title: A Scheme for the Evaluation of Electron Delocalization and Conjugation Efficiency in Linearly π-Conjugated Systems.
abstract: In this study, we present a scheme for the evaluation of electron delocalization and conjugation efficiency in lineraly π-conjugated systems. The scheme, based on the natural bond orbital theory, allows monitoring the evolution of electron delocalization along an extended conjugation path as well as its response to chemical modification. The scheme presented is evaluated and illustrated by means of a computational investigation of π-conjugation in all-trans polyacetylene [PA; H(-CH═CH)n-H], polydiacetylene [PDA, H(-C≡C-CH═CH)n-H], and polytriacetylene [PTA, H(-C≡C-CH═CH-C≡C)n-H] with up to 180 carbon atoms, all related by the number of ethynyl units incorporated in the chain. We are able to show that for short oligomers the incorporation of ethynyl spacers into the PA chain increases the π-delocalization energy, but, on the other hand, reduces the efficiency with which π-electron delocalization is promoted along the backbone. This explains the generally shorter effective conjugation lengths observed for the properties of the polyeneynes (PDA and PTA) relative to the polyenes (PA). It will also be shown that the reduced conjugation efficiency, within the NBO-based model presented in this work, can be related to the orbital interaction pattern along the π-conjugated chain. We will show that the orbital interaction energy pattern is characteristic for the type and the length of the backbone and may therefore serve as a descriptor for linearly π-conjugated chains.
authors.affiliation: Department of Environmental Science, Università degli Studi di Milano-Bicocca, Piazza della Scienza 1, I-20126 Milano, Italy; Laboratory for Physical Chemistry, ETH Zürich, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland; and Physical Chemisty Institute, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Bruschi
authors.affiliation: Department of Environmental Science, Università degli Studi di Milano-Bicocca, Piazza della Scienza 1, I-20126 Milano, Italy; Laboratory for Physical Chemistry, ETH Zürich, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland; and Physical Chemisty Institute, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.
authors.forename: Peter A
authors.initials: PA
authors.lastname: Limacher
authors.affiliation: Department of Environmental Science, Università degli Studi di Milano-Bicocca, Piazza della Scienza 1, I-20126 Milano, Italy; Laboratory for Physical Chemistry, ETH Zürich, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland; and Physical Chemisty Institute, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.
authors.forename: Jürg
authors.initials: J
authors.lastname: Hutter
authors.affiliation: Department of Environmental Science, Università degli Studi di Milano-Bicocca, Piazza della Scienza 1, I-20126 Milano, Italy; Laboratory for Physical Chemistry, ETH Zürich, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland; and Physical Chemisty Institute, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.
authors.forename: Hans Peter
authors.initials: HP
authors.lastname: Lüthi