The new cylindrical molecule L containing two tetraazamacrocyclic rings linked by two azobenzene pillars displays photoelastic properties, Light absorption at 366 nm gives rise to trans --> cis isomerization of the azobenzene moieties producing two isomers containing one or two cis-azobenzenes, respectively, The three trans-trans (E-E), trans-cis (E-Z) and cis-cis (Z-Z) isomers have been identified and characterized by H-1 NMR spectroscopy, allowing the dependence of their formation percentages with irradiation time to be determined, The sequence of photochemical reactions E-E --> E-Z --> Z-Z allows almost complete conversion of the E-E into the Z-Z isomer at 366 nm and 298 K, Both thermal (k = 1.75 x 10(-5) s(-1) at 313 K) and photo-induced (at 436 and 313 nm) back-isomerization reactions have been studied, The protonation constants of the three isomers in equimolar solutions of water-DMSO indicate a decreasing basicity in the order E-E > E-Z > Z-Z, in agreement with increasing electrostatic repulsion between the positive charges caused by a reduction in the separation between the protonation sites occurring upon Z --> E isomerization.

The structural transformations and photochromic properties of the 7-hydroxyflavylium ion have been investigated by means of the pH jump technique and continuous and pulsed light excitation. The primary photoproduct of UV irradiation of the colorless trans-chalcone form, which is the predominant species at pH 4, is its colorless cis isomer, which rapidly disappears on a time scale of seconds through two competitive processes: i) back-reaction to yield the trans-chalcone form, and ii) formation of the colored flavylium ion and its conjugated quinoidal base. Over minutes or hours (depending on pH), the system reverts quantitatively to its original state. The rate constants and equilibrium constants of the various processes have been obtained and compared with those previously reported for the 4'-hydroxyflavylium and 4',7-dihydroxyflavylium ions. This comparison demonstrates the substituent effect on the rate and equilibrium constants; the effect on the rate constant of the cis-->trans thermal isomerization reaction is particularly strong. For the 7-hydroxyflavylium and 4',7-dihydroxyflavylium ions the pH of the solution plays the role of a tap for the color intensity generated by light excitation. This also means that this system can be viewed as a light-switchable pH indicator.

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.

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 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 absorption and excited state properties of 9-cyanoanthracene are strongly modified upon inclusion into an octaimine hemicarcerand; the walls of the host do not transfer excitation to the guest and quench its fluorescent excited state.