Effect of aromatic ring substitution on the optical properties, emission dynamics, and solid-state behavior of fluorinated oligophenylenevinylenes

Several substituted oligopbenylenevinylenes were synthesized using the Witt ig-Horner-Emmons reaction to produce the trans isomers. Optical properties of these compounds were evaluated using absorption and steady-state fluores cence spectroscopy. Fluorescence quantum yields, Phi(f), decrease with incr easing solvent polarity and approach unity in nonpolar solvents In the case of substituted trans,trans-1,4-bis[2-(2',5'-difluoro)-phenylethenyl]benzen es and trans,trans-1,4-bis[2-(2',5'-dialkoxy)phenylethenyl)-2,3,5,6-tetrafl uorobenzenes. The compounds show a strong solvatochromic shift as a functio n 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-tetratfluorobenzene (6d) i n 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 tau(1) and tau(2): 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,tr ans-1,4-bis[2-(2',5'-difluoro)phenylethenyl] 2,5-diheptyloxybenzene (3b). T his was confirmed by polarization microscopy and thermal analysis. Both the long alkoxy chains and fluorine substitution are responsible for the forma tion of mesophases. Photoluminescence studies of 3b and 6d in the solid sta te indicated an intense emission that was yellow for 3b.