The electronic absorption spectrum of ferrocene has been investigated in the vapor, in liquid solutions, and in glassy matrices. Temperatures used ranged from 420° to 77°K. The absorption spectrum contains at least 11 distinct electronic absorption bands. Three of these appear to be triplet←singlet in nature, and to be made allowed by spin—vibronic perturbations (i.e., spin—orbit coupling of the triplet manifold with vibronically coupled singlets). It is shown that a spin—orbit coupling factor 300≤ζ≤400 cm−1 can account for the observed intercombination intensities. The singlet←singlet absorption spectrum is dominated by one allowed transition at ∼50 000 cm−1 with f∼0.69. It is possible that all other electronic transitions, the 53 000-cm−1 band excepted, are dipole forbidden and obtain intensity by first-order vibronic stealing from the ∼50 000-cm−1 band. The higher-energy absorption bands show vibrational structure, and this structure is analyzed herein; unfortunately, the resolution is restricted by ``molecular'' reasons associated with vibrational ``richness,'' hindered rotations, and vibrational ``hot bands.'' The higher-energy absorption bands are heavily localized on the aromatic rings in contrast to the three low-energy diffuse electronic systems in Regions IV, V, and VI which contain much d-orbital and intramolecular charge-transfer character. No phosphorescence emission of ferrocene has been observed here.
A four-electron MO treatment of the interaction of the D2h polyacenes naphthalene, anthracene, pyrene, and perylene to yield D2h dimers is developed, and the energies of these dimers are calculated as functions of the effective nuclear charge to be used in a Slater orbital exponent, and of the intermolecular distance D. Agreement with excimer luminescence energies is obtained for values of Z∼3 and values of D between 3.0 and 3.6 Å. Contact absorption processes are predicted to occur and are discussed. An Appendix which provides simple analytic expressions for four different types of penetration integrals is included.
Nitrite salts of the post-transition-series metals exhibit a long-lived luminescence centered at ∼5500 Å. This emission is characteristic of both the solid state and frozen glassy solutions. It is suggested that this emission originates in a triplet state (T1) of the system, and that, in fact, it is a phosphorescence of T1→S0 molecular type connecting with the ground singlet state (S0). It is shown that the spin–orbit coupling responsible for the enhanced intersystem crossing is introduced by the metal counterion and that such introduction is also dependent on the presence of nitrite-counterion covalency. The increased spin–orbit coupling is manifested in the following ways: (i) a decrease in the phosphorescence lifetime, τp, (ii) an increase in the relative quantum yield ratio Φp / Φf of phosphorescence to fluorescence, (iii) an increase in the T1←S0 absorptivity—which absorptivity is responsible for the yellow-orange colors of the heavy-metal nitrites.
Charge self-consistent semiempirical calculations, including all valence sigma orbitals of the ligands are reported for ferrocene, aminoferrocene, chloroferrocene, and nickelocene. The computations follow the Wolfsberg-Helmholz scheme, and use Slater atomic orbitals which mimic the overlaps, including distance dependence, of the SCF atomic orbitals. An attempt is made to assign electronic excitation energies in a manner consistent with computation and experiment. The only assignment which may be considered reasonably secure is that of the very intense absorption at 51 200 cm-1 which is taken to be the allowed component of the ela<--elu orbital excitation (f •• lclfob.=0.549/0.691). The ionizing orbital in ferrocene was found to be ala (-9.78 eV) and in nickelocene was el a (-6.874 eV). The formal charge on the metal was found to be positive in all cases. A discussion of metal-ligand bonding is included.1. Chern. Phys. 45, 2777 (1966.
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