From Organic Fragments to Photoswitchable Catalysts: The OFF-ON Structural Repository for Transferable Kernel-Based Potentials.

Structurally and conformationally diverse databases are needed to train accurate neural networks or kernel-based potentials capable of exploring the complex free energy landscape of flexible functional organic molecules. Curating such databases for species beyond "simple" drug-like compounds or molecules composed of well-defined building blocks (e.g., peptides) is challenging as it requires thorough chemical space mapping and evaluation of both chemical and conformational diversities. Here, we introduce the OFF-ON (organic fragments from organocatalysts that are non-modular) database, a repository of 7869 equilibrium and 67,457 nonequilibrium geometries of organic compounds and dimers aimed at describing conformationally flexible functional organic molecules, with an emphasis on photoswitchable organocatalysts. The relevance of this database is then demonstrated by training a local kernel regression model on a low-cost semiempirical baseline and comparing it with a PBE0-D3 reference for several known catalysts, notably the free energy surfaces of exemplary photoswitchable organocatalysts. Our results demonstrate that the OFF-ON data set offers reliable predictions for simulating the conformational behavior of virtually any (photoswitchable) organocatalyst or organic compound composed of H, C, N, O, F, and S atoms, thereby opening a computationally feasible route to explore complex free energy surfaces in order to rationalize and predict catalytic behavior.

Data-Driven Discovery of Organic Electronic Materials Enabled by Hybrid Top-Down/Bottom-Up Design.

The high-throughput exploration and screening of molecules for organic electronics involves either a 'top-down' curation and mining of existing repositories, or a 'bottom-up' assembly of user-defined fragments based on known synthetic templates. Both are time-consuming approaches requiring significant resources to compute electronic properties accurately. Here, 'top-down' is combined with 'bottom-up' through automatic assembly and statistical models, thus providing a platform for the fragment-based discovery of organic electronic materials. This study generates a top-down set of 117K synthesized molecules containing structures, electronic and topological properties and chemical composition, and uses them as building blocks for bottom-up design. A tool is developed to automate the coupling of these building blocks at their C(sp2/sp)-H bonds, providing a fundamental link between the two dataset construction philosophies. Statistical models are trained on this dataset and a subset of resulting top-down/bottom-up compounds, enabling on-the-fly prediction of ground and excited state properties with high accuracy across organic compound space. With access to ab initio-quality optical properties, this bottom-up pipeline may be applied to any materials design campaign using existing compounds as building blocks. To illustrate this, over a million molecules are screened for singlet fission. tThe leading candidates provide insight into the features promoting this multiexciton-generating process.

OSCAR: an extensive repository of chemically and functionally diverse organocatalysts.

The automated construction of datasets has become increasingly relevant in computational chemistry. While transition-metal catalysis has greatly benefitted from bottom-up or top-down strategies for the curation of organometallic complexes libraries, the field of organocatalysis is mostly dominated by case-by-case studies, with a lack of transferable data-driven tools that facilitate both the exploration of a wider range of catalyst space and the optimization of reaction properties. For these reasons, we introduce OSCAR, a repository of 4000 experimentally derived organocatalysts along with their corresponding building blocks and combinatorially enriched structures. We outline the fragment-based approach used for database generation and showcase the chemical diversity, in terms of functions and molecular properties, covered in OSCAR. The structures and corresponding stereoelectronic properties are publicly available (https://archive.materialscloud.org/record/2022.106) and constitute the starting point to build generative and predictive models for organocatalyst performance.

Heteroatom oxidation controls singlet-triplet energy splitting in singlet fission building blocks.

Singlet fission (SF) is a promising multiexciton-generating process. Its demanding energy splitting criterion - that the S1 energy must be at least twice that of T1 - has limited the range of materials capable of SF. We propose heteroatom oxidation as a robust strategy to achieve sufficient S1/T1 splitting, and demonstrate the potential of this approach for intramolecular SF.

Learning the Exciton Properties of Azo-dyes.

The ab initio determination of electronic excited state (ES) properties is the cornerstone of theoretical photochemistry. Yet, traditional ES methods become impractical when applied to fairly large molecules, or when used on thousands of systems. Machine learning (ML) techniques have demonstrated their accuracy at retrieving ES properties of large molecular databases at a reduced computational cost. For these applications, nonlinear algorithms tend to be specialized in targeting individual properties. Learning fundamental quantum objects potentially represents a more efficient, yet complex, alternative as a variety of molecular properties could be extracted through postprocessing. Herein, we report a general framework able to learn three fundamental objects: the hole and particle densities, as well as the transition density. We demonstrate the advantages of targeting those outputs and apply our predictions to obtain properties, including the state character and the exciton topological descriptors, for the two bands (nπ* and ππ*) of 3427 azoheteroarene photoswitches.

