Porphyrins electronic transitions

Raman (R) and resonance Raman (RR) spectroscopy detects vibrational modes involving a change in polarizability. For RR, enhancement of modes is coupled with electronic transition excited by a laser light source. This technique is complementary to IR and is used for detection of v(O-O) and v(M-0), especially in metalloproteins. In porphyrins, one may identify oxidation and spin states. 

Electronic spectra of metalloproteins find their origins in (i) internal ligand absorption bands, such as n->n electronic transitions in porphyrins (ii) transitions associated entirely with metal orbitals (d-d transitions) (iii) charge-transfer bands between the ligand and the metal, such as the S ->Fe(II) and S ->Cu(II) charge-transfer bands seen in the optical spectra of Fe-S proteins and blue copper proteins, respectively. Figure 6.3a presents the characteristic spectrum of cytochrome c, one of the electron-transport haemoproteins of the mitochondrial... 

Unfortunately the optical spectrum (159) of VEPI in the visible and ultraviolet appears to be dominated by electronic transitions of the porphyrin group such that the spectrum of the 3d electron is masked, and direct comparison of the experimental gi-factors with the theoretical ones (Equation 40) cannot be made. The value of k necessary to account for the anisotropy of the hfs is 0.71 with the result that... 

State energies depend to a large degree on the energies of the MOs involved in an electronic transition. Thus, by taking proper account of the nodal structure of the relevant MOs it should be possible to determine, at least qualitatively, where substituents should be placed to achieve optimal differential stabilization effects. More detailed Cl calculations can then be carried out to determine whether the expected effects are likely in fact to occur. In addition, the results of numerous experimental studies of substituted porphyrins (37, ) will also provide a useful guide for the design of porphyrin dimers with the desirable properties. 

Up to now the four-orbital model-based theory of porphyrin absorption electronic spectra ( ) has not taken into account the influence of NH-tautomerism in non-symmetrical porphyrins on the positions and intensities of electronic transitions in the visible region. The theoretical consideration of this problem involves solving the fundamental question of the absolute orientation of electronic transition oscillators for each tautomer. First of all. 

In order to consider the inversion of Qx(0,0) and Qy(0,0) electronic transition intensities in NH-tautomers of non-symmetrical free-base porphyrins we calculated the ground-state orbital energies of the investigated molecules by a CNDO/2 method using the symmetrized crystal geometry of porphyrin molecule (37,38). On the basis of the above experimental results we must introduce a motionless system of molecular X and Y axes, identically fixed in both tautomers. Then using theoretical MO calculations and the analysis... 

The nature of the two electronic transitions of the porphyrin from the ground state orbitals, a u and a%u, to the excited orbital, eg, is such... 

If the resonant electronic transition is associated with a site of biological activity, then the technique offers a sensitive probe for structural features of the site. Heme proteins afford particularly informative resonance Raman spectra, with a rich assortment of porphyrin ring vibrations, which can be classified and analyzed via their symmetry properties [132-134], Some of these frequencies are sensitive to the structural features of spin- and oxidation-state changes of the heme group. These can be used to monitor the structural consequences of ligation or electron transfer in heme proteins [66, 135-138]. 

Five-coordinate Mo(RC=CR)(SBu )2(CNBu )2 complexes exhibit strong charge transfer electronic transitions at high energy (133). Only the lowest energy absorption in the visible spectra of these complexes is included in Table VIII, with Amax near 550 nm and e =102 A/-1 cm-1. Although lucid molecular orbital descriptions of these molecules have been presented, no assignments of observed transitions have been made. The five-coordinate Mo(PhC=CPh)(TTP) porphyrin complex has Amax values of 624, 544, and 426 nm with e values of 2300, 9500, and 160,000 M l cm-1, respectively (81). 

A typical electronic spectrum of a M(4-TCPyP) complex is shown in Fig. 16 (39,123,170,176,182,183). In general, the electronic transitions in the porphyrin center exhibit many similarities with those observed in the spectra of the M(4-TRPyP) species, with the Soret band typically in the range of 414- 75 nm, and the Qi and Qo o bands in the range of 557-584 and 611-645 nm, respectively. In the formal Ru(III)Ru(III)Ru(III) oxidation state, the characteristic intracluster band is observed in the 685-707-nm range, while the RusO py MLCT band can be found in the 314—351 -nm range. The spectral data of a series of M-4TCPyP derivatives are listed in Table II. 

In the case of porphyrin systems, there is an agreement that electron injection occurs from the lowest singlet excited state (438, 439). In the tetraruthenated porphyrins, the mechanisms involved in ET are more complex, since both components are responsible for photocurrent generation. As already discussed, from the HOMO and LUMO compositions, the peripheral ruthenium complexes can effectively transfer electronic charge to the porphyrin center via Ru( Jtt) porphyrin MLCT transitions. In addition, the direct interaction between the ZnTPyP core and Ti02 plays an important role in the photoresponse efficiency. 

Based on our knowledge of the structure of chlorophyll as well as the results of studies on the photo behavior of chlorophyll in vitro, it is possible to summarize some of the features of the chlorophyll system which enhance its usefulness as a pigment in photosynthesis. First, there is extensive conjugation of the porphyrin ring. This lowers the energy of the electronic transitions and shifts the absorption maximum into... 

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