High-mobility holes

We forewarn the reader that the formation of high-mobility holes is not peculiar to these four cycloalkanes For instance, cyclooctane [61], squalane [62,63,64], and CCI4 [65] also yield such holes. However, in these other liquids, the holes are unstable and, consequently, more difficult to study (the lifetimes are 5-20 nsec vs. 1-3 psec). This explains why convincing demonstrations for the occurrence of high-mobility holes are slow to come. For example, squalane (by virtue of its high viscosity) has been frequently... 

Other solvents Hydrocarbons were intensively studied in the last couple of decades. The debate around the lifetime and trapping ability of the high mobility holes seems to have been settled (56-58). Apparently fast electron transfer equilibrium between at least two conformers in cyclohexane, and perhaps other hydrocarbons, could account for most of the kinetic observations from both spectrophotometric and dc-conductivity observations (59). However, the rather nondescript spectra of the intermediates in these solvents hampers further elucidation of the primary processes in hydrocarbons. 

The phase shifts of quantum beats have been studied in other alkanes as well [30], As expected, in linear alkanes the rate constants of hole trapping by acceptors, determined from the shifts, were close to the diffusion-controlled ones. In cyclic alkanes (cyclohexane, czs-decalin, and trans-decalin), the hole mobility is known to considerably exceed the mobility of molecular ions [31]. In these solvents the observed phase shifts had an intermediate value between that expected for the highly mobile holes and that assumed for molecular ions. Both types of ions are likely to take part in the formation of diphenylsulphide radical cation in cyclic alkanes. 

The quantum beats method has been also applied for studying viscous squalane solutions [19], where highly mobile holes were found [32] in pulse radiolysis experiments. The rate constant of hole capture was determined from the slope to be 6.1x10 NT s", which confirms conclusion on the high mobility of holes in squalane. The rate constant of squalane hole decay was estimated from the intercept to be (8.3 2)xl0 s , which is also in fair agreement with experiments on pulse radiolysis [32]. 

The formation of high-mobility holes and "satellite ions". As was briefly mentioned above, radiolysis of hydrocarbons results in the formation of several types of cationic species besides the solvent holes. Most of these "satellite ions" are generated within the first nanosecond after the radiolytic pulse. 

Class (i) reactions were observed in all four cycloalkanes that exhibit long-lived high-mobility holes [4-8,10,13,14,17]. These reactions were also observed in squalane [24, 31] and cyclooctane [26]. The reaction rates linearly scale with the hole mobility as a function of temperature (with exception of trans-decalin) and the fraction of cycloalkane in the solvent mixture [14]. The highest rate constants were observed for reactions of cyclohexane hole with low-IP aromatic solutes, (3-4.5)xl0ll M-1 s-l at 25°C [13,14]. In these irreversible reactions, a solute radical cation is generated. 

In concluding this section, we observe that though the nature and the migration mechanism for the high-mobility holes are not yet fully understood, a consistent picture of their chemical, dynamic, and magnetic properties begins to emerge. 

Redecker, M., et al. 1999. High mobility hole transport fluorene-triarylamine copolymers. Adv Mater 11 241. 

Redecker, M., Bradley, D., Inbasekaran, M., Wu, W., Woo, E. High Mobility hole transport fluorene-triarylamine copolymers. Adv. Mater. 11, 241-246 (1999)... 

The rate constants in this collection include both electron transfer from the radical R to the substrate S as well as transfer in the opposite direction. The latter constitutes an oxidative action of the radical R. For cationic radicals this is usually referred to as charge transfer. In irradiated solvents, particularly of non-polar nature, the primary radical cation often exhibits the property of a highly mobile hole. Reactions of such species which involve electron transfer from a substrate molecule to such a hole have also been included. 

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