Jeremy R. M. Giles

Electronic properties of conjugated polymers

The nature of the proposed defect states in conjugated polymers (solitons and polarons in polyacetylene, polarons and bipolarons in other materials) is expected to be strongly dependent on the morphology and chain conjugation length of the polymer. Polyacetylene prepared by the Durham ‘precursor’ route has a lower conjugation length than that prepared by the Shirakawa route, and it is expected that polarons or bipolarons rather than solitons should be the defect state introduced during chemical doping. We discuss here a wide range of physical measurements made on Durham polyacetylene. We present data for two other, processible, polymers. Poly(phenylenesulphide) shows strong optical absorption features below the band gap when reversibly doped with AsF5. Poly(diphenylenediphenylvinylene) shows similar behaviour and evidence for photogeneration and chemical generation of polaron-like states in this material is presented.

An electron spin resonance study of sulphuranyl radicals of the type R2S–SX [X = CF3 or R′C(O)]

Photochemically generated trifluoromethyl- or acyl-thiyl radicals add to dialkyl sulphides to form the sulphuranyl radicals R2S1–S2X [X = CF3 or R′C(O)], the e.s.r. spectra of which have been detected. These adducts appear to be non-planar at the three-co-ordinate sulphur and the unpaired electron is thought to occupy an S–S σ* orbital. The β-hydrogen splittings from the S1-alkyl groups probably arise mainly through a hyperconjugative mechanism of spin transmission and appear to depend upon the spin density (ρ) in the S1-3pσ orbital and the dihedral angle (θ) between the S1CHβ and S2S1C planes according to the approximate equation a(Hβ)= 25.2 ρcos2θ G. The g factors of the sulphuranyl radicals are markedly dependent upon temperature and the nature of R. At low temperatures, the adducts generally decay by a second-order process, presumably bimolecular self-reaction. Different behaviour is observed when R2S = [graphic omitted] or (Et2N)2S and rapid ring-opening or S–N cleavage, respectively, occurs even at low temperatures. Temperature-dependent lineshape effects are evident in the spectra of R2S–SX, and several different processes appear to be responsible for these effects, including efficient spin–rotation relaxation, conformational interconversion, and the bimolecular sulphide exchange reaction R2S + R2S–SX ⇌ R2S–SX + R2S.

An electron spin resonance study of the reactions of oxidising radicals with dialkyl sulphides. Radical cations derived from anodic oxidation of R2S, (R2N)2S, and (R2N)2SO

Constant current electrochemical oxidation of R2S, (R2N)2S, and (R2N)2SO gives rise to radical cations which have been detected by e.s.r. spectroscopy. Dialkyl sulphides, except di-t-butyl sulphide which afforded the spectrum of But2S+˙, gave the dimer cations (R2SSR2)+˙. Bis(dialkylamino) sulphides or sulphoxides gave monomer cations in which a large fraction of the spin density is on nitrogen. The g factors of (R2SSR2)+˙ increase with the bulk of R, and the g factors of both (R2SSR2)+˙ and But2S+˙ are dependent on the temperature, the solvent, and the counteranion. The sulphide cation radicals have also been generated by photolysis of solutions containing R2S, ButOOBut, and CF3CO2H, and formation probably involves oxidation of the sulphide by ButȮH. The neutral radical ButO· and Me3SiO· react with dialkyl sulphides by abstraction of hydrogen from an α-C–H group or by competing addition to sulphur to form a sulphuranyl radical. The e.s.r. spectra of the adducts R2ṠOSiMe3 have been detected and these radicals appear to be non-planar at sulphur with the unpaired electron probably confined to an S–O σ* orbital.

An optical study of the arsenic pentafluoride doping of poly(p-phenylene sulphide): polaron and bipolaron transitions

The oxidative doping of thin films of poly(p-phenylene sulphide) by arsenic pentafluoride has been followed by transmission absorption spectroscopy (near i.r. to u.v.) and transitions are assigned involving cationic states, poloran and bipolaron, formed within the π–π* band gap.

