Electron delocalization and electron density of small polycyclic aromatic hydrocarbons in singlet excited states

Seeking for Optimal Excited States in Photoinduced Electron-Transfer Processes─The Case Study of Brooker's Merocyanine

Material design enters an era in which control of electrons in atoms, molecules, and materials is an essential property to be predicted and thoroughly understood in view of discovering new compounds with properties optimized toward specific optical/optoelectronic applications. π-electronic delocalization and charge separation/recombination enter notably into the set of features that are highly desirable to tailor. Diverse domains are particularly relying on photoinduced electron-transfer (PET), including fields of paramount importance such as energy production through light-harvesting, efficient chemoreceptive sensors, or organic field-effect transistors. In view of completing the arsenal of strategies in this area, we selected Brooker's merocyanine─a typical [D-π-A] compound─as the case study and examined from time-dependent density functional theory the opportunity offered by selected excited states to reach a suited manipulation of the charge transfer (CT) extent. In addition to the consideration of diagnostic tools able to spot the charge amount (i.e., magnitude of electron fraction) transferred upon excitation (q_CT), the spatial extent associated with such an electronic transition or CT length (D_CT), as well as the corresponding variation in dipole moment between the ground and the excited states (μ_CT), further analysis of the excitation process was undertaken. The advantage of going beyond the above-mentioned molecular indicators─which can be considered as PET global indices─was explored on the basis of a partitioning of the electron density. Relevant insight was gained on the relation these global indices have with the evolution of (local) features characterizing either chemical bond or electron delocalization upon vertical excitations.

Late-Stage Functionalisation of Polycyclic (N-Hetero-) Aromatic Hydrocarbons by Detoxifying CYP5035S7 Monooxygenase of the White-Rot Fungus Polyporus arcularius

Functionalisation of polycyclic aromatic hydrocarbons (PAHs) and their N-heteroarene analogues (NPAHs) is a tedious synthetic endeavour that requires diverse bottom-up approaches. Cytochrome P450 enzymes of white-rot fungi were shown to participate in the fungal detoxification of xenobiotics and environmental hazards via hydroxylation of PAH compounds. In this paper, the recently discovered activity of the monooxygenase CYP5035S7 towards (N)PAHs was investigated in detail, and products formed from the substrates azulene, acenaphthene, fluorene, anthracene, and phenanthrene by whole-cell biocatalysis were isolated and characterised. The observed regioselectivity of CYP5035S7 could be explained by a combination of the substrate's electron density and steric factors influencing the substrate orientation giving insight into the active-site geometry of the enzyme.

Experimental study on the biodegradation of naphthalene and phenanthrene by functional bacterial strains in the riparian soil of a binary system

Polycyclic aromatic hydrocarbons (PAHs) such as naphthalene (Nap) and phenanthrene (Phe) are organic pollutants of concern owing to their toxicity, carcinogenicity, and teratogenicity. Biodegradation is considered the most economical and efficient process to remediate Nap and Phe. The riparian zone between a river and a riparian aquifer, which is rich in indigenous microorganisms, may be important for PAH remediation. However, few studies have evaluated the ability of indigenous microorganisms to remove Nap and Phe. In this study, focusing on the typical PAHs (Nap and Phe) as target pollutants, the genus-level community structure of Nap- and Phe-degrading bacteria was identified. Batch static and dynamic biodegradation experiments were conducted to explore the biodegradation mechanisms of Nap and Phe in the riparian zone and identify the factors influencing Nap and Phe biodegradation in the binary system (i.e., where Nap and Phe are simultaneously present). According to the genus-level community structure test results, the dominant bacterial genus in the binary system was mainly the Phe-degrading bacteria. The Nap and Phe-biodegradation percentages were 19.20% lower and 19.49% higher, respectively, in the binary system than in the unitary system. The results indicated that functional bacteria can degrade Nap and Phe, and that Nap weakly promoted Phe biodegradation. Additionally, the initial Nap and Phe concentration ratio, hydraulic gradient, and temperature affected Nap and Phe biodegradation. Dynamic biodegradation experiments showed that the biodegradation percentage decreased as the hydraulic gradient increased, and biodegradation percentage of Phe was always higher than that of Nap. According to the results of the dynamic laboratory experiments, the removal percentages of Nap and Phe by indigenous riparian-zone microorganisms were 6.21-16.73% and 13.95-24.45%, respectively. The findings in this study will be useful for alleviation of Nap and Phe pollution in groundwater and will facilitate determination of appropriate treatment measures for groundwater exposed to this type of pollution.

