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CIDNP (chemically induced dynamic nuclear polarization) effects in the sensitized photoreactions of triethylamine DH (sensitizers A, 9,10-anthraquinone and 2,7-dinitrofluorenone) are investigated in a series of aprotic solvents of widely varying relative permittivity ε, including mixtures of acetonitrile and chloroform. The dependence of the polarization pattern, that is, the relative polarization intensities of the olefinic α- and β-protons in the reaction product N,N-diethylvinylamine, on ε is studied quantitatively by evaluating the polarization ratio r of these protons. The factor responsible for the change of the polarization pattern is the rate, relative to the kinetic window of CIDNP, of in-cage deprotonation of the triethylamine radical cation by the sensitizer radical anion (rate constant k dep), which converts the initially formed radical ion pair into a neutral radical pair . A theory of pair substitution taking place solely during encounters of the radicals is presented. Based on this, an expression for the dependence of r on k dep is derived, which in turn allows the extraction of k dep from the experimentally observed polarization ratio. The accessible range of k dep in these systems is from about 108 to about 1010 M-1 s-1.

SummaryThe photoreactions of diazabicyclo[2,2,2]octane (DABCO) and triisopropylamine (TIPA) with the sensitizers anthraquinone (AQ) and xanthone (XA) or benzophenone (BP) were investigated by time-resolved photo-CIDNP (photochemically induced dynamic nuclear polarization) experiments. By varying the radical-pair concentration, it was ensured that these measurements respond only to self-exchange reactions of the free amine-derived radicals (radical cations DH • + or α-amino alkyl radicals D •) with the parent amine DH; the acid–base equilibrium between DH • + and D • also plays no role. Although the sensitizer does not at all participate in the observed processes, it has a pronounced influence on the CIDNP kinetics because the reaction occurs through successive radical pairs. With AQ, the polarizations stem from the initially formed radical-ion pairs, and escaping DH • + then undergoes electron self-exchange with DH. In the reaction sensitized with XA (or BP), the polarizations arise in a secondary pair of neutral radicals that is rapidly produced by in-cage proton transfer, and the CIDNP kinetics are due to hydrogen self-exchange between escaping D • and DH. For TIPA, the activation parameters of both self-exchange reactions were determined. Outer-sphere reorganization energies obtained with the Marcus theory gave very good agreement between experimental and calculated values of ∆G ‡ 298.

We have studied the photoreaction of triethylamine DH with the triplet sensitizer 9,lO-anthraquinone in a series of aprotic solvents with relative permittivity E, varying between 2 and 50. The results are compared with those of an earlier investigation in which a series of triplet sensitizers in acetonitrile was used. All these reactions are two-step processes: an aminium cation D H t is formed by photoinduced electron transfer, and then deprotonated at C , to give an a-aminoalkyl radical D'. The deprotonation of DH? can either occur within the cage, by the sensitizer radical anion, or outside the cage, by surplus amine. The final reaction products (e.g. N,N-diethylvinylamine) are independent of the deprotonation route. Nevertheless, a distinction between the two mechanistic alternatives is possible by using measurements of chemically induced dynamic nuclear polarization (CIDNP). This technique is sensitive to radical pairs only, and the different aminebased intermediates contained in the two possible kinds of radical pairs (radical-ion pairs for both deprotonation routes, and pairs of neutral radicals for in-cage deprotonation only) give rise to characteristically different signal patterns. The influence of solvent polarity as well as sensitizer oxidation potential E,, on the deprotonation pathway (exclusive incage deprotonation at low values of E , or high E,,, and exclusive out-of-cage deprotonation at high E,, or low Eox) can be quantitatively explained by the dependence of the in-cage deprotonation rate on the driving force -AGS,, of this process, which shows a marked threshold behavior. If AGOdep is more negative than -125 kJ/mol, proton transfer from DHt to the sensitizer radical anion is faster than separation of the primarily formed radical-ion pair, so the aminium cations are deprotonated within the cage. For AGZ,, more positive than -100 kJ/mol, this reaction is too slow to compete with escape from the cage. By the latter process, free aminium cations are formed, which are then deprotonated outside the cage by surplus amine. As we recently reportedL21, two alternative routes exist for the deprotonation of DH?. The proton is either removed within the solvent cage, by the sensitizer radical anion A;formed by the primary electron transfer, or it is abstracted outside the cage, by surplus amine. In experiments with a series of sensitizers in acetonitrile, we found that for sterically unhindered amines the pathway taken is determined by a single parameter, the driving force -AG& of the incage deprotonation. In the present work, we vary the relative permittivity E , of the solvent to fine-tune Aa,,. As we will show, the deprotonation route may also be selected by these relatively weak perturbations. This can be explained quantitatively by the dependence of AG$-, on E,.The two-step hydrogen abstraction must involve a radical-ion pair DHtATI31 as a result of the primary electron transfer. If, and only if, the deprotonation of DH? occurs within the cage, we may in addition obtain a pair of neutral radicals D", ...

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