Effect of Aromatic Ring Substitution on the Optical Properties, Emission Dynamics, and Solid-State Behavior of Fluorinated Oligophenylenevinylenes

Several substituted oligophenylenevinylenes were synthesized using the Wittig−Horner−Emmons reaction to produce the trans isomers. Optical properties of these compounds were evaluated using absorption and steady-state fluorescence spectroscopy. Fluorescence quantum yields, Φf, decrease with increasing solvent polarity and approach unity in nonpolar solvents in the case of substituted trans,trans-1,4-bis[2-(2‘,5‘-difluoro)phenylethenyl]benzenes and trans,trans-1,4-bis[2-(2‘,5‘-dialkoxy)phenylethenyl)-2,3,5,6-tetrafluorobenzenes. The compounds show a strong solvatochromic shift as a function of solvent polarity, yielding a slope of −13 300 cm- 1 according to the Lippert−Mataga equation and indicating the emission of an additional charge-transfer species. A two-state reaction model was confirmed for trans,trans-1,4-bis[2-(2‘,5‘-dialkoxy)phenylethenyl)-2,3,5,6-tetrafluorobenzene (6d) in different solvents by time-correlated single-photon counting using global analysis. A dependence of the kinetic data on solvent polarity was found (global fitted decay times in picoseconds for τ1 and τ2: 381/1281 in n-hexane; 101/1590 in toluene; 27/2974 in acetonitrile). Investigations of the solid state showed liquid crystalline behavior for 6d and for trans,trans-1,4-bis[2-(2‘,5‘-difluoro)phenylethenyl]-2,5-diheptyloxybenzene (3b). This was confirmed by polarization microscopy and thermal analysis. Both the long alkoxy chains and fluorine substitution are responsible for the formation of mesophases. Photoluminescence studies of 3b and 6d in the solid state indicated an intense emission that was yellow for 3b.