Metal-free Carbazole-Based D-π-A Systems: Substituent-Modulated Acceptor Effects in Deep-Blue HLCT Emitters

Abstract

The development of efficient deep-blue emitters is crucial for next-generation optoelectronics materials. In this work, three donor-engineered hybridized local and charge-transfer (HLCT) emitters were rationally designed and synthesized by integrating carbazole (CBZ) and imidazole donor units within a conjugated framework. The molecular design enables effective modulation of the excited-state characteristics through the coexistence of locally excited (LE) and charge-transfer (CT) states, which is beneficial for achieving efficient deep-blue emission. Photophysical investigations reveal that the fluorophores exhibit strong deep-blue fluorescence with high emission efficiency arising from the HLCT nature of the excited states. Theoretical calculations further confirm the hybridized LE/CT character, indicating an appropriate balance between radiative decay and exciton utilization. Owing to their favorable photophysical properties, the emitters were successfully employed as organic downconverter material for hybrid LEDs. The fabricated devices display stable deep-blue electroluminescence with promising device performance. This work demonstrates that strategic donor engineering of imidazole-based frameworks is an effective approach for developing metal-free HLCT emitters for high-performance optoelectronic materials.