Low-Temperature Photophysics of Permethylated n-Heptasilane: The Borderline Between Excitation Localization and Delocalization in a Conjugated Chain

Raymond, Mary Katherine; Magnera, Th. F.; Zharov, Ilya; West, Robert; Dreczewski, B.; Nozik, A. J.; Sprague, Julian; Ellingson, Randy J.; Michl, Josef

doi:10.1007/978-3-642-59903-3_4

Cited by 7 publications

(11 citation statements)

References 25 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Only qualitative experimental information is available for n -Si 5 Me 12 . At temperatures below ∼120 K the all-anti conformer is the only detectable form of n -Si 5 Me 12 present in a hydrocarbon solution, and it seems clear that under these conditions it is the most stable of all conformers, as it was in n -Si 4 Me 10 . Inspection of the results in Table shows that the pattern of relative differences between the computational methods established for n -Si 4 Me 10 carries over, in that the more compact twisted g and o conformers are energetically unfavorable relative to the a conformer in the HF and MM2 approximations, in qualitative agreement with solution results, while in the MM3 and MP2 approximations only the o bond conformation is unfavorable and the g conformer is comparable to the a conformer.…”

Section: Resultsmentioning

confidence: 97%

Conformers of n-Si₅Me₁₂: A Comparison of ab Initio and Molecular Mechanics Methods

et al. 1999

View full text Add to dashboard Cite

Optimized geometries of the conformers of permethylated linear pentasilane, n-Si5Me12, were calculated by the HF/3-21G*, MM3, MM2, and MM+ methods, which predict eight, nine, six, and six energetically distinct enantiomeric conformer pairs, respectively, at geometries representing various combinations of the anti (∼165°), ortho (∼90°), and gauche (∼55°) SiSiSiSi dihedral angles in the backbone. The results of the MM2 and MM+ methods, based on the same force field, differ insignificantly. The barriers between conformers appear to be exaggerated by the molecular mechanics methods, particularly MM2. Contour maps showing the ground-state energy as a function of the full range of two backbone SiSiSiSi dihedral angles, with all other geometrical variables optimized, computed by each of the methods (only a limited range of angles near the anti,anti geometry in the case of HF/3-21G*) are compared with each other and with analogous results for a model compound, Si4Me10. Conformer interconversion paths are discussed, and two meso transition states for enantiomer interconversion have been located at the HF/3-21G* level of calculation. At the eight HF/3-21G* optimized geometries, single-point energies (HF/6-31G* and MP2/6-31G*) and vibrational frequencies (HF/3-21G*) were computed. The predicted IR and Raman spectra suggest that about half of the expected conformers will be identifiable by vibrational spectroscopy under conditions of matrix isolation. Relative conformer energies calculated by the MM2 and HF methods are similar and favor the anti dihedral angles over gauche and ortho, in agreement with results of solution experiments. Those calculated by the MM3 and MP2 methods are similar to each other and favor both anti and gauche dihedral angles nearly equally over ortho, in agreement with indications provided by gas-phase experiments. A rationalization of these solvent effects is proposed. The energies of the conformers of Si4Me10 and Si5Me12 were used to set up a system of additive increments at the MM2, MM3, HF/3-21G*, HF/6-31G*, and MP2/6-31G* levels of calculation, which can be used to predict conformational energies of longer permethylated oligosilanes. An intrinsic energy value is assigned to each of the a, o, and g dihedral angles, and interaction energy values are assigned to each combination of two dihedral angles. The interaction values follow the expected rules in that equal twist sense is favored for adjacent aa, ag, oo, and gg pairs, whereas opposite twist sense is generally favored for adjacent ao and go pairs. The MM3-derived set of increments has been tested against results computed for Si6Me14 and found to perform well.

show abstract

Section: Resultsmentioning

confidence: 97%

Conformers of n-Si₅Me₁₂: A Comparison of ab Initio and Molecular Mechanics Methods

et al. 1999

View full text Add to dashboard Cite

show abstract

“…It has been known for some time that the permethylated linear oligosilane series Si n Me 2 n +2 represents an exemplary realization of the classically anticipated response of singlet excitation in one dimension to quantum confinement. , In very long chains electronic excitation is present as a large-radius exciton with an extrapolated excitation energy of 30 000 cm –1 (Table ). The exciton extends over two or three dozen Si–Si bonds , and emits with a nearly vanishing Stokes shift (400 cm –1 in permethylated hexadecasilane and less in longer chains, cf.…”

Section: Discussionmentioning

confidence: 99%

“…18 The yield, however, increases to almost unity upon sufficient cooling. 17 Permethylated hexasilane, pentasilane, and tetrasilane exhibit the same FC-forbidden blue fluorescence. It shifts to slightly longer wavelengths as the chain is shortened, and cooling to still lower temperatures is then necessary for the quantum yield to approach unity.…”

Section: ■ Introductionmentioning

confidence: 96%

“…While at least one conformer continues the steady trend of gradual change in fluorescence properties, others display a very different strongly Stokes-shifted FC-forbidden broad blue emission with a lower room-temperature quantum yield . The yield, however, increases to almost unity upon sufficient cooling …”

Section: Introductionmentioning

confidence: 98%

“…The emission spectra of extended conformations of short oligosilanes vary in a quite striking fashion as a function of the chain length. , For permethylated decasilane and longer chains, the FC-allowed shape, very small Stokes shift, and high quantum yield are the same as in the high polymers. As the chain is shortened, the Stokes shift increases, and the emission band broadens, very slowly at first.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

From Ordinary to Blue Emission in Peralkylated n-Oligosilanes: The Calculated Structure of Delocalized and Localized Singlet Excitons

MacLeod

Michl

2014

J. Phys. Chem. A

View full text Add to dashboard Cite

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.

show abstract

Electronic Structure and Spectroscopy of Polysilanes

Michl

West

2000

Silicon-Containing Polymers

View full text Add to dashboard Cite

Low-Temperature Photophysics of Permethylated n-Heptasilane: The Borderline Between Excitation Localization and Delocalization in a Conjugated Chain

Cited by 7 publications

References 25 publications

Conformers of n-Si₅Me₁₂: A Comparison of ab Initio and Molecular Mechanics Methods

Conformers of n-Si₅Me₁₂: A Comparison of ab Initio and Molecular Mechanics Methods

From Ordinary to Blue Emission in Peralkylated n-Oligosilanes: The Calculated Structure of Delocalized and Localized Singlet Excitons

Electronic Structure and Spectroscopy of Polysilanes

Contact Info

Product

Resources

About

Low-Temperature Photophysics of Permethylated n-Heptasilane: The Borderline Between Excitation Localization and Delocalization in a Conjugated Chain

Cited by 7 publications

References 25 publications

Conformers of n-Si5Me12: A Comparison of ab Initio and Molecular Mechanics Methods

Conformers of n-Si5Me12: A Comparison of ab Initio and Molecular Mechanics Methods

From Ordinary to Blue Emission in Peralkylated n-Oligosilanes: The Calculated Structure of Delocalized and Localized Singlet Excitons

Electronic Structure and Spectroscopy of Polysilanes

Contact Info

Product

Resources

About

Conformers of n-Si₅Me₁₂: A Comparison of ab Initio and Molecular Mechanics Methods

Conformers of n-Si₅Me₁₂: A Comparison of ab Initio and Molecular Mechanics Methods