Porphyrins optical spectra

Because of space limitations, we will only briefly discuss the most basic application in the area of porphyrin optical spectra. Much work has already been done in this area, both with TDDFT and related methods. The TDDFT applications include free base porphin and its ss-octahalogenated derivatives, the porphyrinato-porphyrazinato-zirconiumhV) complex, NiP, NiPz, NiTBP, and NiPc, zinc phthalocyanine, chlorophyll a, zinc complexes of porphyrin, tetraazaporphyrin, tetrabenzoporphyrin, phthalocyanine, phenylene-linked free-base and zinc porphyrin dimers, metal bis(porphyrin) complexes a series of porphyrin-type molecules, and many more. We refer to ref. 75 for an extensive discussion of TDDFT calculations on the spectra of porphyrins and porphyrazines, as well as their interpretation. For further theoretical work on porphyrines, we mention ref. 76 and other papers in that special issue. 

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... 

The dithionite reduction of the micelle encapsulated aqua (hydroxo) ferric hemes at pH 10 (in inert atmosphere) gives an iron (II) porphyrin complex whose optical spectrum [21] shows two well-defined visible bands at 524 and 567 nm and a Soret band split into four bands (Fig. 10). Such spectral features are typical of four-coordinate iron (II) porphyrins. The magnetic moment (p = 3.8 + 0.2 Pb) of this sample in the micellar solution is also typical of intermediate spin iron(II) system and is similar to that reported for four-coordinate S = 1 iron(II) porphyrins and phthalocyanine [54-56]. The large orbital-contribution (ps.o. = 2.83 p for S = 1) observed in this iron(II) porphyrin... 

The micelle-encapsulated six coordinated bis(pyridinato) iron(II) complexes of protoporphyrin and OEP have been reported by addition of pyridine to the four coordinate ferrous complex in aqueous micellar solution. The optical spectrum of [Fe(II)(PP)(Py)2] in micelle (Fig. 10) is identical to S = 0 six-coordinate bis(pyridinato) iron(II) porphyrin complex [3]. The magnetic moment measurements in solution confirm their diamagnetic nature. The HNMR spectra are also characteristic low-spin iron(II) resonances (S = 0) with shifts lying in the diamagnetic region (Table 2). 

Metalloporphyrins, MP, represent derivatives of porphyrin, P, in which four pyrrole fragments are bound together by methine bridges (Fig. 13). The diversity of porphyrins is due to the possibility of variation for substituents R in the periphery of the porphyrin ring. A typical optical spectrum of a P solution is presented in Fig. 14. One can point out quite a number of characteristic bands in it. The most intensive short-wave peak in the P absorption spectrum (/max 400 nm) corresponds to the transition S0 -+ S2 and is referred to as Soret band. The extinction coefficient of this band is very large, as a rule, and amounts to 10 -106 M 1 cm-1. The less intensive long-wave bands of P absorption correspond to the S0 - Sx transition (bands I-IV in Fig. 14). Complexation with the metal results in a rise of the symmetry of the molecule, due to which MP molecules have only two bands in the long-wave part of the absorption spectra. Most of the metalloporphyrins are characterized by intense luminescence. The time of MP fluorescence decay (transition Si - S0) is short and amounts to 10"8 to 10 9 s. Besides the transition... 

Fig. 14. Characteristic shape of the optical spectrum of a porphyrin solution, t is extinction coefficient in M m-1.

Calculations of such spectra have been reported assuming the molecule to be planar with C2v symmetry [36]. The lower symmetry of corrole removes the degeneracy present in porphyrins so that degenerate transitions are no longer expected and more transitions are allowed in the case of corrole than in that of porphyrin. The la2 and 8fc, levels in corrole correspond to the two components of the lowest vacant MO in porphyrins, the 4eg pair (LUMO). Similarly the 6a2 and 7 >1 orbitals correspond to the highest filled porphyrin orbital, alu and a2u (HOMO). In the case of porphyrins the Q and Soret (thus called because it was discovered in 1883 by Soret in the optical spectrum of hemoglobin) bands arise mainly from the alu, a2u and eg pair and the spectra of corroles closely resemble those of porphyrins (Table 6). 