Mesomorphic Behavior of Symmetric Azomethine Dimers Containing Different Chromophore Groups

A series of new azomethine dimers was synthesized by the condensation reaction of flexible bis-benzaldehydes with four aromatic amines containing phenyl, naphthyl, anthracene and pyrene groups. Their right structure was confirmed by FTIR and ¹H-NMR spectroscopy and their thermal properties were investigated by thermogravimetric analysis, differential scanning calorimetry and polarized light optical microscopy. A view on their photophysical behavior was gained by UV-vis and photoluminescence spectroscopy. The dimers containing pyrene and anthracene presented liquid crystalline behavior, while the other dimers were crystalline compounds. Two dimers containing pyrene moieties formed mesomorphic glasses and had intense luminescence, promising properties for applications in building optoelectronic devices.

Extending the ASS1ST Active Space Selection Scheme to Large Molecules and Excited States

Multireference electronic structure methods based on the CAS (complete active space) ansatz are well-established as a means to provide reliable predictions of physical properties of strongly correlated systems. A critical aspect of every CAS calculation is the selection of an adequate active space, in particular as the boundaries for tractable active spaces have been shifted significantly with the emergence of efficient approximations to the Full-CI problem like the density matrix renormalization group and full-CI quantum Monte Carlo. Recently, we proposed an active space selection based on first-order perturbation theory (ASS1ST) that yields satisfactory results for the electronic ground state of a variety of strongly correlated systems. In this work, we present a state-averaged extension of ASS1ST (SA-ASS1ST) that determines suitable active spaces when electronically excited states are targeted. Furthermore, the computational costs of the single state and state-averaged variants are significantly reduced by a simple approximation that avoids the most expensive step of the original method, the evaluation of active space four-electron reduced density matrices, altogether. After the applicability of the approximation is established, test calculations on a biomimetic Mn₄O₄ cluster demonstrate the enhanced range of ASS1ST in terms of system size and complexity. Furthermore, calculations on [VOCl₄]₂^-, MeMn(CO)₃-α-diimine, and anthracene show that SA-ASS1ST suggests well-suited active spaces to describe d → d and charge-transfer excitations in transition-metal complexes as well as π → π* excitations in aryl compounds. Finally, the application of ASS1ST on multiple points of the potential energy surface of Cr₂ illustrates the applicability of the method even when extremely complicated bonding patterns are met. More importantly, however, it highlights the necessity to use special strategies when different points of a potential energy surface are investigated, e.g., during chemical reactions.

Analysis of lowest energy transitions at TD-DFT of pyrene in vacuum and solvent

Polycyclic aromatic hydrocarbons are present in interstellar medium and trapped in water ice. Among these compounds, pyrene has a controversial theoretical excitation spectrum. We carried out time-dependent density functional theory, including the long-range correction functionals, with the aim to help to understand the inversion of the first two UV bands of lower energies. The pyrene molecule was optimized at TD-DFT functionals with Def2svp basis set. The spectrum of pyrene molecule was calculated using implicit and explicit solvent models. The explicit solvent effect was studied, including a cluster of 51 water molecules. The implicit solvent PCM model was used with water and benzene as solvents. CAM-B3LYP and ωB97XD give correct band positions and the ω parameter was also optimized. NBO and frontier molecular orbitals were used to study the UV band inversion. Graphical abstract Analysis of frontier orbitals involved in the lowest energy transitions at TD-DFT of pyrene.

