Synthesis and optical spectroscopy of oligo(1,6-heptadiynes) with a single basic structure prepared through adamantylimido-based molybdenum Wittig and metathesis chemistry.

Linear oligoenes of 1,6-heptadiynes (derived from dialkyl dipropargylmalonates) with a single basic structure and up to 23 conjugated double bonds were synthesized through Wittig-like reactions between bimetallic Mo-alkylidene compounds and aldehyde-capped oligoenes. The relatively rigid and isomerically pure oligoenes have structures with alternating cis,trans conjugated double bonds in which the cis double bond is part of a cyclopentene ring. Molecular weights have been confirmed through MALDI-MS measurements of samples purified by HPLC. Optical spectra of the purified samples show significant vibronic resolution, even in room temperature samples, and are remarkably similar to those of simple polyenes and carotenoids. Therefore, a systematic investigation of the dependence of the allowed electronic transition energies (electronic origins) on conjugation lengths has become possible. Studies of seven allowed transitions for molecules with 5-23 double bonds (= N) indicate asymptotic convergence (with approximately a 1/N dependence) to a common long polyene limit at approximately 16,000 cm(-1). The convergence of these electronic transitions agrees with theoretical treatments of polyene excited-state energies and is consistent with the absorption spectra of analogous diethyl dipropargylmalonate polymers (1/N approximately 0).