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Tdble 2 Fluoresccnce quantum yields ($cm = 620 nm) of 1 b. 1 c, 2, and [Zn(tpp) almost no electron transfer occurred. In the more polar solvents CH,CI, and CHC1, the fluorescence quantum yield of 2 was much lower than those of l b and lc, which can be explained by a fast electron transfer from the excited porphyrin (570 nm) to the quinone.l'" In line with these experiments, time-resolved single-photon-counting fluorescence (SPC) studies in CCI, showed that the decay profiles of l c and 2 were virtually the same. The decay profile of 2 in CH,CI, was different and showed an additional rapid process (52 ps), which was the major contribution (90 %) to the fluorescence decay.In CCI, almost no quenching of the fluorescence of 2 due to electron transfer processes was observed. After the addition of hexyl 3,5-dihydroxybenzoate, however, 75 YO of the intensity was quenched. (A similar effect is expected in CH,Cl,; however the binding constant is much lower in this solvent, and the effect cannot be observed). Under the same conditions this quenching was not observed for either [Zn(TPP)], lb, or lc, and hence this process is likely to be the result of the guest molecule's presence between the donor and acceptor functions of 2. Further confirmation comes from the quenching of 2, which is highly dependent upon the guest concentration. The lack of fluorescence quenching for l b and l c also indicates that the quenching process for 2 is not due to a proton transfer mechanism from the guest to the host, since the most basic sites (the quinoxaline nitrogen atoms) are present in all three molecules. Preliminary SPC measurements indicate that the electron transfer between the porphyrin and the quinone in 2, responsible for the multiexponential decay of the fluorescence intensity, is substantially faster for the host -guest complex. This is in contrast to a recently constructed porphyrin-quinone system, in which a covalently linked phenyl moiety is positioned between the two chromophores. In this covalently linked system no rate enhancement occurred by through-space electron transfer across the interspaced aromatic rnoiety.['l The enhanced electron transfer by the aromatic guest molecule in our system is probably partly a result of a local polarity effect and buttressing or contact effects and partly due to a superexchange mechanism. The latter effect has previously been observed for porphyrin quinone model systems that have aromatic units linking the two chromophores@I and in solvent-mediated donor-acceptor systems.['71 The addition of hexyl3,5-dihydroxybenzoate to a solution of l b or l c in CCI, resulted in the appearance of an extra emission band, indicating that the emission process was affected by specific interactions between the host and guest (Table 2). More time-resolved fluorescence studies and transient absorption measurements are in progress to provide a better insight into the mechanisms involved in these new model systems. These results will be presented in a full paper. than polarons.['] Radical ions are the chemica...

In this paper, we report on the synthesis of 0-carotenes of variable chain length with between 5 and 23 double bonds (1-7). These oligoenes were prepared by McMurry and Wittig reactions. The tetradecapreno-P-carotene ? would seem to be the longest 0-carotene yet reported. Furthermore, we investigated the electronic properties using cyclic voltammetry and photoelectron spectroscopy (UPS) to generate openshell and closed-shell ions of carotenoids in solution and in the solid state, respectively. With increasing chain length ( 2 11 double bonds), even the generation of tetracations and tetraanions could be observed by cyclic voltammetry. Extending the number of conjugated bonds causes the potentials to converge to limiting values. All electron-transfer processes occur in one-electron steps, which are close to each other in pairs. The potential difference between the first oxidation potential and the first reduction potential is a linear function of the reciprocal chain length. Despite the different techniques used (CV and UPS) and the different condensed phases, there is an excellent correspondence between the energies of the radical cation states generated by the two methods. This shows that the radical cation formation is principally determined by the chain length. The structures of the ion states were investigated using semiempirical methods at the NDDO level. Charge delocalization and bond relaxation are not identical and do not utilize the same number or the same kind of atoms. It can be shown that from the length of 20 double bonds onwards, the effective conjugation length for doubly-charged cations converges slowly to a limiting value.