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 photos ite specific pressure effect is observed at low temperature for the tr iclinic (P less-than-or-equal-to 5.5 kbar) phase of para-terphenyl. In addition, the fluorescence lifetime of pentacene in the monoclinic (h igh pressure) crystal phase at low temperature is obtained. We also ob serve unique fluorescence decay dynamics in the pressure region (5.5-6 .5 kbar) intermediate to the triclinic and the monoclinic crystal phas es. The effect of pressure on the triclinic host crystal is discussed in terms of a lowering of the energy of the lowest singlet excited sta te (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-terp henyl host crystal.