The thermal aquation rate constant and the photoaquation quantum yield of [Cr(CN)(6)](3-) are reduced by a factor of 40 and 3, respectively, when the complex forms a 1:1 adduct with the protonated form of the polyaza macrocycle [32]ane-N-8. On the other hand, the same macrocycle has practically no effect on the photoaquation of [Cr(CN)(5)(H2O)](2-) and a very small effect on the thermal reaction of this complex. These results are discussed in relation to the thermal and photochemical reaction mechanisms and to the steric configuration of the adducts between complexes and macrocycle.

The steady-state fluorescence emission spectra of the azacyclophanes 2,5,8,11-tetraaza[12] paracyclophane (L(1)), 2,6,9,13-tetraaza[14]paracyclophane (L(2)), 14,15,17,18-tetramethyl-2,5,8,11-tetraaza-[12]paracyclophane (L(3)) and 16,17,19,20-tetramethyl-2,6,9,13-tetraaza[14]paracyclophane (L(4)) as a function of pH have been measured. The fully protonated species of each cyclophane gives the highest fluorescence-emission quantum yield. The shapes of the titration curves have been explained by the existence of an electron-transfer quenching effect from a non-protonated amine to the benzene chromophore. This effect is greater for macrocycles in which the first deprotonated amine group is closer to the benzene. The association constants for the interaction of the four fully protonated macrocycles with K-3[Co(CN)(6)] have been measured either by potentiometry or from fluorescence-emission measurements, and increase in the order L(3) approximate to L(4) < L(1) approximate to L(2). The photoaquation quantum yields of K-3[Co(CN)(6)] have been measured in the presence of the macrocycles L(1) and L(2). and indicate that three of the CN nitrogens of the complex are involved in adduct formation with the fully protonated macrocycles, as supported by molecular modelling.

When biacetyl is imprisoned into Cram's hemicarcerand 1, its absorption, fluorescence, and phosphorescence maxima are red shifted compared to the values obtained for solutions of free biacetyl in any solvent. Furthermore, the lifetime of the T-1 excited state of imprisoned biacetyl is unaffected by solvent nature and presence of dioxygen. These results show that inclusion into the hemicarcerand (i) shields biacetyl from interaction with the solvent molecules and (ii) prevents deactivation of its long-lived T-1 excited state by energy transfer to dioxygen. The perturbation provided by the cavity on the spectroscopic properties of biacetyl is much smaller than that provided by even the most ''innocent'' solvent. The consequent picture is that of a biacetyl molecule which is contained in a not-too-tight cavity where no specific host-guest interaction takes place. The peculiar spectroscopic and excited-state behavior of biacetyl imprisoned in hemicarcerand 1 supports Cram's view that the inner phase of carcerands and hemicarcerands is to be considered as a new phase of matter.

The synthetic compound 3,4'-dimethoxy-7-hydroxyflavylium chloride gives rise, in aqueous solution at moderately acidic pH, to a pH-dependent equilibrium between the flavylium cation, hemiacetal, (Z)-chalcone and a small amount of quinonoidal base. The distribution, as a function of pH, of the molar fractions of the several species present in solution have been calculated on the basis of H-1 NMR and pH jump experiments monitored by stopped-flow and conventional UV-VIS spectrophotometry, and highperformance liquid chromatography (HPLC). The compound shows interesting photochemical properties: (i) at pH 4.0 it presents a photochromic effect that converts (Z)-chalcone into hemiacetal, the reaction being reversible in the dark and (ii) excited-state proton transfer is observed between the flavylium cation and quinonoidal base. An appropriate formalism to quantify the experimental results has been developed. The formalism allows determination of the pH-dependent molar fraction distribution of the several anthocyanin forms present at equilibrium, as well as predicting the distribution of the molar fractions prior to equilibrium.

The energy- and electron-transfer quenching processes of the lowest triplet excited state of biacetyl (2,3-butanedione) imprisoned in a hemicarcerand have been systematically investigated in CH2Cl2 solution at room temperature. Twenty potential quenchers have been used, including ten triplet energy accepters (mostly, aromatic hydrocarbons) and nine electron donors (mostly, aromatic amines). Bimolecular rate constants for the quenching processes were obtained by Stern-Volmer analysis and compared with those found for the quenching of free biacetyl. In the electron-transfer processes, aromatic amines with oxidation potential from +0.015 V (N,N,N',N'-tetramethyl-p-phenylenediamine) to +0.83 V (diphenylamine) quench free biacetyl at the diffusion-controlled limit, whereas for imprisoned biacetyl the rate constant decreases (roughly in a linear manner) from 4.0 x 10(8) to 1.2 x 10(5) M(-1) s(-1) As far as energy-transfer is concerned, the rate constant for the quenching of free biacetyl increases with decreasing Delta G degrees and reaches the diffusion-controlled plateau value (k(q) similar to 10(10) M(-1) s(-1)) for Delta G degrees similar to 0.1 eV, whereas for imprisoned biacetyl a scattered, bell-shaped log k(q) vs Delta G degrees plot is obtained, with a maximum value (similar to 10(6) M(-1) s(-1)) much below the diffusion-controlled limit. The results obtained show that the walls of the hemicarcerand allow only very weak electronic interaction between incarcerated triplet biacetyl and external quenchers. A brief discussion of the results obtained in the light of current energy- and electron-transfer theories is presented.

