The making and breaking of σ bonds is an integral part of almost all photochemical reactions. Yet, the electronic states of σ electrons are not nearly as well understood as the states of π-electron systems. Efforts in our laboratory to enhance the current state of their understanding are described, using the specific example of oligosilanes. We address the intrinsically cyclic nature of σ delocalization and its dependence on chain length and conformation, both in terms of theory and spectroscopic experiments, from the simplest disilane chromophore to the spectral properties of the individual conformers of permethylated heptasilane. We also describe a new low-energy luminescence from certain conformers of permethylated oligosilanes.

Photoelectron spectra and solution UV absorption and magnetic circular dichroism (MCD) of hexamethyldisilane (1), hexaethyldisilane (2), hexa-tert-butyldisilane (3), and the 1,(n+2)-disila[n.n.n]propellanes [n = 4 (4) and 5 (5)] were measured, as was the linear dichroism (LD) of 3 and 4 partially aligned in stretched polyethylene. The results support the assignment of the lowest energy electronic absorption band of the disilanes 1−5 to a doubly degenerate σSiSi(HOMO) → π*SiC(LUMO) transition and of the next band, observed in the solution spectra of 2−4 and in the gas-phase spectrum of 1, to a σSiSi → σ*SiSi transition. MP2/VTZ optimized geometries of 1−5 and ab initio molecular orbital energies (HF/VTZ//MP2/VTZ) and ionization potentials (ROVGF/VTZ//MP2/VTZ) of these disilanes reproduce the reported geometries and the trends observed in the photoelectron spectra, respectively. B3LYP/6-31G(d,p) calculations of the Kohn−Sham orbital energies and TD B3LYP/6-31G(d,p) calculations of transition energies and intensities of 1 as a function of Si−Si bond length suggest that many of the features of the UV absorption spectrum of 3, including the small energy difference between the two transitions observed and the large extinction coefficient of the band peaking at higher energy (σSiSi → σ*SiSi), are due to its very long Si−Si bond.

The photophysical properties of oligosilanes show unique conformational dependence due to σ-electron delocalization. The excited states of the SAS, AAS, and AEA conformations of peralkylated n-hexasilanes, in which the SiSiSiSi dihedral angles are controlled into a syn (S), anti (A), or eclipsed (E) conformation, were investigated by using UV absorption, magnetic circular dichroism (MCD), and linear dichroism spectroscopy. Simultaneous Gaussian fitting of all three spectra identified a minimal set of transitions and the wavenumbers, oscillator strengths, and MCD B terms in all three compounds. The results compare well with those obtained by using the symmetry-adapted-cluster configuration interaction method and almost as well with those obtained by time-dependent density functional theory with the PBE0 functional. The conformational dependence of the transition energies and other properties of free-chain permethylated n-hexasilane, n-Si Me , was also examined as a function of dihedral angles, and the striking effects found were attributed to avoided crossings between configurations of σσ* and σπ* character.

The authors noticed that in their Full Paper one of the units in Table 1 has been labeled incorrectly. In Table 1, the heading for the fifth column states that the units of B are 10 2 b e /cm À1. This is wrong, the units are 10 4 Debye 2 b e /cm À1. This mistake has no effect anywhere in the rest of the manuscript. The authors apologize for the oversight , and would like to thanks Professors Raul Crespo and Carmen Piqueras of the University of Valencia, Spain, for bringing the error to their attention.

An overview of magnetic circular dichroism (MCD) spectroscopy of π‐electron systems derived from a 4N‐electron perimeter is provided, with emphasis on the hypothetical parent cycloocta‐1,3,5,7‐tetraene of D8h symmetry (1) and its D4h symmetry derivatives. UV‐visible absorption and MCD spectra of 2, a D4h symmetric cycloocta‐1,3,5,7‐tetraene planarized by the effect of 4 bicyclo[2.1.1]hexeno units fused to its 8‐membered ring, are reported and interpreted. The perimeter model is applied to obtain an understanding of the nature of electronic states in 1 and 2 and to predict general trends in the spectra. The electronic excitation patterns are found to be different in the antiaromatic D8h and unaromatic D4h species, and their states cannot be unequivocally correlated. The results of time‐dependent density functional theory and extended multistate complete active space second‐order perturbation theory (XMS‐CASPT2) computations agree with the algebraic perimeter model analysis and reproduce the spectra of 2 well, including three of the four observed MCD signs of A and B terms.

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-SiO(2) and by UV-vis absorption spectroscopy on SiO(2). 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.

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We report the synthesis and photophysical characterization of 7-dimethylamino-3-methyl-N-methyl-d(3)-4-phenylethynylcarbostyril, a chromophore of interest as a rotator in surface-mounted molecular rotors. Measurement of UV-vis absorption and fluorescence spectra, steady state fluorescence and excitation anisotropy, and linear dichroism in the IR and UV-vis permitted a determination of absolute vibrational and electronic transition moment directions in this previously unreported chromophore. The first singlet-singlet absorption and fluorescence are polarized perpendicular to the axle of the rotator. Density functional theory calculations of electronic excitation and vibrational frequencies gave results in very good agreement with those observed. Calculated IR transition moment directions showed rather poor agreement with experiment.