Enhanced Conductance via Induced Π-Stacking Interactions in Cobalt

Nov 14, 2008 - Trilisa M. Perrine, Timothy Berto and Barry D. Dunietz*. The University of Michigan, Ann Arbor, Michigan 48109. J. Phys. Chem. B , 2008...
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J. Phys. Chem. B 2008, 112, 16070–16075

Enhanced Conductance via Induced Π-Stacking Interactions in Cobalt(II) Terpyridine Bridged Complexes† Trilisa M. Perrine,‡ Timothy Berto, and Barry D. Dunietz* The UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed: August 25, 2008; ReVised Manuscript ReceiVed: September 25, 2008

Computational model systems are used to explore improving the transmission through a molecular device based on bridged cobalt(II) complexes. The bridging ligands and the organic conjugated molecular ligands are altered to improve the current flow through both an enhanced π-stacking interaction as well as involving the metal ions directly in the conduction pathway. With terpyridine as the organic ligand, both acetate and NH2- produce conductive devices, while a terpyridine complex bridged by Cl- is not conductive. The addition of a fused ring on either end of the conjugated molecule has a complex effect which is sensitive to the bridged ligand and the particular geometry of the complex. 1. Introduction Experimental techniques in nanoscience are constantly improving and furthering progress toward the realization of singlemolecule electronic devices. This possibility is an exciting step toward the further miniaturization of electronic devices and computer components. The use of molecular systems provides for an inherent flexibility through the chemistry of the molecules, which can be used to design and determine the electronic properties of the device. While the potential uses of single-molecule devices are many, the ability to reliably and consistently create single-molecule devices is still a problem at the forefront of experimental research in this area.1-3 Recently, molecular transistors with specific chemical reactivity were fabricated using self-aligned lithography and self-assembled monolayers (SAMs).4,5 These molecular transistors function as chemical switches by allowing electric current to flow when a chemical bridging agent is present (the on state) to connect two molecules from opposite SAMs across the gap and no current to flow without the bridging agent (the off state). We previously described by computational modeling an atomic-scale mechanism based on chemical reactivity to explain the experimental observations.6 Briefly, the suggested mechanism of current flow in the on state of the experimental system primarily involves a π-stacking interaction between the two organic terpyridine-based monolayers. π-stacking interactions, in general, are important for the assembly of many systems7,8 and, in particular, for π electronic materials.9,10 The dimer of benzene rings has been the focus of most of the theoretical work aimed at understanding the nature of π-π interactions between aromatic systems.11-19 Additional studies consider other aromatic systems in the context of charge transfer.20-24 For the conisdered Co(II)-terpyridine systems, the π-stacking interacations are produced by the combination of the confinement effects due to the geometric consequences of the fabrication scheme and the chemical environement, which includes the acetate and Co(II) ions. In this system, as indicated by our calculations,6 the metal ions themselves have little direct † Part of the “Karl Freed Festschrift”. * To whom correspondence should be addressed. ‡ Current Address: Ohio Northern University, Ada, Ohio 45810.

involvement in the current flow pathway, though the ions are necessary to chemically create the conductive link between the two SAMs. These confinement aspects make the role of the π-stacking interactions in enabling the conductance rather unique when compared to other cases involving conductance enabled through π-stacking interactions. The π-stacking in such devices is typically utilized to allow for current flow through the molecular layers in a direction perpendicular to the molecular planes. In the cobalt(II) terpyridine based devices, the π-stacking interaction, which is perpendicular to the molecular axis, facilitates current flow in a single molecule parallel to its molecular axis. Here, we explore the chemical aspects of the system to further improve the conductance within a similar experimental setup. The bridging ligands and the SAM molecules (socket ligands) are altered to improve the current flow in the linked system through both an enhanced π-stacking interaction as well as involving the metal ions directly in the conduction pathway. This larger current in the on state of the device is sought to improve the on/off ratio of the potential molecular transistors. 2. Computational and Model Details The self-aligned lithography technique produces gaps involving roughly parallel platinum surfaces upon which the selfassembled monolayers are bound.4,5 In our previous analysis of the experiment,6 the metal contacts were therefore modeled as parallel flat Pt(111) planes with the bulk represented by subsequent layers of six platinum atoms. The distance between the plates was parametrized to simulate the experimental conditions in which the bridging occurs across the fabricated gap. In order to focus on the effect of the molecular portion of the device, a simpler model of the metallic leads is used in this study. The bulks are represented by ideal Pt(111)-based models of scanning tunneling microscope (STM) tips with Pt-Pt bond lengths of 2.775 Å. These metal lead models have a single Pt atom at the tip and three atoms in the next layer, and the remaining bulk is represented by addition of layers of six platinum atoms. In addition, the optimized geometries for the molecular systems are obtained with models including a single platinum atom bonded to each thiol terminal. The calculations

