Donor- and/or Acceptor-Substituted Expanded Radialenes: Theory, Synthesis, and Properties Sharwatie Ramsaywack, ⊥,† Sila Karaca, ⊥,‡ Mojtaba Gholami, † Adrian H. Murray, † Frank Hampel, § Robert McDonald, † Nuran Elmaci,* ,‡ Hans Peter Lü thi, ∥ and Rik R. Tykwinski* ,§ † Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada ‡ Department of Chemistry, I ̇ zmir Institute of Technology, Urla, 35430 I ̇ zmir, Turkey § Department of Chemistry and Pharmacy & Interdisciplinary Center of Molecular Materials (ICMM), Friedrich-Alexander-Universitä t Erlangen-Nü rnberg (FAU), Henkestrasse 42, 91054 Erlangen, Germany ∥ Department of Chemistry and Applied Biosciences, ETH Zü rich, Vladimir-Prelog-Weg 2, 8093 Zü rich, Switzerland * S Supporting Information ABSTRACT: The synthesis of donor- (D) and/or acceptor (A)-expanded [4]radialenes has been developed on the basis of readily available dibro- moolefin(7), tetraethynylethene (10 and 20), and vinyl triflate (12) building blocks. The successful formation of D/A radialenes relies especially on (1) effective use of a series alkynyl protecting groups, (2) Sonogashira cross- coupling reactions, and (3) the development of ring closing reactions to form the desired macrocyclic products. The expanded [4]radialene pro- ducts have been investigated by spectroscopic (UV−vis absorption and emission) and quantum chemical computational methods (density func- tional theory and time dependent DFT). The combined use of theory and experiment provides a basis to evaluate the extent of D/A interactions via the cross-conjugated radialene framework as well as an interpretation of the origin of D/A interactions at an orbital level. ■ INTRODUCTION Conjugated carbon-rich macrocycles are intriguing targets of study for both theoretical and experimental chemists because of their often symmetrical shape and aesthetically appealing struc- tures. They are, however, useful molecules as well, with potential as the organic component for electronic, optical, and nonlinear optical applications. 1−8 A specific subcategory of conjugated macrocycles are the [n]radialenes, which are cyclic, carbon-rich molecules with a general formula C 2n H 2n that contain n ring atoms and n exocyclic double bonds (1, Figure 1). “Expanded radialenes” are derivatives of radialenes that originate by formal insertion of an unsaturated spacer between each pair of exo- methylene fragments of a radialene, giving rise to macrocycles such as 2 and 3 (Figure 1). 8−12 Work with expanded radialenes was pioneered by Diederich and co-workers 13−16 via the introduction of diacetylene moieties into the radialene frame- work to give derivatives with the general structure 3 17,18 as well as structurally related radiannulenes. 19−27 More recently, expanded radialenes 2 composed of repeating enyne units have been realized and studied. 28−30 The two-dimensionally (2D) conjugated structure of ex- panded [n]radialenes is rather special because it combines a number of linearly and cross-conjugated pathways placed on a nonbenzoid carbon framework. 31−39 One key question concern- ing the properties of expanded [n]radialenes has been the role played by cross-conjugation 9,40,41 to the overall electronic makeup of these unique molecules. The influence of D−A or D/A interactions 42 via cross conjugation in acyclic systems has been explored by a number of groups. 9,10,43−46 To date, however, only a few examples of donor- or acceptor-expanded radialenes have been reported. 18,28 Finally, donor−acceptor-expanded radialenes remain unknown, and there have been no attempts to document cross-conjugated interactions in these derivatives using theory. Our group has recently reported a modular approach for the synthesis of perphenylated expanded [n]radialenes 29 and radiaannulenes. 22,29 This approach has been especially useful in Received: July 17, 2014 Published: September 26, 2014 Figure 1. Schematic structure of [n]radialenes and expanded [n]- radialenes. Article pubs.acs.org/joc © 2014 American Chemical Society 10013 dx.doi.org/10.1021/jo5016085 | J. Org. Chem. 2014, 79, 10013−10029