A novel design for porphyrin based D–s–A systems as molecular rectifiers

Electro-grafted porphyrin-based D–s–A monolayers on Si behave as rectifiers.


Introduction
Miniaturization is a vital need of the electronics industry, but it is limited by changes in the bulk properties of materials as they move to nanoscale dimensions. Most of the successes in this eld have focused on the electrical properties of organic molecules placed between metal electrodes. [1][2][3][4][5] In particular, selfassembled monolayers (SAMs) of alkanes and aromatic thiols on gold substrates have been very popular for constructing metal-molecule-metal (MMM) junctions. [6][7][8][9][10] Concurrently, efforts aimed at synthesizing metal-molecule-semiconductor (MMS) junctions by covalent linking of organic molecules to semiconductor surfaces are gaining momentum. 11 Such assemblies present opportunities for novel molecular electronic charge transport mechanisms, and are potentially compatible with conventional metal oxide-semiconductor (MOS) technology. To this end, there is a burgeoning interest in small organic molecules capable of switching their redox status, which, in association with semi-conductors such as Si, may scale down the size of the molecular electronic devices. [12][13][14] Here, surface potential tailoring can be achieved by chemically-graing organic molecules onto Si to develop improved hybrid molecular devices. For example, the p-n junction threshold voltage for rectication can be adjusted by changing the electronic nature of the organic p group molecules, instead of via the classical doping method. 14 Different techniques such as making Langmuir-Blodgett (LB) lms 15 or SAMs of organic molecules on solid substrates via MMM junctions 16 are most commonly used for this purpose. Compared to the LB lms, SAMs are easy to prepare and may be more robust as the organic molecules are sturdily anchored onto the metal substrates at xed distances. Chemically bonded monolayers on Si surfaces can be prepared either on Si oxide (SiOx) surfaces or on oxidefree Si, 11,17 the latter being preferred due to better electronic coupling of the Si molecules and the lack of charging effect. Indepth reviews with excellent analyses of the different methods of fabrication and characterization of SAM junctions on Hterminated Si surfaces are available. 18,19 The protocols usually adopted for constructing densely packed Si-organic hybrids involve the deposition of functionalized alkenes/alkynes using heat, 20 light, 21,22 electrochemical techniques, radical initiators 23 or Lewis acids, 24 as well as alkylhalides via either a Grignard route or lithiation. 25 This is followed by attachment of the electro-active organic molecules to the terminal functionality of the resultant alkane/alkene-Si hybrids by esterication or amidation. However, due to steric factors, not all of the deposited alkane/alkene moieties can be modied with organic molecules. This may produce non-uniform organic-Si hybrids. In our previous work, we found that cathodic electro-graing of pre-synthesized alkenylated electro-active organic molecules onto a Si-H surface can conveniently provide SAM-based molecular electronics devices with the following advantages: 26 the process is simple; it can be monitored in situ to ensure completion of deposition; it can exclude oxidation and/or hydrolysis at the Si surface due to the negative potential bias of the Si wafers; and it can produce materials where the Si-H surface is modied only by the chosen molecules.
Amongst the many electron-rich organic molecules, porphyrins 27 are ideally suited for fabricating molecular devices because they: (i) can form stable p-cation radicals and exhibit two accessible cationic states in their monomeric forms; [28][29][30][31][32] (ii) have long charge retention times, resulting in lower power consumption; (iii) are highly stable, 33 and (iv) can form selfassembled structures. 34 In view of these favourable attributes, porphyrins have been extensively used as p molecules for the construction of storage devices, molecular wires and memory devices. Reports on current rectication using C 60 -porphyrin combinations also exist. 35 Molecules exhibiting rectication behavior with a high rectication ratio (RR) are very useful for making diodes. According to Aviram and Ratner, a single molecule with a donor-spacer-acceptor (D-s-A) structure should behave as a rectifying diode when placed between two electrodes, where the s-bond bridge prevents the direct overlap of the donor (D) and acceptor (A) energy levels to allow unidirectional ow of current. 1 Several groups have experimentally veried this model, but porphyrins have never been used for this purpose in silicon hybrid systems. 36,37 In the present investigation, two such single molecules (5a/5b) were synthesized, where porphyrin and aniline moieties served as the (A) and (D) units respectively, while a -CH 2 -NHmoiety was anticipated to be a suitable spacer. These molecules were electro-graed onto Si-surfaces using the C-6/C-11 alkenyl chain of 5a/5b as the linker to construct the respective MMS heterostructures. Measurement of their I-V behavior revealed high current RRs for these assemblies. Moreover, a subtle change in the linker length signicantly changed the monolayer packing on the Si-surface, resulting in a pronounced alteration in the current rectication properties.

