Excited States Dynamics at Pentacene/Perfluoropentacene Interfaces: A Femtosecond Time-Resolved Second Harmonic Generation Study.

Understanding the dynamics of excited states after optical excitation at donor-acceptor (D/A) interfaces is of paramount importance for improving the efficiency and performance of optoelectronic devices. Here, we studied the ultrafast excited state dynamics after optical excitation at interfaces between the electron donor (D) pentacene (PEN) and the electron acceptor (A) perfuoropentacene (PFP) as well as within the single compounds (PEN and PFP) using femtosecond (fs) time-resolved second harmonic generation (SHG). In the single compounds singlet fission is observed on a time scale of around 200 fs. In the bilayer systems a huge SHG intensity rise is observed due to the creation of charge transfer states at the interface and accordingly to formation of a local electric field within tens of picoseconds. The local electric field and therefore the SHG signal intensity from the interface of PEN/PFP bilayer is much more intense compared to the PFP/PEN system because the PFP and PEN intermixing at the PEN/PFP interface is higher. Accordingly a population of defect states on a time scale of 55±12 ps has been proposed for PEN/PFP. Our study provides important insights into D/A charge transfer properties, which is needed for the understanding of the interfacial photophysics of pentacene-based organic compounds.

Determination of energetic positions of electronic states and the exciton dynamics in a π-expanded N-heterotriangulene derivative adsorbed on Au(111).

Bridged triarylamines, so-called N-heterotriangulenes (N-HTAs) are promising organic semiconductors for applications in optoelectronic devices. Thereby the electronic structure at organic/metal interfaces and within thin films as well as the electronically excited states dynamics after optical excitation is essential for the performance of organic-molecule-based devices. Here, we investigated the energy level alignment and the excited state dynamics of a N-HTA derivative adsorbed on Au(111) by means of energy- and time-resolved two-photon photoemission spectroscopy. We quantitatively determined the energetic positions of several occupied and unoccupied molecular (transport levels) and excitonic states (optical gap) in detail. A transport gap of 3.20 eV and an optical gap of 2.58 eV is determined, resulting in an exciton binding energy of 0.62 eV. With the first time-resolved investigation on a N-HTA compound we gained insights into the exciton dynamics and resolved processes on the femtosecond to picosecond timescale.

Characteristics and long-term kinetics of an azobenzene derivative and a donor-acceptor Stenhouse adduct as orthogonal photoswitches.

Light-triggered molecular switches are extensively researched for their applications in medicine, chemistry and material science and, if combined, particularly for their use in multifunctional smart materials, for which orthogonally, i.e. individually, addressable photoswitches are needed. In such a multifunctional mixture, the switching properties, efficiencies and the overall performance may be impaired by undesired mutual dependences of the photoswitches on each other. Within this study, we compare the performance of the pure photoswitches, namely an azobenzene derivative (Azo) and a donor-acceptor Stenhouse adduct (DASA), with the switching properties of their mixture using time-resolved temperature-dependent UV/VIS absorption spectroscopy, time-resolved IR absorption spectroscopy at room temperature and quantum mechanical calculations to determine effective cross sections, switching kinetics as well as activation energies of thermally induced steps. We find slightly improved effective cross sections, percentages of switched molecules and no increased activation barriers of the equimolar mixture compared to the single compounds. Thus, the studied mixture Azo + DASA is very promising for future applications in multifunctional smart materials.

Influence of N-introduction on the electronic structure and properties of polyacenes: experiment and quantum chemistry in concert.

N-Heteropolycycles (NHPCs) represent a promising substance class for applications in functional organic materials, since their electronic structure and the resulting individual molecular properties are efficiently tuneable by number and position of nitrogen atoms in the aromatic structural backbone. The isosteric replacement of a C-H unit by N leaves the geometric structure unchanged, while ionization potential, electron affinity and absorption spectra are altered. In this prespective, we present the potent combination of two-photon photoelectron spectroscopy (2PPE) and high-resolution electron energy loss spectroscopy (HREELS) with quantum chemical calculations for the investigation of the electronic structure of NHCPs. In contrast to conventional optical spectroscopies, 2PPE provides insight into electron-detached and attached electronic states of NHCPs, while HREELS delivers the energetic position of the lowest triplet states. Based on our comprehensive investigations, an extension of Platt's famous nomenclature of the low-lying excited ππ* states could be suggested for NHPCs based on the physical properties of the corresponding excitons. Also, the influence of N-introduction onto the occurrence of the so-called α-band in NHPCs compared to the parent polycyclic aromatic hydrocarbons could be explained in detail. While N-substitution of C-H in polycyclic aromatic hydrocarbons (PAHs) is often seen as a simple isosteric replacement, it has a strong influence on the electronic structure and the resulting properties. Therefore rules derived for PAHs can often only be transferred to a limited extent or not at all.

