Benzimidazole-Modified Single-Stranded DNA - American Chemical

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J. Phys. Chem. B 2008, 112, 16982–16989

Benzimidazole-Modified Single-Stranded DNA: Stable Scaffolds for 1-Dimensional Spintronics Constructs Sairam S. Mallajosyula and Swapan K. Pati* Theoretical Sciences Unit and DST Unit on Nanoscience, Jawaharlal Nehru Centre For AdVanced Scientific Research, Jakkur, Bangalore, India 560064 ReceiVed: September 11, 2008; ReVised Manuscript ReceiVed: October 17, 2008

We investigate the electronic and magnetic properties of the proposed one-dimensional TMn(benzimidazole)n+1 (TM ) Sc, Ti, V, Cr, Mn) systems by means of density functional theory. We find that the rigid benzimidazole (Bzim) scaffold can stabilize the Ti, V, and Cr transition metal (TM) atoms, while still retaining the helical and one-dimensional characteristics of DNA. The strong coupling between the TM d-orbitals and the HOMO, HOMO-1, and LUMO orbitals of Bzim are found to govern the electronic and magnetic properties of these systems. Under the application of an external electric field, a robust half-metallic behavior is predicted for the V8(Bzim)9 system, which is traced back to the pinning of certain orbitals in the presence of an external field. The ease of attaching a thiol linker to the 5′ and 3′ ends of the DNA strands and formation of stable self-assembled monolayers (SAM) of DNA on metal substrates make these TMn(Bzim)n+1 DNA constructs promising materials for advanced spintronics applications. 1. Introduction The new millennium has seen rapid strides being made in the fields of nanotechnology and molecular electronics.1 With the Nobel prize in the year 2007 being awarded for the discovery of the GMR (giant magnetoresistance) effect, spintronics has come forth as an area of active interest, as it holds promises for the next quantum leap in technological advancement.2 The spin component of an electron, which has, so far, been underutilized in electronics, promises the technology to improve increased data processing speed and decreased electrical power consumption.3 An ideal device for spin-polarized transport should have several key ingredients. First, it should work well at room temperature and should offer as high a magnetoresistance (MR) ratio as possible. In this sense, a half-metallic (HM) ferromagnet with the Curie temperature higher than room temperature is highly desirable since there would be only one electronic spin channel open at the Fermi energy for spin-polarized current.4 HM behavior has been reported in three-dimensional materials such as CrO2, manganite perovskites, and two-dimensional materials such as graphene nanoribbons.5 However, for technological applications it becomes essential to search for one-dimensional (1-D) materials that can be easily manipulated at the nanometer scale. In this regard, it has been found that multidecker organometallic systems, efficiently synthesized as polynuclear clusters in molecular beams via the gas-phase reaction of laser-vaporized metal atoms with benzene (Bz), are promising precursors for building novel 1-D magnetic materials. Recent theoretical and experimental studies have predicted ferromagnetic and HM behavior for infinite 1-D gas-phase sandwich clusters, thus making them potential targets for spintronics applications.6 While discussing their technological applications, the stability of these clusters often becomes an issue. Addressing this issue, recent experimental studies have successfully immobilized V2(Bz)3 clusters on an organic SAM * Corresponding author. E-mail: [email protected].

(self-assembled monolayer) matrix using “soft landing” techniques. However, due to the weak V-Bz interactions, it was found that some of the deposited V2(Bz)3 clusters dissociated into V(Bz)2 and V2(Bz)2 during the landing process.6 Moreover, it has been experimentally found that these clusters tend to form the energetically unstable riceball structures, which is devoid of the multidecker geometry.7 Added to this, the gas-phase preference of the benzene dimers to adopt the perpendicular T-shaped and parallel displaced geometries hinders the formation of the theoretically proposed infinite one-dimensional half-metallic [V(Bz)]∞ structures.8 A way to overcome the instability associated with the Vn(Bz)n+1 clusters is to use a rigid scaffold in which all of the benzene rings are attached to each other.9 Such a scaffold would increase the stability of the organometallic system, thus avoiding the dissociation upon “soft landing”. We also note that a rigid scaffold also overcomes the energetic destabilization associated with the formation of the “rice-ball structures” and “benzene dimers”. The benzaimidazole (Bzim)-modified single-stranded DNA (sDNA), where a Bzim unit is used in the place of a natural DNA base, satisfies the requirements of such a scaffold.10 Interestingly, such a structural modification involving a Bzim moiety serves 2-fold purposes: first, it retains the imidazole (the five-membered ring) connection to the sugar phosphate backbone (as in DNA), and second, it provides the benzene extension for coordinating the transition metal (TM) atoms as investigated experimentally. In this work, we address the applicability of the rigid Bzim scaffold for stabilizing various TM atom constructs. We have chosen Sc, Ti, V, Cr, and Mn as the TM atoms for our extensive theoretical study. Our choice of the TM atoms is based on the rich electronic and magnetic properties exhibited by the corresponding TM-benzene clusters (TMn(Bz)n+1) in the gas phase.6,7 Although, the TM-benzene clusters show novel properties like HM behavior, ferromagnetism, etc., they are highly unstable and dissociate readily on deposition onto SAM, which hinders the realization of stable working devices using the TM-benzene clusters. We note that the stability

