  We investigate strategies for increasing the thermopower of
crown-ether-bridged anthraquinones. The novel design feature of these molecules
is the presence of either (1) crown-ether or (2) diaza-crown-ether bridges
attached to the side of the current-carrying anthraquinone wire. The
crown-ether side groups selectively bind alkali- metal cations and when
combined with TCNE or TTF dopants, provide a large phase-space for optimising
thermoelectric properties. We find that the optimum combination of cations and
dopants depends on the temperature range of interest. The thermopowers of both
1 and 2 are negative and at room temperature are optimised by binding with TTF
alone, achieving thermpowers of -600 microvolts/K and -285 microvolts/K
respectively. At much lower temperatures, which are relevant to cascade
coolers, we find that for 1, a combination of TTF and Na+ yields a maximum
thermopower of -710 microvolts/K at 70K, whereas a combination of TTF and Li+
yields a maximum thermopower of -600 microvolts/K at 90K. For 2, we find that
TTF doping yields a maximum thermopower of -800 microvolts/K at 90K, whereas at
50K, the largest thermopower (of -600 microvolts/K) is obtain by a combination
TTF and K+ doping. At room temperature, we obtain power factors of 73
microwatts/m.K2 for 1 (in combination with TTF and Na+ ) and 90 microwatts/m.K2
for 2 (with TTF). These are higher or comparable with reported power factors of
other organic materials.
