Unifying the Photophysical Profile of Carbazoles: A Comprehensive Study of Triplet-State Properties and Sensitized Singlet Oxygen Yields

Carbazole (Cz) and its derivatives represent a class of organic chromophores with wide-ranging applications in photochemistry, photobiology, and materials science. Although the absorption and fluorescence properties of many Cz derivatives are known, a unified framework connecting this information to triplet-state properties and photosensitizing efficacy is lacking. This current report provides a comprehensive photophysical characterization of 36 Cz derivatives, systematically unifying older data with new experimental measurements to establish robust structure–property relationships. The photophysical behavior was characterized by using steady-state and time-resolved spectroscopic measurements in acetonitrile at 298 K and in a 2-methyl-THF glassy matrix at 77 K. The temperature-dependence (ranging from 4 to 300 K) of emissive properties of selected Cz derivatives in poly(methyl methacrylate) films was also measured. We also report Cz photosensitized singlet dioxygen (O₂(a¹Δ_g)) quantum yields and rate constants for Cz-mediated O₂(a¹Δ_g) removal/deactivation for the entire series of derivatives in acetonitrile. We complement experimental data with conceptual density functional theory calculations of relevant excited state parameters (i.e., vertical ionization potential, vertical electron affinity, electronegativity, electrophilicity, and nucleophilicity). The results presented herein facilitate a better understanding of Cz photophysics and, thereby, better set the stage for the rational development of new functional materials and compounds.