10.1021/jp8080782 CCC: $40.75  2008 American Chemical Society Published on Web 12/03/2008

Benzimidazole-Modified Single-Stranded DNA imparted by the rigid DNA scaffold would allow us to overcome the technical difficulties experienced with the benzene clusters. We present here a critical analysis of the stability of the Bzim scaffold and discuss in detail the magnetic properties of each of the TMn(Bzim)n+1 systems. We find that the stability of the TMn(Bzim)n+1 system critically depends on the hybridization of the d-orbitals of the TM with the orbitals of the Bzim unit. In fact, the Bzim-Bzim interplanar distance serves as a structural constraint for stability. The magnetic coupling between the TM centers is governed by the orbital interactions between the d-orbitals of the TM atom and the Bzim molecular orbitals. We find that the orbitals of the sugar-phosphate backbone lie very far from the Fermi level and do not affect the electronic and magnetic properties of these onedimensional (1-D) systems. 2. Discussion In this work, we perform a comprehensive first-principles study on the electronic and magnetic properties of the 1-D TMn(Bzim)n+1 systems. We have used the SIESTA package for our density functional theory (DFT) calculations.11 The PBE version of the generalized gradient approximation (GGA) functional is adopted for exchange correlation.12 A double-ζ basis set with the polarization orbitals has been included for all the atoms.13 The semicore 3p orbital has been included in the valance orbitals for the TM atoms. A real space mesh cutoff of 300 Ry was used for all the calculations. All of the calculations were carried out in supercells chosen such that the interactions between the neighboring fragments were negligible.14 Atomic relaxations in all the calculations were performed until the forces on the atoms were not larger than 0.04 eV/Å. Following our previous work, we neutralize the DNA phosphate backbone using a methyl phosphonate modification, which does not alter the structural stability of the DNA.15 We have also previously shown that this modification does not affect the magnetic coupling between the TM centers.10 Since we are interested in using the sDNA as a rigid 1-D scaffold for stabilizing the TM atoms, we set a distancedependent criterion for determining the stability of the TMn(Bzim)n+1 systems. For all the TM systems, we first analyze the Bzim-Bzim interplanar distance (dIP) in TM(Bzim)2. If the dIP, which corresponds to the distance between the planes of the benzene rings in Bzim units, is greater than 3.50 Å (upper limit of π-π stacking distance in DNA), then the scaffold cannot be used to stabilize the TM system.16 For all the TM systems that satisfy the distance criterion, we analyze the dimer species for the nature of the magnetic interactions. Further, we analyze the TM8(Bzim)9 system, which describes almost a complete turn of the DNA helix, for all TM which fulfill the distance criterion. We note that, since we are interested in understanding an infinite one-dimensional case, we require a unit cell that can be repeated to infinity in one direction with no interactions in the other two directions. However, since DNA is a dynamically fluctuating system, it is difficult to describe such a periodically repeating unit cell. Thus, we restrict our analysis to the TM8(Bzim)9 system which describes almost a complete turn of the DNA helix and thus can be envisaged as a unit cell. 3. Results To analyze the stability of the modified DNA scaffold, we have calculated the binding energy and the dIP for all the TM(Bzim)2 monomers. We define the binding energy (Eb)

J. Phys. Chem. B, Vol. 112, No. 51, 2008 16983 TABLE 1: Binding Energy (Eb) Calculated for Benzene and Benzaimidazole Monomersa TM

