Ramón Valencia

Thermodynamically consistent equation of state (EOS) of two-temperature aluminum across a wide range of parameter space (compression ratio ratios V₀/V up to 4, electronic temperatures T_e up to 1 500 000 K, and ionic temperature T_ion up to 10 000 000 K for T_e up to 40 000 K) is investigated from the free energy calculations using density functional theory (DFT) based first-principles electronic structure methods. Our results can serve as a stringent benchmark for the present EOS model and database, where various approximations are adopted, used in hydrodynamic simulations as well as developing new EOS models. We find that the Thomas-Fermi model for the electronic pressure overestimates the EOS within the present parameter space, whereas the Thomas-Fermi model with exchange corrections are in good agreement with our results for T_e greater than 600 000 K. The ionic pressure for a given ionic temperature T_ion is found to be nearly independent of the electronic temperature at high temperatures, which can be modeled with kinetic theory for T_ion larger than 1 000 000 K for various T_e. The asymptotic behavior of the electronic contributions to the plasma pressure is further analyzed and casted into a compact analytical form with a few fitting parameters. This analytical form is physically well motivated and reproduces the desired asymptotic behaviors of the EOS within the interested parameter space. Therefore, our results can be conveniently used for modeling important properties and processes of high energy density systems.

In this study, we have strategically designed and convergently synthesized two novel, symmetrical, and linear A–D–A-type π-conjugated donor molecules (TBDTCNR, TBDTCN), each containing a planar electron-rich 2-octylthiene-5-yl-substituted benzodithiophene (TBDT) unit as the core, flanked by octylthiophene units and end-capped with electron-deficient cyanoacetate (CNR) or dicyanovinyl (CN) units. We thoroughly characterized both of these materials and investigated the effects of the end groups (CNR, CN) on their optical, electrochemical, morphological, and photovoltaic properties. We then fabricated solution-processed bulk heterojunction organic solar cells incorporating TBDTCNR and TBDTCN. Among our tested devices, the one containing TBDTCNR and [6,6]-phenyl-C₆₁-butyric acid methyl ester in a 1:0.40 ratio (w/w) exhibited the highest power conversion efficiency (5.42%) with a short-circuit current density (J_sc) of 9.08 mA cm^–2, an open circuit voltage (V_oc) of 0.90 V, and an impressive fill factor (FF) of 0.66 under AM 1.5G irradiation (100 mW cm^–2). The FFs of these solution-processed small-molecule organic solar cells (SMOSCs) are outstanding when compared with those recently reported for benzodithiophene (BDT)-based SMOSCs, because of the high crystallinity and excellent stacking properties of the TBDT-based compounds.

In this work a detailed investigation of the exohedral reactivity of the most important and abundant endohedral metallofullerene (EMF) is provided, that is, Sc₃N@I_h‐C₈₀ and its D_5h counterpart Sc₃N@D_5h‐C₈₀, and the (bio)chemically relevant lutetium‐ and gadolinium‐based M₃N@I_h/D_5h‐C₈₀ EMFs (M=Sc, Lu, Gd). In particular, we analyze the thermodynamics and kinetics of the Diels–Alder cycloaddition of s‐cis‐1,3‐butadiene on all the different bonds of the I_h‐C₈₀ and D_5h‐C₈₀ cages and their endohedral derivatives. First, we discuss the thermodynamic and kinetic aspects of the cycloaddition reaction on the hollow fullerenes and the two isomers of Sc₃N@C₈₀. Afterwards, the effect of the nature of the metal nitride is analyzed in detail. In general, our BP86/TZP//BP86/DZP calculations indicate that [5,6] bonds are more reactive than [6,6] bonds for the two isomers. The [5,6] bond D_5h‐b, which is the most similar to the unique [5,6] bond type in the icosahedral cage, I_h‐a, is the most reactive bond in M₃N@D_5h‐C₈₀ regardless of M. Sc₃N@C₈₀ and Lu₃N@C₈₀ give similar results; the regioselectivity is, however, significantly reduced for the larger and more electropositive M=Gd, as previously found in similar metallofullerenes. Calculations also show that the D_5h isomer is more reactive from the kinetic point of view than the I_h one in all cases which is in good agreement with experiments.

The new endohedral fullerene, Sc₂(μ₂-O)@C_s(6)-C₈₂, has been isolated from the carbon soot obtained by electric arc generation of fullerenes utilizing graphite rods doped with 90% Sc₂O₃ and 10% Cu (w/w). Sc₂(μ₂-O)@C_s(6)-C₈₂ has been characterized by single crystal X-ray diffraction, mass spectrometry, and UV/vis spectroscopy. Computational studies have shown that, among the nine isomers that follow the isolated pentagon rule (IPR) for C₈₂, cage 6 with C_s symmetry is the most favorable to encapsulate the cluster at T > 1200 K. Sc₂(μ₂-O)@C_s(6)-C₈₂ is the first example in which the relevance of the thermal and entropic contributions to the stability of the fullerene isomer has been clearly confirmed through the characterization of the X-ray crystal structure.

By introducing SO₂ as the sulfur source, the synthesis of an extensive family of cluster endohedral fullerenes Sc₂S@C_2n (n = 40–50) was demonstrated and a new isomer of Sc₂S@C₈₂ (C_s : 6) was isolated and characterized by spectroscopic and electrochemical methods for the first time.

An extensive study of the redox properties of metal nitride endohedral fullerenes (MNEFs) based on DFT computational calculations has been performed. The electronic structure of the singly oxidized and reduced MNEFs has been thoroughly analyzed and the first anodic and cathodic potentials, as well as the electrochemical gaps, have been predicted for a large number of M₃N@C_2n systems (M=Sc, Y, La, and Gd; 2n=80, 84, 88, 92, and 96). In particular, calculations that include thermal and entropic effects correctly predict the different anodic behavior of the two isomers (I_h and D_5h) of Sc₃N@C₈₀, which is the basis for their electrochemical separation. Important differences were found in the electronic structure of reduced M₃N@C₈₀ when M=Sc or when M is a more electropositive metal, such as Y or Gd. Moreover, the changes in the electrochemical gaps within the Gd₃N@C_2n series (2n=80, 84, and 88) have been rationalized and the use of Y‐based computational models to study the Gd‐based systems has been justified. The redox properties of the largest MNEFs characterized so far, La₃N@C_2n (2n=92 and 96), were also correctly predicted. Finally, the quality of these predictions and their usefulness in distinguishing the carbon cages for MNEFs with unknown structures is discussed.

Density functional theory computations for Sc₄(μ₃-O)₂@I_h-C₈₀ and Sc₄(μ₃-O)₃@I_h-C₈₀ (which is more stable than the alternative Sc₄(μ₃-O)₂@I_h-C₈₀O) reveal that the electronic structures of these two Sc-oxide endohedral metallofulerenes are different. Sc₄(μ₃-O)₂@I_h-C₈₀ involves a mixed valence cluster with the highest occupied molecular orbital (HOMO) localized on the metal cluster while in Sc₄(μ₃-O)₃@I_h-C₈₀ the HOMO is localized on the carbon cage and the electronic structure resembles that of Sc₃[email protected]I_h-C₈₀.

Size doesn't matter: Metallic nitride endohedral fullerenes (MNEFs) exhibit redox properties that are not dependent on the size of the carbon cage, but on its electronic properties. These findings support the ionic model of MNEFs. magnified image

Structures are proposed for six endohedral metallofullerenes with large carbon cages (from C₉₂ to C₁₀₀) on the basis of sizeable (LUMO−4)–(LUMO−3) gap and the formal transfer of six electrons to the cages.