LASER FLASH-PHOTOLYSIS OF 9-FLUORENOL - PRODUCTION AND REACTIVITIES OF THE 9-FLUORENOL RADICAL CATION AND THE 9-FLUORENYL CATION

Two recent flash photolysis investigations of 9-fluorenol (FOH) in aqueous methanol have reached conflicting conclusions regarding the spectrum and lifetime of the 9-fluorenyl cation (F+). Gaillard, Fox, and Wan (GFW) (J. Am. Chem. Soc. 1989, 111, 2180) attributed to F+a transient at 640 nm with lifetimes in the microsecond range, while Mecklenburg and Hilinski (MH) (J. Am. Chem. Soc. 1989, 111, 5471) concluded that F+was the transient they observed at 515 nm which formed and decayed in <20 ps. MH ascribed the 640-nm transient to the 9-fluorenol radical cation (FOḢ+). In the present study FOḢ+has been produced in aqueous trifluoroethanol and in aqueous acetonitrile by reacting FOH with SO4̇-produced by 248-nm photolysis of the S2O82-ion. The FOḢ+so formed has absorptions with λmaxat 395, 595, and 645 nm and decays with lifetimes of 10-100 μs. We conclude therefore that the 640-nm transient is FOḢ+, and not F+FOḢ+forms in the direct photolysis by two-photon ionization, a conclusion reached on the basis of the quadratic dependency of the 640-nm absorbance on the intensity of the exciting light. The radical cation of fluorene has been produced in aqueous acetonitrile as well, both by photoionization and by reaction with SO4̇-; this species has λmaxat 365, 590, and 645 nm, and a lifetime similar to that of FOḢ+The 515-nm transient is observed as a relatively long-lived species (30 μs) upon photolysis of FOH in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP). This transient is not quenched by oxygen, its decay is accelerated by nucleophiles such as water and trifluoroethanol, and 1,1,1,3,3,3-hexafluorisopropyl 9-fluorenyl ether is obtained as the only product of photolysis. We conclude therefore that the 515-nm transient is the ground-state 9-fluorenyl cation, which has also been observed by MH in aqueous methanol where its lifetime is <20 ps. The remarkably weak nucleophilicity of HFIP is further demonstrated by kinetic and product experiments that show that F+undergoes electrophilic substitution of benzene in competition with capture by this solvent. © 1990, American Chemical Society. All rights reserved.