Paramagnetic probes in an organic semiconductor: μSR and DFT calculations of the Mu adducts of Alq 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) (Alq) have measured electron hopping rates of ∼10s, 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 Alq and 8-hydroxyquinoline (8hq), which is meant to model the muoniated radicals present in Alq when they are not in an OSC. These are supplemented by benchmarked DFT calculations. The ALC-μSR and TF-μSR spectra of 8hq and Alq 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 Alq 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 Alq 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 ∼10s.