Here we report on the direct observation of a long olithiophene wire on Au(111) by using low-temperature STM. The oligothiophene (24T-Si-Dod, M=7183.89) consists of a 10 nm long 24-thiophene wire with 8 N-silyl groups and 16 dodecyl chains, which is synthesized for the single-molecule devices[1]. The 24T-Si-Dod molecules were directly injected with the tetrahydrofuran solvent into an ultrahigh-vacuum (UHV) chamber by using the pulse injection technique, avoiding any heating procedure for the molecular deposition. After the deposition of 24T-Si-Dod on Au(111), the sample was transferred to the cold STM stage in UHV. All STM images were obtained in a constant-current mode at 63 K.
A large-scale STM image shows that individual 24T-Si-Dod wires are adsorbed on Au(111), each of which is composed of eight bright protrusions corresponding to the bulky N-silyl groups. We find that most of the molecules do not exhibit the linear conformation, but are bended at some positions, in which three kinds of the bending angles, 110, 140, and 180 degree have been obserbed. This should be caused by the cis-trans isomerization between thiophene rings[2]. We will present a detailed conformational analysis of 24T-Si-Dod.
[1] S.Tanaka and Y.Yamashita, Synth.Met. 119, 67(2001).
[2] T.Yokoyama, S.Kurata, and S.Tanaka, J.Phys.Chem. 110, 18133(2006). |