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This study delves into the enhancement of two-photon absorption (2PA) properties in diazaacene-bithiophene derivatives through a synergistic approach combining theoretical analysis and experimental validation. By investigating the structural modifications and their impact on 2PA cross sections, we identify key factors that significantly influence the 2PA efficiency. For all molecular systems studied, our state-of-the-art quantum chemical calculations show a very high involvement of the first excited singlet state (S1) in the 2PA processes into higher excited states, even if this state itself has only a small 2PA cross section for symmetry reasons. Consequently, both the oscillator strength of S1 and the transition dipole moments between S1 and other excited states are of importance, underscoring the role of electronic polarizability in facilitating effective two-photon interactions. The investigated compounds exhibit large 2PA cross sections over a wide near-infrared spectral range reaching giant values of 42000 GM. The introduction of diazine and diazaacene moieties into bithiophene derivatives not only induces charge transfer but also opens up pathways for the creation of materials with tailored nonlinear optical responses, suggesting potential applications in nonlinear optics., Comment: 26 pages; 12 figures

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 generic ones to those capable of accepting electrons, such as chloroform, a vastly extended and multifaceted photochemistry is observed. Following the initial excitation, two concurrent charge transfer processes are identified. Firstly, when the TPA derivative and solvent molecules are correctly positioned, an electron transfer to the solvent molecule with immediate charge separation takes place. Consequently, this process gives rise to the formation of the corresponding radical cation of the TPA derivative. This highly reactive species can subsequently combine with other TPA derivative molecules to yield dimeric species. Secondly, when the molecular positioning upon photoexcitation is not optimal, relaxation back to the $\mathrm{S_1}$ state occurs. From this state, an electron transfer process leads to the formation of a charge transfer complex. In this complex, the negatively charged solvent molecule remains closely associated with the positively charged TPA derivative. Within 30 picoseconds, the charges within this complex recombine, yielding a triplet state. This transition to the triplet state is driven by a lower reaction barrier for charge separation compared to the formation of the singlet state., Comment: 14 pages, 7 figures, 1 table

N-heterotriangulenes (N-HTAs) represent a class of functional molecules with high potential for optoelectronic materials, for example as electron donating compounds in donor/acceptor (D/A) systems. The capability of two different N-HTAs, N-HTA 550 and N-HTA 557, the latter containing an additional 7-membered ring, to act as electron donors at interfaces with strong tetracyanoquinodimethane (TCNQ and F4TCNQ) acceptors is studied using high-resolution electron energy loss spectroscopy in combination with state-of-the-art quantum chemical calculations. For TCNQ/N-HTA bilayer systems adsorbed on Au(111) Low-energy (< 2.5 eV) electronic transitions which are attributed to charge transfer (CT) states for all four D/A combinations are identified. Based on substantial quantum chemical calculations a generation of ground state CT complexes is excluded. Instead, CT in the excited state, in which an electron-stimulated CT from the N-HTAs to TCNQs is the underlying process, is proposed. The energies of the CT states are determined by the values of the ionization potential and electron affinity of the involved donor and acceptor., Comment: 22 pages, 6 figures, 3 tables

Benzobisthiadiazoles (BBTs) are promising organic semiconductors for applications in field effect transistors and solar cells, since they possess a strong electron-accepting character. Thereby the electronic structure of organic/metal interfaces and within thin films is essential for the performance of organic electronic devices. Here, we study the structural and the electronic properties of two BBTs, with different core substitution pattern, a phenyl (BBT-Ph) and thiophene (BBT-Th) derivative adsorbed on Au(111) using vibrational and electronic high-resolution electron energy loss spectroscopy in combination with state-of-the-art quantum chemical calculations. In the mono- and multilayer both BBTs adopt a planar adsorption geometry with the molecular backbone as well as the phenyl and thiophene side groups are oriented parallel to the gold substrate. The energies of the lowest excited electronic singlet states and the first triplet state (T1) are determined. The optical gap is found to be 2.2 eV for BBT-Ph and 1.6 eV for BBT-Th. The energy of T1 is identified to be 1.2 eV in BBT-Ph and in the case of BBT-Th 0.7 eV. Thus, both the optical gap size as well as the T1 energy are drastically reduced in BBT-Th compared to BBT-Ph. Based on our quantum chemical calculations this is attributed to the electron-rich nature of the five-membered thiophene rings in conjunction with their preference for planar geometries. Variation of the substitution pattern in BBTs opens the opportunity for tailoring their electronic properties.

Orbital-free density functional theory (OF-DFT) holds the promise to compute ground state molecular properties at minimal cost. However, it has been held back by our inability to compute the kinetic energy as a functional of the electron density only. We here set out to learn the kinetic energy functional from ground truth provided by the more expensive Kohn-Sham density functional theory. Such learning is confronted with two key challenges: Giving the model sufficient expressivity and spatial context while limiting the memory footprint to afford computations on a GPU; and creating a sufficiently broad distribution of training data to enable iterative density optimization even when starting from a poor initial guess. In response, we introduce KineticNet, an equivariant deep neural network architecture based on point convolutions adapted to the prediction of quantities on molecular quadrature grids. Important contributions include convolution filters with sufficient spatial resolution in the vicinity of the nuclear cusp, an atom-centric sparse but expressive architecture that relays information across multiple bond lengths; and a new strategy to generate varied training data by finding ground state densities in the face of perturbations by a random external potential. KineticNet achieves, for the first time, chemical accuracy of the learned functionals across input densities and geometries of tiny molecules. For two electron systems, we additionally demonstrate OF-DFT density optimization with chemical accuracy., Comment: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in The Journal of Chemical Physics 159, 144113 (2023) and may be found at https://doi.org/10.1063/5.0158275

We present an implementation of the B term of Magnetic Circular Dichroism within the Algebraic Diagrammatic Construction (ADC) scheme of the polarization propagator and its Intermediate State Representation. As illustrative results, the MCD spectra of the ADC variants ADC(2), ADC(2)-x and ADC(3) of the molecular systems uracil, 2-thiouracil, 4-thiouracil, purine, hypoxanthine, 1,4-naphthoquinone, 9,10-anthraquinone and 1-naphthylamine, are computed and compared with results obtained using the Coupled-Cluster Singles and Approximate Doubles (CC2) method, literature TD-DFT results as well as with available experimental data.