Photoswitching of arylazopyrazoles upon S1 (nπ*) excitation studied by transient absorption spectroscopy and ab initio molecular dynamics.

Arylazopyrazoles (AAPs) are an important class of molecular photoswitches with high photostationary states (PSS) and long thermal lifetimes. The ultrafast photoisomerization of four water-soluble arylazopyrazoles, all of them featuring an ortho-dimethylated pyrazole ring, is studied by narrowband femtosecond transient absorption spectroscopy and ab initio molecular dynamics simulations. Upon S1 (nπ*) photoexcitation of the planar E-isomers (E-AAPs), excited-state bi-exponential decays with time constants τ1 in the 220-440 fs range and τ2 in the 1.4-1.8 ps range are observed, comparable to those reported for azobenzene (AB). This is indicative of the same photoisomerization mechanism as has been reported for ABs. In contrast to the planar E-AAPs, a twisted E-AAP with two methyl groups in ortho-position of the phenyl ring displays faster initial photoswitching with τ1 = 170 ± 10 fs and τ2 = 1.6 ± 0.1 ps. Our static DFT calculations and ab initio molecular dynamics simulations of E-AAPs on the S0 and S1 potential energy surfaces suggest that twisted E-isomer azo photoswitches exhibit faster initial photoisomerization dynamics out of the Franck-Condon region due to a weaker π-coordination of the central CNNC unit to the aromatic ligands.

Understanding the formation of surface relief gratings in azopolymers: A combined molecular dynamics and experimental study.

The formation of surface relief gratings in thin azopolymeric films is investigated using atomistic molecular dynamics simulations and compared to experimental results for the specific case of poly-disperse-orange3-methyl-methacrylate. For this purpose, the film is illuminated with a light pattern of alternating bright and dark stripes in both cases. The simulations use a molecular mechanics switching potential to explicitly describe the photoisomerization dynamics between the E and Z isomers of the azo-units and take into account the orientation of the transition dipole moment with respect to the light polarization. Local heating and elevation of the illuminated regions with the subsequent movement of molecules into the neighboring dark regions are observed. This leads to the formation of valleys in the bright areas after re-cooling and is independent of the polarization direction. To verify these observations experimentally, the azopolymer film is illuminated with bright stripes of varying width using a spatial light modulator. Atomic force microscopy images confirm that the elevated areas correspond to the previously dark areas. In the experiment, the polarization of the incident light makes only a small difference since tiny grain-like structures form in the valleys only when the polarization is parallel to the stripes.

Reversible, Red-Shifted Photoisomerization in Protonated Azobenzenes.

Azobenzenes are among the best-studied molecular photoswitches and play a key role in the search for red-shifted photoresponsive materials for extended applications. Currently, most approaches deal with aromatic substitution patterns to achieve visible light application, on occasion paired with protonation to yield red-shifted absorption of the azonium species. Appropriate substitution patterns are essential to stabilize the latter approach, as conventional acids are known to induce a fast Z- to E-conversion. Here, we show that steady-state protonation of the azo-bridge instead is possible in simple azobenzenes when the pKa of the acid is low enough, yielding both the Z- and E-azonium as supported by UV-vis- and 1H NMR spectroscopy as well as density functional theory calculations. Moreover, the steady-state protonation of para-methoxyazobenzene, specifically, yields photoisomerizable azonium ions in which the direction of switching is essentially reversed, that is, visible light produces the out-of-equilibrium Z-azonium. Although the current conditions render the visible light photoswitch unsuitable for in vivo and material application, the demonstrated understanding of simple azobenzenes paves the way for a great range of further work on this already widely studied photoswitch.

Self-Assembled Monolayers of Arylazopyrazoles on Glass and Silicon Oxide: Photoisomerization and Photoresponsive Wettability.

Surface coatings that respond to external influences and change their physical properties upon application of external stimuli are of great interest, with light being a particularly desirable choice. Photoswitches such as azobenzenes have been employed in a range of photoresponsive coatings. One striking change in physical property of many photoresponsive coatings is their responsive wettability upon illumination. In this work, we present photoswitchable self-assembled monolayers based on arylazopyrazoles (AAPs). In solution, AAPs offer significant improvements in terms of the photostationary state, thermal stability, and fatigue resistance. The AAP photoswitch is coupled to triethoxysilanes for an easy, one-step functionalization of glass and silicon oxide surfaces. We show the synthesis of AAP-based silanes and the successful surface functionalization, and we confirm the excellent photoswitchability of the AAPs in a self-assembled monolayer upon alternating irradiation with UV (365 nm) and green (520 nm) light. The self-assembled monolayers are investigated by UV/vis spectroscopy, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and contact angle goniometry. We furthermore investigate the effect of substitution of the AAPs on the photoresponsive wetting behavior and compare this with density functional theory (DFT) calculations of the dipole moments of the AAPs.

