Development of robust chiroptical systems through spirobifluorenes.

Chiroptical responses are valuable for the structural determination of dissymmetric molecules. However, the development of everyday applications based on chiroptical systems is yet to come. We have been earlier using axially chiral allenes for the construction of linear, cyclic, and cage-shaped molecules that present remarkable chiroptical responses. Additionally, we have developed chiral surfaces through upstanding chiral architectures. Since the goal is to obtain robust chiroptical materials, more recently we have been studying spirobifluorenes (SBFs), a well-established building block in optoelectronic applications. After theoretical and experimental demonstration, the suitability of chiral SBFs for the development of robust chiroptical systems was certified by the construction all-carbon double helices, flexible shape-persistent macrocycles, chiral frameworks for surface functionalization, and structures featuring helical or spiroconjugated molecular orbitals. Here, we give an overview of our contribution to these matters.

Common Reactivity and Properties of Heme Peroxidases: A DFT Study of Their Origin.

Electronic structure calculations using the density-functional theory (DFT) have been performed to analyse the effect of water molecules and protonation on the heme group of peroxidases in different redox (ferric, ferrous, compounds I and II) and spin states. Shared geometries, spectroscopic properties at the Soret region, and the thermodynamics of peroxidases are discussed. B3LYP and M06-2X density functionals with different basis sets were employed on a common molecular model of the active site (Fe-centred porphine and proximal imidazole). Computed Gibbs free energies indicate that the corresponding aquo complexes are not thermodynamically stable, supporting the five-coordinate Fe(III) centre in native ferric peroxidases, with a water molecule located at a non-bonding distance. Protonation of the ferryl oxygen of compound II is discussed in terms of thermodynamics, Fe-O bond distances, and redox properties. It is demonstrated that this protonation is necessary to account for the experimental data, and computed Gibbs free energies reveal pKa values of compound II about 8.5-9.0. Computation indicates that the general oxidative properties of peroxidase intermediates, as well as their reactivity towards water and protons and Soret bands, are mainly controlled by the iron porphyrin and its proximal histidine ligand.

Optical and electrochemical properties of spirobifluorene iridanaphthalene complexes.

Three new spirobifluorene iridaaromatic compounds bearing electron-withdrawing or electron-donor substituents or another iridanaphthalene moiety have been synthesized and structurally characterized. Thorough experimental and theoretical evaluation revealed that these novel systems present a high thermal, air and electrochemical stability as well as low optical and electronic energy gap values with a significant redshift of the absorption maximum in the UV-Vis spectra and predicted remarkably higher first hyperpolarizabilities compared to their organic counterparts. Therefore, the combination of a metallaaromatic system with a spirobifluorene moiety leads to the design and development of new spirobifluorene derivatives. These new systems have shown interesting optical and electronic properties making them of interest for future applications in optoelectronics.

ON/OFF Spiroconjugation through Peripheral Functionalization: Impact on the Reactivity and Chiroptical Properties of Spirobifluorenes.

Spirobifluorenes are an important class of spiro compounds frequently used in the field of organic electronics. However, harnessing spiroconjugation to obtain high-performance in such structural motifs remains unexplored. We herein propose that peripheral functionalization may serve as a useful tool to control spiroconjugation in an ON/OFF manner on both chemical reactivity and photophysical properties. In particular, the ratio of mono- and di-functionalized spirobifluorenes found experimentally during their synthesis were found to be 3/2, 7/2, and 12/2 for phenyl, nitro-phenyl and amino-phenyl analogues, respectively. These remarkable reactivity differences correlate with the spiroconjugation character evaluated theoretically at the CAM-B3LYP/6-31G(d,p) level of theory. Additionally, comparison of experimental and predicted optical and chiroptical responses shows that spiroconjugated molecular orbitals have a significant or negligible involvement on the main electronic transitions depending on the peripheral functionality of the spirobifluorene.

Tracking the Transition from Pericyclic to Pseudopericyclic Reaction Mechanisms Using Multicenter Electron Delocalization Analysis: The [1,3] Sigmatropic Rearrangement.

