A Time‐Dependent Picture of the Ultrafast Deactivation of keto‐Cytosine Including Three‐State Conical Intersections

Using mixed quantum–classical dynamics, the lowest part of the UV absorption spectrum and the first deactivation steps of keto‐cytosine have been investigated. The spectrum shows several strong peaks, which mainly come from the S₁ and S₂ states, with minor contributions from the S₃. The semiclassical trajectories, launched from these three states, clearly indicate that at least four states are involved in the relaxation of keto‐cytosine to the ground state. Non‐adiabatic transfer between the ππ* and nπ* excited states and deactivation via three‐state conical intersections is observed in the very early stage of the dynamics. In less than 100 fs, a large amount of population is deactivated to the ground state via several mechanisms; some population remains trapped in the S₂ state. The latter two events can be connected to the fs and ps transients observed experimentally.     

Photophysics of Schiff Bases: Theoretical Study of Salicylidene Methylamine

The proton‐transfer reaction in a model aromatic Schiff base, salicylidene methylamine (SMA), in the ground and in the lowest electronically‐excited singlet states, is theoretically analyzed with the aid of second‐order approximate coupled‐cluster model CC2, time‐dependent density functional theory (TD‐DFT) using the Becke, three‐parameter Lee–Yang–Parr (B3LYP) functional, and complete active space perturbation theory CASPT2 electronic structure methods. Computed vertical‐absorption spectra for the stable ground‐state isomers of SMA fully confirm the photochromism of SMA. The potential‐energy profiles of the ground and the lowest excited singlet state are calculated and four photophysically relevant isomeric forms of SMA; α, β, γ, and δ are discussed. The calculations indicate two S₁/S₀ conical intersections which provide non‐adiabatic gates for a radiationless decay to the ground state. The photophysical scheme which emerges from the theoretical study is related to recent experimental results obtained for SMA and its derivatives in the low‐temperature argon matrices (J. Grzegorzek, A. Filarowski, Z. Mielke, Phys. Chem. Chem. Phys. 2011 , 13, 16596–16605). Our results suggest that aromatic Schiff bases are potential candidates for optically driven molecular switches.     

Thin‐Film Properties of DNA and RNA Bases: A Combined Experimental and Theoretical Study

The structures of the DNA and RNA bases cytosine, uracil, and thymine in thin films with a nominal film thickness of about 20 nm are studied by using X‐ray photoemission spectroscopy (XPS) and Fourier‐transform infrared spectroscopy. The molecules are evaporated in situ from powder on a gold foil. The experimental results indicate that cytosine is composed of two energetically close tautomeric forms, whereas uracil and thymine exist in only one tautomeric form. Additionally, quantum chemical calculations are performed to complement the experimental results. The relative energies of the tautomeric forms of cytosine, uracil, and thymine are calculated using Hartree–Fock (HF), density functional theory (DFT), and post‐HF methods. Furthermore, the assignment of the XPS spectra is supported by using simple model considerations employing Koopmans ionization energies and Mulliken net atomic charges.     

Photoinduced Nonadiabatic Dynamics of 9H‐Guanine

Surface‐hopping simulations are used to study the nonradiative relaxation of 9H‐guanine. Two distinct S₁→S₀ (ππ*→gs) decay channels, both of which pass through a conical intersection (CI), are found to be responsible for the experimentally observed double‐decay behavior (see schematic diagram).