An electron spin resonance study of the generation and reactions of borane radical anions in solution

Photochemically or thermally generated t-butoxyl radicals rapidly abstract hydrogen from borohydride or cyanoborohydride anions to form H3B–˙ or H2[graphic omitted]CN–, respectively, which can be detected and their reactions studied in fluid solution using e.s.r. spectroscopy. The spectroscopic parameters confirm that the equilibrium geometry of H3B–˙ is planar, as is that of the isoelectronic H3C·, and the temperature dependences of a(11B) and a(3H) result from Boltzmann population of out-of-plane vibrational states. The extent of conjugative delocalisation of the unpaired electron onto nitrogen is similar in H2[graphic omitted]CN– and in the isoelectronic H2ĊCN. H3B–˙ adds to ethylene, trimethylvinylsilane, benzene, and pyridine, but H2[graphic omitted]CN– is less reactive and addition to only Me3SiCHCH2 was detected. The B–C bond eclipses the orbital of the unpaired electron in the preferred conformation of the ethylene adduct H3H2ĊH2, in contrast to the staggered conformation adopted by the isoelectronic propyl radical. Addition of H3B–˙ to electron deficient arenes is faster than to benzene, and the regioselectivities of these reactions also show that the borane radical anion is highly nucleophilic. Both H3B–˙ and H2[graphic omitted]CN– add to alkyl azides and cyanides to give triazenyl and iminyl radical adducts, respectively, and both borane radical anions displace alkyl radicals from alkyl isocyanides, perhaps through the intermediacy of a transient imidoyl radical adduct. H3B–˙ abstracts a halogen atom from alkyl chlorides, bromides, and iodides, while H2[graphic omitted]CN– reacts rapidly with only bromides and iodides; neither reacts with alkyl fluorides. The reactivities of H3B–˙ and H2[graphic omitted]CN– resemble those of H3Si· and R3Si· much more than those of H3C· and R3C·.

Electron spin resonance studies of thiophosphoranyl radicals. The mechanism of ligand permutation in phosphoranyl radicals

A series of thiophosphoranyl radicals have been generated in solution by addition of thiyl radicals (RS˙, from photolysis of RSSR or H2S) to phosphorus(III) compounds and their e.s.r. spectra have been studied. The phosphoranyl radicals [graphic omitted](OR′)SR have trigonal bipyramidal (TBP) structures in which the ring bridges apical and equatorial sites: the apicophilicity of RS is greater than that of R′O. Apical–equatorial (a–e) ring proton exchange is evident from e.s.r lineshape effects and it is proposed that the exchange takes place via a σ*(P–S) intermediate or transition state. In general, the rate of a–e ligand exchange in TBP phosphoranyl radicals appears to increase as the energy difference between TBP and σ* structures decreases. TBP (C2v) and σ*(C3v) configurations represent limiting structures for hypervalent phosphoranyl radicals and the geometry between these two extremes adopted by a particular radical appears to depend upon the nature of the ligands. It is suggested that the apicophilicity of ligands in TBP or σ* isomers is related to the ease of heterolytic dissociation of the P–ligand bonds, rather than to ligand electronegativity alone. The phosphoranyl radicals Ph3ṖSR are hypervalent and probably have σ*(P–S) structures, whereas the RS ligands are equivalent in (MeS)2ṖPh2 and (RS)2ṖH2, indicating TBP structures. The radical [ButOPPh3]˙ has a ligand–π-electronic structure and the low-temperature e.s.r. spectrum shows that the unpaired electron is centred on one ring in a π-type orbital. At high temperatures electron exchange between the rings renders them magnetically equivalent.

Electron spin resonance spectra of selenuranyl radicals R2Se–X

E.s.r. spectra assigned to the σ* selenuranyl radicals R2Se[graphic omitted]X (R = alkyl or aryl) are detected in solution during photochemical generation of X·[CF3S·, R′C(O)S·, Me3CO·, or Me3SiO·] in the presence of dialkyl or alkyl aryl selenides.

An e.s.r. study of the generation and reactivity of the borane radical anion (H3B·–) in solution

t-Butoxyl radicals are shown to abstract hydrogen rapidly from H4B– and H3BCN to give H3B·– and H2BĊN, respectively, and the reactions of H3B·– with alkyl halides, isocyanides, and azides are reported.

E.s.r. spectra and reactivity of alane radical anions in solution

The structures and reactions of H3Al·– and (ButO)3Al·–, generated by abstraction of hydrogen from H4Al– and (ButO)3AlH–, respectively, have been investigated in solution using e.s.r. spectroscopy.

An e.s.r. study of some reactions of acylthiyl radicals

U.v. photolysis of diacyl disulphides gives acylthiyl radicals [RC(O)S·] which react with alkenes, trialkyl phosphites, and dialkyl sulphides to give adducts which have been detected by e.s.r. spectroscopy.