Piet G. Wiering

Three State Redox-Active Molecular Shuttle That Switches in Solution and on a Surface

Although the desirability of developing synthetic molecular machine systems that can function on surfaces is widely recognized, to date the only well-characterized examples of electrochemically switchable rotaxane-based molecular shuttles which can do so are based on the tetracationic viologen macrocycle pioneered by Stoddart. Here, we report on a [2]rotaxane which features succinamide and naphthalene diimide hydrogen-bonding stations for a benzylic amide macrocycle that can shuttle and switch its net position both in solution and in a monolayer. Three oxidation states of the naphthalene diimide unit can be accessed electrochemically in solution, each one with a different binding affinity for the macrocycle and, hence, corresponding to a different distribution of the rings between the two stations in the molecular shuttle. Cyclic voltammetry experiments show the switching to be both reversible and cyclable and allow quantification of the translational isomer ratios (thermodynamics) and shuttling dynamics (kinetics) for their interconversion in each state. Overall, the binding affinity of the naphthalene diimide station can be changed by 6 orders of magnitude over the three states. Unlike previous electrochemically active amide-based molecular shuttles, the reduction potential of the naphthalene diimide unit is sufficiently positive (-0.68 V) for the process to be compatible with operation in self-assembled monolayers on gold. Incorporating pyridine units into the macrocycle allowed attachment of the shuttles to an acid-terminated self-assembled monolayer of alkane thiols on gold. The molecular shuttle monolayers were characterized by X-ray photoelectron spectroscopy and their electrochemical behavior probed by electrochemical impedance spectroscopy and double-potential step chronoamperometry, which demonstrated that the redox-switched shuttling was maintained in this environment, occurring on the millisecond time scale.

Induction of Motion in a Synthetic Molecular Machine: Effect of Tuning the Driving Force

Rotaxane molecular shuttles were studied in which a tetralactam macrocyclic ring moves between a succinamide station and a second station in which the structure is varied. Station 2 in all cases is an aromatic imide, which is a poor hydrogen-bond acceptor in the neutral form, but a strong one when reduced with one or two electrons. When the charge density on the hydrogen-bond-accepting carbonyl groups in station 2 is reduced by changing a naphthalimide into a naphthalene diimide radical anion, the shuttling rate changes only slightly. When station 2 is a pyromellitimide radical anion, however, the shuttling rate is significantly reduced. This implies that the shuttling rate is not only determined by the initial unbinding of the ring from the first station, as previously supposed. An alternative reaction mechanism is proposed in which the ring binds to both stations in the transition state.

Multistate Photo-Induced Relaxation and Photoisomerization Ability of Fumaramide Threads: A Computational and Experimental Study

Fumaric and maleic amides are the photoactive units of an important and widely investigated class of photocontrollable rotaxanes as they trigger ring shuttling via a cis-trans photoisomerization. Here, ultrafast decay and photoinduced isomerization in isolated fumaramide and solvated nitrogen-substituted fumaramides (that are employed as threads in those rotaxanes) have been investigated by means of CASPT2//CASSCF computational and time-resolved spectroscopic techniques, respectively. A complex multistate network of competitive deactivation channels, involving both internal conversion and intersystem crossing (ISC) processes, has been detected and characterized that accounts for the picosecond decay and photochemical/photophysical properties observed in the singlet as well as triplet (photosensitized) photochemistry of fumaramides threads. Interestingly, singlet photochemistry appears to follow a non-Kasha rule model, where nonequilibrium dynamical factors control the outcome of the photochemical process: accessible high energy portions of extended crossing seams turn out to drive the deactivation process and ground-state recovery. Concurrently, extended singlet/triplet degenerate regions of twisted molecular structures with significant spin-orbit-coupling values account for ultrafast (picosecond time scale) ISC processes that lead to higher photoisomerization efficiencies. This model discloses the principles behind the intrinsic photochemical reactivity of fumaramide and its control.

