloroform solution was more stable than that of unmodified NFC-OPEFB (unmodNFC) as observed via ultraviolet–visible spectrophotometry and based on visual appearance. Observation through transmission electron microscopy confirmed that modNFC was completely dispersed in chloroform but undispersed in water. Furthermore, Fourier-transform infrared (FTIR) spectroscopy analysis revealed that the functional groups of NFC-OPEFB were successfully modified using CTAC-PA, justifying the increase in their hydrophobicity characteristics. Peaks were observed at 2892 and 2921 cm−1 for symmetrical and asymmetrical CH2 from the long alkyl chain of CTAC, at 1480 cm−1 for the trimethyl groups of the quaternary ammonium, and at approximately 805 cm−1 for the C–Cl bond on the FTIR spectra of modNFC which were absent from the FTIR spectra of the unmodNFC. This work provides an eco-friendly way to enhance the hydrophobicity characteristics of NFC-OPEFB and expand the use of NFC-OPEFB in hydrophobic polymer matrices, such as poly (lactic acid), polyethylene, etc.

Synthesis and optical properties of 1-ethyl-indol-3-yl-substituted aza-BODIPY dyes at the 1,7-positions

New aza-BODIPYs with significant bathochromic shifts in their UV–vis absorption spectrum were synthesized by the installation of 1-ethyl-indol-3-yl moieties at the 1,7-positions of the aza-BODIPY core bearing 4-substituted phenyl or thien-2-yl groups at the 3,5-positions. The compounds described herein show strong NIR absorptions up to 779 nm, and emissions up to 805 nm in chloroform solution, and present large molar extinction coefficients. The absorption and emission bands of aza–BODIPYs were significantly red-shifted, by ca 100 nm, relative to the 3,5-dianisyl-1,7-diphenyl aza-BODIPY. TD-DFT calculations indicate an effective electronic conjugation between the aza-BODIPY core and the substituents. Absorption of the studied aza-BODIPY derivatives corresponds to a charge transfer from donor substituents toward the boron-centered core. Moreover, computational results show that all the compounds present HOMO–LUMO band gap values that are in the organic semiconductor range. • A series of new aza-BODIPYs with 1-ethyl-indol-3-yl substituents were synthesized. • These dyes show strong NIR absorptions up to 779 nm, and emissions up to 805 nm in chloroform solution. • DFT-calculated and optical band gap values are found to lie in the organic semiconductor range.

Thermochemistry of Solution, Solvation and Hydrogen Bonding of Chloroform in Linear and Cyclic Ethers

In this work the thermochemistry of solution, solvation and hydrogen bond formation of chloroform in linear and cyclic ethers was studied. The infinite dilution solution enthalpies of chloroform in diethyl ether, diglyme, 1,4-dioxane, tetrahydrofuran, 12-crown-4 and 15-crown-5 were measured at 298.15 K. The solvation and hydrogen bonding enthalpies of chloroform in the ethers were calculated. It was found that the hydrogen bonding enthalpies of chloroform in the ethers are significantly higher for linear than for cyclic ethers. The hydrogen bonding of ethers with chloroform was discussed in cases when ethers act as a solute or solvent.

Optical Properties of New Terbium(III) Ternary Complexes Containing Anionic 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione and Neutral Sensitizers in Solution, Solid and PMMA Thin Films: Intra and Interphase Colour Tuning.

Four new terbium(III) ternary complexes, [Tb(fod)3 (indazole)] (1), [Tb(fod)3 (tptz)] (2), [Tb(fod)3 (impy)] (3) and [Tb(fod)3 (tppo)2 ] (4) with 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione, (Hfod) and N and O donors are synthesized and structurally characterized. The photoluminescence of the complexes are studied in solid, chloroform solution and thin PMMA hybrid films (PMMA is a poly(methyl methacrylate)). The ligand-based emission is dominant in solution of 3 and 4. The effect of ancillary ligands on the photoluminescence of terbium(III) ion is investigated. The mechanism of energy transfer is discussed and correlates the luminescence of acceptor terbium ion with the triplet state energy level of donor ligands. Among the four complexes studied, the luminescence from 1 is most intense followed by 2, 4 and 3. The Commission International de I'Eclairage (CIE) color coordinates of these complexes are calculated and presented. The complexes 3 and 4 show intraphase color tuning when excited under different UV wavelengths. The emission color of [Tb(fod)3 (tppo)2 ] (4) changes from pure blue to turquoise green via almost pure white under 360, 328 and 280nm UV excitation wavelengths respectively. The [Tb(fod)3 (impy)] (3) show similar results. The complex [Tb(fod)3 (tptz)] (2) displays interphase color tuning from yellow (solution) to turquoise green (solid) and fascinatingly incorporation of complex in PMMA polymer generates white light under same excitation wavelength (360nm).