10.1021/jp8075854 CCC: $40.75  2008 American Chemical Society Published on Web 11/14/2008

Π-Stacking Interactions in Cobalt(II) Terpyridine

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Figure 1. (a) The experimental system in refs 1 and 2. Socket ligands utilized in the model calculations functionalized with thiols and bonded to Pt are (b) the molecule used in the experimental work, (c) terpyridine (terpy), and (d) terpyridine with additional fused rings (terpy-plus).

are performed using DFT by employing QChem 3.1.25 In all computations, the B3LYP26,27 functional and the LANL2DZ28 basis set are utilized. Following the geometry optimization, single-point energies were then computed for the extended models as described above. Their electronic structures were analyzed using the Green’s function (GF) formalism, as outlined by others, to calculate the electron transmission functions.29-35 We followed the scatteringbased picture of molecular conductance36-38 to obtain the transmission and current. The related equations are well described in the references, in our previous study of the related system6 and in our other studies.6,39-44 These calculations include a representation of a semi-infinite bulk by efficiently solving a tight-binding (TB) model of the bulk at every energy, where the surface and bulk GFs are solved iteratively and simultaneously.45,46 The TB parameters are extracted from the electronic structure calculations described above. 3. Results and Discussion Our previous work6 focused on modeling the system utilized in the reported experiment.4,5 These models were used to provide an atomic-level electronic structure explanation of the conducting devices. A scheme of this experiment is shown in Figure 1a, and the organic terpyridine-based molecule utilized in the experiment is shown in Figure 1b. This molecule is selfassembled onto both platinum surfaces, and a solution of cobalt(II) acetate is then added to create the molecular junctions which allow for the conductance pathways. We have made another model simplification in addition to the STM tip model described above. The phenyl ring between the terpyridine portion of the socket molecule and the thiol (labeled in Figure 1b) is primarily a spacer which lengthens the overall system in order to be able to link the two sides of the approximately 3 nm gap and is part of the overall conjugation of the molecule. For simplicity, this phenyl ring is removed from the model systems considered here. The conjugated π molecular orbitals (MOs) of the remaining molecule are similar to the original system, and the chemistry between the terpyridine (terpy), the metal ions, and the bridging ligands is not substantially affected by the absence of the spacer phenyl ring. This smaller model ligand is shown in Figure 1c. These model choices (Pt STM tips and removal of the spacer phenyl) are first validated. The transmission through a terpy-

Figure 2. (a) Complex of two terpyridine units and a single cobalt(II) ion. (b) Legend describing the model systems studied and the N′, N′′, C1, and C2 atoms labeled for reference. Model systems utilizing (c) acetate, (d) chloride, and (e) amide as the bridging ligands. The N′-Co+2-Co+2-N′′ dihedral angle is listed as well as the C1-C2 and Co+2-Co+2 distances. The distance between the cobalt(II) ions and the bridging ligand’s bonding atoms are also listed in each case.

ridine device where the two socket ligand are linked through a single Co+2 ion, which utilizes no bridging ligand (mono-Co+2, Figure 2a), and a device linked with two Co +2 ions and bridged with acetate ions (OAc-) (Figure 2c) are compared. As expected, the computed transmission through the acetate bridged complex is significantly larger than that of the single Co+2 complex. This, therefore, qualitatively agrees with the more complete model systems which include the spacing phenyl groups and large flat Pt surfaces reported previously.6 The modeling choice to remove the spacer phenyl ring is also tested using two model systems which utilize the platinum STM tip bulk models and are bridged by acetate ions. A comparison of the transmission through two systems either with or without the spacer phenyl ring in the socket ligand confirms that the spacer ring does not drastically affect the transmission properties. The transmission functions of these two systems are very similar in the height and broadening of the peaks and vary only by a slight shift in the energies of the peaks. While the only bridging ligand available in the experiment was acetate (water was tested as a possible bridging ligand in this system and was shown to be ineffective),6 other bridged cobalt systems are well-known.47-50 We therefore consider the bridging of two cobalt(II) ions bound to tridentate terpyridine

