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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.

FeNPc(l.2) impregnations were investigated at various loadings using a rotating-disk electrode, voltammetry and X-ray photoelectron spectroscopy (XPS) techniques. Optimal activity takes place at monolayer coverage, and major demetallation occurs after prolonged exposure to acid solution. The substitution of Fe by Co enables the electrochemical performances to reach those presented by 10% Pt on Vulcan and stops the demetallation process. The application of fast atomic bombardment secondary ion mass spectrometry (FABS) together with XPS sheds light on the detrimental role of some electrophilic groups attached to the carbon black-catalyst interface.

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