Photocontrolled Reversible Amyloid Fibril Formation of Parathyroid Hormone-Derived Peptides.

Peptide fibrillization is crucial in biological processes such as amyloid-related diseases and hormone storage, involving complex transitions between folded, unfolded, and aggregated states. We here employ light to induce reversible transitions between aggregated and nonaggregated states of a peptide, linked to the parathyroid hormone (PTH). The artificial light-switch 3-{[(4-aminomethyl)phenyl]diazenyl}benzoic acid (AMPB) is embedded into a segment of PTH, the peptide PTH25-37, to control aggregation, revealing position-dependent effects. Through in silico design, synthesis, and experimental validation of 11 novel PTH25-37-derived peptides, we predict and confirm the amyloid-forming capabilities of the AMPB-containing peptides. Quantum-chemical studies shed light on the photoswitching mechanism. Solid-state NMR studies suggest that ß-strands are aligned parallel in fibrils of PTH25-37, while in one of the AMPB-containing peptides, ß-strands are antiparallel. Simulations further highlight the significance of π-π interactions in the latter. This multifaceted approach enabled the identification of a peptide that can undergo repeated phototriggered transitions between fibrillated and defibrillated states, as demonstrated by different spectroscopic techniques. With this strategy, we unlock the potential to manipulate PTH to reversibly switch between active and inactive aggregated states, representing the first observation of a photostimulus-responsive hormone.

Functionality optimization for effective singlet fission coupling screening in the full-dimensional molecular and intermolecular coordinate space.

In computational chemistry, accurately predicting molecular configurations that exhibit specific properties remains a critical challenge. Its intricacies become especially evident in the study of molecular aggregates, where the light-induced functionality is tied to highly structure-dependent electronic couplings between molecules. Here, we present an efficient strategy for the targeted screening of the structural space employing a "functionality optimization" technique, in which a chosen descriptor, constrained by the ground state energy expression, is optimized. The chosen algorithmic differentiation (AD) framework allows one to automatically obtain gradients without its tedious implementation. We demonstrate the effectiveness of the approach by identifying perylene bisimide (PBI) dimer motifs with enhanced effective SF coupling. Our findings reveal that certain structural modifications of the PBI monomer, such as helical twisting and bending as well as slipped-rotated packing arrangements, can significantly increase the effective SF coupling.

Impact of isoelectronic substitution on the excited state processes in polycyclic aromatic hydrocarbons: a joint experimental and theoretical study of 4a,8a-azaboranaphthalene.

Substituting CC with the isoelectronic BN units is a promising approach to modify the optoelectronic properties of polycyclic aromatic hydrocarbons. While computational studies have already addressed trends in the electronic structure of the various isosteres, experimental data are still scarce. Here, the excited state spectroscopy and dynamics of 4a,8a-azaboranaphthalene were studied by picosecond time-resolved photoionization in a supersonic jet and analyzed with the aid of XMS-CASPT2 and time-dependent DFT calculations. A resonance-enhanced multiphoton ionization spectrum (REMPI) reveals the S1 origin at  = 33 830 ± 12 cm-1. Several vibrational bands were resolved and assigned by comparison with the computations. A [1+1] photoelectron spectrum via the S1 origin yielded an adiabatic ionization energy of 8.27 eV. Selected vibrational bands were subsequently investigated by pump-probe photoionization. While the origin as well as several low-lying vibronic states exhibit lifetimes in the ns-range, a monoexponential decay is observed at higher excitation energies, ranging from 400 ps at +1710 cm-1 to 13 ps at +3360 cm-1. The deactivation is attributed to an internal conversion of the optically excited S1 state via a barrier that gives access to a conical intersection (CI) to the S0 state. The doping significantly changes the energetic ordering of CIs and lowers the corresponding energy barrier for the associated deactivation pathway, as revealed by nudged elastic band (NEB) calculations.