NEW 3,5-DISUBSTITUTED 2,6-DICYANOANILINES CONTAINING 3,4-ETHYLENEDIOXYTHIOPHENE FRAGMENTS: SYNTHESIS AND FURTHER TRANSFORMATION

Some novel 3,5-disubstituted 2,6-dicyanoanilines incorporating 3,4-ethylenedioxythiophene (EDOT) fragment were prepared and investigated. Two different synthetic procedures have been applied for this purpose. One of them consists in the cyclization of chalcones with the malononitrile excess, another one relies on the one-pot three-component strategy. Condensation of thus prepared 3,5-disubstituted 2,6-dicyanoanilines with 3,4-ethylenedioxythiophene-2-carboxaldehyde (EDOT-CHO) has successfully given rise to the corresponding Schiff bases. UV absorption and fluorescence spectra were recorded for the chloroform solutions of the synthesized compounds. The influence of the changes in the compounds’ molecular structure on the UV absorption and fluorescence maxima as well as on the absorption edge position has been analyzed. It has been revealed that the π-conjugation chain elongation results in the red shift of the UV absorption and fluorescence maxima (λmax). The absorption red edge values (λonset) of the Schiff bases’ solutions were found to be crucially shifted to the longwave region if compared with the ones obtained for the solutions of the initial 2,6-dicyanoanilines (142 and 170 nm). The strong bathochromic shifts were also detected for their fluorescence maxima (72 and 114 nm). The optical bandgaps (Egopt) calculated on the basis of the λonset values were found to be less than 2.5 eV for all prepared compounds. Cyclic voltammetry method was used to study the electrochemical behavior of the Schiff bases, as a result the values of the onset oxidation potentials (Eoxonset) were determined and used for calculation of such important characteristics of molecules as the frontier orbital energies (EHOMO и ELUMO). The structures of the synthesized compounds were confirmed by elemental analysis and by IR and 1H NMR spectral data.

Preparation of a dicationic, p-cymene ruthenium(II) dimer, [(p-cymene)Ru(µ-Cl)(P{OCH2}3CEt)]2 2+: structure, characterization and reactivity

Reaction of (p-cymene)RuCl2(P{OCH2}3CEt) (P{OCH2}3CEt = 4-ethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane) with silver tetrakis[(3,5-trifluoromethyl)phenyl]borate (Ag[BArF′]) in the presence of acetonitrile produces the monomeric, cationic, acetonitrile ruthenium(II) compound, [(p-cymene)RuCl(P{OCH2}3CEt)(NCMe)][BArF′] (1), in near quantitative yields (97%). Chloroform solutions of 1 cause precipitation of the binuclear, dicationic ruthenium(II) dimer complex, [(p-cymene)Ru(µ-Cl(P{OCH2}3CEt)]2[BArF′]2 (2), with bridging chloride ligands (73%). Complex 2 can be readily converted back to 1 by treatment with acetonitrile. Complexes 1 and 2 were characterized by 1H, 13C{1H}, 31P{1H}, 19F{1H} nuclear magnetic resonance spectroscopy and infrared spectroscopy. Complex 2 was also characterized by single crystal X-ray crystallography, which revealed a rhomboid Ru2Cl2 core. The two ruthenium ions in 2 are displaced at 3.668 Å, which precludes Ru–Ru bonding. To our knowledge, this is the first cymene ruthenium(II) dimeric complex with bridging halide ligands supported by a monodentate phosphite ligand.

Co-crystal structures and Hirshfeld surface analysis of mesitylene and/or p-xylene solvated pseudopolymorphs of fully fluorinated Pd(II) complex

The fully fluorinated complex, Pd(L)2, comprised of one Pd2+ ion and two ligands, namely bis(pentafluorobenzoyl)methanido− (L), was well crystallized with mesitylene (C9H12) and p-xylene (C8H10) as guest molecules to form unique co-crystals, Pd(L)2·2C9H12 and Pd(L)2·2C8H10, respectively, using a chloroform or ethyl acetate solution. In the crystals, the π-planes of the guest molecules closely interacted with the pentafluorophenyl groups of the complex through an π···π-hole interaction. By the combination of two guest molecules, a new crystal, Pd(L)2·2C9H12·C8H10, was obtained and two mesitylene and one p-xylene molecules are quantitatively inserted into the crystal without any structural disorder, showing the good affinity of the guest molecules and the flexible shape changes of the complex.