Eb(Bz) (eV)

Eb(Bzim) (eV)

dIP(Bz) (Å)

dIP(Bzim) (Å)

Sc Ti V Cr Mn

5.318 5.542 6.621 2.72 2.193

5.322 6.091 5.387 4.833 1.991

3.84 3.52 3.43 3.27 3.60

3.92 3.47* 3.50* 3.36* 3.62

a

Interplanar distance (dIP) calculated between the benzene and benzimidazole planes. All energies are reported in eV and distances in Å. Positive Eb indicates the stability of the monomer species (TM(Bz)2 and TM(Bzim)2) over the isolated scaffold.

as E(Bzim)2 + E(TM) - E(TM(Bzim)2), where E(TM) is the energy of the isolated TM atom and E(Bzim)2 is the energy of the monomer scaffold. For comparison, we have also calculated the binding energy and interplanar distance for the benzene monomer systems TM(Bz)2. In Table 1, we summarize the results of the monomer systems. From our analysis, we find that the dIP for Ti, V, and Cr falls within the cutoff limit of 3.50 Å (upper limit of π-π stacking distance in DNA).16 On the other hand, Sc and Mn do not satisfy this distance criterion (dIP > 3.50 Å). The stability of the benzaimidazole scaffold is found to be substantial and in some cases even greater than that of their benzene analogues. For example, Ti(Bzim)2 and Cr(Bzim)2 are stabilized by 0.549 and 2.113 eV, respectively, in comparison to their benzene counterparts. An analysis of the monomer structure highlights other salient features of the benzaimidazole scaffold. In Figure 1, we present the optimized geometries of Ti(Bzim)2, V(Bzim)2, and Cr(Bzim)2. As can be seen, the TM atom binds to the fused benzene rings of the Bzim unit. The TM atom is nearly equidistant from the planes of the two benzimidazole rings, with the distance on either side being 1.77, 1.72 Å for Ti, 1.79, 1.76 Å for V, and 1.69, 1.66 Å for Cr. An important difference between the benzimidazole scaffold and the benzene monomers is the rigidity imparted to the benzimidazole scaffold due to the DNA backbone. The effect of this rigidity is reflected in the staggered conformation adopted by the Bzim units, which is a direct consequence of the finite helical twist of the sugar-phosphate backbone. Thus, we note that while the benzene rings can adopt the eclipsed or staggered conformation in TMn(Bz)n+1 clusters, the Bzim units in TMn(Bzim)n+1 can adopt only the staggered conformation. Before discussing the magnetic properties of these systems, we describe the molecular orbital (MO) ordering observed in these systems, which in turn governs the magnetic properties. We find that under the influence of the strong crystalline field, the five 3d orbitals of the TM atoms are split into one singlet dσ (dz2) and two doublet dδ (dxy and dx2-y2) and dπ (dxz and dyz) orbitals. The classification of the orbitals into σ, π, and δ is based on the orbital symmetry of the interacting benzene/Bzim HOMO-1, HOMO, and LUMO orbitals.6,17 In Figure 2, a and b, we present the orbital ordering for Ti(Bz)2 and Ti(Bzim)2 and the corresponding orbitals of benzene and Bzim, respectively, to illustrate the origin of the σ, π, and δ orbital symmetry (see Supporting Information file for orbital plots corresponding to the dσ (dz2), dδ (dxy and dx2-y2), and dπ (dxz and dyz) orbitals of Ti(Bz)2). The HOMO-1 orbital for both Bzim and Bz has an extended in-plane delocalization synonymous with the σ symmetry, while the degenerate HOMO and LUMO orbitals exhibit an out-of-plane delocalization with the π and δ symmetries,

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Mallajosyula and Pati

Figure 1. Optimized geometry of TM(Bzim)2 systems: (a) Ti(Bzim)2 side view (SV), (b) Ti(Bzim)2 top view (TV), (c) V(Bzim)2 SV, (d) V(Bzim)2 TV, (e) Cr(Bzim)2 SV, and (f) Cr(Bzim)2 TV. All of the distances are given in angstroms, and the dihedral angle is given in degrees. The dihedral angle is defined between the hydrogens marked by blue arrows.

