Electronic absorption spectra porphyrin, with

The next important information on the product is derived from the electronic absorption spectrum. This can usually be obtained by electrolysis of a dilute solution of the metalloporphyrin at a constant potential or by oxidometric titrations directly in the absorption cell. Most redox reactions with metalloporphyrins give good isosbestic points when absorption spectra are taken at various stages of oxidation, and they are fully reversible when no chemical addition reaction to the porphyrin ligand has occurred. Ten typical absorption spectra of metalloporphyrins are given below and correlated with the various metalloporphyrin oxidation states. 

The electronic absorption spectrum recorded for 16 in N,N-dimethylformamide (DMF) is similar to that of the corresponding monomeric porphyrin and there was no indication of exciton interaction between the two porphyrin rings [16]. Fluorescence was not observed, presumably because the excited singlet state lifetime is shortened due to the internal heavy-atom effect, but weak phosphorescence was detected in a methanol glass at 77K with a maximum at 690 nm. Excitation of 16 with a... 

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

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). 

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... 

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. 

Significant effects of metal ions on photoinduced FT in a zinc porphyrin-naphthalenediimide (ZnP NIm) dyad were reported to attain the long-lived CS state (50). A transient absorption spectrum observed at 0.1 Xs after the laser pulse excitation of a PhCN solution of ZnP NIm is shown in Fig. 10(a) (50). The transient absorption bands at479,531,583,620,685 (sh), and 763 nm are assigned to NIm" by comparison with those of NIm" produced independently by the one-electron reduction of ZnP NIm with tetramethylsemiquinone radical anion [Fig. 10(Z))] (50). The absorption band due to ZnP" + is also observed at 410 nm. Thus, the photoexcitation of ZnP NIm results in FT from ZnP to NIm to give the CS state (ZnP —NIm" ). Fach absorption band decays at the same rate, obeying first-order kinetics [inset of Fig. 10(a)]. The rate constant (Atcr) of the CR process of the CS state is obtained from the first-order plot ZnP —NIm as 7.7 x 10 s (the lifetime x= 1.3 xs) (50). 

The conclusion that the cobalt and iron complexes 2.182 and 2.183 are formally TT-radical species is supported by a wealth of spectroscopic evidence. For instance, the H NMR spectrum of the cobalt complex 2.182 indicated the presence of a paramagnetic system with resonances that are consistent with the proposed cobalt(III) formulation (as opposed to a low-spin, paramagnetic cobalt(IV) corrole). Further, the UV-vis absorption spectrum recorded for complex 2.182 was found to be remarkably similar to those of porphyrin 7r-radicals. In the case of the iron complex 2.183, Mdssbauer spectroscopy was used to confirm the assignment of the complex as having a formally tetravalent metal and a vr-radical carbon skeleton. Here, measurements at 120 K revealed that the formal removal of one electron from the neutral species 2.177 had very little effect on the Mdssbauer spectrum. This was interpreted as an indication that oxidation had occurred at the corrole ligand, and not at the metal center. Had metal oxidation occurred, more dramatic differences in the Mdssbauer spectrum would have been observed. 

The aromatic nature of the [18]porphyrin-(2.1.0.1) derivative 3.135, as well as analogs 3.136, 3.137, and 3.144 (systems we will hereafter refer to as corrphycenes), was confirmed by the observation of sustained diamagnetic ring current effects in its H NMR spectrum. The UV-vis absorption spectrum (in benzene) is also consistent with the proposed aromaticity and with an 18-7t-electron formulation. Specifically, a... 

Absorbance attributed to the CT state decayed with a lifetime of 20 ps (Figure 26d) to leave a residual species having the characteristic differential absorption spectrum of the Au porphyrin neutral radical (Figure 26a and b). Deactivation of the CT state may involve direct reverse electron transfer between porphyrinic species ... 

The CT state was found to decay rapidly (Figure 27c), with a lifetime of 50 ps, leaving a residual absorbance which was identified as the Au porphyrin neutral radical by virtue of its differential absorption spectrum. This latter species decayed relatively slowly, with a lifetime of 2.5 ns, to re-form the ground state of Cu.20. As above, the rapid deactivation of the CT state is ascribed to a combination of direct reverse electron transfer (Eq. 18) and oxidation of the central copper(I) complex by the Zn porphyrin 7r-radical cation (Eq. 19). The yield of the Au(III) porphyrin neutral radical which escaped direct electron transfer was estimated from the transient absorption spectral changes to be 90 %. Thus, direct reverse electron transfer (/ i8 = 2.0 X 10 s ) accounts for only 10 %, and electron abstraction from the central copper(I) complex k g = 1.8 x 10 s ) is the dominant decay route. The residual Au porphyrin neutral radical decays over several nanoseconds due to electron donation to the copper(II) complex (Eq. 20). 

---