The hydrogen/deuterium primary kinetic isotope effect provides useful information about the degradation mechanism of OLED host materials.
Thus, replacement of labile C–H bonds in the host with C–D bonds increases the device lifetime by a factor of five without loss of efficiency,
and replacement with C–C bonds by a factor of 22.5. Organic light-emitting diodes (OLEDs) being innovative devices for display and illumination,
their device lifetime and hence the mechanism of device degradation remain the most pressing subjects of research. Many of the studies reported thus
far have focused on the analysis of the degradation products, while a few have reported on the kinetics of the molecular processes of the degradation.
We focused on the hydrogen/deuterium (H/D) primary kinetic isotope effect (KIE) as a mechanistic probe, where the rate of a chemical reaction slows down
by a factor of 6.5 (a theoretical value at 298 K)8 when the cleavage of the C–H(D) bond in question is the rate determining step in the overall reaction
sequence. We report here and finding that deuteration of the benzylic methyl groups in a green phosphorescent host material (i.e., CH3 CZBDF vs. CD3CZBDF
in Fig. 1a) increases the device lifetime by a factor of five (from 0.2 h to 1.0 h) without affecting the other properties of the device much. This rate
retardation factor of five is consistent with the KIE value reported for the heterolysis of a benzylic C–H bond. Taking together the isotope effect and
the resonance-stabilizing effect of a furan ring (Fig. 1b), we consider the degradation of the methyl group in CH3CZBDF to be involved as a critical step
of the degradation process; that is, the molecule is oxidized to a radical cation (i.e., hole formation) during operation of the OLED device and suffers
from the loss of either a proton or a hydrogen radical. Guided by this analysis, we replaced the heterolytically labile C–H bonds in CH3CZBDF with more
stable C–CH3 bonds (t-BuCZBDF) and found the lifetime to increase by a factor of 22.5 under the same device configuration. The lifetime enhancement by a
factor of five through H to D change in the OLED performance makes an interesting contrast to a reported decrease of solar cell performance by 50% where
no C–H(D) cleavage is involved. These data indicate that isotope effects are of considerable practical and mechanistic values in organic electronic research.