Structure and Electrical Bistability of a New Class of Diphenyl

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J. Phys. Chem. C 2008, 112, 18628–18637

Structure and Electrical Bistability of a New Class of Diphenyl-bithiophenes: A Combined Theoretical and Experimental Study Daniele Fazzi,*,† Chiara Castiglioni,† Fabrizia Negri,*,‡ Chiara Bertarelli,† Antonino Famulari,† Stefano Valdo Meille,† and Giuseppe Zerbi† Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, P.zza Leonardo da Vinci 32, 20133 Milano, Italy, and INSTM, UdR Politecnico di Milano, Italy, and Dipartimento di Chimica “G. Ciamician”, UniVersita` di Bologna, Via F. Selmi 2, 40126 Bologna, Italy, and INSTM, UdR Bologna, Italy ReceiVed: July 17, 2008; ReVised Manuscript ReceiVed: September 12, 2008

The more stable conformers of recently synthesized diphenyl-bithiophene (DPBT) derivatives, a new class of relatively flexible conjugated molecules displaying electrical bistability, are investigated with the help of density functional theory calculations and by single crystal X-ray diffraction. The electronic structures of the neutral and positively charged species are computed for the most relevant isomeric forms along with intramolecular reorganization energies associated with charging. Two major mechanisms, charge injection at the interface and bulk charge transport, are considered to rationalize the observed electrical bistability and the efficiency of the electrical phenomenon for different DPBT species, in terms of computed molecular parameters. It is suggested that bistability is governed by an interplay of the two charge transport processes, with the OFF state being determined by activated hole injection. I. Introduction Organic semiconductors are considered promising materials for new applications in electronics, specifically in the emerging and innovative field of nonvolatile organic electrical memories (NVM).1 In NVM, when a threshold bias (switching potential VTh) is reached by applying a voltage to the electrodes, the electrical conductivity of the organic material changes from a low conductive state (i.e., the OFF state of the memory cell 0) to a higher one (i.e., the ON state - 1), thus showing an electrical bistability behavior. Because of easy and low-cost processing techniques, and of a relatively low switching bias (of the order of a few V), organic materials can be considered good candidates for memory devices.2 Several experimental studies3-7 were carried out to define the key parameters (at the molecular level) governing the device performance in terms of ON/OFF current ratio, write and erase cycles, retention time, and switching bias of the organic memory. Changes in conductivity have been explained in terms of structural rearrangements (conformational changes)5 or as induced by electro-reduction (or oxidation).6 However, because of the complexity of the phenomena involved, an exhaustive rationalization of the different conductive states occurring in NVM has not yet been obtained. Recently, a new class of diphenyl-bithiophene (DPBT) derivatives have been synthesized,8 and, when tested for memory devices, they have shown electrical bistability.9,10 The structures of L DPBT (5,5′-bis-3,5-di-tert-butyl-4-hydroxyphenyl-2,2′-bithiophene and 5,5′bis-3,5-di-tert-butyl-4-methoxyphenyl-2,2′-bithiophene) and of Z DPBT derivatives (3,3′-bis-3,5-di-tert-butyl-4-hydroxyphenyl-2,2′bithiophene and 3,3′-bis-3,5-di-tert-butyl-4-methoxyphenyl-2,2′bithiophene) are shown in Figure 1. Different DPBT structures (the * Corresponding authors. E-mail: [email protected] (D.F.), [email protected] (F.N.). † Politecnico di Milano and INSTM. ‡ Universita ` di Bologna and INSTM.

Figure 1. Molecular structure of L and Z DPBT molecules; X indicates different functional groups: X ) H or CH3. Numbers correspond to the CC bonds sequence considered for the study of the bond length alternation (BLA).

L and Z isomers) with different functional groups on the phenyl moiety (i.e., OH or OCH3) have been experimentally tested: their different structures are associated with different I-V characteristics. For instance, it has been shown11 that memory cells with L DPBT film as active layer exhibit low performances in terms of retention time, ON/OFF current ratio, and write/erase cycles, whereas Z DPBT species present higher retention times (>48 h), a greater number of write/erase cycles (>200), and an acceptable ON/OFF current ratio (>100).9 Whithin the same molecular species (L or Z), different substituents (OH or OCH3) confer different electrical switching behavior passing from a two level memory (i.e., Z-OCH3 with one OFF and one ON level)9 to a three level one (i.e., Z-OH with one OFF and two ON levels).10 In the same study, on the basis of density functional theory calculations on the Z-OH species, a correspondence between conductive states and molecular oxidative states was proposed.10 Although this represents a preliminary attempt to correlate molecular parameters with the electrical behavior, an exhaustive theoretical study on the molecular mechanisms determining the electrical bistability has not been reported, so far, for these and other organic systems, and the

