Maria Fumanal

Keys for the Existence of Stable Dimers of Bis-tetrathiafulvalene (bis-TTF)-Functionalized Molecular Clips Presenting [TTF]•+···[TTF]•+ Long, Multicenter Bonds at Room Temperature

A systematic theoretical and computational investigation is performed to determine the keys governing the existence, in acetonitrile solutions, of dimers of bis-tetrathiafulvalene (bis-TTF)-functionalized diphenylglycoluril molecular clips (clip2(n+)) that are stable at room temperature for n ≤ 4. Although the experimental structure of these dimers in solution is unknown, electronic absorption studies suggest that they have [TTF](l+)···[TTF](m+) interactions that are preserved at room temperature (note that when l = m = 1 these interactions become long, multicenter bonds). In good agreement with the interpretation of the experimental spectroscopic data, all clip2(n+) dimers whose charge is ≤4 present an optimum geometry that, in all cases, has three short interfragment [TTF](l+)···[TTF](m+) interactions. The computed ΔG(298 K) for these optimum structures matches the available experimental data on the stability of these dimers. Such optimum geometry, combined with the zwitterionic character of the electron distribution in monomers and dimers (most of the net positive charge is equally distributed among the TTF groups, while a 1- au charge is located in the central fused five-membered rings) allows the formation of a maximum of two long, multicenter [TTF](•+)···[TTF](•+) bonds when all TTF groups host a 1+ au of charge, as in clip2(4+). However, these long, multicenter bonds alone do not account for the stability of clip2(n+) dimers at room temperature. Instead, the studies carried out here trace the origin of their stability to (1) the zwitterionic character of their charge distribution, (2) the proper geometrical shape of the interacting monomers, which allows the intercalation of their arms, thus making possible the simultaneous formation of two short contacts, both involving the positively charged TTF group of one monomer and the negatively charged central ring of the other, (3) the simultaneous presence of three short contacts among the TTF groups in the optimum geometry of the clip2(n+) dimers, which become two long, multicenter bonds and one van der Waals interaction when the four TTF groups host a 1+ charge, and (4) the net stabilizing effect of the solvent.

Description of excited states in [Re(Imidazole)(CO)3 (Phen)](+) including solvent and spin-orbit coupling effects: Density functional theory versus multiconfigurational wavefunction approach.

The low‐lying electronic excited states of [Re(imidazole)(CO)3(phen)]+ (phen = 1,10‐phenanthroline) ranging between 420 nm and 330 nm have been calculated by means of relativistic spin‐orbit time‐dependent density functional theory (TD‐DFT) and wavefunction approaches (state‐average‐CASSCF/CASPT2). A direct comparison between the theoretical absorption spectra obtained with different methods including SOC and solvent corrections for water points to the difficulties at describing on the same footing the bands generated by metal‐to‐ligand charge transfer (MLCT), intraligand (IL) transition, and ligand‐to‐Ligand‐ charge transfer (LLCT). While TD‐DFT and three‐roots‐state‐average CASSCF (10,10) reproduce rather well the lowest broad MLCT band observed in the experimental spectrum between 420 nm and 330 nm, more flexible wavefunctions enlarged either by the number of roots or by the number of active orbitals and electrons destabilize the MLCT states by introducing IL and LLCT character in the lowest part of the absorption spectrum.

On the zeroth‐order hamiltonian for CASPT2 calculations of spin crossover compounds

Complete active space self‐consistent field theory (CASSCF) calculations and subsequent second‐order perturbation theory treatment (CASPT2) are discussed in the evaluation of the spin‐states energy difference (ΔHelec) of a series of seven spin crossover (SCO) compounds. The reference values have been extracted from a combination of experimental measurements and DFT + U calculations, as discussed in a recent article (Vela et al., Phys Chem Chem Phys 2015, 17, 16306). It is definitely proven that the critical IPEA parameter used in CASPT2 calculations of ΔHelec, a key parameter in the design of SCO compounds, should be modified with respect to its default value of 0.25 a.u. and increased up to 0.50 a.u. The satisfactory agreement observed previously in the literature might result from an error cancellation originated in the default IPEA, which overestimates the stability of the HS state, and the erroneous atomic orbital basis set contraction of carbon atoms, which stabilizes the LS states.

