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Helical Electronic Transitions of Spiroconjugated Molecules

preprint
revised on 27.01.2021, 15:23 and posted on 28.01.2021, 12:38 by Marc Hamilton Garner, Clemence Corminboeuf
The two perpendicularly oriented π-systems of allene mix into helical molecular orbitals (MOs) when the symmetry of the molecule is reduced. However, the π-π transitions of allenes are linear combinations of two excitations that always consist of both helicities; consequently, the electronic transitions are not helical. Here, we examine the electronic structure of spiroconjugated molecules, which have the same parent symmetry as allene but with different relative orientation of the two π-systems. We show how the π-mixing in spiropentadiene is analogous to the helical π-mixing in allene. However, in spiroconjugated systems only half the π-MOs become helical. Due to this difference, the π-π transitions in substituted spiropentadiene come in near-degenerate pairs where the helicity is symmetry protected, and consequently there is no significant mixing between excitations involving MOs of opposite helicity. This inherent helicity of the π-π* transitions is verified by computation of the change of electron density. These transitions have big rotatory strengths where the sign correlates with the helicity of the transition. The electronic helicity of spiroconjugated molecules thus manifests itself in observable electronic and optical properties.

Funding

Independent Research Fund Denmark, case no. 9056-00009B

History

Email Address of Submitting Author

marc.garner@epfl.ch

Institution

Laboratory for Computational Molecular Design, Ecole Polytechnique Federale de Lausanne (EPFL)

Country

Switzerland

ORCID For Submitting Author

0000-0002-7270-8353

Declaration of Conflict of Interest

No conflict of interest.

Version Notes

Minor corrections and additions from first upload.

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