Tuning the Thermal Stability and Photoisomerization of Azoheteroarenes through Macrocycle Strain*.

Azobenzene and its derivatives are one of the most widespread molecular scaffolds used in a range of modern applications, as well as in fundamental research. After photoexcitation, azo-based photoswitches revert back to the most stable isomer on a timescale ( t 1 / 2 ) that determines the range of potential applications. Attempts to bring t 1 / 2 to extreme values prompted the development of azobenzene and azoheteroarene derivatives that either rebalance the E- and Z-isomer stabilities, or exploit unconventional thermal isomerization mechanisms. In the former case, one successful strategy has been the creation of macrocycle strain, which tends to impact the E/Z stability asymmetrically, and thus significantly modify t 1 / 2 . On the bright side, bridged derivatives have shown an improved optical switching owing to the higher quantum yields and absence of degradation. However, in most (if not all) cases, bridged derivatives display a reversed thermal stability (more stable Z-isomer), and smaller t 1 / 2 than the acyclic counterparts, which restricts their potential interest to applications requiring a fast forward and backwards switch. In this paper, the impact of alkyl bridges on the thermal stability of phenyl-azoheteroarenes is investigated by using computational methods, and it is revealed that it is indeed possible to combine such improved photoswitching characteristics while preserving the regular thermal stability (more stable E-isomer), and increased t 1 / 2 values under the appropriate connectivity and bridge length.

The Photoisomerization Pathway(s) of Push-Pull Phenylazoheteroarenes*.

Azoheteroarenes are the most recent derivatives targeted to further improve the properties of azo-based photoswitches. Their light-induced mechanism for trans-cis isomerization is assumed to be very similar to that of the parent azobenzene. As such, they inherited the controversy about the dominant isomerization pathway (rotation vs. inversion) depending on the excited state (nπ* vs. ππ*). Although the controversy seems settled in azobenzene, the extent to which the same conclusions apply to the more structurally diverse family of azoheteroarenes is unclear. Here, by means of non-adiabatic molecular dynamics, the photoisomerization mechanism of three prototypical phenyl-azoheteroarenes with increasing push-pull character is unraveled. The evolution of the rotational and inversion conical intersection energies, the preferred pathway, and the associated kinetics upon both nπ* and ππ* excitations can be linked directly with the push-pull substitution effects. Overall, the working conditions of this family of azo-dyes is clarified and a possibility to exploit push-pull substituents to tune their photoisomerization mechanism is identified, with potential impact on their quantum yield.

Thermal spin crossover in Fe(ii) and Fe(iii). Accurate spin state energetics at the solid state.

The thermal spin crossover (SCO) phenomenon refers to an entropy-driven spin transition in some materials based on d6-d9 transition metal complexes. While its molecular origin is well known, intricate SCO behaviours are increasingly common, in which the spin transition occurs concomitantly to e.g. phase transformations, solvent absorption/desorption, or order-disorder processes. The computational modelling of such cases is challenging, as it requires accurate spin state energies in the solid state. Density Functional Theory (DFT) is the best framework, but most DFT functionals are unable to balance the spin state energies. While a few hybrid functionals perform better, they are still too expensive for solid-state minima searches in moderate-size systems. The best alternative is to dress cheap local (LDA) or semi-local (GGA) DFT functionals with a Hubbard-type correction (DFT+U). However, the parametrization of U is not straightforward due to the lack of reference values, and because ab initio parametrization methods perform poorly. Moreover, SCO complexes undergo notable structural changes upon transition, so intra- and inter-molecular interactions might play an important role in stabilizing either spin state. As a consequence, the U parameter depends strongly on the dispersion correction scheme that is used. In this paper, we parametrize U for nine reported SCO compounds (five based on FeII, 1-5 and four based on FeIII, 6-9) when using the D3 and D3-BJ dispersion corrections. We analyze the impact of the dispersion correction treatments on the SCO energetics, structure, and the unit cell dimensions. The average U values are different for each type of metal ion (FeIIvs. FeIII), and dispersion correction scheme (D3 vs. D3-BJ) but they all show excellent transferability, with mean absolute errors (MAE) below chemical accuracy (i.e. MAE <4 kJ mol-1). This enables a better description of SCO processes and, more generally, of spin state energetics, in materials containing FeII and FeIII ions.

Bi-stable spin-crossover in charge-neutral [Fe(R-ptp)2] (ptp = 2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine) complexes.

