Accurate and Efficient Computation of Optical Absorption Spectra of Molecular Crystals: The Case of the Polymorphs of ROY

Predicting the Color Polymorphism of ROY from a Time-Dependent Optimally Tuned Screened Range-Separated Hybrid Functional

Polymorphism is a well-known property of molecular crystals, which allows the same molecule to form solids with several crystalline structures that can differ significantly in physical properties. Polymorphs that possess different optical absorption properties in the visible range may exhibit different perceived colors, a phenomenon known as color polymorphism. One striking example of color polymorphism is given by 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile, known as ROY for its red-orange-yellow colors. First-principles prediction of color polymorphism may help in polymorph assignment and design but has proven to be challenging. Here, we predict the absorption spectra and simulate the colors of 12 ROY polymorphs using the general, nonempirical method of time-dependent (TD) optimally tuned screened range-separated hybrid (OT-SRSH) functional. For 5 ROY polymorphs with known experimental absorption spectra, we show that the TD-OT-SRSH approach predicts absorption spectra in quantitative agreement with experiment. For all polymorphs, we show that an accurate simulation of the colors is obtained, paving the way to a fully predictive, low-cost calculation of color polymorphism.

A Δ-learning strategy for interpretation of spectroscopic observables

Accurate computations of experimental observables are essential for interpreting the high information content held within x-ray spectra. However, for complicated systems this can be difficult, a challenge compounded when dynamics becomes important owing to the large number of calculations required to capture the time-evolving observable. While machine learning architectures have been shown to represent a promising approach for rapidly predicting spectral lineshapes, achieving simultaneously accurate and sufficiently comprehensive training data is challenging. Herein, we introduce Δ-learning for x-ray spectroscopy. Instead of directly learning the structure-spectrum relationship, the Δ-model learns the structure dependent difference between a higher and lower level of theory. Consequently, once developed these models can be used to translate spectral shapes obtained from lower levels of theory to mimic those corresponding to higher levels of theory. Ultimately, this achieves accurate simulations with a much reduced computational burden as only the lower level of theory is computed, while the model can instantaneously transform this to a spectrum equivalent to a higher level of theory. Our present model, demonstrated herein, learns the difference between TDDFT(BLYP) and TDDFT(B3LYP) spectra. Its effectiveness is illustrated using simulations of Rh L3-edge spectra tracking the C-H activation of octane by a cyclopentadienyl rhodium carbonyl complex.

Metalating 5-Methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY): Understanding the Denticity and Speciation of Complexes of the ROY Anion

5-Methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY) is considered to be the most crystalline polymorphic organic molecule discovered to date with 12 fully characterized crystal structures present in the Cambridge Structural Database (CSD). However, metal complexes of ROY have not previously been described. Here, we explore the synthetic chemistry of ROY (denoted as H-ROY hereafter for the purpose of our study) and demonstrate that it can be deprotonated using either NaH or KH and that the resulting sodium and potassium salts of H-ROY can be cleanly isolated. Furthermore, we introduce two new metal complexes of the ROY anion (ROY^-) with Co(II) and Ni(II) cations, formed by the reaction of the sodium salt of ROY, Na(ROY), with the respective transition-metal chloride salts. Solid-state X-ray diffraction studies confirm the presence of Co(II) or Ni(II) centers, with the ROY^- ligand in a 1:2 ratio forming neutral trinuclear clusters of the forms [Co₃ROY₆] (Co-ROY) and [Ni₃ROY₆] (Ni-ROY) in both cases. Here, the ROY^- moiety interacts with the metal center through the anionic N atom, an O atom of the -NO₂ group, and the N atom of the -CN group. IR and electronic absorption spectroscopies reveal the influence of the Co(II) and Ni(II) centers on the properties of the complexes. Taken together, our results show that the metal complexes of the H-ROY proligand can be prepared with late 3d transition metals. The results of these structural chemistry studies may contribute to resolving polymorphism in H-ROY and related compounds.

Probing disorder in 2CzPN using core and valence states

Molecules which exhibit thermally activated delayed fluorescence (TADF) show great promise for use in efficient, environmentally-friendly OLEDs, and thus the design of new TADF emitters is an active area of research. However, when used in devices, they are typically in the form of disordered thin films, where both the external molecular environment and thermally-induced internal variations in parameters such as the torsion angle can strongly influence their electronic structure. In this work, we use density functional theory and X-ray photoelectron spectroscopy to investigate the impact of disorder on both core and valence states in the TADF emitter 2CzPN (1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene). By simulating gas phase molecules displaying varying levels of disorder, we assess the relative sensitivity of the different states to factors such as varying torsion angle. The theoretical results for both core and valence states show good agreement with experiment, thereby also highlighting the advantages of our approach for interpreting experimental spectra of large aromatic molecules, which are too complex to interpret based solely on experimental data.