The synthesis of 3‐(phenylcarbonyl)‐1H‐indazole derivatives via the intramolecular cyclization of triazene derivatives substituted by a 2‐(phenylethynyl)phenyl group at the position 3 was developed. The azonium salt, the key intermediate in the reaction, was prepared under mild conditions from trimethylsilyl iodide (TMSI) obtained from an in situ mixture of trimethylsilyl chloride (TMSCl) and sodium iodide (NaI). The obtained 3‐(phenylcarbonyl)‐1H‐indazole derivatives exhibited interesting optical properties including phosphorescence, which was verified by theoretical calculations.