Bi-stable charge-neutral iron(ii) spin-crossover (SCO) complexes are a class of switchable molecular materials proposed for molecule-based switching and memory applications. In this study, we report on the SCO behavior of a series of iron(ii) complexes composed of rationally designed 2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine (ptp) ligands. The powder forms of [Fe2+(R-ptp-)2]0 complexes tethered with less-bulky substituents-R = H (1), R = CH2OH (2), and R = COOCH3 (3; previously reported)-at the 4-position of the pyridine ring of the ptp skeleton showed abrupt and hysteretic SCO at or above room temperature (RT), whereas complex 5 featuring a bulky pyrene substituent showed incomplete and gradual SCO behavior. The role of intermolecular interactions, lattice solvent, and electronic nature of the chemical substituents (R) in tuning the SCO of the complexes is elucidated.

Exploring chemical space in the search for improved azoheteroarene-based photoswitches.

In quest of improved photo switches, azoheteroarenes have emerged as a potential alternative to azobenzenes. However, to date the number and types of these species that have been subjected to study are insufficient to provide an in-depth understanding of the photochemical effects brought about by different substituents. Here, we computationally screen the optical properties and thermal stabilities of 512 azoheteroarenes that consist of eight different N-containing heteroarenes combined with 64 substitution patterns. The most promising compounds are identified and their properties rationalized based on the nature of the azoheteroarene core and the location and type of substitution patterns.

Revising the common understanding of metamagnetism in the molecule-based bisdithiazolyl BDTMe compound.

The BDTMe molecule-based material is the first example of a thiazyl radical to exhibit metamagnetic behavior. Contrary to the common idea that metamagnetism occurs in low-dimensional systems, it is found that BDTMe magnetic topology consists of a complex 3D network of almost isotropic ferromagnetic spin-ladders that are coupled ferromagnetically and further connected by some weaker antiferromagnetic interactions. Calculated magnetic susceptibility χT(T) data is in agreement with experiment. Calculated M(H) data clearly show the typical sigmoidal shape of a metamagnet at temperatures below 2 K. The calculated critical field becomes more apparent in the dM/dH(H) plot, being in very good agreement with experiment. Our computational study concludes that the magnetic topology of BDTMe is preserved throughout the entire experimental range of temperatures (0-100 K). Accordingly, the ground state is the same irrespective of the temperature at which we study the BDTMe crystal. Revising the commonly accepted understanding of a metamagnet explained as ground state changing from antiferromagnetic to ferromagnetic, the Boltzmann population of the different states is here suggested to be the key concept: at 2 K the ground singlet state has more weight (24%) than at 10 K (1.5%), where excited states have an important role. Changes in the antiferromagnetic interactions that couple the ferromagnetic skeleton of BDTMe will directly affect the population of the distinct states that belong to a given magnetic topology and thus its magnetic response. Accordingly, this strategy could be valid for a wide range of bisdithiazolyl BDT-compounds whose magnetism can be tuned by means of weak antiferromagnetic interactions.

Bi-stable spin-crossover characteristics of a highly distorted [Fe(1-BPP-COOC2H5)2](ClO4)2·CH3CN complex.

A highly distorted high spin Fe(ii)-complex, [Fe(1-BPP-COOC2H5)2](ClO4)2·CH3CN, with a trans-N(pyridine)-Fe-N(pyridine) angle (φ) of 158.83(17)° showed lattice solvent dependent bi-stable spin-state switching characteristics with T1/2 = ca. 233 K and a high thermal hysteresis width (ΔT) of 101 K, for the first cooling and heating cycle, unprecedented for the [Fe(BPP)2]2+ series of complexes; the results presented in this study are fundamentally important and have implications towards the realization of device architectures based on bi-stable SCO complexes.

Deciphering crystal packing effects in the spin crossover of six [FeII(2-pic)3]Cl2 solvatomorphs.

