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Evidence that closely coupled porphyrin-related structures are involved in bacterial1 or green plant2 photosynthesis has led to much recent work on such associated systems in vitro, in which a number of covalently linked porphyrin "dimers" or higher complexes have been synthesized by various means.3 We now

Proton NMR spectroscopy has been used to study the protonation of meso-tetraphenylporphyrins with zero, one, or four para dimethylamino substituents. Changes in the spectra are similar for corresponding protons through this series of compounds. We give the following interpretations. The internal pyrrolenine nitrogen atoms always add two protons before any peripheral dimethylamino groups react. There is no evidence for a monoprotonated intermediate between the free base and diprotonated porphyrin in the chloroform-trifluoroacetic acid solvent system used. The chemical shifts of the NMR signals of the internal N-H protons are strongly acid-dependent and suggest a loss of aromatic ring current at the porphyrin core a t intermediate acid concentrations. Changes in chemical shifts of protons on the periphery of these aromatic systems are much smaller and in the opposite sense. The signals of the pyrrole Hp protons move discontinuously upfield upon diprotonation and then drift continuously downfield as acid concentration is increased beyond that required for diprotonation. These changes are also in the directions to be expected if aromatic ring currents of the porphyrin decrease to a minimum a t the acid concentration a t which the internal nitrogen atoms are fully protonated. W e suggest that this is due to the known distortion from planarity of the diprotonated tetraarylporphyrin aromatic system. Signals of protons attached to the meso-substituted aromatic rings move unidirectionally downfield as acid concentration increases.The changes in optical spectra of porphyrins on addition of acid have been used extensively to follow the evolution of solute species as basic sites are protonated.' Careful analysis of the shapes of these spectrophotometric titration curves can establish the sequence of distinguishable states and in favorable cases the number of protons added if successive sets of isosbestic points can be identified. However, there can be problems in determining the concentration of free acid in the nonaqueous solvents generally used for these studies. Furthermore, if several competing basic sites are available, the spectrophotometric data can at best tell only indirectly which sites are protonated. Although proton NMR spectra of porphyrins have been routinely recorded as a step in characterization, they often have not been used to count the number of protons at specific basic sites in these molecules, by integration of appropriate NMR signals. Also, with some exceptions, the spectra have been run only in a nonpolar neutral solvent or in trifluoroacetic acid but not at intermediate acid concentrations.2 Theexpectation has been that porphyrins without basic groups on the periphery of the ring system will add two protons on the two pyrrolenine nitrogen atoms which lack them in the free base. This expectation was confirmed by early acidbase titrations3 and NMR studies4 which have been interpreted as demonstrations of diprotonation.In this paper we report proton NMR studies on mesotetraphenylporphyrin with from zero...

Spin-polarized transient EPR spectra measured at 9 GHz (X-band) and 24 GHz (K-band) are observed for the metalloporphyrin heterodimers formed by MTTAP (metal/free base meso-tetrakis[4-trimethylanilinium]porphyrin, M = Zn, Mg or H2) and CuTSPP (copper meso-tetrakis[p-sulfonatophenyl]porphyrin). In the temperature range 5−120 K, the EPR transients obtained following pulsed light excitation at 532 nm show a strong temperature dependence. The dimers exhibit two distinct types of EPR signals which may be separated on the basis of their different decay kinetics. The spectrum of one of the signals can be assigned to a triplet state of the dimers, whereas the other spin-polarized spectrum clearly shows involvement of the Cu(II) metal ion. This spectrum is tentatively assigned to the doublet ground state of the CuTSPP moiety in the dimers. It is proposed that the observed spin polarization is generated by a radical triplet pair mechanism (RTPM). In ZnTTAP−CuTSPP and H2TTAP−CuTSPP both spectra are observed, whereas only the latter spectrum is obtained in MgTTAP−CuTSPP. In ZnTTAP−CuTSPP, the triplet-state spectrum shows only small but significant differences from that of the isolated ZnTTAP monomer, indicating that the triplet state is localized on the ZnTTAP component of the dimer. The corresponding spectrum in the free-base dimer is considerably narrower than that of the monomer H2TTAP which suggests that, in this case, the triplet state is partly delocalized over both halves of the dimer.