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

Deng-Gao Chen; Tzu-Chieh Lin; Chi-Lin Chen; Yi-Ting Chen; Yi-An Chen; Gene-Hsiang Lee; Pi-Tai Chou; Chia-Wei Liao; Po-Chen Chiu; Chih-Hao Chang; Yi-Jyun Lien; Yun Chi

doi:10.1021/acsami.8b00053

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

ACS Appl Mater Interfaces. 2018 Apr 18;10(15):12886-12896. doi: 10.1021/acsami.8b00053. Epub 2018 Apr 4.

Authors

Affiliations

¹ Department of Chemistry , National Taiwan University , Taipei 10617 , Taiwan.
² Department of Photonics Engineering , Yuan Ze University , Chungli, Taoyuan 32003 , Taiwan.
³ Department of Chemistry , National Tsing Hua University , Hsinchu 30013 , Taiwan.
⁴ Department of Materials Science and Engineering and Department of Chemistry , City University of Hong Kong , Kowloon , Hong Kong SAR.

PMID: 29582654
DOI: 10.1021/acsami.8b00053

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^-1) 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 CH₂Cl₂. 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^-1 and 18.7 lm W^-1) and 22.6% (67.7 cd A^-1 and 50.0 lm W^-1).

Keywords: N-donor; borane; charge transfer; phenoxazine; solvatochromism; structural relaxation; thermally activated delayed fluorescence.