Electron and hydrogen self-exchange of free radicals of sterically hindered tertiary aliphatic amines investigated by photo-CIDNP

Abstract

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.

Protic Ionic Liquids as p-Dopant for Organic Hole Transporting Materials and Their Application in High Efficiency Hybrid Solar Cells

“…Indeed, protonated organic cations (spiro-OMeTAD-NH + ) can promote a single electron transfer from the parent neutral molecule (spiro-OMeTAD) to the protonated molecule via radical formation (spiro-OMeTAD + and spiro-OMeTAD-NH • , Figure 2d). 40,43 We highlight here that in this particular system, the electron transfer is not spontaneous, as it needs to be thermally activated, described in the Experimental Section. Then, even if we can still probe spiro-OMeTAD-NH • in the OS film (absorption at 575 nm in Figure 1d), this species could spontaneously react to release hydrogen gas and regenerate neutral spiro-OMeTAD.…”

“…36 As previously reported, 37−39 such specific quenching may be ascribed to oneelectron oxidation of the protonated molecule, which generates paramagnetic radical species in addition to spiro-OMeTAD + . 40 However, radical species may also strongly deform the 1 H signals of the remaining molecules in solution, depending on their relative concentration. 37 Subsequently, in order to reduce the concentration of the radical species, we focus our 1 H NMR experiments on Himi-TFSI, which is a significantly weaker acid than H-TFSI (see Figure 1).…”

Electron self-exchange activation parameters of diethyl sulfide and tetrahydrothiophene

“…The activation enthalpies are unusually small, but much more striking are the strongly negative activation entropies because it is well known that outer-sphere electron transfer of organic substrates in polar solvents is usually accompanied by small positive activation entropies [43] (e.g., our recent study on triisopropylamine [20] gave an activation entropy of +7.6 JK −1 mol −1 ). Our experimental values indicate transitions states that are much more ordered than usual, which is strong evidence that in these systems the self-exchange does not involve the monomeric radical cations D • + but the dimeric ones , for which the formation of the new two–center three–electron bond greatly restricts the number of possible orientations of the reacting molecules.…”

“…A radiofrequency pulse (typical duration: microseconds for protons) applied at a certain point of time after the laser flash converts the polarizations present in D at that precise moment into observable coherences and isolates them from the further cancelation, which only operates for magnetizations. Hence, a series of such time-resolved CIDNP experiments [13–16] with different delays between laser flash and observation pulse provides a direct way of measuring the self-exchange rates [17–20]. Practically all published studies of electron self-exchange – apart from indirect determinations based on the Marcus cross relationship [2] – have been carried out on aromatic or olefinic substrates.…”

“…The main complication arises from fast deprotonation of D • + at the α carbon to give a neutral radical which can occur at the radical-pair stage in a direct reaction (base, A • − ) or even at any stage of the reaction by a relayed proton transfer (relay base, D ) [21–23] and turns the gross reaction into a hydrogen abstraction. In a previous report on two aliphatic amines [20], we have used time-resolved CIDNP experiments to distinguish between the ensuing electron and hydrogen self-exchanges of D • + and . In the present work, we investigate two structurally similar aliphatic sulfides, diethyl sulfide DES and tetrahydrothiophene THTP , for which we have barred the direct deprotonation at C α by the choice of a suitable sensitizer; for these substrates, a relayed deprotonation is also impossible because they are not sufficiently basic; however, D • + can stabilize by dimer formation with surplus D to give [24].…”