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What is it about?
Why is it important?
Carbazoles are tricyclic aromatic heterocycles that have attracted significant attention due to their presence in natural products and their wide-ranging biological activities. Synthetic methodologies to access carbazoles and their fused-ring derivatives include direct annulation, cyclization reactions and transition-metal catalysis. Linear and angular fused carbazoles, such as pyrido-, pyrazolo-, pyrimido- and pyridazinocarbazoles, possess pharmacological applications, including antitumour and anti-HIV activities, as well as an ability to act as DNA intercalating agents. Among these, pyrazoloannulated heterocycles like pyrazolopyridopyrimidines stand out for their structural complexity, containing five N atoms and three fused rings, which combine the properties of pyrazole, pyridine and pyrimidine. Beyond therapeutic applications, carbazole derivatives have emerged as versatile fluorescent chemosensors, enabling bioimaging of ionic species, reactive oxygen and sulfur species, biomacromolecules and microenvironments.