Fully Bridged Triphenylamines Comprising Five- and Seven-Membered Rings.

A family of fully bridged triphenylamines with embedded 5- and 7-membered rings is presented. The compounds are potent electron donors capable to undergo donor/acceptor interactions with strong cyano-based acceptors both in the solid state and solution. These interactions were evaluated by IR and UV/vis spectroscopy as well as X-ray crystallography. The vinylene-bridged compound was oxidized to the corresponding 1,2-diketone which readily underwent acid-catalyzed condensation with selected 1,2-phenylenediamines. The resulting π-extended quinoxaline derivatives represent valuable building blocks for the development of functional chromophores upon appropriate functionalization.

Short-range organization and photophysical properties of CdSe quantum dots coupled with aryleneethynylenes.

Hybrid organic-inorganic nanomaterials composed of organic semiconductors and inorganic quantum dots (QDs) are promising candidates for opto-electronic devices in a sustainable internet of things. Especially their ability to combine the advantages of both compounds in one material with new functionality, the energy-efficient production possibility and the applicability in thin films with little resource consumption are key benefits of these materials. However, a major challenge one is facing for these hybrid materials is the lack of a detailed understanding of the organic-inorganic interface which hampers the widespread application in devices. We advance the understanding of this interface by studying the short-range organization and binding motif of aryleneethynylenes coupled to CdSe QDs as an example system with various experimental methods. Clear evidence for an incorporation of the organic ligands in between the inorganic QDs is found, and polarization-modulation infrared reflection-absorption spectroscopy is shown to be a powerful technique to directly detect the binding in such hybrid thin-film systems. A monodentate binding and a connection of neighboring QDs by the aryleneethynylene molecules is identified. Using steady-state and time resolved spectroscopy, we further investigated the photophysics of these hybrid systems. Different passivation capabilities resulting in different decay dynamics of the QDs turned out to be the main influence of the ligands on the photophysics.

Influence of N-introduction in pentacene on the electronic structure and excited electronic states.

N-Heteropolycyclic aromatic compounds are promising organic semiconductors for applications in field effect transistors and solar cells. Thereby the electronic structure of organic/metal interfaces and thin films is essential for the performance of organic-molecule-based devices. Here, we studied the structural and the electronic properties of 6,7,12,13-tetraazapentacene (TAP) adsorbed on Au(111) using vibrational and electronic high-resolution electron energy loss spectroscopy in combination with state-of-the-art quantum chemical calculations. In the mono- and multilayer TAP adsorbs in a planar adsorption geometry with the molecular backbone oriented parallel to the gold substrate. The energies of the lowest excited electronic singlet states (S) as well as the triplet state (T) are assigned. The optical gap (S0 → S1 transition) is found to be 1.6 eV and the T1 energy 1.2 eV. In addition, thorough comparison to previously studied pentacene (PEN) and 6,13-diazapentacene (6,13-DAP) is made explaining in detail the influence of nitrogen substitution on the electronic structure and in particular on the intensity of the α-band in the UV/vis absorption spectrum. In the series PEN, 6,13-DAP, and TAP, the α-band (S0 → S2 transition) gains significantly in intensity due to individual effects of the introduced nitrogen atoms on the orbital energies.

Binding energies of benzene on coinage metal surfaces: Equal stability on different metals.

Interfaces between organic molecules and inorganic solids adapt a prominent role in fundamental science, catalysis, molecular sensors, and molecular electronics. The molecular adsorption geometry, which is dictated by the strength of lateral and vertical interactions, determines the electronic structure of the molecule/substrate system. In this study, we investigate the binding properties of benzene on the noble metal surfaces Au(111), Ag(111), and Cu(111), respectively, using temperature-programmed desorption and first-principles calculations that account for non-locality of both electronic exchange and correlation effects. In the monolayer regime, we observed for all three systems a decrease of the binding energy with increasing coverage due to repulsive adsorbate/adsorbate interactions. Although the electronic properties of the noble metal surfaces are rather different, the binding strength of benzene on these surfaces is equal within the experimental error (accuracy of 0.05 eV), in excellent agreement with our calculations. This points toward the existence of a universal trend for the binding energy of aromatic molecules resulting from a subtle balance between Pauli repulsion and many-body van der Waals attraction.

Differing Isomerization Kinetics of Azobenzene-Functionalized Self-Assembled Monolayers in Ambient Air and in Vacuum.

