Magnetic circular dichroism . excited-state

Compounds 1,2,3,5,10,11,12,13,14 were dissolved in EPIP (diethyl ether, petroleum ether, isopropanol 5 5 2)whereas compounds 4,6,7,8,9,15 were dissolved in THF-DE (tetrahydrofurane, diethyl ether 1 1). These solvent mixtures can be frozen as glassy samples at 77 K. The absorption spectra were recorded on a standard spectrophotometer SF-10 or Beckman-5270. The measurements of fluorescence excitation and emission spectra were made with the aid of a spectrofluorometer SLM-4800 with automatic correction of spectral response. Fluorescence lifetimes were measured with the aid of a pulse fluorometer PRA-3000. Magnetic circular dichroism (MCD) measurements were carried out in a 8 kG magnetic field using a JASCO J-20 circular dichrometer. Triplet state formation was observed for investigated compounds at the experimental set up, whose detailed description can be found in our paper (27). The optical experiments were carried out with a porphyrin concentration of 4.10- - 4.10 mol.l". In NMR investigations (Bruker WM-360) we used higher concentrations ( 5.10" raol.l ) and dried solvents (CDCl, C 2 and toluene-d0). 

Several theoretical studies concluded that the two bands originate from electronic transitions from one ground state to two excited states [47-51,55], The D3-symmetry conformation was confirmed by an X-ray diffraction study of crystal samples [56], resonance Raman studies [46,51,57], and magnetic circular dichroism measurements [55], Yet, no C2-symmetry conformation has been identified yet. 

The electronic spectra of the five-membered ring compounds have been intensively studied by the experimental and theoretical works. These molecules are fundamental units in many important biological systems. Furthermore, their excitation spectra are benchmark examples for theoretical studies of molecular excited states [51,55-58]. For furan and thiophene, various types of excitation spectra were measured the vacuum ultraviolet (VUV) spectrum, electron energy-loss (EEL) spectrum and magnetic circular dichroism (MCD) spectrum. The SAC-Cl method offered consistent interpretations of these electronic spectra [51-53]. 

Fig. 9. Magnetic circular dichroism selection rules. (A) Transitions between Zeeman-split y (or S) = i paramagnetic ground (G) and excited (E) states. Right circularly polarized (RCP) and left circularly polarized (LCP) transitions are indicated. (B) MCD spectra associated with the energy scheme in (A). The dashed bands correspond to individual left and right polarized transitions, whereas the solid line spectra give the dichroism. The C-term spectrum corresponds to a Gaussian band shape, which peaks at the absorption maximum, whereas an A-term reflects its first derivative which crosses zero at the absorption maximum.

Six symmetrically disubstituted derivatives of r-tetrazine have been studied by linear dichroism spectroscopy in the UV-Vis region, magnetic circular dichroism spectroscopy, and theoretical calculations of structures and spectra <1995CPH(200)201>. A detailed discussion of the sensitivity and of the optothermal detection scheme for the study of nonfluorescing excited states of the r-tetrazine-argon van der Waals complex has been presented <2000MI183>. 

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