Stable negative differential resistance in porphyrin based σ-π-σ monolayers grafted on silicon

Article abstract of DOI:10.1039/c5ra09484d

Two Si-based hybrid self-assembled monolayers were synthesized by electro-grafting two di-O-alkylated porphyrins as the σ-π-σ systems. The monolayers showed a stable and reversible negative differential resistance (NDR) property at room temperature. The monolayer, fabricated using the porphyrin with fluorophenyl groups was more compact and showed a tenfold peak-to-valley ratio (PVR) relative to the other similar system devoid of the fluorine atoms in the porphyrin moiety. This suggested better pre-organization of the former, possibly by hydrogen bonding through the electro-negative fluorine atoms.

Full text of DOI:10.1039/c5ra09484d

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PAPER
Stable negative differential resistance in porphyrin
based s–p–s monolayers grafted on silicon†
Cite this: RSC Adv., 2015, 5, 50234
Kavita Garg,a Chiranjib Majumder,b Shiv Kumar Gupta,c Dinesh Kumar Aswal,c
a
a
*
Sandip Kumar Nayak and Subrata Chattopadhyay
Two Si-based hybrid self-assembled monolayers were synthesized by electro-grafting two di-O-alkylated
porphyrins as the s–p–s systems. The monolayers showed a stable and reversible negative differential
resistance (NDR) property at room temperature. The monolayer, fabricated using the porphyrin with
fluorophenyl groups was more compact and showed a tenfold peak-to-valley ratio (PVR) relative to the
other similar system devoid of the fluorine atoms in the porphyrin moiety. This suggested better pre-
organization of the former, possibly by hydrogen bonding through the electro-negative fluorine atoms.
Received 20th May 2015
Accepted 1st June 2015
DOI: 10.1039/c5ra09484d
www.rsc.org/advances
doped Si (111) surfaces,8a,b diamond lms,8c and thiols on Au
surfaces (all at room temperature),8d (ii) 20-amino-4,40-di(eth-
ynylphenyl)-50-nitro-1-benzenethiol, sandwiched between two
Introduction
Extensive research is being carried out on electron-transfer (ET)
processes through molecular scaffolds due to their potential
technological applications in molecular electronic devices.1
Studies have demonstrated that besides the junction geometry,
the structures of the incorporated molecules also dictate the
electron-transfer rates, current–voltage (J–V) curves and the
behaviour of the resulting devices.2a–g Self-assembled systems of
organic molecules on metal/semiconductor substrates,
prepared by Langmuir–Blodgett (LB) lms, or chemically-
graed monolayers of organic molecules junction is a power-
ful ‘bottom-up’ approach for the fabrication of devices for
molecular-scale electronics.3 Integration of molecular compo-
nents into electronic circuits by graing on Si is expected to
miniaturize the electronic circuits to nanoscale.4 This approach
offers the advantage of tailoring the surface potential for
improved hybrid molecular devices, changing the p-n junction
threshold voltage by adjustment of the electronic nature and/or
use of multiple oxidation states of the organic p group mole-
cules, instead of classical doping of silicon.5 The negative
differential resistance (NDR) behavior (i.e., an initial rise in
current and its subsequent sharp drop even with progressively
augmented voltage, as opposed to Ohm's law) has drawn
signicant attention because of its potential application in
realization of logic devices and memory circuits,2b,6 and is found
in a variety of molecular devices.4b,7 NDR effects have been
reported using various organic molecules and different types of
junctions. Some representative examples include (i) boron
metal electrodes (observed at 60 K),2b (iii) Pd/ferrocene self-
assembled layer/Au structure,8e (iv) cyclopentene molecules,
deposited on p-type hydrogen free Si(001) (observed at 80 K),2b
(v) styrene and 2,2,6,6-tetramethyl-1-piperidinyloxy, deposited
on degenerately doped Si (100) reconstructed surfaces,8f and (vi)
disulde molecules deposited on Si.8g In an interesting study,
organic molecules deposited on doped Si (100) surfaces by
ultrahigh vacuum scanning tunnelling microscope (STM)
showed room temperature NDR behaviour that was decided by
the nature of the molecules and the dopants.8h In all these cases,
the resonant and off-resonant electronic tunneling mechanism
provide satisfactory explanations for the NDR behavior.9 A large
peak-to-valley ratio (PVR) in the NDR effect, required for fast
switching and functioning of the device at room temperature
with high reproducibility are the prerequisites for applications
in hybrid nanoelectronics. Most of the reported molecular
hybrids, exhibiting the NDR behaviour did not full many of
these criteria, and the measurements were carried out under
ultra-high vacuum in certain cases. Some of the devices were
made using expensive techniques such as STM, and require
appropriate biasing and doping. Due to the variation of the
electronic structures of the STM tips during experimentation,
NDR may occur at different bias magnitudes and polarities.
Also, routine impurity doping is problematic due to uncertainty
of its distribution.10
Thus, despite impressive progress in molecular electronics,
search for alternate single molecules or a nite ensemble of self-
assembled molecules showing NDR property at a lower bias is
currently an active research area in molecular electronics.
Amongst the electron-rich organic molecules, porphyrins11 are
used extensively in fabricating molecular devices because they
(i) form stable p-cation radicals, and exhibit two accessible
aBio-Organic Division, Bhabha Atomic Research Centre, Mumbai, 400085, India.
E-mail: schatt@barc.gov.in
bChemistry Division, Bhabha Atomic Research Centre, Mumbai, 400085, India
cTechnical Physics Division, Bhabha Atomic Research Centre, Mumbai, 400085, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra09484d
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cationic states in monomeric forms;12a–e (ii) have long charge and a C11-alkenyl chain as the barriers. The choice of the alkenyl
retention times, hence less power consumption, (iii) are highly chain as one of the barriers was a part of the molecular design,
stable,12f and (iv) can form self-assembled structures.12g Despite because this helped in electro-graing monolayers of the
all these attributes and theoretical proposition,8h,13 scarce molecules on Si (111) surfaces. Subsequently, the possibility of
attention has been paid to construct porphyrin-based NDR using the oxidation states of the porphyrin molecules as
devices experimentally. Previously, the J–V curves of some molecular-scale devices was investigated using the fabricated
porphyrin-metal ions combinations showed NDR-like effect.14a–c porphyrin–Si hybrids.
