Absorption, spectra porphyrins

The various porphyrinogens are colorless, whereas the various porphyrins are all colored. In the smdy of porphyrins or porphyrin derivatives, the characteristic absorption spectrum that each exhibits—in both the visible and the ultraviolet regions of the spectrum—is of great value. An example is the absorption curve for a solution of porphyrin in 5% hydrochloric acid (Figure 32-10). Note particularly the sharp absorption band near 400 nm. This is a distinguishing feamre of the porphin ring and is characteristic of all porphyrins regardless of the... 

Sanders (14) has exploited the strong and selective coordination of phosphine donor groups to Ru(II) to construct hetero-dimetallic porphyrin dimers (17, Fig. 5). An alkyne-phosphine moiety introduced on the periphery of a free base or metalloporphyrin (M = Zn or Ni) spontaneously coordinates to a Ru(II)(CO) porphyrin when the two porphyrins are mixed in a 1 1 ratio. Coordination is characterized by a downfield shift of the 31P resonance (A<531P = 19 ppm). There is no evidence of self-coordination of the zinc porphyrin at 10 6 m in toluene, there is no shift in the Soret band in the UV-Vis absorption spectrum. The Ni-Ru dimer was observed by MALDI-TOF mass spectrometry. Heating the Ru(II)CO porphyrin with 2 equivalents of the phosphine porphyrins led to quantitative formation of trimeric assemblies. 

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 dication [(oep)Pb(IV)]2+ is unstable. Its absorption spectrum differs from that of doubly oxidized porphyrins of other metals like Zn or Mg, where the ligand is doubly oxidized. Spectrometry and voltammetry indicate that PblV binds weakly to the unoxidized ligand (the Soret and adjacent lines are those of the unoxidized ligand). Another unique aspect of the lead complex is its instability, which stands in contrast to the stable Sn(IV) analog171, implying the inability of PblV to accommodate into the central cavity of the porphyrin plane. 

The structure of HRP-I has been identified as an Fe(IV) porphyrin -ir-cation radical by a variety of spectroscopic methods (71-74). The oxidized forms of HRP present differences in their visible absorption spectra (75-77). These distinct spectral characteristics of HRP have made this a very useful redox protein for studying one-electron transfers in alkaloid reactions. An example is illustrated in Fig. 2 where the one-electron oxidation of vindoline is followed by observing the oxidation of native HRP (curve A) with equimolar H202 to HRP-compound I (curve B). Addition of vindoline to the reaction mixture yields the absorption spectrum of HRP-compound II (curve C) (78). This methodology can yield useful information on the stoichiometry and kinetics of electron transfer from an alkaloid substrate to HRP. Several excellent reviews on the properties, mechanism, and oxidation states of peroxidases have been published (79-81). 

However, in contrast to the human His25Ala HO-l heme complex, which has no detectable activity in the absence of imidazole (78), the His20Ala Hmu O rheme complex in the presence of NAD PH and NADPH-cytochrome P450 reductase was foimd to catalyze the initial meso-hydroxylation of the heme (151). The product of the reaction was Fe verdoheme, as judged by the electronic absorption spectrum and the detection of carbon monoxide as a product of the reaction. Hydrolytic conversion of the verdoheme product to biliverdin and subsequent HPLC analysis confirmed that the oxidative cleavage of the porphyrin macrocycle was specific for the a-meso-carbon. 

Figure 3. Transient absorption spectrum of a 2 x 10 M solution i in butyronitrile at 100 ps following a 0.3 mJ, 0.5 ps, 600 nm laser flash. Filters that reject stray excitation light cut out the 580-620 nm wavelength region, while the sharp cutoff at 440 nm is due to the intense absorption of the porphyrin Soret band at 419 nm.

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. 

First major band in porphyrin absorption spectrum. 

Porphyrin [125-128] and phthalocyanine [129] derivatives are also employed as photosensitizers in DSSCs. A nanocrystalline Ti02 solar cell sensitized by Cu chlorophyllin produced 2.6% efficiency (./ . = 9.4 mA/cm2 and Voc = 0.52 V) under 100 mW/cm2 [127]. In order to develop new, efficient, metal dye photosensitizers, both an increase in the absorption coefficient of the metal complex and a greater red shift of the absorption spectrum is required. 

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... 

While the typical bands of porphyrins with general structure 2 appear at 419, 515, 548, and 591 nm in the UV-vis absorption spectrum, the... 

Taking advantage of the fact that the triplet state of metalloporphyrins with tetrabenzopor-phyrin (H2TBP or H2I, fig. 10) lies around 12 500 cm-1, a team from Minsk succeeded in sensitizing Ybm luminescence in solutions of [Yb(TBP)L], where L = Cl or acac (acetyl-acetonate), in several solvents like benzene, dimethylformamide (dmf), pyridine, quinoline or a mixture of octane and benzene (Kachura et al., 1974). Energy transfer from the porphyrin chromophore was ascertained by the excitation spectrum of the Ybm luminescence being identical to the absorption spectrum of the complex. In benzene, the quantum yield of... 

In this connection, the authors also report an analog of 14 in which the two porphyrins and the acceptor assume a more extended conformation wherein the moieties are side-by-side, rather than stacked. The electronic coupling is not as good in this case, as demonstrated by the absorption spectrum. In this analog, both the photoinitiated electron transfer and the recombination of the P+-P-AT state are substantially retarded [66]. 

Triad 25 is another example of this general type [75]. As was the case with the previously discussed triads 15—18, the absorption spectrum of 25 indicates some degree of excitonic interaction between the porphyrins. The fluorescence quantum yield of 25 is 5 5 x 10-6, which indicates efficient quenching of the porphyrin singlet states, presumably by electron transfer. No information concerning the lifetime of any charge separated state was presented, but one would predict that it would be extremely short. 

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