Site-selective deuteration modulates vibronic coupling and excited-state dynamics in deep-blue thermally activated delayed fluorescence emitters

Abstract

The development of efficient and stable deep-blue emitters has long remained a critical challenge for organic light-emitting diodes (OLEDs), with deuteration emerging as a promising strategy. Here, site-selective deuteration of a donor–acceptor thermally activated delayed fluorescence (TADF) molecule incorporating an oxa-boraphenalene (BO) acceptor and a diphenylamino (DPA) donor demonstrates that deuteration selectively modulates vibrational dynamics while leaving electronic properties essentially invariant. Non-adiabatic coupling analysis reveals that mid-frequency modes (300–1500 cm⁻¹) dominate internal conversion, with DPA-site deuteration suppressing non-radiative loss more effectively than BO-site substitution. Huang–Rhys factor analysis identifies two DPA-derived low-frequency modes that provide two additional vibronic relaxation paths, accelerating the reverse intersystem crossing rate of the S₁–T₂ path beyond other pathways. Conversely, BO deuteration enhances the Herzberg–Teller contribution to the effective spin–orbit coupling through increased first-order vibronic coupling matrix elements. The observed spectral broadening is attributed to transitions into a quasi-continuum of high-lying vibrational levels involving vibronic progressions of modes 1 and 4. Consequently, a 3.94-fold reduction in C–D bond dissociation rate corresponds to the significantly extended device operational lifetime. These findings establish site-selective deuteration as an effective strategy for decoupling radiative and non-radiative vibrational pathways, providing molecular-level guidance for the design of stable deep-blue TADF emitters.