We have also found electrical bistability and current rectica-
tion properties of some porphyrin monolayers on Si surfa-
ces.14d,e More recently, metal-free and Zn-bis-porphyrin
molecules have been used as efficient chemosensors for Cl2
and NH3 gases in air.15 Presently, we fabricated redox-active
porphyrin monolayers on Si and investigated the possibility of
using the oxidation states of the porphyrin molecules as
molecular-scale information storage systems. The prime aim
was to develop devices that are environmentally stable, and
show high repeatability and easy processability. Studies on the
J–V characteristics of the hybrid systems revealed stable,
reversible, reproducible, and room temperature NDR behaviour
by both the systems. We also demonstrate that the NDR prop-
erty can be tuned by subtle changes in the porphyrin structure
by incorporating an electro-negative substituent (F) at the meso-
phenyl groups.
Synthesis of porphyrins 5a and 5b
The classical one-pot method of Adler and Longo involving an
acid-catalyzed reaction between pyrrole and an aryl aldehyde
under reuxing conditions is suitable for the synthesis of
symmetric (A4-Por) or partially symmetric (A3B-Por) porphyrins,
where A and B are the meso-aryl substituents.20a Extension of the
method to synthesize porphyrins, bearing two or more types of
meso-substituents provides a statistical mixture of six porphy-
rins from which isolation of pure compounds becomes
extremely difficult. Hence, we adopted Lindsey's method
involving a “2 + 2” route using a dipyrromethane-1,9-dicarbinol
and a dipyrromethane (bearing ABC- and D-substituents,
respectively) for the synthesis of the target porphyrins 5a and
5b.20b This involved a base-catalyzed alkylation of 4-hydrox-
ybenzaldehyde with 1-bromohexane and 11-bromoundec-1-ene
to furnish the aldehydes 1a and 1b respectively. The alde-
hydes were individually subjected to a triuoroacetic acid
(TFA)-catalyzed condensation with pyrrole to yield the dipyrro-
methanes 2a and 2b respectively. Compound 2a was subse-
quently acylated with benzoyl chloride (3a) or 4-uorobenzoyl
chloride (3b) using EtMgBr as the base to afford the ketones 4a
and 4b respectively. This reaction is tricky and produces both
mono- and di-acylated products if the stoichiometry and reac-
tion temperature are not maintained carefully. The ketones 4a
and 4b were subsequently reduced with NaBH4 and the resul-
tant unstable diols were converted to the porphyrins 5a and 5b
via a TFA-catalyzed condensation with 2b followed by a 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) oxidation, the
three-steps process being carried out in one-pot (Scheme 1).
Results and discussions
Given the importance of the molecular bridges in nano-devices,
design of the organic molecule is crucial in attaining our
objectives. The molecular design was conceived keeping in
mind that the energy gap (DE) between the energy states
(LUMO/HOMO) of the molecular bridges and the Fermi levels of
the donor and acceptor units control the electron-transfer rate
and current ow.16 For large DE, the ET process is dominated by
a “through-bond” non-resonant tunnelling mechanism, where
the organic molecules generally act as poor electron conductors.
However, alteration of their electronic structures can induce the
ET process via a resonant tunnelling or a hopping mechanism.
The change from a non-resonant to resonant tunnelling would
result in an abrupt increase in the current, and the measured
J–V curves would show NDR characteristics.17 Resonant
Preparation of the graed organic assembly
tunnelling requires a double potential barrier along the electron Physisorption of organic molecules as Pockels–Langmuir (PL)
transfer coordinates. Earlier, we have proposed a possible lms or by vapor-phase deposition on electrodes is oen used to
physical origin for such a double potential barrier, and fabricate hybrid organic electronic devices. However, ordering
hypothesized that the s–p–s monolayers, graed on Si might of PL lms is usually achievable with amphiphilic molecules
show NDR effect.14d The s–p–s molecular architecture is anal- only, restricting the molecular design. On the other hand, the
ogous to the tunnel diode, with a ‘quantum well’ surrounded by vapor-phase deposition method oen results in poor deposition
thin layer barriers.18 Here, the p-moiety (a conjugated molecule) yields, disordered packing and random orientations. Further,
acts as a quantum well and the s-moieties (alkyl chains) as the the physisorbed molecules oen move to seek a lower energetic
tunnel barriers. The NDR effect in such a monolayer is expected, state on the surface, or in response to an applied electric eld.
if electrons tunnel through some resonant states of the Instead, covalent linking of organic molecules to metal/
p-moiety. Our previous studies with N-(2-(4-diazoniophenyl) semiconductor surfaces provides a better alternative.21 Exten-
ethyl)-N0-hexylnaphthalene-1,8,4,5 tetracarboxy diimide tetra- sive work has been carried out by attaching organic thiol
uoroborate as a s–p–s molecule showed poor NDR effect (PVR molecules to Au electrodes. But, the Au surfaces are reported to
ꢀ 10) with hysteresis, but established the hypothesis.19 Hence, be thermally unstable.22 Monolayers of alkyl silanes have been
we synthesized two new di-O-alkylatedporphyrins 5a and 5b graed on SiO2 surface, but the multi-steps protocol required
with tetraphenylporphyrin (TPP) and a uoro-TPP derivatives as stringent reactions conditions such as control of the optimized
the respective quantum wells (p moiety), and a C6-alkyl chain temperature and anhydrous conditions. In addition, many of
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Scheme 1 Syntheses of the prophyrins.
the alkyl silanes have to be synthesized separately.23 Hence, we surface. Using different number of scans (5, 10, 20, 25 and 30), a
followed electro-graing for an easy attachment of the compact monolayer, as revealed by AFM (Fig. 2) was prepared at
porphyrins on Si (111) surface through the strong Si–C bond 25 and 30 scans respectively with 5a and 5b. At higher scans,
(Si–C ꢀ76 kcal molꢁ1).23b Organic molecules having a cleavable formation of multilayers was evident by AFM analysis (data not
group such as vinyl (C]C), ethynyl (C^C), halide (Cl, Br, I), shown).
tetraalkylammonium, and diazonium silane reacts with
H-terminated Si, and can be deposited using electrograing
process. The advantage of the process is that the deposition
Monolayer characterization
process can be monitored in situ by measuring the redox peak of
the electrograing reaction.
