On the first Electronic Transitions in Molecular Spectra of Conjugated Diphenylpolyenes: A reappraisal

Abstract The first UV/Vis absorption band of the diphenylpolyenes (DPPn) from one (DPP1) to seven (DPP7) conjugated double-bonds, dissolved in 2-methyltetrahydrofuran, was recorded by slowly lowering down the temperature from 338 to 138 K. By analyzing these spectra under conditions of constant concentration with the aid of the Mulliken-Rieke rule, it is concluded that the molecular structure of these compounds changes significantly when temperature increases. Unlike trans -stilbene (DPP1) and 1,4-diphenylbutadiene (DPP2), which keep up the onset wavelength for the first peak absorption on going from 338 to 77 K, the longer-chain polyenes miss it and generate a weak absorption with an onset that shows up at a longer wavelength – ca. 700 cm −1 to the red. These new onsets cancel out the energy gap between the absorption and emission for these conjugated diphenylpolyenes.