A.S. Jeevarajan

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.

Simultaneous electrochemical and electron paramagnetic resonance studies of keto and hydroxy carotenoids

Cyclic voltammogram (CV) and simultaneous electrochemical and electron paramagnetic resonance (SEEPR) measurements have been carried out on the oxygenated carotenoids: echinenone, canthaxanthin, isozeaxanthin and rhodoxanthin in dichloromethane. The CV displays are markedly different from that of β-carotene. Comproportionation constants, deduced from EPR spin concentration measurements of electrochemically oxidized dihydroxy β-carotene and several keto carotenoids vary by four orders of magnitude. ΔHpp values deduced from the SEEPR spectra of the cation radicals are in the range 13.2 to 14.5 G and the g-factors are 2.0027 ± 0.0002. These EPR parameters are in accordance with a polyene π-cation radical structure. Theoretical CVs are calculated using DigiSim, a CV simulation program, and the proposed mechanism involves three electrode and two homogeneous reactions.

Resonance Raman study of carotenoid cation radicals

Abstract The resonance Raman (RR) spectra of the neutral and the cation radical of 8′-apo-β-caroten-8′-al (I), 7′,7′-dicyano-7′-apo-β-carotene (II) and canthaxanthin (III) were measured using 752.6 nm laser excitation. Optical and RR spectra of the electrochemically prepared cation radicals of I–III were measured in situ at different time intervals during the electrolysis. The RR spectra of the cation radicals are considerably different from those of the neutral species. The frequencies of the CC stretching vibrations are decreased by 30–40 cm −1 , whereas those of CC stretching vibrations are increased by 15–30 cm −1 . These results suggest delocalization of the unpaired electron density throughout the entire carbon backbone which is consistent with our previous EPR and ENDOR results.

An ENDOR study of carotenoid cation radicals on silica—alumina solid supports

Abstract The cation radicals of canthaxanthin, β-caroten and 8′-apo-β-caroten-8′-al were chemically prepared on activated silica—alumina solid supports. An ENDOR study of these cation radicals revealed the hyperfine coupling constants of the methyl protons and indicated a carotenoid-dependent orientation of the cation radicals bound to the solid support. The Lewis acidic sites of the solid support appear to be responsible for the chemical formation of the cation radicals and the Bronsted acidic sites are responsible for the stability of the cation radical produced on the support.

Electrochemical, EPR and AM1 studies of acetylenic and ethylenic carotenoids

It has been shown, by cyclic voltammetry (CV) and simultaneous electrochemical and electron paramagnetic resonance (SEEPR) measurements of β-carotene (I) and 15,15′-didehydro-β-carotene (II) in dichloromethane solution, that the central triple bond increases the oxidation potentials by 237 ± 5 mV and decreases the solution EPR peak-to-peak linewidth (ΔHpp) by 1.7 G. Two synthetic carotenoids, 7,7′-diphenyl-7,7′-diapocarotene (III) and 7,7′-diphenyl-15,15′-didehydro-7,7′-diapocarotene (IV), close analogues of I and II, were also studied by CV and SEEPR methods. In contrast to compound I, two separate oxidation waves are observed for III, a feature previously found only for carotenoids containing electron-withdrawing substituents. The increase in oxidation potential of IV compared to III was similar to that of IIvs.I, but the decrease in ΔHpp(2.7 G) was considerably larger. Because the radical cation of III is adsorbed strongly on the platinum electrode in dichloromethane, complete analysis using the DigiSim® program was not possible; however, the oxidation potentials could be estimated. Structural parameters of neutral I–IV and their dications were calculated using the AM1 method, and the bond lengths and bond angles of I and II are in good agreement with crystallographic data. The magnitude of the EPR parameters, ΔHpp and the g factors, for the carotenoid radical cations of I–IV indicate a polyene π-radical cation structure with little unpaired electron density occurring near the terminal groups. The radical cation and dication deprotonate by loss of a proton from the chain methyl groups, with the former forming a polyene π-radical.

An ENDOR study of the canthaxanthin cation radical in solution

Abstract The cation radical of canthaxanthin was generated electrochemically in dichloromethane and its solution ENDOR spectrum is reported for the first time. The isotropic proton hyperfine coupling constants are in reasonable agreement with those calculated by RHF-INDO/SP molecular orbital methods for a planar all-trans-polyene conformation. Proton couplings less than 1.1 MHz were not detected due in part to incomplete hyperfine separation and chemical exchange processes.

Geometrical isomerization of carotenoids in dichloromethane

In a 3 mmol dm–3 solution of all-trans-canthaxanthin (I) in HPLC grade dichloromethane cis-isomers are formed. Optical absorptions due to cis-isomers in addition to those due to an intermediate are observed in 3 mmol dm–3 solutions of all-trans-β-carotene(II) and 8′-apo-β-caroten-8′-al (III), as a result of 0.1–0.2 mmol dm–3 acid normally found in HPLC grade dichloromethane. The cis-isomers were separated by HPLC and characterized by optical and NMR spectroscopic techniques. AM1 calculations performed on the ground state and protonated I showed that 9-cis and 13-cis-isomers will be formed by acid induced isomerization in greater yield than other cis and cis,cis-isomers. This was observed experimentally. If an excess amount of hydrochloric acid (∼ 1 mol dm–3) is added to the solution of dichloromethane containing the carotenoids, radical cations of the carotenoids are formed.

ENDOR study of the (7′,7′-dicyano)- and (7′-phenyl)-7′-apo-β-carotene radical cations formed by UV photolysis of carotenoids adsorbed on silica gel

The radical cations of two synthetic carotenoids, 7′-7′-dicyano-7′-apo-β-carotene (I) and 7′-phenyl-7′-apo-β-carotene (II), have been prepared by UV irradiation of samples adsorbed on silica gel. The methyl proton hyperfine coupling constants were deduced from ENDOR powder spectra and assigned by RHF-INDO/SP calculations. The powder EPR spectra of the carotenoid radical cations formed photolytically by electron transfer are unresolved, with a peak-to-peak linewidth of 15 ± 1 G, irrespective of the donor or acceptor character of the terminal substituents.

Studies of new organic conductors based on thiophene derived annulene complexes With TCNQF4

Abstract Two newly synthesized thiophene derived annulenes 1 and 2 have been studied as electron donors. Stable 1:1 complexes of them have been found with the electron acceptor TCNQF 4 . Their pressed-pellet room-temperature conductivities are 9.5×10 −3 S/cm for complex 1. TCNQF 4 , and 1.2×10 −4 S/cm for complex 2. TCNQF 4 . The temperature-dependent EPR (100 K - 300 K) reveals a narrow asymmetric line with g =2.0027 for both complexes, and low activation energies of 4.1 meV for 1. TCNQF 4 and 9.7meV for 2. TCNQF 4 .

Cidep Studies of Carotenoid Radical Cations

Time-resolved electron paramagnetic resonance (TREPR) studies of radical cations formed by 308 nm photoexcitation of several carotene analogues are reported. The spinpolarized EPR spectra are weak in intensity and have very broad line widths. Assignment is based on polarization patterns and on the time scales of the signals. Comparisons of g factors and line widths are made to the values of radical cations generated electrochemically from the same compounds and the results are discussed in terms of proposed radical structures. A dramatic solvent dependence of the signals is observed.