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ionic) character peculiar to the S, perp minima. The latter avoided crossing is similar in nature to the 'B-'A (vibronically) avoided crossing involved in the trans -* cis photoisomerism of stilbene.39 The observed decrease of the activation energies with increasing solvent polarity can easily be explained within the proposed mechanism. In fact, because of their high dipole moments (~1 0 D) S, perp minima may be markedly stabilized in highly polar solvents and this may cause appreciable lowering of the trans -* perp barriers.4. The photoreaction mechanism is well interpretable in the light of both the experimental results and theoretical calculations. The nonadiabatic singlet mechanism (activated twisting to the funnel 'perp, internal conversion to °perp, and relaxation to the two geometrical isomers), favored in the case of stilbene because of the small A£tp, is operative here only at high temperatures because of the high barriers. At room temperature or below room temperature (depending on the solvent), where and rF remain constant, the fluorescent state is no longer the isomerizable state and the photoreaction proceeds through a different pathway. According to the calculation results which do not show any evidence for the existence of other singlet states below the fluorescent St state (Figure 5), the most reasonable and least speculative assumption is that the triplet pathway becomes the prevalent mechanism for isomerization. Fluorescence and ISC seem then to offer the predominant deactivation pathways in these experimental conditions. The smaller photoisomerization quantum yields obtained, compared to stilbene, are thus easily understood since kF is here of the same order of magnitude as kISC, the rate for the reactive deactivation at or below room temperature. The comparison of the rate parameters for the competitive deactivation processes of S, clearly indicate the relative weight of the two reaction mechanisms at any temperature.5. The results obtained, different from those of stilbene but similar to those of StNs, indicate that the observed behavior is probably of general character. They show that, when one of the phenyl groups of stilbene is replaced by a polycyclic group (naphthyl, phenanthryl, etc.) of low S, energy, the lowest excited states acquire an increasing aromatic character thus leading to high activation energies for the 'trans -'perp rotation and correspondingly to high values. Consequently, the photoisomerization mechanism gradually change from singlet to triplet and the radiative decay becomes the main deactivation path of excited styrylaromatics.Acknowledgment. Financial support by the Consiglio Nazionale delle Ricerche and Ministero per la Pubblica Istruzione (Roma) and by the Centro di Calcólo Elettronico (University of Modena) is gratefully acknowledged. A.S. thanks the Associazione Italiana per la Ricerca Industríale for the award of a fellowship.

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The synthesis, optical spectra, and x-ray pattern of iron naphthalocyanine %S the pure powder are reported. The electrochemical properties for 02 reduction and evolution o~ this compound loaded on carbon Dlack supports have been determined by ultratnin and rotating ring-disk electrode techniques. While on the carbon black support 02 reduction occurs through a two electron pathway followed by HO2-decomposition, four electron reduction is obtained on the loadings with a high efficiency. Stability tests proved their applicability as useful catalysts as the air cathode in metal-air generators.While the capacity of monomeric phthalocyanine materials for nearly reversible charge transfer reaction (1) has been demonstrated, the main objection to their use as practical catalysts m metal-air or fuel cell generators has been their lack of stability. However, by synthesizing polymeric materials, high stability (2) during continuous operation m ~V~ S(941-i2 for times up to 3000 hr has been obtained. The working potentials of the oxygen electrode remain about 400 mV below the four electron reversible potential. In these types of compounds the effect of the enlargement of the 3electron system on their activity has been emphasized by several groups (3a, b).One might hope to be able to apply such materials to metal-air systems due to the lower current densities (C.D.) involved. On different types of polymeric iron phthalocyanines (4) spectroscopic and electrochemical investigations have been performed in alkaline media and correlations between the activity and the structural properties have been found. The role of the 3electron delocalization in the optimization of the free energy of oxygen adsorption on the iron sites (due to ~-d hybridation) has been confirmed by ESCA techniques (5). In the case of compounds of larger sizes the ;~-electron delocalization is inhibited due to spinspin interactions. Such effects upon chemical reactivity have already been noticed in the chemistry of conjugated ring systems (6), the activity decreasing i1~ the order anthracene > naphthalene > benzene.Besides this :t-electron density condition, many factors can affect the electrochemical activity, for example, the junction with the support, the wettability, or the d orbital configuration of the iron which is very critical (2-4). The difficult problem is to meet all these requirements simultaneously. Several types of synthesis have been worked out. A rapid technique for the comparison of the electrochemical activity is the ultrathin electrode (7). The preparations were tested * Electrochemical Society Active Member.