Bond Rotations and Heteroatom Effects in Donor–Acceptor–Donor Molecules: Implications for Thermally Activated Delayed Fluorescence and Room Temperature Phosphorescence

Abstract

New thermally activated delayed fluorescence (TADF) blue emitter molecules based on the known donor−acceptor−donor (D−A−D)type TADF molecule, 2,7-bis(9,9-dimethylacridin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DDMA-TXO2), are reported. The motivation for the present investigation is via the use of rational molecular design, based on DDMA-TXO2, to elevate the organic light emitting diode (OLED) performance and obtain deeper blue color coordinates. To achieve this goal, the strength of the donor (D) unit and acceptor (A) units have been tuned with methyl substituents. The methyl functionality on the acceptor was also expected to modulate the D−A torsion angle in order to obtain a blue shift in the electroluminescence. The effect of regioisomeric structures has also been investigated. Herein, we report the photophysical, electrochemical, and single-crystal X-ray crystallography data to assist with the successful OLED design. The methyl substituents on the DDMA-TXO2 framework have profound effects on the photophysics and color coordinates of the emitters. The weak electron-donating methyl groups alter the redox properties of the D and A units and consequently affect the singlet and triplet levels but not the energy gap (ΔE ST ). By systematically manipulating all of the aforementioned factors, devices have been obtained with acceptor-substituted III with a maximum external quantum efficiency of 22.6% and Commission Internationale de l'Ećlairage coordinates of (0.15, 0.18) at 1000 cd m −2 .

The influence of molecular geometry on the efficiency of thermally activated delayed fluorescence

“…Molecule 1 can interconvert between the ax., eq., and mixed forms in solution, as the barrier that separates these conformers is relatively low (o0.15 eV). This calculated barrier is consistent with previous calculations on 1, 35 and highlights the suitability of the DFT parameters used in this more expanded study. In molecule 5 however, such interconversion is more difficult as the energy barrier is higher (0.33 eV between ax.…”

“…This is important, as it has been recently proposed that the different conformers of phenothiazine have profound influence on the electronic structure, and on TADF efficiency. 25,35 All the compounds show oxidation and reduction processes in cyclic voltammetry (CV) studies ( While care should be exercised in interpreting excited state energies from DFT HOMO-LUMO energy gaps, this data set is consistent with the higher CT energy observed for the axial conformers. 25 The stabilisation of the HOMO is due to its ability to delocalise over both the donor and acceptor groups, as shown in Fig.…”

On the geometry dependence of tuned‐range separated hybrid functionals

“…Important in the context of the present work is that the electronic ground state of this molecule can exist in two dominant conformers, equatorial ( eq ) and axial ( ax ) . These conformers have different electronic character leading to contrasting TADF performance . This is primarily associated with the varying overlap between the HOMO and LUMO observed in each conformer.…”

Dual emission in purely organic materials for optoelectronic applications

“…Fig. 9 Examples of PF/DF DE in the systems with (a) conformational heterogeneity (27) and (b) multicomponent emitter (32). (a) Single-crystal structure and luminescence picture of 27; emission spectra of 27 before and after grinding (the left inset shows the pXRD spectra of the original sample, ground sample, and fumed sample; the right inset shows the luminescence photographs (under UV light) of the original and ground samples); reproduced with permission.…”

“…26 Hence, DE comprised of simultaneous fast fluorescence and slower phosphorescence with lifetimes in microsecond (ms) or millisecond (ms) scales can be observed at room temperature and sometimes under ambient conditions. 27 In contrast, a reverse ISC (RISC) process, occurring from the lowest triplet (T 1 ) to singlet (S 1 ) states with assistance of a small energy gap (DE ST r 0.3 eV) at room temperature, leads to the well-known TADF phenomenon, with a lifetime in the microsecond range. [28][29][30] TADF processes rarely lead to DE, as both the prompt fluorescence (PF) and delayed fluorescence (DF) arise from the same excited state and generate the same emission spectrum.…”