Daniel Escudero

Very Long-Range Effects: Cooperativity between Anion–π and Hydrogen-Bonding Interactions

The interplay between two important non-covalent interactions involving aromatic rings (namely anion-pi and hydrogen bonding) is investigated. Very interesting cooperativity effects are present in complexes where anion-pi and hydrogen bonding interactions coexist. These effects are found in systems where the distance between the anion and the hydrogen-bond donor/acceptor molecule is as long as approximately 11 A. These effects are studied theoretically using the energetic and geometric features of the complexes, which were computed using ab initio calculations. We use and discuss several criteria to analyze the mutual influence of the non-covalent interactions studied herein. In addition we use Baders theory of atoms-in-molecules to characterize the interactions and to analyze the strengthening or weakening of the interactions depending upon the variation of the charge density at the critical points.

Interplay between anion‐π and hydrogen bonding interactions

The interplay between two important noncovalent interactions involving aromatic rings is studied by means of high level ab initio calculations. They demonstrate that synergistic effects are present in complexes where anion‐π and hydrogen bonding interactions coexist. These synergistic effects have been studied using the “atoms‐in‐molecules” theory and the Molecular Interaction Potential with polarization partition scheme. The present study examines how these two interactions mutually influence each other.

Ruthenium(II) photosensitizers of tridentate click-derived cyclometalating ligands: a joint experimental and computational study.

A systematic series of heteroleptic bis(tridentate)ruthenium(II) complexes of click-derived 1,3-bis(1,2,3-triazol-4-yl)benzene N^C^N-coordinating ligands was synthesized, analyzed by single crystal X-ray diffraction, investigated photophysically and electrochemically, and studied by computational methods. The presented comprehensive characterization allows a more detailed understanding of the radiationless deactivation mechanisms. Furthermore, we provide a fully optimized synthesis and systematic variations towards redox-matched, broadly and intensely absorbing, cyclometalated ruthenium(II) complexes. Most of them show a weak room-temperature emission and a prolonged excited-state lifetime. They display a broad absorption up to 700 nm and high molar extinction coefficients up to 20 000 M(-1)cm(-1) of the metal-to-ligand charge transfer bands, resulting in a black color. Thus, the complexes reveal great potential for dye-sensitized solar-cell applications.

RASPT2/RASSCF vs Range-Separated/Hybrid DFT Methods: Assessing the Excited States of a Ru(II)bipyridyl Complex.

The excited states of the trans(Cl)-Ru(bpy)Cl2(CO)2 (bpy = bypyridyl) transition-metal (TM) complex are assessed using the newly developed second-order perturbation theory restricted active space (RASPT2/RASSCF) method. The delicate problem of partitioning the RAS subspaces (RAS1, RAS2, and RAS3) is addressed, being the choice of the RAS2 the bottleneck to obtain a balanced description of the excited states of different nature when TMs are present. We find that the RAS2 should be composed by the correlation orbitals involved in covalent metal-ligand bonds. The level of excitations within the RAS1 and RAS3 subspaces is also examined. The performance of different flavors of time-dependent density functional theory including pure, hybrid, meta-hybrid, and range-separated functionals in the presence of solvent effects is also evaluated. It is found that none of the functionals can optimally describe all the excited states simultaneously. However, the hybrid M06, B3LYP, and PBE0 functionals seem to be the best compromise to obtain a balanced description of the excited states of trans(Cl)-Ru(bpy)Cl2(CO)2, when comparing with the experimental spectrum. The conclusions obtained in this molecule should pave the road to properly treat excited states of larger Ru-polypyridyl complexes, which are of particular interest in supramolecular chemistry.

Molecular Structure–Intersystem Crossing Relationship of Heavy-Atom-Free BODIPY Triplet Photosensitizers

A thiophene-fused BODIPY chromophore displays a large triplet-state quantum yield (ΦT = 63.7%). In contrast, when the two thienyl moieties are not fused into the BODIPY core, intersystem crossing (ISC) becomes inefficient and ΦT remains low (ΦT = 6.1%). First-principles calculations including spin-orbit coupling (SOC) were performed to quantify the ISC. We found larger SOC and smaller singlet-triplet energy gaps for the thiophene-fused BODIPY derivative. Our results are useful for studies of the photochemistry of organic chromophores.