Azobenzenealkanethiols in self-assembled monolayers (SAMs) on Au(111) exhibit reversible trans-cis photoisomerization when diluted with alkanethiol spacers. Using these mixed SAMs, we show switching of the linear optical and second-harmonic response. The effective switching of these surface optical properties relies on a reasonably large cross section and a high photoisomerization yield as well as a long lifetime of the metastable cis isomer. We quantified the switching process by X-ray absorption spectroscopy. The cross sections for the trans-cis and cis-trans photoisomerization with 365 and 455 nm light, respectively, are 1 order of magnitude smaller than in solution. In vacuum, the 365 nm photostationary state comprises 50-74% of the molecules in the cis form, limited by their rapid thermal isomerization back to the trans state. In contrast, the 455 nm photostationary state contains nearly 100% trans-azobenzene. We determined time constants for the thermal cis-trans isomerization of only a few minutes in vacuum and in a dry nitrogen atmosphere but of more than 1 day in ambient air. Our results suggest that adventitious water adsorbed on the surface of the SAM stabilizes the polar cis configuration of azobenzene under ambient conditions. The back reaction rate constants differing by 2 orders of magnitude underline the huge influence of the environment and, accordingly, its importance when comparing various experiments.

Quantitative Prediction of Molecular Adsorption: Structure and Binding of Benzene on Coinage Metals.

Interfaces between organic molecules and solid surfaces play a prominent role in heterogeneous catalysis, molecular sensors and switches, light-emitting diodes, and photovoltaics. The properties and the ensuing function of such hybrid interfaces often depend exponentially on molecular adsorption heights and binding strengths, calling for well-established benchmarks of these two quantities. Here we present systematic measurements that enable us to quantify the interaction of benzene with the Ag(111) coinage metal substrate with unprecedented accuracy (0.02 Å in the vertical adsorption height and 0.05 eV in the binding strength) by means of normal-incidence x-ray standing waves and temperature-programed desorption techniques. Based on these accurate experimental benchmarks for a prototypical molecule-solid interface, we demonstrate that recently developed first-principles calculations that explicitly account for the nonlocality of electronic exchange and correlation effects are able to determine the structure and stability of benzene on the Ag(111) surface within experimental error bars. Remarkably, such precise experiments and calculations demonstrate that despite different electronic properties of copper, silver, and gold, the binding strength of benzene is equal on the (111) surface of these three coinage metals. Our results suggest the existence of universal binding energy trends for aromatic molecules on surfaces.

Core Halogenation as a Construction Principle in Tuning the Material Properties of Tetraazaperopyrenes.

A detailed study on the effects of core halogenation of tetraazaperopyrene (TAPP) derivatives is presented. Its impact on the solid structure, as well as the photophysical and electrochemical properties, has been probed by the means of X-ray crystallography, UV/Vis and fluorescence spectroscopy, high-resolution electron energy loss spectroscopy (HREELS), cyclic voltammetry (CV), and DFT modeling. The aim was to assess the potential of this approach as a construction principle for organic electron-conducting materials of the type studied in this work. Although halogenation leads to a stabilization of the LUMOs compared to the unsubstituted parent compound, the nature of the halide barely affects the LUMO energy while strongly influencing the HOMO energies. In terms of band-gap engineering, it was demonstrated that the HOMO-LUMO gap is decreased by substitution of the TAPP core with halides, the effect being found to be most pronounced for the iodinated derivative. The performance of the recently reported core-fluorinated and core-iodinated TAPP derivatives in organic thin-film transistors (TFTs) was investigated on both a glass substrate, as well as on a flexible plastic substrate (PEN). Field-effect mobilities of up to 0.17 cm(2) Vs(-1) and on/off current ratio of >10(6) were established.

Electronic structure and excited state dynamics in a dicyanovinyl-substituted oligothiophene on Au(111).

Dicyanovinyl (DCV)-substituted oligothiophenes are promising donor materials in vacuum-processed small-molecule organic solar cells. Here, we studied the structural and the electronic properties of DCV-dimethyl-pentathiophene (DCV5T-Me2) adsorbed on Au(111) from submonolayer to multilayer coverages. Using a multi-technique experimental approach (low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), and two-photon photoemission (2PPE) spectroscopy), we determined the energetic position of several affinity levels as well as ionization potentials originating from the lowest unoccupied molecular orbitals (LUMO) and the highest occupied molecular orbitals (HOMO), evidencing a transport gap of 1.4 eV. Proof of an excitonic state was found to be a spectroscopic feature located at 0.6 eV below the LUMO affinity level. With increasing coverage photoemission from excitonic states gains importance. We were able to track the dynamics of several electronically excited states of multilayers by means of femtosecond time-resolved 2PPE. We resolved an intriguing relaxation dynamics involving four processes, ranging from sub-picosecond (ps) to several hundred ps time spans. These show a tendency to increase with increasing coverage. The present study provides important parameters such as energetic positions of transport levels as well as lifetimes of electronically excited states, which are essential for designing organic-molecule-based optoelectronic devices.