Herein, the power of multicenter electron delocalization analysis to elucidate the intricacies of concerted reaction mechanisms is brought to light by tracking the transition of [1,3] sigmatropic rearrangements from the high-barrier pericyclic mechanism in 1-butene to the barrierless pseudopericyclic mechanism in 1,2-diamino-1-nitrosooxyethane. This transition has been progressively achieved by substituting the migrating group, changing the donor and acceptor atoms, and functionalizing the alkene unit with weak and strong electron-donating and electron-withdrawing groups. Fourteen [1,3] sigmatropic reactions with electronic energy barriers ranging from 1 to 89 kcal/mol have been investigated. A very good correlation has been found between the barrier and the four-center electron delocalization at the transition state, the latter calculated for the atoms involved in the four-centered ring adduct formed along the reaction path. Surprisingly, the barrier has been found to be independent of the bond strength between the migrating group and the donor atom so that only the changes induced in the multicenter bonding control the kinetics of the reaction. Additional insights into the effect of atom substitution and group functionalization have also been extracted from the analysis of the multicenter electron delocalization profiles along the reaction path and qualitatively supported by the topological analysis of the electron density.

Key factors in the synthesis of polycyclic iridaaromatics via the methoxyalkenylcarbene pathway.

Polycyclic iridaaromatic compounds are of great interest not only because of the contributions made in "aromatic chemistry", but also because of the possibility of improving the results of the applications of the corresponding organic analogues in different fields. Therefore, understanding the requirements necessary to build on demand this type of compound with specific properties is of great importance. In this work, the keys to successfully synthesize iridaaromatic complexes via methoxyalkenylcarbenes are established. Experimental and theoretical results show (i) that bearing two aromatic substituents on the gamma carbon of the methoxyalkylcarbene promotes the C-H bond activation; (ii) the need for large steric hindrance of the second substituent for a selective synthesis and, (iii) the selectivity in the C-H bond activation towards the less sterically hindered system.

The effect of spin polarization on the electron transport of molecular wires with diradical character.

Some of the most promising materials for application in molecular electronics and spintronics are based on diradical chains. Herein, the proposed relation between increasing conductance with length and diradical character is revisited using ab initio methods that account for the static electron correlation effects. Electron transmission was previously obtained from restricted single determinant wavefuntions or tight-binding approximations, which are unable to account for static correlation. Broken Symmetry Unrestricted Kohn-Sham Density Functional Theory (BS-UKS-DFT) in combination with electron transport analysis based on electron deformation orbitals (EDOs) reflects an exponential decay of the electrical conductance with length. Also, other important effects such as quantum interference are correctly accounted for, leading to a decrease of the conductance as the diradical character increases. As a proof-of-concept, the electrical conductance obtained from BS-UKS-DFT and CASSCF(2,2) wavefunctions were compared in diradical graphene strips in the frame of the pseudo-π approach, obtaining very similar results.

Clar Goblet and Aromaticity Driven Multiradical Nanographenes.

The Clar Goblet, the first radical bowtie nanographene proposed by Erich Clar nearly 50 years ago, was recently synthesized. Bowtie nanographenes present quasi-degenerate magnetic ground states, which make them so elusive as unique. A thorough analysis is presented of the spin-state energetics of Clar Goblet and bowtie nanographenes by a battery of existing and novel ab initio procedures ranging from density functional theory to complete active space Hamiltonians. With this, it was proven that π radicals of bowtie nanographenes sit on BP (Benzo[cd]Pyrene) moieties driven by their local aromaticity, a purely chemical concept, which confers global stability to the whole structure. Besides, a novel Pauli energy densities analysis provided a visual intuitive explanation for this preference. These findings allow envisioning that analogous bowtie nanographenes with arbitrary polyradical character are not only feasible at the molecular scale but will share Clar Goblet's peculiar properties.

Distinct Helical Molecular Orbitals through Conformational Lock*.

Several theoretical studies have proposed strategies to generate helical molecular orbitals (Hel-MOs) in [n]cumulenes and oligoynes. While chiral even-[n] cumulenes feature Hel-MOs, odd-[n] cumulenes may also present them if the terminal groups lie in different planes. However, the proposed systems have been either experimentally unfeasible or resulted in opposite pseudo-degenerated Hel-MOs. We hereby demonstrate the introduction of a remarkable energy difference between helical orbitals of opposite twist by fixing the torsion angle between the terminal groups in butadiyne fragments. To experimentally lock the conformation of the terminal groups, we designed and synthesized cyclic architectures by combining acetylenes with chiral spirobifluorenes. The high stability of these systems with distinct helical orbitals allowed their isolation and full characterization. In our view, these results constitute a step further in the development of real systems presenting helical molecular orbitals.

Assessing the Reversed Exponential Decay of the Electrical Conductance in Molecular Wires: The Undeniable Effect of Static Electron Correlation.