In order to ascertain the monolayer deposition on Si, the
electro-graed materials were characterized by contact angle
measurement, polarized FT-IR spectroscopy, X-ray reectivity
(XRR), ellipsometry, AFM, secondary ion mass spectrometry
(SIMS) and electrochemistry. The contact angles of deionized
water in case of Si wafers, graed with 5a and 5b were ꢀ58ꢂ and
64ꢂ respectively, whereas for the cleaned Si wafer it was 84ꢂ. The
observed contact angles were much less than the reported
values (97–108ꢂ) of the methyl terminated alkyl chains.24 This
suggested interaction of the water molecule with the porphy-
rins, possibly through their pyrrole rings, which is possible only
when the molecules are tilted. This was also conrmed by
ellipsometry, where the average thicknesses of respective
monolayers were found to be ꢀ2.3 ꢃ 0.2 nm in case of 5a and
2.9 ꢃ 0.2 nm with 5b. The XRR experiments, carried out at room
temperature with the monolayers further conrmed their
thicknesses. The reectivity data (Fig. 3) was tted in MOTOFIT
soware, using Parratt's formalism. The scattering length
density (SLD) values of the monolayers were calculated from the
density of monolayer and molecular formula of molecule
according to eqn (1).
Presently electrograing of the molecules 5a and 5b on
highly-doped, commercially available Si (111) surfaces was
achieved by conventional cyclic voltammetry (CV). The CVs
(Fig. 1), recorded during electrochemical deposition of the
molecules 5a and 5b on Si showed an irreversible peak at
ꢀ0.3 V, which was earlier assigned for the bonding of an alkene
with the H-terminated Si surfaces.14d Moreover, no peak at 0.3 V
appeared when the CV was run using the TBAP solution alone,
but similar peak was observed with 1-undecene (taken as
reference). This indicated covalent attachment of the porphyrin
molecules at the H-terminated Si surfaces is through terminal
double bond. The possible electrochemical reactions in the
process is schematically shown in ESI (Fig. SL1†). In the rst
step, application of a negative potential to the working electrode
produces a radical on silicon and a proton. The Si radical
subsequently reacts with the alkene function of 5a and 5b to
form the Si–C bond, and generates a C-centred radical b to the
Si atom. A subsequent transfer H atom from another Si–H bond
generates a new Si radical to propagate the process. Formation
of the Si–C bond resulted in the irreversible oxidation peak at
ꢀ0.3 V. As the number of scans increased, the peak diminished
owing to the non-availability of nucleophilic Si atoms at the
n
X
NNarmass
r ¼
ꢄ
bci
(1)
MR
i¼1
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Fig. 1 CVs indicating electrografting of the molecules on silicon (n++) wafers. (a) 5a; (b) 5b. The deposition was carried out under N2 atmosphere
at a scan rate of 0.05 V sꢁ1 using Si wafers as the WE, Pt as the CE, Ag/AgCl as the RE, 0.1 M Bu4NP as the electrolyte and the porphyrins (1 mM) in
dry CH2Cl2.
nm respectively (Fig. 3 inset). These values are lower than the
e2
bi ¼
f1i
(2)
theoretically calculated (using Molkel soware) length ꢀ3.9 nm
4p30mc2
of the porphyrins. The rouꢁg6hness values for the monolayers of
ꢁ2, c5a ¼ 0.04913) and 5b 29.7 A
2
˚
˚
2
˚
where Na is Avogadro number, rmass is the mass density of the
material, MR is its relative molecular mass and bci is the bound
coherent scattering length of the ith atom of a molecule with n
atoms. For X-rays, the scattering length for each atom was
calculated using eqn (2), where e is the charge on a single
electron, 30 is the permittivity of free space, m is the mass of an
electron and c is the speed of light. The scattering factor (f1i) for
an atom of element i is available in literature.25 The SLD prole
was calculated using eqn (3), where N is the total number of
layers, z is the distance from the top interface and erf is the error
function.
5a 24.0 A (SLD ¼ 0.22 ꢄ 10
A
(SLD ¼ 2.23 ꢄ 10ꢁ6
c5a ¼ 0.03049) were close to their
ꢁ2
˚
A
thicknesses estimated by ellipsometry. The XRR data also
indicated that the tilt angles of the monolayers were ꢀ39ꢂ and
51.4ꢂ for 5a and 5b respectively.
The AFM analyses revealed that the monolayers were more
organized with lesser number of voids and hillocks, and the
void depths were ꢀ2.3 nm for 5a and 2.9 nm for 5b (Fig. 2). The
RMS and average roughnesses values of the monolayers were
1.45 and 1.16 nm for 5a, and 0.89 and 0.73 nm for 5b. Compared
to 5a, the monolayers of 5b were more compact and uniform
with larger grain size. The fast scan (10 V sꢁ1) CVs (Fig. 4) of the
monolayers exhibited a reversible peak at +0.8 V for the
respective porphyrin moieties and no such peak was observed
in bare silicon and undecene deposited silicon. The net charge
transferred during the oxidation process, calculated from the
area under the oxidation peak were 4.5 ꢄ 10ꢁ8 C and 8.8 ꢄ 10ꢁ7
ꢀ
ꢀ
ꢁꢁ
N
X
ri ꢁ riþ1
z ꢁ zi
pffiffi
r ¼
1 þ erf
(3)
2
2si
i¼0
From the plot of SLD vs. interface distance, the thicknesses
for 5a and 5b were found to be 2.54 ꢃ 0.02 nm and 3.05 ꢃ 0.03
Fig. 2 AFM images (1 mm ꢄ 1 mm) for the monolayers of (a) 5a; (b) 5b, electro-grafted on silicon (n++) wafers.
