Single azopyridine-substituted porphyrin molecules for configurational and electronic switching

Article abstract of DOI:10.1039/c0cc00959h

A new azopyridine functionalized Ni-porphyrin was synthesized as a model compound for deposition and switch on surfaces. Two geometrically and electronically different states of single molecules on Au(111) were found by scanning tunneling microscopy/spectroscopy and analyzed with density functional calculations.

Full text of DOI:10.1039/c0cc00959h

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Single azopyridine-substituted porphyrin molecules for configurational
and electronic switchingw
a
b
c
d
a
¨
Francesca Matino,* Guillaume Schull, Umasish Jana,* Felix Kohler, Richard Berndt and
Rainer Hergesd
Received 15th April 2010, Accepted 30th July 2010
DOI: 10.1039/c0cc00959h
A new azopyridine functionalized Ni–porphyrin was synthesized
as a model compound for deposition and switch on surfaces. Two
geometrically and electronically different states of single molecules
on Au(111) were found by scanning tunneling microscopy/
spectroscopy and analyzed with density functional calculations.
surfaces, further chemical methodologies for a rational vertical
design of complex surface architectures are needed.
To combine the cis–trans switching capability of azopyridine
with the property of porphyrins to form ordered self-assembled
monolayers we covalently attached a 3-azopyridine unit to a
Ni(II) tetraphenylporphyrin (Ni–TPP). The porphyrin platform
provides bonding to the surface, electronic decoupling and a
fixed orientation of the azopyridine unit. Azobenzene
functionalized porphyrins have been synthesized previously.
However, the azo group was attached in the para position to
the meso phenyl substituents, thus lying within the porphyrin
molecular plane.9 We attached an azopyridine unit in the ortho
position to one of the meso phenyl groups of tetraphenyl-
porphyrin, thus allowing the azopyridine to protrude from the
surface. Direct synthesis from a mixture of pyrrole, benzaldehyde,
and an azopyridine functionalized benzaldehyde in a 4 : 3 : 1
ratio failed. Using prefabricated dipyrromethane units for
cyclization furnished the monosubstituted tetraphenyl-
porphyrin in 6.6% yield (Fig. 1, for details see ESIw). Irradiation
of the trans compound with 310 nm or 520 nm light in
acetonitrile solution gave 15–20% and 35–40% cis isomer.
Re-isomerisation to the trans compound is achieved by heating
to 70–75 1C for 2 h.
Switching is an elementary step in many sophisticated
functions, such as directed motion, pumping, information
storage and processing, in the macroscopic world as well as
at the molecular scale.1 For nanoscale applications of such
functions, molecules have to be immobilized on solid supports
in a well defined geometry and orientation, in order to achieve
advanced and reproducible dynamic functions. Moreover, a
controlled electronic coupling with the surface as well as
sufficient space for molecular movement during an isomerization
process are required to avoid charge transfer and hybridization
with the surface or intermolecular interactions.2 Immobilization
and decoupling have been achieved by anchoring functional
molecules with a thiol group and an alkane spacer on gold
surfaces.3 Sterical hindrance within a densely packed self-
assembled monolayer can be avoided by dilution of the
functional molecules using short alkane thiols.4 However,
usually stochastic rather than ordered monolayers are
obtained. To achieve decoupling from the surface, bulky
tert-butyl substituents were attached to a molecular framework
to lift it away from the surface.5,6 Vertical alignment at a large
distance from the surface and control over the distance of the
functional groups with respect to each other were attempted
by constructing molecular tripods with thiol feet.7 Anyway,
ordered monolayers are difficult to obtain and it has been
shown that not all feet touch the gold surface.7i Modular
platforms and spacers have been recently introduced to
control distance from the surface, orientation and packing
density.8 In view of the construction of controllable functional
The molecules were deposited from a heated tantalum
crucible onto a previously cleaned Au(111) surface kept at
room temperature and investigated by means of a UHV
scanning tunnelling microscope (STM) at 5 K. Calculations
at the B3LYP/6-311+G* level of density functional
theory (DFT) were performed to determine the structure
and electronic properties of the molecule, neglecting the
substrate.
a Institut fur Experimentelle und Angewandte Physik,
¨
Christian-Albrechts-Universitat zu Kiel, 24098 Kiel, Germany.
¨
E-mail: matino@physik.uni-kiel.de; Fax: +49 431 880 2510;
Tel: +49 431 880 2667
b
´
Institut de Physique et Chimie des Materiaux de Strasbourg,
UMR 7504 (CNRS – Universite de Strasbourg), 67034 Strasbourg,
´
France
c Department of Chemistry, Jadavpur University, Kolkata 700032,
India. E-mail: jumasish2004@yahoo.co.in
d Institut fur Organische Chemie, Christian-Albrechts-Universitat zu
Kiel, 24098 Kiel, Germany
¨
¨
w Electronic supplementary information (ESI) available: Experimental
and characterization data for the synthesis of azopyridine functionalized
porphyrin. Molecular energy levels from DFT calculation. See DOI:
10.1039/c0cc00959h
Fig. 1 Synthesis of the azo-functionalized porphyrin 1 (azo/Ni–TPP).
c
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Fig. 2 Pseudo three-dimensional constant current STM images of azo/Ni–TPP adsorbed on Au(111): (a) 12 Â 10 nm2, sample voltage V = 1 V;
(b) 2.9 Â 2.9 nm2, V = 1.4 V; (c) 3.4 Â 2.9 nm2, V = 1.4 V. (d) and (e) Space filling models of optimized geometries for trans and cis conformations.
(f) I(V) characteristic acquired by placing the tip over the azopyridine related protrusion. STM images were treated with a Laplacian filter to
enhance small molecular features.10
At high coverage, molecules tend to aggregate in long islands
aligned along the fcc sites of the herringbone reconstruction. As a