10.1021/jp8063239 CCC: $40.75  2008 American Chemical Society Published on Web 11/04/2008

Structure of a New Class of Diphenyl-bithiophenes

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Figure 2. Molecular fragments of DPBT molecules: 2Th, Th-Ph (Z and L shape), and Ph-tBu unit.

rationalization of different electrical responses is currently unclear. However, such mechanisms cannot be identified just by experimental studies.12-14 Prompted by the intriguing open questions discussed above, we have carried out a combined theoretical and experimental investigation encompassing several structural and mechanistic issues: (i) the most stable DPBT conformers have been identified, for both L and Z species, as the more likely molecular species involved in the electrical phenomenon; (ii) the predicted molecular structures have been compared to the experimental structure obtained from single crystal X-ray measurements on the Z-OCH3 molecule; and (iii) the electronic structure (molecular orbitals and energies) of the dominant isomers and (iv) the intramolecular reorganization energies λ have been evaluated with density functional theory (DFT) calculations. Finally (v), the computed molecular parameters have been employed to discuss two major processes that can influence the electrical bistability behavior, the charge injection at the electrode-organic material interface and the bulk charge transport. II. Methods A. Experimental. Crystal Data and Structure Refinement for Z-OCH3. Crystals of Z-OCH3 were obtained by slow evaporation of a diethyl ether solution. Data from a single crystal were collected on a Bruker P4 diffractometer at room temperature, using graphite monochromated Cu KR radiation (λ ) 1.54179 Å) and θ-2θ scans. An analytical absorption correction was applied using crystal faces indexing procedures. Crystal data: C38H50O2S2, Mr ) 602.90, monoclinic, space group C2/c (No. 15), a ) 20.0401(16), b ) 11.8903(9), c ) 17.7905(11) Å, R ) 123.480(8)°, V ) 3535.8(4) Å3, Z ) 4, Dc ) 1.133 Mg m-3, µ ) 1.583 mm-1, F(000) ) 1304; 3563 reflections (2903 unique, Rint ) 0.0769). The final refinement, for 219 refined parameters, with 7 restraints converged to wR(F2) ) 0.2150 (R ) 0.0860) for 2903 unique reflections and to wR(F2) ) 0.1948 (R ) 0.0684) for 2125 unique reflections with I > 2σ(I), with

a final goodness of fit of 1.041. The structure was solved by direct methods using the SIR97 program15 and refined by fullmatrix least-squares with SHELXL97.16 Non-hydrogen atoms were treated anisotropically. Hydrogen atoms bound to aromatic carbons were located by Fourier methods and refined with a group temperature factor. B. Computational Details. The determination of the lowest energy structures of DPBT was performed in two steps. First, three molecular fragments were identified (see below), and their conformations were studied with DFT calculations. Second, the information collected on fragments was used to build a set of reasonable initial structures for DPBT that were then refined by performing AM1 semiempirical17 geometry optimizations. The more stable conformers derived from the AM1 analysis were finally optimized with DFT, employing the B3LYP hybrid functional18 with the 6-31G**19 basis set, the same level of theory employed for molecular fragments. In addition to neutral DPBT species, charged species were also optimized at the same level of theory, starting from the corresponding structures of the neutral forms. One electron energy levels (more specifically, HOMO energies) resulting from DFT calculations were employed to discuss the alignment with the work function of the electrodes, assuming the validity of the Schottky-Mott rule.20 Reorganization energies λ were obtained by performing single point calculations following the procedure reported in refs 21, 22, according to the adiabatic potential (AP) approach. All calculations were carried out using the Gaussian 0323 package. In Figures 1 and 2, we collect the structures of L and Z DPBT and their molecular fragments. DPBT are flexible molecules, because of the presence of exocyclic CC single bonds connecting the aromatic rings or the tert-butyl (tBu) units to the aromatic rings. The dihedral angles associated with these CC bonds are expected to determine the more relevant DPBT conformations. An additional soft degree of freedom to be considered is the torsion around the CO bond. The four dihedral angles, associated with the above-discussed degrees of freedom, are depicted in

