Efficient Long-Wavelength Through-Space Charge-Transfer Thermally Activated Delayed Fluorescence in Conformationally Adaptive Emitters without Conformational Locking

Abstract Through-space charge-transfer thermally activated delayed fluorescence (TSCT-TADF) emitters often employ conformationally constrained donor–acceptor architectures to control through-space electronic coupling, particularly for long-wavelength emission. Here, we report conformationally adaptive orange-to-red TSCT-TADF emitters based on an S-shaped donor−π–acceptor−π–donor framework composed of phenoxazine donors, xanthene linkers, and a dibenzo[a,j]phenazine acceptor. Structural and NMR analyses reveal substantial conformational mobility, while systematic conformational searches and excited-state calculations identify thermally accessible conformers with distinct donor–acceptor geometries and excited-state characteristics. Comparative studies with reference compounds and fragment-resolved transition-density analysis show that the xanthene linker primarily acts as a conformational regulator and that the low-energy CT states have predominant TSCT character with only minor linker involvement. Temperature-dependent early-time photoluminescence measurements reveal continuous spectral relaxation in fluid toluene that slows upon cooling and is absent in rigid Zeonex and mCBP hosts, providing strong evidence for structural relaxation following photoexcitation within a pre-existing conformational ensemble. The emitters exhibit orange-to-red TADF with reverse intersystem crossing rates on the order of 105–106 s–1. Organic light-emitting diodes employing these emitters show electroluminescence at 598–624 nm, external quantum efficiencies up to 14.9%, and limited efficiency roll-off over a broad luminance range. These results demonstrate that efficient long-wavelength TSCT-TADF can be realized without permanent conformational locking and establish a framework for understanding the interplay between ground-state conformational heterogeneity and excited-state structural relaxation.