Magnetic field dependence of monomer and excimer delayed fluorescence of aromatics in fluid solution

We have extended our study of the magnetic field dependence of monomer (M_M) and excimer (I_D) delayed fluorescence of aromatics in fluid solution. I_M and I_D show a monotonic decrease with increasing field strength at room temperature in agreement with theoretical predictions. At lower temperatures both I_M and I_D show an initial increase followed by a monotonic decrease. This marked temperature dependence can be attributed to a “freezing-out” of the effect of molecular motion on the field effect. At even lower temperatures the field effect curves of I_M and I_D start to diverge. Two models have been proposed to explain the mechanism of excimer formation in triplet—triplet annihilation. Qualitatively at least the divergence can be rationalized in terms of both models.     

Study of properties on non-protected room temperature phosphorescence and delayed excimer fluorescence of pyrene solution

A strong and stable room temperature phosphorescence (RTP) and delayed excimer fluorescence signal located at 596 and 475 nm, respectively, can be induced for pyrene solution in the absence of any protective medium only use KI or TlNO3 as a heavy atom perturber (HAP) and Na2SO3 as a deoxygenator. Both lifetimes of RTP and the delayed fluorescence are in the order of X-ms and the intensities are changed with kind and amount of HAP, but the peak positions are same and there is a iso-luminescent point in the emission spectra corresponding to emission at 475 nm and at 596 nm. The optimum conditions and the effects of kind and amount of HAP and organic solvents on luminescence properties of pyrene solution were studies in detail, and the photophysical process in the presence of KI or TlNO3 for phosphorescence and delayed excimer fluorescence emission of pyrene solution was discussed.     

Unexpected sense dependence of the effect of a magnetic field on delayed fluorescence

Delayed fluorescence intensity from pyrene and methylene phenanthrene crystals alters when an applied, steady magnetic field is reversed. This sense dependence vanishes at high fields. It is maximum along the pyrene two-fold axis and zero in the mirror plane.     

Magnetic field-induced polarization inversion in delayed fluorescence

It is shown that a recent theory for the magnetic field dependence of delayed fluorescence radiation of aromatic hydrocarbons in liquid solutions leads to the theoretical prediction of a new effect according to which the polarized intensity branches of the relative field effect exhibit a crossover at a certain magnetic field H_c. As a general feature, the branch polarized parallel to the external field H lies above the branch polarized perpendicularly for H < H_c whereas this order is reversed for H >H_c. The magnitude of the polarization splitting may become quite large at sufficiently low temperatures and will easily be measurable experimentally, particularly at higher fields.     

External electric field effects on fluorescence of pyrene butyric acid in a polymer film: concentration dependence and temperature dependence

Fluorescence spectra and electrofluorescence spectra (plots of the electric field-induced change in fluorescence intensity as a function of wavelength) have been measured at different temperatures for pyrene butyric acid (PBA) in a PMMA film at different concentrations. At a low concentration of 0.5 mol % where fluorescence emitted from the locally excited state of PBA (LE fluorescence) is dominant, LE fluorescence spectra show only the Stark shift in the presence of an electric field (F), which results from the difference in molecular polarizability between the ground and emitting states. At a high concentration of 10 mol % where the so-called sandwich-type excimer fluorescence (EX(1)) is dominant, both EX(1) and LE fluorescence are quenched by F. Another fluorescence assigned to a partially overlapped excimer (EX(2)) also exists at room temperature, and this emission is enhanced by F. As the temperature decreases, three fluorescence emissions whose electric field effects are different from each other become clear besides EX(1) and LE fluorescence, indicating that at least five fluorescence components exist at high concentrations at low temperatures. At a medium concentration of 5 mol % where EX(1) is comparable in intensity to the LE fluorescence, the intensity of EX(1) is not affected by F at any temperature, but LE fluorescence and EX(2) are markedly influenced by F at room temperature, and four fluorescence emissions are confirmed at low temperatures.     

Intramolecular excimer formation and delayed fluorescence in sterically constrained pyrene dimers

The synthesis is described for a series of five molecular dyads comprising pyrene-based terminals covalently linked through a 1,3-disubstituted phenylene spacer. The extent of through-space communication between the pyrene units is modulated by steric interactions imposed by bulky moieties attached at the 6,8-positions of each pyrene unit. For the control compound, only hydrogen atoms occupy the 6,8 positions (DP1), whereas the remaining compounds incorporate ethynylene groups terminated with either triisopropylsilyl (DP2), 1-tert-butylbenzene (DP3), 2,6-di-tert-butylbenzene (DP4) or 1-tert-butyl-3,5-dimethylbenzene (DP5) units. Each compound shows a mixture of monomer and excimer fluorescence in fluid solution at room temperature, but only monomer emission in a glassy matrix at 77 K. The ratio of monomer to excimer fluorescence depends markedly on the molecular structure; DP1 is heavily biased in favour of the excimer and DP4 is enriched with monomer fluorescence. Photophysical properties, including laser induced and delayed fluorescence data, are reported for each compound. Delayed fluorescence occurs by both intramolecular and bimolecular steps, but these events take place on different timescales. The possibility is raised for using intramolecular triplet-triplet annihilation as a means of molecular imaging.     