Acid-catalysed liquid-to-solid transitioning of arylazoisoxazole photoswitches.

Molecular photoswitches play a vital role in the development of responsive materials. These molecular building blocks are particularly attractive when multiple stimuli can be combined to bring about physical changes, sometimes leading to unexpected properties and functions. The arylazoisoxazole molecular switch was recently shown to be capable of efficient photoreversible solid-to-liquid phase transitions with application in photoswitchable surface adhesion. Here, we show that the arylazoisoxazole forms thermally stable and photoisomerisable protonated Z- and E-isomers in an apolar aprotic solvent when the pK a of the applied acid is sufficiently low. The tuning of isomerisation kinetics from days to seconds by the pK a of the acid not only opens up new reactivity in solution, but also the solid-state photoswitching of azoisoxazoles can be efficiently reversed with selected acid vapours, enabling acid-gated photoswitchable surface adhesion.

An Arylazopyrazole-Based N-Heterocyclic Carbene as a Photoswitch on Gold Surfaces: Light-Switchable Wettability, Work Function, and Conductance.

A novel photoresponsive and fully conjugated N-heterocyclic carbene (NHC) has been synthesized that combines the excellent photophysical properties of arylazopyrazoles (AAPs) with an NHC that acts as a robust surface anchor (AAP-BIMe). The formation of self-assembled monolayers (SAMs) on gold was proven by ToF-SIMS and XPS, and the organic film displayed a very high stability at elevated temperatures. This stability was also reflected in a high desorption energy, which was determined by temperature-programmed SIMS measurements. E-/Z-AAP-BIMe@Au photoisomerization resulted in reversible alterations of the surface energy (i.e. wettability), the surface potential (i.e. work function), and the conductance (i.e. resistance). The effects could be explained by the difference in the dipole moment of the isomers. Furthermore, sequential application of a dummy ligand by microcontact printing and subsequent backfilling with AAP-BIMe allowed its patterning on gold. To the best of our knowledge, this is the first example of a photoswitchable NHC on a gold surface. These properties of AAP-BIMe@Au illustrate its suitability as a molecular switch for electronic devices.

Perylene Diimide-Embedded Double [8]Helicenes.

The rational design and modification of the helix is of significance for fully promoting properties of configurationally stable materials for various applications in chiral science. Herein, a straightforward, sterically less demanding synthetic approach involving hybridization between two [6]helicene subunits and a perylene diimide (PDI) scaffold are presented, affording perylene diimide-embedded double [8]helicenes (PD8Hs) which represent the highest double carbohelicenes reported thus far. Due to the structural features of PDI and [6]helicene, the PD8Hs have six stereoisomers including two pairs of enantiomers and two mesomers. Such structural diversity is unprecedented in the realm of double helicenes. The absolute configuration of these PD8Hs was unambiguously confirmed by single-crystal X-ray diffraction analyses, revealing that the subtle configurational differences lead to great variation in the superhelical structure and molecular packing arrangement. Due to the embedding of the PDI chromophore, the PD8Hs possess outstanding fluorescence quantum yields of approximately 30%. Two pairs of enantiomers were resolved by chiral HPLC, and the chiroptical properties were evaluated using circular dichroism and circularly polarized luminescence spectroscopy, of which PD8H-6R exhibited excellent chiroptical performances in both the absorption and emission ranges with dissymmetry factors |gabs| of 0.012 and |glum| of 0.002.

Arylazopyrazole Photoswitches in Aqueous Solution: Substituent Effects, Photophysical Properties, and Host-Guest Chemistry.

Getting the green light! Substituted arylazopyrazoles (AAPs) have been investigated as supramolecular photoswitches in aqueous solution. Selective photostationary states (PSSs) and improved binding affinities to ß-cyclodextrin have been determined. The experimental findings are supported by results from DFT calculations.

Towards understanding photomigration: Insights from atomistic simulations of azopolymer films explicitly including light-induced isomerization dynamics.

The light-induced surface modification of a thin film of poly-(disperse orange-3-methylmethacrylate) is investigated computationally using atomistic molecular dynamics simulations specifically tailored to include photoisomerization dynamics. For a model surface consisting of a periodic pattern of alternating irradiated and dark spots, it is shown that repeated photoisomerization in the irradiated areas initially leads to a local temperature increase and a raised surface profile accompanied by a migration of molecules away from the bright spots. After switching off the light source and letting the system cool down, this leads to an inversion of the surface profile, i.e., dips in the bright spots and bumps in the dark spots. To separate the effect of photoisomerization from the pure heating effect, a second simulation is performed in which no photoisomerization is allowed to occur in the bright spots, but the equivalent amount of energy is introduced there locally in the form of heat. This also leads to a raised surface in these areas; however, no outward migration of molecules is observed and the surface pattern practically vanishes when the system is subsequently cooled back to room temperature.