Perspectives in Dye Chemistry: A Rational Approach toward Functional Materials by Understanding the Aggregate State.

The past 20 years have witnessed a renaissance of dye chemistry, moving from traditional colorant research toward functional materials. Different from traditional colorant research, the properties of functional materials are governed extensively by intermolecular interactions, thereby entailing significant limitations to the classical approach based on molecular structure-molecular property (color, emission, redox properties, etc.) relationships for the respective dye molecules. However, as discussed in this Perspective, such an approach can be pursued for dye aggregates, and in many cases already well-tailored dimers are sufficient to understand the influence of supramolecular organization on the functional properties of ground and photoexcited states. Illustrative examples will be given for exciton coupling and charge-transfer coupling and how these properties relate to desirable functions such as fluorescence, symmetry-breaking charge separation, and singlet fission in molecular aggregates. While the progress in this research so far mostly originated from studies on well-defined folded and self-assembled structures composed of only two dye molecules, future work will have to advance toward larger oligomers of specific size and geometry. Furthermore, future experimental studies should be guided to a larger extent by theoretical predictions that may be supported by machine learning algorithms and new concepts from artificial intelligence. Beyond already pursued calculations of potential energy landscapes, we suggest the development of theoretical approaches that identify the most desirable dye aggregate structures for a particular property on functional energy landscapes.

Endohedral Hydrogen Bonding Templates the Formation of a Highly Strained Covalent Organic Cage Compound*.

A highly strained covalent organic cage compound was synthesized from hexahydroxy tribenzotriquinacene (TBTQ) and a meta-terphenyl-based diboronic acid with an additional benzoic acid substituent in 2'-position. Usually, a 120° bite angle in the unsubstituted ditopic linker favors the formation of a [4+6] cage assembly. Here, the introduction of the benzoic acid group is shown to lead to a perfectly preorganized circular hydrogen-bonding array in the cavity of a trigonal-bipyramidal [2+3] cage, which energetically overcompensates the additional strain energy caused by the larger mismatch in bite angles for the smaller assembly. The strained cage compound was analyzed by mass spectrometry and 1 H, 13 C and DOSY NMR spectroscopy. DFT calculations revealed the energetic contribution of the hydrogen-bonding template to the cage stability. Furthermore, molecular dynamics simulations on early intermediates indicate an additional kinetic effect, as hydrogen bonding also preorganizes and rigidifies small oligomers to facilitate the exclusive formation of smaller and more strained macrocycles and cages.

Energetics and optimal molecular packing for singlet fission in BN-doped perylenes: electronic adiabatic state basis screening.

Singlet fission has the potential to increase the efficiency of photovoltaic devices, but the design of suitable chromophores is notoriously difficult. Both the electronic properties of the monomer and the packing motif in the crystal have a big impact on the singlet fission efficiency. Using perylene as an example, it is shown that doping with boron and nitrogen not only helps to align the energy levels but also shifts the stacking position that is optimal for singlet fission. Among all perylene derivatives doped with one or two BN groups, we identify the most suitable isomer for singlet fission with the help of TD-DFT and CASPT2 calculations. The optimal relative disposition of the two monomer units in a cofacially stacked homodimer is explored using two semiempirical models for the singlet fission rate: The first one is the well-known diabatic frontier orbital model, while the second treats singlet fission as a non-adiabatic transition and approximates the rate as the length squared of the non-adiabatic coupling vector between eigenfunctions of the diabatic Hamiltonian.

A Self-Assembled Unit Comprising 12 Squaraine Dyes Built Up from Two Star-Shaped Hexasquarainyl-Benzene Molecules.

