  We have measured the interlayer and in-plane (needle axis) thermal
diffusivities of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn). The
needle axis value is comparable to the phonon thermal conductivities of
quasi-one dimensional organic metals with excellent pi-orbital overlap, and its
value suggests that a significant fraction of heat is carried by optical
phonons. Furthermore, the interlayer (c-axis) thermal diffusivity is at least
an order of magnitude larger, and this unusual anisotropy implies very strong
dispersion of optical modes in the interlayer direction, presumably due to
interactions between the silyl-containing side groups. Similar values for both
in-plane and interlayer diffusivities have been observed for several other
functionalized pentacene semiconductors with related structures.
