Spiro[1H-azulenium-1,9′-fluorene] perchlorate. Intramolecular charge-transfer interaction between orthogonally arranged units of the azulenium cation and fluorene

Spiro[1H-azulenium-1,9′-fluorene] (3) perchlorate was synthesized in five steps starting from fluorenone and trimethylsilyl enol ether of 1-acetylcycloheptatriene. The pKR+ value of 3 was determined to be 7.8. A characteristic absorption band of 3 in acetonitrile was observed at 430 nm, which shows a bathochromic shift in dichloromethane, indicating the first example of the intramolecular charge-transfer interaction between orthogonally arranged units of an azulenium ion having a tropylium ion ring and fluorene having two benzene rings.