A new class of rod-shaped strongly dipolar molecular rotors for insertion into channels of hexagonal tris(o-phenylenedioxy)cyclotriphosphazene (TPP) has been examined. Seven different 3,6-disubstituted pyridazines and one singly 3-substituted system have been prepared and studied by solid-state nuclear magnetic resonance (NMR), X-ray powder diffraction, and dielectric spectroscopy. NMR and X-ray diffraction both show that all but one of these molecular rotors form hexagonal bulk inclusion compounds with TPP. In-plane lattice parameters for the hexagonal phases increase with the size of the end group, which also controls the energy barriers for rotation of the pyridazine dipole. The barriers range from ≈4 kcal mol⁻¹ for small or flexible end groups to less than 0.7 kcal mol⁻¹ for 3-methylbicyclo[1.1.1]pent-1-yl end groups after annealing to 235 °C, and an interpretation of these differences is offered. Computer modeling of the relaxed TPP channels followed by density functional calculation of the environment for one of the rotors provides quantitative agreement with the observed barrier. The systems with the lowest rotational barriers show signs of collective behavior, discussed in terms of antiferroelectric intrachannel and ferroelectric interchannel dipole–dipole interactions. A Curie temperature of 22 K is deduced for 3,6-diadamant-1′-ylpyridazine, but no ordered dielectric phases are found. Conclusions have been drawn for improved rotor design.

The choice of chromophores and of their mutual geometrical arrangement for optimized singlet fission (SF) rates are considered. The electronic matrix element that enters the Fermi golden rule for the rate of SF is worked out algebraically for a simple model, but the density of states factor is not analyzed here. The model treats only the highest occupied and lowest unoccupied orbitals of the partners. It provides an approximate formula that requires only the knowledge of the expansion coefficients of these orbitals and of overlap integrals between atomic orbitals on the partners to obtain an estimate of the electronic matrix element. An illustrative application to a pair of ethylene molecules suggests that favored geometries will be those in which one of the AOs on the first ethylene overlaps with both AOs on the second ethylene, while the other AO on the first ethylene overlaps with at least one, and preferably both, AOs of the second ethylene as little as possible.

In toluene containing LiCB₁₁(CH₃)₁₂ as the supporting electrolyte, the oxidation potential of H₂ on a platinized Pt electrode is a strong function of the composition of a basic buffer, lithium 2,2,6,6-tetramethylpiperidinate / 2,2,6,6-tetramethylpiperidine, and can approach the onset of the reduction of Li⁺. This suggests that only a moderate voltage might be needed to use H₂ as a surrogate for Li and to take advantage of the specific chemical reactivity of the latter.

Unlike π-electron chromophores, the peralkylated n-tetrasilane σ-electron chromophore resembles a chameleon in that its electronic spectrum changes dramatically as its silicon backbone is twisted almost effortlessly from the syn to the anti conformation (changing the SiSiSiSi dihedral angle ω from 0 to 180°). A combination of UV absorption, magnetic circular dichroism (MCD), and linear dichroism (LD) spectroscopy on conformationally controlled tetrasilanes 1–9, which cover fairly evenly the full range of angles ω, permitted a construction of an experimental correlation diagram for three to four lowest valence electronic states. The free chain tetrasilane n-Si₄Me₁₀ (10), normally present as a mixture of three enantiomeric conformer pairs of widely different angles ω, has also been included in our study. The spectral trends are interpreted in terms of avoided crossings of 1B with 2B and 2A with 3A states, in agreement with SAC-CI calculations on the excited states of 1–7 and conformers of 10.

The trideuteriomethylation of BH vertices in CB₁₁H₁₂⁻ and its derivatives with CD₃OTf (OTf=triflate, trifluoromethanesulfonate) yields a mixture of BCD₃ and BCHD₂ substitution products, thus demonstrating the intermediacy of a species with a long enough lifetime for hydrogen scrambling between the boron vertex and the methyl substituent. No such scrambling is observed if CD₃OTf is used to methylate toluene. According to density functional theory calculations, the intermediate in BH vertex methylation is a three-center bonded σ adduct of a methyl cation to the BH bond and the proton scrambling occurs via a transition structure containing a distorted square-pyramidal methane attached axially to a “naked” boron vertex. The subsequent proton or deuteron loss is presently not understood in detail. A general comparison of electrophilic substitution on closo-boranes and arenes is provided and similarities as well as differences are discussed. A recalculation of the optimized geometry of the CB₁₁Me₁₂^. radical produced a second Jahn–Teller distorted minimum and resulted in a somewhat improved agreement between calculated and measured proton hyperfine coupling constants.

The effects of σ-electron delocalization on optical properties of saturated linear chains of permethylated oligosilanes are strongly conformation dependent. We analyze the origin of the conformational dependence of the energies of molecular orbitals and of electronic excitations in simple intuitively understandable terms by using a first-order approximation to the Hückel version of the “Ladder C” model. The analysis is supported by comparison with results of numerical calculations from time-dependent density functional theory, which agree well with experiment. To facilitate the comparison, a simple procedure has been developed that defines the overall and local fractional σ and π characters of a backbone molecular orbital and a fractional overall and local σσ* and σπ* characters of an excited state for any conformation of a linear chain.