Electron transfer to triplet fullerene C60 and subsequent radical ion reactions studied by time-resolved CIDNP

Abstract

Absolute net nuclear polarization and their time dependencies are reported for the products of geminate and nongeminate radical ion pairs involved in the electron-transfer reaction between 3C60 and the donor N,Ndimethyl-1-naphthylamine. Their analysis yields rate constants and reaction probabilities and shows that the electron back transfer between C60' -and D ' + is very efficient in benzonitrile/benzene. This is attributed to fast spin relaxation of C60' -. IntroductionBy many important publications, Closs and co-workers1 contributed to the understanding of the phenomenon of chemically induced dynamic nuclear polarization (CIDNP) and to its development for quantitative studies of radical reaction mechanisms and rates. Following earlier investigations along similar lines,2we here present a quantitative time-resolved CIDNP study of the electron-transfer reactions occurring after laser pulse generation of 3Cm in the presence of the electron donor N,Ndimethyl-1-naphthylamine (D). It is well-known from optical3 and ESR observations4 that excitation of Cm produces the triplet within nanoseconds in high yield. It has a rather long first-order lifetime and also decays by a diffusion-controlled triplet-triplet annihilation and a slower ground-state quenching. For a variety

Singlet and Triplet State Back Electron Transfer from Photogenerated Radical Ion Pairs Studied by Time-Resolved CIDNP

“…(e) Finally, nuclear relaxation times of protons in radicals are usually larger than 100 μs, ,, i.e., cannot lead to an effective loss of polarization on the time scales considered here (Figures −9). Hence, we adopted a common value of 125 μs for all systems and note that this is equivalent to the neglect of the relaxation terms in eq 21.…”

“…Further, for λ ) 308 nm (excimer laser) the fraction of the irradiated volume to the detection volume of the sample tubes was determined as 0.49 and used in the conversion of CIDNP intensities to absolute nuclear polarizations. 5,8,10 For the analysis of the CIDNP phases following Kaptein's rules 2b,4a one needs the ESR hyperfine coupling constants and the g factors of all radical ions involved. In the literature these were found for the donor cations (N,N-dimethyl-1-naphthylamine: 5 g ) 2.0029, a(CH 3 ) ) 0.8 mT, a(N) ) 0.7 mT; 1,4-dimethoxynaphthalene: 14 g ) 2.003 21, a(H,OCH 3 ) ) 0.216 mT, a(H 2,3 ) ) -0.335 mT; naphthalene: 15 g ) 2.0025, a(H 1,4,6,8 ) ) -0.74 mT, a(H 2,3,5,6 ) ) -0.187 mT); and the anion of (E)-1,2-dicyanoethene: 5 g ) 2.0028, a(H 1,2 ) ) -0.779 mT, a(N 1,2 ) ) 0.23 mT.…”

“…Proton CIDNP studies were performed as described earlier , with a 200 MHz FT instrument (Bruker CXP 200). For excitation of the donors N , N -dimethyl-1-naphthylamine and 1,4-dimethoxynaphthalene, a pulsed laser operating at 308 nm (Lambda Physics EMG 100, XeCl) was used whereas the donor naphthalene was also excited by 266 nm pulses (Spectra Physics GCR 18S, Nd:YAG, fourth harmonic).…”

“…For other laser powers (typically 28−40 mJ) and lower optical densities the singlet concentration [ 1 D 1 ] 0 was estimated from the above value using Beer's law. Further, for λ = 308 nm (excimer laser) the fraction of the irradiated volume to the detection volume of the sample tubes was determined as 0.49 and used in the conversion of CIDNP intensities to absolute nuclear polarizations. ,, …”

Time-Resolved CIDNP Investigation of the Cross-Relaxation Mechanism of 1H Nuclear Polarization

“…The 90° pulses detect net and second-order multiplet effects alone, and the multiplet effects shown in the spectra are due to the latter and/or to imperfect 90° pulses. The pulse scheme described in ref was used to suppress the normal acetone NMR signal for which a simple presaturation pulse was insufficient. A delay of approximately 30 s was maintained between the laser pulses in order to avoid significant buildup of the enol, which is known to react with triplet acetone …”