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 Si_nMe_2n+2 (n = 2–8, 10), deduced from temperature-dependent measurements on conformer mixtures in hydrocarbon solvents, by a nearly constant amount (∼2000 cm⁻¹). The TD B3LYP/6-31G(d) results are less satisfactory. The first band is calculated to be due to a σπ* excitation in Si₂Me₆ and to a superposition of overlapping σσ* and σπ* excitations in the longer oligosilanes. The σπ* excitation is calculated to lie a little below the σσ* excitation up to Si₄Me₁₀, the two transitions are nearly degenerate in Si₅Me₁₂, and the σσ* excitation drops increasingly below the σπ* as the chain length is extended. The dipole strength of the σσ* 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 σπ* 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 σπ* origin using intensity borrowing σ–π mixing vibrations (b₁ in Si₃Me₈). As usual, the red shift is attributed to the disappearance of twisted conformers, which have higher calculated σσ* excitation energies.