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Light-triggered molecular switches are extensively researched for their applications in medicine, chemistry and material science and, if combined, particularly for their use in multifunctional smart materials, for which orthogonally, i.e. individually, addressable photoswitches are needed. In such a multifunctional mixture, the switching properties, efficiencies and the overall performance may be impaired by undesired mutual dependences of the photoswitches on each other. Within this study, we compare the performance of the pure photoswitches, namely an azobenzene derivative (Azo) and a donor–acceptor Stenhouse adduct (DASA), with the switching properties of their mixture using time-resolved temperature-dependent UV/VIS absorption spectroscopy, time-resolved IR absorption spectroscopy at room temperature and quantum mechanical calculations to determine effective cross sections, switching kinetics as well as activation energies of thermally induced steps. We find slightly improved effective cross sections, percentages of switched molecules and no increased activation barriers of the equimolar mixture compared to the single compounds. Thus, the studied mixture Azo + DASA is very promising for future applications in multifunctional smart materials. [ABSTRACT FROM AUTHOR]

N-Heteropolycycles (NHPCs) represent a promising substance class for applications in functional organic materials, since their electronic structure and the resulting individual molecular properties are efficiently tuneable by number and position of nitrogen atoms in the aromatic structural backbone. The isosteric replacement of a C–H unit by N leaves the geometric structure unchanged, while ionization potential, electron affinity and absorption spectra are altered. In this prespective, we present the potent combination of two-photon photoelectron spectroscopy (2PPE) and high-resolution electron energy loss spectroscopy (HREELS) with quantum chemical calculations for the investigation of the electronic structure of NHCPs. In contrast to conventional optical spectroscopies, 2PPE provides insight into electron-detached and attached electronic states of NHCPs, while HREELS delivers the energetic position of the lowest triplet states. Based on our comprehensive investigations, an extension of Platt's famous nomenclature of the low-lying excited ππ* states could be suggested for NHPCs based on the physical properties of the corresponding excitons. Also, the influence of N-introduction onto the occurrence of the so-called α-band in NHPCs compared to the parent polycyclic aromatic hydrocarbons could be explained in detail. While N-substitution of C–H in polycyclic aromatic hydrocarbons (PAHs) is often seen as a simple isosteric replacement, it has a strong influence on the electronic structure and the resulting properties. Therefore rules derived for PAHs can often only be transferred to a limited extent or not at all. [ABSTRACT FROM AUTHOR]

For the calculation of core-ionization energies (IEs), X-ray photoelectron spectra (XPS), and X-ray emission spectra (XES), a commonly applied approach is to use non-Aufbau reference states with a core-hole as either final (IE and XPS) or initial (XES) state. However, such reference states can introduce numerical instabilities in post-HF methods, relating to the denominator of the energy corrections involved. This may become arbitrarily close to zero if a negative virtual orbital is present, e.g. a core-hole, leading to near-singularities. The resulting instabilities lead to severe convergence issues of the calculation schemes and, in addition, can strongly affect both energies and intensities, with oscillator strengths seen to reach values up to 4 × 107. For the K-edge we propose freezing the highest-energy virtual orbitals which contribute to any denominator below a threshold of 0.1 Hartree. Stable and reliable spectra are then produced, with minimal influence due to freezing energetically high-lying virtual orbitals (typically removing <5% of the total number of MOs). The developed protocol is here tested for Møller–Plesset perturbation theory and for the algebraic diagrammatic construction scheme for the polarization propagator, and it is also relevant for coupled cluster theory and other related methods. [ABSTRACT FROM AUTHOR]

The photochemistry of fluorenols has been of special interest for many years. This is because both the fluorenol and the fluorenyl cation are antiaromatic in the ground state due to their 4n π-electrons according to the Hückel rule. The photolysis reaction of various fluorene derivatives takes place via a cation intermediate and is preferred due to its excited state aromaticity. Here we present an extremely long-lived fluorenyl cation and its effects on the uncaging of various leaving groups. We analyze the relationship between uncaging quantum yields of fluorene-based cages and the longevity of their fluorenyl cations with different spectroscopic methods in the steady state and on an ultrafast time scale and find that the uncaging quantum yield rises with the stability of the cation. In contrast to previous reports, the cation can be observed on a time scale of minutes, even in moderately protic solvents as methanol and ethanol. The stability of this cation depends on the dimethylamino-substituents on the fluorene scaffold and the properties of the solvent. In addition, with bis-dimethylamino fluorenol, a photobase is introduced that expands the small group of known photoinduced hydroxide emitters. [ABSTRACT FROM AUTHOR]

N-Heteropolycyclic aromatic compounds are promising organic semiconductors for applications in field effect transistors and solar cells. Thereby the electronic structure of organic/metal interfaces and thin films is essential for the performance of organic-molecule-based devices. Here, we studied the structural and the electronic properties of 6,7,12,13-tetraazapentacene (TAP) 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 TAP adsorbs in a planar adsorption geometry with the molecular backbone oriented parallel to the gold substrate. The energies of the lowest excited electronic singlet states (S) as well as the triplet state (T) are assigned. The optical gap (S0 → S1 transition) is found to be 1.6 eV and the T1 energy 1.2 eV. In addition, thorough comparison to previously studied pentacene (PEN) and 6,13-diazapentacene (6,13-DAP) is made explaining in detail the influence of nitrogen substitution on the electronic structure and in particular on the intensity of the α-band in the UV/vis absorption spectrum. In the series PEN, 6,13-DAP, and TAP, the α-band (S0 → S2 transition) gains significantly in intensity due to individual effects of the introduced nitrogen atoms on the orbital energies. [ABSTRACT FROM AUTHOR]