Probing solution behaviour of metallosupramolecular complexes using pyrene fluorescence

A new method for assessing the topology of metallosupramolecular assemblies using pyrene-appended ligands is reported. Two potentially tetradentate ligands containing one (L(1)) and two (L(2)) terminal pyrene moieties were synthesised and their complexes with Cu(+) and Cd(2+) were characterised. Photophysical measurements demonstrate that in [Cu(2)(L(1))(2)](2+), [CdL(1)](2+) and [Cu(2)(L(2))(2)](2+) the emission spectra are dominated by monomeric emission but in the cadmium complex of L(2) (where the pyrene units are in close proximity) a quenching of the luminescence coupled with weak emission at 540 nm is indicative of excimer formation.     

十二烷基磺酸钠增强的吡啶锉 对芘的荧光猝灭

水溶液中三种吡啶 盐(吡啶盐酸盐, HP+;N-苄基吡啶,BP^+; 苄基紫精, BV^2^+)对芘的荧光猝灭因十二烷基磺酸钠(SLS)的引入而增强, 且猝灭常数对SLS浓度的敏感性BV^2^+>BP^+>HP^+, 电导实验表明体系中无簇集体形成。认为SLS与吡啶 的静电作用及表面活性剂分子中烷基链的绕曲是导致猝灭增强的原因。     

Polymer conformation via magnetic field modulation of delayed fluorescence

Magnetic-field modulation of delayed fluorescence from poly(2-vinylnaphthalen) in a low-temperature glass at 77 K is reported. The observations are interpreted in terms of local ordering of the naphthalene chromophores with their normal axes aligned approximately parallel to one another.     

Magnetic field effect on the fluorescence of glyoxal

Fluorescence from the ¹A_u state of glyoxal is quenched by a magnetic field and the pre-exponential factor of phosphorescence from the ³A_u state increases with increasing magnetic field. These findings are explained by the enhancement of intramolecular intersystem crossing by the magnetic field.     

Non-quadratic-intensity dependence of two-photon absorption induced fluorescence of organic chromophores in solution

We have provided a model to interpret the non-quadratic-intensity dependence behavior commonly observed in the two-photon fluorescence (TPF) experiment excited with high laser intensity. The model also provides one with a different technique to measure the two-photon absorption cross section of an organic chromophore in solution. In contrast to the commonly used low intensity technique that depends on the quadratic-intensity law, the present technique is based on the non-quadratic-intensity dependence of two-photon fluorescence. Auxiliary data such as two-photon quantum efficiency and fluorescence collection efficiency, essential in the low intensity method, are not required in the present technique. TPF measurements of Rhodamine B in methanol are carried out to demonstrate the validity of the present method. The method is used to determine the two-photon absorption cross section of a new chromophore attached with tricyano-derivatized furan as the electron acceptor. The two-photon absorption cross section measured using this method is also compared with that using a conventional transmission technique.     

New approaches to characterize polymeric oil additives in solution based on pyrene excimer fluorescence

Maleated polyolefins (MaPOs) such as maleated ethylene propylene copolymers or polyisobutylene terminated at one end with a succininic anhydride can be used as polymeric dispersants in engine oils after reaction with polyamines while unmodified EP copolymers improve the viscosity index of oils. MaPOs can also be labeled with pyrene derivatives to generate pyrene‐labeled polyolefins (PyLPOs) as fluorescent mimics of oil additives and pyrene excimer fluorescence (PEF) can be applied to probe their behavior in solution. This review describes new methodology that was recently implemented to characterize the complex fluorescence signal emitted by PyLPOs by using steady‐state and time‐resolved fluorescence. This methodology enables one to gain quantitative information about the level of clustering of the succinic pendants along a maleated polyolefins and intra‐ and intermolecular aggregation of polyolefins in solution. Such information is relevant to scientists aiming to characterize polymeric oil additives used in engine oil. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 7–18     

Magnetic field effects on the fluorescence of intramolecular electron-donor-acceptor systems

The external magnetic field effects on the exciplex fluorescence of α-(4-dimethylaminophenyl)-ω-(9-phenanthryl)alkanes have been studied by photostationary, time-resolved, and magnetic field modulation fluorescence spectroscopy. The singlet-triplet degeneracy of the radical ion pair is suggested to occur at a methylene chain length of about ten.     

