Frank Ortmann

We investigate the carrier dynamics in graphene junctions under mid-infrared excitation using ultrafast pump–probe photocurrent spectroscopy. We utilize dual split gate devices to demonstrate that the photothermoelectric effect can dominate the photocurrent response of graphene also for a mid-infrared femtosecond excitation. Graphene retains its broadband photocurrent response in this spectral region, but the photocurrent relaxation time increases from ∼2 ps below 8–9 μm up to 3 ps at longer mid-infrared wavelengths. The absence of a pronounced phonon bottleneck in the decay dynamics at room temperature suggests an efficient interplay of electron–electron and electron–phonon scattering even for photon energies below the optical phonon energy in graphene. The observed wavelength dependence of the photocurrent relaxation times is consistent with energy-dependent theoretical relaxation times as derived from a microscopic transport theory of graphene that includes electron–phonon coupling within a Holstein-Peierls Hamiltonian.

While covalent organic frameworks (COFs) have attracted enormous attention in diverse fields due to their large molecular design space, tuning their optoelectronic properties is challenging because of complex synthetic pathways. Herein, we establish a combinatorial synthetic strategy employing reactive donor and acceptor modules for the generation of extended linear COF building blocks, which already encode the key optoelectronic features of corresponding COFs. This approach enables optoelectronic functionality to be rationally preprogrammed at the building-block level, allowing systematic control over conjugation length, donor–acceptor composition, and energy-level alignment in the resulting COFs. As a great diversity of COFs can be generated from linear building blocks and easily accessible multifunctional nodes, this strategy offers a novel entry into the synthesis of numerous COFs with tunable optoelectronic properties and enables us to construct highly crystalline and porous photoactive COFs and oriented COF films. These pore systems allow for the inclusion of fullerene acceptor phases and the construction of periodic bulk heterojunctions, offering a blueprint for generating novel COFs with emerging optoelectronic features and device architectures.

Strategies for tuning structural and (opto-)­electronic properties are fundamental to the rational design of functional materials. Here, we present a molecular-design approach that enables precise modulation of the optoelectronic properties of covalent organic frameworks (COFs) through single-atom halogen substitution on π-extended anthracene linkers. Using a Wurster-type tetratopic amine (W–NH₂) and a series of anthracene-based dialdehydes bearing H, Cl, Br, or I at the 2-position, a family of imine-linked COFs, W-A-X (X = H, Cl, Br, I), was synthesized. The halogen substituent strongly influences framework formation, with brominated COFs forming substantially larger crystalline domains than the unsubstituted analogue. UV–vis absorption and photoluminescence measurements reveal a systematic redshift across the series (H < Cl < Br < I), demonstrating that a single-atom modification tunes the optical response. In addition, the significantly longer excited-state lifetime of W-A-Cl COF highlights the strengthened donor–acceptor interactions induced by the more electron-withdrawing chlorine substituent, further confirming the decisive role of single-atom functionalization in controlling excited-state dynamics. Time-dependent density functional theory calculations on both isolated fragments and extended COF models attribute the observed trends in optical response to halogen-induced changes in the COF band structure and provide a mechanistic understanding of how a single-atom substitution influences the optoelectronic properties of the extended π-framework. Overall, this study establishes single-atom halogen substitution as a powerful and modular tool for tailoring the structural and optical properties of anthracene-based COFs.

1. Biological function emerges from processes that span a remarkable range of temporal and spatial scales [1–3]. At one extreme, the fundamental events underlying enzymatic catalysis [4–6], electro...