In this work we investigate how the crystal packing affects the spin crossover (SCO) of [FeII(2-pic)3]Cl2. Four alcohol solvatomorphs of this compound have been reported to present a transition from a singlet low spin (LS, S = 0) to a quintet high spin (HS, S = 5) state at different temperatures, whereas two other solvatomorphs remain in the HS state along the entire range of temperatures explored. Given that all these solvatomorphs are isostructural, this behavior hints at crystal packing effects playing an important role in the spin transition. With the aim of deciphering their origin, we performed periodic DFT+U+D2 computations on the crystal lattices of the six solvatomorphs to quantify the importance of all possible molecular and intermolecular contributions to their spin-state energetics. We demonstrate that the spin crossover of the alcohol solvatomorphs of [FeII(2-pic)3]Cl2 (1) is strongly influenced by the change of intermolecular interactions. We prove that isolated molecules of 1 would not undergo SCO, whereas intermolecular interactions are crucial to allow the spin transition in the solid state. The key interactions are those between (i) different SCO molecules (stabilizing the LS) and (ii) between SCO molecules and counterions (stabilizing the HS). Moreover, we show that these contributions are proportional to the crystal expansion associated with the phase change, in a way that larger volume expansions imply larger changes in these contributions, with the overall effect of stabilizing the LS state of the material. Finally, we discuss how our results challenge the common perspective that associates the cooperativity of a spin transition with the number- and strength- of intermolecular contacts in the crystal.

Pairing-up viologen cations and dications: a microscopic investigation of van der Waals interactions.

The microscopic origin of van der Waals- and magnetic-interactions in 4,4' methyl viologen cation-based units (MV+˙ and MV2+) was inspected using wave function (variational DDCI and perturbative MP2, CASPT2) and density functional theory (DFT) calculations. The analysis deepens the comprehension of the magnetic behavior of experimental bis-viologen cyclophanes ([CYC]2(+˙)), in which the MV+˙ units are connected through alkyl linkers of different lengths. The formation of the so-called long-multicenter bonds in such radical dimers, responsible for the quenching of the magnetic response, was analyzed in [MV2]2(+˙). Dynamical correlation effects, accessible from second-order perturbation corrections, were decisive in observing a bonding regime characterized by an equilibrium distance of 3.3 Šand a 45 kJ mol-1 dissociation energy. At larger intermolecular distances, our calculations on [MV2]2(+˙) indicate that the singlet and triplet states are energetically competing (i.e. weak exchange interactions, JAB). Despite the absence of any clear bonding regime at the MP2 level, the puzzling association of two di-cations into [MV2]4+ is anticipated at 3.3 Šusing weakly screened point charges (ε = 1.5) to account for the Coulomb interactions between the solvated subunits. The main conclusion is that both dispersion interactions and environment effects are required to overcome the Coulomb repulsion associated with doubly-charged species. All these data provide some complementary insights into the nature and amplitude of interactions between cation and dication units, and their relevance in various experimental manifestations.

A Spin-Crossover Molecular Material Describing Four Distinct Thermal Pathways.

Spin-crossover (SCO) molecular solids are valued switchable materials for their common abrupt and reversible thermal transitions, large thermal hysteresis, or guest-dependent effects. These properties usually involve crystallographic transitions coupled to the SCO events. These phenomena are of great value for the understanding of solid-state transformations and also for exploiting them. We present here a lattice of the complex [FeL(bbp)](ClO4)2 (1; L and bbp are tris-imine ligands) featuring an unprecedented rich succession of SCO and crystallographic phase transformations. Magnetometry measurements unveil a thermally irreversible sequence of spin conversions that delineate four different thermal pathways. All of these are single-crystal-to-single-crystal processes and can thus be monitored by single crystal X-ray diffraction using one unique specimen. Fresh crystals of 1 contain one molecule of acetone per Fe center (1·ac) that abandons the lattice upon warming at the same time that a SCO from an ordered mixed spin state (1:1 high spin/low spin; HS/LS) to a fully HS state, 1α, occurs. This crystallographic phase, accessed through a template effect by the solvent, converts into another one, 1ß, upon cooling, as triggered by a HS to LS SCO. Warming of 1ß induces a new SCO (LS to ordered HS/LS) coupled to another crystallographic phase transition, 1ß â†’ 1γ. The fully HS state of 1γ can not be reached before decomposition of the compound. Instead, this phase cycles between the HS/LS and the LS states through superimposable pathways, different from that of the prerequired 1ß â†’ 1γ phase change. Analysis of the thermal variation of the free energy, G, through density functional theory methods provides trends in agreement with the observation of these transformations and clarifies the possible metastable nature of the various phases identified. This unique behavior allows the access to four different magnetic responses depending on the thermal history of the sample, within a given range of temperatures near the ambient conditions.

Cooperativity in Spin Crossover Systems. An Atomistic Perspective on the Devil's Staircase.