     

The Origin of the Selectivity and Activity of Ruthenium‐Cluster Catalysts for Fuel‐Cell Feed‐Gas Purification: A Gas‐Phase Approach

Gas‐phase ruthenium clusters Ru_n⁺ (n=2–6) are employed as model systems to discover the origin of the outstanding performance of supported sub‐nanometer ruthenium particles in the catalytic CO methanation reaction with relevance to the hydrogen feed‐gas purification for advanced fuel‐cell applications. Using ion‐trap mass spectrometry in conjunction with first‐principles density functional theory calculations three fundamental properties of these clusters are identified which determine the selectivity and catalytic activity: high reactivity toward CO in contrast to inertness in the reaction with CO₂; promotion of cooperatively enhanced H₂ coadsorption and dissociation on pre‐formed ruthenium carbonyl clusters, that is, no CO poisoning occurs; and the presence of Ru‐atom sites with a low number of metal–metal bonds, which are particularly active for H₂ coadsorption and activation. Furthermore, comprehensive theoretical investigations provide mechanistic insight into the CO methanation reaction and discover a reaction route involving the formation of a formyl‐type intermediate.     

Excited‐State Dynamics of Isolated DNA Bases: A Case Study of Adenine

We present a summary of recent advances in the understanding of the UV photophysics of the isolated DNA base adenine, emphasizing a discussion of the mechanisms behind the ultrafast relaxation following excitation to the ππ* band. Drawing on our femtosecond time‐resolved photoelectron spectroscopy experiments, we discuss differences in the ultrafast relaxation of adenine and 9‐methyladenine and consider the relative merits of the various proposed mechanisms.     

Study of the Photochemical Properties and Conical Intersections of [2,2′‐Bipyridyl]‐3‐amine‐3′‐ol

The two isoelectronic bipyridyl derivatives [2,2′‐bipyridyl]‐3,3′‐diamine and [2,2′‐bipyridyl]‐3,3′‐diol are experimentally known to undergo very different excited‐state double‐proton‐transfer processes, which result in fluorescence quantum yields that differ by four orders of magnitude. In a previous study, these differences were explained from a theoretical point of view, because of topographical features in the potential energy surface and the presence of conical intersections (CIs). Here, we analyze the photochemical properties of a new molecule, [2,2′‐bipyridyl]‐3‐amine‐3′‐ol [BP(OH)(NH₂)], which is, in fact, a hybrid of the former two. Our density functional theory (DFT), time‐dependent DFT (TDDFT), and complete active space self‐consistent field (CASSCF) calculations indicate that the double‐proton‐transfer process in the ground and first singlet π→π* excited state in BP(OH)(NH₂) presents features that are between those of their “parents”. The presence of two CIs and the role they may play in the actual photochemistry of BP(OH)(NH₂) and other bipyridyl derivatives are also discussed.     

The Three‐Component Biginelli Reaction: A Combined Experimental and Theoretical Mechanistic Investigation

Biginelli reactions have been monitored by direct infusion electrospray ionization mass spectrometry (ESI‐MS) and key cationic intermediates involved in this three‐component reaction have been intercepted and further characterized by tandem MS experiments (ESI‐MS/MS). Density functional theory calculations were also used to investigate the feasibility of the major competing mechanisms proposed for the Biginelli reaction. The experimental and theoretical results were found to corroborate the iminium mechanism proposed by Folkers and Johnson, whereas no intermediates directly associated with either the more energy demanding Knoevenagel or enamine mechanisms could be intercepted.     

Relationship between Electron Affinity and Half‐Wave Reduction Potential: A Theoretical Study on Cyclic Electron‐Acceptor Compounds

A high‐level ab initio protocol to compute accurate electron affinities and half‐wave reduction potentials is presented and applied for a series of electron‐acceptor compounds with potential interest in organic electronics and redox flow batteries. The comprehensive comparison between the theoretical and experimental electron affinities not only proves the reliability of the theoretical G3(MP2) approach employed but also calls into question certain experimental measurements, which need to be revised. By using the thermodynamic cycle for the one‐electron attachment reaction A+e^?→A^?, theoretical estimates for the first half‐wave reduction potential have been computed along the series of electron‐acceptor systems investigated, with maximum deviations from experiment of only 0.2 V. The precise inspection of the terms contributing to the half‐wave reduction potential shows that the difference in the free energy of solvation between the neutral and the anionic species (ΔΔG_solv) plays a crucial role in accurately estimating the electron‐acceptor properties in solution, and thus it cannot be considered constant even in a family of related compounds. This term, which can be used to explain the occasional lack of correlation between electron affinities and reduction potentials, is rationalized by the (de)localization of the additional electron involved in the reduction process along the π‐conjugated chemical structure.     