respectively. Interestingly, we find that the orbital symmetries of the six-membered ring of Bzim and Bz are similar for the HOMO-1, HOMO, and LUMO orbitals, which suggest that the imidazole unit (five-membered ring) does not alter the orbital picture of the six-membered ring. The effect of the imidazole ring is seen on the orbital ordering and d-orbitals splitting. To quantify this effect, we probe the splitting of the d-orbitals under the influence of the molecular field. The dπ-dσ, dσ-dδ, and dπ-dδ gaps for Ti(Bz)2 and Ti(Bzim)2 turn out to be 2.66, 1.05, and 3.71 eV, and 2.65, 0.81, and 3.46 eV, respectively. Note that, the dσ-dδ and dπ-dδ gaps in Ti(Bz)2 are 0.24 and 0.25 eV greater than the same in Ti(Bzim)2, which indicates that the d-p coupling of the intervening orbitals of δ symmetry is stronger in Bz when compared to Bzim. This is because, in the case of Ti(Bzim)2, the in-plane delocalization of the benzene-type orbitals with the orbitals of the imidazole unit extends the delocalization length, thereby reducing the d-p coupling and hence the observed gap. We also note that the dπ-dσ gap remains

nearly constant in both Ti(Bz)2 (2.66 eV) and Ti(Bzim)2 (2.65 eV), indicating that the d-p coupling strength for dπ and dσ orbitals does not vary among the Bz and Bzim systems. The ordering of these orbitals with respect to the center of the HOMO-LUMO gap (Ecenter ) (EHOMO + ELUMO)/2) is an indicator of the strength of hybridization. We find that due to strong hybridization the dπ (dxz and dyz) energy levels move away from Ecenter, while due to weak hybridization, the dσ (dz2) and dδ (dxy and dx2-y2) orbitals remain close to Ecenter. We also note that the number of the TM d-electrons governs the orbital ordering. From our analysis, we find that the doublet dδ orbitals are filled before the singlet dσ orbital followed by the doublet dπ orbitals. This becomes clear when we analyze the orbital ordering in V(Bzim)2 and Cr(Bzim)2 presented in Figure 2, c and d, respectively. As can be seen, due to the odd electron count for V (5 d electrons), the symmetrical splitting of the up-spin and down-spin orbitals is lost and we obtain an unsymmetrical orbital ordering, while for both Ti (4 d electrons) and Cr (6 d electrons), we obtain

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Figure 2. Orbital ordering of the dσ, dπ, and dδ orbitals in the TM(Bzim)2 systems. (a) Orbital ordering for Ti(Bz)2 and the corresponding HOMO, HOMO-1, and LUMO orbitals of benzene to illustrate the origin of the σ, π, and δ orbital symmetry. (b) Orbital ordering for Ti(Bzim)2 and the corresponding HOMO, HOMO-1, and LUMO orbitals of benzimidazole to illustrate the origin of the σ, π, and δ orbital symmetry. (c) Orbital ordering in V(Bzim)2; the ordering of the two spin channels is shown separately. (d) Orbital ordering in Cr(Bzim)2.

a symmetrical orbital ordering of the up-spin and down-spin orbitals. We note that the dπ orbitals remain unoccupied in all the cases, as the 4, 5, and 6 d electrons of Ti, V, and Cr occupy the doublet dδ orbitals and partially or completely occupy the singlet dσ orbital. To probe the magnetic exchange interactions in these systems, we analyze the spin states of TM(Bzim)2, TM2(Bzim)3, and TM8(Bzim)9. Specifically, we concentrate on the dimer TM2(Bzim)3 system where we can easily analyze the stability of the corresponding highest spin (HS) and lowest spin (LS) spin states, the HS state being the state with all the spins aligned parallel to each other and the LS state being the singlet (doublet) state for even (odd) number of TM atoms. For Ti, we find the net ground-state spin of Ti(Bzim)2 to be 0, indicating the system to be nonmagnetic. This is

clear from the MO ordering as the 4 d electrons fill up the two degenerate dδ (dxy and dx2-y2) orbitals. Interestingly, however, Ti2(Bzim)3 and Ti8(Bzim)9 are found to show net nonzero spin density on each Ti atom, although the LS state is always found to be stable over the HS state, with the stabilization energy EHS-LS being 64.5 and 14.0 meV for Ti2(Bzim)3 and Ti8(Bzim)9, respectively. The reduction of the stabilization energy on going from Ti2(Bzim)3 to Ti8(Bzimn)9 is consistent with the stabilization energy of 6 meV for the LS state predicted theoretically for a magnetic [Ti(Bz)]∞ cluster.6c Thus, we note that even though the ground state is nonmagnetic, the spin density/TM atom is not zero.18 These results are consistent with the findings of the Stern-Gerlach experiment on the Tin(Bz)n+1 clusters, where Ti(Bz)2 was found to be nonmagnetic while Ti2(Bz)3 gave

