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

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 (ΔQ) is essential for achieving narrow emission bandwidth. Here we challenge this notion by demonstrating that ΔQ and molecular vibrational frequency (ω) act as independent, rather than coupled, determinants of the emission full width at half maximum (FWHM). Through systematic ortho (oDB-BN), meta (mDB-BN), and para (pDB-BN) positional isomerism of a sterically bulky dimesitylboryl acceptor on an archetypal MR scaffold, we reveal that small ΔQ 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 ΔQ, 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.