An extraordinary new family of molecular junctions, inaccurately referred to as "anti-Ohmic" wires in the recent literature, has been proposed based on theoretical predictions. The unusual electron transport observed for these systems, characterized by a reversed exponential decay of their electrical conductance, might revolutionize the design of molecular electronic devices. This behavior, which has been associated with intrinsic diradical nature, is reexamined in this work. Since the diradical character arises from a near-degeneracy of the frontier orbitals, the employment of a multireference approach is mandatory. CASSCF calculations on a set of nanowires based on polycyclic aromatic hydrocarbons (PAHs) demonstrate that, in the frame of an appropriate multireference treatment, the ground state of these systems shows the expected exponential decay of the conductance. Interestingly, these calculations do evidence a reversed exponential decay of the conductance, although now in several excited states. Similar results have been obtained for other recently proposed candidates to "anti-Ohmic" wires. These findings open new horizons for possible applications in molecular electronics of these promising systems.

Spirobifluorene Metallaaromatics.

Spirobifluorene derivatives find use in many end-user applications. Therefore, further expansion of their scope is the focus of many research studies. However, although the optical properties of spirobifluorenes can be greatly tuned through incorporation of metal complexes, to date, spirobifluorene metallaaromatics remain unknown. Taking advantage of the versatility of our methodology for the synthesis of metallaaromatic systems, this work reports the first metallaaromatic spirobifluorene compound. The presence of an Ir atom was found to redshift the absorption maximum by ca. 1.1 eV compared to bare spirobifluorene. Additionally, X-ray analysis as well as anisotropy of the current-induced density calculations revealed this compound to be of aromatic nature. The high stability in solution, solid state, under air, and at high temperature, as well as distinct optical properties of this new class of compounds are expected to open new frontiers for chiroptical and optoelectronic applications.

Chiroptical Symmetry Analysis: Exciton Chirality-Based Formulae to Understand the Chiroptical Responses of Cn and Dn Symmetric Systems.

The high sensitivity of chiroptical responses to conformational changes and supramolecular interactions has prompted an increasing interest in the development of chiroptical applications. However, prediction of and understanding the chiroptical responses of the necessary large systems may not be affordable for calculations at high levels of theory. In order to facilitate the development of chiroptical applications, methodologies capable of evaluating the chiroptical responses of large systems are necessary. The exciton chirality method has been extensively used for the interaction between two independent chromophores through the Davydov model. For systems presenting C2 or D2 symmetry, one can get the same results by applying the selection rules. In the present article, the analysis of the selection rules for systems with symmetries Cn and Dn with n = 3 and 4 is used to uncover the origin of their chiroptical responses. We foresee that the use of the Chiroptical Symmetry Analysis (CSA) for systems presenting the symmetries explored herein, as well as for systems presenting higher symmetries will serve as a useful tool for the development of chiroptical applications.

Theoretical study of the decomposition of ethyl and ethyl 3-phenyl glycidate.

The mechanism of the decomposition of ethyl and ethyl 3-phenyl glycidate in gas phase was studied by density functional theory (DFT) and MP2 methods. A proposed mechanism for the reaction indicates that the ethyl side of the ester is eliminated as ethylene through a concerted six-membered cyclic transition state, and the unstable intermediate glycidic acid decarboxylates rapidly to give the corresponding aldehyde. Two possible pathways for glycidic acid decarboxylation were studied: one via a five-membered cyclic transition state, and the other via a four-membered cyclic transition state. The results of the calculations indicate that the decarboxylation reaction occurs via a mechanism with five-membered cyclic transition state.

A MP2 and DFT study of the influence of complexation on the aromatic character of phosphole.

This work is focused in three topical subjects: intermolecular interactions, metal ions, and aromaticity. A comprehensive MP2/6-31 + G and B3LYP/6-31 + G study of the influence of cation-π interactions on the aromatic character of phosphole was conducted. For this purpose, the structures of complexes were optimized at both theoretical level and different magnetic properties were evaluated. The main conclusion is the increase of the aromatic character of the phosphole when complexes with Li(+), Be(2+), and Al(3+) are formed.

Effect of microhydration on the guanidinium···benzene interaction.

The effect of microhydration on the interaction of guanidinium cation with benzene has been studied by employing ab initio calculations. Four different structural arrangements were considered for the guanidinium···benzene interaction to which up to six water molecules were added. T-shaped structures are usually the most stable, but as water molecules are included the energy differences with the parallel structures decrease, reaching a point where parallel complexes are even more stable than T-shaped ones. Therefore, the inclusion of water molecules promotes a change in the structure of the cation···π contact. The analysis reveals that these stability changes are more related with the structure of the hydrating water molecules than to a modulation of the cation···π interaction. Already with three water molecules, one water molecule in the T-shaped complex has to be located in the second solvation shell, whereas in parallel structures this occurs with four water molecules. As a consequence energy differences among structures decrease. The calculations show that the nature of the interaction is almost unaffected in T-shaped structures, whereas an important dispersion increment is observed in parallel ones, though its overall effect is small.