16072 J. Phys. Chem. B, Vol. 112, No. 50, 2008 units by alternative ligands. We consider below the transmission when the acetate ions are replaced by Cl- and NH2-, which are known to serve as bridging ligands.47-50 The aim is to identify ligation chemistry leading to a further enhancement of the conductance. For all of the considered bridged complexes (including the OAc- bridged system), the ground-state is a triplet, which is not a surprising result as there are two five-coordinate cobalt(II) ions in each complex. The geometries of the Cl- and NH2bridged complexes feature substantial differences from the geometry which arises when acetate functions as the bridging ligand. When acetate is the bridging ligand, each acetate ion bonds to the two cobalt(II) ions through the use of both of its oxygen atoms (µ-acetato-κ2O,O′ bonding).6 When the terpyridine units are bridged by acetate, two of the rings (one on each terpy) overlap substantially and are thus in a π-stacking interaction, which results in a delocalized molecular orbital across the entire complex. The inherent O-C-O bond angle and distance between the acetate oxygen atoms allows for a twist in the N′-Co+2-Co+2-N′′ dihedral angle. This angle is given in Figure 2 for the various model systems along with some other pertinent bond distances and angles. Note that N′ and N′′ are the central ligating nitrogen atoms on each of the socket ligands. Both Cl- and NH2- bond the two cobalt(II) ions through a single atom and result in a more linear (symmetric, see below) device. These bridging ligands impose a ∼180° N′-Co+2Co+2-N′′ dihedral angle since each of the five-coordinate cobalt(II) ions are square pyramidal with three of the four positions in the plane occupied by the tridentate terpyridine. Binding through a single atom also leads to less flexibility in the distance between the terpyridine socket than when they are bridged by OAc- ions. The distance between the terpyridine units is represented by the C1-C2 distances listed in Figure 2. For the Cl- and NH2- systems, the terpy plane-terpy plane distance (C1-C2) is determined primarily by the Co+2-N or Co+2-Cl bond lengths. The OAc- complex is more flexible, and this distance is determined by the C-O bond lengths, the Co+2-O bond length, the O-C-O angle, and the N′-Co+2Co+2-N′′ dihedral angle. Figure 2 shows the Cl-, NH2-, and OAc- optimized geometries with some relevant distances and angles labeled. The thiol-platinum bonds are not oriented the same way in each of the systems shown. The bonded platinum atoms are in the plane of the terpy molecule in the case of the NH2- and acetate bridged systems and out of the plane for mono-Co+2 and the Cl- bridged system. For all systems, each of the two possible thiol-platinum bond orientations corresponds to a local minimum. For all considered cases, the geometries shown in the figures and further analyzed below correspond to both the geometrical minimum (with the lower energy of the two) and the geometry that features greater transmission. The computed transmission for the terpy systems bridged by each of the bridging ligands is shown in Figure 3 as well as the transmission through the mono-Co+2 complex. The transmission functions of the mono-Co+2 complex and the acetate bridged complex are at similar locations in the energy scale, with the peak of the transmission for the acetate complex being approximately twice as high as that of the mono-Co+2 complex. This agrees with the previously reported computations as the two π systems of the terpyridine molecules in the mono-Co+2 complex are orthogonal to each other and therefore do not form a delocalized π system across the metal center. The terpyridine molecules in the OAc- bridged system participate in π-stacking

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Figure 3. Transmission functions of the terpyridine model systems with no bridging ligand (the single cobalt(II) complex) and amide, chloride, and acetate as the bridging ligands. The right panel shows the detail of the peaks in the region of interest.

