Lowell D. Kispert

Photocurrent generated on a carotenoid-sensitized TiO2 nanocrystalline mesoporous electrode

Photocurrent was observed upon monochromatic illumination of an ITO electrode coated with a TiO2 nanocrystalline mesoporous membrane with carotenoid 8 0 -apo-b-caroten-8 0 -oic acid (ACOA) deposited as a sensitizer (illuminated area 0.25 cm 2 ) and immersed in an aqueous 10 mM hydroquinone (H2Q), 0.1 M NaH2PO4 solution (pH = 7.4) purged with argon, using a platinum flag counter electrode (area 3.3 cm 2 ) and a SCE reference electrode. The carotenoid-sensitized short-circuit photocurrent reached 4.6 mA/cm 2 upon a 40mW/cm 2 incident light beam at 426 nm, with an IPCE (%, incident monochromatic photon-to-photocurrent conversion efficiency) as high as 34%. The short-circuit photocurrent was stable during 1 h of continuous illumination with only a 10% decrease. An open-circuit voltage of 0.15 V was obtained (upon 426 nm, 40mW/cm 2 illumination) which remained at a constant value for hours. The observed open-circuit voltage is close to the theoretical value (0.22 V) expected in such a system. The action spectrum resembled the absorption spectrum of ACOA bound on the TiO2 membrane with a maximum near 426 nm. No decay of the ACOA on the TiO 2 surface was observed after 12 h, presumably because of rapid regeneration of ACOA from ACOA C at the surface by electron transfer from H2Q. ©2000 Elsevier Science

Temperature dependence of the lowest excited singlet‐state lifetime of all‐trans‐β‐carotene and fully deuterated all‐trans‐β‐carotene

A 4 ps, 450 nm laser pulse was used to electronically excite all‐trans‐β‐carotene and all‐trans‐β‐carotene‐d56 in 3‐methylpentane. The transient absorption spectra of these molecules were measured as a function of temperature down to 20 K. In all cases the 400–500 nm electronic absorption band of each carotene bleaches and a new absorption band near 560 nm appears immediately upon excitation. These bands recover with single exponential kinetics: τ=8.1±0.5 ps for all‐trans‐β‐carotene, and τ=10.5±0.6 ps for all‐trans‐β‐carotene‐d56 at 294 K. These recovery times increase by about a factor of 2 in glassy 3‐methylpentane, and are nearly independent of temperature from 100 to 20 K. The weak dependencies of the lowest excited single‐state lifetime of all‐trans‐β‐carotene on deuteration and temperature are discussed in terms of nonradiative decay mechanisms within carotenoids.

An EPR study of the reaction between poly(p‐phenylene sulfide) and electron‐acceptor dopants

EPR spectra for electron‐acceptor doped poly(p‐phenylene sulfide) are reported. The g values obtained from these spectra correspond to an electron density centered around a divalent sulfur radical cation R–S–R. This assignment is supported by corresponding EPR data from an oligomeric model compound φ–S–φ–S–φ (φ=phenyl). The g value of the sulfur‐based radical cation of doped poly(p‐phenylene sulfide) distinctly contrasts the nearly free electron g values obtained from EPR measurements on poly(p‐phenylene) and its oligomer p‐terphenyl.

Electrochemical properties of natural carotenoids

Abstract Carotenoids are widely distributed among plants, certain bacteria and animals, serving for light harvesting and photoprotection in photosynthesis and as antioxidants. Electrochemical data (such as oxidation potentials, reaction rate constants, kinetic equilibrium constants) for 12 naturally occurring carotenoids are summarized in this paper. Electrochemical parameters were estimated by simulating experimental cyclic voltammograms (CVs). Conventional electrochemical techniques (cyclic voltammetry and Osteryoung square-wave voltammetry) were used in these studies. CVs of some natural carotenoids are presented and discussed. The dependence of redox potentials and other kinetic parameters on the structures of carotenoids is summarized. The oxidation potential of neutral carotenoid ( E ° 1 ), which corresponds to the formation of cation radicals, varies from 0.50 to 0.72 V versus SCE. The oxidation potential of carotenoid cation radical ( E ° 2 ), which corresponds to the formation of a dication, usually lies in the region of 0.52–0.95 V versus SCE. The electrochemical data presented in this article should be helpful in the study of reaction processes in model systems or actual photosynthesis reaction centers.

Carotenoids as antioxidants: spin trapping EPR andoptical study

The role of several natural and synthetic carotenoids as scavengers of free radicals was studied in homogeneous solutions. A set of free radicals: *OH, *OOH, and *CH(3) were generated by using the Fenton reaction in dimethyl sulfoxide. It was shown that the spin trapping technique is more informative than optical methods for the experimental conditions under study. 5,5-Dimethyl-pyrroline-N-oxide (DMPO) and N-tert-butyl-alpha-phenylnitrone (PBN) were used as spin traps for the EPR studies. The results show that the scavenging ability of the carotenoids towards radical *OOH correlates with their redox properties.