The Lowest Excited Singlet States of 1-Azaadamantane and 1-Azabicyclo[2.2.2]octane: Fluorescence Excitation Spectroscopy and Density Functional Calculations

The lowest excited singlet states of the structurally rigid amines 1-azaadamantane and 1-azabicyclo[2.2.2]octane have been investigated by using fluorescence excitation spectroscopy on samples seeded in supersonic expansions. Based upon the notion that in both species the lowest excited singlet state is a Rydberg state with the ground state of the radical cation as its ionic core, excitation spectra have been analyzed by employing density functional calculations of the equilibrium geometries and force fields of the ground state of the neutral species and its radical cation. A good agreement is obtained between experimentally observed and theoretically predicted frequencies and intensities of vibronic transitions. Subsequent refinements of the geometry of the lowest excited singlet state are shown to account adequately for the minor differences between experiment and the computational results obtained by using the radical cation as a model for the lowest excited singlet state. From our analysis it also bec...

Successive translocation of the rings in a [3]rotaxane

A [2]rotaxane, a [3]rotaxane and the corresponding thread containing two succinamide (succ) binding stations and a central redox-active pyromellitimide (pmi) station were studied. Infrared spectroelectrochemical experiments revealed the translocation of the macrocycle between the succinamide station and the electrochemically reduced pmi station (radical anion and dianion). Remarkably, in the [3]rotaxane, the rings can be selectively translocated. One-electron reduction leads to the translocation of one of the two macrocycles from the succinamide to the pyromellitimide station, whereas activation of the shuttle through two-electron reduction results in the translocation of both macrocycles: the dianion, due to its higher electron density and hence greater hydrogen-bond accepting affinity, is hydrogen bonded to both macrocycles. Systems with such an on-command contraction are known as molecular muscles. The relative strengths of the binding between the macrocycle and the imide anions could be estimated from the hydrogen-bond-induced shifts in the C=O stretching frequencies of hydrogen-bond accepting amide groups of the macrocycle.

Multistep photoinduced electron transfer in a photoacceptor terminated molecular triode

A molecular triode (3) is described containing a nonconjugatively interconnected array (D 2 -D 1 -P A ) of two potential one-electron donor (D) moieties and a powerful photoacceptor (P a ). In a related diode (2) consisting of the array D 1 -P a excitation of the photoacceptor, P a , induces charge-separation as detected by time resolved microwave conductivity (TRMC) measurements and as evidenced from the observation of charge transfer emission resulting from radiative recombination. The latter emission is quenched in the triode 3 indicating occurrence of virtually complete consecutive electron transfer D 2 to D 1 . Instead 3 displays a much weaker charge transfer emission at longer wavelength attributed to direct radiative recombination from D 2 to P a . Fluorescence lifetime measurements and TRMC data show that the recombination in 3 is significantly slowed down as compared to the shorter range recombination in 2

Two-step photoinduced charge separation and unexpectedly fast one-step charge recombination in a linked donor2-donor1-acceptor system

accompanied by a characteristic charge transfer fluorescence D2+ -D1-A-+D2-D1-A thVCT (~~~42 ns, &=O.OlS in benzene, a,,,=635 nm). The relatively fast charge recombination between D2 and A and the partly radiative nature of this process indicate that there is substantial electronic coupling between the two units. Although the D2-D:-A- state is thermodynamically accessible in la, as shown by the behaviour of the corresponding D,-A compound [6] it is not observed as an intermediate state. In order to establish the involvement of the Dz-D:-A- state in the charge separation between DZ and A, D1 was replaced by a much weaker donor, i.e. in lb. In this compound the D,-D: -A- state is of higher energy than the A* state (except in very polar solvents [ 71) thereby rendering the D2-D1A*+D,-D: -A- charge separation process endothermic. Nevertheless the long range charge separation and charge recombination processes in lb (b= 27 ns in benzene) occur with rates similar to those in la. This indicates that the intermediate D2-D: -A- state probably plays no role in the charge separation between Dz and A, and single step long range electron transfer prevails, In order to reduce the direct electronic coupling between D2 and A system 2 was now synthesised, in which the D2/A distance is increased as compared

Highly solvatochromic emission of electron donor-acceptor compounds containing propanedioato boron electron acceptors

Light-induced electron transfer occurs in bifunctional compounds consisting of 1,3-diphenylpropanedioato boron oxalate or fluoride electron acceptors and simple aromatic electron-donor groups, linked by a methylene bridge; fluorescence from the highly polar charge-transfer excited state is remarkably efficient and shows an exceptionally large solvatochromicity.