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Fig. 3 XRR curves of the porphyrins-grafted monolayers on silicon (n++) wafers. (a) 5a; (b) 5b; insets: SLD plots.
C respectively for 5a and 5b. These amounted to surface
The SIMS of the monolayer of 5a showed peaks due to the
coverages of 4.3 ꢄ 1011 and 3.4 ꢄ 1012 molecules cmꢁ2 respec- porphyrin fragments at m/z 665, 646, 461, 400, 356 and 324 amu.
tively for 5a and 5b. Thus, the surface covered by 5b was In case of 5b, the peaks appeared at a lower mass range viz. m/z
ꢀ8 times that by 5a. These data are consistent with the AFM 457, 407, 387 and 334 amu. Nevertheless, the SIMS data
analyses, both revealing more compact monolayers with 5b (Fig. 5(a) and (b)) of the monolayers of 5a and 5b conrmed
than 5a. This may be because of hydrogen bonding amongst the deposition of their respective monolayers on the Si wafers. In
F and H atoms of the porphyrin phenyl moieties.
case of 5a monolayers, the secondary ions knocked down the
Identifying the C–H/F–C interaction as a hydrogen bond is porphyrin moiety from the alkyl spacer, attached to the Si
questioned due to the poor acceptor ability of C-bonded F atoms surface. Subsequent ionization of the released porphyrin moiety
compared to the O- and N-atoms, if present.26 However, distinct provided the mass fragments at higher masses. Possibly, the
hydrogen bond character has been reported in the layered secondary ions cannot penetrate the more compact 5b mono-
crystal structure of uoroaromatics, where C–H/F–C interac- layers, resulting in the fragmentation of the porphyrin moiety in
tions contribute signicantly in stabilizing the layers. This has the middle to generate the low molecular weight mass peaks of
been attributed to activation of the ortho-aromatic protons by the truncated porphyrin moiety.
the F atom that may override the poor acceptor nature of the
The polarized FTIR spectra (Fig. 6) for the monolayers of 5a
C-bonded halogen.27 In addition, the face-to-face noncovalent exhibited N–H stretching frequency at 3249 cmꢁ1, symmetric
interaction in arene–peruoroarene system is ubiquitous, and (ns) and asymmetric stretching modes (na) of CH2 group at
widely recognised as one of the major driving forces in forming 2842 and 2910 cmꢁ1 and of CH3 group at 2877 and 2949 cmꢁ1
.
robust supramolecular assemblies.28 This primarily involves In contrast, the respective IR absorption peaks of the mono-
stabilizing Coulombic interactions, and has been reported with layers of 5b were at 3255 cmꢁ1, 2855 and 2925 cmꢁ1, and at
several uoroaromatic compounds.29 In the present case, the 2871 and 2961 cmꢁ1. In pure solid alkane monolayers, the
parallel offset disposition of the uorophenyl moieties of hydrocarbon chains exist in an all-trans conguration such that
adjacent porphyrin molecules may also be responsible for the the carbon backbone of each molecule lies in single planes.
compact monolayers of 5b.
However, in liquid form, there is a substantial twisting about
Fig. 4 Fast scan CVs for the monolayers of (a) 5a; (b) 5b; electro-grafted on silicon (n++) wafers. The CVs were recorded under N2 atmosphere at
a scan rate of 10 V sꢁ1 using the respective monolayer-grafted Si as the WE, Pt as the CE and Ag/AgCl as the RE, and 0.1 M Bu4NP as the
electrolyte. The reversible peaks are indicated by circles. Insets show the magnified redox peaks, after background correction and converting the
potential scale into time scale.
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Fig. 5 SIMS of the porphyrins-grafted monolayers on silicon (n++) wafers. (a) 5a; (b) enlarged plot for 5a; (c) 5b; (d) enlarged plot for 5b.
the individual bonds; these out-of-plane twists alter the
frequency of the CH2 vibrational modes.30 Thus, the IR peaks
due to the CH2 vibrational modes can provide better insights
about the proposed van der Waals interactions between the
porphyrin rings, parallely anchored on Si. Our IR data showed
that the alkyl chains in the monolayers of 5a are more rigid like
in pure solid alkanes, while that in the monolayers of 5b are
more liquid like and twisted. Presumably, in case of 5b, the
phenyl rings of the porphyrin moiety are more tightly packed
setting the alkyl chains free to twist. But in case of 5a, proper
packing requires stacking of the porphyrins as well as the alkyl
moieties at a tilt angle of 39ꢂ. This rigidies the alkyl chains
in 5a.
I–V characteristics
Typical current voltage (I–V) curves of Hg/molecule/Si (111)
wafers are shown in Fig. 7. The hybrid assemblies, prepared
from 5a and 5b showed reversible NDR behaviour at room
temperature with current PVRs of 10 and 100, and peak posi-
tions (voltage) at 1.18 V and 1.09 V respectively. The details of
the I–V curves are shown in Table Sl1.† Interestingly majority of
the devices, constructed with both the systems were stable
during repetitive voltage scanning for 8 h in positive and
negative bias voltages, without any reduction in current or the
NDR effect. However, the reversible NDR effect showed only a
marginal hysteresis. The Si-alkyl//Hg junctions, used in the
studies are very stable, and exclude any possibility of penetra-
tion of Hg drops through pinholes or diffusion of mercury vapor
through the SAM. Thus, the measured I–V is expected to be
direct. The statistical analyses of data, and junction yields are
extremely valuable to discriminate artifacts from real data.31 In
the present work, we constructed 96 and 48 devices, respectively
with the compounds 5a and 5b. Of these, 80 and 43 devices,
made of 5a and 5b showed stable (up to 8 h), and reversible NDR
property, although 94 and 46 of these devices showed reversible
NDR behavior.
