Publications

Export 278 results:
Sort by: Author Title Type [ Year  (Desc)]
1998
Lima, JC, Abreu I, Brouillard R, Macanita AL.  1998.  Kinetics of ultra-fast excited state proton transfer from 7-hydroxy-4-methylflavylium chloride to water. Chemical Physics Letters. 298:189-195., Number 1-3 Abstract
n/a
Pina, F, Roque A, Melo MJ, Maestri I, Belladelli L, Balzani V.  1998.  Multislate/multifunctional molecular-level systems: Light and pH switching between the various forms of a synthetic flavylium salt. Chemistry-a European Journal. 4:1184-1191., Number 7 Abstract
n/a
Vigil, MR, Renamayor CS, Pierola I, Lima JC, Melo EC, Macanita AL.  1998.  Non-diffusion-controlled excimer formation with indane and acenaphthene. Kinetics and thermodynamics from picosecond-time-resolved fluorescence. Chemical Physics Letters. 287:379-387., Number 3-4 Abstract
n/a
1997
Pina, F, Parola AJ, SaintMaurice A, Manfrin MF, Moggi L, Indelli T, Scandola F.  1997.  Electron transfer between Fe(CN)(6)(3-) and iodide promoted by supercomplexation with a polyammonium macrocycle, 1997. Journal of the Chemical Society-Dalton Transactions. :2327-2330. Abstract

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.

Pina, F, Benedito L, Melo JM, Parola AJ, Lima JC, Macanita AL.  1997.  Structural transformations of the synthetic salt 4',7-dihydroxyflavylium chloride in acid and basic aqueous solutions .1. Ground state, 1997. Anales De Quimica. 93:111-118. Abstract

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.

Maestri, M, Ballardini R, Pina F, Melo MJ.  1997.  An easy and inexpensive flash spectroscopy experiment. Journal of Chemical Education. 74:1314-1316., Number 11 Abstract
n/a
Pina, F, Melo MJ, Ballardini R, Flamigni L, Maestri M.  1997.  Flash photolysis of 4',7-dihydroxyflavylium perchlorate. New Journal of Chemistry. 21:969-976., Number 9 Abstract
n/a
Pina, F, Melo MJ, Maestri M, Ballardini R, Balzani V.  1997.  Photochromism of 4'-methoxyflavylium perchlorate. A ''write-lock-read-unlock-erase'' molecular switching system. Journal of the American Chemical Society. 119:5556-5561., Number 24 Abstract
n/a
1996
Pina, F, Benedito L, Melo MJ, Parola AJ, Bernardo A.  1996.  Photochemistry of 3,4'-dimethoxy-7-hydroxyflavylium chloride - Photochromism and excited-state proton transfer, 1996. Journal of the Chemical Society-Faraday Transactions. 92:1693-1699. Abstract

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.

Parola, AJ, Pina F, Ferreira E, Maestri M, Balzani V.  1996.  Photoinduced electron- and energy-transfer processes of biacetyl imprisoned in a hemicarcerand, 1996. Journal of the American Chemical Society. 118:11610-11616. Abstract

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.

Melo, MJ, Bernardo MA, Melo EC, Pina F.  1996.  Shape of acid-base fluorescence emission titration curves in the presence of buffer and quenching effects. Journal of the Chemical Society-Faraday Transactions. 92:957-968., Number 6 Abstract
n/a
1995
Sotomayor, J, Parola AJ, Pina F, Zinato E, Riccieri P, Manfrin MF, Moggi L.  1995.  Control of the thermal and photochemical aquation of the hexacyanochromate(III) and aquapentacyanochromate(III) ions by formation of a supramolecular structure with 32 ane-N-8, 1995. Inorganic Chemistry. 34:6532-6537. Abstract

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.

Bernardo, MA, Parola AJ, Pina F, Garciaespana E, Marcelino V, Luis SV, Miravet JF.  1995.  STEADY-STATE FLUORESCENCE EMISSION STUDIES ON POLYAZACYCLOPHANE MACROCYCLIC RECEPTORS AND ON THEIR ADDUCTS WITH HEXACYANOCOBALTATE(III), 1995. Journal of the Chemical Society-Dalton Transactions. :993-997. Abstract

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.

Pina, F, Parola AJ, Ferreira E, Maestri M, Armaroli N, Ballardini R, Balzani V.  1995.  SUPRAMOLECULAR PHOTOCHEMISTRY AND PHOTOPHYSICS - BIACETYL IMPRISONED IN A HEMICARCERAND, 1995. Journal of Physical Chemistry. 99:12701-12703. Abstract

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.

