Interplay between Diradical Character, Aromaticity and Conductance in Oligothiophenes

Abstract

The fundamental relationships of the diradical character-aromaticity-conductance triangle are elucidated across a diverse set of oligothiophenes capable of adopting both aromatic and quinoidal structures. The electronic ground-state was determined using open-shell/closed-shell and singlet-triplet adiabatic energy gaps, while the diradical character (y0) was quantified using Yamaguchi’s approach. All oligomers exhibit a singlet ground state, with only 13 being open-shell with non-zero diradical character. A strong correlation between the multicenter index of the central thiophene ring(s) and y0 indicates that the diradical character and aromaticity vertices are intrinsically linked: modulation of one invariably impacts the other. The diradical character-conductance relationship reveals two competing effects: while higher y0 generally enhances conductance, this trend is counterbalanced by length-dependent attenuation, leading to near-zero conductance in sufficiently long oligomers (~20 Å). Additionally, systems with y0≈1 exhibit reduced conductance regardless of molecular length. Conductance also correlates with local aromaticity: annulated rings that most strongly disrupt the aromaticity of the backbone thiophene units yield higher conductance compared to parent structures. In terms of the linear aromaticity, approximately 70% of local transmission plots could be identified through pathways derived from the AVmin index and delocalization indices, underscoring their predictive value for local charge transport behavior. This study extends the traditional diradical–aromaticity–conductance triangle by incorporating the electronic ground-state structure as a fourth vertex and distinguishing between local and global aromaticity–conductance relationships. The resulting framework provides deeper insight into the shared electronic origins of key molecular properties and their influence on charge transport.