The complete PVR statistics of the devices are shown as the
ESI (Table Sl2† and the accompanying pi-chart), and summa-
rized here. With compound 5a, the PVR values of 36% of the
devices were 8–10, while an additional 37% of the devices
Fig. 6 Polarized FTIR spectra of the monolayers of 5a and 5b on
silicon (n++) wafers.
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Fig. 7 Device design and characteristics. (a) I–V measurement set up; (b) and (c) I–V plots of the monolayers of 5a and 5b on silicon (n++) wafers
respectively.
Thus, the NDR effect in forward bias is a result of alignment of
the HOMO levels of the molecules with the Fermi-levels of the
electrodes. Various mechanisms such as charge transfer-
induced change of the charge state, and chemical/
conformational changes under nite bias have been proposed
to explain NDR phenomenon.32 It is possible that presently, the
NDR behaviour depends on a match (resonant tunnelling)
between the Fermi levels of electrodes and the HOMO levels of
Fig. 8 CV of the porphyrins in solution phase using ferrocene as the molecules sandwiched between the electrodes, followed by a
standard. (a) 5a; (b) 5b.
mismatch of HOMO levels of the oxidized molecules with the
Fermi-levels of electrodes. The hypothesis is consistent with the
Aviram–Ratner model of molecular rectication.4a However,
involvement of additional mechanisms can't be excluded.
showed PVR values 5–8. The device statistics of the monolayers
of 5b were more impressive with 39% of the devices showing
PVR values of 80–100, an additional 33% with PVR of 50–80, and
an additional 24% with PVR of 10–50. The I–V characteristics of
Theoretical interpretation
the solid state devices can be understood in terms of the
molecular properties observed in the solution. Presently, the
current ow in both the solid-state devices (Fig. 4) as well as in
the respective porphyrin solutions (Fig. 8) showed same oxida-
tion peaks.
The correspondence between the solution and solid-state
results suggested that the fundamental molecular electronic
properties of the porphyrins are retained in the solid-state
devices. Hence, the forward bias current-ow should be deter-
mined by the HOMO states of the molecules, while their
respective LUMO states would dictate the reverse bias current.
For further clarication, the theoretical calculations of the
electronic transport behaviour were carried out. The ground
state geometries of the molecules 5a and 5b were optimized by
ab initio molecular orbital calculations. The ionic optimization
of molecules 5a and 5b was carried out without any symmetry
constraint at the B3LYP/6-31G (d,p) level of theory. The
geometrical parameters of both molecules were found to be
same, except for the C–H and C–F bond lengths, which were
˚
˚
1.09 A and 1.39 A, respectively.
Our experimental results revealed that on applying voltage,
initially there was a slow rise in the current due to tunnelling.
Fig. 9 Diagramatic presentation of the NDR mechanism for a s–p–s system.
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But at Vonset the HOMO level of the molecule would align in Bu4NI (10 mol%) in acetone (100 mL) was reuxed till the
resonance with the Fermi level of Hg. This can explain the sharp reaction was complete (cf. TLC, ꢀ16 h). The mixture was ltered,
increase in current at Vonset. At Vp, the molecules get oxidized to and concentrated in vacuum. The residue was taken in Et2O
the +1 state, causing the misalignment with the Fermi-levels of (40 mL) and washed with H2O (2 ꢄ 10 mL) and brine (1 ꢄ 20
Hg, and resulting in the current drops. When the voltage is mL), dried, and concentrated in vacuo. The residue was puried
reduced in the reverse scan, the new device will be Si/ by column chromatography (silica gel, 5% EtOAc/hexane) to
porphyrin+1/Hg. At Vonset-rev, the Fermi level of Hg would align give pure 1a (6.1 g, 91.2%) and 1b (8.1 g, 91%).
with the LUMO of porphyrin+1. This induces a sharp increase in
Compound 1a. colorless liquid; IR (lm, n/cmꢁ1): 3019 (s),
the current due to resonance tunnelling through the molecule. 2928 (s), 2856 (s), 1687 (s); 1H NMR (200 MHz, CDCl3): d 0.90 (t,
It again drops at Vp-rev as the molecule gets misaligned with the J ¼ 6.4 Hz, 3H), 1.15–1.49 (m, 6H), 1.71–1.88 (m, 2H), 4.01 (t, J ¼
Fermi levels of Hg during its reduction. The observed small 6.0 Hz, 2H), 6.97 (d, J ¼ 9.5 Hz, 2H), 7.80 (d, J ¼ 9.5 Hz, 2H), 9.85
hysteresis may be due to conformational changes in the mole- (s, 1H) ppm; 13C NMR (50 MHz, CDCl3): d 13.6, 22.2, 25.3, 28.7,
cule aer oxidation. The experimentally observed voltages are in 31.2, 68.1, 114.4, 129.6, 131.5, 163.9, 190.04 ppm; MSMS (m/z):
qualitative agreement with the theoretically calculated HOMO– 207 (100) [M + H]+; anal. calcd for C13H18O2: C, 75.69; H, 8.80.
LUMO values of 5a and 5b and their respective +1 oxidation Found: C, 75.34; H, 9.06%.
states, using ab initio (GAMESS soware) (data shown in ESI,
Table SL3†). The mechanism of NDR effect in 5a and 5b is 2928, 2856 (s), 1687 (s); 1H NMR (200 MHz, CDCl3): d 1.21–1.57
Compound 1b. Colorless liquid; IR (lm, n/cmꢁ1): 3019 (s),
explained schematically in Fig. 9.
