Supporting Information
Theoretical Investigation on Molecular Rectification
on the Basis of Asymmetric Substitution and Proton
Transfer Reaction
Hongmei Liu,1 Peng Li,1 Jianwei Zhao,1* Xing Yin1 and Haoli Zhang2
1 Key
Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210008, P. R. China
2State
Key Laboratory of Applied Organic Chemistry, College of Chemistry & Chemical Engineering, Lanzhou
University, Lanzhou 730000, P. R. China
E-mail: zhaojw@nju.edu.cn
1. The torsional features of the molecular wires
2. Influence of molecular rotation and the scattering region size
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1. The torsional features of the molecular wires
In the present study, two phenyl rings are separated by an acetylene linkage (triple
bond) or ethylene linkage (double bond). The first structure may facilitate the rotation
around the conjugated connection. The domination of the twist angle in the molecular
properties have been emphasized by GraveError! Reference source not found. and
Venkataraman et al.,Error! Reference source not found. revealing a twist angle
dependence of single molecule junction conductance. A detailed twist
angle-dependence of the molecular wire conductance has also been studied in our
previous work.Error! Reference source not found. To precisely understand the
influence of substituents on the molecular conformation, we used conformational
analysis,Error! Reference source not found. to investigate the torsional effect. The
relaxed potential energy surface (PES) scanning was performed by changing the twist
angle (dihedral angle , C1-C2-C3-C4 as defined in Figure 1a) by a 5 step between
0 and 180. The PES was fitted to the following six-term truncated Fourier
expansion,Error! Reference source not found.
6
Vi
[1  cos i (   )]  V0
i 1 2
V ( )  
(1)
where V(θ) is the relative energy at the rotational angle θ, V0 is relative energy at 0,
the energy of the planar conformation was referred to as 0.
As expected, the planar conformations of these three systems show much lower
energy. For the T0, T1, and TN systems, the PES curves are very similar and are
symmetric with respect to 90. Three specific points on the PES profiles indicate two
energy minima at 0 and 180 and one energy maximum at 90. At zero-EF, the PES
2
gives a barrier height of 3.78, 4.07, and 3.62 kJmol-1 for the T0, T1 and neutral TN
systems, respectively. The barrier height of T0 is slightly larger than that of
diphenylacetylene
method,Error!
(3.30
Reference
kJmol-1)
source
as
calculated
not
found.
by
and
B3LYP/6-311+G**
that
obtained
at
B3PW91/6-311G** (3.60 kJmol-1) level.Error! Reference source not found. The
similar barrier heights for T0, T1 and neutral TN demonstrated that the influence of
substituents on the torsional barrier is negligible.
However, in the case of the ionic form, TI, the barrier height becomes much higher
(18.85 kJ/mol), indicating that the ionic groups significantly affect the torsion barrier.
Therefore, the molecule is hard to rotate around the molecular axis at room
temperature. The ionic state TI favors a conformation that the amino and carboxylic
groups are located at the same side of the benzene rings, and the energy is 6.50 kJ/mol
lower than the conformation with two substituents at the opposite sides. This is
presumably because of the electrostatic attraction between the positive and negative
groups.
From the PES analysis, we can conclude that the stable planar conformation is
more favorable because the long rigid triple bond may dramatically reduce the steric
hindrance. As demonstrated by previous analysis,Error! Reference source not
found. the transmission coefficient radically decreases with increasing the torsional
angle, the planar conformation is especially beneficial to the electron transport. The
present model systems are based on the most conductive conformation. This is the
3
important relevance between the tortional feature of the system with its transportation
behavior.
(a)
(a)
X
1
2
T0
T1
TN
TI
X=
X=
X=
X=
4

3
-H,
-NO2,
-COOH,
-COO,
Y
Y= -H,
Y= -NH2,
Y= -NH2,
Y= -NH3+
(b)
(b)
20
Relative Energy / kJ mol
-1
T0
T1
15
TN
TI
10
5
0
0
20
40
60
80
100
120
140
160
180
Twist angle / degree
Figure SI-1 The torsional barrier for the molecular wire with different substituents,
4
calculated at the B3LYP level with 6-31G* basis set.
5
2. Influence of molecular rotation and the scattering region size
In order to test the influence from the molecular rotation around the long axis, we
rotated the molecule about 15, and then calculated the current-voltage curve. In
addition, the current of TN with different gold layers were calculated to confirm the
reliability of scattering region, shown in Figure SI-2a. A supercell consisting of 3
layers to the left and 3 layers to the right of the scattering region was used to perform
the calculation. Figure SI-2b displays the I-V curves of model A, B and C, which are
almost identical.
Figure SI-1a
A 2 layers
3 layers
Figure SI-1b
Electric field
2
A
B
C
Current / A
1
B rotate 15
C 3 layers
0
-1
3 layers
-2
-1.0
-0.5
0.0
0.5
1.0
Bias / V
15
Figure SI-2 (a) The test models, A is the model with 2 and 3 surface layers in the
scattering region, B is the model with molecule rotating 15, C is the model with 3
and 3 surface layers in scattering region, respectively. The I-V curves of neutral state
TN of A, B and C model.
6
Reference
Error! Reference source not found.
7