Fluorescence of poly(di-n-alkylsilane)s in room-temperature solution

A comprehensive study of the electronic absorption and emission spectra and the fluorescence quantum yield and lifetime of seven poly(di-n-alkylsilane)s and of three isotopically labelled poly(di-n-hexylsilane)s in hydrocarbon solution at room temperature is reported. Also reported are fluorescence polarization and carbon tetrachloride quenching of fluorescence of poly(di-n-hexylsilane). The observed fluorescence spectra, quantum yield, and polarization depend on the selected excitation energy in a very characteristic fashion, whereas the fluorescence lifetime does not; however, it depends on the selected emission energy. These characteristic dependencies are qualitatively accounted for by the previously proposed segment distribution model if one assumes that the photophysical behavior at higher excitation energies is strongly affected by the presence of a low-lying weakly allowed state in short-segment chromophores and the behavior at lower excitation energies is dictated by the selective excitation of emitting long-segment chromophores.     

Luminescence and photo-oxidation of commercial poly(4-methylpent-1-ene)

The fluorescence and phosphorescence excitation and emission spectra of commercial poly(4-methylpent-1-ene) are examined using a fully compensated spectrofluorometer. The excitation spectra of the polymer are compared with the absorption spectra of model chromophores of those believed to be present in the polymer. The fluorescence emission is primarily associated with the presence of enone and the phosphorescence with dienone impurity chromophoric units. Bromination of cold hexane extracts of the polymer significantly reduces the intensity of the fluorescence, confirming the presence of ethylenic unsaturation. The behavior of the luminescent enone and dienone groups during irradiation under sunlight-simulated conditions is also examined. Possible mechanisms for the participation of these chromophoric units in the photo-oxidation of poly(4-methylpent-1-ene) are discussed.     

Excitation Dependent Fluorescence Quantum Yield in Hydrocarbon Fuels Containing Polycyclic Aromatic Compounds

Fluorescence emission spectra and quantum yields have been measured for neat sample of six different hydrocarbon fuels over different possible excitation wavelengths. Hydrocarbon fuels show different excitation fluorescence cutoff depending on their commercial use and applications in the UV ～ visible region. The fluorescence emission maximum of these fuels shows a red shift with excitation wavelength. Resonance energy transfer and self-quenching through solvent collision of fluorophores play an important role on the fluorescence characteristics of multifluorophoric mixture like petroleum fuels at higher concentration. The fluorescence quantum yield of these fuels is a function of the excitation wavelength and is different for different fuels. The change in quantum yield with excitation wavelength for petroleum fuels is different from that of crude oils. The change in fluorescence quantum yield at lower excitation wavelength provides a promising tool to estimate contamination of diesel and petrol by kerosene.     

Photoyellowing of milled Wood Lignin and Peroxide-Bleached Milled Wood Lignin in Solid 2-Hydroxypropylcellulose Films After Sodium Borohydride Reduction and Catalytic Hydrogenation in Solution: a Fluorescence Spectroscopic

Fluorescence spectra and emission quantum yields of 2-hydroxypropyl-cellulose films, incorporating milled wood lignin that had been treated in solution by HgOg/NaOH and/or NaBfy and/or H₂ (Pd/C), were measured before and after irradiation by UV light (λ> 300 nm). Bleaching, reduction (NaBH₄), and hydrogenation (H₂, Pd/C) increase the quantum yield of fluorescence and emission in the blue region (400 nm). The destruction of carbonyl chromophores (α-carbonyl, coniferaldehyde, and quinones), which quench the fluorescence of biphenyl groups, the main structures emitting in this part of the spectrum, appeared to be mainly responsible for this increase. Irradiation restores i) emission in the long wavelength part of the spectra (maximum emission: 500 nm, maximum excitation: 400 nm), and ii) quenching of the fluorescence in the blue part (400 nm) of the spectra, even for hydrogenated films. These results are interpretated in relation to the formation, under irradiation, of coniferaldehyde groups and also the generation of complex structures formed by photooxidation of phenolic biphenyls.     

