Right On Time: Ultrafast Charge Separation Before Hybrid Exciton Formation.

Organic/inorganic hybrid systems offer great potential for novel solar cell design combining the tunability of organic chromophore absorption properties with high charge carrier mobilities of inorganic semiconductors. However, often such material combinations do not show the expected performance: while ZnO, for example, basically exhibits all necessary properties for a successful application in light-harvesting, it was clearly outpaced by TiO2 in terms of charge separation efficiency. The origin of this deficiency has long been debated. This study employs femtosecond time-resolved photoelectron spectroscopy and many-body ab initio calculations to identify and quantify all elementary steps leading to the suppression of charge separation at an exemplary organic/ZnO interface. It is demonstrated that charge separation indeed occurs efficiently on ultrafast (350 fs) timescales, but that electrons are recaptured at the interface on a 100 ps timescale and subsequently trapped in a strongly bound (0.7 eV) hybrid exciton state with a lifetime exceeding 5 µs. Thus, initially successful charge separation is followed by delayed electron capture at the interface, leading to apparently low charge separation efficiencies. This finding provides a sufficiently large time frame for counter-measures in device design to successfully implement specifically ZnO and, moreover, invites material scientists to revisit charge separation in various kinds of previously discarded hybrid systems.

Xolography for linear volumetric 3D printing.

The range of applications for additive manufacturing is expanding quickly, including mass production of athletic footwear parts1, dental ceramics2 and aerospace components3 as well as fabrication of microfluidics4, medical devices5, and artificial organs6. The light-induced additive manufacturing techniques7 used are particularly successful owing to their high spatial and temporal control, but such techniques still share the common motifs of pointwise or layered generation, as do stereolithography8, laser powder bed fusion9, and continuous liquid interface production10 and its successors11,12. Volumetric 3D printing13-20 is the next step onward from sequential additive manufacturing methods. Here we introduce xolography, a dual colour technique using photoswitchable photoinitiators to induce local polymerization inside a confined monomer volume upon linear excitation by intersecting light beams of different wavelengths. We demonstrate this concept with a volumetric printer designed to generate three-dimensional objects with complex structural features as well as mechanical and optical functions. Compared to state-of-the-art volumetric printing methods, our technique has a resolution about ten times higher than computed axial lithography without feedback optimization, and a volume generation rate four to five orders of magnitude higher than two-photon photopolymerization. We expect this technology to transform rapid volumetric production for objects at the nanoscopic to macroscopic length scales.

Highly Cooperative Photoswitching in Dihydropyrene Dimers.

We present a strategy to achieve highly cooperative photoswitching, where the initial switching event greatly facilitates subsequent switching of the neighboring unit. By linking donor/acceptor substituted dihydropyrenes via suitable π-conjugated bridges, the quantum yield of the second photochemical ring-opening process could be enhanced by more than two orders of magnitude as compared to the first ring-opening. As a result, the intermediate mixed switching state is not detected during photoisomerization although it is formed during the thermal back reaction. Comparing the switching behavior of various dimers, both experimentally and computationally, helped to unravel the crucial role of the bridging moiety connecting both photochromic units. The presented dihydropyrene dimer serves as model system for longer cooperative switching chains, which, in principle, should enable efficient and directional transfer of information along a molecularly defined path. Moreover, our concept allows to enhance the photosensitivity in oligomeric and polymeric systems and materials thereof.

Donor-Acceptor Dihydropyrenes Switchable with Near-Infrared Light.

The use of low-intensity NIR light to operate molecular switches offers several potential advantages including enhanced penetration into bulk materials, in particular biological tissues, and reduced radiation damage due to the limited photon energies. The latter, however, pose a challenge for designing reasonably bistable systems. We have developed a general design strategy for direct one-photon NIR photoswitches based on negative photochromic dihydropyrenes carrying opposing strong donor-acceptor substituents either along the long axis of the molecule or across it. Thus, two series of 2,7- and 4,9-disubstituted dihydropyrenes were synthesized, and their photothermal properties investigated as a function of the type, strength, and position of the attached donor and acceptor substituents as well as the polarity of the environment. By shifting the excitation wavelength deep into the NIR, both NIR one-photon absorption cross-section and photoisomerization efficiency could be maximized while retaining a reasonable thermal stability of the metastable cyclophanediene isomer. Thus, the lowest optical transition was shifted beyond 900 nm, the NIR cross-section was enhanced by two orders of magnitude, and the thermal half-lives vary between milliseconds and hours. These unique features open up ample opportunities for noninvasive, optically addressable materials and material systems.

Indexing grazing-incidence X-ray diffraction patterns of thin films: lattices of higher symmetry.