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Figure 3. DOS and pDOS for Ti8(Bzim)9 (upper panel), V8(Bzim)9 (middle panel), and Cr8(Bzim)9 (lower panel). The plot is scaled for EF to lie at 0 eV. Solid v and broken V arrows depict the majority spin and minority spin channels, respectively. Code: (broken black line) total DOS; (red line with shaded area) pDOS from the TM orbitals; and (blue line) pDOS from Bzim orbitals.

magnetic deflections.6b The main point is that the Bzim scaffold stabilizes Ti atoms with finite spin density in a LS ground state for Ti2(Bzim)3 and higher system sizes.

Mallajosyula and Pati For V systems, we find that the total spin density (M) increases linearly from V(Bzim)2 to V8(Bzim)9 in integral multiplies of the spin 1/2 moment with the number of vanadium atoms (see Supporting Information file for an analysis of the total spin density (M) with increasing system size n). From our analysis of the V centers, we find that the localized net spin density value at each V atom (M/TM) varies from 0.530 to 0.773, which is comparable to the calculated M/TM value of 0.556 for the V atoms in an infinite cluster [V(Bz)]∞.6c The linear increase in the spin density is due to the presence of an unpaired d electron per V atom, which is accommodated in the dσ (dz2) orbital. The HS state is stabilized over the LS state in V2(Bzim)3 and V8(Bzim)9, with the stabilization energy EHS-LS being -2.2 and -102.0 meV, respectively (negative sign indicates the stability of the HS state). This increase in the stabilization energy clearly indicates that, with increasing system sizes, the Bzim scaffold can be used to create stable ferromagnets. Note that our calculations are consistent with previous theoretical studies and experimental results.6,7 For Cr systems, we find that the dσ (dz2) and dδ (dxy and dx2-y2) are completely filled and thus the systems are nonmagnetic for all system sizes. At this point, we would like to point out that various experimental groups have reported the formation of stable SAM of DNA strands on gold surfaces,19 which have been tested for molecular electronics applications.20 Thus, it is easy to envisage the use of these TMn(Bzim)n+1 one-dimensional constructs in electronic circuitry. To quantify the electronic nature of these

Figure 4. pDOS contour analysis for V8(Bzim)9: (a) contour analysis for the majority spin channel, (b) contour analysis for the minority spin channel. The energy range has been plotted on the x axis and the applied external electric field (E-field) has been plotted on the y axis. Red arrows indicate important changes in the contour. Energies are reported in eV and the fields in eV/Å.

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Figure 5. (a) Orbital-segregated pDOS of the V 3d orbitals for V8(Bzim)9. The plot is scaled for EF to lie at 0 eV. Solid v and broken V arrows depict the majority spin and minority spin channels, respectively. Code: (s) dσ (dz2) pDOS, (---) dπ (dxz and dyz) pDOS, and (- · · - · · ) dδ (dxy and dx2-y2) pDOS. Orbitals with maximum contribution form dδ (dxy and dx2-y2), dσ (dz2), and dπ (dxz and dyz) are shown; the corresponding positions of these orbitals have been marked in the pDOS plot. (b) xDNA base pair modification to accommodate H-bonding. Possible aggregate structures are also shown.

