SPECTROSCOPY AND PHOTOCHEMISTRY OF 8-SUBSTITUTED 5-DEAZAFLAVINS

Abstract— Absorption, fluorescence and phosphorescence spectra as well as fluorescence and phosphorescence quantum yields of 8-X-5-deazaflavins (X = C1, NO₂, p -NO₂-C₆H₄, N(CH₃)₂, NH₂, p -NH₂-C₆H₄, p -N(CH₃)₂-C₆H₄-N=N) were determined. It was found that all these data are highly influenced by the substituent at position 8 of the 5-deazaisoalloxazine skeleton. Also the photoreduction of 8-X-5-deazaflavins in the presence of electron donors was studied. It was established that the photoreduction leads to the formation of a 5,5'-dimer and/or a 6,7-dihydro compound. Reduction of the C(6)-C(7) bond is promoted by strong electron-donating substituents and bulky electron donors. 5-Deazaftavins with a reducible substituent at position 8 exhibit reduction of the substituent prior to the reduction of the 5-deazaisoalloxazine skeleton.     

Particle tunnelling in the 7-azaindole dimer

A model has been developed that attempts to calculate the particle (proton and deuteron) tunnelling probability in the molecular dimer of 7-azaindole.     

Using Lasers to Demonstrate the Concept of Polarizability: Variations in the Refractive Indices of the O-Halobenzenes

Using a simple 633-nm He/Ne or diode laser, the variation of the refractive indices of four ortho-substituted dihalogenated benzenes can be obtained, demonstrating the phenomenon of polarizability. In addition, this exercise is an excellent display of the correspondence between the properties of a chemical system on the macroscopic level and the characteristics of the molecule on a microscopic scale.     

Synthesis, characterization and magnetism of μ-chloranilato bi-nuclear chromium (III) complexes

Five new chloranilato-bridged binuclear chromium (III) complexes have been synthesized and identified as [Cr2(CA)L4]-(ClO4)4[L denotes 5-methyl-1,10-phenanthroline (Me-phen); 2,9-dimethyl-1, 10-phenanthroline ( Me2-phen); 5-chloro-1,10-phenanthroline(Cl-phen); diaminoethane (en) or 1,3-diaminopropane (pn)], where CA represents the dianion of chloranilic acid. Based on elemental analyses, molar conductivity and magnetic moment of room-temperature measurements, and IR and electronic spectral studies, it is proposed that these complexes have CA-bridged structures and consist of two chromium (III) ions, each in an octahedral environment. The complexes [Cr2(CA)(Me-phen)4](ClO4)4(1) and [Cr2(CA)(Me2-phen)4](ClO4)4(2) were further characterized by variable temperature (4.2-300 K) magnetic susceptibility measurements and the observed data were successfully simulated by the equation based on the spin Hamiltonian operator, , giving the exchange parameter J = -7.8 cm-1 for (1) and J= -6.5 cm*1 for (2). This result     

Luminescence spectroscopy of high-energy 4f 11 levels of Er3+ in fluorides

The 4f¹¹ energy levels of Er³⁺ in LiYF₄ in the spectral region 39000–65000 cm^?1 have been studied. The agreement between experimental energy levels, obtained from luminescence excitation spectra, and calculated energy levels is good. Luminescence originating from high-lying energy levels has been investigated. Emission from the states ⁴D_1/2 (～47 200cm^?1), ²F(2)_7/2 (～54700cm^?1) and ²F(2)_5/2 (～63 300cm^?1) is observed. ²F(2)_5/2 emission occurs for Er³⁺ in LaF₃, where the ²F(2)_5/2 level is situated just below the lowest 4f¹⁰5d state, but also for Er³⁺ in LiYF₄, where it lies in between the two lowest 4f¹⁰5d states.     

Test of the stability of a three-phase contact line with negative line tension

The equilibrium structure of a three-phase contact line with negative line tension using a mean-field free-energy functional is calculated in the square-gradient approximation. The equilibrium density profiles are found by solving the Euler—Lagrange equations on a square grid of N ² points covering an area of L ². The fluctuations about the equilibrium structure are analysed via the spectrum of the free energy's second functional derivative. The equilibrium configuration is found to be stable with respect to fluctuations in the structure of the three-phase line and of the interfaces that meet at this line. In addition, the behaviour is investigated numerically of the lowest eigenvalue as the area of the grid is increased. The lowest eigenvalue is always positive and vanishes as 1/L ².     

Optical pumping in an organic crystal: quinoxaline in durene

Experiments have been performed to determine the path of entry into and exit from the phosphorescent triplet state T₀ of quinoxaline in a durene host. First of all the decay of phosphorescence after flash excitation was followed at 4.2 and 1.34 °K. It was found that for both perdeutero- and perhydroquinoxaline the lifetime is shortened by a factor of about three when the temperature is lowered from 4.2 to 1.34 °K. At 1.34 °K relaxation between the spin components (i.e. re-orientation of the triplet spin angular momentum) is slow relative to the decay, and the observed reduction in lifetime indicates that entry into and exit from T₀, are through the same spin component. Similar decay experiments were then carried out at 1.34 °K in a 10 kG magnetic field or in a somewhat weaker field so chosen that the effect of microwave saturation of one of the E.S.R. transitions between the components could be observed. From the results it follows that on intersystem crossing the molecules enter the manifold T₀ through the top zero-field component and thus initially have their spins aligned. Decay departs almost exclusively from the same component, even in the case of perhydroquinoxaline, where at least 45 per cent of it must be radiationless. The decay route agrees with out-of-plane polarization of phosphorescence for the free molecule. Finally, spin alignment on intersystem crossing is discussed from the theoretical point of view. It appears that the phenomenon is clear-cut only in molecules such as those of the aza-aromatics, where strong spin-orbit coupling of the atomic type occurs betwen ππ? and nπ? states.