PRESSURE-DEPENDENCE OF THE PHOTOLUMINESCENCE OF POLYPARAPHENYLENE

The photoluminescence and optical-absorption spectra for polyparaphenylene (PPP) and I2-doped PPP are measured. The photoluminescence of PPP can be attributed to bipolarons by comparing the luminescence and the absorption spectra for PPP and I2-doped PPP. The photoluminescence spectrum of PPP has been studied as a function of hydrostatic pressure, in the range of 0-6.0 GPa. We find a redshift and then a blueshift of the spectrum with an increase in pressure. We can therefore draw the conclusion that the band gap of PPP decreases and then increases as the interchain coupling increases because of the quite small confinement energy due to the interchain coupling compared to that of a bipolaron due to the energy difference between nondegenerate ground states. We suggest that the variation of the band gap with interchain coupling arises from the dependence of the interchain coupling on both the dimerization amplitude and the energy bandwidth. As a result, the decrease and then increase of the band gap of one-dimensional polymers with interchain coupling can, in principle, be expected. In addition, resolved structures that are observed in the luminescence spectrum decrease in magnitude and broaden at high pressure.