1994
Parola, AJ, Pina F, Maestri M, Armaroli N, Balzani V.  1994.  SUPRAMOLECULAR PHOTOCHEMISTRY AND PHOTOPHYSICS - 9-CYANOANTHRACENE IMPRISONED IN A HEMICARCERAND, 1994. New Journal of Chemistry. 18:659-661. Abstract

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.

Lima, JC, DANESH P, FIGUEIREDO P, PINA FS, MACANITA A.  1994.  EXCITED-STATES OF ANTHOCYANINS - THE CHALCONE ISOMERS OF MALVIDIN 3,5-DIGLUCOSIDE. Photochemistry and Photobiology. 59:412-418., Number 4 Abstract
n/a
FIGUEIREDO, P, Lima JC, Santos H, WIGAND MC, Brouillard R, Macanita AL, Pina F.  1994.  PHOTOCHROMISM OF THE SYNTHETIC 4',7-DIHYDROXYFLAVYLIUM CHLORIDE. Journal of the American Chemical Society. 116:1249-1254., Number 4 Abstract
n/a
Melo, MJ, Macanita AL, Melo E, Wamhoff H, Pina F.  1994.  PHOTOPHYSICAL PROPERTIES AND PHOTODEGRADATION MECHANISM OF 2-(2'-FURANYL)-1H-BENZIMIDAZOLE (FUBERIDAZOLE). Journal of Photochemistry and Photobiology a-Chemistry. 83:237-244., Number 3 Abstract
n/a
1993
Santos, H, Turner DL, Lima JC, FIGUEIREDO P, PINA FS, Macanita AL.  1993.  ELUCIDATION OF THE MULTIPLE EQUILIBRIA OF MALVIN IN AQUEOUS-SOLUTION BY ONE-DIMENSIONAL AND 2-DIMENSIONAL NMR. Phytochemistry. 33:1227-1232., Number 5 Abstract
n/a
1992
Pina, F, Parola AJ, Bencini A, Micheloni M, Manfrin MF, Moggi L.  1992.  CHARGE EFFECTS ON THE PHOTOCHEMISTRY OF THE CO(EDTA) .1. SYSTEM IN THE PRESENCE OF POLYAMMONIUM MACROCYCLIC RECEPTORS, 1992. Inorganica Chimica Acta. 195:139-143. Abstract

The effects of polyammonium macrocycles on the spectroscopic and photochemical properties of the Co(EDTA)- . I- ion-pair have been investigated. The addition of a macrocycle to aqueous solutions containing Co(EDTA)- and I- causes an increase of the absorbance in the region of the ion-pair charge-transfer band, as well as an increase of the quantum yield for the intramolecular photooxidation reduction of the ion-pair. Both these effects are mainly, if not only, due to an increase of the association constant between Co(EDTA)- and I-, caused by the positive charge of the macrocycle bound to the complex. On the contrary no change was observed on the intrinsic photoreactivity of the excited ion-pair. This last result is discussed in comparison with the effects already observed on the ligand photodissociation of MC excited states of Co(III) cyanide complexes.

Melo, MJ, Pina F, Macanita AL, Melo EC, Herrmann C, Forster R, Koch H, Wamhoff H.  1992.  PHOTOCHEMISTRY OF 2-(2-FURYL)-BENZIMIDAZOLE (FUBERIDAZOLE). Zeitschrift Fur Naturforschung Section B-a Journal of Chemical Sciences. 47:1431-1437., Number 10 Abstract
n/a
1990
FIGUEIREDO, P, Pina F, Vilasboas L, Macanita AL.  1990.  FLUORESCENCE-SPECTRA AND DECAYS OF MALVIDIN 3,5-DIGLUCOSIDE IN AQUEOUS-SOLUTIONS. Journal of Photochemistry and Photobiology a-Chemistry. 52:411-424., Number 3 Abstract
n/a
Pina, F, Sotomayor J, Moggi L.  1990.  PHOTOCHEMISTRY OF THE CO(SEP)3+-I- SYSTEM - EVALUATION OF THE QUANTUM YIELD FOR THE PHOTOCHEMICAL REDOX REACTION OF THE ION-PAIR. Journal of Photochemistry and Photobiology a-Chemistry. 53:411-422., Number 3 Abstract
n/a
1989
Pina, F, Moggi L, Manfrin MF, Balzani V, Hosseini MW, Lehn JM.  1989.  PHOTOCHEMISTRY OF SUPRAMOLECULAR SYSTEMS - SIZE AND CHARGE EFFECTS IN THE PHOTOAQUATION OF ADDUCTS OF THE HEXACYANOCOBALTATE(III) ANION WITH POLYAMMONIUM MACROCYCLIC RECEPTORS. Gazzetta Chimica Italiana. 119:65-67., Number 1 Abstract
n/a
loading