Optically Triggered Planarization of Boryl-Substituted Phenoxazine: Another Horizon of TADF Molecules and High-Performance OLEDs

Abstract

We report the unprecedented dual properties of excited-state structural planarization and thermally activated delayed fluorescence (TADF) of 10-dimesitylboryl phenoxazine, i.e., PXZBM. Bearing a nonplanar phenoxazine moiety, PXZBM shows the lowest lying absorption onset at ∼390 nm in nonpolar solvents such as cyclohexane but reveals an anomalously large Stokes-shifted (∼14 500 cm) emission maximized at 595 nm. In sharp contrast, when a phenylene spacer is added between phenoxazine and dimesitylboryl moieties of PXZBM, the 10-(4-dimesitylborylphenyl)phenoxazine PXZPBM in cyclohexane reveals a much blue-shifted emission at 470 nm despite its red-shifted absorption maximized at 420 nm (cf. PXZBM). The emission of PXZBM further reveals solvent polarity dependence, being red-shifted from 595 nm in cyclohexane to 645 nm in CHCl. For rationalization, the steric hindrance between phenoxazine and the dimesitylboryl unit in PXZBM caused a puckered arrangement of phenoxazine at the ground state. Upon electronic excitation, as supported by the femtosecond early relaxation dynamics, spectral-temporal evolution and energetics calculated along the reaction potential energy surfaces, the diminution of N → B electron transfer reduces π-conjugation and elongates the N-B bond length, inducing the fast phenoxazine planarization with a time constant of 890 ± 100 fs. The associated charge-transfer reaction from phenoxazine (donor) to dimesitylboryl unit (acceptor) results in a further red-shifted emission in polar solvents. In stark contrast, PXZPBM shows a planar phenoxazine and undergoes excited-state charge transfer only. Despite the distinct difference in excited-state relaxation dynamics, both PXZBM and PXZPBM exhibit efficient TADF capable of producing highly efficient orange and green organic light emitting diodes with peak efficiencies of 10.9% (30.3 cd A and 18.7 lm W) and 22.6% (67.7 cd A and 50.0 lm W).

Insights into the Optical Properties of Triarylboranes with Strongly Electron‐Accepting Bis(fluoromesityl)boryl Groups: when Theory Meets Experiment

“…In THF solvent, the energies of the singlet and triplet excited states are very close to each other only for compound 2 , which may explain why 2 shows phosphorescence at 77 K in contrast to 3 and 4 . Finally, D‐π‐A triarylboranes with small singlet‐triplet gaps are already proving useful in thermally activated delayed fluorescence (TADF) based materials . The use of trifluoromethylated aryl groups in three coordinate boron compounds for optical applications provides a very efficient tool to fine tune properties.…”

Facile Generation of Thermally Activated Delayed Fluorescence and Fabrication of Highly Efficient Non‐Doped OLEDs Based on Triazine Derivatives

“…Table ) . The short lifetimes of these delayed fluorescence components and the large TADF/RISC rate constants indicate the effective TADF of the compounds . It worth mentioning that the exhibited delayed components of the neat films were akin to their doped film counterparts, implying the possibility of fabricating high‐efficiency non‐doped OLEDs.…”

Design, Synthesis, and Temperature-Driven Molecular Conformation-Dependent Delayed Fluorescence Characteristics of Dianthrylboron-Based Donor–Acceptor Systems

“…On the other hand, coordinatively unsaturated and Lewis acid characteristics of tricoordinate born-based D-A systems have been extensively exploited for developing sensors (Hudnall et al, 2009;Galbraith and James, 2010;Jäkle, 2010), luminescence materials for non-linear optics (Yuan et al, 1990;Del Rey et al, 1998), and OLEDs (Suzuki et al, 2015;Ji et al, 2017;Mellerup and Wang, 2019). Recently, aminoboranebased molecular systems have been successfully utilized in optoelectronics (Hatakeyama et al, 2012;Wang et al, 2013;Hashimoto et al, 2014;Ayhan et al, 2016Ayhan et al, , 2018Wang and Pei, 2016;Lien et al, 2017;Liu et al, 2017Liu et al, , 2019Chen et al, 2018). We have been actively involved in developing luminescent materials by judiciously altering the molecular conformations of boron-based D-A systems (Sudhakar et al, 2013(Sudhakar et al, , 2017Swamy et al, 2014;Neena and Thilagar, 2016;Kalluvettukuzhy and Thilagar, 2017).…”