We
expose significant changes in the emission color of carbazole-based
thermally activated delayed fluorescence (TADF) emitters that arise
from the presence of persistent dimer states in thin films and organic
light-emitting diodes (OLEDs). Direct photoexcitation of this dimer
state in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) reveals the significant influence of dimer species
on the color purity of its photoluminescence and electroluminescence.
The dimer species is sensitive to the sample preparation method, and
its enduring presence contributes to the widely reported concentration-mediated
red shift in the photoluminescence and electroluminescence of evaporated
thin films. This discovery has implications on the usability of these,
and similar, molecules for OLEDs and explains disparate electroluminescence
spectra presented in the literature for these compounds. The dimerization-controlled
changes observed in the TADF process and photoluminescence efficiency
mean that careful consideration of dimer states is imperative in the
design of future TADF emitters and the interpretation of previously
reported studies of carbazole-based TADF materials.
A simplified state model and associated rate equations are used to extract the reverse intersystem crossing and other key rate constants from transient photoluminescence measurements of two high performance thermally activated delayed fluorescence materials. The values of the reverse intersystem crossing rate constant are in close agreement with established methods, but do not require a priori assumption of exponential decay kinetics, nor any additional steady state measurements. The model is also applied to measurements at different temperatures and found to reproduce previously reported thermal activation energies for the thermally activated delayed fluorescence process. Transient absorption measurements provide independent confirmation that triplet decay channels (neglected here) have no adverse effect on the fitting.
The realization of efficient organic electronic devices requires the controlled preparation of molecular thin films and heterostructures. As top-down structuring methods such as lithography cannot be applied to van der Waals bound materials, surface diffusion becomes a structure-determining factor that requires microscopic understanding. Scanning probe techniques provide atomic resolution, but are limited to observations of slow movements, and therefore constrained to low temperatures. In contrast, the helium-3 spin-echo (HeSE) technique achieves spatial and time resolution on the nm and ps scale, respectively, thus enabling measurements at elevated temperatures. Here we use HeSE to unveil the intricate motion of pentacene admolecules diffusing on a chemisorbed monolayer of pentacene on Cu(110) that serves as a stable, well-ordered organic model surface. We find that pentacene moves along rails parallel and perpendicular to the surface molecules. The experimental data are explained by admolecule rotation that enables a switching between diffusion directions, which extends our molecular level understanding of diffusion in complex organic systems.
New charge transfer crystals of π-conjugated, aromatic molecules (phenanthrene and picene) as donors were obtained by physical vapor transport. The melting behavior, optimization of crystal growth and the crystal structure is reported for charge transfer salts with (fluorinated) tetracyanoquinodimethane (TCNQ-F x , x=0, 2, 4), which was used as acceptor material. The crystal structures were determined by single-crystal X-ray diffraction. Growth conditions for different vapor pressures in closed ampules were applied and the effect of these starting conditions for crystal size and quality is reported. The process of charge transfer was investigated by geometrical analysis of the crystal structure and by infrared spectroscopy on single crystals. With these three different acceptor strengths and the two sets of donor materials, it is possible to investigate the distribution of the charge transfer systematically. This helps to understand the charge transfer process in this class of materials with π-conjugated donor molecules.
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