EPR study of polarons in a conducting polymer with nondegenerate ground states: Alkali metal complexes of poly (p‐phenylene) and phenylene oligomers

EPR measurements are used to characterize electronic states relevant for carrier transport in alkali metal doped poly(p‐phenylene), PPP, fully deuterated poly(p‐phenylene), DPPP, and phenylene oligomers. Observed spin concentrations per carbon are at least one decade higher than the Curie spin concentration for Na‐doped polyacetylene. The number of these spins, which likely corresponds to polarons (mobile radical anions), is much less than the amount of alkali metal dopant, suggesting that much of the charge on the polymer chains is in bipolarons (spinless dianions). Relevant to the interaction between spins on the polymer chain and the metal cations, the observed g values are close to the free electron value and do not substantially vary with the donor dopant, temperature, or the molecular weight of the phenylene chain. Although the spin‐orbit effect on g values is small, room temperature linewidth tends to increase with increasing atomic number of dopant—suggesting some interaction, albeit a smaller mag...

Electrochemical oxidation of carotenoids containing donor/acceptor substituents

Abstract Cyclic voltammetry and square wave voltammetry of the carotenoids β-carotene (I), 7′,7′-dimethyl-7′-apo-β-carotene (II), 7′,7′-hexadeuterodimethyl-7′-apo-β-carotene (III), 7′-cyano-7′-ethoxycarbonyl-7′-apo-β-carotene (IV), 7′,7′-dicyano-7′-apo-β-carotene (V) and canthaxanthin (VI) show that the electrochemical properties of the synthetic dimethyl and hexadeuterodimethyl compounds are similar to those of the naturally occurring β-carotene, while the properties of the synthetic dicyano carotenoid are similar to those of the naturally occurring canthaxanthin, and those of the cyanoester compound are of an intermediate nature. The difference in oxidation potentials for the formation of the cation radical and the dication is small for the first set of compounds (less than 36 mV) and large (up to about 200 mV) for the second. Simulation of the CVs with the DigiSim® program led to evaluation of the half-wave potentials, heterogeneous rate constants and homogeneous equilibrium constants which describe the oxidation properties of the carotenoids.

ENDOR of biradicals

The ENDOR spectra of Tschitschibabins hydrocarbon and certain of its specifically deuterated derivatives have been measured. To interpret these spectra the factors determining the ENDOR spectrum of a biradical are discussed in detail. The analysis reveals that the paramagnetic species present in solutions of Tschitschibabins hydrocarbon and responsible for the solution electron resonance spectrum is a substituted p-biphenyldiphenylmethyl radical, thus resolving the biradical paradox. The substituent is thought to be the diphenylmethyl group in which case the radical could be formed through hydrogen abstraction by a thermally populated triplet.

Water Soluble Complexes of Carotenoids with Arabinogalactan

We present the first example of water soluble complexes of carotenoids. The stability and reactivity of carotenoids in the complexes with natural polysaccharide arabinogalactan were investigated by different physicochemical techniques: optical absorption, HPLC, and pulsed EPR spectroscopy. Compared to pure carotenoids, polysaccharide complexes of carotenoids showed enhanced photostability by a factor of 10 in water solutions. A significant decrease by a factor of 20 in the reactivity toward metal ions (Fe(3+)) and reactive oxygen species in solution was detected. On the other hand, the yield and stability of carotenoid radical cations photoproduced on titanium dioxide (TiO(2)) were greatly increased. EPR measurements demonstrated efficient charge separation on complex-modified TiO(2) nanoparticles (7 nm). Canthaxanthin radical cations are stable for approximately 10 days at room temperature in this system. The results are important for a variety of carotenoid applications, in the design of artificial light-harvesting, photoredox, and catalytic devices.

Spectroelectrochemistry of carotenoids in solution

Abstract The D0→D2 optical absorption spectra of the cation radicals of β-carotene and canthaxanthin in solution prepared using in situ spectroelectrochemical techniques at 300 K are reported. The weak and usually not reported D0→D1 transition of the cation radicals of the carotenoids are also observed in the optical spectra. The upper limit of the apparent molar extinction coefficient of carotenoid dications in solution is estimated to be on the order of 103 to 104. Under equilibrium conditions, the contribution of the dication absorption spectrum to that of the cation radical spectrum is negligible for the canthaxanthin cation radical and estimated to be approximately 10% for β-carotene.