Influence of solvent polarity and temperature on dual fluorescence of 10,10′-dibromo, 9,9′-bianthryl

Fluorescence of 10,10′-dibromo, 9,9′-bianthryl (DBrBA) in solvents of different polarities (n-hexane, dibutyl ether, tetrahydrofuran, and acetone) has been investigated as a function of temperature. Changing of the solvent and variation of temperature modifies the ratio of local (LE) and charge transfer (CT) fluorescence quantum yields. From the basic fluorescence data (quantum yields, lifetimes, ratio of CT to LE fluorescence quantum yields) the temperature-dependent equilibrium constants for the charge transfer process in the excited singlet state were calculated and discussed on the basis of the modern electron transfer theories. It has been found that the intersystem crossing in DBrBA in nonpolar n-hexane, leading to the population of the lowest triplet state, proceeds via the third triplet state. It has been confirmed by the fluorescence measurements and quantum mechanical calculations. Surprisingly, the experimentally obtained intersystem crossing rate constants are very weakly dependent on temperature. Thus, the electron transfer reaction leading to the population of the molecular triplet state is probably an adiabatic reaction with a rate constant controlled by the dielectric relaxation of the solvent.     

Fluorescence Quenching Studies of 1,3‐Diphenyl Benzene

Fluorescence quenching of 1,3‐diphenyl benzene (m‐terphenyl) by carbon tetrachloride (CCl₄) at steady state in different solvents, namely n‐hexane, n‐heptane, cyclohexane, toluene, benzene acetonitrile, 1,4‐dioxane, and with a transient method in benzene has been done at room temperature to understand the role of quenching mechanisms. The Stern–Volmer plot was found to be linear for all the solvents studied. The probability of quenching per encounter p was determined in all the solvents and was found to be less than unity. Further, from the studies of rate parameters and lifetime measurements in benzene at different temperatures (30–60°C), it was shown that the phenomenon of quenching is generally governed by the well‐known Stern–Volmer (S‐V) plot. The activation energy E _a (E′_a) of quenching was determined using literature values of activation energy of diffusion E _d, and it was found to be greater than E _d, which confirms the fact that the quenching mechanism is not solely due to material diffusion but there is also contribution from activation energy.     

Influence of donor and acceptor groups on the S-T energy gap for thermally activated delayed fluorescence emitters

Molecules composed of different donors and acceptors are theoretically designed as potential thermally activated delayed fluorescence emitters, and their singlet–triplet (S-T) energy gap is studied using the optimal Hartree–Fork method. It is found that the S-T energy gap is in reverse proportional to the electron-donating ability. Stronger electron-donating ability of donors will induce smaller highest occupied molecular orbital–lowest unoccupied molecular orbital overlap and also a smaller S-T energy gap. Based on our calculation results, three molecules are proposed to have great potential to be used as thermally activated delayed fluorescence emitters in organic light-emitting diodes.     

Magnetic field dependence of vortex activation energy: A comparison between MgB2, NbSe2 and Bi2Sr2Ca2Cu3O10 superconductors

The dissipative mechanism at low current density is compared in three different classes of superconductors. This is achieved by measuring the resistance as a function of temperature and magnetic field in clean polycrystalline samples of NbSe₂, MgB₂ and Bi₂Sr₂Ca₂Cu₃O₁₀ (BSCCO) superconductors. Thermally activated flux flow behaviour is seen in all the three systems and clearly identified in bulk MgB₂. While the activation energy at low fields for MgB₂ is comparable to Bi₂Sr₂Ca₂Cu₃O₁₀, its field dependence follows a parabolic behaviour unlike a power-law dependence seen in Bi₂Sr₂Ca₂Cu₃O₁₀. We analyse our results based on Kramer’s scaling for grain boundary pinning in MgB₂ and NbSe₂.      

Energy,fluence and temperature dependence of MeV nitrogen implantation profiles in steel

Abstract

1 to 2 MeV nitrogen (N⁺ and N₂ ⁺) ions were implanted at high fluences in stainless steel, and their depth distributions were measured subsequently by Rutherford backscattering and thermal neutron depth profiling. The range profiles were broader than theoretically expected. With increasing fluence, deviations from ballistic computer codes increase. These deviations can well be described by the assumption of radiation enhanced diffusion for which a simple analytical model is presented. The thermal mobility shows a different behavior for low, and for high implanted fluences.