Alternating Orthogonal Switching in a Thiophenyl-Phenyl-Bis-Azobenzene Switch.

To design efficient molecular information storage systems with multi-photoswitchable entities, orthogonal isomerization of the different switchable moieties is essential. Various challenges, like unintentional energy transfer, spectral overlap, and other energy dissipation channels, have to be addressed by intelligent molecule design. In this context, we took advantage of calculations to design a bis-azobenzene switch, which consists of a phenyl- and a thiophenylazobenzene moiety in meta-connection to reduce π-conjugation.

Molecular Excitons and Plasmons in Acenes and Their Radical Cations.

For existing and potential applications of acenes and acene derivatives, properties of their excited states play a central role. In describing these, the molecular orbital picture can reach its limits, and consideration within the quasi-particle picture can enable further insight. In this work, exciton size ( ), hole and electron size ( and ) and correlation coefficient ( ) of excited states of acenes and acene cations are investigated using TD-DFT at the TDA/CAM-B3LYP/6-311G* level, with a focus on their development with increasing acene length.

Tuning Electrocatalytic Energy Release in Norbornadiene Based Molecular Solar Thermal Systems Through Substituent Effects.

Molecular solar thermal (MOST) systems, such as the norbornadiene/quadricyclane (NBD/QC) pair, combine solar energy conversion, storage, and release in a simple one-molecule process. The energy-releasing reaction QC to NBD can be controlled electrochemically. In this study, we used in-situ photoelectrochemical infrared spectroscopy (PEC-IRRAS) together with density functional theory (DFT) calculations to investigate how electron donating (EDG) and electron withdrawing (EWG) groups in the push-pull system of the MOST pair affect the electrocatalytic properties of the electrochemically triggered back-conversion.

Stable and Accurate Orbital-Free Density Functional Theory Powered by Machine Learning.

Hohenberg and Kohn have proven that the electronic energy and the one-particle electron density can, in principle, be obtained by minimizing an energy functional with respect to the density. While decades of theoretical work have produced increasingly faithful approximations to this elusive exact energy functional, their accuracy is still insufficient for many applications, making it reasonable to try and learn it empirically. Using rotationally equivariant atomistic machine learning, we obtain for the first time a density functional that, when applied to the organic molecules in QM9, yields energies with chemical accuracy relative to the Kohn-Sham reference while also converging to meaningful electron densities.

Unitary coupled-cluster theory for the electron propagator: electron attachment and physical properties the intermediate state representation.

A scheme for the calculation of electron-attachment (EA) processes within the framework of unitary coupled-cluster (UCC) theory is presented. Analogous to the description of electron-detachment, the intermediate state representation (ISR) approach is used for the formulation and its relation to the algebraic-diagrammatic construction scheme is pointed out. Due to the UCC ansatz, the resulting equations cannot be given by closed-form expressions, but need to be approximated.

Low Molecular Weight Multistate Photoswitches Based on Simple Norbornadiene-Triazine Scaffolds.

We have synthesized and characterized a series of simple norbornadiene(NBD)-triazine architectures, including multistate photoswitches with unprecedentedly high information storage densities. The simple mono-NBDs served as suitable model systems to investigate the underlying absorption and switching characteristics. To increase the complexity stepwise, a bis-NBD derivative with a symmetric substitution pattern was investigated next.

Two-Photon Circular Dichroism in the Algebraic Diagrammatic Construction Framework.

We present the calculation of the rotatory strengths of two-photon circular dichroism (TPCD) in the framework of algebraic diagrammatic construction (ADC) using the intermediate state representation (ISR). First, the differences between the length-gauge formulation of TPCD and three gauge-origin invariant approaches are investigated depending on the order of perturbation theory and the basis set. Subsequently, our ADC/ISR results are compared with results of corresponding coupled cluster methods from the literature.

Rational design of red-shifted 1,2-azaborinine-based molecular solar thermals.

1,2-Azaborinines are a promising molecular class for application as molecular solar thermal (MOST) systems offering a solution for solar energy storage. Our computational screening of 4,5-substituted derivatives using density functional theory targeting excitation and storage energies revealed push-pull systems with significantly red-shifted absorption bands, enhancing their spectral overlap with the solar spectrum. Among these, the 4-cyano-5-hydroxy derivative, with a 1.

