Interplay between diradical character, aromaticity and conductance in oligothiophenes

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 (y ₀) 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 y ₀ 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 y ₀ 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 y ₀ ≈ 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 AV_min 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.