Perdeuteration of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (d-MEH-PPV): control of microscopic charge-carrier spin–spin coupling and of magnetic-field effects in optoelectronic devices

Abstract

Replacing all protons on a polymer by deuterium has a dramatic impact on spin-dependent properties of the material in devices.

Isotope Effect in the Magneto‐Optoelectronic Response of Organic Light‐Emitting Diodes Based on Donor–Acceptor Exciplexes

“…The FWHM of both EDMR(B) spectra are very similar (2.1 ± 0.1 mT), with the H-TADF spectrum being marginally broader than that in the D-TADF spectrum, yet this difference is not significant and far from the ratio expected from the proton to deuteron HFI ratio of ≈6. Thus, in contrast to previously measured EDMR(B) spectra of OLEDs based on traditional polymers, [36,37] the isotope effect on the line width does not play a significant role here, and thus is not governed by random HFI fields, in agreement with the isotope independent MEL(B) responses at RT. This shows that the magnetic resonant transitions studied here are between Zeeman split energy levels which are governed by interactions that are substantially stronger than the HFI.…”

Perdeuterated Conjugated Polymers for Ultralow‐Frequency Magnetic Resonance of OLEDs

“…Figure a shows the structure of the perdeuterated version of poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene] (d‐MEHPPV) . Unlike in previous reports of magnetic‐field effects in OLEDs, this material has the hydrogen atoms of the side chains and polymer backbone replaced by deuterium, rather than merely those associated with the side chains.…”

“…The effective hyperfine fields, B hyp , characterized by the standard deviation of a Gaussian distribution of hyperfine fields for electron and hole are therefore reduced from 0.72 mT and 0.19 mT for the protonated MEHPPV (h‐MEHPPV) to 0.24 mT and 0.08 mT for d‐MEHPPV. These values were extracted from a multi‐frequency analysis of EDMR spectra . Similar values are obtained by analyzing EDMR measurements taken at an excitation frequency of 280 MHz, as detailed in the Supporting Information.…”

“…In this case, the RF radiation modulates the magnetoresistance rather than inducing a true magnetic resonance. Since the curvature of the magnetoresistance of d‐MEHPPV is much larger than for h‐MEHPPV (see Figure S1), the zero‐field peak and the inversion of the signal at finite B 0 are more pronounced for the deuterated polymer. This effect is likely linked to an increase of the zero‐field peak with increasing device current (not shown), which presumably lowers the carrier‐pair lifetime.…”

“…While OLEDs offer a method of exploring the hyperfine field‐induced spin‐mixing processes invoked in the radical‐pair mechanism of magnetoreception, even removing 95 % of the hydrogen nuclei in the perdeuterated MEHPPV used here does not allow us to conclusively resolve Zeeman resonances at geomagnetic field strengths. Suitable molecular candidate complexes for the radical‐pair mechanism in biological systems, free of any hydrogen isotopes, have been identified—at least for one of the two spins .…”