Electron-vibrational coupling on the absorption spectrum of dithienyl-diketopyrrolopyrrole dye

The S1←S0 electronic spectrum of DPP2T in a chloroform solution was investigated using the vertical gradient (VG) and adiabatic shift (AS) models, which were compared with experimental results. The solvent effects were accounted through the polarizable continuum model (PCM) in non-equilibrium regime. Using the Condon approximation, linear coupling contribution for DPP2T's spectrum is mainly composed of zero-phonon and fundamental transitions. The most intense fundamental transition is related to symmetrical and in-plane normal mode stretching of carbon-carbon bonds belonging π-conjugated skeleton of DPP2T, which is quinoidized upon S1←S0 electronic transition. DPP2T's low-frequency torsional modes couple quadratically to the transition. Their thermal excitation at room temperature broadens the spectrum, shifting it slightly to higher energies.

Viscosity-induced dual-emission of europium ions containing metallopolymer

A hybrid structure poly[(9,9′-dihexyl-9H-fluoren-2,7-yl)-6,6′-(2,2′:6,2″-terpyridine)] complexed with europium tris[3-trifluorometyhydroxymethylene]-(+)-camphorate, with luminescence sensitive to viscous chemical environments is presented. Four solvents were tested: chloroform, toluene, DMF and DMSO. The material depicts a dual-emission profile with red luminescence from europium ions in the DMF and DMSO media, whereas in chloroform and toluene solution the structure presents blue emission only. In this study, absorption, excitation, emission and lifetime measurements are presented and discussed. It is suggested that DMSO and DMF restrict intramolecular motion / rotation on the polymer backbone, minimizing non-radioactive energy losses and promoting the europium luminescence via aggregation–induced emission (AIE), caused by the viscous environments.

Chemical synthesis of polyindole

Polyindole is an important electroactive polymer that can be obtained by electrochemical or chemical oxidation of indole. Polyindole an important role in organic synthesis due to its catalytic properties, its use as a catalyst makes it possible to synthesize organic compounds and composites for various functional purposes. The aim of the work was to research the features of chemical polymerization of indole, as well as the structure and properties of the obtained polymer. In our work, the possibilities of chemical synthesis of polyindole by the interaction of 0.1 M indole solution with 0.1 M of oxidant (ammonium persulfate) in acetonitrile in an inert medium (Ar atmosphere) at a temperature of 0 ± 2 o C were investigated. The structure, optical and electrical properties of the obtained compounds have been studied. It is determined that during the chemical synthesis the relative yield of dark green polyindole powder is 71 %. According to electron microscopy in the polymerization process the formation of ordered structures of polyhedral type takes place, with nanosizes of 30–50 nm. When dissolving the polymer, a solvatochromic effect is observed – a change in the color of the solution and the absorption spectrum depending on the nature of the solvent in the range λ = 310…900 nm. In the optical spectra of polyindole solutions, absorption bands are observed at λ = 390 and 510 nm (dioxane), λ = 390, 440 and 510 nm (DMF) and λ = 400, 510 and 540 nm (tetrachlormethane). The color of the solutions is markedly different, the solution in chloroform is pale pink, in DMF is yellow, in dioxane is light green. The electrical conductivity of the obtained polymer was experimentally determined, which turned out to be at 5 *10 -3 S /сm. According to the temperature dependence of the resistivity (in the coordinates of the Arrhenius equation), the activation energy of conductivity E a = 0.087 eV is determined, which indicates the semiconductor properties of the synthesized polymer. Keywords : polyindole, chemical synthesis, structure, solvatochromic effect, electrical conductivity.

Биодеградация ультраволокнистых материалов на основе смесей полигидроксибутирата и полилактида

Comparative studies of ozone and oxygen effects, hydrolytic destruction, biodegradation of nonwoven fibrous and film materials based on mixtures of natural polymers - polyhydroxybutyrate and polylactide are carried out in this work. Fibers were obtained from a solution in chloroform via electrospinning method. The films were formed by pouring a solution onto a glass substrate. The structure of materials was investigated by methods of electron and optical microscopy, DSC, EPR spectroscopy. It has been shown that materials enriched with polylactide have greater resistance to oxidation, low resistance to hydrolysis and UV irradiation, in comparison with polyhydroxybutyrate.

Unusual temperature dependence of the fluorescence decay in heterostructured stilbene.

Fluorescence spectra as well as the fluorescence decay kinetics of hot-pressed and sublimated films of stilbene have been studied in a wide temperature range, from 15 K up to room temperature. The fluorescence decay kinetics demonstrate unusual elongation of the excitation lifetime with a temperature increase. This is in contrast to the corresponding data of stilbene solutions in chloroform and in a polystyrene (PS) matrix. It is well known that the excitation dynamics of stilbene in solution and in a PS matrix is controlled by the molecular isomerization/twisting process of separate molecules. The data analysis and quantum chemistry calculations of stilbene aggregates suggest that the temperature dependence of the fluorescence kinetics of bulk stilbene solids can be explained by fast exciton diffusion, which yields a thermalized exciton distribution in a relatively small number of fluorescence centres. The temperature dependence of the distribution can thus explain the observed fluorescence decay lifetimes.