CL-140687
Received: July 22, 2014 | Accepted: August 20, 2014 | Web Released: August 27, 2014
Substituent Site Effect Induced Assemblies of Porphyrin Derivatives on Graphite Surface
Characterized Using a Scanning Probe Microscope
Min Li,1,2,³ Haijun Xu,3,4,³ Yanlian Yang,2 Lina Zhao,1 Zhen Shen,*3 Qingdao Zeng,*2 and Chen Wang*2
1CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics,
the Chinese Academy of Sciences, Beijing 100049, P. R. China
2CAS Key Laboratory of Standardization and Measurement for Nanotechnology,
National Center for Nanoscience and Technology, Beijing 100190, P. R. China
3State Key Laboratory of Coordination Chemistry, Nanjing National University of Microstructures,
Nanjing University, Nanjing 210093, P. R. China
4College of Chemical Engineering, Jiangsu Key Lab of Biomass-based Green Fuels and Chemicals,
Nanjing Forestry University, Nanjing 210037, P. R. China
(E-mail: zengqd@nanoctr.cn, wangch@nanoctr.cn, zshen@nju.edu.cn)
The assembly behavior of both symmetrically substituted
tetraphenylporphyrin (TPP) (sym-16OTPP, meso-tetra(3,5-dihexa-
decyloxyphenyl)) and asymmetrically substituted tetraphenyl-
porphyrin (TPP) (asym-16OTPP, meso-tetra(3,4-dihexadecyloxy-
phenyl)) on a highly oriented pyrolytic graphite (HOPG) surface has
been studied by a scanning probe microscope in this work. Scanning
tunneling microscope results show that asym-16OTPP and sym-
16OTPP form different arrangements on the graphite surface in a
“face-on” manner due to different site substitution. The atomic force
microscope detection further confirms the site effect on one-
dimensional ordered assembling structures formed from sym- and
asym-16OTPP on HOPG by adopting the “edge-on” mode.
Figure 1. Chemical structures of the adsorbate molecules: (a) asym-
16OTPP; (b) sym-16OTPP.
Asym- and sym-16OTPPs (molecular structures shown in
Figure 1) were synthesized according to a previously reported
method.16,17 Details of the synthesis and characterization are
described in the Supporting Information. Asym- or sym-16OTPP
was dissolved in toluene (HPLC grade, Aldrich) at a concentration
less than 10¹5 M. The samples for both STM and AFM measure-
ments were prepared by depositing a droplet of the above solution
on the basal plane of a freshly cleaved graphite (HOPG, grade ZYB)
surface. STM experiments were carried out with a Nanoscope IIIa
scanning probe microscope system (Bruker, USA) under ambient
conditions. Tips were mechanically cut from Pt-Ir wires (80/20).
All STM images were recorded in the constant-current mode.
The imaging parameters are indicated in the figure captions. AFM
detection was performed in the tapping mode using the tapping Si
tip with the following parameters of the cantilever: f0: 302-357 kHz;
k: 20-80 N m¹1, no coating on either the front or the back side.
The adsorption and assembling behavior of alkyl-substituted
TTPPs on HOPG surface have been well studied by STM in ambient
conditions and reported in our previous work.6,12 Similar packing
was formed by TTPPs on the graphite surface, with the porphyrin
cores arranged side by side and the alkyl chains interdigitating with
each other to form a uniform alkane lamella. These results indicate
the importance of 2D crystallization of alkyl chains to the
arrangement of porphyrins on graphite surfaces. Asym-16OTPP is
an alkyl-substituted porphyrin with eight alkyl chains surrounding
four phenyl groups as shown in Figure 1a.
As a typical functional material, porphyrins have high potential
in terms of their electronic, photonic, and catalytic properties and
thus has been attracting increasing attention.1,2 Fabricating ordered
porphyrin adlayers on solid surfaces is of great significance and has
been extensively pursued in recent years.3-8 Many reports have
studied the assembly behavior of porphyrin-based molecules with-
out modification on metal surfaces by STM.3-5,9-11 By modification
of porphyrin with alkylated derivatives or coadsorption with
alkanes, one can get ordered two-dimensional assemblies on highly
oriented pyrolytic graphite (HOPG) surfaces.6,12,13 Among those,
the assembly behavior of tetraalkyl-substituted tetraphenylporphyrin
(TTPP) has been widely investigated by researchers.6,14,15 Normally
in these cases, the porphyrins are aligned side-by-side and are
separated by the insulated alkane lamellae, which indicates that the
assembly structure is strongly affected by the coordination of the
alkyl substituents. Another study of ours shows that introducing an
ester group into the aliphatic chain leads to different interaction and
consequently different molecular packing among alkyl chains in the
lamella on HOPG. The porphyrin cores in some rows rotate by a
certain degree, finally resulting in an alternating single and double
layer pattern on the graphite surface.14 That is, changing the
interaction among alkyl chains would result in a different arrange-
ment of porphyrins on the surface. One should note that the previous
studies were mainly focused on the molecular arrangement of TTPP
on adsorbed substrates. Rare work has been done on the self-
assembly behavior of polyalkyl-substituted porphyrin on surfaces,
like eight alkyl substituents. In the present work, we investigate
and for the first time report the site effect of substituents on the
assembling behavior of octaalkyl-substituted porphyrin (OTPP, The
molecular structures are shown in Figure 1) on a graphite surface
and characterized it using both scanning tunneling microscope
(STM) and atomic force microscope (AFM) techniques.
Two substituted alkyl chains occupy the 3¤- and 4¤-positions at
each phenyl group, which was defined as asymmetric substitution
(Figure 1a). When adsorbing on the graphite surface, asym-
16OTPPs were observed to form a self-assembly structure similar
to that of TTPP, as shown in Figure 2a. The porphyrin cores (bright-
featured squares) adsorb side by side on HOPG while the alkyl
chains (dark-featured part) arrange uniformly in between the core
© 2014 The Chemical Society of Japan