Vinylogous β‐Carotenes: Generation, Storage, and Delocalization of Charge in Carotenoids

Abstract

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.

The effect of structural changes on charge transfer states in a light-harvesting carotenoid-diaryl-porphyrin-C60 molecular triad

“…50,78 However, for the excited state the formation of a radical cation on the carotenoid component in the charge separated state may facilitate rotation about the carbon-nitrogen amide bond to generate various cis-conformations in the carotenoid backbone. 79,80 Since the oneelectron oxidation of a carotenoid may lead to bond length equalization between single and double bonds in the conjugated amide/carotenoid backbone, a reduction in the rotational barrier for bonds that are formally double bonds in the ground state allows for more conformational flexibility in the cationic carotenoid component. 79,80 In the present study, we undertook a systematic structural search for conformers resulting from torsions using the DFT method.…”

Anti- and Prooxidant Properties of Carotenoids

“…___________________________________________________________________________ FULL PAPER Synthesis, characterization, and properties of the carotenoids are described in several published articles [7][8][9], all other compounds will be introduced in this publication (see Experimental). β-Carotene (1a), canthaxanthin (4a), zeaxanthin (3a), astaxanthin (5a), lycopene (2a), tetrahydrolycopene (2c), 13-cis-Phytoene (2e) and all-trans-Phytoene (2d) were provided by BASF AG.…”

The McMurry Coupling and Related Reactions

“…β-Carotene (67) and lycopene (68) are synthesized by treatment of retinal and γ-retinal in 84 and 75% yields, respectively, with the TiCl 3 -LiAlH 4 system. 201 Other titanium reagents such as TiCl 4 -Zn, 202 titanium powder-TMSCl, 83 and TiCl 3 -Li 27 have been used for the β-carotene synthesis. Isorenieratenes (69) (carotenide light-harvesting pigments) are prepared in a similar manner.…”