Linking Emission Bandwidth to Charge-Resonance Character in B/O-Based Multiple-Resonance Emitters

Narrowband emission is essential for high-color-purity organic light-emitting diodes, yet rapid prediction of the emission full width at half-maximum (fwhm) from molecular electronic structure remains challenging. Here, we adapt the cyanine limit two-state framework to define c_eff², an effective descriptor of charge-resonance balance in oxygen-bridged triarylborane multiple-resonance (MR) emitters. This descriptor is constructed from the state and transition dipole moments at the Franck-Condon geometry and, therefore, provides an axis-independent measure of excitation-induced dipolar reorganization without requiring excited-state geometry optimization. For 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (DOBNA) derivatives in dichloromethane, $\sqrt{c_{\mathrm{eff}}^2}$ shows a strong inverse linear correlation with experimental fwhm, giving R² = 0.997. The same negative trend is retained in toluene, with R² = 0.925, although the correlation is more scattered because the cyanine-like charge-resonance balance is less fully stabilized in the less polar medium. Analysis of the underlying dipole quantities indicates that fwhm variation is governed primarily by the magnitude of excitation-induced dipolar reorganization rather than by the transition dipole moment alone. These results establish c_eff² as a physically interpretable and computationally economical descriptor for assessing spectral narrowing in DOBNA-based MR emitters.