σ3-Endo 1,4-Azaphosphinine-Embedded Polycyclic Aromatic Hydrocarbons: P-Hyperconjugation-Regulated Excited-State Symmetry and Thermally Activated Delayed Fluorescence

Abstract

This work reports a new family of 1,4-azaphosphinine-based polycyclic aromatic hydrocarbons (AP-PAHs) via efficient tandem phosphorylation. Being the first hyperconjugation-induced multiple-resonance (MR) emitters with the σ3-endo, σn-exo P-center, AP-PAHs exhibited thermally activated delayed fluorescence (TADF) with a narrow band (full width at half-maximum: 20 nm) and high photoluminescence quantum yields (PLQYs: 72% in CH2Cl2, 99% in doped films). Driven by the P-hybridization and P-hyperconjugation effects, the theoretical computations revealed that P(III)/PB-PAHs demonstrated excited-state symmetry breaking (ESSB) characters, which induced a strong structural relaxation or a reduced transition overlap integral, thus decreasing the PLQYs. Such EESB is distinct from the symmetrical excited-state structure of the classical BN MR emitters. Switching to the strong PO/PMe-induced π–σ(P–O/Me)* coupling enabled more symmetric excited-state structures and large transition overlap integrals and kRISCs, consequently boosting the PLQYs from 2% to 72% (99% in the 1% doped film). Leveraging on the cis-/trans-hyperconjugation effects, emission ranges of the double PO-derivatives were efficiently extended to 477 nm for the cis-isomer and 620 nm for the trans-isomer. Although having suppressed TADF via the double PO-induced backbone twisting, the cis-isomer maintained PLQYs as high as 73% in CH2Cl2 and 99% in the 1% doped film. As a proof of concept, two narrow-emitting AP-PAHs were applied in organic lighting-emitting diodes that showed narrow-band blue-violet and deep-blue emission with an EQE as high as 10%. This study revealed a new P-hyperconjugation-induced EESB mechanism for PN-type MR emitters, which opens an avenue for designing new efficient light-emitting materials.