3‐Azafluorenone Derivatives with Small HOMO–LUMO Gaps, Allowing Visible‐Light Absorption

A versatile synthetic strategy for 3-azafluorenone derivatives was developed. The strategy enabled efficient access to previously inaccessible structural analogs. Crucially, tert-butyl protection of ketocarboxylic acid intermediates significantly improved isolated yields and enabled the incorporation of electron-donating, electron-withdrawing, and π-extended substituents. The photophysical properties of the compounds were characterized by UV-Vis absorption and fluorescence spectroscopic measurements, and the electrochemical behavior was monitored using cyclic voltammetry. The electron-donating and π-extended substituents induced pronounced bathochromic shifts in absorption and emission, consistent with the narrowing of the HOMO-LUMO gap, whereas strong electron-withdrawing groups or 6-methoxy substitution caused a slight hypsochromic shift. Nitrogen incorporation and electron-withdrawing substituents stabilized the LUMO, affording more positive ground- and excited-state reduction potentials. Most derivatives exhibited higher excited-state reduction potentials than fluorenone, with the majority exceeding +1.6 V versus saturated calomel electrode (SCE). Further, time-dependent density functional theory (TD-DFT) calculations reproduced the observed spectral trends and oscillator strengths, confirming the π-π* nature of the lowest-energy transitions. All derivatives exhibited intense, red-shifted visible light absorption, and most showed high excited-state reduction potentials, demonstrating their potential for photoredox applications. These results establish detailed structure-property relationships for 3-azafluorenones, providing design principles for the rational tuning of optical and redox properties.