Revealing the excited-state emission pathways in indolizine-derived TADF molecules by photoluminescence and electroluminescence

Abstract

Thermally activated delayed fluorescence (TADF) enables the harvesting of triplet excitons in purely organic emitters, yet the relationship between photoluminescence (PL) and electroluminescence (EL) pathways resulting in emissive states remains poorly understood. Herein, we explore indolizine derivatives as TADF emitters by reporting the synthesis and a comprehensive investigation of their photophysical properties. While the parent indolizine (Z) and the carbazole-substituted derivative (ZCz) do not exhibit TADF, phenoxazine- and phenothiazine-based compounds (ZPX and ZPT) display TADF behavior in rigid environments. Photophysical studies performed in polymer matrices (PS and PMMA) and in host–guest systems using mCP reveal the participation of locally excited (1LE, 3LE) and charge-transfer (1CT, 3CT) states, giving rise to two delayed fluorescence pathways mediated by reverse intersystem crossing (RISC). Temperature-dependent emission and kinetic analysis indicate that distinct triplet states contribute differently to delayed emission, supported by DFT/TD-DFT calculations and small singlet–triplet energy gaps (ΔEST). A direct comparison between PL and EL demonstrates that electrical excitation preferentially accesses the CT-states, leading to measurable differences in emission profiles despite a common emissive 1CT state. Organic light-emitting diodes (OLEDs) incorporating ZPX and ZPT achieve external quantum efficiencies (EQE) of up to 8.7%. These results highlight indolizine-based architectures as promising TADF emitters and demonstrate that excitation-dependent excited-state pathways are a critical design consideration for OLEDs.