Importance of intramolecular electron spin relaxation in small molecule semiconductors

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 intramolecular phenomenon general to many organic semiconductors, in contrast to the commonly assumed spin relaxation models based on intermolecular charge-carrier transport.

Observation of a large spin-dependent transport length in organic spin valves at room temperature

“…The intermolecular distance and interactions may affect the orbital overlap of the p electrons in OSs 40 , which determine the transport efficiency of the charge carriers. Previous studies have also reported that temperature affects the lattice fluctuations 41,42 and intramolecular interactions 30,43 of C 60 , which may cause variations in the s-orbital. The s-orbital affects the SDT length in organic OSs 38 .…”

Temporal mapping of photochemical reactions and molecular excited states with carbon specificity

“…4). We also believe that electron spin relaxation and/or spin exchange cannot account for these changes-as it is known to affect all ALCs in a similar fashion in the solid state 32,33 , there is no associated increase in relaxation rate and it cannot account for all of the features in the time-dependent data (see the Supplementary Information). To explain our observations, one must invoke the mechanism of radical formation from the solvated muonium.…”

“…There could be a change in hyperfine coupling constant due to a structural or conformational difference 31 , an increased electron spin relaxation or exchange [32][33][34] , or a modification of the muonium reaction rate for a given carbon 31 . It is extremely unlikely that a conformational change or unpaired spin can account for this data, as the ALC at around 300 mT for site 4 is identical for the light-on and light-off cases.…”

Muonium avoided level crossing measurement of electron spin relaxation rate in a series of substituted anthradithiophene based molecules

“…This latter species can then chemically bind to an unsaturated organic molecule, in a similar way to hydrogenation, and is an extremely sensitive probe of spin dynamics intrinsic to the molecule [5][6][7][8][9][10]. If an external magnetic field is applied in the direction of the initial muon spin, as it is in so called longitudinal field experiments, the Hamiltonian for muonium can be written as…”

“…However, spin phenomena in these materials are not yet fully understood, partly due to the lack of suitable characterisation techniques, since those extensively used to characterise inorganic semiconductors are not always applicable to organic materials [1][2][3]. Muon spin relaxation (mSR) has been proven to be an applicable probe in this field, as it is sensitive to spin dynamics in organic semiconductors on a molecular length scale [4][5][6][7]. In organic semiconductors the implanted muons form a hydrogen-like system, called muonium, by capturing an electron.…”