Synthesis of the porphyrins
Porphyrin-based functional molecules are, by and large, synthesized via functionalization of the aryl moieties of unsymmetrical meso-tetraryl porphyrins. However, the synthesis of unsymmetrical porphyrins is fraught with limitations such as poor yields and tedious isolation procedures. Instead, functionalization of the pyrrole units of the porphyrins offers a better alternative to alter the porphyrin scaffold. However, this strategy is rarely used because the porphyrin pyrrole units are inert towards most electrophilic reactions such as Friedel-Cras alkylation and acylation, while halogenation 38 and nitration 38,39 oen lead to di-or higher substituted products. An exception to this is the Vilsmeier-Haack reaction, which can provide mono-formyl porphyrins in appreciable yields. 39 We reasoned that the resulting formyl group could subsequently be used to construct the desired D-s-A structure for the present studies. So, following Bonfantini's method, 39 tetraphenylporphyrin (TPP, 1) was converted to Cu(II)-TPP and then subjected to the Vilsmeier-Haack reaction to obtain bformyl-TPP (2). For the synthesis of the donor part of the molecule, p-nitrophenol was o-alkylated with either 1-bromohexene or 1-bromo-10-undecene to furnish compounds 3a and 3b, respectively. These were converted to aniline derivatives 4a and 4b by reduction with Zn/HCO 2 NH 4 . Next, aldehyde 2 was separately subjected to a reductive amination using 4a or 4b to obtain the target porphyrins 5a and 5b, respectively. Previously, Welch et al. 40 synthesized the Schiff's base of 2 in toluene aer 72 hr, using a Dean-Stark apparatus for simultaneous removal of water. We performed the reductive amination in THF in the presence of 4Å molecular sieves followed by a one-pot reduction of the intermediate imine to obtain 5a and 5b in improved yields ($78%) in only 6 h (Scheme 1).

Device fabrication
Preparation of the Si-hybrids. Molecules 5a and 5b were electrochemically deposited on H-terminated silicon via a twostep process, which is schematically shown in the ESI (Fig. SL1 †). In the rst step, application of a negative potential to the working electrode releases H free radicals from the Si-H surface. The newly generated nucleophilic Si atoms subsequently react with the alkene functionalities of 5a and 5b to form Si-C bonds, resulting in an irreversible oxidation peak at $0.3 V. A similar oxidation peak was observed with 1-undecene ( Fig. SL2 †), but not with the blank Si sample (electrolyte only), conrming our interpretation. Cyclic voltammograms (CVs) (Fig. 1), recorded during this electrochemical deposition helped to monitor the extent of deposition. Disappearance of the oxidation peak indicated completion of the process. AFM analysis revealed the formation of homogeneous monolayers with both 5a and 5b aer 25 scans.
Characterization of the monolayers. To ensure monolayer deposition on Si, the electro-graed materials were characterized by contact angle measurements, polarized FT-IR spectroscopy, ellipsometry, AFM, secondary ion mass spectrometry (SIMS) and electrochemistry. The contact angles of deionized water at the Si surface graed with 5a and 5b were 55 and 64 , respectively. For the cleaned Si wafer and the C-11 alkyl-graed Si surfaces, the angles were 84 and 112 , respectively. The value for the cleaned Si wafer is consistent with several previous reports. [41][42][43][44][45] The low contact angles of the porphyrin monolayers suggested they were tilted on the Si-surface, exposing the pyrrole and amine nitrogen atoms for interaction with water droplets. The observed contact angles of the porphyrin monolayers are in close proximity to the reported contact values (66-74 ) for thiophene-terminated alkyl monolayers on Si-surfaces that were prepared by a late-stage attachment of the aryl moieties. 46 This established the suitability of our direct attachment protocol for the preparation of the monolayers. The average thicknesses of the monolayers, estimated by ellipsometry were found to be 2.4 AE 0.1 nm and 2.9 AE 0.2 nm in case of 5a and 5b, respectively. AFM analysis revealed that the monolayers formed aer 25 scans were organized with the least number of voids and hillocks. The void depth and RMS roughness of the 5a monolayers were $2.5 nm and 0.91 nm, respectively, while for 5b they were 3 nm and 0.7 nm, respectively (Fig. 2). Compared to 5a, the monolayers of 5b were more compact and uniform with a larger grain size.
