arXiv subjects

Merle I. S. Röhr

Publications and source records attributed to Merle I. S. Röhr.

8 recordsLinked to original sources

arXiv2026

Hidden excimer formation in the gas-phase photodynamics of a BN-doped phenanthrene

Replacing CC units by isoelectronic BN motifs provides a powerful strategy to tune the electronic structure and excited-state chemistry of polycyclic aromatic hydrocarbons (PAHs). Here, we combine multiphoton ionization spectroscopy, time-resolved photoelectron imaging, ion velocity-map imaging, and quantum-chemical calculations to disentangle the monomer and dimer photophysics of 4a,4b-azaboraphenanthrene. The monomer exhibits a structured S$_1 \leftarrow$ S$_0$ spectrum with an origin at $22880 \pm 15\,\mathrm{cm}^{-1}$, corresponding to $2.837 \,\mathrm{eV}$, and pronounced activity in low-wavenumber deformation modes. Photoelectron spectroscopy yields an adiabatic ionization energy of $7.18 \pm 0.02\,\mathrm{eV}$. While the structured spectrum, high fluorescence quantum yield, small computed geometry changes, and weak spin-orbit couplings all point to a long-lived monomer S$_1$ state, time-resolved photoelectron images reveal an additional picosecond component. Ion imaging shows that this component originates from dissociative ionization of the molecular dimer, which projects dimer excited-state dynamics into the monomer mass channel. Computations identify the initially excited dimer state as a bright H-aggregate-like exciton, followed by ultrafast S$_2 \rightarrow$ S$_1$ internal conversion and subsequent structural relaxation toward an excimeric S$_1$ minimum. The experimentally observed $\approx 15\,\mathrm{ps}$ time constant is therefore assigned to excimer formation in the neutral dimer.

physics.chem-ph↗

arXiv2026

Asymmetric Planar-to-Dewar Isomerisation in BN-Doped Naphthalene: Mechanistic Implications for Molecular Solar Thermal Storage

The planar to Dewar valence isomerisation of 4a,8a-azaboranaphthalene (BN$_\text{Naph}$), a $π$ extended BN-doped analogue of azaborine, is investigated to evaluate how BN incorporation reshapes the minimum energy pathway on the ground state. This process is, for example, relevant in the context of molecular solar thermal (MOST) energy storage, where absorbed sunlight is converted into chemical energy through reversible photoisomerisation. Structures and vertical excitations were computed using DFT and TD-DFT, minimum energy pathways were mapped with nudged elastic band (NEB) calculations, and pathway energetics were refined with state averaged XMS-CASPT2. In addition, azaborine was examined as a comparison system, with particular emphasis on whether substituents at nitrogen and boron promote Dewar formation. The effect of BN doping on the system was analysed in detail. Compared with the carbon analogue, the conversion pathway becomes asymmetric with a metastable intermediate stabilized by a transient boron to carbon contact. The transition structure closely resembles an S$_0$/S$_1$ conical intersection, which is consistent with a vibrationally activated nonradiative funnel. For tuning MOST properties, screening of single substituents across the whole molecule reveals predominantly red shifted S$_1$ energies together with increased oscillator strengths and indicates that appropriate substitution can improve Dewar formation in azaborine derivatives.

physics.chem-ph↗

arXiv2026

Fragment-Based Configuration Interaction: Towards a Unifying Description of Biexcitonic Processes in Molecular Aggregates

Biexcitonic states govern singlet fission, triplet-triplet and exciton-exciton annihilation, yet a unified understanding of how these processes compete within a shared electronic manifold remains elusive. We outline a conceptual framework based on fragment-based configuration-interaction that systematically constructs diabatic Hamiltonians spanning the full one-particle (LE, CT) and two-particle (LELE, CTCT, TT, CTX with X = LE, CT, or T) manifolds from monomer-local building blocks, preserving physical interpretability throughout. SymbolicCI provides analytic Hamiltonian matrix elements for efficient large-scale calculations; NOCI-F delivers benchmark-quality couplings. The resulting diabatic Hamiltonians can be coupled to quantum dynamics simulations. Applications to ethylene aggregates and the anthracene crystal reveal CTX configurations as electronic gateways bridging excitonic manifolds, with CT-mediated relaxation pathways competing with conventional annihilation. In H-type aggregates, LECT admixture stabilizes a "bi-excimer" analogous to one-particle excimers. By providing first-principles access to biexciton formation, separation, and transport, we hope to stimulate further exchange between electronic structure and quantum dynamics communities toward a predictive understanding of multiexcitonic photophysics.

physics.chem-ph↗

arXiv2024

Functionality Optimization for Singlet Fission Rate 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 to automatically obtain gradients without its tedious implementation. We demonstrate the effectiveness of the approach by identifying Perylene Bisiimide (PBI) dimer motifs with enhanced SF rate. 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 SF rate.

physics.comp-ph↗

arXiv2023

Formation Pathways of the Spin-Correlated, Spatially Separated $^{1}$(T...T) State in the Singlet Fission Process of Perylene Diimide Stacks

For the theoretical screening of Singlet Fission (SF) rates in molecular aggregates, commonly dimer model systems are employed. However, there is experimental evidence, that the SF process proceeds from the $^{1}$(TT) state via an triplet-triplet energy transfer process to a further intermediate: a $^{1}$(T...T) state with two non-adjacent, spin-correlated triplets, which cannot be captured by the dimer models. In this work, we extend Michl's diabatic frontier orbital model to trimer systems, for which we automatically generate the diabatic two and three-center couplings using symbolic algebra. We apply this method to study the packing dependence of the $^{1}$(T...T) formation in the perylene diimide (PDI) trimer stack. We find that efficient triplet-triplet energy transfer is facilitated by structural motifs for which also significant excimer character can be observed. Furthermore, the coupling shows a local maximum for the structural motif that has been assigned to efficient $^{1}$(TT) population. Employing second order perturbation theory, we study the interference of the individual electronic pathways that arise in the PDI trimer system, allowing us to reproduce the packing dependence of the SF rates derived from the Redfield simulations in the work by Mirjani et. al. (Phys. Chem. C 2014, 118, 26, 14192-14199).

physics.chem-ph↗

arXiv2019

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 \textit{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.

physics.chem-ph↗

arXiv2018

Multistate metadynamics for automatic exploration of conical intersections

We introduce multistate metadynamics for automatic exploration of conical intersection seams between adiabatic Born-Oppenheimer potential energy surfaces in molecular systems. By choosing the energy gap between the electronic states as a collective variable the metadynamics drives the system from an arbitrary ground-state configuration toward the intersection seam. Upon reaching the seam, the multistate electronic Hamiltonian is extended by introducing biasing potentials into the off-diagonal elements, and the molecular dynamics is continued on a modified potential energy surface obtained by diagonalization of the latter. The off-diagonal bias serves to locally open the energy gap and push the system to the next intersection point. In this way, the conical intersection energy landscape can be explored, identifying minimum energy crossing points and the barriers separating them. We illustrate the method on the example of furan, a prototype organic molecule exhibiting rich photophysics. The multistate metadynamics reveals plateaus on the conical intersection energy landscape from which the minimum energy crossing points with characteristic geometries can be extracted. The method can be combined with the broad spectrum of electronic structure methods and represents a generally applicable tool for the exploration of photophysics and photochemistry in complex molecules and materials.

physics.chem-ph↗

arXiv2014

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.

physics.chem-ph↗