result of both porphyrin–porphyrin interaction and hydrogen-
like bonds, the islands are composed of trimers and rectangular
structures (the lateral dimensions, 1.4 nm  1.5 nm, are in good
agreement with the unit cell parameters reported for Ni–TPP,11
Fig. 2(a)). Fig. 2(b) shows a detailed image of a single molecule.
Its shape in STM images compares well with the DFT calculated
structure of the trans configuration of the azo/Ni–TPP
(Fig. 2(d)). The phenyl rings attached to the porphyrin core as
well as the azopyridine arm are clearly discernible.12 The lower
intensity inside the porphyrin ring is assigned to the nickel dz2
filled orbital lying far below the Fermi level.11 According to the
calculations the phenyl rings and the azo group of the
azopyridine unit are pointing away from the surface (i.e. towards
the STM tip), which is consistent with the appearance of the
latter as a prominent protrusion in STM images (Fig. 2(b)).
As reported in Fig. 2(f), the current acquired over the
sidearm of the azo/Ni–TPP can become unstable at sample
voltages exceeding B1.4 V, which may indicate that the arm
switches between different conformations. Indeed, controlled
manipulation of the arm was achieved by increasing the
sample voltage to B2 V with the tip positioned over the
sidearm of the molecule. Fig. 2(c) shows the molecule from
Fig. 2(b) after manipulation. The double lobed azopyridine
related feature is now bent and exhibits an additional lobe.
Such bending of the azopyridine sidearm is expected for a
trans–cis isomerization. The corresponding geometry change is
very well represented by the calculations (Fig. 2(d) and (e)).
We note that molecules found with a bent sidearm can be
switched to a straight arm conformation by applying similar
voltages. To further the assignment of trans and cis
configurations, scanning tunneling spectroscopy (STS) was
performed. The measured differential conductances dI/dV of
straight (red) and bent (green) sidearm molecules (Fig. 3(a))
may be compared to the calculated densities of states of trans
(red) and cis (green) (Fig. 3(b)). The straight and bent arm
spectra are markedly different. Clear features at +1.6 V, À1.0 V
and À1.7 V are visible for the bent configuration. The straight
arm configuration exhibits a strong peak at +1.5 V and
smaller peaks at À0.9 V and À1.2 V. The energies of these
peaks are in good agreement with the calculated densities of
states for the cis and the trans configurations. The peaks at
positive voltage (1.6 V, green curve, and 1.5 V, red curve) can
be assigned to the LUMOs of the cis and the trans configurations.
The energy difference of these peaks agrees with the calculation,
which predicts the LUMO of the trans isomer to be 0.1 eV
lower in energy than in cis isomer (Fig. 3(b)). The strong
features of the bent configuration at À1.0 V and À1.7 V are
attributed to the first occupied orbitals of the cis isomer,
HOMO and HOMO À 1 (mainly located on the porphyrin
frame), and the HOMO À 2, which has large coefficients at the
lone pairs of the NQN group (Fig. 3(b)). For the straight arm
configuration, HOMO and HOMO À 1, with small coefficients
on the azopyridine arm, are barely discernible in the STS data
(low intense features at À0.9 V and À1.2 V, Fig. 3(a)).13 MOs
with large coefficients in the trans configured azopyridine arm
are very low in energy (HOMO À 5 and HOMO À 6) and
beyond the range of the STS measurements (Fig. 3(b), see also
Table S1 in ESIw). The energy difference between HOMO and
LUMO in the STS data for the straight arm conformation is
reduced by B0.3 V with respect to the bent conformation,
consistent with the calculation (Fig. 3(a) and (b)). A similar
reduction of the energy gap for cis and trans conformers was
reported from UV spectroscopy in the parent azobenzene.14
The correspondence between the LUMO energies and the
voltage instability shown in Fig. 2(f) suggests that these
changes occurred in the azo/Ni–TPP as a result of tunneling
electrons interacting with the LUMO states. These orbitals are
very close in energy in the two configurations, which explains
why the switching occurs at similar voltages for both
molecules.
A detailed analysis of cis or trans molecules found on the
surface after deposition with those resulting from manipulation
revealed a slight decrease of their apparent heights in STM
images and the switching could not usually be reversed. The
observations suggest that the three-dimensional structure of
the sidearm is essential for the switching. While an azopyridine
arm protruding from the surface may be switched, an arm on
the surface leads to significant interaction between the lone
pair at the pyridine nitrogen atom with the metal which
effectively prevents further switching.15
c
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Fig. 3 Electronic structure of the azo/Ni–TPP in the bent azopyridine arm or cis (green) and straight arm or trans (red) configurations.
(a) Experimental STS data. STM feedback was opened at V = 2 V, I = 1 nA. The bent arm data are vertically offset for clarity. The dotted lines
highlight the reduction of the HOMO–LUMO energy distance between the two conformations. (b) Calculated density of states and most
important molecular orbitals. The molecular energy levels are broadened by Gaussian (full width at half maximum = 0.4). The grey area is beyond
the voltage range covered by the STS data.
In summary, a new azopyridine functionalized porphyrin
was synthesized as a model switch for deposition on surfaces.
Two geometrically and electronically different states of single
molecules on Au(111) were found and analyzed by STM and
STS. Switching between two conformational states occurs
upon tunneling of electrons through LUMO orbitals.
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Funding by the Deutsche Forschungsgemeinschaft (SFB 677)
and by the Schleswig-Holstein-Fonds is gratefully acknowledged.
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c
6782 Chem. Commun., 2010, 46, 6780–6782
This journal is The Royal Society of Chemistry 2010