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Groupe d'Optique Appliquee, Centre de Recherches Nucleaires et Universite Louis Pasteur, 23, rue duRuthenium(1i) and osmium(i1) complexes with 2,Z': 6',2"-terpyridine (terpy), 4'-(p-tolyl)-Z,Z': 6',2"terpyridine (tterpy) and 4,4',4"-triphenyl-2,Zf: 6',2"-terpyridine (tpterpy) have been prepared and characterized. Their electrochemical, photophysical and photochemical properties (room temperature) have been studied. Metal-to-ligand charge-transfer excited-state quenching measurements using various redox active species clearly show that [Ru(tpterpy)J2+ is a good electron-transfer agent under light irradiation.The room-temperature emitting character of numerous ruthenium(I1) complexes bearing bidentate ligands of the 2,2'-bipyridine (bipy) type is now well established and documented. 1.2 By contrast, the corresponding ruthenium(I1) and osmium(r1) complexes containing tridentate ligands of the 2,2' : 6',2''-terpyridine (terpy) type have been much less investigated.3-9 Among others, a possible explanation for this is the delicate synthetic problems attached to terpy derivatives and to their complexes [in particular for osmium(II)].l Another reason is that the luminescence properties of both families of ruthenium(r1) compounds are drastically different. For instance, it is striking that [ R~( t e r p y )~]~+ , the parent compound of the so universally utilized [Ru(bipy),]' + complex, becomes luminescent only at low temperatures.Some systems containing terpy derivatives have been made so as to render the corresponding ruthenium(I1) complexes more compact or with lower lying metal-to-ligand charge-transfer (m.1.c.t.) excited states.6 However, none of the ruthenium complexes examined turned out to luminesce at room temperature, except for [ R~( t p t e r p y )~]~ + (tpterpy = 4,4',4"-triphenyl-2,2' : 6',2"-terpyridine) which was briefly reported as a light emitter in ethanol-methanol with a lifetime of ~0 . 2 p6 Intrigued by these results we have repeated the synthesis of [ R~( t p t e r p y )~]~ + and prepared the new complex [Os-( t~t e r p y )~]~' . We now report some of their photophysical and photochemical properties. For comparison, the lesssubstituted complexes [Ru( terpy) +, [ Ru(tterpy),] + [tterpy = 4'-(p-tolyl)-2,2': 6',2"-terpyridine], [Os(terpy),12 + and [Os(tterpy)J2 + have been studied under identical conditions. Results and DiscussionSjwtheses qf Ligands and Complexes.-The synthesis of terpy was first developed by Case and Kasper" and has been improved recently by Spahni and Calzaferri. l 2 The overall yield of tpterpy can be dramatically improved (387)) using this threestep procedure derived from Krohnke's method.' Various complexes of Ru" and 0 s " were prepared by procedures derived Table 1 SCE) ( k 0.002 V) * Electrochemical data for complexes of Ru and Os, E,/V ( u s . Complexes MIll/ll M"/I Ml/o CRu(terpy)212+ 1.30 -1.24 -1.49 CRu(tterPY)*I2 + 1.26 -1.22 -1.46 [ Ru(tpterpy),]' + 1.22 -1.19 -1.42 COs(terPY)212 + 0.97 -1.23 -1.52 COs(tterPY)212 + 0.93 -1.22 -1.54 COs(tPterPY)*l2 + 0.90 -1.15 -1.40 * In...

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However, bioavailability considerations seem to suggest that the reactive surface, S, is of greater relevance.34 The dissolution of some drugs is summarized in 26-29: The Z>R -2 values for all indicate that the dissolution occurs at the outer exposed regions of the drug crystal.Summary. The reactive regions of many heterogeneous reactions have a characteristic reaction dimension, Z)R. Z)R may be larger, smaller, or equal to the surface fractal dimension, D (see the recently suggested mutlifractal concept35). Z)R is sensitive to reaction conditions (compare the following pairs: 1, 2; 3, 4; 15, 16; 23, 24). The absolute and the relative values of Z)R add to the understanding of the reaction mechanism. An important conclusion made is that the reaction efficiency can be increased by reducing the particle size for DK < D, m > 1 cases and increasing particle size for Z)R > D, m < 1 reactions. This can be analyzed by the generalized Wenzel law (eq 5).The ideas presented in this report have been already utilized successfully by other groups, e.g., in coal gasification (30).36 We •