The hydrostatic pressure-induced shift of the zero-phonon line of sing
le pentacene molecules in a p-terphenyl host crystal was studied at cr
yogenic temperatures. Five molecules in the inhomogeneously broadened
S1<--S0 absorption band of the O1 site (592.32 nm) were selected. In t
he low-pressure regime (DELTAp less-than-or-equal-to 600 hPa), all mol
ecules showed a linear and reversible frequency shift to the red, vari
ed from -0.74 to -1.0 MHz/hPa.