Ultrafast Excited-State Decays in [Re(CO)3(N,N)(L)]n+: Nonadiabatic Quantum Dynamics

The ultrafast luminescent decay of [Re(CO)3(phen)(im)]+, representative of Re(I) carbonyl α-diimine photosensitizers, is investigated by means of wavepacket propagations based on the multiconfiguration time-dependent Hartree (MCTDH) method. On the basis of electronic structure data obtained at the time-dependent density functional theory (TD-DFT) level, the luminescence decay is simulated by solving a 14 electronic states multimode problem including both vibronic and spin-orbit coupling (SOC) up to 15 vibrational modes. A careful analysis of the results provides the key features of the mechanism of the intersystem crossing (ISC) in this complex. The intermediate state, detected by means of fs - ps time-resolved spectroscopies, is assigned to the T3 state corresponding to the triplet intraligand (3IL) transition localized on the phen ligand. By switching off/on SOC and vibronic coupling in the model it is shown that efficient population transfer occurs from the optically active metal-to-ligand-charge-transfer1,3MLCT states to T3 and to the lowest long-lived phosphorescent 3MLCT (T1) state. The early ultrafast SOC-driven decay followed by a T3/T1 equilibration controlled by vibronic coupling underlies the photoluminescent properties of [Re(CO)3(phen)(im)]+. The impact of the axial and N,N ligands on the photophysics of this class of Re(I) complexes is further rationalized on the basis of their calculated optical properties. The relative position of the 3IL and upper 3MLCT states with respect to the optically active singlet state is influenced by the N,N ligand and affects the relaxation dynamics.

Three Redox States of a Diradical Acceptor-Donor-Acceptor Triad: Gating the Magnetic Coupling and the Electron Delocalization.

The diradical acceptor-donor-acceptor triad 1(••), based on two polychlorotriphenylmethyl (PTM) radicals connected through a tetrathiafulvalene(TTF)-vinylene bridge, has been synthesized. The generation of the mixed-valence radical anion, 1(•-), and triradical cation species, 1(•••+), obtained upon electrochemical reduction and oxidation, respectively, was monitored by optical and ESR spectroscopy. Interestingly, the modification of electron delocalization and magnetic coupling was observed when the charged species were generated and the changes have been rationalized by theoretical calculations.

On the importance of thermal effects and crystalline disorder in the magnetism of benzotriazinyl-derived organic radicals.

Recent experiments suggest that benzotriazinyl-derived radicals are promising building blocks for the design of new functional materials. Herein, a detailed computational study of the main structural and magnetic features of two prototypes of this family of radicals is presented. By means of several computational techniques within the DFT framework, this work unveils the key importance of the thermal contraction of the crystal to quantitatively predict the magnetism of the studied compounds. In this sense, for the first time in the context of molecular magnetism, we propose to use variable-cell geometry optimizations as an efficient alternative to obtain an estimation of low-temperature crystal structures. The crucial role of crystalline disorder in defining the structure present at low temperature, and thus, the magnetic response, is revealed. Altogether, these are important elements for the rational design of future materials of this family of compounds.

Unravelling the Key Driving Forces of the Spin Transition in π-Dimers of Spiro-biphenalenyl-Based Radicals.

Spiro-biphenalenyl (SBP) boron radicals constitute an important family of molecules for the preparation of functional organic materials. The building blocks of several SBP-based crystals are π-dimers of these radicals, in which two phenalenyl (PLY) rings face each other and the other two PLYs point away from the superimposed PLYs. The dimers of ethyl-SBP and butyl-SBP undergo a spin transition between a diamagnetic and a paramagnetic state upon heating, while other dimers exhibit paramagnetism at all temperatures. Here, we present a computational study aimed at establishing the driving forces of the spin-transition undergone by ethyl-SBP at ∼140 K. The ground state of the π-dimers below 140 K is a singlet state in which the SBP unpaired electrons are partially localized in the superimposed PLYs. Above 140 K, the unpaired electrons are localized in the nonsuperimposed PLYs. These high-temperature structures are exclusively governed by the ground triplet state because the open-shell singlet with the unpaired electrons localized in the nonsuperimposed PLYs does not feature any minimum in the potential energy surface of the system. Furthermore, we show that the electrostatic component of the interaction energy between SBP radicals in the π-dimers is more attractive in the triplet than in the singlet, thereby partially counteracting the bonding and dispersion components, which favor the singlet. This electrostatic stabilization of the triplet state is a key driving force of the spin transition of ethyl-SBP and a key factor explaining the paramagnetic response of the π-dimers of other SBP-based crystals.

