The Effect of Pressure and of Controlled Stretch on the Luminescent Properties of 9,10-Bis(phenylethynyl) Anthracene‡

The luminescence properties of 9,10-bis(phenylethynyl) anthracene have been investigated in solid poly(vinyl acetate) (PVAc) and a copolymer of vinyl chloride (VCl) and vinyl acetate (VAc) as a function of pressure to 65 kbar and of controlled stretch over a range of up to 4 times the original length using the 400 nm line of a frequency-doubled Ti:sapphire laser. With increasing pressure, the intensity (corrected for the absorption at 400 nm) decreases significantly and there is a distinct change in peak shape in the region of 20-40 kbar. In this region, the lifetime increases from 3.5 ′ 0.2 to 4.0 ′ 0.2 ns. The result can be explained on the basis of the analysis of Levitus and Garcia-Garibay. The red shift of the absorption peak is equivalent to a blue shift of the excitation laser, which results in excitation of a different polarization of the molecule. Up to a stretch of 2, the controlled stretch produces an increase of intensity for laser excitation polarized in the direction of stretch and a decrease for excitation polarized at 90° to the stretch, indicating that molecules are being oriented preferentially with the stretch. At larger stretches, there is a very similar increase in emission intensity for both orientations of the exciting light. Various aspects of the observations are considered but there appears to be no simple explanation for the apparent increase in oscillator strength.