Work

Positional Isomerism Reveals Intrinsic Decoupling of Structural Relaxation and Vibrational Frequency in MR‐TADF Emitters

Year: 2026

Type: article

Abstract: ABSTRACT The pursuit of high‐color‐purity organic light‐emitting diodes (OLEDs) has spurred the development of multiple resonance (MR) emitters. Yet a long‐standing assumption in the field holds that minimizing excited‐state structural displacement (Δ Q ) is essential for achieving narrow emission bandwidth. Here we challenge this notion by demonstrating that Δ Q and molecular vibrational frequency ( ω ) act as independent, rather than coupled, determinants of the emission full width at half maximum (FWHM). Through systematic ortho ( o DB‐BN), meta ( m DB‐BN), and para ( p DB‐BN) positional isomerism of a sterically bulky dimesitylboryl acceptor on an archetypal MR scaffold, we reveal that small Δ Q reduces the Stokes shift but does not guarantee narrow FWHM; instead, low ω emerges as the decisive factor. The meta‐substituted isomer, despite exhibiting the smallest Δ Q , displays the broadest FWHM owing to a prominent low‐frequency torsional vibration. Conversely, the ortho ‐ and para ‐isomers retain ultranarrow emission despite pronounced molecular distortion. The resulting OLEDs achieve excellent external quantum efficiencies of up to 33.3% with suppressed roll‐off, and pDB‐BN delivers a remarkable luminance of 109 600 cd m −2 . This work provides an alternative perspective on the design of narrowband emitters, suggesting that controlling vibrational coupling, in addition to minimizing structural displacement, is a valuable consideration in molecular engineering.

Source: Angewandte Chemie International Edition

DOI: 10.1002/anie.2538716