Thermoelectric enhancement by intramolecular rotation in tetraphenylporphyrin-based molecular junctions

Mechanical control of intramolecular rotation offers an effective means to modulate charge and thermoelectric (TE) transport at the molecular scale. In this work, first principles calculations combined with the non-equilibrium Green's function (NEGF) are employed to investigate the impact of intramolecular rotation on the TE transport properties of tetraphenylporphyrin (TPP) molecular junctions. Our results reveal that rotation of the internal pyrrole unit leads to a pronounced enhancement of the electronic conductance, which can be attributed to the progressive shift of the frontier molecular orbitals toward the Fermi level (E_F). Notably, at a rotation angle of 90°, the molecular structure undergoes a transition to an open-shell electronic configuration, giving rise to resonant transmission features at E_F and a substantial increase in the Seebeck coefficients. Consequently, the overall TE performance is significantly improved, with the maximum ZT being approximately twice that of the non-rotated junction. These results highlight intramolecular rotation control as an effective strategy for regulating TE transport in single-molecule junctions.