We describe the aggregate formation and optical properties of a star-shaped hexaarylbenzene with six squaraine chromophores (=hexasquarainyl benzene). Comprehensive concentration-dependence studies in acetone/CHCl3 mixtures reveal a strong propensity to form discrete dimeric aggregates with a high binding constant in excess of 106 m-1 . In this context, a large hypsochromic shift of almost 2700 cm-1 was found in the absorption spectrum, indicating H-type exciton coupling. The aggregate band is characterised by a very small band width of only 560 cm-1 , probably caused by exchange narrowing. Both experimental and computational methods were used to elucidate the supramolecular aggregate structure, which is assumed to consist of two stacked hexasquarainyl benzene monomers.

Metadynamics for automatic sampling of quantum property manifolds: exploration of molecular biradicality landscapes.

We present a general extension of the metadynamics allowing for an automatic sampling of quantum property manifolds (ASQPM) giving rise to functional landscapes that are analogous to the potential energy surfaces in the frame of the Born-Oppenheimer approximation. For this purpose, we employ generalized electronic collective variables to carry out biased molecular dynamics simulations in the framework of quantum chemical methods that explore the desired property manifold. We illustrate our method on the example of the "biradicality landscapes", which we explore by introducing the natural orbital occupation numbers (NOONs) as the electronic collective variable driving the dynamics. We demonstrate the applicability of the method on the simulation of p-xylylene and [8]annulene allowing to automatically extract the biradical geometries. In the case of [8]annulene the ASQPM metadynamics leads to the prediction of biradical scaffolds that can be stabilized by a suitable chemical substitution, leading to the design of novel functional molecules exhibiting biradical functionality.

Dynamic exciton localisation in a pyrene-BODIPY-pyrene dye conjugate.

The photophysics of a molecular triad consisting of a BODIPY dye and two pyrene chromophores attached in 2-position are investigated by steady state and fs-time resolved transient absorption spectroscopy as well as by field induced surface hopping (FISH) simulations. While the steady state measurements indicate moderate chromophore interactions within the triad, the time resolved measurements show upon pyrene excitation a delocalised excited state which localises onto the BODIPY chromophore with a time constant of 0.12 ps. This could either be interpreted as an internal conversion process within the excitonically coupled chromophores or as an energy transfer from the pyrenes to the BODIPY dye. The analysis of FISH-trajectories reveals an oscillatory behaviour where the excitation hops between the pyrene units and the BODIPY dye several times until finally they become localised on the BODIPY chromophore within 100 fs. This is accompanied by an ultrafast nonradiative relaxation within the excitonic manifold mediated by the nonadiabatic coupling. Averaging over an ensemble of trajectories allowed us to simulate the electronic state population dynamics and determine the time constants for the nonradiative transitions that mediate the ultrafast energy transfer and exciton localisation on BODIPY.

The mechanism of excimer formation: an experimental and theoretical study on the pyrene dimer.

The understanding of excimer formation in organic materials is of fundamental importance, since excimers profoundly influence their functional performance in applications such as light-harvesting, photovoltaics or organic electronics. We present a joint experimental and theoretical study of the ultrafast dynamics of excimer formation in the pyrene dimer in a supersonic jet, which is the archetype of an excimer forming system. We perform simulations of the nonadiabatic photodynamics in the frame of TDDFT that reveal two distinct excimer formation pathways in the gas-phase dimer. The first pathway involves local excited state relaxation close to the initial Franck-Condon geometry that is characterized by a strong excitation of the stacking coordinate exhibiting damped oscillations with a period of 350 fs that persist for several picoseconds. The second excimer forming pathway involves large amplitude oscillations along the parallel shift coordinate with a period of ≈900 fs that after intramolecular vibrational energy redistribution leads to the formation of a perfectly stacked dimer. The electronic relaxation within the excitonic manifold is mediated by the presence of intermolecular conical intersections formed between fully delocalized excitonic states. Such conical intersections may generally arise in stacked π-conjugated aggregates due to the interplay between the long-range and short-range electronic coupling. The simulations are supported by picosecond photoionization experiments in a supersonic jet that provide a time-constant for the excimer formation of around 6-7 ps, in good agreement with theory. Finally, in order to explore how the crystal environment influences the excimer formation dynamics we perform large scale QM/MM nonadiabatic dynamics simulations on a pyrene crystal in the framework of the long-range corrected tight-binding TDDFT. In contrast to the isolated dimer, the excimer formation in the crystal follows a single reaction pathway in which the initially excited parallel slip motion is strongly damped by the interaction with the surrounding molecules leading to the slow excimer stabilization on a picosecond time scale.

