Stepwise Assembly of Site Specifically Functionalized Dehydrobenzo[18]annulenes

Abstract

Site specifically functionalized dehydrobenzo[18]annulenes (DBAs) with previously inaccessible topologies (C2v and Cs symmetry) were prepared by utilizing an in situ protiodesilylation/alkynylation reaction. By application of a stepwise synthetic route, donor and/or acceptor functional groups were introduced to the annulenic core in a designed manner. The electronic absorption spectra of the DBAs revealed moderate to dramatic changes in the electronic structure of the [18]annulene core when subtle changes were made in substitution patterns and/or functional groups. Macrocycles containing C2v symmetry exhibited progressive and predictable bathochromic shifts ranging from 20 to 80 nm along with increasing intramolecular excited state change transfer (CT) behavior. Asymmetric (Cs) molecules presented more interesting absorption behavior, such as accentuation of bands in the visible region. X-ray crystal structures of [18]DBAs 20, 22, 23, and 25 showed normal bond lengths and bond angles within the planar annulenic core. A solid-state thermal study indicated ordered polymerization of the DBAs between 164 and 240 °C.