  Electron spin relaxation rate (eSR) is investigated on several organic
semiconductors of different morphologies and molecular structures, using
avoided level crossing muon spectroscopy as a local spin probe. We find that
two functionalized acenes (polycrystalline tri(isopropyl)silyl-pentacene and
amorphous 5,6,11,12-tetraphenyltetracene) exhibit eSRs with an Arrhenius-like
temperature dependence, each with two characteristic energy scales similar to
those expected from vibrations. Polycrystalline tris(8-hydroxyquinolate)gallium
shows a similar behavior. The observed eSR for these molecules is no greater
than 0.85 MHz at 300 K. The variety of crystal structures and transport regimes
that these molecules possess, as well as the local nature of the probe,
strongly suggest an intra-molecular phenomenon general to many organic
semiconductors, contrasting the commonly assumed spin relaxation models based
on inter-molecular charge carrier transport.
