Revealing weak spin-orbit coupling effects on charge carriers in a $\pi$-conjugated polymer

We measure electrically detected magnetic resonance on organic light-emitting diodes made of the polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] at room temperature and high magnetic fields where spectral broadening of the resonance due to spin-orbit coupling (SOC) exceeds that due to the local hyperfine fields. Density-functional-theory calculations on an open-shell model of the material reveal $g$-tensors of charge-carrier spins in the lowest unoccupied (electron) and highest occupied (hole) molecular orbitals. These tensors are used for simulations of magnetic resonance line shapes. Besides providing the first quantification and direct observation of SOC effects on charge-carrier states in these weakly SO-coupled hydrocarbons, this procedure demonstrates that spin-related phenomena in these materials are fundamentally monomolecular in nature.

Figure 1

(a) Room-temperature EDMR spectrum of MEH-PPV with a double-Gaussian fit (red). (b) and (c) Plots of the single-carrier probability density in the (b) LUMO and (c) HOMO of a model of MEH-PPV.

Figure 2

(a) Sketch of the high-magnetic-field EDMR probe head. (b) Photograph of the probe head when mounted to the waveguide. (c) The OLED sample for high-field EDMR. (d) Sketch of the sample design (the encapsulation is omitted).

Figure 3

Lower panels: EDMR spectra at MW frequencies of 120, 240, and 336 GHz along with a global fit to a double-Gaussian model with frequency-dependent broadening. Upper panels: residuals along with coefficients of determination.

Figure 4

Measured and simulated spectra. The red lines represent simulations based on DFT where the narrow linewidth is assigned to the electron and the broad linewidth is assigned to the hole. The green lines show opposite assignment. The solid lines correspond to a geometry where the substituents are located on alternating sides of each monomer unit, whereas the dashed lines show the all-trans conformation.