Nonlinear optical response of photochromic azobenzene-functionalized self-assembled monolayers.

The combination of photochromic and nonlinear optical (NLO) properties of azobenzene-functionalized self-assembled monolayers (SAMs) constitutes an intriguing step towards novel photonic and optoelectronic devices. By utilizing the second-order NLO process of second harmonic generation (SHG), supported by density-functional theory and correlated wave function method calculations, we demonstrate that the photochromic interface provides the necessary prerequisites en route towards possible future technical applications: we find a high NLO contrast on the order of 16% between the switching states. These are furthermore accessible reversibly and with high efficiencies in terms of cross sections on the order of 10(-18) cm(2) for both photoisomerization reactions, i.e., drivable by means of low-power LED light sources. Finally, both photostationary states (PSSs) are thermally stable at ambient conditions.

Electronic structure changes during the surface-assisted formation of a graphene nanoribbon.

High conductivity and a tunability of the band gap make quasi-one-dimensional graphene nanoribbons (GNRs) highly interesting materials for the use in field effect transistors. Especially bottom-up fabricated GNRs possess well-defined edges which is important for the electronic structure and accordingly the band gap. In this study we investigate the formation of a sub-nanometer wide armchair GNR generated on a Au(111) surface. The on-surface synthesis is thermally activated and involves an intermediate non-aromatic polymer in which the molecular precursor forms polyanthrylene chains. Employing angle-resolved two-photon photoemission in combination with density functional theory calculations we find that the polymer exhibits two dispersing states which we attribute to the valence and the conduction band, respectively. While the band gap of the non-aromatic polymer obtained in this way is relatively large, namely 5.25 ± 0.06 eV, the gap of the corresponding aromatic GNR is strongly reduced which we attribute to the different degree of electron delocalization in the two systems.

Digital Light 3D Printing of Double Thermoplastics with Customizable Mechanical Properties and Versatile Reprocessability.

Digital light processing (DLP) is a 3D printing technology offering high resolution and speed. Printable materials are commonly based on multifunctional monomers, resulting in the formation of thermosets that usually cannot be reprocessed or recycled. Some efforts are made in DLP 3D printing of thermoplastic materials. However, these materials exhibit limited and poor mechanical properties. Here, a new strategy is presented for DLP 3D printing of thermoplastics based on a sequential construction of two linear polymers with contrasting (stiff and flexible) mechanical properties. The inks consist of two vinyl monomers, which lead to the stiff linear polymer, and α-lipoic acid, which forms the flexible linear polymer via thermal ring-opening polymerization in a second step. By varying the ratio of stiff and flexible linear polymers, the mechanical properties can be tuned with Young's modulus ranging from 1.1 GPa to 0.7 MPa, while the strain at break increased from 4% to 574%. Furthermore, these printed thermoplastics allow for a variety of reprocessability pathways including self-healing, solvent casting, reprinting, and closed-loop recycling of the flexible polymer, contributing to the development of a sustainable materials economy. Last, the potential of the new material in applications ranging from soft robotics to electronics is demonstrated.

Coverage- and temperature-controlled isomerization of an imine derivative on Au(111).

The isomerization behavior of photochromic molecular switches is strongly influenced by adsorption on metal surfaces. For (E)-3,5-di-tert-butyl-N-(3,5-di-tert-butylbenzylidene)aniline (abbreviated as TBI for tetra-tert-butyl imine), it is found that a layer adsorbed on Au(111) can undergo an isomerization from the trans to the cis and back to the trans configuration when continuously increasing the sample temperature and accordingly decreasing the sample coverage. The conformation and adsorption geometry of TBI are determined from near-edge X-ray absorption fine structure measurements in agreement with density functional theory calculations taking into account the van der Waals interaction between adsorbate and metal surface. The coverage- and temperature-controlled conformational transitions are reversible and are driven by the higher packing density of the less stable cis-isomer in combination with the low thermal activation barrier of the trans- to cis-isomerization typical for imine derivatives. This unexpected scenario is corroborated by thermal desorption and vibrational spectroscopy as well as scanning tunneling microscopy.

Adsorption Structures Affecting the Electronic Properties and Photoinduced Charge Transfer at Perylene-Based Molecular Interfaces.

