Symmetrical or non-symmetrical luminescent turnstiles based on hydroquinone stators and rotors bearing pyridyl or p-dimethylaminopyridyl coordinating units

Abstract

The design and synthesis of a novel family of molecular turnstiles T1-T5 were achieved. All five turnstiles are based on a stator and a rotor covalently interconnected. While turnstiles T1-T2 are based on a symmetric stator equipped with two coordinating pyridyl units and a rotor bearing either a pyridyl or p-dimethylaminopyridyl coordinating moiety, the two non-symmetric turnstiles T4 and T5 are based on a stator bearing only a single pyridyl unit and the same rotor as T1 and T2 mentioned above. The switching between the open (T1-T5) and the closed (T-M) states of the turnstiles by metal cations (M = Ag or Pd) was investigated in solution by using 1D and 2D NMR techniques. The locking of the rotational movement leading to the closed state of the turnstile was achieved upon addition of the Ag cation through its simultaneous binding by both pyridyl moieties of the stator and the rotor. The unlocking process leading back to the open state was achieved by the addition of EtNBr. For the symmetric turnstiles T1 and T2, bearing two pyridyl units on the stator, the binding of the Ag cation leads to an oscillating phenomenon between the two energetically equivalent closed states. However, in the case of turnstile T1, the oscillating process could be prevented by blocking the rotational movement using PdCl as the locking agent. Owing to the emissive nature of the stator, the open and closed states of the turnstiles were investigated by steady state and time-resolved photophysical methods. The photo-excitation of the turnstiles in their open state leads to an intense near-UV to deep-blue emission with short-lived excited states and a singlet intra-ligand charge transfer (ILCT) character. Upon binding of the Ag cation, sizeable bathochromic shifts and a substantial decrease of PLQY were observed. Finally, the coordination of PdCl, which possesses lower-lying excited states with metal-centered (MC) and ligand-to-metal charge transfer (LMCT) character, completely quenches the photoluminescence.

Off-Equilibrium Speed Control of a Multistage Molecular Rotor: 2-Fold Chemical Fueling by Acid or Silver(I)

“…The present work addresses several fields of topical chemistry, including fueling well-defined up/down speed oscillations of conformations, a new chemical fuel system, optimization of fueled off-equilibrium systems by using a waste product, and the fueled interconversion of rotors. While in other work, thermal rotation in rotors has been stopped by using a metal ion as a brakestone betweeen two ligands (two-state switching), , the rotational frequency in rotor 1 was up- and down-regulated in permanent oscillations by pulsing the rotor with chemical fuels, e.g., acid 2 or silver­(I) ions, as monitored by fluorescence spectroscopy, for up to 10 successive cycles. The duration of the silver­(I)-pulsed speed oscillation may be conveniently varied by changing the substrates of the substitution reaction or their concentrations.…”

“…Finally, the proton waste generated during the silver­(I) fueling was used in an in situ pseudorotaxane clipping as shown over 10 cycles. The reliability and easy handling of the off-equilibrium oscillations using silver­(I) fuel is expected to prompt its further utilization in silver­(I)-activated materials, supramolecular assembly, turnstiles, , switches, and molecular machines …”

The Motion of an Azobenzene Light‐Controlled Switch: A Joint Theoretical and Experimental Approach

“…Still, we felt that theoretical inspection might be desirable to support the mechanistic views and to rationalize the actual molecular rotation. In the meantime, Hosseini and co‐workers reported on molecular turnstiles, constructed on a rotor (typically a tetraethylene glycol (TEG) chain grafted onto a porphyrin, an organometallic or an aromatic moiety) and a stator . With the possibility to introduce a photo‐sensitive group almost at will on the stator, we designed a system where the macrocycle structure would be maintained upon irradiation.…”

“…The design of the AzoT macrocycle (Scheme ) stems from the molecular turnstiles previously reported, to which a pyridyl moiety was introduced to better follow its dynamical behavior by 1 H NMR spectroscopy. On the other side the light‐responsive Azo moiety can be isomerized between E (thermodynamically stable) and Z (metastable) using different excitation wavelengths, with concomitant large geometrical change of the compound.…”

“…This finding is most likely due to photo‐isomerization occurring under ambient light. The 1 H NMR spectrum is displayed in Figure S1, where attributions of the 1 H resonances were made on the basis of similar Azo derivatives . Expectantly, full conversion of the E / Z ‐ mixture into the thermodynamically stable E isomer can be attained by thermal treatment of a CD 3 CN solution (Figure S2), a procedure that was systematically followed for all samples.…”