A Crystalline π-Stack Containing Five Stereoisomers: Insights into Conformational Isomorphism, Chirality Inversion, and Disorder.

An unprecedented crystal-packing arrangement of a tetramethoxy-bay-substituted perylene bisimide (PBI) consists of three crystallographically independent molecules, that is, an achiral (AC) PBI of saddle-shaped geometry along with two pairs of propeller-like twisted (P)- and (M)-enantiomeric PBI frameworks. All these five conformations are observed within a single π-stack revealing an intriguing packing sequence with an inversion of chirality from P to M via AC. Nudged elastic band calculations for the isolated molecule show that AC is a local minimum of the P to M interconversion path. In addition, two minor conformations were observed in the crystal, one of which resembles a transition-state molecule. Theoretical studies of dimeric and trimeric stacks reveal that the coexistence of all these structures in the crystal lattice is aided by the strong dispersion interactions between PBI cores and perfectly interdigitated dodecyl chains which stabilize energetically higher conformations.

Vibrationally resolved optical spectra and ultrafast electronic relaxation dynamics of diamantane.

We present theoretical simulations of the vibrationally resolved photoabsorption and photoemission spectra of diamantane combined with nonadiabatic dynamics simulations in order to identify the state responsible for the measured photoluminescence of diamantane and to determine the mechanism and the time-scales of the electronic state relaxation. Diamantane is a prototype representative of the diamondoid class of hydrocarbons which have recently gained significant interest due to their unique electronic properties. This molecule is characterised by an almost dark first excited state, which therefore cannot be directly excited. Moreover, the calculated vertical transition from the geometrically relaxed first excited state to the ground state also bears no intensity. However, recent experiments suggest that the observed photoluminescence originates from the lowest excited state. We have performed spectral simulations in the frame of the Herzberg-Teller approximation for vibronic transitions, which goes beyond the Franck-Condon approximation of constant transition dipole moments and takes into account their linear dependence on the geometrical deformations. In this way, the available experimental spectrum could be fully reproduced, resolving the issue about the origin of the photoluminescence. Moreover, the photoemission from the first excited state also implies that ultrafast nonradiative processes have to take place after the initial excitation of the bright electronic states. We have determined the mechanism and time-scales of these relaxation processes by performing nonadiabatic dynamics simulations in the manifold of s- and p-type Rydberg excited states. The simulations demonstrate that the lowest excited electronic state of diamantane gains significant population from higher-lying states already after several hundreds of femtoseconds. Thus, our dynamics simulations combined with spectra calculated using the Herzberg-Teller approximation allow us to fully explain the observed photoabsorption and photoemission properties of diamantane.

Excitonic Properties of Ordered Metal Nanocluster Arrays: 2D Silver Clusters at Multiporphyrin Templates.

The design of ordered arrays of metal nanoclusters such as, for example, two-dimensional cluster-organic frameworks might open a new route toward the development of materials with tailored optical properties. Such systems could serve as plasmonically enhanced light-harvesting materials, sensors, or catalysts. We present here a theoretical approach for the simulation of the optical properties of ordered arrays of metal clusters that is based on the ab initio parametrized Frenkel exciton model. We demonstrate that small atomically precise silver clusters can be assembled in one- and two-dimensional arrays on suitably designed porphyrin templates exhibiting remarkable optical properties. By employing explicit time-dependent density functional theory calculations on smaller homologues, we show that the intrinsic optical properties of metal clusters are largely preserved but undergo J- and H-type excitonic coupling that results in controllable splitting of their excited states. Furthermore, ab initio parametrized Frenkel exciton model calculations allow us to predict an energetic splitting of up to 0.77 eV in extended two-dimensional square arrays and 0.79 eV in tilted square aggregates containing up to 25 cluster-porphyrin subunits.

