Conjugated oligomers are envisioned as basic functional units in molecular electronics, due to their versatility, flexibility and multi-functionality. There are challenges in understanding basic phenomena of charge transport and spin manipulation in conjugated oligomers. This thesis focuses on on-surface synthesis and functionality characterization of porphyrin-based conjugated oligomers utilizing low-temperature scanning tunneling microscopy and spectroscopy.

This thesis consists of three parts: on-surface synthesis, charge transport and spin manipulation of porphyrin-based oligomers.

In the part of on-surface synthesis, the biphenyl-linked porphyrin oligomers (bp-ppo) is synthesized by homo-coupling of Br₂-TPP, the phenyl-linked porphyrin oligomers (p-ppo) is synthesized by c...[ Read more ]

Conjugated oligomers are envisioned as basic functional units in molecular electronics, due to their versatility, flexibility and multi-functionality. There are challenges in understanding basic phenomena of charge transport and spin manipulation in conjugated oligomers. This thesis focuses on on-surface synthesis and functionality characterization of porphyrin-based conjugated oligomers utilizing low-temperature scanning tunneling microscopy and spectroscopy.

This thesis consists of three parts: on-surface synthesis, charge transport and spin manipulation of porphyrin-based oligomers.

In the part of on-surface synthesis, the biphenyl-linked porphyrin oligomers (bp-ppo) is synthesized by homo-coupling of Br₂-TPP, the phenyl-linked porphyrin oligomers (p-ppo) is synthesized by cross-coupling of Br₂-TPP and Br₂-DPP and porphyrin-dimers (ppo) can not be synthesized on surface.

In the part of charge transport, we studied the charge transport behavior of bp-ppo. Firstly, the long-range nearly length-independent resonant charge transport is observed in straight bp-ppo using STM vertical manipulation technique. We found that the electrons transfer through delocalized LUMO, and the lowest attenuation factor ( β < 0.001Å^-1) of differential conductance is achieved. Secondly, the negative differential conductance (NDC) was observed in oligomers of bp-ppo with a 90°-kink, and the mechanism is proposed to be localization of delocalized molecular orbital at higher bias.

In the part of spin manipulation, we studied the mediation of Fe spin by molecular backbone. Firstly, in the system of double-Fe metalated and three C-C bonds linked ppo, we found that the spin-excitation on individual spin is quenched due to the formation of spin-spin singlet by indirect coupling mediated by molecular backbone. Secondly, we demonstrated that the spin of Fe is closely related to the conformation of porphyrin hosting the Fe: the transformation from saddle to planar changes the Fe spin state from S=2 to S=1.

In summary, we studied on-surface synthesis, charge transport and spin manipulation of porphyrin-based conjugated oligomers. Our results may contribute to the development of molecular electronics.