Lattice-Solvent Effects in the Spin-Crossover of an Fe(II)-Based Material. The Key Role of Intermolecular Interactions between Solvent Molecules

The spin transition of Fe(II) complexes is the subject of intensive synthetic and computational efforts. In this manuscript, we analyze the spin crossover (SCO) of [Fe(E-dpsp)2]2+ (1), which features a spin transition depending on the cocrystallizing solvent molecules. Whereas the use of acetone results in a hysteretic spin transition at ∼170 K, the use of propylene carbonate (PC) results in a permanent diamagnetic signal up to 300 K. By means of DFT+U+D2 calculations in the solid state of the material, we unravel the reasons for such different behavior. Our results allow us to ascribe the relatively low transition temperature of 1(BF4)2·acetone to the distorted arrangement of the SCO molecules in the low-spin state of the material. In turn, intermolecular interactions play the primary role in the case of 1(BF4)2·2PC. In particular, we found that solvent-solvent interactions actively promote the stability of the low-spin state due to the formation of PC dimers. These dimers would appear at larger distances in the high-spin phase, with the subsequent loss of phase stability. This is yet another proof of how subtle is the spin transition phenomenon in Fe(II)-based architectures.

Diffusion Monte Carlo Perspective on the Spin-State Energetics of [Fe(NCH)6]2+

The energy difference between the high spin and the low spin state of the model compound [Fe(NCH)6](2+) is investigated by means of Diffusion Monte Carlo (DMC), where special attention is dedicated to analyzing the effect of the fix node approximation on the accuracy of the results. For this purpose, we compare several Slater-Jastrow and multireference Slater-Jastrow trial wave functions. We found that a Slater-Jastrow trial wave function constructed with the generalized Kohn-Sham orbitals from hybrid DFT represents the optimal choice. This is understood by observing that hybrid functionals account for the subtle balance between exchange and correlation effects and the respective orbitals accurately describe the ligand-metal hybridization as well as the charge reorganization accompanying the spin transition. Finally the DMC results are compared with those obtained by Hartree-Fock, DFT, CASSCF, and CASPT2. While there is no clear reference value for the high spin-low spin energy difference, DMC and high level CCSD(T) calculations agree within around 0.3 eV.

Quantitative wave function analysis for excited states of transition metal complexes

The character of an electronically excited state is one of the most important descriptors employed to discuss the photophysics and photochemistry of transition metal complexes. In transition metal complexes, the interaction between the metal and the different ligands gives rise to a rich variety of excited states, including metal-centered, intra-ligand, metal-to-ligand charge transfer, ligand-to-metal charge transfer, and ligand-to-ligand charge transfer states. Most often, these excited states are identified by considering the most important wave function excitation coefficients and inspecting visually the involved orbitals. This procedure is tedious, subjective, and imprecise. Instead, automatic and quantitative techniques for excited-state characterization are desirable. In this contribution we review the concept of charge transfer numbers---as implemented in the TheoDORE package---and show its wide applicability to characterize the excited states of transition metal complexes. Charge transfer numbers are a formal way to analyze an excited state in terms of electron transitions between groups of atoms based only on the well-defined transition density matrix. Its advantages are many: it can be fully automatized for many excited states, is objective and reproducible, and provides quantitative data useful for the discussion of trends or patterns. We also introduce a formalism for spin-orbit-mixed states and a method for statistical analysis of charge transfer numbers. The potential of this technique is demonstrated for a number of prototypical transition metal complexes containing Ir, Ru, and Re. Topics discussed include orbital delocalization between metal and carbonyl ligands, nonradiative decay through metal-centered states, effect of spin-orbit couplings on state character, and comparison among results obtained from different electronic structure methods.