Emission of pyridine and polypyridine chromium(III) complexes in rigid and fluid media

Abstract

by the Cl atom through occupation of the d22 orbital, while in passing from the quartet to sextet, the additional charge loss is absorbed by the four porphyrin nitrogen atoms via covalent mixing with the dxx.yy orbital.In passing from the doublets through the quartets to the sextets there is also a systematic increase in 4S population. This increase seems to be reflected in the observed Mossbauer isomer shifts <5 that increases roughly from about 0.17 ± 0.07 mm/s for the doublets to about 0.40 ± 10 mm/s for the sextet. ConclusionsIn this study we examine the ground and excited states properties of (porphinato)iron(III) chloride as a model for ferric porphyrin complexes. For the geometry and chelation we assume, 6Alg and 1234A2g states are predicted to lie very near in energy. As suggested by many others, and as discussed in the text, two states so close in energy could both be thermally populated or they might mix directly through spin-orbit coupling to form Kramers doublets in which only Jz is a good quantum number. The nature of these two states, and indeed their mixing, will be a sensitive function of the details of the model assumed.Four spin states of 2Eg and 2B2g symmetry are predicted to lie lowest under stronger ligand fields. For our model structure the 2Eg state is predicted about 1800 cm'1 lower in energy than the 2B2g. The calculated Mossbauer quadrupole splitting for the 2Eg state of 1.91 mm/s is in reasonable agreement with the range of values observed for low spin complexes, 1.7-2.3 mm/s. The electronic spectra of (porphinato)iron(III) chloride is calculated assuming ground-state symmetries 6Alg, 4A2g, 2Eg, and 2B2g. The low-lying excitations are in good agreement with experimental values when they are known. The Q band in the visible is well reproduced. Porphyrin triplet states are predicted to gain intensity through spin coupling through the central metal, and we suggest their role in the spectra observed below the Q band. The 6Alg has considerable mixing between eg(rr) and metal dT (dX2 and d>2) orbitals leading to a predicted intensity pattern among the many allowed Eu excited states that is quite sensitive to details of geometry and calculation. In our calculation, as well as in the ab initio calculations of Rawlings, et al.26 a "split Soret" is predicted for the 6Alg. The B or Soret band is predicted some 6000 cm'1 higher in energy than observed.No reliable spectra for a spin 3/2 complex is available but we are not certain these calculations could be used to distinguish the 4A2g and 6Alg spectra. Both suggest low-lying bands about 10400 cm'1, weakly allowed bands at about 15 000 cm'1, a Q band about 16 500 cm"1 and even one component of a split Soret at about 24000 cm"1 (see Tables VI and VII). The calculated spectra from the 2Eg state are in reasonable agreement with that observed and again suggest the 2Eg state is the lowest doublet.Acknowledgment. This work was supported in part through grants from the CRDEC Chemical Systems Technology Research Center at the University of Florida, DAAA15-8...

The photophysics of chromium(III) complexes

“…The sharp rate increase at 150 K is associated with solvent motions that reduces the 2EQ -4A2 transition energy. 91 This behavior is a clear indication of a transition region between two thexi states. Above 230 K the decay rate increase is well fitted by a single-term Arrhenius expression.…”

“…85,87 The room temperature emission might also be 2EQ -*• 4A2. 91 The transition energies listed in Tables I-IV correspond to the 0-0 band positions for the 2E -4A2 spectra that comprise the bulk of the tabulated data.…”

“…On the other hand, there is a red shift of the 2EQ -* 4A2 band when strong solvent-solute interactions are reduced by solvent motions, e.g., trans-Cr(py)4F2+. 91 This has been ascribed to an increase in A(t2) that leads to a lowering of 2EQ. In alcohol-water solutions of cis-Cr(phen)2F2+, level inversion occurs during the rigidfluid transition.…”

(Pyridine)pentaamminechromium(III). Synthesis, Characterization, and Photochemistry

“…A number of pyridine complexes of chromium(III) have been known since the early times of coordination chemistry. , Although some have received spectroscopic − and photophysical − attention, no photochemistry has been reported to date for any of these compounds.…”