The formation of a supercomplex between the Ru(bpy)(CN)(4)(2-) (bpy = 2,2'-bipyridine) complex and the [32]-ane-N8H88+ macrocycle (1) has been studied in water and in acetonitrile. In acetonitrile, supercomplex formation is accompanied by (i) large hypsochromic shifts in the absorption spectrum (color changes from deep violet to yellow) and in the emission spectrum, (ii) large anodic shifts in standard oxidation (0.73 V) and reduction (0.37 V) potentials, (iii) typical shifts of H-1-NMR signals for the macrocycle N-bound protons and the complex bipyridine protons, and (iv) a large increase in the MLCT excited-state lifetime of the complex. In water, the spectral shifts and the changes in standard potential are much less pronounced, but supercomplex formation is evidenced by C-13-NMR (and H-1-NMR) and by emission lifetime changes. In both solvents, supercomplex formation is complete in 1:1, 1.0 x 10(-4) M solutions, indicating very large stability constant values. A structure of the supercomplex with the macrocycle bound in a ''boat'' conformation to the four cyanide ligands of the complex, plausible in terms of molecular models, is consistent with all the experimental data. In water, the supercomplex further associates with added negative species containing carboxylate functions, as shown by partial reversal of the lifetime changes. When the added species is also a potential electron transfer quencher (such as, e.g., Rh(dcb)(3)(3-), dcb = 4,4'-dicarboxy-2,2'-bipyridine), however, association is not accompanied by quenching. This behavior is attributed to the structure of the supercomplex-quencher adduct, in which the macrocycle acts as an insulating spacer between the excited complex and the quencher.

Some new properties promoted by the formation of a supercomplex between iron hexacyanometallates and the polyazamacrocycle [32]aneN(8) (1,5,9,13,17,21,25,29-octaazacyclodotrane) are described. In the presence of the polyazamacrocycle, thermal and photoinduced electron transfer from iodide to Fe(CN)(6)(3-) were observed in moderately acidic media. The thermal reaction is slow (k(obs) = 8.9 x 10(-4) s(-1), at 25 degrees C) and proceeds to an equilibrium (K = 7 M-2, at 25 degrees C). The reaction is almost isoergonic, with favorable enthalpy and unfavorable entropy changes (Delta G degrees = -4.8 kJ mol(-1), Delta H degrees = -160 kJ mol(-1), Delta S degrees = -0.54 kJ mol(-1) K-1). A photoinduced electron-transfer process, leading to additional iodide oxidation, was observed upon flash irradiation of equilibrated solutions. Following the photoinduced process, the system reverts to the thermal equilibrium in the dark. The promoting role of the macrocycle is thermodynamic for the thermal process (anodic shift in the Fe-II/III potential upon supercomplex formation) and kinetic for the photoinduced process [formation of ion-paired species between hexacyanoferrate(III) and iodide upon supercomplex formation]. The thermal reaction is reversible in basic media (where the macrocycle deprotonates and supercomplex formation is prevented), providing an example of on/off switching by pH changes of an electron-transfer reaction.

A complete study of the structural pH dependent transformations of the synthetic flavylium salt 4',7-dihydroxyflavylium chloride (DHF), occurring in aqueous solutions, including the basic region, is described. The kinetic study of the transformations occurring in acidic media (quinoidal base (A) reversible arrow flavylium cation (AH(+)) reversible arrow hemiacetal (B) reversible arrow cis-chalone (C-cis) reversible arrow trans-chalcone (C-trans)) allowed to conclude that the cis-trans isomerization is faster than the tautomerization and the hydration processes, which is unique in the anthocyanins family. Results obtained with the parent compound 4'7-dimethoxyglavylium chloride (DMF) with relevance to this study are also presented. In equilibrated basic solution the existence of acid-base equilibria involving the trans-Chalcone (C-trans) and its conjugated bases, (C-trans(-) and C-trans(2)), was detected. Freshly prepared solutions at pH >7 show also the presence of a transient species identified as the ionized quinoidal base (A(-)), which is almost completely converted into C-trans(2-) with a pH dependent rate constant.