Figure 4. (a) Molecular orbitals of a C2V terpy-Co+2 unit. (b) Transmission function of the terpy complex bridged by NH2- and the molecular orbitals responsible for the transmission peaks. All orbitals are drawn at the same isosurface.

interactions and have MOs delocalized across the entire complex. Bridging by Cl- ions causes the significant transmission peak of the system to be closer to the Fermi energy (Ef) of the platinum leads than the mono-Co+2 complex or the OAcbridged complex. However, the transmission peak for the Clsystem is only as high as that of the single cobalt(II) ion complex. The reason underlying the low transmission of the Cl- bridged system is explained below. While the chloride ion complex shows a reduction in the transmission compared to that of the acetate bridged compound, bridging by NH2- yields a complex far more conductive than the acetate bridged compound. The transmission peak for the NH2- compound is located in a similar energy region, but it is both higher and broader than the acetate compound’s transmission peak. The R spin channel transmission function for the NH2- bridged compound can be analyzed by an examination of the molecular orbitals responsible for this transmission. These R molecular orbitals are shown in Figure 4 and are assigned to the appropriate peaks in the transmission function. While all of these orbitals are highly delocalized across both halves of the complex, the heights of the transmission peaks are very different. The MOs of the NH2- complex are essentially linear combinations of the MOs (LCMOs) of a single terpy-Co+2 system. The frontier orbitals of terpy-Co+2 are given in Figure 4 with the appropriate C2V symmetry labels assigned. The transmitting orbitals of the NH2- complex are LCMOs of the b2 (LUMO+1) and a2 (LUMO+2) unoccupied orbitals on terpy-

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Figure 6. Molecular orbitals of a Cs cobalt(II) terpyridine complex bridged by chloride ions. All orbitals are drawn at the same isosurface. Figure 5. The highest molecular orbital and low-lying unoccupied molecular orbitals for a Cs cobalt(II) terpyridine complex bridged by NH2-. All orbitals are drawn at the same isosurface.

Co+2. The transmitting ability of these LCMOs is determined by their symmetries along the conductance axis.51 The bonding and antibonding linear combinations of the a2 MOs of terpyCo+2 (iii and iv) transmit very little. The a2 MO and the linear combinations of it (iii and iv) have a nodal plane down the molecular axis of the complex (this plane is labeled in Figure 2). The nodal plane includes both sulfur atoms which contact the platinum bulks. There is thus no electron density at the contact of these MOs, and little transmission occurs. The bonding and antibonding combinations of the b2 MO of terpyCo+2 (orbitals i and ii), however, are associated with high transmission. The b2 MO of terpy-Co+2 and the linear combinations have density at the contacting S atoms, and the transmission through these MOs is substantial. To illustrate this further, the molecular orbitals of a complex of two terpy-Co+2 units bridged by NH2- ions with Cs symmetry (without the thiol functionalization) are drawn in Figure 5. The molecular orbitals of the complex are shown with their Cs symmetries assigned in parentheses. The linear combinations of the C2V symmetry molecular orbitals of the isolated terpy-Co+2 units are also listed in square brackets. An examination of the MOs of the Cl- bridged model system reveals the reason for the small transmission through this system. While the NH2- complex has a geometry which allows the overlap of similar MOs on the two halves of the complex, this is not true for the Cl- bridged geometry. The Co+2-Cl bond (2.5 Å) is longer than the Co+2-N bond (2.0 Å), and therefore, the two terpyridine units are farther from each other in the Clbridged model. This is evidenced by the difference in the C1-C2 distances for the two systems, 4.1 Å for the Cl- bridged system and 3.4 Å for the NH2- system. The consequence of this geometrical difference is that the π MOs on the two halves do not interact properly to result in a delocalization across the complex. Instead, the MOs remain localized on one-half of the system, allowing for only minimal transmission. This is illustrated by the frontier molecular orbitals of a complex of two terpy-Co+2 units bridged by Cl- ions drawn in Figure 6, and the MOs of the full model system (including the thiols and Pt STM tips) are similar in shape and span. The transmission peak intensity in this case is similar in strength to the transmission through the mono-Co+2 complex since in both cases the MOs are primarily localized on one-half of the complex.

