Visible‐Light‐Driven Photoisomerization and Increased Rotation Speed of a Molecular Motor Acting as a Ligand in a Ruthenium(II) Complex

Abstract

Toward the development of visible-light-driven molecular rotary motors, an overcrowded alkene-based ligand and the corresponding ruthenium(II) complex is presented. In our design, a 4,5-diazafluorenyl coordination motif is directly integrated into the motor function. The photochemical and thermal isomerization behavior has been studied by UV/Vis and NMR spectroscopy. Upon coordination to a Ru(II) bipyridine complex, the photoisomerization process can be driven by visible (λmax = 450 nm) instead of UV light and furthermore, a large increase of the speed of rotation is noted. DFT calculations point to a contraction of the diazafluorenyl lower half upon metal-coordination resulting in reduced steric hindrance in the "fjord region" of the molecule. Consequently, it is shown that metal-ligand interactions can play an important role in the adjustment of both photophysical and thermodynamic properties of molecular motors.

Artificial molecular motors

“…. 107 Dube and co-workers designed a novel hemithioindigo based molecular motor 48 to achieve visible light driven rotation of molecular motors (Figure 18). 108 The introduction of a hemithioindigo moiety, originally used in photoswitches, resulted in a shift of the excitation wavelength up to 500 nm light, which offers important opportunities compared to the early 2nd generation overcrowded alkene molecular motors, which are generally fuelled with <400 nm light.…”

Allosteric Regulation of the Rotational Speed in a Light-Driven Molecular Motor

“…The second transition state (TS2), found for Pd(Cl) 2 ( L1 ) and Pt(Cl) 2 ( L1 ), is slightly higher in energy (3–4 kJ mol –1 ) and is structurally similar to that calculated earlier for the uncomplexed ligand L1 . 19 It has a slightly bent diazafluorenyl lower half with C(2′)H and C(2′)CH 3 positioned on opposite sides of this unit. The most important structural parameters of all the transition states obtained are summarized in Table 1.…”

“…19 In the UV–vis spectrum (Figure 2), the absorption maxima of Zn(Cl) 2 ( L1 ) and Pd(Cl) 2 ( L1 ) are situated around λ = 405 nm, whereas Pt(Cl) 2 ( L1 ) has a maximum absorption at λ = 422 nm. These maxima are all bathochromically shifted relative to the parent uncomplexed L1 (λ max = 390 nm).…”

“…Recently, the molecular motor L1 was developed (see Scheme 1A), 19 which incorporates a 4,5-diazafluorenyl lower half that can bind metal ions. Our initial studies focused on ruthenium bipyridine complex formation with the goal of shifting the excitation wavelength to the visible region, but interestingly, also an effect on the energy barrier for THI was observed.…”

Toward Fast and Efficient Visible‐Light‐Driven Molecular Motors: A Minimal Design

“…From the point of view of applications, adding to these challenges is the fact that most overcrowded‐alkene motors are driven by energetic UV light, which is more damaging to the motors and their environment than visible light. Thus, a key goal in the development of light‐driven molecular motors is to make them responsive to visible light, so as to facilitate their usage in biological systems and other soft materials …”