Fast scan (10 V s À1 ) CVs (Fig. 3) of the respective porphyrin monolayers exhibited a reversible peak at +0.8 V, conrming attachment of the porphyrin moieties. This was absent in the blank Si sample and the C-11 alkyl monolayers. The net charge transferred during the oxidation process, calculated from the area under the oxidation peak divided by the scan rate were 8.6 Â 10 À7 C and 2.45 Â 10 À6 C for 5a and 5b, respectively. Using these values, the surface coverages for the monolayers were calculated using the formula: surface coverage ¼ total charge/(F Â area dipped in electrolyte). The surface coverages were 1.11 Â 10 12 and 4.5 Â 10 14 molecules per cm 2 for 5a and 5b, respectively. Thus, the areas occupied by each molecule in the 5a and 5b monolayers were 90 nm 2 and 22Å 2 , respectively, indicating that 5b formed more compact monolayers than 5a. The signicantly higher value for 5a compared to that previously reported 19 for monolayers of simple C 18 -, C 16 -, and C 12alkanes on Si (100) revealed poor packing. This may be due to the edge-on orientation of the porphyrins. 47 On the other hand, the value for the 5b monolayers matched well with the theoretically calculated diameter (14.8Å) of TPP, 48 indicating that the molecules were tightly packed due to p-p stacking, which was also revealed in the AFM images (Fig. 2). Consistent with the AFM analysis, the surface area covered by 5b was several fold that covered by 5a.
SIMS of the 5a monolayers showed mass peaks at m/z 795, 691, 675 and 596 amu, while for the 5b monolayers peaks appeared at m/z 777 and 386 amu (Fig. 4), revealing that the molecules remained intact during the graing process. The observed higher mass fragments, in the case of the C-11 monolayers, was consistent with its longer alkyl chain length vis-à-vis that of the C-6 monolayers.
IR peaks due to -CH 2 vibrational modes can provide better insight into the van der Waals interactions between the alkylated porphyrin rings anchored parallel on the Si surface. This, in turn, may help explain the better packing of the 5b monolayers vis-à-vis that of 5a. In pure solid alkane monolayers, the hydrocarbon chains exist in an all-trans conguration such that    the carbon backbone of each molecule lies in a single plane. However, in liquid form, there is substantial out-of-plane twisting around the individual bonds, altering the frequency of the -CH 2 vibrational modes. 45,49 The polarized FTIR spectrum ( Fig. 5) of the 5a monolayers exhibited a N-H stretching frequency at 3251 cm À1 along with symmetric (n s ) and asymmetric stretching (n a ) vibrational modes for the CH 2 groups at 2856 and 2927 cm À1 , respectively. In contrast, the respective IR absorption peaks of the 5b monolayers were at 3255, 2840 and 2921 cm À1 . Our results showed that the alkyl chains in the monolayers of 5b are more rigid like those in pure solid alkanes, while those in the monolayers of 5a are twisted. This clearly explained the observed improvement in packing for the 5b (C-11 linker) monolayers over the 5a (C-6 linker) layers. 50 From the Xray photoelectron spectroscopy (XPS) data, the peak for the monolayers at 99.5 eV could be attributed to the Si-C bonds, while the absence of a SiO 2 peak at 103 eV conrmed that the monolayers were free of SiO 2 (Fig. SL3 †).
I-V measurements. In order to measure the I-V characteristics, a metal-molecule-Si (n++) structure was constructed ( Fig. 6(a)), using a tiny drop of liquid Hg (40 mm diameter) as the counter electrode. The area in contact with the graed monolayer, measured using a goniometer, was 0.002 mm 2 .
