Generation-Dependent\nIntramolecular Charge Transfer\nExcited-State Dynamics in Thermally Activated Delayed Fluorescence\nCarbazole Dendrimers

Organic light-emitting diodes (OLEDs)\nincorporating dendrimers\nas emissive layers are attracting significant attention due to their\nprecisely controllable molecular structures. Carbazole dendrimers\nhave been successfully employed in luminescent devices exhibiting\nthermally activated delayed fluorescence (TADF). TADF is a mechanism\nthat efficiently converts nonemissive triplet states to emissive singlet\nstates, thereby enhancing the performance of the OLED performance.\nTo optimize and manipulate emission properties, it is essential to\ngain a molecular-level understanding of the relevant excited states,\nspecifically the lowest singlet excited state (S<sub>1</sub>) and\ntriplet excited state (T<sub>1</sub>). This study investigates triazine-cored\ncarbazole dendrimers of three generations, examining their excited-state\nproperties through time-resolved photoluminescence and transient absorption\nspectroscopy. Our research revealed notable generation-dependent charge-transfer\ncharacteristics in the singlet excited states, as evidenced by solvent-dependent\nsteady-state and time-resolved photoluminescence spectroscopy. Higher-generation\ndendrimers exhibited an enhanced charge-transfer character in the\nS<sub>1</sub> state. While the transient absorption spectra of the\nS<sub>1</sub> states were significantly influenced by dendron elongation,\nthose of the T<sub>1</sub> states remained relatively insensitive\nto generation changes. These observations suggest that the electron–hole\ndistribution in the T<sub>1</sub> state is more localized near the\ninner core, whereas the hole distribution extends to the outer carbazole\nunits in the S<sub>1</sub> state. This comprehensive investigation\nelucidates the unique photophysical behaviors of these dendrimers,\npotentially guiding future advancements in OLED technology.