Stepwise assembly of site specifically functionalized dehydrobenzo[18]annulenes

Site specifically functionalized dehydrobenzo[18]annulenes (DBAs) with previously inaccessible topologies (C-2 upsilon and C-s 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 Ct, symmetry exhibited progressive and predictable bathochromic shifts ranging from 20 to 80 nm along with increasing intramolecular excited state change transfer (CT) behavior. Asymmetric (C-s) 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 degrees C.