Photochemistry and emission behaviour of the iron(II) mixed metallocene complexes [C5R5Fe(arene)]PF6(R = H or Me)

Abstract

Mixed metallocenes of general formula [(η⁵-C₅R₅)Fe⁺(η⁶-arene)](R = H or Me), irradiated at long wavelengths (> 320 nm) show a fluorescence band centred at 520 nm. Fluorescence yields are strongly dependent upon the arene, and range from 1.2 × 10^–3 for [CpFe⁺(p-dimethoxybenzene)](Cp = C₅H₅), to less than 0.2 × 10^–4 when the arene is benzene, toluene, mesitylene, or pentamethylbenzene. The fluorescence quantum yields obtained upon irradiation of [CpFe⁺(p-xylene)] increase when hexamethylbenzene (HMB) is added, and approach, at high HMB concentrations, a value of 8 × 10^–4, similar to that obtained when [CpFe⁺(HMB)] is irradiated. These results are explained in terms of an efficient ligand exchange on the excited surface. Hexamethyl(dewar benzene)(HMDB) increases the fluorescence of [CpFe⁺(p-xylene)] and decreases the fluorescence emitted by [CpFe⁺(p-dimethoxybenzene)]. In both systems the fluorescence yields can be extrapolated, at high [HMDB], to the value observed when [CpFe⁺(HMB)] is irradiated. These results are compatible with both the occurrence of an efficient valence isomerization of HMDB and ligand exchange on the excited surface.