Steric Enhancement of the Chemiluminescence of Luminols

A surprising 20‐fold increase in chemiluminescence efficiency was observed for dialkyl luminol derivatives in comparison with the parent compound. This effect could be a direct consequence of steric gearing which facilitates the transition from the intermediate endoperoxide to the electronically excited phthalate. Mechanistic aspects of this process have been supported by computational calculations (CASPT2//CASSCF).     

Computational Exploration of the Photoprotective Potential of Gadusol

Gadusol shows one of the simplest structures among a series of natural UV-absorbing compounds that have been related to the photoprotective and antioxidant functions in aquatic organisms. CASPT2//CASSCF methodology was used to carry out a theoretical study on this basic structure in order to describe the underlying features responsible for the photoprotective capacity of the molecule. The influence of the enol–enolate equilibrium on the photophysical properties was explored. The results confirm that both forms undergo a rapid deactivation, which very efficiently dissipates light energy as heat. This work highlights the potential of molecular-level studies to provide an understanding of natural photoprotective mechanisms and gives support to the future design of structurally related new synthetic sunscreens.     

The Low‐Lying Excited States of 2,2′‐Bithiophene: A Theoretical Analysis

The low‐energy regions of the singlet→singlet, singlet→triplet, and triplet→triplet electronic spectra of 2,2′‐bithiophene are studied using multiconfigurational second‐order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1¹B_u and 2¹B_u, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2³A_g and 2³B_u, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states.     

Ultrafast Decay of the Excited Singlet States of Thioxanthone by Internal Conversion and Intersystem Crossing

The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high‐level ab initio CASPT2 calculations of the singlet‐ and triplet‐state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin–orbit coupling terms. The initially populated singlet ππ* state is shown to decay through internal conversion and intersystem crossing processes via intermediate nπ* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet–triplet crossing near the singlet ππ* minimum and the large spin–orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity.     

Global versus Local Aromaticity in Porphyrinoid Macrocycles: Experimental and Theoretical Study of “Imidacene”, an Imidazole‐Containing Analogue of Porphycene

The synthesis, spectroscopic properties, and computational analysis of an imidazole‐based analogue of porphycene are described. The macrocycle, given the trivial name “imidacene”, was prepared by reductive coupling of a diformyl‐substituted 2,2′‐biimidazole using low‐valent titanium, followed by treatment with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone. Imidacene displays a porphyrin‐like electronic structure, as judged by its ¹H NMR, ¹³C NMR, and UV/Vis spectral characteristics. Despite a cyclic 18 π‐electron pathway, dichloromethane or ethyl acetate solutions of imidacene were found to undergo rapid decomposition, even in the absence of light and air. A series of high‐level theoretical calculations, performed to probe the origin of this instability, revealed that the presence of a delocalized 18 π‐electron pathway in both imidacene and porphycene provides less aromatic stabilization energy than locally aromatic 6 π‐electron heterocycles in their reduced counterparts. That reduction of imidacene occurs on perimeter nitrogen atoms allows it to maintain its planarity and two stabilizing intramolecular hydrogen bonds, thereby distinguishing it from porphycene and, more generally, from porphyrin. Despite the presence of both 18 π‐ and 22 π‐electron pathways in the planar, reduced form of imidacene, aromaticity is evident only in the 6 π‐electron five‐membered rings. Our computational analysis predicts that routine ¹H NMR spectroscopy can be used to distinguish between local and global aromaticity in planar porphyrinoid macrocycles; the difference in the chemical shift for the internal NH protons is expected to be on the order of 19 ppm for these two electronically disparate sets of ostensibly similar compounds.