Theoretical Study on Selective Isotope Substitution Boosting High-Performance Circularly Polarized Delayed Fluorescence (CP-TADF) for Planar-Chiral Emitters

Recently, the isotope effect has emerged as a promising strategy for improving OLED performance. However, a comprehensive theoretical understanding is still lacking to elucidate how isotopic engineering can simultaneously amplify the performance of circularly polarized thermally activated delayed fluorescence (CP-TADF) emitters. Herein, we designed four isotopically substituted isomers (Czp-DiKTa, Czp-DiKTa-¹⁸O/D, Czp-DiKTa-D, and Czp-DiKTa-both) by selecting different functional groups based on the resonance Raman (rR) spectroscopic analysis. In the Herzberg-Teller (HT) limit, the <i>k</i>_CPL rates of all four isotopic isomers exhibited an enhancement of 3 orders of magnitude over the FC approximation prediction. This enhancement was particularly pronounced in donor-acceptor substituted isomers Czp-DiKTa-¹⁸O/D (3.71 × 10⁶ s^-1), and Czp-DiKTa-D (2.22 × 10⁶ s^-1), accompanied by substantially enhanced circularly polarized luminescence (CPL) dissymmetry factor <i>g</i>_CPL factors of 1.96 × 10^-1 and 1.21 × 10^-1, respectively. Furthermore, by suppressing the nonradiative internal conversion rate <i>k</i>_IC rate, isotopic substitution significantly enhances the delayed circularly polarized quantum yield (Φ_DCPL) for Czp-DiKTa-¹⁸O/D (93%) and Czp-DiKTa-D (91%). In contrast, the donor-acceptor doubly isotope-substituted Czp-DiKTa-both shows no notable improvement (79%). This trend was observed in <i>k</i>_IC and <i>g</i>_CPL, demonstrating that selective isotope substitution is a promising strategy for boosting device efficiency. These findings provide valuable insights for the rational design of advanced CPL optoelectronic materials.