Time-dependent density functional theory treatment of the first UV absorption band in all-transoid permethyloligosilanes SinMe2n + 2 (n = 2-8, 10)

Understanding the Effect of Conformation on Hole Delocalization in Poly(dimethylsilane)

Density functional theory calculations confirm that the simple explanation of the origin of the striking conformational dependence of σ-electron localization/delocalization in polysilanes offered by the extremely simple Ladder C model is correct.

Asymmetric charge separation and recombination in symmetrically functionalized σ–π hybrid oligosilanes

The flexibility of σ-conjugated silanes presents new opportunities for controlling charge transfer via changes in molecular conformation.

σ-Bond Electron Delocalization in Oligosilanes as Function of Substitution Pattern, Chain Length, and Spatial Orientation

Polysilanes are known to exhibit the interesting property of σ-bond electron delocalization. By employing optical spectroscopy (UV-vis), it is possible to judge the degree of delocalization and also differentiate parts of the molecules which are conjugated or not. The current study compares oligosilanes of similar chain length but different substitution pattern. The size of the substituents determines the spatial orientation of the main chain and also controls the conformational flexibility. The chemical nature of the substituents affects the orbital energies of the molecules and thus the positions of the absorption bands.

From ordinary to blue emission in peralkylated n-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(n)Me(2n+2)) have been found and characterized with time-dependent density functional (TD-DFT) methods for chain lengths 4 ≤ n ≤ 16. For chain lengths with n > 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 n < 8 more strongly distorted structures with one long Si-Si bond built from silicon 3p 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 n = 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.

Electronic transitions in conformationally controlled tetrasilanes with a wide range of SiSiSiSi dihedral angles

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-Si4 Me10 (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.

Fifth stereoactive orbital on silicon: relaxation of the lowest singlet excited state of octamethyltrisilane

We address relaxation pathways in the excited singlet states S(1) of saturated molecules, specifically alkylated oligosilanes. Unlike their longer peralkylated homologues, disilanes and trisilanes do not fluoresce even at low temperatures. An examination of the S(1) potential energy surface of Si(3)Me(8) with density functional (TDDFT, LC-TDDFT), and ab initio (RICC2, RIADC(2)) methods with TZVP basis sets revealed only extremely shallow minima in the vicinity of funnels, accounting for the absence of fluorescence, rapid internal conversion, and photoproducts. Relaxed singlet excited state structures either contain one approximately trigonal bipyramidal Si atom or two that are halfway between tetrahedral and trigonal bipyramidal. Four of the ligands are those that the Si atom had in the ground state. Natural bond orbital analysis suggests that the fifth one is a nonbonding hybrid orbital of the lone-pair type and size intermediate between valence and Rydberg, with an only very small occupancy, yet stereochemically active. The fifth natural hybrid orbital is composed primarily of 4s, 4p, and usually to a lesser degree, also 3d atomic orbitals. The trigonal bipyramidal structure allows an optimal accommodation of the presence of both a negative and a positive charge in the Lewis structures. The excess negative charge on the distorted Si atom is shared between the nonbonding fifth hybrid orbital and σ* antibonding orbitals associated with its bonds. The positive charge resides in an adjacent σ SiSi bond orbital. A Rydberg minimum also occurs on the S(1) surface at the geometry of the radical cation.

Conformational dependence of sigma-electron delocalization in linear chains: permethylated oligosilanes

The effects of sigma-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 sigma and pi characters of a backbone molecular orbital and a fractional overall and local sigma sigma* and sigma pi* characters of an excited state for any conformation of a linear chain.

Interpretation of the electronic spectra of four disilanes

Time-dependent density functional theory (TD-DFT/B3LYP(AC)/cc-pVTZ/cc-pVTZ/6-311G//MP2/cc-pVTZ/cc-pVTZ/6-31G**) has been used to compute vertical excitation energies and oscillator strengths of the six low-lying excited states of four peralkylated disilanes, hexamethyldisilane (1), hexa-tert-butyldisilane (2), 1,6-disila[4.4.4]propellane (3), and 1,7-disila[5.5.5]propellane (4). The results provide an accurate interpretation of the reported UV absorption spectra of 1-4 in solution, and for 1 also in the gas phase up to 62,000 cm(-1). The excellent agreement of the calculated with the available experimental energies and oscillator strengths, and with magnetic circular (MCD) and linear (LD) dichroism, gives us confidence that the method will be useful for dependable interpretation of the electronic spectra of longer oligosilanes. Although the disilane chromophore finds itself in quite different environments in 1-4, its fundamental characteristics remain the same, with one important exception. In all four compounds, the first valence excited state is due to an electron promotion from the sigma(1) HOMO to the pi(1)* orbital, and the second valence excited state to a promotion from the sigma(1) HOMO to the sigma(1)* orbital. Surprisingly, however, it is only in 2, which has an extraordinarily long SiSi bond, that the terminating sigma(1)* orbital is the sigma*(SiSi) antibond, as anticipated, and the sigma sigma* transition has the expected very high oscillator strength. In 1, 3, and 4, the sigma*(SiSi) antibonding orbital is high in energy and does not play any role in low-energy excitations. Instead, the terminating orbital of the sigma(1)sigma(1)* excitation is represented by Si-alkyl antibonds, combined symmetrically with respect to rotation around the SiSi axis and antisymmetrically with respect to operations that interchange the two Si atoms. The common assumption that the characteristic intense sigma sigma* transitions of longer peralkylated oligosilanes extrapolate to the lowest sigma sigma* transition in common peralkylated disilanes is incorrect, and only the weak sigma pi* transitions extrapolate simply.

all-anti-Octasilane: Conformation Control of Silicon Chains Using the Bicyclic Trisilane as a Building Block

The perfect all-anti-octasilane composed of two bicyclic trisilane units with trimethylsilyl groups at the termini has been synthesized. Its X-ray crystal structure and spectroscopic data demonstrate the effective sigma-delocalization over the silicon framework, which is a definitive difference from the unconstrained n-Si8Me18.

Magnetic circular dichroism of peralkylated tetrasilane conformers

Magnetic circular dichroism (MCD) of five peralkylated tetrasilanes (1-5) conformationally constrained to angles ranging from nearly 0 degrees to 180 degrees and of the open chain tetrasilane Si(4)Me(10) (6) shows a clear conformational dependence and permits the detection of previously hidden transitions. In the tetrasilane CH(2)Si(4)Me(8) (1), with the smallest dihedral angle, comparison of MCD with absorption spectra reveals four low-energy electronic transitions. In the tetrasilanes 2-4, three distinct transitions are apparent. In tetrasilanes 5 and 6, MCD reveals the very weak transition that has been predicted to be buried under the first intense peak and to which the anomalous thermochromism of 6 and other short-chain oligosilanes has been attributed.