(m, 12H), 1.72–1.95 (m, 2H), 1.96–2.15 (m, 2H), 4.05 (t, J ¼ 6.0
Hz, 2H), 4.85–5.08 (m, 2H), 5.67–5.99 (m, 1H), 6.99 (d, J ¼ 8.0
Hz, 2H), 7.83 (d, J ¼ 8.0 Hz, 2H), 9.88 (s, 1H) ppm; 13C NMR (50
MHz, CDCl3): d 25.9, 28.9, 29.0, 29.3, 29.4, 33.7, 68.4, 114.1,
Conclusions
In summary, we have synthesized two di-O-alkylated porphyrin 114.8, 129.9, 131.9, 139.1, 164.3, 190.6 ppm; MSMS (m/z): 275.1
molecules as prototype s–p–s systems, and electro-graed (100) [M + H]+; anal. calcd for C18H26O2: C, 78.79; H, 9.55.
them on H-terminated Si to form monolayers. The I–V charac- Found: C, 79.02; H, 9.55%.
teristics of the monolayers revealed pronounced reversible NDR
Dipyrromethanes 2a and 2b. A mixture of pyrrole
effects with peak-to-valley current ratio of ꢀ10 and 100. The (250 mmol), compound 1a or 1b (10 mmol) and TFA (1 mmol)
NDR effects were relatively stable during repetitive voltage was stirred under Ar for 5–10 min. Aer completion of the
scanning for 8 h in the positive and negative bias, without any reaction, it was quenched with 0.1 M aqueous NaOH (40 mL)
reduction in current or in the NDR effect. The higher PVR, and extracted with EtOAc (100 mL). The organic layer was
observed with the device containing the uorophenylporphyrin washed with H2O (3 ꢄ 10 mL) and brine (1 ꢄ 5 mL), dried, and
moiety 5b suggested its better pre-organization possibly by concentrated in vacuum. Excess pyrrole was removed by
hydrogen bonding through the F atoms, compared to the vacuum distillation at room temperature, and the residue
device, fabricated using the non-uorinated porphyrin, 5a. column chromatographed (neutral alumina, 20% EtOAc/
Theoretical simulations of Si/porphyrin/Hg structure showed hexane) to give the respective products 2a (1.4 g, 42%) and 2b
that the NDR effect is intrinsic to the porphyrin molecules. The (2.0 g, 52%), which were crystallized from hexane.
Compound 2a. White crystals; mp: 58 C; IR (n/cmꢁ1): 3463
ꢂ
NDR effect was explained using the ab initio molecular-orbital
theoretical calculations.
1
(m), 3019 (s), 2956 (s), 2859 (s), 2399 (w); H NMR (200 MHz,
CDCl3): d 0.92 (t, J ¼ 6.2 Hz, 3H), 1.25–1.50 (m, 6H), 1.68–1.85
(m, 2H), 3.93 (t, J ¼ 6.4 Hz, 2H), 5.42 (s, 1H), 5.85–5.93 (m, 2H),
6.12–6.22 (m, 2H), 6.61–6.68 (m, 2H), 6.84 (d, J ¼ 7.8 Hz, 2H),
7.12 (d, J ¼ 7.8 Hz, 2H), 7.89 (broad s, 2H) ppm; 13C NMR
Experimental section
General methods in synthesis
Reagents and solvents (Sigma-Aldrich and Fluka) were of (50 MHz, CDCl3): d 14.1, 22.7, 25.9, 29.4, 31.7, 43.4, 68.3, 107.2,
synthetic grade. Pyrrole and benzoyl chloride used aer distil- 108.6, 114.8, 117.2, 129.5, 133.1, 134.2, 158.4 ppm; MSMS (CI,
lation. 4-Hydroxybenzaldehyde was used aer crystallization. m/z): 321.2 (100) [M ꢁ H]+; anal. calcd for C21H26N2O: C, 78.22;
All solvents were dried and distilled before use. Tetrahydro- H, 8.13; N, 8.69. Found: C, 78.60; H, 8.17; N 8.54%.
furan (THF) was distilled from Na under argon. Acetone was
Compound 2b. White crystals; mp: 64 C; IR (n/cmꢁ1): 3463
ꢂ
dried over Na2CO3 and HPLC grade acetonitrile was used. The (m), 3019 (s), 2928 (s), 2856 (s), 2399 (w), 1639 (w); 1H NMR
1H NMR and 13C NMR spectra were recorded with 200/300/500 (200 MHz, CDCl3): d 1.21–1.59 (m, 12H), 1.69–1.84 (m, 2H),
(50/75/100) MHz spectrometers using deuterated solvents as the 1.95–2.17 (m, 2H), 3.94 (t, J ¼ 6.0 Hz, 2H), 4.85–5.09 (m, 2H),
internal standards. The mass spectrometry was carried out with 5.43 (s, 1H), 5.65–5.98 (m, 3H), 6.10–6.21 (m, 2H), 6.65–6.74 (m,
a MSMS (410 Prostar Binary LC with 500 MS IT PDA Detectors, 2H), 6.85 (d, J ¼ 8.5 Hz, 2H), 7.12 (d, J ¼ 8.5 Hz, 2H), 7.95 (broad
Varian Inc, USA) and MALDI-TOF/TOF (BrukerUltraex II) data s, 2H) ppm; 13C NMR (50 MHz, CDCl3): d 26.2, 29.1, 29.3, 29.5,
systems.