Energy Transfer Between Squaraine Polymer Sections: From Helix to Zigzag and All the Way Back.

We provide a joint experimental and theoretical study of squaraine polymers in solution. The absorption spectra show evidence that two different conformations are present in the polymer: a helix and a zigzag structure. This unique situation allows investigating ultrafast energy-transfer processes between different structural segments within a single polymer chain in solution. The understanding of the underlying dynamics is of fundamental importance for the development of novel materials for light-harvesting and optoelectronic applications. Here, we combine femtosecond transient absorption spectroscopy with time-resolved 2D electronic spectroscopy in order to demonstrate that ultrafast energy transfer within the squaraine polymer chains proceeds from initially excited helix segments to zigzag segments or vice versa, depending on the solvent as well as on the excitation wavenumber. These observations contrast other conjugated polymers such as MEH-PPV where much slower intrachain energy transfer was reported. The reason for the very fast energy transfer in squaraine polymers is most likely a close matching of the density of states between donor and acceptor polymer segments because of the very small reorganization energy in these cyanine-like chromophores.

Site-dependence of van der Waals interaction explains exciton spectra of double-walled tubular J-aggregates.

The simulation of the optical properties of supramolecular aggregates requires the development of methods, which are able to treat a large number of coupled chromophores interacting with the environment. Since it is currently not possible to treat large systems by quantum chemistry, the Frenkel exciton model is a valuable alternative. In this work we show how the Frenkel exciton model can be extended in order to explain the excitonic spectra of a specific double-walled tubular dye aggregate explicitly taking into account dispersive energy shifts of ground and excited states due to van der Waals interaction with all surrounding molecules. The experimentally observed splitting is well explained by the site-dependent energy shift of molecules placed at the inner or outer side of the double-walled tube, respectively. Therefore we can conclude that inclusion of the site-dependent dispersive effect in the theoretical description of optical properties of nanoscaled dye aggregates is mandatory.

Laser-induced fluorescence of free diamondoid molecules.

We observe the fluorescence of pristine diamondoids in the gas phase, excited using narrow band ultraviolet laser light. The emission spectra show well-defined features, which can be attributed to transitions from the excited electronic state into different vibrational modes of the electronic ground state. We assign the normal modes responsible for the vibrational bands, and determine the geometry of the excited states. Calculations indicate that for large diamondoids, the spectral bands do not result from progressions of single modes, but rather from combination bands composed of a large number of Δv = 1 transitions. The vibrational modes determining the spectral envelope can mainly be assigned to wagging and twisting modes of the surface atoms. We conclude that our theoretical approach accurately describes the photophysics in diamondoids and possibly other hydrocarbons in general.

Nonlinear absorption dynamics using field-induced surface hopping: zinc porphyrin in water.

We wish to present the application of our field-induced surface-hopping (FISH) method to simulate nonlinear absorption dynamics induced by strong nonresonant laser fields. We provide a systematic comparison of the FISH approach with exact quantum dynamics simulations on a multistate model system and demonstrate that FISH allows for accurate simulations of nonlinear excitation processes including multiphoton electronic transitions. In particular, two different approaches for simulating two-photon transitions are compared. The first approach is essentially exact and involves the solution of the time-dependent Schrödinger equation in an extended manifold of excited states, while in the second one only transiently populated nonessential states are replaced by an effective quadratic coupling term, and dynamics is performed in a considerably smaller manifold of states. We illustrate the applicability of our method to complex molecular systems by simulating the linear and nonlinear laser-driven dynamics in zinc (Zn) porphyrin in the gas phase and in water. For this purpose, the FISH approach is connected with the quantum mechanical-molecular mechanical approach (QM/MM) which is generally applicable to large classes of complex systems. Our findings that multiphoton absorption and dynamics increase the population of higher excited states of Zn porphyrin in the nonlinear regime, in particular in solution, provides a means for manipulating excited-state properties, such as transient absorption dynamics and electronic relaxation.

