Coherent Spin Manipulation in Molecular Semiconductors: Getting a Handle on Organic Spintronics

Organic semiconductors offer expansive grounds to explore fundamental questions of spin physics in condensed matter systems. With the emergence of organic spintronics and renewed interest in magnetoresistive effects, which exploit the electron spin degree of freedom to encode and transmit information, there is much need to illuminate the underlying properties of spins in molecular electronic materials. For example, one may wish to identify over what length of time a spin maintains its orientation with respect to an external reference field. In addition, it is crucial to understand how adjacent spins arising, for example, in electrostatically coupled charge‐carrier pairs, interact with each other. A periodic perturbation of the field may cause the spins to precess or oscillate, akin to a spinning top experiencing a torque. The quantum mechanical characteristic of the spin is then defined as the coherence time, the time over which an oscillating spin, or spin pair, maintains a fixed phase with respect to the driving field. Electron spins in organic semiconductors provide a remarkable route to performing “hands‐on” quantum mechanics since permutation symmetries are controlled directly. Herein, we review some of the recent advances in organic spintronics and organic magnetoresistance, and offer an introductory description of the concept of pulsed, electrically detected magnetic resonance as a technique to manipulate and thus characterize the fundamental properties of electron spins. Spin‐dependent dissociation and recombination allow the observation of coherent spin motion in a working device, such as an organic light‐emitting diode. Remarkably, it is possible to distinguish between electron and hole spin resonances. The ubiquitous presence of hydrogen nuclei gives rise to strong hyperfine interactions, which appear to provide the basis for many of the magnetoresistive effects observed in these materials. Since hyperfine coupling causes quantum spin beating in electron–hole pairs, an extraordinarily sensitive probe for hyperfine fields in such pairs is given. “Messy” organic semiconductors seem the least likely materials for high‐resolution physics experiments. Yet their unique material properties open a route to explore fundamental aspects of spin physics in the condensed phase. The control of the spin population in organic light‐emitting diodes (OLEDs; see picture) may herald a new device concept: quantum mechanically coherent organic spintronics.