29.7, 33.9, 43.4, 68.3, 107.2, 108.6, 114.3, 114.9, 117.2, 129.5,
4-Hexyloxybenzaldehyde (1a) and 4-(10-undecenyloxy)benz- 133.1, 134.2, 139.4, 158.4 ppm; MSMS (CI, m/z): 391.1(100) [M +
aldehyde (1b). A mixture of 4-hydroxybenzaldehyde (4.0 g, H]+; anal. calcd for C26H34N2O: C, 79.96; H, 8.77; N, 7.17. Found:
32.7 mmol), 1-bromohexane (6.50 g, 39.3 mmol) or 1-bromo-10- C, 79.62, H, 8.77, N, 7.26%.
undecene (9.16 g, 39.3 mmol), K2CO3 (5.52 g, 40 mmol) and
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give the porphyrins 5a (0.069 g, 10%) and 5b (0.143 g, 20%) as
Diacyldipyrromethanes 4a and 4b. A solution of EtMgBr in
THF (8.1 mmol) was slowly injected to a stirred solution of 2a
(0.524 g, 1.62 mmol) in toluene (25 mL) under argon. Aer
stirring for 0.5 h at room temperature, the acid chloride 3a or 3b
(4.05 mmol) in toluene (2 mL) was injected into the resulting
brown solution over 10 min, and stirring continued for an
additional 10 min. The reaction was quenched with aqueous
saturated NH4Cl (10 mL) and the mixture extracted with EtOAc
(20 mL). The organic extract was washed with H2O (2 ꢄ 10 mL)
and brine (1 ꢄ 5 mL), dried, and concentrated in vacuum. The
residue was column chromatographed (neutral alumina, 25%
EtOAc/hexane) to obtain brown oil as a 4 : 1 mixture of diacetyl
and monoacetyl derivatives of 2a. The required compounds 4a
(0.532 g, 62%) and 4b (0.422 g, 46%) were obtained in pure form
by triturating the oils with MeOH.
purple solids, which were recrystallized from CHCl3/MeOH.
Compound 5a. Purple crystals; mp: 230 C; IR (n/cmꢁ1): 3433
ꢂ
(s), 3019 (s), 2928 (s), 2399 (w), 1643 (w); 1H NMR (200 MHz,
CDCl3): d ꢁ2.79 (s, 2H), 0.97 (t, J ¼ 6.6 Hz, 3H), 1.12–1.75 (m,
18H), 1.88–2.20 (m, 6H), 4.23 (t, J ¼ 6.4 Hz, 4H), 4.82–5.08 (m,
2H), 5.75–5.98 (m, 1H), 7.21–7.38 (m, 4H), 7.61–7.79 (m, 6H),
8.02–8.28 (m, 8H), 8.78–8.96 (m, 8H) ppm; 13C NMR (50 MHz,
CDCl3): d 14.1, 22.7, 25.9, 26.2, 29.0, 29.2, 29.5, 29.6, 31.7, 33.9,
68.4, 112.8, 113.6, 113.8, 114.2, 118.7, 120.2, 130.8, 134.2, 135.6,
135.8, 138.2, 139.3, 159.1, 161.2164.5 ppm; MALDI-TOF (m/z):
882 [M]+; anal. calcd for C61H62N4O2: C, 82.96; H, 7.08; N, 6.34.
Found: C, 82.94; H, 7.03; N, 6.04%.
Compound 5b. Purple crystals; mp: 225 ꢂC; IR (n/cmꢁ1): 3434
(s), 3019 (s), 2927 (s), 2850 (s), 1643 (w); 1H NMR (200 MHz,
CDCl3): d ꢁ2.81 (s, 2H), 1.07 (t, J ¼ 6.6 Hz, 3H), 1.25–1.59 (m,
18H), 1.85–2.17 (m, 6H), 4.23 (t, J ¼ 6.0 Hz, 4H), 4.86–5.12 (m,
2H), 5.72–5.97 (m, 1H), 7.15–7.32 (m, 4H), 7.35–7.5 (m, 4H),
8.03–8.25 (m, 8H), 8.75–8.99 (m, 8H) ppm; 13C NMR (50 MHz,
CDCl3): d 14.2, 22.7, 25.9, 26.3, 29.5, 29.7, 31.7, 32.0, 68.3,
112.8, 113.6, 113.8, 118.7, 120.2, 130.9, 134.2, 135.6, 135.8,
138.2, 159.1, 161.2, 164.5 ppm; MALDI-TOF (m/z): 918 [M]+.
anal. calcd for C61H60N4O2F2: C, 79.71; H, 6.58; N, 6.10. Found:
C, 79.84; H, 6.64; N, 6.03%. The 13C–19F couplings were not
analysed.
Compound 4a. Light brown powder; mp: 150 ꢂC; IR (n/cmꢁ1):
3225 (m), 3017 (s), 2928 (s), 2856 (s), 1610 (s); 1H NMR (200
MHz, CDCl3): d 0.89 (t, J ¼ 6.5 Hz, 3H), 1.21–1.45 (m, 6H), 1.71–
1.88 (m, 2H), 3.94 (t, J ¼ 6.5 Hz, 2H), 5.60 (s, 1H), 5.91–6.05 (m,
2H), 6.52–6.66 (m, 2H), 6.88 (d, J ¼ 6.8 Hz, 2H), 7.25–7.65 (m,
8H), 7.77 (d, J ¼ 6.8 Hz, 4H), 11.08 (s, 2H) ppm; 13C NMR (50
MHz, CDCl3): d 14.0, 22.6, 26.1, 29.2, 29.3, 31.8, 44.1, 68.1, 111.0,
114.9, 120.7, 128.0, 129.4, 129.7, 131.0, 131.6, 138.4, 141.1,
158.6, 184.4 ppm; MS (DI, m/z): 530 (100) [M]+; anal. calcd for
C
35H34N2O3: C, 79.22; H, 6.46; N, 5.28. Found: C, 79.04, H, 6.52,
N, 5.52%.