The I-V curves ( Fig. 6(b)) of the devices constructed using the monolayers of 5a and 5b showed current rectication in the reverse bias. The maximum RR was observed at AE1 V for both of the devices. The ratio for the monolayers prepared using 5b was very high (10 7 ), while that for 5a (C-6 linker) was $100. However, while both systems were stable during repeated voltage scanning up to 100 scans at a scan rate of 0.01V s À1 , the RRs reduced gradually from their original values to $10 000 for 5b and $10 for 5a aer 50 scans.
It is already well known that when electrodes are asymmetric (and especially when the work functions of the electrode materials are different) any molecule can show current rectication. 51,52 Therefore, to see the effect of using asymmetric electrodes (if any), we also recorded the I-V curves of two control devices made of (n++) Si/Hg (SL4 (a) †) and (n++) Si/C-11 alkyl monolayers/Hg (SL4 (b) †). These showed marginal rectications with RR I + /I À values of 0.75 and 2, respectively. Recently, we constructed a n + -Si/pyrene C-11 monolayers/Hg device, which exhibited current rectication in the positive bias with RR of 100 at 1 V. 53 Furthermore, a n + -Si/5-(4-undecenyloxyphenyl)-10,15,20-triphenyl porphyrin (TPP C-11) monolayers/Hg system constructed by our group showed a marginally asymmetric I-V with signicant hysteresis. In the positive bias scan (0 to +0.8 V), the current jumped by an order of magnitude at +0.6 V. However, on the reverse scan (+0.8 to 0 V) the current did not retrace the curve and remained at a higher value. 54 Taken together, the I-V curves of all these devices clearly indicated that the results for the present devices were not due to electrode asymmetry. Moreover, the I-V results for the n + -Si/TPP C-11 monolayers/Hg device indicated that the observed rectications in the negative bias using the 5a/5b molecules were not due to resonance tunnelling through the TPP moiety.
The AFM and fast scan CV results showed better molecular stacking of the 5b monolayers, which may be due to the longer alkyl bridge in 5b than in 5a. 55 This may contribute to the better electrical characteristics of the 5b monolayers because the overlapping of electron clouds favours the generation and transport of charge carriers to induce intrinsic conductivity. Consequently, a signicantly higher maximum RR was observed for the 5b monolayers. Control experiments, carried out with a blank Si sample as well as C-11 alkyl chain-graed Siwafers showed nearly symmetrical sigmoidal I-V curves (Fig. SL4 †), eliminating any doubt about artifacts.
The void sizes ($0.2-0.4 nm) of the present Si-alkyl porphyrin/Hg junctions were small compared to the size of the Hg drops ($40 mm). Therefore, Hg drops are unable to penetrate through the pinholes of the SAMs and the measured I-V is expected to be direct. Statistical analyses of the data and junction yields are extremely valuable to discriminate artifacts from the real data. Previously, Kim et al., 56 and Nijhuis et al., 57,58 employed extensive statistical analyses to assess the performance of SAM-based devices. In the present work, we constructed only 80 devices for each of compounds 5a and 5b. Nevertheless, we analyzed the statistics of our I-V results as shown in Fig. 7 and Table 1, and summarized below. For compound 5a, only 25% of the devices showed RR values of 80-100, while an additional 15% of the devices showed RR values of 50-80. However, the RR values of 44% of the devices were <50, while 16% of the devices didn't show any rectication. The  device statistics for the monolayers of compound 5b were very impressive. The RR values for the majority (35%) of the devices were 10 5 -10 6 , while 21% of the devices showed RR values of 10 6 -10 7 . Another 20% showed RR values of 10 4 -10 5 , and the rest had RR values of 1-10 4 . The performance of the devices made of compounds 5a and 5b were satisfactory. In particular, the RR values of the Hg/5b/Si (n++) devices were far superior to that of molecular rectiers reported so far. 53,57,58 The current rectication properties of various D-s-A-based SAMs and LB lms in contact with noble metal electrodes have been described. 