A series of photophysical measurements and semiempirical calculations were carried out with 1,4-bis(phenylethynyl)benzene in search of evidence on the effects of phenyl group rotation and chromophore aggregation of oligo- and poly(phenyleneethynylene)s. It is suggested that planarization gives rise to relatively modest shifts of ca. 20-30 nm, which preserve the vibronic structure of the monomer and retain a high emission quantum yield. In contrast, it is proposed that aggregation gives rise to larger shifts and loss of vibronic structure.

We report the synthesis and photophysical properties of 1,4-bis(9-ethynylanthracenyl)benzene. Results in dilute hexane solutions give a very high fluorescence quantum yield (Φ ) 0.97) and a lifetime of τ ) 1.8 ns. Studies at different temperatures, viscosities, and micromorphologies, as well as excited-state semiempirical calculations, indicate that conformations having aryl groups coplanar absorb and emit at wavelengths that are longer than those that have their aryl groups orthogonal or twisted. A simple MO picture is consistent with essentially frictionless aryl group rotation in the ground state and a relatively high rotational barrier in cummulene-like excited states.

Changes in the excitation and emission spectra of 1,4-bis(phenylethynyl)benzene (compound 3) in dilute methylcyclohexane solutions at 298 K, in glassy matrixes at 77 K, and in polyethylene films at 298 and 77 K were interpreted in terms of structural changes associated with rotation about the phenyl-alkyne single bonds. We deduced the coexistence of two distinguishable conformer populations that we referred to as coplanar and twisted. After publication of this work, Prof. Andrew Beeby from the University of Durham in the United Kingdom brought to our attention the possible presence of an emissive impurity in the samples used in our study. Analyis of the original samples confirmed his suggestions and showed a small amount of a similar chromophore with a red shift of ca. 20 nm.

Crystals of 1,4-bis(2-hydroxy-2-methyl-3-butynyl)-2-fluorobenzene 4 have a rich packing structure with four distinct molecules in the unit cell. A complex hydrogen bonding network results in the formation of cofacial trimers, cofacial dimers, and monomers within the same unit cell. Given a remarkable opportunity to investigate the effect of aggregation on the photophysics of 1,4-diethynylbenzenes, we analyzed the absorption, diffuse reflectance, and emission spectra of compound 4 in solutions and in crystals. Diffuse reflectance and fluorescence excitation revealed a red-shifted absorption that is absent in dilute solution but becomes observable at high concentrations and low temperatures. The fluorescence emission in the solid state is dual with components assigned to monomers and aggregates. The excitation and emission assigned to the monomer are nearly identical in crystals and dilute solutions. The absorption and emission bands assigned to aggregates are broad and red-shifted by 60--80 nm. As expected for a sample with absorbers and emitters with different energies and incomplete equilibration, efficient monomer-to-aggregate energy transfer was observed by a proper selection of excitation wavelengths. The fluorescence quantum yield of 4 in solution is relatively low (Phi(F) = 0.15) and the singlet lifetime short (tau(F) = 3.8 ns). A lower limit for the triplet yield of Phi(T) = 0.64 was determined indirectly in solution by (1)O(2) sensitization, and a relatively strong and long-lived phosphorescence was observed in low-temperature glasses and in crystals at 77 K.