A new series of donor-acceptor fused dyads consisting of a C 60 doubly tethered to a substituted TTF moiety has been synthesized. Cyclic voltammetry of the new fullerene derivatives in solution shows a modulation in the difference between the first reduction potential of the fullerene moiety and the first oxidation potential of the TTF moiety with the substituents of the TTF addend. Along with the neutral bichromophoric compounds, we also report the generation and characterization by EPR and absorption spectroscopies of the corresponding persistent open-shell species obtained electrochemically, namely their radical cations and radical anions. Spin density distributions of radical cations and radical anions derived from dyads 1a-c are mainly located on the TTF and fullerene moieties, respectively, as ascertained from the g values and 33 S hyperfine coupling constants. Interestingly, the EPR of the radical anion derived from the bisadduct 1d exhibits a structured signal (g ) 2.0005) arising from the coupling of the unpaired electron with the hydrogen atoms of the addends. The modification of the donor strength of the TTF moiety allows the tuning of the HOMO-LUMO gap of dyads, permitting a study of the interaction between the two electroactive centers of the molecules as a function of the donor strength. Nanosecond-resolved flash photolysis in the UV-vis region of dyads 1a-c in degassed benzonitrile shows a rapid quenching of the corresponding excited triplet states, which indicates different lifetimes depending on the donor ability of their TTF addends. Excited triplet states of 1b and 1c evolve toward transient charge-separated open-shell species that have remarkably long lifetimes (1b, τ ) 75 × 10 -6 s; 1c, τ ) 79 × 10 -6 s) and show absorptions around 460 and 620 nm due to the radical cation on the TTF moiety. These biradical species are also observed by LESR, having in frozen solution spectra consistent with strong exchange coupling between both electrons. S0022-3263(98)00498-8 CCC: $15.00

The quadruple bacteriorhodopsin (BR) mutant E9Q+E74Q+E194Q+E204Q shows a V V max of about 500 nm in water at neutral pH and a great influence of pH and salts on the visible absorption spectrum. Accessibility to the Schiff base is strongly increased, as detected by the rapid bleaching effect of hydroxylamine in the dark as well as in light. Both the proton release kinetics and the photocycle are altered, as indicated by a delayed proton release after proton uptake and changed M kinetics. Moreover, affinity of the color-controlling cation(s) is found to be decreased. We suggest that the four Glu side chains are essential elements of the extracellular structure of BR.z 1999 Federation of European Biochemical Societies.

Chiral pentenoates 1-3 in both Z and E isomeric forms underwent stationary irradiations in several solvents and in the presence of different photosensitizers. The photostationary-state ratio has been determined for each Z/E couple showing a predominance of the thermodynamically more stable isomer for 1 and 3. Moreover, transient species were generated by pulsed laser excitation and detected by their characteristic ultraviolet absorptions, being the first time that enoate-originated triplets are detected. Stern-Volmer quenching studies afforded a quantitative measure for the efficiency of the photosensitization processes induced by benzophenone or acetophenone and allowed the determination of the corresponding quenching rate constants. Density functional calculations permitted the determination of the geometries and the energies of the diastereomeric excited states. Two diastereomeric orthogonal and two diastereomeric planar structures result as a consequence of the presence of a chiral substituent. The orthogonal triplets are the energy minima in all cases, whereas the planar triplets are the transition states linking these orthogonal structures, the corresponding energy barriers being 8-10 kcal mol(-1) for enoates 1-3. The computed S(0) to T(1) excitation energies show a trend which is consistent with the quenching rate constants. On the other hand, the triplet lifetimes determined for 1 and 2 are unusually long (1-20 micros) if compared with the data already described for several enones, in the range of nanoseconds. This fact has been rationalized from calculations of spin-orbit coupling at several points of the T(1) potential energy surface. This coupling is maximum for structures with a torsional angle close to 45 degrees, which are 4-5 kcal mol(-1) above the minima of T(1). Calculations done on the hypothetical aldehyde 4 and methyl vinyl ketone show much lower energy barriers, thus accounting for the shorter lifetimes reported for enone triplets.

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