THE EFFECT OF PRESSURE ON THE FLUORESCENCE LIFETIME OF PENTACENE IN PARA-TERPHENYL AT LOW-TEMPERATURE

Time resolved fluorescence measurements are presented as a function of pressure for pentacene in para-terphenyl at low temperature. A photosite specific pressure effect is observed at low temperature for the triclinic (P less-than-or-equal-to 5.5 kbar) phase of para-terphenyl. In addition, the fluorescence lifetime of pentacene in the monoclinic (high pressure) crystal phase at low temperature is obtained. We also observe unique fluorescence decay dynamics in the pressure region (5.5-6.5 kbar) intermediate to the triclinic and the monoclinic crystal phases. The effect of pressure on the triclinic host crystal is discussed in terms of a lowering of the energy of the lowest singlet excited state (S1) relative to the nearby triplet level (T2) with a corresponding decrease in the intersystem crossing rate. The fluorescence dynamics also provide information on the nature of the pentacene environment in the higher pressure (i.e. > 5.5 kbar) structural phases of the p-terphenyl host crystal.