A study is made of the bonding in sandwich compounds by means of ESR and susceptibility measurements. Data obtained for vanadocene and nickelocene are discussed in terms of various calculated MO schemes. The experimental results are compatible with the level order e2g ≲ a1g < e1g* for the highest-bonding and lowest-antibonding orbitals, all orbitals having a strong metal 3d character. The assignments given for the ground-state configurations are quantitatively checked by comparison of calculated and experimental D and g values. Both spin–orbit and spin–spin coupling are shown to give a substantial contribution to the value of D. The excitation energies used in the perturbation calculation of the DLS and g values are obtained from a ligand-field treatment of the low-energy bands in the optical absorption spectra.

Abstract— –Spectral properties of isoalloxazines in organic solvents of low polarity are determined at 300 and 77 K. Vibrational structure in the spectra reveals a vibrational mode of 1250cm‐. The pure electronic transition energies are established to a greater accuracy than was done previously and comparison to theoretical data is made. Actual lifetimes up to 10 ns for fluorescence and 300 ms for phosphorescence are found. The ratio of the actual fluorescence lifetime and the radiative lifetime is found to agree well with the quantum yield. Solvent interactions hardly shift the energy of the first electronically excited singlet state but merely affect the Franck‐Condon envelope of the spectrum and the non radiative decay of the chromophore. In albne solutions at 77 K isoalloxazine clusters are formed exhibiting P‐type delayed fluorescence.

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