Extremely efficient photoisomerization of water-soluble diphenylbutadiene dendrimers

A water-soluble diphenylbutadiene-cored poly(aryl ether) dendrimer, together with a corresponding lipophilic diphenylbutadiene dendrimer and a standard compound, (1E,3E)-1,4-bis(3,5-dimethoxyphenyl)buta-1,3-diene, were prepared and their photochemical properties were examined. The water-soluble dendrimer in aqueous solution exhibited an extremely high photoisomerization quantum yield (Φ_iso = 0.64), whereas the quantum yield of the bis(3,5-dimethoxyphenyl)butadiene reference compound in THF was much lower (Φ_iso = 0.26). The excitation spectra of the water-soluble dendrimer strongly depended on the emission wavelength and the fluorescence lifetime contained several components. These results indicate that the core unit in the water-soluble dendrimer adopts multiple conformations, one of which is distorted and can undergo photoisomerization quite easily.     

Chain organization and photophysics of conjugated polymer in poor solvents: Aggregates, agglomerates and collapsed coils

UV/vis absorption, excitation, and emission spectroscopies are utilized to explore the association behavior of conjugated polymer, poly[2-methoxy, 5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV), in a homologous series of linear alcohols and solvent-nonsolvent systems. We demonstrate that the aggregation, agglomeration, and collapse of isolated chains, which all take place in the poor solvents, exhibit different optical signatures. The aggregate, in which chromophores interact electronically, causes a red shift of absorption and emission spectra while the weak interaction of chromophores in the agglomerates leaves electronic properties unaltered. The collapse of isolated chain is accompanied by a blue shift of the spectra. Energy transfer from excited chromophores of non-aggregated chains to the lower energy aggregates is significantly diminished in the system of alcohol solvents, allowing separate emissions from various chromophores to occur. The optical signatures and extent of energy transfer are used as tools to elucidate the association mechanism and chain organization of isolated chains into large particles upon decreasing solvent quality. The extreme collapse of isolated chains in the very poor solvents is found to inhibit the chain organization into aggregates.     

The fluorescence of dihydroxyphenylalanine: the effects of protonation-deprotonation

The fluorescence excitation and emission spectra and quantum yields of 3,4-dihydroxyphenylalanine (DOPA), its methyl ester (DOPAM) and the N -acetylated derivative (DOPANA) in aqueous solution at pH values ranging from 3.0 to 10.0 have been determined and the excitation spectra related to their absorption spectra. The fluorescence quantum yield of the phenolic form of DOPA is 0.09. However, the fluorescence yield is substantially reduced for the conjugate base forms, i.e. deprotonation of the amine group and the phenolic hydroxyl group. The fluorescence spectra are dominated by the phenolic forms of DOPA, DOPAM and DOPANA from pH 3.0 to 10.0 with excitation and emission spectral maxima at 280 and 315 nm respectively. The excitation maxima for all the compounds correspond to the absorption maxima of their phenolic forms. The phenolate anions of DOPA and DOPAM exhibit weak fluorescence with emission maxima at 325 and 330 nm respectively.     

In situ study of nanostructure and morphological development during the crystal-mesophase transition of poly(di-n-hexylsilane) and poly(di-n-butylsilane) by X-ray and hot-stage AFM

Nanostructure and morphology and their development of poly(di-n-hexylsilane) (PDHS) and poly(di-n-butylsilane) (PDBS) during the crystal-mesophase transition are investigated using small angle X-ray scattering (SAXS), wide angle X-ray diffraction and hot-stage atomic force microscopy. At room temperature, PDHS consists of stacks of lamellae separated by mesophase layers, which can be well accounted using an ideal two-phase model. During the crystal-mesophase transition, obvious morphological changes are observed due to the marked changes in main chain conformation and intermolecular distances between crystalline phase and mesophase. In contrast to PDHS, the lamellae in PDBS barely show anisotropy in dimensions at room temperature. The nonperiodic structure and rather small electronic density fluctuation in PDBS lead to the much weak SAXS. The nonperiodic structure is preserved during the crystal-mesophase transition because of the similarity of main chain conformation and intermolecular distances between crystalline phase and mesophase.     

Synthesis,Characterization and Fluorescence Properties of Poly(styrene-co-n-caprylamide maleic acid) and Its Europium(III) and Terbium(III) Complexes