Exploring the Triplet Excited State Potential Energy Surfaces of a Cyclometalated Pt(II) Complex: Is There Non-Kasha Emissive Behavior?

In this Article, we address the complexity of the emissive processes of a square-planar heteroleptic Pt(II) complex bearing 2-phenylpyridine (ppy) as cyclometalated ligand and an acetylacetonate derivative (dbm) as ancillary ligand. The origins of emission were identified with the help of density functional theory (DFT) and quadratic response (QR) time-dependent (TD)-DFT calculations including spin-orbit coupling (SOC). To unveil the photodeactivation mechanisms, we explored the triplet potential energy surfaces and computed the SOCs and the radiative decay rates (kr) from possible emissive states. We find that emission likely originates from a higher-lying (3)MLCT/(3)LLCT state and not from the Kasha-like (3)MLCT/(3)LCdbm state. The temperature-dependent nonradiative deactivation mechanisms were also elucidated. The active role of metal-centered ((3)MC) triplet excited states is confirmed for these deactivation pathways.

Assessing the density functional theory-based multireference configuration interaction (DFT/MRCI) method for transition metal complexes

We report an assessment of the performance of density functional theory-based multireference configuration interaction (DFT/MRCI) calculations for a set of 3d- and 4d-transition metal (TM) complexes. The DFT/MRCI results are compared to published reference data from reliable high-level multi-configurational ab initio studies. The assessment covers the relative energies of different ground-state minima of the highly correlated CrF6 complex, the singlet and triplet electronically excited states of seven typical TM complexes (MnO4(-), Cr(CO)6, [Fe(CN)6](4-), four larger Fe and Ru complexes), and the corresponding electronic spectra (vertical excitation energies and oscillator strengths). It includes comparisons with results from different flavors of time-dependent DFT (TD-DFT) calculations using pure, hybrid, and long-range corrected functionals. The DFT/MRCI method is found to be superior to the tested TD-DFT approaches and is thus recommended for exploring the excited-state properties of TM complexes.

Design of acidochromic dyes for facile preparation of pH sensor layers

AbstractEight new acidochromic dyes have been synthesised that can be used for optically monitoring pH in the range from 3 to 12. Their corresponding pKa values have been both measured and calculated theoretically by means of density functional theory. The synthesis of these new dyes is facile without the need for chromatographic purification. The dyes can be covalently linked to polymers containing hydroxyl functions such as cellulose, polyurethane hydrogel, and hydroxyalkyl methacrylate. The resulting sensor layers exhibit significant colour changes both in the UV and in the visible spectral range. FigureSensor layers based on covalently immobilised pH indicator dyes show fast and fully reversible colour changes.

Spectroscopic Properties of Azobenzene-Based pH Indicator Dyes: A Quantum Chemical and Experimental Study

The UV-visible absorption spectra of six new optical sensors based on acidochromic azobenzenes have been measured and assigned with the help of quantum chemical calculations. The investigated compounds are able to monitor the pH in the range from pH 3-10. Using the hybrid density functional PBE0 and including solvent effects with a polarized continuum model, the agreement between the experimental and theoretical UV/vis spectra of the dyes in their neutral and anionic forms is very good. The spectroscopic ππ* states, responsible for the optical properties of the sensors, are described within an accuracy of 0.1 eV. Similar accuracy is demonstrated in the nπ* states. The ππ* states can be assigned as a charge transfer from the aromatic π orbital localized in the azo-phenol moiety to the antibonding π* of the azo group. Under basic conditions, the spectrum is bathochromically shifted and more intense than in acid media. Upon substitution in the phenyl moiety, red- or blue-shifts of the UV-visible bands are observed depending on whether the substituent is electron-donor or -withdrawing, respectively. These effects are stronger at high pH values and can be rationalized in terms of the stabilization and/or destabilization of the involved frontier orbitals.

Interplay between edge-to-face aromatic and hydrogen-bonding interactions.

The interplay between two important noncovalent interactions involving aromatic rings is studied by means of MP2/6-31++G** ab initio calculations. They indicate that synergistic effects are present in complexes where edge-to-face aromatic interactions and hydrogen-bonding interactions coexist. These synergistic effects have been studied bu using the atoms in molecules theory and the molecular interaction potential with polarization partition scheme. Experimental evidence for such interactions has been obtained from the Cambridge Structural Database.