<b>Update 1 of:</b><b>Chemistry of the Carba-</b><i><b>closo</b></i><b>-dodecaborate(−) Anion, CB</b>_<b>11</b><b>H</b>_<b>12</b>^<b>–</b>

<b>Update 1 of:</b><b>Chemistry of the Carba-</b><i><b>closo</b></i><b>-dodecaborate(−) Anion, CB</b>_<b>11</b><b>H</b>_<b>12</b><sup><b>–</b></sup

From Ordinary to Blue Emission in Peralkylated <i>n</i>‑Oligosilanes: The Calculated Structure of Delocalized and Localized Singlet Excitons

Excited singlet state structures believed to be responsible for the Franck–Condon-allowed and the strongly Stokes-shifted (blue) emissions in linear permethylated oligosilanes (Si_<i>n</i>Me_2<i>n</i>+2) have been found and characterized with time-dependent density functional (TD-DFT) methods for chain lengths 4 ≤ <i>n</i> ≤ 16. For chain lengths with <i>n</i> > 7, the S₁ relaxed structures closely resemble the S₀ equilibrium structures where all valence angles are tetrahedral and all backbone dihedral angles are transoid. At chain lengths with <i>n</i> < 8 more strongly distorted structures with one long Si–Si bond built from silicon 3<i>p</i> orbitals are encountered. The large Stokes shift is due more to a large destabilization of the ground state than the relaxation in the S₁ excited state. For <i>n</i> = 7, both types of minima were located, exactly reproducing the borderline between the large-radius and the small-radius self-trapped excitons known from experiments

Five Stereoactive Orbitals on Silicon: Charge and Spin Localization in the <i>n</i>‑Si₄Me₁₀^–• Radical Anion by Trigonal Bipyramidalization

RIUMP2/def2-TZVPPD calculations show that in addition to its usual conformation with charge and spin delocalized over the Si backbone, the isolated Si₄Me₁₀^–• radical anion also has isomeric conformations with localized charge and spin. A structure with localization on a terminal Si atom has been examined in detail. In vacuum, it is calculated to lie 11.5 kcal/mol higher in energy than the charge-and-spin delocalized conformation, and in water the difference is as little as 1.6 kcal/mol. According to natural orbital and localized orbital analyses, the charge-and-spin-carrying terminal Si atom uses five stereoactive hybrid orbitals in a trigonal bipyramidal geometry. Four are built mostly from 3s and 3p atomic orbitals (AOs) and are used to attach a Si₃(CH₃)₇ and three CH₃ groups, whereas the larger equatorial fifth orbital is constructed from 4s and 4p AOs and acts as a nonbonding (radical) hybrid orbital with an occupancy of about 0.65 <i>e</i>

Search for a Small Chromophore with Efficient Singlet Fission: Biradicaloid Heterocycles

Of the five small biradicaloid heterocycles whose S₁, S₂, T₁, and T₂ adiabatic excitation energies were examined by the CASPT2/ANO-L-VTZP method, two have been found to meet the state energy criterion for efficient singlet fission and are recommended to the attention of synthetic chemists and photophysicists

Appendice Quinta meditazione

Friction in molecular rotors is examined by classical molecular dynamics simulations for grid-mounted azimuthal dipolar molecular rotors, whose rotation is either allowed to decay freely or is driven at GHz frequencies by a flow of rare gas or by a rotating electric field. The rotating parts (rotators) are propeller-shaped. Their two to six blades consist of condensed aromatic rings and are attached to a deltahedral carborane hub, whose antipodal carbons carry [<i>n</i>]staffane axles mounted on a square molecular grid. The dynamic friction constant η has been derived in several independent ways with similar results. Analysis of free rotation decay yields η as a continuous exponentially decreasing function of rotor frequency. The calculated dependence of friction torque on frequency resembles the classical macroscopic Stribeck curve. Its relation to rotational potential energy barriers and the key role of the rate of intramolecular vibrational redistribution (IVR) of energy and angular momentum from rotator rotation to other modes are considered in two limiting regimes. (i) In the strongly overdamped regime, rotation is much slower than IVR, and effective friction can be expressed through potential barriers to rotation. (ii) In the strongly underdamped regime, rotation is much faster than IVR, whose rate then determines friction

Synthesis of Hexadehydrotribenzo[<i>a,e,i</i>][12]annulenes by Acetylene Insertion into an Open-Chain Precursor

