Mikael Sundahl

C60 embedded in γ-cyclodextrin: a water-soluble fullerene

A water-soluble complex of C60 is formed on refluxing a solution of γ-cyclodextrin with solid C60; the lifetime of the triplet excited state of C60 in the complex is 83 µs in an oxygen free solution.

A gas phase container for C60; a γ-cyclodextrin dimer

Abstract A molecular complex consisting of two molecules of γ-cyclodextrin and one molecule of C 60 has been identified by negative FAB/LSIMS mass spectroscopy.

Clusters of C60-fullerene in a water solution containing γ-cyclodextrin : a photophysical study

Abstract γ-CD has been used to dissolve C 60 in water. Depending on the concentration of cyclodextrin two forms of solutions can exist. At high γ -CD:C 60 ratio a complex that is monomeric in C 60 is stable whereas at lower γ -CD:C 60 ratio a cluster of several C 60 molecules surrounded by γ-CD can exist. We propose that clusters of different sizes can be formed. The γ -CD:C 60 complex have an electronic absorption spectrum similar to that of an organic solution of C 60 and the cluster have an electronic absorption spectrum similar to that of a thin film of C 60 . For the γ -CD:C 60 complex the photophysical properties are similar to those of an organic solution with the exceptions for quenching of the triplet state by molecular oxygen and annihilation of the triplet state. The rate constant for quenching of the triplet is reduced by a factor two and the rate constant for annihilation of triplets is reduced by a factor four when compared to those expected for a “free” C 60 , respectively. The properties of the clusters are different from those of an organic solution of C 60 . For water solutions of small clusters, decay of the excited state of the cluster is clearly dependent on the intensity of the excitation laser pulse; at high laser intensity there is both a fast and a slow component in the decay process whereas at low laser intensity only the slow process is observed. For excitation of larger clusters we could only detect a fast decay process.

Host-guest chemistry of fullerenes; a water-soluble complex between C70 and γ—cyclodextrin

Abstract A water-soluble complex between C70 and γ—cyclodextrin has been prepared by boiling a conc. solution of γ—cyclodextrin (0.8M) in water with solid C70 suspended in water. The complex has been characterised by its UV/VIS spectrum and by photophysical methods.

NMR and UV–VIS Investigation of water-soluble fullerene-60–γ-cyclodextrin complex

Fullerene-60 can be made water-soluble by forming an inclusion complex with γ-cyclodextrin (γ-CD). The complex between C60 and γ-CD is selectively formed; neither β- or α-CD, nor C70 takes part in complex formation. The UV–VIS and 13C NMR spectra of C60 in the complex are almost identical to those of C60 in organic solvents. The 1H NMR spectrum of the host, γ-CD, in the complex has been identified. Only small differences to the spectrum of the free host are observed; this is interpreted as being due mainly to a conformational change to a more conical structure in the host. Both 2 : 1 and 1 : 1 complexes between γ-CD and C60 are believed to exist in water. On heating in water, the complexes are transformed into water-soluble aggregates containing several fullerenes and γ-CD molecules. On addition of excess γ-CD to this solution and further heating, the initial complexes, monomeric in C60, are reformed.

Triplet-state quenching in complexes between Zn-cytochrome c and cytochrome oxidase or its CuA domain

The quenching of the triplet state of Zn-cytochrome c in electrostatic complexes with cytochrome oxidase and its soluble CuA domain has been studied by laser flash photolysis. The triplet state of free Zn-cytochrome c decayed with a rate of about 200 s-1. With the oxidase, biphasic decay with rate constants of 2 x 10(5) and 2 x 10(3) s-1, respectively, was observed. At high ionic strength (I = 0.2) the decay was the same as with free Zn-cytochrome c. The quenching was also eliminated by reduction of the oxidase. The decay rate in the complex with the CuA domain was 4 x 10(4) s-1. The results are interpreted in terms of rapid electron transfer to CuA and a slower one to cytochrome a. No electron transfer products were detected, because the backward reaction is faster than the forward one. This can be explained by the high driving force (1.1 eV) for the forward electron transfer, taking the system into the inverted Marcus region. The distance in the electrostatic complex between cytochrome c and the electron acceptor, presumed to be CuA, is calculated to be 16 A.

Intraprotein electron transfer in a ruthenium-modified Tyr83-His plastocyanin mutant: evidence for strong electronic coupling

Abstract A site-directed mutant of spinach plastocyanin, Pc(Tyr83-His), has been modified by covalent attachment of a photoactive [Ru(bpy)2(im)]2+ complex to the His83 residue. The residue is surface exposed and located about 10–12 Å from the copper ion at the entrance of a proposed natural electron transfer pathway from cytochrome f. Electron transfer within the Ru-Pc complex has been studied with time-resolved optical spectroscopy using two different approaches. In the first, the fully reduced [Cu(I), Ru(II)] protein was photoexcited and subsequently oxidized by an external quencher, forming the [Cu(I), Ru(III)] protein. This was followed by an electron transfer from reduced Cu(I) to Ru(III). In the second method, the initially oxidized Cu(II) ion acted as an internal quencher for excited Ru(II) and the photoinduced reduction of the Cu(II) ion was followed by a thermal recombination with the Ru(III) ion. The reoxidation of the Cu ion, which has an estimated driving force of 0.56 eV, occured with a rate constant ket = (9.5±1.0)×106 s–1, observed with both methods. The results suggest a strong electronic coupling (HDA>0.3 cm–1) along the Ru-His(83)-Cys(84)-Cu pathway.

One-way isomerization of a [26]orthoparacyclophene: Spectroscopic evidence for adiabatic sixfold Z/E-photoisomerization

Abstract On laser flash photolyses, with biacetyl as sensitizer, the all-Z and all-E isomers of [2 6 ]orthoparacyclophene show T 1 -T n spectra in the region 450–800 nm (λ max =520 nm, τ T =45 μs) which are identical (within experimental error) and assigned to the planar all-E triplet ( 3 all-E * ). The energy of 3 all-E * was estimated to ≈41 kcal mol −1 from quenching experiments with azulene and anthracene. Direct excitation gave similar transient absorption spectra, to those observed on biacetyl sensitization. The observation of T 1 -T n absorption spectra on direct excitation and the temperature effect on the quantum yields of direct photoisomerization and fluorescence point toward a triplet manifold mechanism also for the unsensitized photoisomerization.

A sixfold triplet-sensitized Z/E isomerization of a π-perimeter macrocycle

Abstract On irradiation, all- Z [2.2.2.2.2.2]orthoparacyclophene ( 1 ) is converted to the all- E isomer ( 2 ). The photo-isomerization occurs via the triplet state and proceeds without any groun state intermediates.

Catalysis of a photochemical reaction; a cis-trans isomerization proceeding by a quantum chain process

Abstract The quantum efficiency for the biacetyl-sensitized isomerization of ( Z , E )-3,3″,5,5″-tetra( tert -butyl)-4′-styrylstilbene to ( E , E )-3,3″,5,5″-tetra( tert -butyl)-4′-styrylstilbene can be increased by more than fifty times by the addition of anthracene to the reaction mixture. The results are explained as being due to a more efficient energy transfer within a quantum chain process. The chain length of the process increases with increasing concentration of anthracene.