Benzo-bis(imidazole) self-assembled monolayers molecular junctions in meta or para conformation: effects of protonation on the electrical and thermal conductances

TL;DR

This study investigates how protonation affects the thermal and electrical conductances of benzo-bis(imidazole) molecular junctions in meta and para conformations, revealing reversible conductance changes linked to molecular organization.

Contribution

It provides experimental evidence of protonation-induced modulation of thermal and electrical conductances in self-assembled monolayer molecular junctions, highlighting structural effects.

Findings

Thermal conductance is lower in meta-connected junctions compared to para-connected ones.

Protonation increases the thermal conductance of meta-connected junctions by about 50%.

Electrical conductance decreases upon protonation, with no change in molecular orbital energy.

Abstract

We report the thermal conductances of molecular junctions made of self-assembled monolayers of benzo-bis(imidazole) molecules, without side groups or functionalized with two phenylamine side groups. In the two cases, when the molecules are connected to the electrodes by thiol anchoring groups in the meta-position, the thermal conductance is decreased compared to the same molecules connected in the para-position (ca. 16-29 nW/K and ca. 37-40 nW/K, respectively) in agreement with the theoretically predicted phonon interference effect in molecular junctions. Upon protonation, the thermal conductances of the meta-connected molecular junction increase by about 50% (reversible behavior upon deprotonation). The fact that only the thermal conductance of the meta-connected molecular junction is sensitive to the protonation/deprotonation is tentatively related to modifications of the structural organization of the molecules in the monolayer, which modifies the thermal conductance at the molecule/electrode interfaces. The electrical conductance is lower for the meta-connected molecule than for the para-connected one, due to destructive quantum interferences, as expected and reported for other molecular junctions. The conductance further decreases (reversibly) upon protonation. The energy position of the molecular orbital involved in the electron transport is not modified by the protonation and the decrease in current is related to changes in the molecule organization in the monolayer, which modulate the electronic coupling energy at the molecule/electrode interfaces.