Chemical bonding analysis of excited states using the adaptive natural density partitioning method

A novel approach to chemical bond analysis for excited states has been developed. Using an extended adaptive natural density partitioning method (AdNDP) as implemented in AdNDP 2.0 code, we obtained chemically intuitive bonding patterns for the excited states of H2O, B5+, and C2H4+ molecules. The deformation pathway in the excited states could be easily predicted based on the analysis of the chemical bond pattern. We expect that this new method of chemical bonding analysis would be very helpful for photochemistry, photoelectron spectroscopy, electron spectroscopy and other chemical applications that involved excited states.

Energetic Analysis of Conjugated Hydrocarbons Using the Interacting Quantum Atoms Method

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.

Singlet La and Lb Bands for N-Acenes (N = 2-7): A CASSCF/CASPT2 Study

In this work CASPT2 calculations of polyacenes (from naphthalene to heptacene) were performed to find a methodology suitable for calculations of the absorption spectra, in particular of the L_a (B_2u state) and L_b (B_3u state) bands, of more extended systems. The effect of the extension of the active space and of freezing σ orbitals was investigated. The MCSCF excitation energy of the B_2u state is not sensitive to the size of the active space used. However, the CASPT2 results depend strongly on the amount of σ orbitals frozen reflecting the ionic character of the B_2u state. On the other hand, the excitation energies of the B_3u state are much more sensitive to the size of the active space used in the calculations reflecting its multiconfigurational character. We found a good agreement with experimental data for both bands by including 14 electrons in 14 π orbitals in the active space followed by the CASPT2(14,14) perturbation scheme in which both σ and π orbitals are included.

Cyclopropyl Group: An Excited-State Aromaticity Indicator?

The cyclopropyl (cPr) group, which is a well-known probe for detecting radical character at atoms to which it is connected, is tested as an indicator for aromaticity in the first ππ* triplet and singlet excited states (T₁ and S₁ ). Baird's rule says that the π-electron counts for aromaticity and antiaromaticity in the T₁ and S₁ states are opposite to Hückel's rule in the ground state (S₀ ). Our hypothesis is that the cPr group, as a result of Baird's rule, will remain closed when attached to an excited-state aromatic ring, enabling it to be used as an indicator to distinguish excited-state aromatic rings from excited-state antiaromatic and nonaromatic rings. Quantum chemical calculations and photoreactivity experiments support our hypothesis; calculated aromaticity indices reveal that openings of cPr substituents on [4n]annulenes ruin the excited-state aromaticity in energetically unfavorable processes. Yet, polycyclic compounds influenced by excited-state aromaticity (e.g., biphenylene), as well as 4nπ-electron heterocycles with two or more heteroatoms represent limitations.

Chemical bonding in excited states: Energy transfer and charge redistribution from a real space perspective

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 C₂ H₄ . 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.

Metal-free photochemical silylations and transfer hydrogenations of benzenoid hydrocarbons and graphene

The first hydrogenation step of benzene, which is endergonic in the electronic ground state (S₀), becomes exergonic in the first triplet state (T₁). This is in line with Baird's rule, which tells that benzene is antiaromatic and destabilized in its T₁ state and also in its first singlet excited state (S₁), opposite to S₀, where it is aromatic and remarkably unreactive. Here we utilized this feature to show that benzene and several polycyclic aromatic hydrocarbons (PAHs) to various extents undergo metal-free photochemical (hydro)silylations and transfer-hydrogenations at mild conditions, with the highest yield for naphthalene (photosilylation: 21%). Quantum chemical computations reveal that T₁-state benzene is excellent at H-atom abstraction, while cyclooctatetraene, aromatic in the T₁ and S₁ states according to Baird's rule, is unreactive. Remarkably, also CVD-graphene on SiO₂ is efficiently transfer-photohydrogenated using formic acid/water mixtures together with white light or solar irradiation under metal-free conditions.

Baird's rules say that the first triplet state of benzene displays antiaromatic character. Here, the authors exploit this to show that aromatic molecules can undergo rapid transfer hydrogenation or silylations without the need for metal catalysts when photochemcially excited into this state.