Optical Spectroscopy of Long Polyenes

We have synthesized a homologous series of soluble, linearly conjugated oligomers and related polymers using molybdenum alkylidene catalysts. We have developed HPLC procedures to isolate the oligomers according to their chain lengths and have obtained the absorption spectra of the purified oligomers in room temperature solutions and in 77 K glasses. The oligomer absorption spectra are structured and remarkably similar to those of simple polyenes with comparable numbers of conjugated double bonds (N). Furthermore, the electronic origins of the low-energy, strongly allowed 11Ag- → 11Bu+ transitions follow the E(0−0) = A + B/N behavior previously noted in simple polyenes and carotenoids. Extrapolation of data for oligomers with N = 3−15 suggests E(0−0) ≈ 14 000 cm-1 (λ ≈ 700 nm) in the long polyene limit. The oligomer spectra exhibit modest red shifts on cooling, suggesting minimal conformational disorder in the room temperature samples. In contrast, the absorption spectrum of the longest soluble polymer (N ...