dπ-pπ共轭对分子电子输运性质影响的理论研究

Theoretical study of effects of dπ-pπ conjugation on electron transport properties of molecules

[目的]探究Ⅳ主族的C、Si、Ge等原子与不同离域基团形成的dπ-pπ共轭对相应分子体系电子输运性质的影响.[方法]采用密度泛函理论结合非平衡格林函数(DFT-NEGF)方法探究了含Ⅳ主族元素(C、Si、Ge)的三类分子:四乙基烷烃分子(C_(9)H_(20)S_(2)、SiC_(8)H_(20)S_(2)、GeC_(8)H_(20)S_(2)),四乙炔基烷烃分子(C_(9)H_(4)S_(2)、SiC_(8)H_(4)S_(2)、GeC_(8)H_(4)S_(2)),四苯基烷烃分子(C_(25)H_(20)S_(2)、SiC_(24)H_(20)S_(2)、GeC_(24)H_(20)S_(2))的电子输运性质.[结果]在四乙基烷烃分子中,中心原子Ge由于具有更为扩展的d轨道与碳氢(C—H)σ键存在dπ-pπ轨道相互作用,四乙炔基烷烃分子中乙炔基与Si的d轨道也存在dπ-pπ共轭,而在四苯基烷烃分子中苯环基团间存在额外的π-π弱相互作用,其均有利于电子离域,增大单分子电导.[结论]该研究一定程度上建立了dπ-pπ共轭作用与分子电子输运性质之间的联系,可为含Ⅳ主族中心原子分子在单分子电子学器件中的应用提供参考.

[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 theⅣA 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-pπconjugation formed by theⅣA 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 theⅣA group elements(C,Si,Ge):tetraethylalkane molecules(C_(9)H_(20)S_(2),SiC_(8)H_(20)S_(2),GeC_(8)H_(20)S_(2)),tetraethynylalkane molecules(C_(9)H_(4)S_(2),SiC_(8)H_(4)S_(2),GeC_(8)H_(4)S_(2)),and tetraphenylalkane molecules(C_(25)H_(20)S_(2)、SiC_(24)H_(20)S_(2)、GeC_(24)H_(20)S_(2)).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(C_(9)H_(20)S_(2),SiC_(8)H_(20)S_(2),GeC_(8)H_(20)S_(2)),among the three molecules highest occupied molecular orbits(HOMOs),the d orbitals contribution of Ge atom in the GeC_(8)H_(20)S_(2) molecule is 0.67%,higher than d orbitals contribution of 0.55%for the Si atom in the SiC_(8)H_(20)S_(2) molecule.However,in the C_(9)H_(20)S_(2) molecule,C atoms do not contribute d orbitals.This suggests that compared to C_(9)H_(20)S_(2),both SiC_(8)H_(20)S_(2) and GeC_(8)H_(20)S_(2) molecules involve the d orbitals of the central atoms Ge and Si in electronic transport.In the case of tetraethynylalkane molecules(C_(9)H_(4)S_(2),SiC_(8)H_(4)S_(2),GeC_(8)H_(4)S_(2)),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-pπconjugation with the connecting ethynyl groups,resulting in greater delocalization and stronger electron transport capabilities compared to C_(9)H_(4)S_(2),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 pπorbitals.For tetraphenylalkane molecules(C_(25)H_(20)S_(2),SiC_(24)H_(20)S_(2),GeC_(24)H_(20)S_(2)),the HOMOs predominantly consist of p orbitals from the anchoring S atoms,πorbitals from the phenyl rings,and a lesser contribution from the central atom s d orbitals.In the case of SiC_(24)H_(20)S_(2),in addition to dπ-pπinteractions between the central Si atom and the attached phenyl rings,there are also weakπ-πinteractions between adjacent phenyl rings,contributing toπelectron 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—Hσ-bonds,leading to the formation of dπ-pπorbital interactions.In tetraethylynylalkane molecules,the ethynyl groups are more prone to establish stronger dπ-pπconjugation with Si d-orbitals.Furthermore,with the introduction of phenyl groups,weakπ-πinteractions occur between the phenyl ring groups,and there are also dπ-pπinteractions between the central atom and the phenyl rings of connecting groups.The presence of dπ-pπconjugation facilitates electron delocalization,thereby increasing the corresponding single-molecule conductivity.In this study,a correlation between dπ-pπconjugation and molecular electronic transport properties is established,offering insights into the potential applications of groupⅣA central atom-containing molecules in single-molecule electronic devices.