The Relation Between the Excited Electronic States of Acene Radical Cations and Neutrals-A Computational Analysis.

Acenes are a class of molecules that enjoy popularity in both experimental and theoretical fields of research for their diverse areas of application and unique electronic structure. One particular aspect of interest lies in their electronic absorption spectra, which have been thoroughly investigated both experimentally and theoretically. In this work, the electronically excited states of radical cations of acenes from naphthalene to dodecacene are investigated using algebraic diagrammatic construction (ADC) methods and different time-dependent density functional theory (TD-DFT) exchange-correlation kernels.

Exceptionally High Two-Photon Absorption Cross Sections in Quinoidal Diazaacene-Bithiophene Derivatives.

This study addresses the two-photon absorption (2PA) properties of (azaacene-annulated) heterophenoquinones through a synergistic approach combining detailed experimental and theoretical analyses. Exceptionally large 2PA cross sections are found over a broad spectral range in the near-infrared spectral region, with values up to 4100 GM in the 1400-1600 nm range and even higher values of up to 51770 GM in the 850-950 nm range, which is outstanding for organic chromophores of this molecular size. Our quantum chemical calculations support the experimental findings and elucidate the underlying absorption mechanism leading to the corresponding 2PA properties.

Photochemistry upon Charge Separation in Triphenylamine Derivatives from fs to μs.

Quantum chemical methods and time-resolved laser spectroscopy are employed to elucidate ultrafast charge-separation processes in triphenylamine (TPA) derivatives upon photoexcitation. When changing the ambient solvent from non-electron-accepting to electron-acceptor solvents, such as chloroform, a vastly extended and multifaceted photochemistry of TPA derivatives is observed. Following initial excitation, two concurrent charge-transfer processes are identified.

Series expansion of a scalable Hermitian excitonic renormalization method.

Utilizing the sparsity of the electronic structure problem, fragmentation methods have been researched for decades with great success, pushing the limits of ab initio quantum chemistry ever further. Recently, this set of methods has been expanded to include a fundamentally different approach called excitonic renormalization, providing promising initial results. It builds a supersystem Hamiltonian in a second-quantized-like representation from transition-density tensors of isolated fragments, contracted with biorthogonalized molecular integrals.

Searching the chemical space of hetero-atom bridged norbornadienes.

The efficient utilization of solar energy as renewable source is a central pillar of societal future energy production. So-called molecular solar thermal energy storage (MOST) systems have attracted considerable attention as storage solution and heat release on demand. Substituted norbornadiene/quadricyclane (NBD/QC) derivatives have been shown to be well suited for this task, in particular when substituted with electron donating and accepting functional groups.

Meta-Connected Oligo-Azobenzenes Outperform Their Para Counterparts.

Systems with multiple photoswitchable units in one molecule have attracted considerable attention in the past years as they are useful for a broad variety of possible applications. Especially, linked azobenzenes sharing one benzene ring are of high interest since their direct linkage introduces an additional photoswitchable unit at only small increase in molecular weight. In this spirit, linear oligo-azobenzenes had been synthesized, though their photochemical properties have only been investigated for short chain lengths.

Diels-Alder Reactivity of Triisopropylsilyl Ethynyl Substituted Acenes.

We investigated the Diels-Alder reaction of 6,13-bis(triisopropylsilylethynyl)pentacene (1) with small dienophiles such as (bridged) dihydronaphthalenes/cyclohexenes that yielded adducts at the central ring, the other dienophiles predominantly or exclusively attacked the unsubstituted off-center ring. The difference in regioselectivity was investigated by DFT calculations. Apart from dispersion interactions, it is due to the steric demand of the dienophiles, which need to fit in between the silylethynyl substituents to react at the central ring.

Breaking Strong Alkynyl-Phenyl Bonds: Poly(-phenylene ethynylene)s under Mechanical Stress.