Grazing-incidence X-ray diffraction studies on organic thin films are often performed on systems showing fibre-textured growth. However, indexing their experimental diffraction patterns is generally challenging, especially if low-symmetry lattices are involved. Recently, analytical mathematical expressions for indexing experimental diffraction patterns of triclinic lattices were provided. In the present work, the corresponding formalism for crystal lattices of higher symmetry is given and procedures for applying these equations for indexing experimental data are described. Two examples are presented to demonstrate the feasibility of the indexing method. For layered crystals of the prototypical organic semiconductors di-indeno-perylene and (ortho-di-fluoro)-sexi-phenyl, as grown on highly oriented pyrolytic graphite, their yet unknown unit-cell parameters are determined and their crystallographic lattices are identified as monoclinic and orthorhombic, respectively.

Taking Photochromism beyond Visible: Direct One-Photon NIR Photoswitches Operating in the Biological Window.

The success of photopharmacology is inevitably tied to the availability of photoswitches, which can be operated within the biological window (λ=650-1450 nm) to maximize penetration in tissue. A general design strategy has been devised and a dihydropyrene derivative is described here that displays negative T-type photochromism, allowing for efficient and nearly quantitative (95 %) switching induced by NIR light λ>800 nm. The thermal half-life of the decolored ring-open meta-cyclophanediene isomer ranges from minutes to hours, depending on the solvent polarity and hence serves as a probe of the local environment. Due to the rather subtle geometrical differences between the two isomers, suitably modified NIR photoswitches are potential candidates for switching when bound in the pocket of the biological target, in principle allowing for reversible light-induced inhibitor deactivation as an alternative approach to externally regulate biological functions.

N,N'-Disubstituted Indigos as Readily Available Red-Light Photoswitches with Tunable Thermal Half-Lives.

Some rare indigo derivatives have been known for a long time to be photochromic upon irradiation with red light, which should be advantageous for many applications. However, the absence of strategies to tune their thermal half-lives by modular molecular design as well as the lack of proper synthetic methods to prepare a variety of such molecules from the parent indigo dye have so far precluded their use. In this work, several synthetic protocols for N-functionalization have been developed, and a variety of N-alkyl and N-aryl indigo derivatives have been prepared. By installation of electron-withdrawing substituents on the N-aryl moieties, the thermal stability of the Z-isomers could be enhanced while maintaining the advantageous photoswitching properties upon irradiation with red light (660 nm LED). Both experimental data and computational results suggest that the ability to tune thermal stability without affecting the dyes' absorption maxima originates from the twisted geometry of the N-aryl groups. The new indigo photoswitches reported are expected to have a large impact on the development of optical methods and applications in both life and material sciences.

Dihydropyrene as an Aromaticity Probe for Partially Quinoid Push-Pull Systems.

Although aromaticity is a key concept in chemistry that is frequently used to explain the structure and reactivity of organic compounds, it remains challenging to actually measure this property. Proper probes allow for an experimental quantification of aromaticity using nuclear magnetic resonance (NMR) spectroscopy and in this study 15,16-dimethyl-15,16-dihydropyrene (DHP) is demonstrated to be particularly well suited for this purpose. DHP has two internal methyl groups, which are positioned within the π cloud of this bridged, planar, and rigid [14]annulene, and are therefore shifted to higher field in proton NMR spectra owing to ring-current effects. A specific DHP derivative bearing strongly electron-donating dimethylamino and strongly electron-accepting nitro groups in a relative pseudo-para orientation has been synthesized and characterized with respect to its quinoid character. Solvatochromism as well as a reduced ring current show that the partial quinoid character in this push-pull DHP derivative is about 12 %. This study extends the scope of DHP as an aromaticity probe and aids in the better understanding of the phenomenon of aromaticity.

Tuning the formation of discrete coordination nanostructures.

Novel surface coordination nanostructures based on cyanosexiphenyl molecules are assembled on a gold surface and investigated by scanning tunneling microscopy and density functional theory. Their formation can be tuned by varying the surface temperature during deposition. Diffusing gold adatoms act as coordination centers for the cyano groups present on one end of the nonsymmetrical molecules.

Making nonsymmetrical bricks: synthesis of insoluble dipolar sexiphenyls.

A versatile synthesis of nonsymmetrical, terminally substituted p-sexiphenyl (6P) derivatives has been developed. The synthesis makes use of a nonsymmetrical starting material as well as modular functionalization using Suzuki cross-coupling to yield a soluble precursor, which finally is converted to the insoluble target 6P derivatives. These derivatives display similar electronic and optical properties to the parent 6P, yet the permanent dipole along their molecular axis allows for tuning of their self-assembly on various substrate surfaces.