A simple synthesis of a hexadehydrotribenzo­[<i>a,e,i</i>]­[12]­annulene by insertion of acetylene into an open-chain diiodo precursor under Sonogashira coupling conditions has been developed and used to prepare a rigid three-armed star connector for testing as a building block for a two-dimensional hexagonal hydrogen-bonding array

Captodatively Stabilized Biradicaloids as Chromophores for Singlet Fission

Singlet fission offers an opportunity to improve solar cell efficiency, but its practical use is hindered by the limited number of known efficient materials. We look for chromophores that satisfy the desirable but rarely encountered adiabatic energy conditions, <i>E</i>(T₂) – <i>E</i>(S₀) > <i>E</i>(S₁) – <i>E</i>(S₀) ≈ 2­[<i>E</i>(T₁) – <i>E</i>(S₀)], and are small enough to permit highly accurate calculations. We provide a rationale for the use of captodative biradicaloids, i.e., biradicals stabilized by direct interaction between their radical centers, which carry both an acceptor and a donor group. A computation of vertical excitation energies of 14 structures of this type by time-dependent density functional theory (TD-DFT) yielded 11 promising candidates. The vertical excitation energies from S₀ and T₁ were recalculated by complete-active-space second-order perturbation theory (CASPT2), and five of the compounds met the above energy criteria. Their adiabatic excitation energies from the S₀ into the S₁, S₂, T₁, and T₂ excited states were subsequently calculated, and three of them look promising. For 2,3-diamino-1,4-benzoquinone, adiabatic <i>E</i>(T₁) and <i>E</i>(S₁) energies were close to optimal (1.12 and 2.23 eV above the S₀ ground state, respectively), and for its more practical <i>N</i>-peralkylated derivative they were even lower (0.63 and 1.06 eV above S₀, respectively). PCM/CASPT2 results suggested that the relative energies can be further tuned by varying the polarity of the environment

Average Orientation of a Molecular Rotor Embedded in a Langmuir–Blodgett Monolayer

A molecular rotor in which a naphthalene rotator is attached through a silicon atom to three fatty acid chains has been synthesized, and Langmuir–Blodgett techniques were used to deposit on silica surfaces monolayers of its calcium salt, both neat and diluted with stearic acid salts. The monolayer films have been characterized by ellipsometry and Fourier transform infrared (FT-IR) grazing-incidence attenuated total internal reflection (GATR) spectroscopy on Si-SiO₂ and by UV–vis absorption spectroscopy on SiO₂. The measurements were combined with calculations of the electronic (INDO/S) and vibrational (DFT) transition moment directions to deduce the average orientation of the rotor molecules, including the naphthalene ring, relative to the surface. In both neat and mixed films, the naphthalene ring is found to preferentially tilt toward the surface, enough that its rotation is most likely hindered. A comparable picture was obtained from molecular mechanics calculations on a mixed film of the naphthalene rotor and stearic acid

Highly Branched Polyisobutylene by Radical Polymerization under Li[CB₁₁(CH₃)₁₂] Catalysis

In the presence of a nonoxidizing radical initiator, azo-<i>tert</i>-butane, and a high concentration of LiCB₁₁(CH₃)₁₂, isobutylene undergoes thermal or light-induced radical polymerization to <i>b</i>-PIB, a highly branched polymer of modest molecular weight (mostly a few thousand and up to ∼25 000 g/mol based on GPC with polystyrene standards). The structure of <i>b</i>-PIB was elucidated by NMR spectroscopy of a low-molecular-weight fraction. The polymer is branched on every carbon atom of the main chain; one chain end carries an isobutenyl group, and the other carries a tert-butyl group originating in the initiator. The branches are short segments of <i>l</i>-PIB (linear polyisobutylene), on the average composed of five IB units. A mechanism of formation if this dendrimer-like structure is proposed

Synthesis of Hexadehydrotribenzo[<i>a,e,i</i>][12]annulenes by Acetylene Insertion into an Open-Chain Precursor

A simple synthesis of a hexadehydrotribenzo­[<i>a,e,i</i>]­[12]­annulene by insertion of acetylene into an open-chain diiodo precursor under Sonogashira coupling conditions has been developed and used to prepare a rigid three-armed star connector for testing as a building block for a two-dimensional hexagonal hydrogen-bonding array