Structures and electronic states of photoexcited states in a system of two nitroxide radicals linked to fullerene studied by two-dimensional pulsed nutation and time-resolved electron paramagnetic resonance spectroscopy

Abstract

Magnetic properties of fulleropyrrolidine adducts with two stable nitroxide radicals (2,2,6,6-tetramethylpiperidine-l-oxyl, TEMPO) were studied in toluene solution by continuous-wave time-resolved (TR) and pulsed electron paramagnetic resonance (EPR) spectroscopy in the ground and photoexcited states. Four isomers of the bisadduct, trans-1, trans-2, trans-3, and equatorial forros, having the second pyrrolidine ring at different [6-6] bonds were synthesized. In the ground states, the exchange interaction between two TEMPOs is so small that the spin state of the bisadduct is a doublet in nature. By means of spectral simulations of the EPR spectra in frozen solution at 70 K, the upper limit of the exchange interaction was estimated to be 5 MHz for the trans-1 and trans-2 and 10 MHz for the trans-3 and equatorial isomers. The simulation was also made to determine relative positions of the two TEMPO groups with respect to the pyrrolidine ring. Photoexcited states of the bisadducts with excitation of the 532 nm laser pulse were studied in frozen toluene solution at 5-100 K by using two-dimensional (2-D) pulsed nutation EPR and TREPR. The spin multiplicity of the excited state was determined by the nutation frequency. Al1 of the four bisadducts showed strong exchange couplings between two TEMPOs and fullerene triplet 3C60 , resulting in the generation of the excited quintet and triplet states. The excited triplet states have been observed and assigned for the first time in strongly coupled triplet-radical systems. The zero-field splittings of the quintet state determined from the 2-D nutation EPR spectra were analyzed as the sum of the spinspin interactions among the three paramagnetic centers, two TEMPOs and 3C~0. On the basis of these analyses, the spin distribution on the 3C60 part and the geometry of two TEMPOs are discussed.

Synthesis and characterization of bispyrrolidine derivatives of H2@C60: differentiation of isomers using 1H NMR spectroscopy of endohedral H2

“…As reported, 8,12 comparison of the UV-Vis spectra between 400 and 700 nm region allowed the correlative assignment of structures of the five isomers of 1 (see Fig. S2-Fig.…”

“…Due to asymmetrical additions to the C 60 cage, each bisadduct isomer of 1 also gives several geometrical isomers 12 caused by orientation change of the substituents: three possible isomers for trans-2, trans-3 and trans-4 (see Fig. S1 in ESIw) and two possible isomers for trans-1 and equatorial-5 (e-5).…”

Recent Developments in Studies of Electronic Excited States by Means of Electron Paramagnetic Resonance Spectroscopy

“…We have met this latter case in several bis-radical adducts of fullerene, where the excited states signals could be observed for the quintet but not for the triplet in the pulsed experiment. 36 In contrast, both the quintet and T 1 signals were observed in the 1D TREPR spectrum. Here the nutation was not observed for the triplet signals but was observed for the quintet ones, which is consistent with the 2D experiment.…”

“…This is the first observation of the T 1 state in the strongly coupled triplet-biradical pair system. 36,37 The observed very fast spin-spin relaxation was interpreted by a very fast inter-conversion occurring between two close-lying triplet (T 1 and T 2 ) states. The 2D nutation technique is also not applicable to systems in solution where the whole excitation is achieved by pulsed microwave excitation.…”

Dual Chromophore‐Nitroxides: Novel Molecular Probes, Photochemical and Photophysical Models and Magnetic Materials

“…The organic synthetic chemistry allows playing with the chemical structure of the dual molecules of different absorption, fluorescence and ESR spectra, and of redox and spin properties with variety bridges (spacers) tethered to the chromophore and nitroxide segments (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Figure 1 shows examples of the dual compounds.…”