Copolymer of poly(styrene-co-n-caprylamide maleic acid) (PSCMA, defined as HL) and its lanthanide complexes Ln(L)₃·6H₂O (Ln = Eu and Tb) had been synthesized and characterized by elemental analysis, X-ray diffraction, Fourier transform infrared spectra, UV-spectrophotometer and thermal analysis (TG–DTA). The fluorescence properties of the HL ligand and the Ln(L)₃·6H₂O complexes in the solid state were investigated. At room temperature, the HL ligand had a strong broad emission band at 410–575 nm (λmax = 458 nm) under excitation at 380 nm, while the respective characteristic emission of Eu(III) and Tb(III) ions was observed in Ln(L)₃·6H₂O complexes. This demonstrated that the HL ligand in the extra-framework channels succeeded in sensitizing Eu(III) and Tb(III) ions emission. Compared with the Eu(L)₃·6H₂O complex, the fluorescence intensity of the Tb(L)₃·6H₂O complex was much stronger. This indicated that the lowest excited triplet state energy level of HL matched well with the excited state energy level of Tb(III). With the increase of the Ln(III) ions content below 15 wt%, the fluorescence intensity increased monotonically. All the Ln(L)₃·6H₂O complexes exhibited high quantum yield, long fluorescence lifetime and good thermal stability.     

Fluorescence spectra of poly(di-n-hexylsilane)/TiO2 nanoparticle hybrid film

The interaction between poly(di-n-hexylsilane) (PDHS) and TiO₂ nanoparticle was studied based on the temperature dependence of the fluorescence of a PDHS/TiO₂ nanoparticle hybrid film. The polysilane is a suitable probe to investigate a guest polymer-host matrix interaction because the photophysical properties of polysilanes remarkably depend on the conformation of the σ-conjugated Si-Si chain. The PDHS/TiO₂ nanoparticle hybrid film showed a fluorescence band assigned to a disordered structure even at 80 K whereas only the fluorescence band of an ordered structure was observed for the PDHS film at 80 K. The disordering of the Si-Si main chain was explained by the perturbation of the n-hexyl side chain in the neighborhood of the TiO₂ nanosurface. The non-radiative deactivation of the excited state via the disorder-induced local potential minima was suggested by the temperature dependence of the fluorescence intensities of the disordered and ordered structures in the temperature region from 80 to 160 K.     

Highly emissive PEG-encapsulated conjugated polymer nanoparticles

A novel bioimaging probe based on a conjugated polymer, poly(9,9-dihexylfluorene-alt-2,1,3-benzoxadiazole) (PFBD), is demonstrated. Transfer of the hydrophobic polymer into water using a short chain poly(ethylene glycol) (PEG) resulted in conjugated polymer nanoparticles (PEG-PFBD) with a fluorescence quantum yield of 46%. The PEG-PFBD nanoparticles possessed several desirable structural and photophysical properties, such as colloidal stability in a broad range of pH values, sub-20 nm particle size, the presence of surface chemical functionality, as well as desirable excitation and emission spectra, for bioimaging applications. PEG-PFBD nanoparticles were conjugated with cyclic RGDfK targeting peptide for labeling of membrane α(V)β(3) integrin receptors on live HT-29 adenocarcinoma cells. Single nanoparticle microscopy revealed that the PEG-capped PFBD nanoparticles exhibit at least ten times higher emitted photon counts than single quantum dots (QD655) of comparable size. In addition, Fluorescence Lifetime Imaging Microscopy (FLIM) of single PEG-PFBD nanoparticles revealed that the nanoparticles display a clearly resolvable single nanoparticle fluorescence lifetime.     

Origin and role of the luminescent species in the photo-oxidation of commercial polypropylene

The fluorescence and phosphorescence excitation and emission spectra of commercial polypropylene have been examined using a fully compensated spectrofluorometer. This has made possible the comparison of the excitation spectra of the polymer with model chromophores of those believed to be present in the polymer. The fluorescence emission is primarily associated with the presence of enone, and the phosphorescence with dienone chromophoric units. The behavior of the luminescent enones and dienones during irradiation under sunlight-simulated conditions has also been examined. Possible mechanisms for the participation of these chromophoric units in the photo-oxidation of polypropylene are discussed.     

Fabrication of fluorescent poly(l‐lactide‐co‐caprolactone) fibers with quantum‐dot incorporation from emulsion electrospinning for chloramphenicol detection

The on‐site rapid detection of antibiotic residues deposited in food or beverage still remains a challenge in daily life. In this study, cadmium tellurium (CdTe) quantum dots (QDs) were incorporated into poly(l ‐lactide‐co‐caprolactone) (PLLACL) fibers with emulsion electrospinning. Water‐soluble CdTe QDs were used as fluorescence agents, and PLLACL was used as filament materials, respectively. A variety of experiments were performed to characterize the structure and properties of the fibrous composite. Ultraviolet–visible and photoluminescence spectra of the fibers showed similar characteristic absorption and emission properties to those of the CdTe QDs. The fibrous QD–PLLACL composite showed stable fluorescence over 30 days at room temperature and could be used to detect chloramphenicol through fluorescence quenching caused by resonance energy transfer. This approach provides a facile shortcut for fabricating fluorescent fibers that simultaneously inherit the mechanical behavior of PLLACL fibers and the fluorescence properties of CdTe QDs for the detection of antibiotics. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44584.     

