Inversion of the rectifying effect in diblock molecular diodes by protonation

Article abstract of DOI:10.1021/ja051332c

A new molecular diode based on biphenyl-co-bispyrimidine was synthesized and showed a pronounced rectifying effect. Further studies indicate that protonation on the nitrogen atoms of the diode molecule by strong acids can reversibly alter the rectifying direction. This phenomenon can be envisioned as a starting point for single molecule detection devices. Copyright

Full text of DOI:10.1021/ja051332c

Published on Web 07/06/2005
Inversion of the Rectifying Effect in Diblock Molecular Diodes by Protonation
Gustavo M. Morales, Ping Jiang, Shenwen Yuan, Youngu Lee, Arturo Sanchez, Wei You, and
Luping Yu*
Department of Chemistry and The James Frank Institute, The UniVersity of Chicago, 5735 South Ellis AVenue,
Chicago, Illinois 60637
Received March 2, 2005; E-mail: lupingyu@uchicago.edu
Organic rectifiers or molecular diodes are asymmetric organic
molecules in which the electrons flow through in one preferential
direction.1 In this paper, we report the effect of protonation on the
rectifying behavior in newly designed and synthesized molecular
diodes. In our previous publications, we have described the synthesis
and characterization of a series of diode molecules.2-4 These diode
molecules are based on diblock oligomers with electron-rich
dithiophene covalently connected with electron-deficient dithiazole.
Scanning tunneling spectroscopy (STS) studies revealed a pro-
nounced rectification effect in single molecular devices.2-4 It was
Figure 1. Preparation of the assembly (a) NaOEt, ethanol/THF, dodec-
anethiol, Au(111); (b) TBAF, THF, Au nanoparticles.
also demonstrated that we could control the charge transport
direction by controlling the orientation of the diode molecules.4
We attributed this to the dipole orientation of the thiophene-
thiazole diblock molecule. Those results point out an approach to
enhance the rectification effect by increasing the dipole moment
of the molecules. With that in mind, we have designed and
successfully synthesized a new diode molecule 1, where a dipy-
rimidinyl moiety was connected with diphenyl groups (Figure 1).
Scanning tunneling microscopy (STM) and STS studies revealed
that the molecules not only have a more pronounced rectifying
behavior but also exhibit a dramatic effect by protonation on the
nitrogen atoms by strong acids, which can reversibly alter the
rectifying direction completely. This is the first example where the
effect of protonation on charge transport through a molecule was
Figure 2. Constant-current STM topography of (a) dodecanethiol/1 SAM
on Au(111) after attachment of AuNP to the top termini of 1. Inset: High-
observed.
Detailed synthesis of molecule 1 is described in the Supporting
Information. Various physical characterizations confirmed the
structure of the molecule. To control the orientation of 1 onto the
surface of the gold substrate and at the same time allow the
connection of a second electrode on the opposite side of the
molecule, we utilized two different thiol-protecting groups (cyano-
ethyl, CNE, and trimethylsilylethyl, TMSE).4 The CNE group was
cleaved in situ by sodium ethoxide (NaOEt) in ethanol to form
free thiol, which was then co-assembled with dodecanethiol (DDT)
in a 1:500 ratio on a Au(111) substrate (step a in Figure 1). Next,
the TMSE group on the top of the molecule was cleaved with a
solution of tetrabutylammonium fluoride (TBAF) in THF. The
resulting thiol was derivatized with a suspension of gold nanopar-
ticles (AuNP) to give the final assembly (step b in Figure 1).
The assembling processes were monitored with STM studies of
the sample at different stages. The STM topography after the self-
assembly process shows two main characteristics, a highly ordered
structure of DDT and randomly distributed bright spots. The
position, shape, and size of these spots were constant during
consecutive scans, except for small variations in the intensity due
to the stochastic behavior of single molecules inserted into the
alkanethiol SAM.5 The average sizes for the spots are approximately
2.5 nm. Finally, the bright spots were not observed in the images
of control samples prepared under identical conditions described
in Figure 1 without the addition of NaOEt. These observations
resolution image of the DDT SAM. (b). Image of a single AuNP. STM
imaging conditions: Vbias ) +1.0 V, It ) 1 pA.
indicate that the bright spots are the image of a single or a small
group of diblock molecules bonded to the Au substrate.
The cleavage of the TMSE protecting group, on the top of the
molecule, was confirmed with grazing incidence FTIR spectroscopy
with a SAM of molecules 1 on the gold substrate. The C-H
symmetric deformation vibration from the -Si-CH3 moiety was
considerably reduced after the TMSE cleavage.6 The free thiol was
immediately treated with a solution of AuNPs to give the final
assembly. Figure 2 shows the constant-current STM image of the
Au surface of the sample after the assembly was completed. The
apparent diameters of the AuNPs were in the range of 7-10 nm,
depending on the STM tip and scan rate used. This value is much
larger than the bright spots due to single molecules. STM images
of control samples prepared under the same conditions without
cleavage of the TMSE group or in the absence of inserted target
molecules do not show images of AuNPs. The evidence presented
above allows us to conclude that the consecutive deprotection and
orientation of the molecules resulted in the formation of an
assembly, as depicted in Figure 1. The current-voltage character-
istics of the resulting unprotonated assembly were studied with STS.
