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A theoretical study of the non‐radiative photophysical relaxation mechanisms of the first singlet excited state of benzene, cyclobutadiene and fulvene is presented. For these molecules, the calculation of the Minimum Energy Path (MEP) leading from the Franck–Condon region to the surface crossing with the ground state is carried out. Subsequently, the decomposition of the electronic energies into atomic and pair contributions is performed using the Interacting Quantum Atom (IQA) method. The IQA approach provides the important mechanistic information necessary to rationalise some relevant aspects of the processes, such as the components that explain the appearance of an energy barrier or that favour the crossing between potential energy surfaces (PES); it also allows to quantify the direct effect on the MEP due to the inclusion of a substituent. In particular, it is shown how the IQA energies allow measuring the extent to which the formation of biradicaloid structures affects the crossing of the PES. The analysis of electron density functions suggests that aromaticity is not a driving force on the relaxation processes. Overall, this work shows the potential of the IQA method as a useful tool for the detailed description of photophysical processes. [ABSTRACT FROM AUTHOR]

Oxazolones or azlactones are small heterocyclic systems that can be easily functionalized with a double bond directly attached to their structure. In addition, they can be substituted on the opposite side of the heterocycle with styrenyl groups to extend their conjugation. This allows for versatile tuning of their linear and non‐linear optical properties. In this work, eight doubly substituted oxazolones including different push‐pull or pull‐pull designs were synthesized, and their linear and non‐linear photophysical properties were studied. As we show, the two‐photon activity of these systems is dependent on the donor‐acceptor pattern, where a methoxy substitution on the styrene end of the molecule and an acetyl group on the other end shows two‐photon Z‐E conversion properties with an important two photon absorption cross section of 308 GM. Most oxazolones shown in this study undergo picosecond relaxation to the electronic ground state, releasing the biphotonic energy to their surrounding environment. The results obtained in this work are indicative of potential applications of these molecules in schemes where high spatial patterning or thermalization is required. [ABSTRACT FROM AUTHOR]

An approach is developed for the fast calculation of the interacting quantum atoms energy decomposition (IQA) from the information contained in the first order reduced density matrix only. The proposed methodology utilizes an approximate exchange‐correlation density from Density Matrix Functional Theory without the need to evaluate the correlation‐exchange contribution directly. Instead, weight factors are estimated to decompose the exact Vxc into atomic and pairwise contributions. In this way, the sum of the IQA contributions recovers the energy obtained from the electronic structure calculation. This method can, hence, be applied to obtain atomic contributions in excited states on the same footing as in their ground states using any method that delivers the reduced first‐order density matrix. In this way, one can locate chromophores from first principles quantum chemical calculations. Test calculations on the ground and excited states of a set of small molecules indicate that the scaled atomic contributions reproduce vertical electronic transition energies calculated exactly. This approach may be useful to extend the applicability of the IQA approach in the study of large photochemical systems especially when the calculations of the second order reduced density matrices is prohibitive or not possible. [ABSTRACT FROM AUTHOR]

The Front Cover illustrates the photophysical relaxation mechanisms of archetypal aromatic/antiaromatic molecules, starting from low‐energy excited states, and their rationalisation in terms of electronic descriptors that allow quantifying the extent to which the formation of biradicaloid structures affects the crossing of the potential energy surfaces. More information can be found in the Research Article by Jesús Jara‐Cortés et al. (DOI 10.1002/cptc.202300291)...By Jesús Jara‐Cortés; José A. Pérez‐Pimienta; Jae Woo Park and Jesús Hernández‐TrujilloReported by Author; Author; Author; Author [Extracted from the article]

Real‐space tools were employed to show that the chemical bonding scenario used routinely to understand ground states lacks the necessary flexibility in excited states. It is shown that, even for two‐center, two‐electron bonds, the real‐space bond orders have exotic values that have never been reported. The nature of these situations was uncovered by using electron‐counting techniques that provide an appealing statistical interpretation of bonding descriptors, together with simple physical models. Bond orders greater than one as well as negative bond orders for a single bonding electron pair emerge in situations in which the electrons in the pair show a gregarious (bosonic) instead of the usual lonely (fermionic) behavior. In the first case the gregarious pair is intra‐atomic, whereas the coupling is interatomic in the second. A number of examples are used to substantiate these claims. [ABSTRACT FROM AUTHOR]

A number of aromatic, antiaromatic, and nonaromatic organic molecules was analyzed in terms of the contributions to the electronic energy defined in the quantum theory of atoms in molecules and the interacting quantum atoms method. Regularities were found in the exchange and electrostatic interatomic energies showing trends that are closely related to those of the delocalization indices defined in the theory. In particular, the CC interaction energies between bonded atoms allow to rationalize the energetic stabilization associated with the bond length alternation in conjugated polyenes. This approach also provides support to Clar's sextet rules devised for aromatic systems. In addition, the H ⋯H bonding found in some of the aromatic molecules studied was of an attractive nature, according to the stabilizing exchange interaction between the bonded H atoms. © 2017 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

This work provides a novel interpretation of elementary processes of photophysical relevance from the standpoint of the electron density using simple model reactions. These include excited states of H2 taken as a prototype for a covalent bond, excimer formation of He2 to analyze non-covalent interactions, charge transfer by an avoided crossing of electronic states in LiF and conical interesections involved in the intramolecular scrambling in C2H4. The changes of the atomic and interaction energy components along the potential energy profiles are described by the interacting quantum atoms approach and the quantum theory of atoms in molecules. Additionally, the topological analysis of one- and two-electron density functions is used to explore basic reaction mechanisms involving excited and degenerate states in connection with the virial theorem. This real space approach allows to describe these processes in a unified way, showing its versatility and utility in the study of chemical systems in excited states. © 2017 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]