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

Abstract

The isotope effect is studied in the magneto‐electroluminescence (MEL) and pulsed electrically detected magnetic resonance of organic light‐emitting diodes based on thermally activated delayed fluorescence (TADF) from donor–acceptor exciplexes that are either protonated (H) or deuterated (D). It is found that at ambient temperature, the exchange of H to D has no effect on the spin‐dependent current and MEL responses in the devices. However, at cryogenic temperatures, where the reverse intersystem crossing (RISC) from triplet to singlet exciplex diminishes, a pronounced isotope effect is observed. These results show that the RISC process is not governed by the hyperfine interaction as thought previously, but proceeds through spin‐mixing in the triplet exciplex. The observations are corroborated by electrically detected transient spin nutation experiments that show relatively long dephasing time at ambient temperature, and interpreted in the context of a model that involves exchange and hyperfine interactions in the spin triplet exciplex. These findings deepen the understanding of the RISC process in TADF materials.

Long-lived spin-polarized intermolecular exciplex states in thermally activated delayed fluorescence-based organic light-emitting diodes

“…The homogeneous linewidth FWHM h = 22 MHz allows to estimate a lower limit for the dephasing time T 2 * > 2/(π ∙ FWHM h ) = 30 ns, while the width of the spike corresponds to the spin-lattice relaxation time. The dephasing time agrees well with that estimated from pulsed Rabi experiments on similar OLED systems ( 55 , 61 ).…”

“…Electron-proton HFI A H is usually weak and unresolved, leading to inhomogeneous broadening ( 50 – 54 ). For exciplexes in a similar TADF system, A H,max < 5.5 MHz was determined as an upper limit ( 55 ), whereas for charge-separated states in polymer:fullerene blends, A H,max of 2 to 3 MHz ( 50 ) or 4 to 5 MHz ( 51 ) was estimated. Considering the large molecular extent of m-MTDATA:BPhen and that protons at the edges of molecules are not part of the conjugated electron system, we can expect A H < 2 MHz.…”

Disorder‐Induced Dispersive Magneto‐Electroluminescence of Blue Emitters in Organic Light Emitting Diodes

“…Xin Pan, Ohyun Kwon,* Dipak Raj Khanal, Byoungki Choi, and Zeev Valy Vardeny* DOI: 10.1002/adom.202101334 rates can be tuned by an external magnetic field B, which modulates the EL emission intensity giving rise to magneto-EL (MEL) of which response is an efficient tool for investigating the e-h pair dynamics in the active layer of the device. [2][3][4][5][6][7][8][9][10][11][12][13][14] The MEL(B) response requires spinmixing activity among the four spin sublevels of the spin-entangled e-h pairs, namely intersystem-crossing between singlet and triplet spin configurations; which is dominated by various spinrelated interactions. [2][3][4][5][6]10,11,[16][17][18] These include the hyperfine interaction (HFI), spin orbit coupling (SOC), and also the difference, Δg of the Landé g-factors of the electron and hole constituents of the e-h species, known as the "Δg mechanism".…”

“…The magnetic field effect has been proven to be a powerful tool for investigating the spin-dependent processes associated with spin-pair species in organic-based devices, especially in organic light emitting diodes (OLEDs). [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Upon carriers' injection into the organic active layer of the device, the principal process that counts is the formation of spin-entangled electron and hole (e-h) pairs which generate singlet and triplet excitons; or, alternatively dissociate into free carries. These OLED processes may be studied indirectly by measuring the device electro-luminescence (EL) versus bias voltage V (namely EL-V) and current density, J versus V (J-V response).…”

Highly selective single and multiple deuteration of unactivated C(sp3)-H bonds

“…The precise deuterium labeling technique is of significant value in the investigation of kinetic isotopic effects 1 , 2 , the discovery of pharmaceutical drugs 3 – 6 , and material modifications 7 as well as biochemical techniques 8 , 9 (Fig. 1a ).…”