59 Results for some representative examples clearly establishes the signicantly superior performance of the devices described in the present study. For example, a LB lm of a pyrenyl carbamate in a M-M-M junction exhibited a RR value of 130 at $2.5 V. 60 Likewise, quinolinium and tetrahydroisoquinolinium iodide-based SAMs deposited on Au substrates showed RR values of 50-150 and 30-80, respectively, at AE1 V. 61,62 Meanwhile, the RR value for SAMs of quinolinium salts joined by a truncated S-C 3 H 6 group on a Au surface was found to be 12 at AE1 V. 63 In another study, a LB monolayer of the D + -p-A À molecule, hexadecylquinolinium tricyanoquinodimethanide on Au electrodes showed a maximum RR of 27.5 at 2.2 V. 64 However, literature reports on metal-porphyrin-semiconductor junctions are scarce. SAMs of 4-aminothiophenol/ ZnTPP/fulleropyrrolidine (PyC 2 C 60 ) on a Au (111) surface showed a modest RR of 24 at 1.8 V. 35 Interestingly, self-assembled layers of Fe(III)-5,15-di[4-(s-acetylthio)phenyl]-10,20diphenyl porphyrin on annealed Au crystal facets on glass substrates showed asymmetric I-V curves with the highest RR up to 9000, but the majority of the devices showed RR ¼ 20-200 at AE1 V. 65 To conrm our current rectication results, we computed the theoretical I-V curve of the device made of 5b. Initially, the ground state (GS) geometry of molecule 5b was optimized using an ab initio molecular orbital theory based LCAO-MO approach as implemented in the GAMESS soware. The ionic optimization of molecule 5b was carried out without any symmetry constraint at the B3LYP/6-31G(d,p) level of theory. To calculate the transport characteristics, a suitable device was constructed using the optimized conguration of the molecule as the central device region between two electrodes. Besides the active parts of the device, the central region also included a sufficient part of the contacts, such that the properties of the electrode regions could be described as bulk materials. This could be ensured by extending the central region into a few layers of the metallic contacts. The calculation of the electron-transport properties of the system was divided into two parts: (i) a self-consistent calculation for the electrodes with periodic boundary conditions in the transport direction, and (ii) a self-consistent open boundary calculation of the properties of the central region, where the electrodes dene the boundary conditions. The complete details of the method are described in the literature. 66 In the present experimental set-up, we have used a highly doped Si substrate that is expected to undergo reconstruction. This restricted accurate modelling of the molecule-substrate interface using the ab initio formalisms of our computational resources. Therefore, a model for a two-probe system was constructed ( Fig. 8(a)) by placing the molecules between two Au electrodes. We modelled the electrodes as part of truncated solid crystals. Unlike Hg, which is liquid under the experimental conditions and has a complicated structure, Au possesses a well-dened face-centred cubic crystal structure. Additionally, the pseudo-potential for Au is robust and it has been tested and used by many groups as a model electrode. 67,68 Understandably, the chosen system is not ideal for verication of the experimental results. However, our calculations were primarily aimed at a qualitative understanding of the electron transport through these molecules, and not a quantitative comparison, justifying our choice of Au electrodes. It is worth noting that Zheng et al. 69 recently reported the NDR properties of C 60 based electronic devices, wherein they claimed that the ndings were independent of the type of electrodes used. For   construction of the theoretical device, a thiol end group was used for attachment of the molecule with the electrode. The interface geometry of the thiol-terminated molecule and the electrode was optimized to ensure good overlap between the device and the electrodes. Previously, we have reported the interaction of methyl thiol, a prototype device molecule, with an extended Au(111) surface using a plane wave based pseudopotential method. 70 The results showed that the terminal S atom binds at the hollow site of the Au(111) surface and the distance between the Au and S atoms is 2.52Å. Using this information, we constructed the model for our present calculations. Two Au(111)-8X8 surfaces were used as the le and right electrodes. The Au/molecule/Au conguration was divided into three parts: le electrode, right electrode, and the central scattering region. In our models, there were three Au layers in each of the le and right electrode unit cells. The scattering region was composed of the isolated molecule together with the respective two Au layers on the le and right sides. The electron-transport properties of the Au/molecule/Au systems were investigated using the ATK 11.2.3 program, where semi-empirical extended Hückel theory, in combination with a rst-principle NEGF, was employed. 71 A k-point sampling of 100 was used in the electron-transport direction (Z direction). 70 Consistent with the experimental results, the theoretical I-V curve also showed rectication in the reverse bias ( Fig. 8(b)).