Phosphorescence emission from poly(di-n-hexylsilane) in a glass: Temperature dependence and optically detected magnetic resonance (ODMR) spectra

The low-temperature steady-state phosphorescence spectrum of poly(di-n-hexylsilane) (PDHS) in a 3-methylpentane (3MP) glass at 1.5 K has been measured. The temperature dependence of the phosphorescence decay was determined and zero-field optically detected magnetic resonance (ODMR) experiments were performed. Evidence of at least two distinct emitting species was found. A narrow phosphorescence spectrum with no distinct vibronic structure was observed for PDHS and interpreted as indicating a delocalized triplet state. The ODMR spectrum is dependent on the detection wavelength, indicating that the triplet splittings are sensitive to the conformation and local environment of the polysilane chains. The phosphorescence disappears completely above 30 K. This is attributed to the presence of a dissociative triplet state.     

Improved photoluminescence quantum yield of CsPbBr3 quantum dots glass ceramics

We have fabricated CsPbBr₃ perovskite quantum dots (QDs) in a multi-component borate glass by melt-quenching technique. Transmission electron microscopy (TEM) reveals a cubic phase CsPbBr₃ crystal for QDs. As the treatment temperature or the treatment time duration increases, the photoluminescence (PL) peak shifts to long wavelength in the range of 510 to 525 nm, and the full width at half-maximum varies in the range of 24 to 18 nm. The absorption edge shifts to low energy side in the range of 2.54 to 2.41 eV. The different photoluminescence excitation spectra (PLE) reflect the change of microstructure for different samples. The PL peak wavelength and line-shape are independent of excitation wavelength. These results of spectra show typical exciton emission characteristics. As treatment conditions strengthens, photoluminescence quantum yield (PLQY) first increases and then decreases, having the best PLQY 86.9%. Bi-exponential fitting curves show that short lifetime τ₁ continuously decreases. Long lifetime τ₂, weight for long lifetime component, and average lifetime τ_avg first increase and then decrease. The PLQY values are affected by both τ₁ and τ₂, which are relative to the crystal quality in the interior and the surface of QDs, respectively. The high PLQY value corresponds to medium treatment condition, which is attributed to a balanced effect of crystal quality in interior and the surface of QDs.     

Effects of absorption overlap on the modulation of the polarized fluorescence of Eu(DBM)<Subscript>3</Subscript>Phen-doped azobenzene polymer films

Two azopolymers, poly(4-(4′-nitrophenylazo)phenoxy)hexyl methacrylate (PNAzoPOH) and poly(6-(N-methyl-N-(4-(4′-nitrophenylazo)phenyl)amino)hexyl methacrylate) (PNAzoPAH), bearing strong push–pull azobenzene chromophores, have been designed and synthesized. Azopolymer films doped with a europium complex, tris(dibenzoylmethanido)(o-phenanthroline)europium(III) (Eu(DBM)₃Phen), were prepared by a drop-casting method. UV-vis spectra indicated that the absorption peak of Eu(DBM)₃Phen at 365 nm overlaps with that of PNAzoPOH, rather than that of PNAzoPAH. Polarized fluorescence from both of the oriented films was observed to depend on the absorption overlap between the complex and the azopolymer. When excited by unpolarized light at 365 nm, the polarization ratio R of Eu(DBM)₃Phen-doped PNAzoPOH film shows an obvious decreasing trend with increasing irradiation time, and the ratio R of Eu(DBM)₃Phen-doped PNAzoPAH film has a relative stable value of about 1.47. When excited by polarized light at 365 nm, the florescence intensities from the two films show different dependences upon the angle between the direction of orientation of the film and the direction of polarization of the excitation light: for Eu(DBM)₃Phen-doped PNAzoPOH film, the strength of the polarized fluorescence depended on the direction of polarization of the excitation light; for Eu(DBM)₃Phen-doped PNAzoPAH film, no such dependence was observed. These different results arise due to differences between the azopolymers in terms of the allocation of energy between the oriented azobenzene groups and the complex.     