systems, we analyze the density of states (DOS) and the projected density of states (pDOS) projected onto the orbitals of TM and Bzim close to EF. The same has been plotted for Ti8(Bzim)9, V8(Bzim)9, and Cr8(Bzim)9 in Figure 3. Before discussing the electronic properties of these systems, we comment on the insulating behavior of the sugar-phosphate backbone. Interestingly, we find that the orbitals of the sugar-phosphate backbone do not contribute to the DOS in the vicinity of EF and the only contributions to the DOS are above 2.00 eV away from EF. Thus, the contributions to the DOS in the vicinity of EF are solely due to the TM and the Bzim moieties. As can be seen, Ti8(Bzim)9 (upper panel in Figure 3) is a metallic system with no gap at EF with finite contributions to the pDOS at EF from the orbitals of both Ti and Bzim. In fact, the strong hybridization of the orbitals of Bzim and Ti is responsible for the closing of the gap in both the spin channels. Since the LS state is stable over the HS state by only 14 meV (162 K), we expect Ti8(Bzim)9 to be a paramagnetic metal under room temperature device conditions. Interestingly, the electronic nature of V8(Bzim)9 (middle panel in Figure 3) is very different when compared to Ti8(Bzim)9. The spin gaps for V8(Bzim)9 are found to be different in the majority and the minority spin channels, which is a direct consequence of the presence of one unpaired electron per vanadium atom. This additional electron is accommodated in the minority spin channel, which leads to a semiconducting gap of 0.40 eV in the minority spin channel when compared to the insulating gap of 0.80 eV in the majority spin channel. Thus, we infer that V8(Bzim)9 is a spin-polarized ferromagnetic semiconductor under normal device conditions. Finally, for the nonmagnetic Cr8(Bzim)9 (lower panel in Figure 3) system, we find an insulating gap of 0.74 eV, which is due to the complete filling of the dσ (dz2) and dδ (dxy and dx2-y2) orbitals, leading to the large separation between the occupied and the unoccupied states as was shown by the orbital ordering

in Figure. 2. It is interesting to note that, using the same scaffold and different TM, we can access the metallic, semiconducting, and insulating states, which is synonymous to the concept of doping. Here, it is important to mention that previous studies on Vn(Bz)n+1 one-dimensional systems have shown that GGA results compare well with hybrid functionals such as B3LYP and calculations performed within the LDA+U (local density approximation and correlation effects) scheme.6 Thus, we note that our results are acceptable within the accuracy of the computational method. We now turn our attention to the V8(Bzim)9 system which exhibits spin-polarized gaps, for further analyses. Recently, for low-dimensional zigzag graphene nanoribbons (ZGNR), it has been shown that, under the influence of a homogeneous external electric field, applied across the zigzag shaped edges, the spin gap in one of the spin channel closes, giving rise to HM behavior.21 The closing of the spin gap was traced back to electrical polarization affecting the magnetic properties. However, such a behavior for one-dimensional systems has not been reported. Here, we present an elaborate electric field analysis and show that the V8(Bzim)9 system exhibits robust half-metallic behavior beyond a critical field (Ec), thus being an ideal candidate for one-dimensional device applications. In Figure 4, we present the results of our contour analysis, where the pDOS contour is plotted against an energy range of -1.5 to 1.5 eV in the x-axis and an applied field strength range of +0.25 eV/Å to -0.25 eV/Å in the y axis. The external electric field (E-field) is applied along the helical axis of the DNA scaffold, with the +ve field being along the 5′-3′ direction and the -ve field being along the 3′-5′ direction. From Figure 4a, we notice that the finite insulating gap of 0.80 eV in the majority spin channel does not close for the entire sweep of the applied E-field, although this gap reduces to 0.64 eV for an applied E-field of +0.25 eV/Å and 0.78 eV for a field strength of -0.25 eV/Å. The contour plot clearly

