  We report results of theoretical studies of thermoelectric efficiency of
single-molecule junctions with long molecular linkers. The linker is simulated
by a chain of identical sites described using a tight-binding model. It is
shown that thermoelectric figure of merit ZT strongly depends on the bridge
length, being controlled by the lineshape of electron transmission function
within the tunnel energy range corresponding to HOMO/LUMO transport channel.
Using the adopted model we demonstrate that ZT may significantly increase as
the linker lengthens, and that gateway states on the bridge (if any) may
noticeably affect the length-dependent ZT. Temperature dependences of ZT for
various bridge lengths are analyzed. It is shown that broad minima emerge in ZT
versus temperature curves whose positions are controlled by the bridge lengths.
