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Electronic excitations of 1,4-disilyl-substituted 1,4-disilabicycloalkanes: A MS-CASPT2 study of the influence of cage size
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Biochemistry and Organic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Biochemistry and Organic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Biochemistry and Organic Chemistry.
2007 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 111, no 14, 2804-2810 p.Article in journal (Refereed) Published
Abstract [en]

We present a multistate complete active space second-order perturbation theory computational study aimed to predict the low-lying electronic excitations of four compounds that can be viewed as two disilane units connected through alkane bridges in a bicyclic cage. The analysis has focused on l,4-disilyl-l,4-disilabicyclo[2.2.1]heptane (la), l,4-bis(trimethylsilyl)-l,4- disilabicyclo[2.2.1]heptane (1b), l,4-disilyl-l,4-disilabicyclo[2.1.1]hexane (2a), and l,4-fcw(trimethylsilyl)-l,4-disilabicyclo[2.1.1]hexane (2b). The aim has been to find out the nature of the lowest excitations with significant oscillator strengths and to investigate how the cage size affects the excitation energies and the strengths of the transitions. Two different substituents on the terminal silicon atoms (H and CH3) were used in order to investigate the end group effects. The calculations show that the lowest allowed excitations are of the same character as that found in disilanes but now redshifted. As the cage size is reduced from a 1,4-disilabicyclo[2.2.1]heptane to a l,4-disilabicyclo[2.1.1]hexane, the Si'"Si through-space distance decreases from approximately 2.70 to 2.50 A and the lowest allowed transitions are red-shifted by up to 0.9 eV, indicating increased interaction between the two Si-Si bonds. The first ionization potential, which corresponds to ionization from the Si-Si a orbitals, is lower in Ib and 2b than in SiiMee by approximately 0.9 and 1.2 eV, respectively. Moreover, Ib and 2b, which have methyl substituents at the terminal Si atoms, have slightly lower excitation energies than the analogous species la and 2a.

Place, publisher, year, edition, pages
2007. Vol. 111, no 14, 2804-2810 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-95451DOI: 10.1021/jp070010nISI: 000245438600021PubMedID: 17388376OAI: oai:DiVA.org:uu-95451DiVA: diva2:169661
Available from: 2007-02-14 Created: 2007-02-14 Last updated: 2011-02-07Bibliographically approved
In thesis
1. Heavy-Core Staffanes: A Computational Study of Their Fundamental Properties of Interest for Molecular Electronics
Open this publication in new window or tab >>Heavy-Core Staffanes: A Computational Study of Their Fundamental Properties of Interest for Molecular Electronics
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The basic building blocks in molecular electronics often correspond to conjugated molecules. A compound class consisting of rigid rod-like staffane molecules with the heavier Group 14 elements Si, Ge, Sn and Pb at their bridgehead positions has now been investigated. Herein these oligomers are called heavy-core or Si-, Ge-, Sn- or Pb-core staffanes. These compounds benefit from interaction through their bicyclo[1.1.1]pentane monomer units. Quantum chemical calculations were performed to probe their geometries, stabilities and electronic properties associated with conjugation.

The stabilities of the bicyclo[n.n.n]alkane and [n.n.n]propellanes (1 ≤ n ≤ 3) with C, Si, Ge and Sn at the bridgehead positions were studied by calculation of homodesmotic ring strain energies. The bicyclic compounds with n = 1 and Si, Ge or Sn at bridgehead positions have lower strain than the all-carbon compound.

A gradually higher polarizability exaltation is found as the bridgehead element is changed from C to Si, Ge, Sn or Pb. The ratio between longitudinal and average polarizability also increases gradually as Group 14 is descended, consistent with enhanced conjugation in the heavier oligomers.

The localization of polarons in C-, Si- and Sn-core staffane radical cations was calculated along with internal reorganization energies. The polaron is less localized in Si- and Sn-core than in C-core staffane radical cation. The reorganization energies are also lower for the heavier staffanes, facilitating hole mobility when compared to the C-core staffanes.

The effect of the bicyclic structure on the low valence excitations in the UV-spectra of compounds with two connected disilyl segments was also investigated. MS-CASPT2 calculations of 1,4-disilyl- and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.2.1]heptanes and 1,4-disilyl- and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.1.1]hexanes revealed that although the bicyclic cage separates the two disilyl chromophores, there is a strong red-shift of the lowest valence excitations when compared to an isolated disilane.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2007. 80 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 271
Keyword
Organic chemistry, conjugation, polarizability, Group 14 elements, cage compounds, electronic excitations, quantum chemistry, molecular electronics, Organisk kemi
Identifiers
urn:nbn:se:uu:diva-7492 (URN)978-91-554-6796-8 (ISBN)
Public defence
2007-03-07, B22, BMC, Husargatan 3, Uppsala, 13:15
Opponent
Supervisors
Available from: 2007-02-14 Created: 2007-02-14 Last updated: 2011-02-07Bibliographically approved

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