16988 J. Phys. Chem. B, Vol. 112, No. 51, 2008 shows the robustness of this spin gap. Interestingly, we find that the spin gap in the minority spin channel (Figure 4b) closes completely in both the directions. As can be seen, the spin gap closes only for applied field strengths greater than (0.1 eV/Å (closing shown by red arrows in Figure 4b). Thus, we note that for any field strength above 0.1 eV/Å (critical field strength) in both the directions, V8(Bzim)9 system behaves as a stable halfmetallic ferromagnet. The origin of this stability can be traced back to the perturbation of the dσ (dz2), dδ (dxy and dx2-y2), and dπ (dxz and dyz) orbitals under the influence of an E-Field applied along the helical axis of the DNA scaffold (z-direction). As can be seen from Figure 4a, for the majority spin channel, the energy levels closest to EF in the -ve energy range (E < 0) are not influenced by the E-field, while the energy levels in the +ve energy range (E > 0) are moderately influenced by the E-field. To identify the orbitals which contribute in the vicinity of EF, we plot the orbital segregated pDOS for the V 3d orbitals in Figure 5a. The asymmetric influence of E-field on the majority spin channel energy levels is due to the fact that for E < 0 the major contributions are from the dδ (dxy and dx2-y2) orbital, while for E > 0 it is the dπ (dxz and dyz) orbital which contributes. Since the applied E-field is along the z-direction, the dδ (dxy and dx2-y2) orbitals are not influenced by the E-field and the energy level remains stringently pinned for the entire range. While, for E > 0 the dπ (dxz and dyz) orbitals which have a finite z-component are influenced by the E-field and hence we observe a reduction of the spin gap. It is to be noted that the dπ (dxz and dyz) orbitals are aligned at a finite angle with respect to the z-axis and thus the perturbation is moderate. The effect of the E-field perturbation is strongest for the dσ orbitals in the minority spin channel. This is because the dσ orbitals are composed of the V dz2 orbitals which are strongly polarized under the influence of an E-field applied along the z-direction, thereby leading to the closing of the spin gap in the minority spin channel (Figure 4b). Hence, we note that the d-p hybridization is essential for the half-metallic behavior in these systems. We would like to point out that, in this case, the E-field couples with the orbital polarization, which in turn affects the magnetic spin state. Thus, there is an indirect coupling between the E-field and magnetic states. The main point is that the pinning of the dδ (dxy and dx2-y2) orbitals in the majority spin channel is crucial for half-metallicity. Before concluding, we discuss a possible route to couple the H-bonding self-assembly of the natural DNA basepairs to these Bzim constructs. Since Bzim is devoid of H-bonding groups, commonly present in the DNA bases, namely the NH, NH2, and CO groups, it is devoid of the H-bonding self-assembly. In this regard, the xDNA purine bases satisfy the structural criteria of Bzim and natural DNA bases. As shown in Figure 5b, xDNA bases are artificial modifications of natural bases, made longer by the addition of an extra benzene ring.22 Thus, we note that via xA-xA interstrand hydrogen bonding and using a full or a half-helical twist, it is possible to stabilize higher aggregates of these TMn(Bzim)n+1 systems, leading to novel technological applications. 4. Conclusion In conclusion, we have performed a comprehensive firstprinciples study on the electronic and magnetic properties of the proposed TMn(Bzim)n+1 systems. We find that the Bzim scaffold can stabilize the Ti, V, and Cr TM atoms. The strong d-p coupling between the TM d-orbitals and the HOMO,