Stronger chemical bonds withstand higher mechanical forces; thus, the rupture of single bonds is preferred over the rupture of double or triple bonds or aromatic rings. We investigated bond scission in poly(dialkyl--phenylene ethynylene)s (PPEs), a fully conjugated polymer. In a scale-bridging approach using electron-paramagnetic resonance spectroscopy and gel permeation chromatography of cryomilled samples, in combination with density functional theory calculations and coarse-grained simulations, we conclude that mechanical force cleaves the sp-sp bond of PPEs (bond dissociation energy as high as 600 kJ mol).

Analytical Gradients for Electron-Attached and Ionized States for the Algebraic-Diagrammatic Construction Scheme for the Electron Propagator up to Third Order.

The derivation and implementation of analytical gradients for methods based on the non-Dyson algebraic diagrammatic construction for the electron propagator, IP-ADC and EA-ADC, up to the third order is presented. Using nuclear gradients, ground-state equilibrium structures for small open-shell systems are calculated. In addition, we investigated the performance of IP/EA-ADC methods for the calculation of adiabatic ionization potentials and electron affinities for medium-sized organic molecules.

Fourth-Order Algebraic Diagrammatic Construction for Electron Detachment and Attachment: The IP- and EA-ADC(4) Methods.

We present a non-Dyson fourth-order algebraic diagrammatic construction formulation of the electron propagator, featuring the distinct IP- and EA-ADC(4) schemes for the treatment of ionization and electron attachment processes. The algebraic expressions have been derived automatically using the intermediate state representation approach and implemented in the Q-Chem quantum-chemical program package. The performance of the novel methods is assessed with respect to high-level reference data for ionization potentials and electron affinities of closed- and open-shell systems.

Azaacene Diradicals Based on Non-Kekulé Meta-Quinodimethane with Large Two-Photon Cross-Sections in the Infrared Spectral Region.

Non-Kekulé quinoidal azaacences m-A (1 a,b) were synthesized and compared to their para- and ortho-quinodimethane analogues. m-A display high diradical characters (1 b: y = 0.88) due to their meta-quinodimethane (m-QDM) topology.

Aggregation of N-Heteropolycyclic Aromatic Molecules: The Acridine Dimer and Trimer.

Polycyclic aromatic hydrocarbons and their nitrogen-substituted analogues are of great interest for various applications in organic electronics. The performance of such devices is determined not only by the properties of the single molecules, but also by the structure of their aggregates, which often form via self-aggregation. Gaining insight into such aggregation processes is a challenging task, but crucial for a fine-tuning of the materials properties.

Dense-sparse quantum Monte Carlo algebraic diagrammatic construction and importance ranking.

Quantum Monte Carlo Algebraic Diagrammatic Construction (QMCADC) has been proposed as a reformulation of the second-order ADC scheme for the polarization propagator within the projection quantum Monte Carlo formalism. Dense-sparse partitioning and importance ranking filtering strategies are now exploited to accelerate its convergence and to alleviate the sign problem inherent in such calculations. By splitting the configuration space into dense and sparse subsets, the corresponding projection operator is decomposed into four distinct blocks.

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, 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.

Efficient and Parallel Implementation of Real and Complex Response Functions Employing the Second-Order Algebraic-Diagrammatic Construction Scheme for the Polarization Propagator.

We present the implementation of an efficient matrix-folded formalism for the evaluation of complex response functions and the calculation of transition properties at the level of the second-order algebraic-diagrammatic construction (ADC(2)) scheme. The underlying algorithms, in combination with the adopted hybrid MPI/OpenMP parallelization strategy, enabled calculations of the UV/vis spectra of a guanine oligomer series ranging up to 1032 contracted basis functions, thereby utilizing vast computational resources from up to 32,768 CPU cores. Further analysis of the convergence behavior of the involved iterative subspace algorithms revealed the superiority of a frequency-separated treatment of response equations even for a large spectral window, including 101 frequencies.

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.

Wavelength Selective Photocontrol of Hybrid Azobenzene-Spiropyran Photoswitches with Overlapping Chromophores.

Compounds with multiple photoswitching units are appealing for complex photochemical control of molecular materials and nanostructures. Herein, we synthesized novel meta- and para- connected (related to the nitrogen of the indoline) azobenzene-spiropyran dyads, in which the central benzene unit is shared by both switches. We investigated their photochemistry using static and time-resolved transient absorption spectroscopy as well as quantum chemical calculations.