Excimer Formation in Oligo[2,5-bis(hexadecyloxy)-1,4-phenylene]s Followed by Fluorescence Spectroscopy

Using the steady-state and time-resolved fluorescence spectroscopy, the behavior of "hairy-rod" oligo- and poly[2,5-bis(hexadecyloxy)-1,4-phenylene]s in tetrahydrofuran solutions was investigated. The materials were prepared by the Yamamoto coupling reaction using zinc as a reducing metal, the nickel(II)/triphenylphosphine complex as a catalyst, and 2,2'-bipyridine as a coligand. The appropriate oligomer fractions were separated by fractional precipitation and characterized by GPC and end group analysis. The fluorescence quantum yield of oligomers and polymers increased with their increasing conjugation length. The fluorescence emission spectra of polymers and longer oligomers exhibited one emission maximum at 390 nm with a single-exponential decay and fluorescence lifetimes (τ) around 1 ns. The substitution in positions 2 and 5 forces the adjacent backbone benzene units out of the plane, which results in twist angles 60-80°, and the bulky substituents exclude the cofacial sandwich-type configuration necessary for excimer formation. However, with shorter oligomers, another emission band at 460 nm appeared. Fluorescence decays at 460 nm were found to be double-exponential with longer excited-state lifetimes [e.g. τ1 = 6.9 ns (76%), τ2 = 2.4 ns (24%)]. With shorter oligomers (dimer, trimer), we assume a sandwich-type configuration with sufficiently close interchain distance and hence the excimer can form. Hydrophobic interactions of long aliphatic side chains in a polar medium play an important role in the excimer formation.