There are now good theoretical descriptions of the electronic structures contributing to the optical absorption bands in spectra of porphyrin radicals and ferryl species [160,167] most charge-transfer bands in the latter are due to a transition from a porphyrin p orbital to an Fe-0 tt orbital [167], However, in the absence of a prior knowledge of the structure around the Felv site (and/or spectra of a variety of synthetic model compounds) it is not straightforward to assign an optical spectrum to a ferryl species. Thus the intermediate assumed to be the ferryl species in the binuclear haem c /Cub centre of cytochrome c oxidase [168] has a spectrum at 580 nm essentially identical [169] to that of low-spin ferric haem a3 compounds (e.g. cyanide). 

Oxidation of the porphyrin ring to generate the radical cation, port, can occur by removal of an electron from either one of the HOMOs ctiu( ) or a2u(7r). This results in a major perturbation of the optical spectrum of the porphyrin ring (73). The detailed assignments of the new feature are uncertain. This radical species can be generated either chemically or electrochemically by one-electron oxidation of metal-free porphyrin. 

Beckmann rearrangement provided the ring-expanded metallopyrazine imide 143, which showed metallochlorine-like optical properties. The demetallated derivative of 143 has a porphyrin-like optical spectrum In terms of further elaboration, the imide in 143 provides a functional group that can be derivatized. 

Electrochemistry and spectroscopy of the tt cation radical of meso-tetraalkylchlorin (tetra-methyl) and various porphyrins (tetramethyl, tetraethyl, and tetra-ra-propyl) indicate that these do not convert to Nim at low temperatures.280 Optical evidence reveals, however, that oxidation of the tt cation radical of [Ni(pEt2N)(TPP)] leads to a Ni111 cation radical which can be further oxidized to a Ni111 porphyrin dication. Similar studies have been carried out for various other derivatives of me.so-tetraarylporphyrins such as /V-oxides of TPP and 5,10,15,20-tetramesitylpro-phyrin (TMP). Addition of trifluoroacetic acid (TFA) to the /V-oxide of [NinTMP] at —25 °C in CH2C12 results in [Nim(TMP)]+ with a rhombic EPR spectrum, g = 2.40, 2.12, and 2.04.281... 

Over the past 20 years, the principal biomedical application for (free or metal-substituted) porphyrins has been Photodynamic Therapy (PDT), with extensive literature in the area including a number of comprehensive reviews (114-116). Porphyrins offer scope for optical imaging as they are potent fluorophores in the red region of the electromagnetic spectrum. Although, to date, the emphasis has been on their therapeutic effects, due to their... 

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... 

The associated optical cis effects at the porphyrin system can be understood according to the inductive transmission path shown in Figure 1, Case A Rh,u causes a hypso spectrum via Rh - (P) 7r-donation increasing in the series I < Br < C =CPh < Ph Me for X which is roughly the series of increasing basicity towards the proton, as expected. A common feature (see Sect. 6) is the small effect of the exchange NH3 - HjO in the a-band the Soret band shows a stronger shift. 

Perhaps the simplest optically controlled switches are single molecules embedded in a solid host matrix. These systems consist of an amorphous, polycrystalline, or crystalline film doped with dilute concentrations of impurity molecules. The most commonly used dopant molecules are fused polycyclic aromatic hydrocarbons and porphyrins. In addition to facile sample preparation, these planar molecules absorb in the visible to near IR regions of the spectrum, possess large extinction coefficients in both the ground and excited states, and have high fluorescence quantum yields. 

Fig. 7. Optical spectra of ferryl iron and free radicals in peroxidases. Optical spectra of intermediates in the catalytic cycle of horse-radish peroxidase (HRP) in the Soret (left) and visible (right) regions (A) Soret and visible spectra of native HRP (B) Soret and visible spectra of HRP compound II (C) Soret spectrum of HRP compound I (D) visible spectrum of HRP compound I. Note the unusual low haem absorbance in the Soret for compound I, where the nature of the porphyrin cation radical dominates the spectrum. Reprinted with permission from Dunford, H.B. (1982) Adv. Inorg. Biochem.

All of these compounds are expected to have ferryl iron with no porphyrin cation radical. As with optical spectroscopy the presence of the distant tryptophan radical in cytochrome c peroxidase compound I appears to have no effect on the MCD spectra. This was confirmed by a direct comparison of cytochrome c peroxidase compounds I and II [172] in the visible region. Tryptophan has a distinct MCD spectrum at 280 nm [173]. However, none of the changes in the UV MCD spectrum that occurred upon compound I formation could be attributed to the formation of the tryptophan radical [174]. 

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