In general, the forward bias current-ow should be determined by the HOMO states of the molecules, while their respective LUMO states would dictate the reverse bias current. The observed rectication in the reverse bias is a result of an alignment of the LUMO levels of the molecules with the Fermilevels of the electrodes. To verify this, we determined the HOMO and LUMO energy levels ( Table 2) of 5b by theoretical calculations using an ab initio method (GAMESS soware). Ionic optimization without any symmetry constraints was carried out at the B3LYP/6-31G(d,p) level of theory where the exchange correlation functions are expressed using hybrid density functional theory. It was observed that the HOMO of the molecule was at À4.707 eV and the LUMO at À2.062 eV. The HOMO of the molecule (located at p-aminophenol group) is in close proximity to the Fermi level of electrode, but due to non contact with the electrode, resonance tunnelling will be difficult. For the LUMO, the energy difference with the electrode Fermi level is too large to undergo resonance tunnelling. Therefore, the observed current rectication is unlikely to be due to resonant tunnelling, but molecular asymmetry (D-s-A), which favours Aviram and Ratner's mechanism for rectication.
From the spatial distribution of the HOMO and LUMO energy levels (Fig. 9), it could be seen that the HOMO was localized on the p-aminophenol segment, while the LUMO was on the porphyrin ring, and their separation by the spacer, CH 2 , allowed a unidirectional ow of electrons. This is consistent with the Aviram and Ratner theory that suggests that a single molecule with a donor-spacer-acceptor (D-s-A) structure should behave as a diode when placed between two electrodes if a non-conjugated s-bond bridge prevents direct overlap of the donor and acceptor energy levels. Moreover, a monolayer will rectify if its molecules are aligned in register between two electrodes such that they work together when electrons ow from the electrode MD (attached to the acceptor) to D, and then exit from A to the electrode MA (attached to the donor).
To underscore the mechanism of electron ow between the donor and acceptor moieties, we also calculated the energy levels of the individual components (p-aminophenol and TPP) of 5b using the same computational approach. The energy level diagram of the constituent species is shown in Fig. 10. The LUMO energy level of p-aminophenol was at a higher energy than the TPP moiety. This will make it the donor. Therefore, under reverse bias, when electrons ow from MD to D, the electrons will move from the HOMO of p-aminophenol to its LUMO, tunnel through the bridge to the vacant LUMO of TPP, and nally transfer to the Hg electrode to complete the reversedirection ow.
In principle, reversing the orientation of the molecular dipole would change the I-V curves of the devices. This would also conrm that the observed rectication is not simply because of the non-symmetric nature of the device system. 72  . 9 Spatial distribution of the HOMO and LUMO energy levels of 5b. Fig. 10 Mechanism of rectification for the devices made with 5b.
However, construction of this type of device is synthetically more demanding for the following reasons. The synthesis has to start with an unsymmetrical porphyrin, wherein the porphyrin needs to have the alkenyl attachment at one of its pyrrole moieties for graing to the Si wafers. Synthesis of the required porphyrin proceeds via mono-bromination of the porphyrin followed by a Stille coupling with 11-trimethylstannyl undecene. Next, the unsymmetrical porphyrin needs to be formylated for the subsequent attachment of the aniline moiety. However, formylation of unsymmetrical porphyrins is never clean, and we experienced the formation of a mixture of products, formylated at different sites, and their purication was extremely cumbersome. Moreover, the graing of such a molecule may not selfassemble in a similar manner to that used in the present work. Thus, the I-V results of the newly constructed device may provide unreliable results, despite having the reverse geometry.
To conrm this hypothesis, the GS geometry of a 5b-congener having the alkenyl attachment at one of the pyrrole moieties in the porphyrin core was theoretically optimized, as done for 5b.
