Wire‐Type Ruthenium(II) Complexes with Terpyridine‐Containing [2]Rotaxanes as Ligands: Synthesis, Characterization, and Photophysical Properties

Abstract

[2]Rotaxanes based on the 1,2-bis(pyridinium)ethane subset[24]crown-8 ether motif were prepared that contain a terminal terpyridine group for coordination to a transition-metal ion. These rotaxane ligands were utilized in the preparation of a series of heteroleptic [Ru(terpy)(terpy-rotaxane)]2+ complexes. The compounds were characterized by 1D and 2D 1H NMR spectroscopy, X-ray crystallography, and high-resolution electrospray ionization mass spectrometry. The effect of using a rotaxane as a ligand was probed by UV/Vis/NIR absorption and emission spectroscopy of the Ru(II) complexes. In contrast with the parent [Ru(terpy)(2)]2+ complex, at room temperature the examined complexes exhibit a luminescence band in the near infrared region and a relatively long lived triplet metal-to-ligand charge-transfer (3MLCT) excited state, owing to the presence of strong-electron-acceptor pyridinium substituents on one of the two terpy ligands. Visible-light excitation of the Ru-based chromophore in acetonitrile at room temperature causes an electron transfer to the covalently linked 4,4'-bipyridinium unit and the quenching of the MLCT luminescence. The 3MLCT excited state, however, is not quenched at all in rigid matrix at 77 K. The rotaxane structure was found to affect the absorption and luminescence properties of the complexes. In particular, when a crown ether surrounds the cationic axle, the photoinduced electron-transfer process is slowed down by a factor from 2 to 3. Such features, together with the synthetic and structural advantages offered by [Ru(terpy)2]2+-type complexes compared to, for example, [Ru(bpy)3]2+-type compounds, render these rotaxane-metal complexes promising candidates for the construction of photochemical molecular devices with a wire-type structure.

Light-powered autonomous and directional molecular motion of a dissipative self-assembling system

“…1a) the macrocyclic ring 2,3-dinaphtho [24]crown-8 ether 1 24 and the molecular axle E-2 + , comprising a photoswitchable E-azobenzene unit at one end, a central ammonium recognition site for the macrocycle and a passive methylcyclopentyl pseudo-stopper at the other end 19 . As shown schematically in Fig.…”

Light-Operated Machines Based on Threaded Molecular Structures

“…The design of the system relies on previously investigated rotaxanes utilizing ruthenium(II) polypyridine-type complexes as electron-transfer photosensitizers [50,61,62]. The investigated rotaxane 1 8+ (Fig.…”

Molecular machines operated by light

“…We have also investigated systems related to 3 6+ in which either the position of the A 1 2+ and A 2 2+ stations is exchanged with respect to the P unit [83], a different Rubased moiety is employed [84], or the photosensitizer is connected noncovalently to an electron-accepting station [85,86].…”