Compound 4b. Light brown powder; mp: 120 ꢂC; IR (n/cmꢁ1):
Preparation of H-terminated Si wafers
3275 (m), 3018 (s), 2932 (s), 2871 (s), 1610 (s); 1H NMR (300
MHz, acetone-D6): d 0.87–0.93 (m, 3H), 1.21–1.38 (m, 6H), 1.68–
1.82 (m, 2H), 3.99 (t, J ¼ 6.2 Hz, 2H), 5.83 (s, 1H), 5.95–6.18 (m,
2H), 6.75–6.97 (m, 4H), 7.16–7.48 (m, 6H), 7.88–8.05 (m, 4H),
11.12 (broad s, 2H) ppm; 13C NMR (75 MHz, CDCl3): d 13.9, 22.5,
25.7, 29.2, 31.6, 43.5, 43.9, 68.1, 111.0, 114.8, 115.0, 115.4, 120.5,
129.3, 129.5, 130.7, 131.3, 131.5, 131.7, 141.0, 158.3, 162.5, 182.9
ppm; MSMS (CIMS, m/z): 567.4 (100) [M + 1]+; anal. calcd for
N-type silicon wafers (orientation: 111; resistivity: 0.001–0.005
Ucm) and 40% NH4F were purchased from Siltronix and
Fluka, respectively. The Si (111) wafers, cut into small pieces
(ꢀ0.5 cm ꢄ 1.5 cm) were cleaned by heating them in 3 : 1 (v/v)
of conc. H2SO4 : 30% H2O2 (piranha) for 10 min at 80 ꢂC. The
wafers were removed, washed with excess H2O and, immersed
successively in a deaerated (purged with Ar for 30 min) 40%
aqueous NH4F for 10 min, and 2% aqueous HF for 2 min. The
wafers were washed with deionized H2O for 1 min, dried
under a stream of N2 and immediately taken into the elec-
trochemical cell to perform the electrograing of the
porphyrins 5a and 5b.
C
35H32F2N2O3: C, 74.19; H, 5.69; N, 4.94. Found: C, 73.79, H,
5.43, N, 4.89%. The 13C–19F couplings were not analysed.
Porphyrins 5a and 5b. To a stirred solution of the respective
diacetyldipyrromethanes 4a or 4b (0.78 mmol) in dry THF/
MeOH (10 : 1, 34.3 mL) was added NaBH4 (1.0 mmol) in
portions. Aer the reduction was complete, the mixture was
poured into aqueous saturated NH4Cl (60 mL) and extracted
with CH2Cl2 (100 mL). The organic layer was washed with H2O
(2 ꢄ 5 mL) and brine (1 ꢄ 5 mL), dried, and concentrated in
vacuum to get the respective dicarbinols as foam like solids.
Mixtures of each of these compounds and dipyrromethane
2b (0.78 mmol) in CH3N (350 mL) were stirred to get a homo-
geneous solution. TFA (9.49 mmol) was slowly added into these
under rapid stirring, followed by addition of DDQ (2.34 mmol)
aer 5 min. The reaction was stirred for 1 h at room tempera-
ture and then quenched with Et3N (9.49 mmol). The mixture
was passed through a pad of alumina and eluted with CH2Cl2
until the eluent was colourless. The resulting solution was
concentrated, passed through a pad of silica gel, and eluted
with CH2Cl2 to remove the non-porphyrinic products. The
purple fractions were combined and concentrated in vacuo to
Monolayer formation
The electrochemical deposition of 5a and 5b was carried out by
CV with a potentiostat/galvanostat system (model: AutolabPG-
STAT 30) using the above Si wafers as the working electrode
(WE), Pt as the counter electrode (CE) and Ag/AgCl as the
reference electrode (RE). The solution contained 0.1 M Bu4NP as
the electrolyte and 5a and 5b (1 mM) in dry CH2Cl2. The CV was
run from 0 to ꢁ1 V for 25–50 cycles at 0.05 V sꢁ1 scan rate under
an inert atmosphere. It was found that homogeneous mono-
layer was formed at 25 scans for 5a and 30 scans for 5b.
Homogeneity of monolayer was determined by AFM. Aer the
CV scans, the WE was sonicated in CH2Cl2 for 10 min to remove
the electrolyte and the unreacted or physisorbed 5a or 5b. The
WE was further washed with acetone, isopropanol and meth-
anol to obtain the respective graed monolayers.
50242 | RSC Adv., 2015, 5, 50234–50244
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RSC Advances
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Characterization of monolayer
The monolayers were characterized in terms of thickness, using
an ellipsometer (Sentech: model SE400adv), surface morphology
by AFM (Multiview 4000, Nanonics) imaging, by de-ionized water
contact angle (Data Physics System, model: OCA20), FT-IR
(Bruker, 3000 Hyperion Microscope with Vertex 80 FTIR
System, LN-MCT 315-025 detector) in polarized ATR (20 ꢄ
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ka as the X-ray source in a xed monochromator mode, and
molecular mass by SIMS (BARC make, Kore's Technology so-
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determined at the minimum value of c2 for the respective
monolayers. The Levenberg–Marquardt algorithm (eqn (4)) was
used for obtaining the minimum value of c2, which denes a
surface in a multidimensional error space. The deepest valley in
the c2 surface signies minimum coefficient values of the tting
function. The CV of the graed monolayer (as WE) was recorded
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from 0 to 1 V for 10 cycles at a scan rate 10 V sꢁ1
.
!
2
L
X
1
yn;obs ꢁ yn;calc
c2 ¼
(4)
L ꢁ P
yn;error
n¼1
Junction and measurement setup
To measure the J–V characteristics, a metal/molecule/Si (n++
structure was completed by using a tiny drop of liquid mercury
of diameter 400 mm as the counter electrode. The contact area in
the graed monolayer was 0.2 mm2. The J–V curves were
recorded at room temperature in a dark box using a pAmeter–dc
voltage source (HP 4140).
)
Acknowledgements
One of the authors (KG) thanks Bhabha Atomic Research Centre
for the nancial support. The technical help by Mr A. Nelson for
the MOTOFIT soware is appreciated.
¨
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