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Efficient photocyclization from a low-lying triplet state is reported for a photochromic dithienylperfluorocyclopentene with Ru(bpy)3 units attached via a phenylene linker to the thiophene rings. The ring-closure reaction in the nanosecond domain is sensitized by the metal complexes. Upon photoexcitation into the lowest Ru-to-bpy 1MLCT state followed by intersystem crossing to emitting 3MLCT states, photoreactive 3IL states are populated by an efficient energy-transfer process. The involvement of these 3IL states explains the quantum yield of the photocyclization, which is independent of the excitation wavelength but decreases strongly in the presence of dioxygen. This behavior differs substantially from the photocyclization of the nonemissive dithienylperfluorocyclopentene free ligand, which occurs from the lowest 1IL state on a picosecond time scale and is insensitive to oxygen quenching. Cyclic voltammetric studies have also been performed to gain further insight into the energetics of the system. The very high photocyclization quantum yields, far above 0.5 in both cases, are ascribed to the strong steric repulsion between the bulky substituents on the dithienylperfluorocyclopentene bridge bearing the chelating bipyridine sites or the Ru(bpy)3 moieties, forcing the system to adopt nearly exclusively the reactive antiparallel conformation. In contrast, replacement of both Ru(II) centers by Os(II) completely prevents the photocyclization reaction upon light excitation into the low-lying Os-to-bpy 1MLCT state. The photoreaction can only be triggered by optical population of the higher lying 1IL excited state of the central photochromic unit, but its yield is low due to efficient energy transfer to the luminescent lowest 3MLCT state.

We have synthesized nine rodlike compounds of nanometric dimension with general formula [M(bpy)3-(ph) n -M‘(bpy)3]4+ (M = M‘ = Ru(II); M = M‘ = Os(II); M = Ru(II), M‘ = Os(II); bpy = 2,2‘-bipyridine; ph = 1,4-phenylene; n = 3, 5, 7; the central phenylene unit bears two alkyl chains for solubility reasons; the metal-to metal distance is 4.2 nm for the longest spacer). The absorption spectra and the luminescence properties (emission spectra, quantum yields, and excited-state lifetimes) of the nine dinuclear complexes have been investigated in acetonitrile solution at 293 K and in butyronitrile rigid matrix at 77 K. The results obtained have been compared with those found for the separated chromophoric units ([Ru(bpy)3]2+, [Os(bpy)3]2+, and oligophenylene derivatives). The absorption spectrum of each dinuclear complex is essentially equal to the sum of the spectra of the component species, showing that intercomponent electronic interactions are weak. In the homodinuclear compounds, the strong fluorescence of the oligophenylene spacers is completely quenched by energy transfer to the metal-based units, which exhibit their characteristic metal-to-ligand charge-transfer (MLCT) phosphorescence. In the heterodinuclear compounds, besides complete quenching of the fluorescence of the oligophenylene spacers, a quenching of the phosphorescence of the [Ru(bpy)3]2+ chromophoric unit and a parallel sensitization of the phosphorescence of the [Os(bpy)3]2+ chromophoric unit are observed, indicating the occurrence of electronic energy transfer. The rate of the energy-transfer process from the [Ru(bpy)3]2+ to the [Os(bpy)3]2+ unit is practically temperature independent and decreases with increasing length of the oligophenylene spacer (in acetonitrile solution at 293 K, k en = 6.7 × 108 s-1 for n = 3; k en = 1.0 × 107 s-1 for n = 5; k en = 1.3 × 106 s-1 for n = 7). It is shown that such an energy-transfer process takes place via a Dexter-type mechanism (superexchange interaction) with an attenuation coefficient of 0.32 per Å and 1.5 per interposed phenylene unit.