Figure 3a summarizes the averaged I(V) data. The STS spectrum
showed asymmetric charge-transport behavior. The I(V) asym-
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C O M M U N I C A T I O N S
Figure 4. Energy diagram for the unprotonated (left) and protonated (right)
assemblies, respectively. ∆Ed is the relative shift in the local VL at the
interface due to the dipole layer. E+ (E-) represents the energy barrier
needed to align the Fermi level with the molecular HOMO (LUMO) level.
Figure 3. Averaged I(V) curves measured over AuNPs at Vbias ) 1.5 V
for (a) assembly deprotonated, I sample before protonation, III sample after
protonation-deprotonation (It ) 0.1 nA) and (b) assembly protonated by
ClO4H (It ) 0.01 nA).
the EF at the middle of the gap between the HOMO and LUMO.9
Thus, for the assembly with the unprotonated diode molecule, the
dipole moment of the molecule will shift the local VL in the organic
layer from that of the metal in ∆Ed. Assuming an elastic resonant
tunneling mechanism for the charge transport, a smaller bias
potential is needed to produce electron-tunneling current through
the HOMO than through the LUMO (E- > E+).
When the molecule in the assembly is protonated, the positive
charge centered on the nitrogen reverses the sign of the electronic
density on each block, that is, the dipyrimidinyl became positively
charged. Theoretical calculation for the molecule 1 in the absence
of the anion revealed that the dipole moment also points in the
opposite direction. The local VL of the organic layer decreases and
lays the EF of the gold substrate electrode closer to the LUMO.
Now, the resonance tunneling through the LUMO becomes more
accessible (E+ > E-). Thus, the protonation inverted the rectification
direction in a reversible manner.10
metry was quantified by the rectification ratio (RR) defined as
I(+1.5 V)/-I(-1.5 V), whereas the reproducibility of the data
acquired for different NPs on the SAM was evaluated from the
RR range. The RR values for the unprotonated assembly range from
4.5 to 9 with an averaged RR around of 7.4. This value is larger
than the previously reported RR of 5 for the dithiophene-dithiazole
diblock oligomer in a similar assembly.4 The increase in the RR
can be explained by the stronger intrinsic polarization of the
dipyrimidinyl-diphenyl in comparison with that of the dithiophene-
dithiazole diblock. This difference is reflected in the theoretically
calculated dipole moment for the compound 1 (6.3 debye) and
dithiophene-dithiazole (1.6 debye).4,6
The most interesting result is the reversible change in the
rectifying direction by protonation/deprotonation of the dipyrim-
idinyl block (Figure 3b). Perchloric acid in a methanol/THF mixture
and NaOEt in methanol were used to protonate/deprotonate the
nitrogen atom in the assembly, respectively. The reversibility of
this process was followed by FTIR in a SAM of 1 and UV-visible
spectroscopy in solution. The FTIR studies demonstrated the
appearance and disappearance of the absorption band due to the
ClO4- anion after the protonation and deprotonation.6 In addition,
the UV-vis spectra of 1 in a THF solution exhibited a 0.2 eV
bathochromic shift and decreases in relative oscillator strength for
the πfπ* absorption band after addition of perchloric acid. The
energy absorption maxima recovered the original position with the
treatment of NaOEt.6,7 We can assume that, at least, monoproto-
nation of the dipyrimidinyl block occurred under the conditions
used. In addition, a more pronounced rectifying effect was observed
with the inverse of the RR (1/RR) in a range of 4-12, with an
average around 9.2. Moreover, if we change the orientation of
molecule in the assembly and let the dipyrimidine moieties remain
embedded in the DDT SAM, the dipyrimidine is not accessible by
protons. We observed that the sample has a reversed rectification
direction from the unprotonated assembly 1. However, the rectifica-
tion direction is not affected by the treatment with strong acid
solution.
In summary, a new molecular diode was synthesized and a
pronounced rectifying effect observed. It was found that protonation
on the nitrogen atoms of the diode molecule by strong acids affects
the charge transport dramatically, which can reversibly alter the
rectifying direction.
Acknowledgment. We gratefully acknowledge the financial
support of the National Science Foundation and the NSF MRSEC
program at the University of Chicago and AFOSR. UC-Argonne
Nanoscience Consortium provided partial support of this research.
Supporting Information Available: Experimental details for the
synthesis of 1, assembly preparation, theoretical calculation, STM, STS,
FTIR, and UV-visible measurement. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
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(10) This model does not pretend to give a quantitative explanation of the
phenomena; it merely offers a rationale for the observed results. More
detailed theoretical computation is needed to gain more insight into the
mechanism of charge transport.
These results and observations can be explained based on a crude
model shown in Figure 4. The electron density of the diblock co-
oligomer is polarized and induces a permanent dipole moment in
the molecule. When the molecule is connected to the electrodes,
the surface dipole layer induces a shift of the vacuum level at the
surface (VL) of the gold electrodes, which alters the Fermi level
(EF) alignment in the interface.8 Although it is difficult to know
exactly the position of EF within the molecular energy gap, as a
first approximation, we assumed a band lineup corresponding to
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