Mechanism of Color Purity Regulation in Multiresonance Thermally Activated Delayed Fluorescence Emitters: Vibronic...

Multiresonance thermally activated delayed fluorescence (MR-TADF) emitters are renowned for their intrinsic narrowband emission characteristics. However, practical MR-TADF materials often exhibit nonnegligible spectral broadening. Herein, we present a theoretical investigation into para-substituted Cz-BDTP derivatives to elucidate how para-substituents modulate vibrational coupling to regulate emission line width and color purity. Systematic studies reveal that para-substitution significantly narrows the singlet-triplet energy gap (ΔEST) by >0.3 eV, thereby markedly enhancing the reverse intersystem crossing (RISC) rate. CB-BDTP-p-Nitro exhibits a RISC rate constant reaching 2.098 × 109 s-1, giving a calculated delayed fluorescence quantum yield (Φd) of 97.04% within the present kinetic model while also showing the broadest adiabatic emission profile in this series with a full width at half-maximum (FWHM) of 45 nm. Through the analysis of Huang-Rhys (HR) factors and fluorescence spectra, we identified the vibrational modes contributing to this broadening. Nonadiabatic dynamics simulations further reveal that substituents modulate the degree of singlet-triplet state mixing on ultrafast time scales. By correlating the electronic structure, vibrational coupling, and nonadiabatic molecular dynamics (NAMD) simulations, this work establishes a physically consistent framework linking molecular structure to emission line width. These findings provide molecular-level guidance for achieving high color purity in MR-TADF emitters through suppression of low-frequency vibrational activation and minimization of excited-state structural relaxation.