Influence of a magnetic field on delayed fluorescence of aromatic hydrocarbons in solution: I. Dependence on temperature,solvent and solute

The relative magnetic field effects on the total triplet—triplet annihilation (TTA) rate constant, on the rate constant for production of a singlet monomer and on the rate constant for production of a singlet excimer have been measured in a magnetic field range from 0 to 6000 gauss for the hydrocarbons pyrene, 3,4-benzopyrene, 1,2-benzanthracene and phenanthrene in solvents of different polarity between room temperature and 120 K. A qualitative discussion of the experimental results yields the following information on the mechanism of TTA: (i) The ratio of singlet to triplet products decreases with decreasing temperature or increasing viscosity of the solvent. (ii) The magnetic field effect depends much more on viscosity than on temperature. (iii) Singlet monomers and excimers are predominantly formed from different initial triplet—triplet pair configurations. (iv) Ionic radical pair states do not seem to play an important role in the TTA mechanism between equal molecules.     

Magnetic field dependence of electron spin polarization generated through radical-triplet interactions

The magnetic field dependence of electron spin polarization (ESP), generated in free radicals when they encounter photoexcited triplets, was measured experimentally and analyzed theoretically. The time-resolved electron paramagnetic resonance measurements were performed with a microwave setup consisting of low-loss dielectric ring resonators with tunable microwave frequencies and the corresponding magnetic fields. The ESP of the radical was found in the magnetic field range of 170-370 mT, and the results of the calculation based on the numerical solution of the stochastic Liouville equation were found to be in line with the experimental data showing that ESP decreases when the magnetic field increases.     

Magnetic field effect on stainless steel corrosion in FeCl3 solution

Corrosion of AISI 303 stainless steel in FeCl₃ solution was studied in the magnetic field, whose direction was perpendicular to the corroding surface. The magnetic field inhibited corrosion in both quiet and stirred solutions. This was evident from the increased repassivation potential, the reduced number of pits and the decreased mass loss. By contrast, an accelerating effect of the magnetic field was observed on the single cathodic reaction of the corrosion process, viz. Fe(III) reduction to Fe(II). This was confirmed by voltammetric measurements on both platinum and stainless steel electrodes. The corrosion magnetoinhibition was explained in terms of field-assisted development of a passive layer, whose passivation capacity was higher than that under the field-free conditions.     

Temperature dependence of intersystem crossing of pyrene

The temperature dependence of the rate constant for intersystem crossing from S₁ of pyrene was determined by means of a method proposed previously. In contrast to the conclusions of Stevens, Thomaz and Jones, and of Kropp, Dawson and Windsor, we conclude that both temperature-independent and temperature-dependent intersystem crossing processes exist. These two processes contribute equally at ≈ 150 K.     

Theory of delayed thermal fluorescence

A theory of nonradiative thermal activation involved in delayed thermal fluorescence has been developed from the viewpoint of the breakdown of the Born-Oppenheimer adiabatic approximation.Supported by the National Science Foundation and ACS Petroleum Research Fund.John Simon Guggenheim Fellow.     

Magnetic field dependence of spatial frequency encoding NMR as probed on an oligosaccharide

The magnetic field dependence of spatial frequency encoding NMR techniques is addressed through a detailed analysis of ¹H NMR spectra acquired under spatial frequency encoding on an oligomeric saccharide sample. In particular, the influence of the strength of the static magnetic field on spectral and spatial resolutions that are key features of this method is investigated. For this purpose, we report the acquisition of correlation experiments implementing broadband homodecoupling or J‐edited spin evolutions, and we discuss the resolution enhancements that are provided by these techniques at two different magnetic fields. We show that performing these experiments at higher field improves the performance of high resolution NMR techniques based on a spatial frequency encoding. The significant resolution enhancements observed on the correlation spectra acquired at very high field make them valuable analytical tools that are suitable for the assignment of ¹H chemical shifts and scalar couplings in molecules with highly crowded spectrum such as carbohydrates. Copyright © 2015 John Wiley & Sons, Ltd.     

Magnetic field dependence of the product yields of cycloheptanone photolysis in the dilute gas phase

The product yields of the gas-phase photolysis of cycloheptanone were measured in magnetic fields up to 10 kG. The magnetic-field effect is explained in terms of the radical pair model. A reaction mechanism is proposed that explains the magnetic field dependence, pressure dependence, and excitation enegy dependence of the gas-phase photochemistry of cycloheptanone.