Stimulus-Driven Tuning of Multipathway Emission in 9-Fluorenone Derivatives: Elucidating Charge Transfer Dynamics in Higher Excited Singlet and Triplet States

The study of luminous materials having the capacity to emit light via many emission pathways has become a priority in materials research, spurred by the demand for increased performance in optoelectronic and medical applications. Traditional luminous materials are usually limited to single emission channels, restricting their performance and applicability. Multiemissive materials, on the other hand, can display fluorescence, charge transfer (CT) emission, room temperature phosphorescence (RTP), and delayed fluorescence (DF), providing a potential means to overcome these limits. In this study, we reported a class of 9-fluorenone derivatives tailored to utilize these diverse emission mechanisms. We acquired exact control over the relative contributions of each emission pathway by purposely modifying the molecular architecture─for example, adding heavy atoms to boost spin–orbit coupling and introducing electron-withdrawing groups to influence electronic states. The resulting compounds possessed high fluorescence quantum yields, extended RTP durations in the microsecond region, and efficient DF lifetimes in the millisecond domain. Furthermore, by altering molecular structure and external environmental circumstances, their emission spectra can be fine-tuned from visible to near-infrared. In addition, time-dependent density functional theory (TDDFT) calculations were performed to investigate the excited states and their roles in the different emission channels, providing deeper insight into the mechanisms underlying the observed photophysical behaviors. The adjustable character of these materials is further emphasized by their sensitivity to external stimuli such as solvent polarity and temperature, allowing for the selective enhancement of specific emissive routes. These 9-fluorenone derivatives are suited for advanced applications in organic light-emitting diodes (OLEDs), bioimaging, and molecular sensing technologies due to their stimuli-responsive features. Our findings emphasize the importance of combining molecular design and environmental factors to optimize multipathway emission, providing a versatile platform for the development of next-generation luminescent materials with broad applicability in both fundamental research and practical applications.