Theoretical study of effects of dπ-pπ conjugation on electron transport properties of molecules
[Objective]As photonic devices continue to trend towards miniaturization and integration,new optoelectronic materials based on carbon,silicon,and germanium are constantly expanding.Although elements of the ⅣA group,such as C,Si,and Ge,share similar electronic structures,they exhibit significant differences in their electronic transport properties.This study aims to explore the impact of dπ-pπ conjugation formed by the Ⅳ A group elements like C,Si,and Ge with different linking group on the electronic transport properties of corresponding molecular systems.[Methods]In this study,we focus on three classes of molecules containing the Ⅳ A group elements(C,Si,Ge):tetraethylalkane molecules(C9H20S2,SiC8H20S2,GeC8H20S2),tetraethynylalkane molecules(C9H4S2,SiC 8H4S2,GeC8H4S2),and tetraphenylalkane molecules(C25H20S2,SiC4H20S2,GeC24H20S2).Geometry optimization and frequency calculations for these molecules were carried out and the corresponding molecular structure models were constructed.Their electron transport properties were explored using Quantum ATK software.[Results]The calculated results indicate that in the case of tetraethylalkane molecules(C9H20S2,SiC8H20S2,GeC8H20S2),among the three molecules'highest occupied molecular orbits(HOMOs),the d orbitals contribution of Ge atom in the GeC8H20S2 molecule is 0.67％,higher than d orbitals contribution of 0.55％for the Si atom in the SiC8H20S2 molecule.However,in the C9H20S2 molecule,C atoms do not contribute d orbitals.This suggests that compared to C9H20S2,both SiC8H20S2 and GeC8H20S2 molecules involve the d orbitals of the central atoms Ge and Si in electronic transport.In the case of tetraethynylalkane molecules(C9H4S2,SiC8H4S2.GeC8H4S2),the calculated projected density of states(PDOS)reveals that their HOMOs are primarily contributed by C-p,Si-d,and Ge-d orbitals.Therefore,the presence of d orbitals and their spatial extension partly determine the strength of electronic transport in these molecules.For Si as the central atom,the existence of d orbitals leads to strong dπ-pπ conjugation with the connecting ethynyl groups,resulting in greater delocalization and stronger electron transport capabilities compared to C9H4S2,which centers around carbon.However,when the central atom is Ge,the highly extended d orbitals of Ge atom are involved,weakening their interaction with the ethynyl pπ orbitals.For tetraphenylalkane molecules(C25H20S2,SiC24H20S2,GeC24H20S2),the HOMOs predominantly consist of p orbitals from the anchoring S atoms,π orbitals from the phenyl rings,and a lesser contribution from the central atom's d orbitals.In the case of SiC24H20S2,in addition to dπ-pπinteractions between the central Si atom and the attached phenyl rings,there are also weak π-πnteractions between adjacent phenyl rings,contributing to π electron delocalization and affecting single-molecule conductivity.[Conclusion]Compared to C and Si atoms,Ge atoms,when serving as central atoms in tetraethylalkane molecules,exhibit a more extensive d-orbital overlap with C—H σbonds,leading to the formation of dπ-pπ orbital interactions.In tetraethylynylalkane molecules,the ethynyl groups are more prone to establish stronger dπ-pπ conjugation with Si d-orbitals.Furthermore,with the introduction of phenyl groups,weak π-π interactions occur between the phenyl ring groups,and there are also dπ-pπ interactions between the central atom and the phenyl rings of connecting groups.The presence of dπ-pπ conjugation facilitates electron delocalization,thereby increasing the corresponding single-molecule conductivity.In this study,a correlation between dπ-pπ conjugation and molecular electronic transport properties is established,offering insights into the potential applications of group Ⅳ A central atom-containing molecules in single-molecule electronic devices.
density functional theorynon-equilibrium Green functional methodⅣ main group elementsingle molecule conductanceorbital composition analysis
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