Influence of local excitation of octupolar Oligophenylenevinylenes on their dipole moments

Absorption and fluorescence spectra of octupolar centrosymmetric oligophenylenevinylene dyes (E,E)-bis[2-(4-hexyloxyphenyl)ethenyl]-(E,E)-3,6-bis-[2-(4-N,N-dipropylaminophenyl)ethenyl]pyrazine and (E,E,E,E)-2,3,5,6- tetra-[2-(4-hexyloxyphenyl)ethenyl]pyrazine were measured in various solvents. An electro-optical absorption method was used to determine their dipole moments as μ _g  = 6.1∙10^–30 and 3.4∙10^–30 C∙m in the equilibrium ground state and the increased values ∆ ^a μ = 11.9∙10^–30 and 8.2∙10^–30 C∙m upon excitation into a Franck–Condon state. Quantum-chemical calculations showed that the molecules had non-planar configurations. The π,π-conjugated system was localized on the most planar part of the molecule that was responsible for light absorption in the range 300–450 nm due to a change in the geometry of the molecules in the ground state. Localized excitation of the molecules caused their dipole moments ∆ ^a μ to change significantly.