The unusually broad and intense visible (Vis) absorption band observed in naphthalenediimide–bithiophene (NDI‐T2) co‐polymers reflects complex electronic and vibronic interactions between donor and acceptor units. While the charge‐transfer character arising from electronic coupling across the moieties likely contributes to the red‐shifted absorption, the pronounced spectral broadness may also originate from strong coupling to vibrations or from conformational diversity within the polymer. Distinguishing the relative roles of these effects—electronic, vibronic, and environmental—requires a quantitative theory that can treat multiple electronic and excitonic states and their coupling to many vibrational modes without resorting to simplified models. Here, we address this challenge by focusing on well‐defined NDI‐T2 co‐oligomers and employing a first‐principles–parameterized linear vibronic coupling model combined with a matrix product state (MPS) approach, which allows us to treat dozens of excitons and strongly coupled vibrational modes in full quantum detail. This enables a direct, quantitative link between molecular geometry, vibrational structure, and the experimentally observed ultraviolet‐visible (UV–vis) absorption and fluorescence anisotropy spectra, going beyond the capabilities of standard excited‐state modeling approaches and providing a microscopic framework for understanding optical line shapes in flexible donor–acceptor materials.

Electron–phonon coupling (EPC) is fundamental for understanding the behavior of molecules and crystals, influencing phenomena such as charge transport, energy transfer, phase transitions, and polaron formation. Accurate computational methods to calculate EPCs from first principles are essential, but their complexity has resulted in a variety of computational strategies, raising concerns about their mutual consistency. In this study, we provide a systematic benchmark of methods for EPC calculation by comparing two fundamentally different ab initio methodologies. We investigate Gaussian-type orbital methods based on the CP2K code and plane-wave-based projector-augmented-wave methods combined with maximally localized Wannier functions, as implemented in VASP and wannier90 . In addition, we further distinguish between the derivative–of–Hamiltonian ( d H ) and derivative–of–states ( d ψ ) approaches for obtaining EPC parameters. The comparison is conducted on a representative set of organic molecules, including pyrazine, pyridine, bithiophene, and quarterthiophene, varying significantly in size and flexibility. We find excellent agreement across implementations and basis sets when employing the same computational approach ( d H or d ψ ), demonstrating robust consistency between the numerical schemes. However, noticeable deviations occur when comparing the d H and d ψ approaches within each code and for specific cases discussed in detail. Our findings emphasize the reliability of EPC computations using the d H method and caution against potential pitfalls associated with the d ψ approach, providing guidance for future EPC calculations and model parameterizations.

Precise structural control is essential for high-performance semiconductors. In organic electronics, traditional methods for tuning the dimensions of device structures often rely on cumbersome, limited-resolution processes such as shadow mask patterning, printing, or viscosity tuning. Here, we report ultraviolet (UV) irradiation in ambient conditions as a transformative approach for tuning structural parameters of organic small molecule hole transport layers (HTLs) in vertical and lateral directions. The method preserves HTL conductivity while facilitating uniform thickness reduction through synergistic photo-induced oligomerization and photo-oxidative layer shrinking. Controlled thinning applies to various organic materials. In cavity architectures, UV-treated organic photodetectors show narrowband detection from 900 to 1200 nm with a full width at half maximum down to 25 nm, and UV-treated organic light-emitting diodes exhibit 75 nm peak tunability. Moreover, this strategy permits micrometer-scale lateral patterning of HTLs. Our work opens new opportunities for precise and practical engineering for organic electronic devices.

Decoupling solar energy conversion and storage in a single material offers a great advantage for off-grid applications. Herein, we disclose a two-dimensional naphthalenediimide (NDI)-based covalent organic framework (COF) exhibiting remarkable solar battery performance when used as a photoanode. Light-induced radicals are stabilized within the framework for several hours, offering on-demand charge extraction for electrical energy production. Our study reveals mechanistic insights into the long-term charge stabilization using optical spectroscopy and (photo)electrochemical measurements, in conjunction with density functional theory (DFT) simulations. Among several solvents, water provides the best dielectric screening and energetically favorable proton exchange to stabilize photoinduced radicals for more than 48 h without the need for additional metal cations. This study provides fundamental insights into the optoionic charge storage mechanism in NDI-COF, while introducing a highly tunable, nanoporous material platform that surpasses related materials, such as carbon nitrides, metal-organic frameworks (MOFs), or metal oxides, in terms of charge storage capacity. This study opens new perspectives for the design of optoionic charge-storing materials and the direct storage of solar energy to overcome the intermittency of solar irradiation.