Cooperativity is key in defining the shape (i.e., gradual, abrupt, or hysteretic) of thermally driven spin transitions in magnetic switches. Despite its importance, there is very little information on its atomistic origin, which hinders the rational design of materials displaying a bistability region (i.e., hysteresis). In this paper, we investigate the spin transition of two solvatomorphs of [Fe(2-pic)3]Cl2, an Fe(II)-complex displaying thermal spin crossover (SCO) from a low-spin (LS) to a high-spin (HS) state with either gradual or abrupt two-step character. To do it, we apply a novel computational protocol to study the cooperativity of SCO compounds from DFT calculations, which does not rely on a priori assumptions on the studied system. The distinct shape of the spin transition is successfully captured, and the atomistic origin of cooperativity is traced back to geometrical distortions of the Fe-N6 core in case of the solvatomorph exhibiting an abrupt transition. According to our calculations, HS and LS molecules contribute differently to cooperativity, which results in a complex energetic evolution of the spin transition that cannot be described by the common Slichter-Drickamer model. The present work opens new avenues for the study of cooperativity of SCO systems having a chemically oriented perspective and demonstrates that quantum chemistry calculations can discriminate the shape of a spin transition, while providing insight into the atomistic underlying factors that contribute to its cooperative behavior.

Absorption Spectroscopy and Photophysics of a ReI -dppz Probe for DNA-Mediated Charge Transport.

Optical properties of [Re(CO)3 (dppz)(py)]+ (dppz=dipyrido[3,2-a:2',3'-c]phenazine; py=pyridine) in acetonitrile, water and DNA have been investigated based on DFT, time-dependent-DFT (TD-DFT)/ conductor-like screening model, with and without explicit solvent molecules, and molecular dynamics. Whereas implicit solvent model is not appropriate to model optical properties of dppz-substituted metal complexes, adding explicit solvent molecules in interaction with dppz stabilizes the metal-to-ligand-charge-transfer (MLCT) transitions. Classical molecular dynamics simulations point to an important conformational flexibility, as evidenced by the coexistence of two conformers A and B. When considering the conformational sampling, the lowest band of the absorption spectrum is red-shifted and broadened up to 500 nm in agreement with the experimental spectra supporting important dynamical effects. The absorption spectra of [Re(CO)3 (dppz)(py-R)]+/ GC-DNA and [Re(CO)3 (dppz)(py-R)]+ /AT-DNA (R=CH2 -CH2 -COO- ) intercalated in both major or minor grooves exhibit a lowest energy charge separated (CS) band at about 600 nm and 500 nm, respectively, corresponding mainly to excitations from guanine and adenine to dppz. These states may play a central role into DNA-mediated charge transport processes. The over stabilization of the lowest 3 ILdppz state of [Re(CO)3 (dppz)(py)]+ in water as compared to acetonitrile could be responsible for the quenching of emission in water.

Spin State Chemistry: Modulation of Ligand p Ka by Spin State Switching in a [2×2] Iron(II) Grid-Type Complex.

The iron(II) [2×2] grid complex Fe-8H has been synthesized and characterized. It undergoes spin-crossover (SCO) upon deprotonation of the hydrazine-based terpyridine-like ligand. The deprotonation patterns have been determined by X-ray crystallography and 1H NMR spectroscopy and discussed in relation to the spin state of the iron(II) centers, which influences greatly the p Ka of the ligand. The synthesis of the magnetically silent zinc(II) analogue is also reported, and its (de)protonation behavior has been characterized to serve as a reference for the study of the FeII grid complexes. DFT computations have also been performed in order to investigate how the successive deprotonation of the bridging ligands affects the SCO behavior within the grid.

Luminescent Dinuclear Copper(I) Complexes as Potential Thermally Activated Delayed Fluorescence (TADF) Emitters: A Theoretical Study.

The excited state properties of a series of binuclear NHetPHOS-Cu(I) complexes (NHetPHOS) have been investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT). It is shown that experimental trends observed in powder, generally explored via S1 and T1 excited state energetics and S1 ⇔ T1 intersystem crossing (ISC) efficiency, are hardly analyzed on the basis of excited state properties calculated in solution. Indeed, several local minima corresponding to various structural deformations are evident on the lowest excited state potential energy surfaces (PES) when solvent correction is applied, leading to a four-state thermally activated delayed fluorescence (TADF) mechanism. In contrast, preliminary simulations performed in the solid point to the reduction of nuclear flexibility and consequently to a rather simple two-state model.

Twisting induces ferromagnetism in homometallic clusters.

A helical chiral cluster bridging two sets of Cu2 units is reported. The two double-stranded ligands induce a distorted tetrahedral environment for one of the two copper(ii) ions whereas the second one remains in a standard octahedral environment. Magnetic measurements and wavefunction calculations demonstrate that the copper(ii) centres are ferromagnetically coupled (J = 7.7 cm-1). This ligand-driven ferromagnetic interaction thus appears as a proof-of-concept of an innovative strategy towards high-spin clusters.