Photochromic Dithienylethene Derivatives Containing Ru(II) or Os(II) Metal Units. Sensitized Photocyclization from a Triplet State

Abstract

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.

Photoswitchable mixed valence

“…[42][43][46][47][48][49] This undesired situation may arise either through direct excitation of the isomerizable bridge or via energy transfer from the donor (or 85 the acceptor) to the bridge. 32,[42][43][46][47][48][49] Since IVCT bands usually occur in the NIR spectral range while shorter wavelengths are required for bridge photoisomerization, this problem is much less prone to occur in mixed valence compounds. While there is considerable room for improvement of the 90 mixed valence properties of photoswitchable systems, it seems clear that much future work will be geared at direct measurement of the conductance of photoisomerizable molecules.…”

Photochromic Switches Incorporated in Bridging Ligands: A New Tool to Modulate Energy‐Transfer Processes

“…In fact the osmium excited state lives 46 ns (Table 2), which is essentially identical to the emission lifetime of Os(l-1o)Os (44 ns). [24] The lifetime of the ruthenium emission is strongly reduced compared to Ru(l-1o)Ru. The emission decay is bi-exponential, with components of 0.07 and 0.61 ns.…”

“…[24] As all three metalcomplex moieties are luminescent and their emission properties depend on the state of the photochromic unit, this could provide a way to monitor the photoreaction and to understand its intimate mechanism.…”

“…The absorption spectrum of Ru(l-1o)Os is the average of the absorption spectra of complexes Ru(l-1o)Ru and Os(l-1o)Os. [24] This suggests that, in the heterometallic complex, the interactions between the Ru-based and Os-based components are weak.…”

Intramolecular Hydrogen Bonding in a Triangular Dithiazolyl‐Azaindole for Efficient Photoreactivity in Polar and Nonpolar Solvents

“…[1] Recently, particular interest has focused on photochromic molecules as an active unit for photo-reversible control of various molecular properties such as color, [2][3][4] fluorescence, [5][6][7][8][9] electrochemistry, [10,11] chemical reactivity, [12] and magnetism.…”