Temperature-dependent (4.3-10 K) photon echo measurements are presented for pentacene doped p-terphenyl at variable high pressure (0-18 kbar). The homogeneous electronic linewidth of pentacene in monoclinic p-terphenyl is observed to narrow at high pressure due to an increase in the pseudolocal phonon frequency. In contrast, pressure induced changes in the Linewidth of pentacene in triclinic p-terphenyl appear to occur largely due to changes in the pseudolocal phonon lifetime. In addition, softening of the pseudolocal phonon frequency is observed near the polymorphic phase transition pressure of 5.5 kbar and photosite specific dynamics are observed at high pressure.