Emission and Absorption Cross Sections at 810 and 860 nm Bands of Er3+ in LiNbO3 Crystal

We have measured the unpolarized and polarized emission spectra of ⁴I_9/2→⁴I_15/2 (810 nm) and ⁴S_3/2→⁴I_13/2 (860 nm) electronic transitions of Er³⁺ in LiNbO₃ crystal under different incident directions and polarization states of excitation beam. From the measured emission spectra, the emission and absorption cross‐section spectra were calculated based upon McCumber theory. It is found that Er³⁺ electronic transition shows interesting excitation beam direction effect in polarization dependence, spectral shape, and cross‐section value. Both transitions are highly π‐polarized as the excitation beam was aligned perpendicular to the optical axis of crystal while being highly σ‐polarized as the excitation beam was oriented parallel to the optical axis of crystal. The spectral shape in the case of the perpendicular excitation is very different from that in the case of parallel excitation. The cross‐section value in the perpendicular excitation case is at least 1.5 times larger than that in the parallel excitation case. These excitation direction effects are independent of the polarization state of excitation light, and are attributed to the selective Er³⁺ site excitation. In addition, the Er³⁺ 860 nm emission lifetime was measured to be 27 ± 5 μs and the quantum efficiency of the emission is 2.5%.     

Studies of the antenna effect in polymer molecules V. Determination of the extent of singlet energy migration in a phenanthrene — Containing polymer by fluorescence quenching

A modification of a Stern-Volmer fluorescence quenching experiment was used to determine the extent of singlet energy migration between phenanthrene chromophores in poly(9-phenanthrylmethyl methacrylate). This system avoids the problems of nonexponential fluorescence decay, inhomogeneous quencher concentrations and complication by the formation of excimers. The contribution to the distance travelled by the singlet excitation during the excited state lifetime as a result of energy migration was shown to correspond to about a third of the distance diffused by an oxygen molecule during the same time.     

Radical copolymerization of sulphur dioxide and styrene: 4. Intramolecular excimer formation in poly(styrene sulphone)s

Intramolecular excimer formation in poly(styrene sulphone)s with various compositions has been investigated based on the characteristic monomer sequence distributions which were determined experimentally. The fluorescence spectra of the poly(styrene sulphone)s show two emission bands at 285 nm and 330 nm as for polystyrene, corresponding to the monomer and the excimer bands, respectively. The ratio of the excimer to the monomer emission intensities (I_e/I_m) is linearly correlated with the mole fraction of styrene. This observation is a consequence of the characteristic sequence distributions in poly(styrene sulphone)s. containing a very small fraction of SMS units (S = SO₂; M = styrene). The efficiency of excimer formation in poly(styrene sulphone)s with regular styrene sequences, e.g. dyad sequence only, was calculated from the sequence distribution and empirical I_e/I_m. It is concluded that the excimer formation in poly(styrene sulphone)s is very efficient. Energy migration along the copolymer chain was also demonstrated by measuring the degree of polarization as a function of copolymer composition. The efficient excimer formation and the depolarization in poly(styrene sulphone)s suggest that the SO₂ unit in poly(styrene sulphone)s does not act as a barrier or trap to energy migration unlike the methyl methacrylate unit.     

Genetically Encoded FRET-Sensor Based on Terbium Chelate and Red Fluorescent Protein for Detection of Caspase-3 Activity

This article describes the genetically encoded caspase-3 FRET-sensor based on the terbium-binding peptide, cleavable linker with caspase-3 recognition site, and red fluorescent protein TagRFP. The engineered construction performs two induction-resonance energy transfer processes: from tryptophan of the terbium-binding peptide to Tb³⁺ and from sensitized Tb³⁺ to acceptor—the chromophore of TagRFP. Long-lived terbium-sensitized emission (microseconds), pulse excitation source, and time-resolved detection were utilized to eliminate directly excited TagRFP fluorescence and background cellular autofluorescence, which lasts a fraction of nanosecond, and thus to improve sensitivity of analyses. Furthermore the technique facilitates selective detection of fluorescence, induced by uncleaved acceptor emission. For the first time it was shown that fluorescence resonance energy transfer between sensitized terbium and TagRFP in the engineered construction can be studied via detection of microsecond TagRFP fluorescence intensities. The lifetime and distance distribution between donor and acceptor were calculated using molecular dynamics simulation. Using this data, quantum yield of terbium ions with binding peptide was estimated.