Organic Radical Ions

BY A. G. DAVIES AND G. GESCHEIDT

1 Introduction

This report covers the two years 1994 and 1995, with a few references taken from earlier years and from 1996 with the aim of improving continuity.

We note that an increasing number of papers are referring to the use of both ESR and ENDOR spectroscopy, and to the simultaneous application of ESR spectroscopy together with another technique such as UV-VIS spectroscopy or electrochemical measurements. More work is also being directed to radical trianions. There are a number of interesting studies of those factors, particularly steric strain, which can affect electron distribution and the ordering of energy levels in conjugated π-systems, and a lot of attention is being paid to counterion interactions. The interest in radical ions derived from the fullerenes has not slackened, and an increasing number of papers are concerned with the development of devices for molecular electronics.

2 Bibliography

There is a very timely review of the use of ENDO R spectroscopy in the study of radical ions in solid matrices. Topics in Current Chemistry have commenced a new series on the topic of electron transfer, and ESR studies are covered in articles on 'Radical ions: where organic chemistry meets material science' and on 'Photophysical and photochemical properties of fullerenes'.

3 General Technique

A simple experimental setup has been described permitting the simultaneous measurement of the ESR and optical absorption spectra of the same sample. It involves a diode array UV-VIS spectrometer which is coupled to an optical ESR cavity by quartz fibre optics. A small (0.2 ml) robust electrochemical cell capable of rapid-scan voltametry allows simultaneous electrochemical and ESR (SEESR) measurements to be carried out in solvents of high or low dielectric constant over a wide range of temperature, and similar measurements can be carried out using a cylindrical rotating cavity.

The pulse-mode product-yield-detected ESR (PYESR) technique measures spin-adduct yield as a function of the delay period for the on/ off time of a microwave pulse, which causes a reduction in the yield from radicals generated by a UV laser. The PYESR response curves, thus obtained, can be reproduced by numerical calculation to determine the radical-pair dynamics.

4 Calculations of Hyperfine Coupling Constants

Obviously the isotropic coupling constants map the electron distribution of radicals. They are inter alia indicative for the singly occupied orbital. Thus almost all papers reporting ESR data also include theoretical calculations. The methods utilised range from Hückel to high level ab initio, including perturbation treatment, but semiempirical calculations prevail. The agreement between theoretical and experimental values however often varies significantly. Recently, density functional theory (DFT) has been used to calculate the Fermi contact interactions of small radicals. The results of these calculations look very promising, and it seems very likely that OFT calculations will become an efficient tool for the calculation of hyperfine data. The good applicability of DFT is due to an improved inclusion of electron correlation compared to semiempirical and less sophisticated ab initio treatment. The major advantage versus sophisticated ab initio methods is a considerably shorter CPU time. Thus it should be possible also to investigate more extended molecules with DFT.

5 Radical Cations

5.1 Reviews

Radical cations of small molecules generated by γ-irradiation in freon matrices and investigated by ESR/ENOOR have been reviewed by Gerson and silicon radical cations by Bock. The isomerisations of radical cations have been compiled by Gebicki and Cope-type rearrangements of hexa-1,5-diene and semibullvalene radical cations by Williams.

5.2 Experimental Methods

The techniques for generating radical cations in fluid and rigid solution are well established. The common methods include oxidation with acids (e.g., trifluoroacetic acid (TFA), H2SO4), Lewis acids (e.g., AlCl3, SbCl3, SbCl5), metal salts (e.g., Pb(OAc)4) or differently substituted triarylaminium radical cation salts. For time-resolved studies strong electron acceptors like chloranil (with narrow ESR signals which do not greatly distort the spectrum of the desired compound) are applied. Predominately the radical cations of small strained molecules with high ionisation potentials (up to ca. 12 eV) are generated by γ-irradiation in freon matrices at low temperatures. Often radical cations can be formed in zeolites either directly or by additional radiolysis.

Phenyl iodine bis(trifluoroacetate) and 4–tolylthallium(III) bis (trifluoroacetate) were shown to serve as efficient oxidants and 1,1,1,3,3,3–hexafluoropropan-2–ol is a solvent with a highly stabilising ability for radical cations and allows them to be studied even at room temperature; a whole variety of oxidising agents can be utilised. Application of 2,3–dichloro-5,6–dicyanobenzoquinone (DDQ) under acid conditions thermally leads to radical cations of substrates with oxidation potentials below ca. 1.6 V (vs. SCE). Irradiation with UV light allows the observation of ESR spectra of radical cations derived from compounds with significantly higher oxidation potentials, e.g. benzocrown ethers. The presence of an acid quenches the radical anion of DDQ which then presumably decomposes to ESR-silent products.

Mechanisms underlying the activity of zeolites were investigated with benzene as a probe. When benzene is adsorbed in ZSM-5 which is activated with a-oxygen (by decomposition of N2O) it is first converted to phenol by the insertion of an O atom into a CH bond, and then the phenol is oxidised to its radical cation. Generation of the benzene (or d6-benzene) radical cation in H-ZSM-5 zeolite by irradiation (2.8 eV, 1000-W Hg lamp) at 93 K yields a paramagnetic precursor. The primarily observed ESR signal is ascribed to a complex consisting of two interacting radicals with a distance of ca. 5.36 Å, but it is not possible from the experimental data to specify its structure.

Ion pairing is a well established phenomenon found for radical anions. In contrast, the interaction of radical cations with their environment is only rarely observed and analysed. For AlCl3 oxidation, a close contact with a π-radical cation may lead to partial breaking of the symmetry. Some evidence about the influence of the...