The importance of intra-molecular electron spin relaxation in small molecular semiconductors

TL;DR

This study reveals that intra-molecular processes significantly influence electron spin relaxation in small molecular semiconductors, challenging the traditional focus on inter-molecular mechanisms and highlighting the role of vibrational modes.

Contribution

It demonstrates that intra-molecular electron spin relaxation is a dominant factor in small molecular semiconductors, based on muon spectroscopy across various molecular structures.

Findings

eSR exhibits Arrhenius-like temperature dependence

eSR is no greater than 0.85 MHz at 300 K

Intra-molecular phenomena are likely key to spin relaxation

Abstract

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.