Excitonic and Molecular Properties of the Triplet T₁-State in Diphenylpolyene Single Crystals

Abstract

Excitonic properties of the excited triplet state in single crystals of diphenylpolyenes all-trans(1, 4-diphenyl)-1, 3-butadiene (DPB) and all-trans(1, 6-diphenyl)-1, 3, 5-hexatriene (DPH) are derived from T ₁←S ₀ photoexcitation spectra at low temperatures.

The 0-0-transition is split into 2 (DPH monoclinic) and 4 (DPB orthorhombic) Davydov components in zero-field. Measurements with an applied external magnetic field made accessible the respective fine-structure components and their assignment to the principal axes of the fine-structure tensor. In the case of DPB orthorhombic the determination of the excitonic D* and E* fine-structure parameters was possible due to the small linewidths occuring in the spectra.

The intensity ratios of the Davydov components were evaluated for different polarizations of the excitation light with respect to the crystal axes. The conclusion is drawn that in both DPH and DPB the transition moment is oriented mainly perpendicular to the molecular plane and that the total symmetry of the T ₁-state is A_u .

The vibrational structure in the photoexcitation spectra was assigned by using selectively deuterated crystal samples for comparison. Since the vibrations involving the phenyl endgroup are absent in the spectra of DPH and carry only very weak intensity in the spectra of DPB it is concluded that the T ₁-excitation energy is localized mainly on the polyene chain.

Key Words:

• Short-chain diphenylpolyenes
• excitonic properties