Electrical Conductance and Thermopower of β-Substituted Porphyrin Molecular Junctions─Synthesis and Transport

Abstract

Molecular junctions offer significant potential for enhancing thermoelectric power generation. Quantum interference effects and associated sharp features in electron transmission are expected to enable the tuning and enhancement of thermoelectric properties in molecular junctions. To systematically explore the effect of quantum interferences, we designed and synthesized two new classes of porphyrins, <b>P1</b> and <b>P2</b>, with two methylthio anchoring groups in the 2,13- and 2,12-positions, respectively, and their Zn complexes, <b>Zn-P1</b> and <b>Zn-P2</b>. Past theory suggests that <b>P1</b> and <b>Zn-P1</b> feature destructive quantum interference in single-molecule junctions with gold electrodes and may thus show high thermopower, while <b>P2</b> and <b>Zn-P2</b> do not. Our detailed experimental single-molecule break-junction studies of conductance and thermopower, the latter being the first ever performed on porphyrin molecular junctions, revealed that the electrical conductance of the <b>P1</b> and <b>Zn-P1</b> junctions is relatively close, and the same holds for <b>P2</b> and <b>Zn-P2</b>, while there is a 6 times reduction in the electrical conductance between <b>P1</b> and <b>P2</b> type junctions. Further, we observed that the thermopower of <b>P1</b> junctions is slightly larger than for <b>P2</b> junctions, while <b>Zn-P1</b> junctions show the largest thermopower and <b>Zn-P2</b> junctions show the lowest. We relate the experimental results to quantum transport theory using first-principles approaches. While the conductance of <b>P1</b> and <b>Zn-P1</b> junctions is robustly predicted to be larger than those of <b>P2</b> and <b>Zn-P2</b>, computed thermopowers depend sensitively on the level of theory and the single-molecule junction geometry. However, the predicted large difference in conductance and thermopower values between <b>Zn-P1</b> and <b>Zn-P2</b> derivatives, suggested in previous model calculations, is not supported by our experimental and theoretical findings.