Steering the multiexciton generation in slip-stacked perylene dye array via exciton coupling.

Dye arrays from dimers up to larger oligomers constitute the functional units of natural light harvesting systems as well as organic photonic and photovoltaic materials. Whilst in the past decades many photophysical studies were devoted to molecular dimers for deriving structure-property relationship to unravel the design principles for ideal optoelectronic materials, they fail to accomplish the subsequent processes of charge carrier generation or the detachment of two triplet species in singlet fission (SF). Here, we present a slip-stacked perylene bisimide trimer, which constitutes a bridge between hitherto studied dimer and solid-state materials, to investigate SF mechanisms. This work showcases multiple pathways towards the multiexciton state through direct or excimer-mediated mechanisms by depending upon interchromophoric interaction. These results suggest the comprehensive role of the exciton coupling, exciton delocalization, and excimer state to facilitate the SF process. In this regard, our observations expand the fundamental understanding the structure-property relationship in dye arrays.

Tuning phenoxyl-substituted diketopyrrolopyrroles from quinoidal to biradical ground states through (hetero-)aromatic linkers.

Strongly fluorescent halochromic 2,6-di-tert-butyl-phenol-functionalised phenyl-, thienyl- and furyl-substituted diketopyrrolopyrrole (DPP) dyes were deprotonated and oxidised to give either phenylene-linked DPP1˙˙ biradical (y 0 = 0.75) with a singlet open shell ground state and a thermally populated triplet state (ΔE ST = 19 meV; 1.8 kJ mol-1; 0.43 kcal mol-1) or thienylene/furylene-linked DPP2q and DPP3q compounds with closed shell quinoidal ground states. Accordingly, we identified the aromaticity of the conjugated (hetero-)aromatic bridge to be key for modulating the electronic character of these biradicaloid compounds and achieved a spin crossover from closed shell quinones DPP2q and DPP3q to open shell biradical DPP1˙˙ as confirmed by optical and magnetic spectroscopic studies (UV/vis/NIR, NMR, EPR) as well as computational investigations (spin-flip TD-DFT calculations in combination with CASSCF(4,4) and harmonic oscillator model of aromaticity (HOMA) analysis). Spectroelectrochemical studies and comproportionation experiments further prove the reversible formation of mixed-valent radical anions for the DPP2q and DPP3q quinoidal compounds with absorption bands edging into the NIR spectral region.

Impact of substituents on molecular properties and catalytic activities of trinuclear Ru macrocycles in water oxidation.

Herein we report a broad series of new trinuclear supramolecular Ru(bda) macrocycles bearing different substituents at the axial or equatorial ligands which enabled investigation of substituent effects on the catalytic activities in chemical and photocatalytic water oxidation. Our detailed investigations revealed that the activities of these functionalized macrocycles in water oxidation are significantly affected by the position at which the substituents were introduced. Interestingly, this effect could not be explained based on the redox properties of the catalysts since these are not markedly influenced by the functionalization of the ligands. Instead, detailed investigations by X-ray crystal structure analysis and theoretical simulations showed that conformational changes imparted by the substituents are responsible for the variation of catalytic activities of the Ru macrocycles. For the first time, macrocyclic structure of this class of water oxidation catalysts is unequivocally confirmed and experimental indication for a hydrogen-bonded water network present in the cavity of the macrocycles is provided by crystal structure analysis. We ascribe the high catalytic efficiency of our Ru(bda) macrocycles to cooperative proton abstractions facilitated by such a network of preorganized water molecules in their cavity, which is reminiscent of catalytic activities of enzymes at active sites.