Functional acetylenic molecular architecture

A diversity of perethynylated building blocks, in particular derivatives of tetraethynylethene (TEE, 3,4-diethynylhex-3-ene-1,5-diyne), were prepared for the construction of multinanometer-sized functional molecular objects. Macrocyclic acetylenic scaffolding yielded perethynylated antiaromatic octadehydro[12]annulenes and aromatic dodecadehydro[18]annulenes as well as highly stable expanded radialenes with extended all-carbon cores. An attempted new synthesis of perethynylated octadehydro[12]annulenes was unsuccessful and led to the isolation of a highly strained permethylenated cycloocta-1,5-diyne instead. Acyclic acetylenic scaffolding using both TEEs and 1,2-diethynylethenes (DEEs (E)-hex-3-ene-1,5-diynes) afforded the first oligomers and polymers with the poly(triacetylene) backbone [PTA, -(C=C-CR=CR-C=C)(n)-]. The effective conjugation length for this third class of linearly conjugated polymers with a nonaromatic all-carbon backbone, besides poly acetylene and poly(diacetylene), was determined to be in the range of 7-10 monomeric units. By lateral attachment of dendrons along the conjugated backbone of PTA oligomers, cylindrically shaped dendrimers were obtained which can be viewed as insulated molecular wires. Donor-acceptor substituted arylated TEEs and DEEs were prepared, and their unusual electrochemical properties and tendency to undergo photochemical trans-->cis isomerization were investigated. Measurements of third-order nonlinear optical effects by third harmonic generation revealed useful structure-function relationships. The highest second hyperpolarizabilities gamma were obtained for two-dimensionally fully conjugated systems with reduced molecular symmetry.