The GS geometry of the congener revealed that both the porphyrin and aniline moieties lie on same side (Fig. SL5 †). Therefore, the Si-graed 5b-congener would be positioned in such a manner that the Hg electrode would preferably interact with the porphyrin core. However, 5b has many possible conformers, and the possibility of touching the mercury electrode by one of its straight conformers cannot be excluded. Nevertheless, based on the I-V results of the TPP C-11 monolayers 54 and that of 5a/5b, it is tempting to propose that the observed current rectication is because of the nature of the molecules.
5a Characterization of the monolayers. The monolayers were characterized in terms of thickness, using an ellipsometer (Sentech, model SE 400adv); surface morphology was measured by AFM imaging (Nanonics, Multiview 4000 system), 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 mode (20Â objective) at an angle of 45 for 500 scans and the data were background corrected with freshly prepared Si-H monolayers. The molecular mass was measured by SIMS (BARC make, Kore's Technology soware) keeping Si-H as a reference. The XPS analysis of the deposited lms was carried out using a Mg Ka (1253.6 eV) source and a MAC-2 electron analyzer. The XPS analysis chamber was maintained at a base vacuum of 10 À9 mbar. The XPS binding energy scale was calibrated to the Au 4f 7/2 line at 83.95 eV.
Preparation of H-terminated Si wafers. n-Type silicon wafers (orientation: 111; resistivity: 0.001-0.005 U cm) and 40% NH 4 F were purchased from Siltronix and Fluka, respectively. The Si (111) wafers were cut into small pieces ($0.5 cm Â 1.5 cm) and cleaned by heating in a 3 : 1 (v/v) mixture of conc. H 2 SO 4 : 30% H 2 O 2 (piranha solution) for 10 min at 80 C. The wafers were washed with excess H 2 O and immersed successively in deaerated (purged with Ar for 30 min) 40% aqueous NH 4 F for 10 min and 2% aqueous HF for 2 min. The wafers were washed with deionized H 2 O for 1 min, dried under a stream of N 2 and immediately taken into the electrochemical cell for electro-graing.
Monolayer formation. The electrochemical deposition of 5a and 5b was carried out by cyclic voltammetry (CV) with a potentiostat/galvanostat system (model: Autolab PGSTAT 30) using 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 Bu 4 NP as the electrolyte and 5a or 5b (1 mM) in dry CH 2 Cl 2 . The CV was run from 0 to À1 V for 30 cycles at a 0.05 V s À1 scan rate under an inert atmosphere. Aer the CV scans, the WE was sonicated in CH 2 Cl 2 for 10 min to remove the electrolyte and any unreacted or physisorbed 5a or 5b. The WE was further washed with acetone, isopropanol and methanol to obtain the respective graed monolayers.
Junction and measurement setup. To measure the I-V characteristics, a metal/molecule/Si (n++) structure was completed by using a tiny drop of liquid mercury of diameter 40 AE 2 mm as the counter electrode. The contact area in the graed monolayer was 0.002 mm 2 . The I-V curves were recorded at room temperature in a dark box using a pA meter-dc voltage source (HP 4140).
Theoretical calculations. The ground state geometry optimization and molecular orbital calculations of molecule 5b, TPP, p-aminophenol and the 5b congener were carried out using an ab initio molecular orbital theory based LCAO-MO approach as implemented in the GAMESS soware. The ionic optimization of the molecules was carried out without any symmetry constraints at the B3LYP/6-31G(d,p) level of theory.

Conclusions
Overall, we have synthesized two porphyrin-based D-s-A prototype systems (5a and 5b) with two alkenylated anilines, differing in the chain length of the alkenyl using C-6 and C-11, respectively. These were individually electro-graed on Hterminated Si surfaces to form monolayers. The I-V characteristics of the monolayers revealed pronounced, stable and reversible current rectication at room temperature in the negative bias. To the best of our knowledge, such high RR values are rare, except for previous devices constructed by C. A. Nijhuis and Whitesides's group, 51,57,58 L. Venkataraman 52 and a recent publication from our own group. 53 The monolayer with the C-11 linker was more compact and showed a 10 5 times high rectication ratio (RR) relative to the other similar system having the C-6 linker, possibly because of the compact packing. The rectication mechanism was explained on the basis of Aviram and Ratner's theory of rectication by using ab initio molecular orbital calculations.