P3HT:DiPBI bulk heterojunction solar cells: morphology and electronic structure probed by multiscale simulation and UV/vis spectroscopy.

Coarse grained molecular dynamics simulations are performed for a mixture of poly(3-hexylthiophene) (P3HT) and diperylene bisimide (DiPBI). The effect of different annealing and cooling protocols on the morphology is investigated and the resulting domain structures are analyzed. In particular, π-stacked clusters of DiPBI molecules are observed whose size decreases with increasing temperature. Domain structure and diffusivity data suggest that the DiPBI subsystem undergoes an order → disorder phase transition between 700 and 900 K. Electronic structure calculations based on density functional theory are carried out after backmapping the coarse grained model onto an atomistic force field representation built upon first principles. UV/vis absorption spectra of the P3HT:DiPBI mixture are computed using time-dependent density functional linear response theory and recorded experimentally for a spin-coated thin film. It is demonstrated that the absorption spectrum depends sensitively on the details of the amorphous structure, thus providing valuable insight into the morphology. In particular, the results show that the tempering procedure has a significant influence on the material's electronic properties. This knowledge may help to develop effective processing routines to enhance the performance of bulk heterojunction solar cells.

Adaptive switching of interaction potentials in the time domain: an extended Lagrangian approach tailored to transmute force field to QM/MM simulations and back.

An extended Lagrangian formalism that allows for a smooth transition between two different descriptions of interactions during a molecular dynamics simulation is presented. This time-adaptive method is particularly useful in the context of multiscale simulation as it provides a sound recipe to switch on demand between different hierarchical levels of theory, for instance between ab initio ("QM") and force field ("MM") descriptions of a given (sub)system in the course of a molecular dynamics simulation. The equations of motion can be integrated straightforwardly using the usual propagators, such as the Verlet algorithm. First test cases include a bath of harmonic oscillators, of which a subset is switched to a different force constant and/or equilibrium position, as well as an all-MM to QM/MM transition in a hydrogen-bonded water dimer. The method is then applied to a smectic 8AB8 liquid crystal and is shown to be able to switch dynamically a preselected 8AB8 molecule from an all-MM to a QM/MM description which involves partition boundaries through covalent bonds. These examples show that the extended Lagrangian approach is not only easy to implement into existing code but that it is also efficient and robust. The technique moreover provides easy access to a conserved energy quantity, also in cases when Nosé-Hoover chain thermostatting is used throughout dynamical switching. A simple quadratic driving potential proves to be sufficient to guarantee a smooth transition whose time scale can be easily tuned by varying the fictitious mass parameter associated with the auxiliary variable used to extend the Lagrangian. The method is general and can be applied to time-adaptive switching on demand between two different levels of theory within the framework of hybrid scale-bridging simulations.

Structure of P3HT crystals, thin films, and solutions by UV/Vis spectral analysis.

Optical absorption spectra of poly(3-hexylthiophene) (P3HT) are calculated in solution, spin-coated thin films, and the bulk crystal using a multiscale simulation approach. The structure of the amorphous thin film is obtained from coarse grained molecular dynamics (MD) simulations and subsequent back-mapping onto an atomistic force field representation. The absorption spectra are computed using TDDFT by statistically averaging over an ensemble of molecules taken from the MD simulations. Experimental UV/Vis spectra of spin-coated thin films and solutions are recorded with varying ratios of 'good' versus 'poor' solvent. The theoretical approach is able to faithfully predict the spectral position in the various phases and offers fundamental insight into the cause of any spectral shifts. The position of the main absorption peak is found to be chiefly determined by the level of torsion between the thiophene rings inside each molecule, while intermolecular effects are less important. Hence, optical absorption spectra hold valuable clues about the microscopic structure of disordered P3HT phases.

Light-controlled macrocyclization of tetrathiafulvalene with azobenzene: designing an optoelectronic molecular switch.

Macrocyclization between tetrathiafulvalene (TTF) dithiolates and bis-bromomethylazobenzenes/bis-bromomethylstilbenes is investigated under high dilution conditions. We show that macrocycles of different size can be formed depending on whether the (Z)- or (E)-isomers of azobenzene (AB) or stilbene are used. This represents the first example of a light-controllable cyclization reaction. The oxidation potential of the small, structurally rigid TTF-AB macrocycle is found to depend on the conformation of the AB moiety, opening the way for the modulation of redox properties by an optical stimulus. DFT calculations show that the out-of-plane distortion of the TTF moiety in this macrocycle is responsible for the variation of its oxidation potential upon photoisomerization of the neighboring AB bridge.

Regioselective functionalization of core-persubstituted perylene diimides.

