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

Abstract

The TD B3LYP/6-311G(d,p) method slightly overestimates the excitation energies of the first UV absorption band of the all-transoid conformers of SinMe2n + 2 (n = 2-8, 10), deduced from temperature-dependent measurements on conformer mixtures in hydrocarbon solvents, by a nearly constant amount (approximately 2000 cm-1). The TD B3LYP/6-31G(d) results are less satisfactory. The first band is calculated to be due to a sigma pi * excitation in Si2Me6 and to a superposition of overlapping sigma sigma * and sigma pi * excitations in the longer oligosilanes. The sigma pi * excitation is calculated to lie a little below the sigma sigma * excitation up to Si4Me10, the two transitions are nearly degenerate in Si5Me12, and the sigma sigma * excitation drops increasingly below the sigma pi * as the chain length is extended. The dipole strength of the sigma sigma * excitation grows by 4.8 D2 (D = debye) per added SiSi bond (more slowly up to n = 5) and the calculation overestimates it by a factor of about three. The sigma pi * excitation is computed to carry no or almost no oscillator strength, but as noted earlier by others, its presence is critical for the interpretation of the observed thermochromism. Upon cooling below room temperature, the first absorption maximum is blue-shifted in short chains and red-shifted in long chains. Unlike the prior investigators, we attribute the blue shift to the disappearance of hot bands built on the sigma pi * origin using intensity borrowing sigma-pi mixing vibrations (b1 in Si3Me8). As usual, the red shift is attributed to the disappearance of twisted conformers, which have higher calculated sigma sigma * excitation energies.

Magnetic circular dichroism of peralkylated tetrasilane conformers

“…It also provides additional confirmation of the presence of at least three transitions in the constrained tetrasilanes 2-4 and allows the detection of a weak lowest-energy transition in 5 and 6 that had been postulated (28,31) to play a crucial role in the thermochromism of short-chain oligosilanes. The spectrum of the open chain, n-Si 4 Me 10 (6), is in agreement with the dominance of the transoid conformer in its room-temperature solution.…”

“…Less reliable time-dependent (TD) density functional theory (DFT) calculations, available at the Becke three-parameter hybrid functional combined with Lee-YangParr correlation functional (B3LYP)͞double zeta (DZ) level for a large number of dihedral angles, produced similar results for the lowest six states of n-Si 4 Me 10 (6). B3LYP͞DZ and B3LYP͞ triple zeta results are also available for certain conformations of permethylated chains up to n-Si 10 Me 22 (28). Results of even more approximate calculations on peralkylated oligosilanes were published earlier (10,25,29).…”

Intuitive Understanding of σ Delocalization in Loose and σ Localization in Tight Helical Conformations of an Oligosilane Chain

“…The only observedi ntensea bsorption maximum gradually shifts from 43 100 cm À1 in Si 4 Me 10 to 34 100 cm À1 in Si 10 Me 22 , and gains af actor of nearly 10 in intensity in the process. The long-axis polarized excitationst ot he first and only calculated strongly allowed excited state of B symmetry are in good agreement with the positions of the intense bands apparent in the observed absorption spectra.A ll calculated energies are slightly too high (perhaps owing to solvent effects).…”

“…None of the four lowest transitions have significant intensity.T his has ad ramatic effect on the appearance of the calculated absorption spectra, as is illustrated for Si 13 Me 28 in Figure 4( the peak half-widtha th alf maximum, 700 cm À1 ,w as chosen to fit roughlyt he shapeo ft he low-temperatures pectrum of all-[t]-1 [10]). [22] Down to 1508,t he transition to the 1B state is the strongest, but at 1208,i ta lready only appearsw eakly and 2B with 3B carry far more oscillator strength.T he trend continues, and the calculated absorption spectra of the all-o and all-g conformers do not show any intense absorption at lowenergies. The increase in the energy of the 1B transition, the gradual shift of oscillator strength to higher energy transitions, and the reduced separation between transitions as w decreases are striking.…”

“…In peralkylated n-tetrasilanes, the spectra of which have been studied in considerable detail in as eries of conformations, these additional transitions have been uncovered and are in impressive agreement with the resultso fT D-DFT/TZ, CAS-PT2, and SAC-CI calculations. [4,22,25] Al ess detaileds tudy is availablefor af ew conformers of peralkylated hexasilanes. [49] The right-hand side of Figure 2s erves only for highlighting the contrast between s-delocalized and a s-localized behavior and will be dealt with in more detail elsewhere.…”