Acid-base responsive molecular switching of a [2]rotaxane incorporating two different stations in an axle component

Interlocked compounds such as rotaxanes and catenanes exhibit unique kinetic properties in response to external chemical or physical stimuli and are therefore expected to be applied to molecular machines and molecular sensors. To develop a novel rotaxane for this application, an isophthalamide macrocycle and a neutral phenanthroline axle were used. Stable pseudorotaxanes are known to be formed using hydrogen bonds and π-π interactions. In this study, we designed a non-symmetric axial molecule and synthesized a [2]rotaxane with the aim of introducing two different stations; a phenanthroline and a secondary amine/ammonium unit. Furthermore, 1H NMR measurements demonstrated that the obtained rotaxane acts as a molecular switch upon application of external acid/base stimuli.

Benchmarking of Density Functionals for Z-Azoarene Half-Lives via Automated Transition State Search

Molecular photoswitches use light to interconvert from a thermodynamically stable isomer into a metastable isomer. Photoswitches have been used in photopharmacology, catalysis, and molecular solar thermal (MOST) materials because of their spatiotemporal activation. Visible-light-absorbing photoswitches are especially attractive because low-energy light minimizes undesired photochemical reactions and enables biological applications. Ideal photoswitches require well-separated absorption spectra for both isomers and long-lived metastable states. However, predicting thermal half-lives with density functional theory is difficult because it requires locating transition structures and chosing an accurate model chemistry. We now report EZ-TS; by automatically calculating activation energies for the thermal Z → E isomerization. We used 28 density functionals [local spin density approximation, generalized gradient approximation, meta-GGA, hybrid GGA, and hybrid meta-GGA] and five basis sets [6-31G(d), 6-31+G(d,p), 6-311+G(d,p), cc-pVDZ, and aug-cc-pVDZ]. The hybrid GGA functionals performed the best among all tested functionals. We demonstrate that the mean absolute errors of 14 model chemistries approach chemical accuracy.

Photophysical Properties of Fluorescent 2-(Phenylamino)-1,10-phenanthroline Derivatives†

The present study details the experimental and theoretical characterization of the photophysical properties of 14 examples of 2-(phenylamino)-1,10-phenanthrolines (1). The absorption spectra of 1 are substituent-dependent but in a general manner present absorption bands at wavelengths of ~230; ~300; ~335 and a shoulder at ~380 nm. Electron-donating groups (EDG) and electron-withdrawing groups (EWG), respectively, result in bathochromic and hypsochromic shifts. Compounds 1 are highly luminescent, in contrast to phenanthroline, and emit in the region between 350 and 500 nm with substituent-dependent λ_max emission. The emission spectra show a redshift for EDG (4-OMe 62 nm; 4-Me 19 nm) and a blueshift for EWG (4-CN 41 nm; 4-CF₃ 38 nm) relative to the emission of the unsubstituted parent compound 1a. Plotting the λ max EM against Hammett σ+ constants gave an excellent linear correlation demonstrating the electron-deficient nature of the excited state and how the substituents (de)stabilize S₁ . Theoretical calculations revealed a HOMO-LUMO π-π* electronic transition to S₁ which in combination with difference (S₁ -S₀ ) in electron density maps revealed charge-transfer character. Strongly electron-withdrawing substituents switch off the charge transfer to give rise to a local excitation.

Template-Free Synthesis of a Phenanthroline-Containing [2]Rotaxane: A Reversible pH-Controllable Molecular Switch

The synthesis of symmetric and asymmetric rotaxanes consisting of neutral axle and ring components without ionic templates is necessary for applications in molecular sensors and molecular switches. A phenanthroline-containing symmetric [2]rotaxane was newly synthesized by inducing hydrogen bonding and π-interaction using a template-free threading-followed-by-stoppering method. The obtained rotaxane serves as a reversible pH-controllable molecular switch.

pH-dependent binding of guests in the cavity of a polyhedral coordination cage: reversible uptake and release of drug molecules

A range of organic molecules with acidic or basic groups exhibit strong pH-dependent binding inside the cavity of a polyhedral coordination cage. Guest binding in aqueous solution is dominated by a hydrophobic contribution which is compensated by stronger solvation when the guests become cationic (by protonation) or anionic (by deprotonation). The Parkinson's drug 1-amino-adamantane ('amantadine') binds with an association constant of 104 M-1 in the neutral form (pH greater than 11), but the stability of the complex is reduced by three orders of magnitude when the guest is protonated at lower pH. Monitoring the uptake of the guests into the cage cavity was facilitated by the large upfield shift for the 1H NMR signals of bound guests due to the paramagnetism of the host. Although the association constants are generally lower, guests of biological significance such as aspirin and nicotine show similar behaviour, with a substantial difference between neutral (strongly binding) and charged (weakly binding) forms, irrespective of the sign of the charged species. pH-dependent binding was observed for a range of guests with different functional groups (primary and tertiary amines, pyridine, imidazole and carboxylic acids), so that the pH-swing can be tuned anywhere in the range of 3.5-11. The structure of the adamantane-1-carboxylic acid complex was determined by X-ray crystallography: the oxygen atoms of the guest form CH···O hydrogen bonds with one of two equivalent pockets on the internal surface of the host. Reversible uptake and release of guests as a function of pH offers interesting possibilities in any application where controlled release of a molecule following an external stimulus is required.

Design and assembly of rotaxane-based molecular switches and machines

Mechanically interlocked molecules, such as catenanes and rotaxanes, are at the heart of the development of molecular machines chemistry. They are able to self-organize, self-assemble, and self-control themselves into new materials with potential application as molecular devices. In this review, an overview of some recent progress on molecular machines is given, including new methodologies for their synthesis and self-assembly and their recent applications as dual or multilevel fluorescent molecular switches, as potential sensors, and even as a molecular-level transporter. In one development, a molecular machine containing a charge-transfer chromophore was designed to generate controllable aggregate structures through the reversible movement of a macrocycle over a thread; this was done in order to better understand the application of a molecular shuttle in solid state. Light is shed on how the novel properties and functions of molecular machines are extended, and examples of the ways in which molecular machines have been applied to the design and process of intelligentized systems are provided.