Electronic Spectra of Polyacetylenes

Abstract

The low‐temperature emission spectra in rigid glass solutions of several substituted polyacetyelnes have been investigated. All the compounds phosphoresced with decay lifetimes ranging from 0.01 to 0.3 second. None of the compounds with less than four triple bonds fluoresced except diphenyl acetylene. All compounds with four triple bonds did fluoresce. An explanation is offered for this phenomenon. Some new absorption measurements were made on dimethyl diacetylene, dimethyl triacetylene, and dimethyl tetraacetylene. The polarization of the transition between 32 000 and 40 000 cm^—1 in a single crystal of dimethyl triacetylene was found to be along the axis of the molecules. These results and those of previous investigators show that the energy level arrays of the molecules can be related and that they vary systematically as the number of triple bonds increases. This is in accord with expectation on the basis of simple molecular orbital theory. An assignment of the symmetries of the electronic wave functions is proposed on the basis of the positions and intensities of the bands.