Regioselective functionalization of core per-substituted perylene diimides has been achieved efficiently based on a new versatile building block, named tetrabromotetrachloro-perylene-3,4:9,10-tetracarboxylic acid dianhydride (Br4Cl4-PTCDA), which affords a series of novel chromophores with impressive optoelectronic properties. Direct palladium-catalyzed fourfold intramolecular ring fusion affords successfully unique propeller-shaped biscarbazole[2,3-b]carbazole diimides with six annulated rings.

Mimicking photoisomerisation of azo-materials by a force field switch derived from nonadiabatic ab initio simulations: application to photoswitchable helical foldamers in solution.

A force field to induce isomerisation of photoswitchable azobenzene groups embedded in molecular materials has been developed in the framework of force field molecular dynamics simulations. A molecular mechanics switching potential has been tuned so as to reproduce both the correct photoisomerisation timescale and mechanism that has been generated by reference nonadiabatic ab initio molecular dynamics. As a first application, we present a force field molecular dynamics study of a prototype photoswitchable foldamer in acetonitrile as solvent. Our analyses reveal that the photoisomerisation of the azobenzene unit embedded in the foldamer occurs via the so-called NN-twist mechanism, and that there exist several distinct unfolding channels for the helix that could be exploited in novel applications of photoresponsive materials.

Enhanced photoswitching of bridged azobenzene studied by nonadiabatic ab initio simulation.

Photoisomerization of a bridged azobenzene derivative (AB-C(2)) is studied by nonadiabatic ab initio molecular dynamics simulation. The effect of the alkyl bridge linking the two phenyl rings on the Z → E and E → Z photoisomerization pathways and efficiencies is analyzed by detailed comparison to the unbridged parent compound. It is found that the bridge makes E → Z photoisomerization considerably faster and increases its quantum yield, whereas Z → E photoswitching is slightly hindered and has a significantly lower quantum yield although still being ultrafast. The simulations reveal that unsuccessful Z → E photoisomerization attempts can interconvert two pro-enantiomeric forms of Z-AB-C(2) via pseudorotation in the excited electronic state.

Unfolding a photoswitchable azo-foldamer reveals a non-covalent reaction mechanism.

Simulations on photoswitching of an azobenzene-functionalized helical mPE foldamer reveal several distinct non-covalent reaction channels. It is expected that distinct products can be selectively (de)stabilized by attaching suitable side chains to the backbone. The methodology presented could be applied to study photoinduced manipulation of AB-functionalized proteins.

Multiscale modelling of mesoscopic phenomena triggered by quantum events: light-driven azo-materials and beyond.

The macroscopic functionality of soft (bio-)materials is often triggered by quantum-mechanical events which are highly local in space and time. In order to arrive at the resulting macroscopically observable phenomena, many orders of magnitude need to be bridged on both the time and the length scale. In the present paper, we first introduce a range of simulation methods at different scales as well as theoretical approaches to form bridges between them. We then outline a strategy to develop an adaptive multiscale simulation approach which connects the quantum to the mesoscopic level by bringing together ab initio molecular dynamics (QM), classical (force field) molecular dynamics (MM), and coarse grained (CG) simulation techniques. With a multitude of photoactive materials in mind, we apply our methodology to a prototypical test case-light-induced phase transitions in a liquid crystal containing the azobenzene photoswitch.

Nonadiabatic hybrid quantum and molecular mechanic simulations of azobenzene photoswitching in bulk liquid environment.

A nonadiabatic hybrid quantum and molecular mechanical (na-QM/MM) molecular dynamics scheme has been implemented recently combining the nonadiabatic Car-Parrinello molecular dynamics method by Doltsinis and Marx [Phys. Rev. Lett. 2002, 88, 166402] with the QM/MM coupling approach by Laio et al. [J. Chem. Phys. 2002, 116, 6941]. Here an extensive validation of the underlying, density functional theory based, electronic structure methods by comparison to CASPT2 ab initio data is presented for the case of azobenzene. The "on the fly" na-QM/MM method is then applied to study Z-->E and E-->Z photoisomerization of azobenzene in a bulk liquid environment. The isomerization mechanism is found to be a pedal motion of the central CN horizontal lineNC group in both cases. While the Z-->E reaction is barely affected by the environment, E-->Z photoisomerization is slowed down considerably in the liquid compared to the gas phase. This effect is due to the fact that reorientation of the phenyl rings is significantly hindered in the liquid by steric nearest neighbor interactions. Nonradiative decay is found to be substantially faster for Z-AB (subpicosecond regime) than for E-AB (picosecond regime). The main molecular motions responsible for nonadiabatic coupling have been identified as the oscillations in the NN and CN bond lengths, the CNN bond angles, and the CNNC dihedral angle.