Computational study of the interaction of indole-like molecules with water and hydrogen sulfide.

The characteristics of the interaction between water and hydrogen sulfide with indole and a series of analogs obtained by substituting the NH group of indole by different heteroatoms have been studied by means of ab initio calculations. In all cases, minima were found corresponding to structures where water and hydrogen sulfide interact by means of X-H···π contacts. The interaction energies for all these π complexes are quite similar, spanning from -13.5 to -18.8 kJ/mol, and exhibiting the stability sequence NH > CH(2) ≈ PH > Se ≈ S > O, for both water and hydrogen sulfide. Though interaction energies are similar, hydrogen sulfide complexes are slightly favored over their water counterparts when interacting with the π cloud. σ-Type complexes were also considered for the systems studied, but only in the case of water complexes this kind of complexes is relevant. Only for complexes formed by water and indole, a significantly more stable σ-type complex was found with an interaction energy amounting to -23.6 kJ/mol. Oxygen and phosphorous derivatives also form σ-type complexes of similar stability as that observed for π ones. Despite the similar interaction energies exhibited by complexes with water and hydrogen sulfide, the nature of the interaction is very different. For π complexes with water the main contributions to the interaction energy are electrostatic and dispersive contributing with similar amounts, though slightly more from electrostatics. On the contrary, in hydrogen sulfide complexes dispersion is by far the main stabilizing contribution. For the σ-type complexes, the interaction is clearly dominated by the electrostatic contribution, especially in the indole-water complex.

A MP2 and DFT study of the aromatic character of polyphosphaphospholes. Is the pyramidality the only factor to take into consideration?

A comprehensive MP2/6-311 + G(d,p) and B3LYP/6-311 + G(d,p) study of the aromatic character of phospholes, P(n)(CH)(4-n )PH with n = 0-4 was conducted. For this purpose, the structures for these compounds were optimized at both theoretical levels and different magnetic properties (magnetic susceptibility anisotropy, χ(anis), and the nucleus-independent chemical shifts, NICS) were evaluated. For comparison, these magnetic properties were also calculated in the optimized structures with planarity constraints. We have also applied the ACID (anisotropy of the current-induced density) method in this analysis. The main conclusions are the aromatic character of these compounds, the relationship between aromaticity and planarity and the importance of other factors in this aromaticity.

A computational study of the mechanism of the unimolecular elimination of α,ß-unsaturated aldehydes in the gas phase.

The mechanism for the decarbonylation of (E)-2-butenal and (E)-2-methyl-3-phenyl-2-propenal was studied with different levels of ab initio and DFT methods. Reactants, products and transition structures were optimized for two kinds of reaction channel: a one-step reaction which involves a three-membered cyclic transition state, and a two-step reaction which involves an initial four-membered cyclic transition state. According to our calculations, these two possible mechanisms entail similar energetic costs, and there are only small differences depending on the reactant. The elimination of (E)-2-methyl-3-phenyl-2-propenal yields different products depending on the channel followed. Only one of the three possible one-step mechanisms leads directly to (E)-ß-methylstyrene (the main product according to experiment). This fact is reasonably well reproduced by our results, since the corresponding transition state gave rise to the lowest activation Gibbs free energy.

Characteristics of the interaction of azulene with water and hydrogen sulfide: A computational study.

A computational study was carried out for studying the characteristics of the interaction between azulene and water or hydrogen sulfide. In azulene...water complex the water molecule is located with both hydrogen atoms pointing toward the aromatic cloud but displaced to the five-membered ring. Hydrogen sulfide adopts a similar arrangement but located roughly over the central C-C bond of azulene. Calculations show that hydrogen sulfide interacts with azulene more strongly (-4.19 kcal/mol) than water (-3.76 kcal/mol), although this is only revealed at the highest levels of calculation. The nature of the interaction is electrostatic and dispersive in the same percentage for water cluster, whereas for hydrogen sulfide dispersion is the dominant contribution. Clusters containing two water molecules are controlled by the possibility of establishing an O-H...O hydrogen bond. As a consequence, the most stable structure corresponds to the interaction between a water dimer and azulene, with an interaction energy amounting to -11.77 kcal/mol. Hydrogen sulfide interaction is stronger with azulene than with itself, so structures with S-H...S contact and others, where H(2)S only interacts with azulene, present similar interaction energies (-8.02 kcal/mol for the most stable one).