Perylene-based organic semiconductors are widely used in organic electronic devices. Here, we studied the ultrafast excited state dynamics after optical excitation at interfaces between the electron donor (D) diindenoperylene (DIP) and the electron acceptor (A) dicyano-perylene-bis(dicarboximide) (PDIR-CN2 ) using femtosecond time-resolved second harmonic generation (SHG) in combination with large scale quantum chemical calculations. Thereby, we varied in bilayer structures of DIP and PDIR-CN2 the interfacial molecular geometry. For an interfacial configuration which contains a edge-on geometry but also additional face-on domains an optically induced charge transfer (CT) is observed, which leads to a pronounced increase of the SHG signal intensity due to electric field induced second harmonic generation. The interfacial CT state decays within 7.5±0.7 ps, while the creation of hot CT states leads to a faster decay (5.3±0.2 ps). For the bilayer structures with mainly edge-on geometries interfacial CT formation is suppressed since π-π overlap perpendicular to the interface is missing. Our combined experimental and theoretical study provides important insights into D/A charge transfer properties, which is needed for the understanding of the interfacial photophysics of these molecules.

3D Printing Hierarchically Nano-Ordered Structures.

Natural materials are composed of a limited number of molecular building blocks and their exceptional properties are governed by their hierarchical structure. However, this level of precision is unattainable with current state-of-the-art materials for 3D printing. Herein, new self-assembled printable materials based on block copolymers (BCPs) enabling precise control of the nanostructure in 3D are presented. In particular, well-defined BCPs consisting of poly(styrene) (PS) and a polymethacrylate-based copolymer decorated with printable units are selected as suitable self-assembled materials and synthesized using controlled radical polymerization. The synthesized library of BCPs are utilized as printable formulations for the fabrication of complex 3D microstructures using two-photon laser printing. By fine-tuning the BCP composition and solvent in the formulations, the fabrication of precise 3D nano-ordered structures is demonstrated for the first time. A key point of this work is the achievement of controlled nano-order within the entire 3D structures. Thus, imaging of the cross-sections of the 3D printed samples is performed, enabling the visualization also from the inside. The presented versatile approach is expected to create new avenues for the precise design of functional polymer materials suitable for high-resolution 3D printing exhibiting tailor-made nanostructures.

Accurate quantification of the stability of the perylene-tetracarboxylic dianhydride on Au(111) molecule-surface interface.

Studying inorganic/organic hybrid systems is a stepping stone towards the design of increasingly complex interfaces. A predictive understanding requires robust experimental and theoretical tools to foster trust in the obtained results. The adsorption energy is particularly challenging in this respect, since experimental methods are scarce and the results have large uncertainties even for the most widely studied systems. Here we combine temperature-programmed desorption (TPD), single-molecule atomic force microscopy (AFM), and nonlocal density-functional theory (DFT) calculations, to accurately characterize the stability of a widely studied interface consisting of perylene-tetracarboxylic dianhydride (PTCDA) molecules on Au(111). This network of methods lets us firmly establish the adsorption energy of PTCDA/Au(111) via TPD (1.74 ± 0.10 eV) and single-molecule AFM (2.00 ± 0.25 eV) experiments which agree within error bars, exemplifying how implicit replicability in a research design can benefit the investigation of complex materials properties.

Cluster-Based Approach Utilizing Optimally Tuned TD-DFT to Calculate Absorption Spectra of Organic Semiconductor Thin Films.

The photophysics of organic semiconductor (OSC) thin films or crystals has garnered significant attention in recent years since a comprehensive theoretical understanding of the various processes occurring upon photoexcitation is crucial for assessing the efficiency of OSC materials. To date, research in this area has relied on methods using Frenkel-Holstein Hamiltonians, calculations of the GW-Bethe-Salpeter equation with periodic boundaries, or cluster-based approaches using quantum chemical methods, with each of the three approaches having distinct advantages and disadvantages. In this work, we introduce an optimally tuned, range-separated time-dependent density functional theory approach to accurately reproduce the total and polarization-resolved absorption spectra of pentacene, tetracene, and perylene thin films, all representative OSC materials. Our approach achieves excellent agreement with experimental data (mostly ≤0.1 eV) when combined with the utilization of clusters comprising multiple monomers and a standard polarizable continuum model to simulate the thin-film environment. Our protocol therefore addresses a major drawback of cluster-based approaches and makes them attractive tools for OSC investigations. Its key advantages include its independence from external, system-specific fitting parameters and its straightforward application with well-known quantum chemical program codes. It demonstrates how chemical intuition can help to reduce computational cost and still arrive at chemically meaningful and almost quantitative results.