Paramagnetic probes in an organic semiconductor: µSR and DFT calculations of the Mu adducts of Alq3 and 8-hydroxyquinoline.

It has been claimed that longitudinal field muon spin relaxation (LF-µSR) experiments on the organic semiconductor (OSC) tris-(8-hydroxyquinoline)aluminum(III) (Alq3) have measured electron hopping rates of ∼1012s-1, while density functional theory (DFT) calculations suggest that electron hopping between a muoniated radical and a neighboring molecule is energetically unfavorable and that the LF-µSR experiments were probing muoniated radicals with localized spin density. We have performed avoided level crossing muon spin resonance (ALC-µSR) and transverse field muon spin rotation (TF-µSR) measurements on Alq3 and 8-hydroxyquinoline (8hq), which is meant to model the muoniated radicals present in Alq3 when they are not in an OSC. These are supplemented by benchmarked DFT calculations. The ALC-µSR and TF-µSR spectra of 8hq and Alq3 are best explained by Mu adding to all six secondary carbons of the quinolate rings with roughly equal yields and localized spin density. There is no evidence in the TF-µSR spectrum of Alq3 for the formation of radicals with muon hyperfine coupling constants of 23 or 91 MHz as reported earlier by others. Our measurements support the view that there is localized spin density on the molecule to which Mu is covalently bound and the muon is not a passive probe in organic systems as it can be incorporated into radicals that have different electronic structures to the parent compounds. The muoniated radicals in Alq3 are more short-lived than in 8hq, which could be due to interactions with mobile electrons in the OSC, but with electron spin flip rates on the order of ∼107s-1.

Dynamics of polystyrene probed by muon spin spectroscopy.

Muon spin spectroscopic measurements were made on atactic low-molecular-weight (LMW) (1.3 kg mol-1) and high-molecular-weight (HMW) (202 kg mol-1) polystyrene. Muoniated cyclohexadienyl radicals, which are formed by muonium addition to the phenyl side groups, are used as local probes of bulk dynamics. Muon spin relaxation is caused by the secondary γ-relaxation process, which involves motion of the phenyl rings, and is sensitive to the glass transition. The activation energy of the γ-relaxation process in the rubbery state is 0.60(2) eV in the HMW sample and 0.37(3) eV in the LMW sample.