Since the π-stacking interactions are the primary pathways for conductance in these compounds, we consider increasing this π-stacking interaction and thus the overall conductance of the devices. This is done through both the previous discussion of the bridging ligands and altering the structure of the socket ligands (organic molecules in the SAMs). In an aim to explore the ability to increase the electronic density of states (DOS), which contributes to the transport through the system, an additional fused phenyl ring is added to each end of the terpyridine molecule. This ligand is shown in Figure 1d and is referred to as terpy-plus from this point on. This extends the conjugated system in the socket ligand and potentially allows for a more extensive π-stacking interaction between the two halves. We find that this extra phenyl ring has a complex effect and does not have the same effect on the transmission in all of the model systems. Plots of the transmission through the bridged terpy and terpy-plus complexes for each of the three bridging ligands are given in Figure 7. In all three cases, the transmission peaks are shifted by about 0.5 eV in the positive direction due to the destabilization of the MOs on the isolated halves of the complex. The transmission peaks in the three models are affected differently by the addition of the fused phenyl rings as seen by comparing the intensities of the peaks for the different cases. As discussed above, the best transmitting model with terpy as the socket ligand is realized with NH2- as the bridging ligand. However, upon addition of a fused ring on each end of the ligand, this transmission is effectively destroyed, Figure 7b. Conversely, the intensity of the transmission through a Clbridged complex is greatly increased by the additional rings, and the transmission through the OAc- bridged complex is affected very little by the addition of the extra ring (other than the shift already discussed). The distances and angles of the bridged complexes with this larger socket ligand are listed in parentheses in Figure 2 where they differ from those in the terpyridine complex. The geometry of the NH2- complexes is very similar for the two socket ligands, but the transmission changes substantially. The cause of this difference is evident from the MOs of the terpy-plus system shown in Figure 8. In the terpy-plus system, the a2 (LUMO+3) and b2 (LUMO+4) orbitals of terpy are reversed in energy due to extra bonding interactions introduced on the additional ring. The interaction between the two halves of the NH2--terpy-plus complex takes place between the 10b2

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Figure 9. Molecular orbitals of a Cs cobalt(II) terpy-plus complex bridged by chloride ions. All orbitals are drawn at the same isosurface.

Figure 7. (a) Transmission functions of the terpy and terpy-plus model systems bridged by amide (b), chloride (c), and acetate (d) ions.

Figure 8. Molecular orbitals of a C2V cobalt(II) terpy-plus unit. All orbitals are drawn at the same isosurface.

(LUMO+1) and 11b2 (LUMO+3) orbitals, instead of a linear combination of the 8b2 (LUMO+1) on each half of the system as in the terpy complex bridged with NH2-. The LUMO+1 of both the terpy and terpy-plus systems has significant density at the carbon atom bonded to the thiol. However, the LUMO+3 of the terpy-plus system has little density on this carbon atom. Therefore, a linear combination of the LUMO+1 and LUMO+3 of the terpy-plus system, while delocalized over the system, is not as effective at transmitting as the LCMO present in the NH2--terpy complex. There is also an additional large peak in the transmission for the NH2--terpy-plus complex at approximately 1.5 eV below the Fermi level. This peak is due to occupied molecular orbitals which are similar to the HOMO shown in Figure 5. These metal-bridging ligand orbitals are present in the other systems as well but are destabilized in the NH2--terpy-plus system and become closer to the Fermi energy than they are in any of the other systems discussed here.

The transmission through a Cl- bridged system has an opposite response to the addition of the extra rings. The geometries of the Cl--terpy-plus and Cl--terpy system are more different than those of the two NH2- systems, as seen in Figure 2, where the Cl--terpy-plus distances and angles are listed in parentheses. In the Cl--terpy system, the two halves of the complex are far enough apart and in a configuration which does not allow the effective overlap of the b2 MOs necessary for good transmission. In the Cl--terpy-plus system, significant overlap occurs between the additional rings on each half of the system. Whereas this interaction is not present in the Cl--terpy system, this additional interaction allows for the delocalization of the π MOs across the entire system for the Cl--terpy-plus complex. Molecular orbitals of a Cs symmetric Cl--terpy-plus complex (without the thiol-Pt part of the system) are shown in Figure 9. The transmission of the Cl--terpy-plus system is therefore far more substantial than that in the Cl--terpy complex. The transmission for the two acetate bridged complexes is remarkably similar, Figure 7d. With this bridging ligand, the MOs for the terpy system and those for the terpy-plus system are very similar and result from linear combinations of the LUMO+1, b2, orbitals in each case. While there is more contact between the two halves of the complex for the terpy-plus system, this does not serve to make the LCMOs more effective as a conductance pathway. The MOs are well delocalized in both systems, and no additional pathways are introduced near the Fermi level of the metal by this interaction. The transmission functions are therefore very similar in the two cases. 4. Conclusions A recent computational study6 associates the observed conductance of the in situ fabrication technique4,5 to confinement-induced π-stacking interactions. These stacking interactions are orientated perpendicularly to the conductance path and are mediated by Co(II) and acetate ions. Here, we have examined three additional bridging ligands and two socket ligands to analyze the effect of altering each portion of the molecular device on the transmission function. Both an enhanced π-stacking interaction and a direct involvement of the metal ions in the conduction pathway are sought to improve the current flow through the pathway via these chemical manipulations. Models of both acetate and NH2- bridged terpyridine systems

