Measuring Ti(III)−Carotenoid Radical Interspin Distances in TiMCM-41 by Pulsed EPR Relaxation Enhancement Method

Abstract

Interspin distances between the Ti³⁺ ions and the carotenoid radicals produced inside TiMCM-41 pores by photoinduced electron transfer from 7′-apo-7′-(4-carboxyphenyl)-β-carotene (coordinated to Ti³⁺), canthaxanthin (formed as a random distribution of isomers), and β-ionone (model for a short-chain polyene) to Ti³⁺ framework sites were determined using the pulsed EPR relaxation enhancement method. To estimate the electron transfer distances, the temperature dependence of relaxation rates was analyzed in both siliceous and metal-substituted siliceous materials. The phase memory times, T_M, of the carotenoid radicals were determined from the best fits of two-pulse ESEEM curves. The spin−lattice relaxation times, T₁, of the Ti³⁺ ion were obtained from the inversion recovery experiment with echo detection on a logarithmic time scale in the temperature range of 10−150 K. The relaxation enhancement for the carotenoid radicals in TiMCM-41 as compared to that in MCM-41 is consistent with an interaction between the radical and the fast relaxing Ti³⁺ ion. For canthaxanthin and β-ionone, a dramatic effect on the carotenoid relaxation rate, 1/T_M, occurs at 125 and 40 K, respectively, whereas for carboxy-β-carotene 1/T_M increases monotonically with increasing temperature. The interspin distances for canthaxanthin and β-ionone were estimated from the 1/T_M − 1/T_M0 difference, which corresponds to the Ti³⁺ contribution at the temperature where the maximum enhancement in the relaxation rate occurs. Determination of the interspin distances is based on calculations of the dipolar interaction, taking into consideration the unpaired spin density distribution along the 20-carbon polyene chain, which makes it possible to obtain a fit over a wider temperature interval. A distribution of the interspin distances between the carotenoid radical and the Ti³⁺ ion was obtained with the best fit at ∼10 Å for canthaxanthin and β-ionone and ∼9 Å for 7′-apo-7′-(4-carboxyphenyl)-β-carotene with an estimated error of ±3 Å. The interspin distances do not depend on 1/T_M − 1/T_M0 for carboxy-β-carotene which shows no prominent peak in the relaxation rate over the temperature range measured.