Mallajosyula and Pati HOMO-1, and LUMO orbitals of Bzim govern the electronic and magnetic properties of these systems. We find that the sugar-phosphate backbone does not affect these properties, as the sugar-phosphate orbitals lie far from EF. This neutral behavior of the sugar-phosphate backbone makes it a very efficient linker for molecular electronics applications. From our calculations, we find Ti8(Bzim)9 to be a paramagnetic metal, V8(Bzim)9 to be a ferromagnetic semiconductor, and Cr8(Bzim)9 to be a nonmagnetic insulator. The electronic behavior of these TM systems is found to be a direct consequence of the number of TM d-electrons, which governs the overall orbital ordering. Furthermore, we find that, under the application of an external electric field, the spin gap closes for the minority spin channel in V8(Bzim)9 leading to halfmetallic behavior. The ease of attaching a thiol linker to the 5′ and 3′ ends of the DNA strands and formation of stable SAM of DNA strands make these TMn(Bzim)n+1 DNA constructs promising materials for spintronics device applications. Calculations are underway to probe the selfassembly properties of xDNA terminated TMn(Bzim)n+1 DNA constructs. Acknowledgment. S.S.M. thanks the CSIR for the SR fellowship. S.K.P. acknowledges the CSIR and DST, Government of India, and AOARD, US Air Force, for the research grants. Supporting Information Available: Orbital plots corresponding to the dσ (dz2), dδ (dxy and dx2-y2), and dπ (dxz and dyz) orbitals of Ti(Bz)2, Analysis of the total spin density (M) with the system size n. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tour, J. M. Molecular Electronics: Commercial Insights, Chemistry, DeVices, Architecture and Programming; World Scientific: River Edge, NJ, 2003. (2) http://nobelprize.org/index.html. (3) (a) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (b) Zutic, A.; Fabian, J.; Sarma, S. D. ReV. Mod. Phys. 2004, 76, 323. (c) Johnson, M. J. Phys. Chem. B 2005, 109, 14278. (4) de Groot, R. A.; Mueller, F. M.; van Engen, P. G.; Buschow, K. H. J. Phys. ReV. Lett. 1983, 50, 2024. (5) (a) Parker, J. S.; Watts, S. M.; Ivanov, P. G.; Xiong, P. Phys. ReV. Lett. 2002, 88, 196601. (b) Park, J.-H.; Vescovo, E.; Kim, H.-J.; Kwon, C.; Ramesh, R.; Venkatesan, T. Nature 1998, 392, 794. (c) Son, Y. W.; Cohen, M. L.; Louie, S. G. Nature 2006, 444, 347. (d) Dutta, S.; Pati, S. K. J. Phys. Chem. B 2008, 112, 1333. (6) (a) Miyajima, K.; Yabushita, S.; Nakajima, A.; Knickelbein, M. B.; Kaya, K. J. Am. Chem. Soc. 2004, 126, 13202. (b) Miyajima, K.; Yabushita, S.; Knickelbein, M. B.; Nakajima, A. J. Am. Chem. Soc. 2007, 129, 8473. (c) Xiang, H.; Yang, J.; Hou, J. G.; Zhu, O. J. Am. Chem. Soc. 2006, 128, 2310. (d) Maslyuk, V. V.; Bagrets, A.; Meded, V.; Arnold, A.; Evers, F.; Brandbyge, M.; Bredow, T.; Mertig, I. Phys. ReV. Lett. 2006, 97, 97201. (7) (a) Mitsui, M.; Nagaoka, S.; Matsumoto, T.; Nakajima, A. J. Phys.Chem. B 2006, 110, 2968. (b) Nagaoka, S.; Matsumoto, T.; Ikemoto, K.; Mitsui, M.; Nakajima, A. J. Am. Chem. Soc. 2007, 129, 1528. (c) Nakajima, A.; Kaya, K. J. Phys. Chem. A 2000, 104, 176. (8) Hobza, P.; Selzle, H. L.; Schlag, E. W. Chem. ReV. 1994, 94, 1767. (9) Braunschweig, H.; Kaupp, M.; Adams, C. J.; Kupfer, T.; Radacki, K.; Schinzel, S. J. Am. Chem. Soc. 2008, 130, 11376. (10) Mallajosyula, S. S.; Pati, S. K. J. Phys. Chem. B 2007, 111, 13877. (11) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon, P.; Sanchez-Portal, D. J. Phys.: Condens. Matter 2002, 14, 2745. (12) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (13) Sankey, O. F.; Niklewski, D. J. Phys. ReV. B 1989, 40, 3979. (14) Supercell sizes. TM(Bzim)2: 25 × 25 × 25 Å3; TM(Bz)2: 10 × 10 × 10 Å3; TM2(Bzim)3: 30 × 30 × 30 Å3; TM2(Bz)3: 15 × 15 × 15 Å3; TM8(Bzim)9: 60 × 60 × 60 Å3. (15) Hamelberg, D.; Williams, L. D.; Wilson, W. D. Nucleic Acids Res. 2002, 30, 3615.

Benzimidazole-Modified Single-Stranded DNA (16) El Hassan, M. A.; Calladine, C. R. Philos. Trans.: Math., Phys. Eng. Sci. 1997, 355, 43. (17) Weng, H.; Ozaki, T.; Terakura, K. J. Phys. Soc. Jpn. 2008, 77, 014301. (18) Our calculations show that each Ti atom has a net M/TM value of 0.560. (19) (a) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (b) Warner, M. G.; Hutchison, J. E. Nat. Mater. 2003, 2, 272. (c) Zhang, S. Nat. Biotechnol. 2003, 21, 1171.

J. Phys. Chem. B, Vol. 112, No. 51, 2008 16989 (20) (a) Porath, D.; Cuniberti, G.; Di FeliceR. Long-Range Charge Transfer in DNA II; Topics in Current Chemistry; Springer: Berlin, 2004. (b) Endres, R. G.; Cox, D. L.; Singh, R. R. P. ReV. Mod. Phys. 2004, 76, 195. (21) (a) Son, Y. W.; Cohen, M. L.; Louie, S. G. Nature 2006, 444, 347. (b) Datta, S.; Pati, S. K. J. Phys. Chem. B 2008, 112, 1333. (22) Lynch, S. R.; Liu, H.; Gao, J.; Kool, E. T. J. Am. Chem. Soc. 2006, 128, 14704.

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