Π-Stacking Interactions in Cobalt(II) Terpyridine are conductive devices, but Cl- bridged terpyridine complexes are not highly conductive. While the bridging ligand has a significant effect on the conductivity of the device, the interactions which occur directly between the two socket ligands are also critical to the functionality of the device. Therefore, an extension of the conjugated system of the organic socket ligand is explored in an attempt to increase the transmission through the device. It is found that it is difficult to increase the DOS of the conductivity channels through the addition of a fused ring to each end of the socket ligand. This addition may have no effect or serve to either improve or destroy the transmission through a molecular device. This difference is due to the geometries imposed by the bridging ligands and how the π systems of the socket ligands interact at these geometries. The effect of the additional fused ring is thus seen to be very sensitive to the specific bridging ligand. Acknowledgment. B.D.D. acknowledges the University of Michigan and the Petroleum Research Fund of the ACS (through Grant 47118-G6) for financial support. We acknowledge the National Energy Research Scientific Computing Center (NERSC) for awarding computing time. References and Notes (1) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571–574. (2) Tao, N. J. Mater. Chem. 2005, 15, 3260–3263. (3) Perrine, T.; Dunietz, B. D. Phys. ReV. B 2007, 75, 195319. (4) Tang, J.; Wang, Y.; Nuckolls, C.; Wind, S. J. J. Vac. Sci. Technol., B 2006, 24, 3227–3229. (5) Tang, J. Y.; Wang, Y. L.; Klare, J. E.; Tulevski, G. S.; Wind, S. J.; Nuckolls, C. Angew. Chem., Int. Ed. 2007, 46, 3892–3895. (6) Perrine, T.; Dunietz, B. D. J. Phys. Chem. A 2008, 112, 2043. (7) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; De Leeuw, D. M. Nature 1999, 401, 685–688. (8) Collings, J. C.; Roscoe, K. P.; Robins, E. G.; Batsanov, A. S.; Stimson, L. M.; Howard, J. A. K.; Clark, S. J.; Marder, T. B. New J. Chem. 2002, 26, 1740. (9) Katz, H. E. J. Mater. Chem. 1997, 7, 369–376. (10) Garnier, F. Chem. Phys. 1998, 227, 253–262. (11) Jaffe, R. L.; Smith, G. D. J. Chem. Phys. 1996, 105, 2780. (12) Pirko, V.; Engkvist, O.; Solda´n, P.; Selzle, H. L.; Schlag, E. W.; Hobza, P. J. Chem. Phys. 1999, 111, 572. (13) Tsuzuki, S.; Uchimaru, T.; Matsumura, K.; Mikami, M.; Tanabe, K. Chem. Phys. Lett. 2000, 319, 547. (14) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M.; Tanabe, K. J. Am. Chem. Soc. 2002, 124, 104. (15) Sinnokrot, M. O.; Valeev, E. F.; Sherrill, C. D. J. Am. Chem. Soc. 2002, 124, 10887. (16) Sinnokrot, M. O.; Sherrill, C. D. J. Phys. Chem. A 2004, 108, 10200. (17) Ye, X.; Li, Z.-H.; Wang, W.; Fan, K.; Xu, W.; W.; Hua, Z. Chem. Phys. Lett. 2004, 397, 56–61. (18) Sinnokrot, M. O.; Sherrill, C. D. J. Phys. Chem. A 2006, 110, 10656. (19) Hobza, P.; Selzle, H. L.; Schlag, E. W. J. Phys. Chem. 1996, 100, 18790. (20) Cornil, J.; Beljonne, D.; Calbert, J. P.; Bredas, J. L. AdV. Mater. 2001, 13, 1053.

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