Porphyrin complexes absorption spectra study

Maldotti (96) studied the kinetics of the formation of the pyrazine-bridged Fe(II) porphyrin shish-kebab polymer by means of flash kinetic experiments. Upon irradiation of a deaerated alkaline water/ethanol solution of Fe(III) protoporphyrin IX and pyrazine with a short intense flash of light, the 2 1 Fe(II) porphyrin (pyrazine)2 complex is formed, but it immediately polymerizes with second-order kinetics. This can be monitored in the UV-Vis absorption spectrum, with the disappearance of a band at 550 nm together with the emergence of a new band due to the polymer at 800 nm. The process is accelerated by the addition of LiCl, which augments hydrophobic interactions, and is diminished by the presence of a surfactant. A shish-kebab polymer is also formed upon photoreduction of Fe(III) porphyrins in presence of piperazine or 4,4 -bipyridine ligands (97). 

The experimental studies of the MCD spectra of porphyrin and TPP complexes (134,137,138) have generally focused on the first two major bands. The first band, the Q band, appears near 2eV and is has low intensity in the absorption spectrum. The second band, the or Soret band, starts at around 3 eV and has greater intensity in the absorption spectrum. Both bands exhibit some fine structure that may indicate that more than one excitation contributes to each band. 

Reactions of NO were also studied with the synthetic heme protein discussed earlier, namely the recombinant human serum albumin (rHSA) with eight incorporated TPPFe derivatives bearing a covalently linked axial base, were also investigated. The UV-vis absorption spectrum of the phosphate buffer solution at physiological pH showed absorption band maxima at 425 and 546 nm upon the addition of NO to form the nitrosyl species, which was also formed when the six-coordinate CO-adducts were reacted with NO gas. EPR spectroscopy revealed that the albumin-incorporated iron(II) porphyrin formed six-coordinate nitrosyl complexes. It was observed that the proximal imidazole moiety does not dissociate from the central iron when NO binds to the trans position. The NO-binding affinity P1 /2no was 1.7 X 10 torr at pH 7.3 and 298 K, significantly lower than that of the porphyrin complex itself, and was interpreted as arising from the decreased association rate constant (kon(NO), 8.9 x 10 M s" -1.5 x 10 M s ). Since NO-association is diffusion controlled, incorporation of the synthetic heme into the albumin matrix appears to restrict NO access to the central iron(II). ... 

McCandlish et al. have isolated a peroxo-iron(III) complex (9) (Fig. 4) in the reaction of Fe(III)TPP(Cl) and KO2 according to Eq. (4) (37). The Soret band of 9 appears at 437 nm with unusually red-shifted a- and -b-bands (565 and 609 nm in DMSO). The EPR spectmm of 9 at 77K showed a relatively narrow, sharp resonance at g = 4.2 and weak resonances at g = 2 and g = 8, typical of rhombic high-spin ferric complexes such as Fe" EDTA (JS). Such a spectrum is not typical of high-spin ferric porphyrin complexes, which usually show resonance at g = 2 and 6, indicative of axial symmetry. An IR band at 806 cm" was observed to shift to 759 cm when K 02 rather than K 02 was used to prepare solutions of 9 these observations suggested the side-on bonding formulation illustrated in Fig. 4. Extended X-ray absorption fine structure (EXAFS) studies of 9 also... 

One of the more extensively studied and discussed structural variables has been equilibrium out-of-plane distortion of the porphyrin. Out-of-plane porphyrin deformation typically results in a bathochromic (red) shift of the optical absorption spectrum and a shift to more negative reduction potentials (easier to oxidize). These tantalizing correlations, based upon studies of model complexes comprising porphyrin ligands such as octaethyltetraphenylporphyrinate (OETPP ), having sterically crowded peripheries that force large out-of-plane deformations, have driven... 

The solvent effects on the absorption spectrum of Gd(TPP)(acac) have been studied by Radzki and Giannotti (1993). As shown in fig. 7, both the Soret and Q(1,0) bands are substantially shifted in different solvents, in particular in solvents with donor atoms. The magnitude is about two times larger than that for metal-free porphyrin H2(TPP) which can be attributed to complex formation between the gadolinium porphyrin and the Lewis base solvents. 

Porphyrin molecules form stable complexes with lanthanide ions, these complexes have intensive absorption in a visible range of spectrum. Erbium, ytterbium and neodymium complexes are characterized by a 4f-luminescence in near IR-range of spectrum [1]. The most studied complexes with porphyrins are ytterbium complexes since Yb has smaller ionic radius in comparison with lanthanum (radius of Yb ion is 1.01 A), which determines higher stability of these metallocomplexes. Distinctive feature of Yb porphyrin complexes is a characteristic narrow and rather intensive luminescence band located in the IR-range at 975-985 nm, in so-called therapeutic window of tissue transparency. ... 

The prosthetic groups of many membrane enzyme systems contain metalloporphyrins. Therefore, it is not surprising that an attempt has been made to reveal and study the catalytic effect of porphyrins during redox processes at the interface between immiscible liquids. Complexes of different metals with different porphyrins exhibited catalytic activity in the oil/water system [50]. First of all, let us consider the transformations of metal complexes of porphyrins in the octane/water system. In wet octane, as a result of the hydrolysis and dimerization reactions, FeEP changes to a //-complex and this results in a change in the absorption spectrum (Fig. 5). With a strong acidification of the aqueous phase, the equihbrium of Eqs. (12) and (13) shifts to the left and at pH 1 the positions of the maxima on the absorption spectra of ethioporphyrin in dry and wet octane coincide. 

Moreover, supra-molecular systems involving crown ethers, fullerene and k-extended systems have been achieved that can mimic the photosynthetic process [9-14]. The fullerene Qo has been used successfully as an electron acceptor in the construction of model photosynthetic systems [9], the r-extended systems, such as porphyrins [12], phthalocyanines [13], r-extended tetrathiafulvalene (w -exTTF) derivatives [9,10], which are utilized as electron donors, while the crown ethers act as a bridge between the electron donor and acceptor. In the absorption spectrum of the complexes, the absorption maxima are associated experimentally and theoretically with the formation of charge-transfer states [14-16]. Consequently, these supramolecular systems have potential for applications in photonic, photocatalytic, and molecular optoelectronic gates and devices [9-14]. As a result, the study of the conformations and the complexation behavior of crown ethers and their derivatives are motivated both by scientific curiosity regarding the specificity of their binding and by potential technological applications. 

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