18630 J. Phys. Chem. C, Vol. 112, No. 47, 2008 Figure 2. The k ) 1-2, i ) 1-4, j ) 1-2 subscripts for τ, β, and δ refer to the numbering of each type of dihedral in the DPBT molecule. As indicated in Figure 2, the three molecular fragments considered to build DPBT structures are (i) the internal bithiophene unit (2Th), (ii) the L and Z thiophene-phenyl units (Th-Ph), and (iii) the phenyl ring with two tert-butyl groups (Ph-tBu) in para with respect to the OH group. For each fragment, a potential energy surface (PES) scan along a selected torsional coordinate was carried out by optimizing all of the internal degrees of freedom except for the chosen dihedral angle. The PES scan provides information on the relative minima along the chosen torsional coordinate, on their energy difference, and it gives an estimate of the barrier height for interconversion among them. III. Structure and Stability of Z- and L-DPBT A. Relevant Conformers of DPBT from Molecular Fragment Analysis. As compared to other organic semiconductors, DPBT molecules and, more specifically, the Z derivatives, those better performing as electric memories, are characterized by a complex three-dimensional structure, because of the bulky tBu substituents and of the presence of relatively flexible exocyclic CC bonds that can lead to different conformations. As discussed in the previous section, molecular fragments were investigated to determine the values of the dihedral angles defined in Figure 2, corresponding to stable conformers. Appropriate combinations of these values were used to build the more relevant DPBT conformers. The discussion is restricted here to the L and Z DPBT derivatives with the OH group, hereafter indicated as L-OH and Z-OH. The first fragment considered is 2Th. The PES scan on 2Th indicates the presence of two minima (separated by less than a kcal/mol) corresponding to the cis ((33.43°) and the trans ((157.60°) bithiophene conformers (for more details, see the Supporting Information) in agreement with the literature data.24,25 The estimated energy barrier for interconversion from cis to trans is about 2.7 kcal/mol, a value that compares well with previous results obtained at a higher level of theory.24 Accordingly, both the cis and the trans conformers have to be considered as major isomeric forms of DPBT (both Z-OH and L-OH species). B3LYP/6-31G** PES scans for the torsion around the exocyclic CC bond of the Th-Ph fragment (see Supporting Information) predict two symmetry related isoenergetic pairs of conformations, which differ from each other for the value of the dihedral angle (τk ) (32.74° and (146.75°) with an estimated energy barrier of 2.6 kcal/mol. Each pair of structures is identical to the other two, due to the local C2V symmetry of the phenyl ring. Thus, in the following, we will consider only the values τk ) (32.74° for building initial DPBT structures. Focusing on the Ph-tBu moiety (Figure 2), we expect different stable conformers, which differ for the positions of the tert-butyl groups with respect to the phenyl plane. Taking as reference the CC bond (e.g., C(10)-C(9) in Figure 2) of the phenyl ring to which the OH and the tBu groups are bound, we label t or c the tBu conformation whose CC bond (e.g., C(12)-C(13) in Figure 2) forms, with the reference CC bond of the phenyl ring, a dihedral angle βi close to 180.00° (trans, i.e., t) or close to 0.00° (cis, i.e., c). From DFT calculations, the trans conformation of the two tBu units (i.e., tt) is 7 kcal/ mol more stable than the cis one (i.e., cc). Because of the steric interactions of the methyl groups, the tBu units control the position of the hydrogen atom of OH groups; when both tBu units, bound to a given Ph ring, are in trans conformation (tt, βi ) 180.00°), the hydrogen atom of the OH group lies in the plane of the ring (δj = 0.00° or =180.00°).26

Fazzi et al. The values of the dihedral angles determined for the fragments were used to build initial structures of DPBT. Possible combinations can be obtained by considering four values for the R angle ((33.00° (cis) and (157.60° (trans)), two values for each τk ((32.74°), one lower energy choice for all of the βi angles (the whole set in trans conformation (tttt)), and two values for each δj (=0.00° and =180.00°). Several combinations lead to identical structures, equivalent in terms of both geometry and electronic structure because they are symmetry related. In other words, some conformers are structurally identical and some are enantiomers. If we focus on the value assumed by the R torsion angle, we can define two classes of conformers for each DPBT species (L and Z), each class encompassing molecules with the same or similar R values but with different combinations of the remaining three dihedral angles. The two classes of conformers will be hereafter labeled as L-OH trans and L-OH cis (and the corresponding Z-OH trans and Z-OH cis). Representative examples of them are illustrated in Figure 3. Inside each class (trans and cis), we can identify subclasses characterized by different sign combinations of the backbone torsion angles in the sequence τ1Rτ2. These are the - - -, - - +, and - + subclasses. For symmetry reasons, other combinations of the τ1Rτ2 sequence give equivalent subclasses (i.e., - + - and + - +). According to the calculations on the fragments, we built one DPBT structure for each subclass, by assuming the four tBu groups in trans conformation (tttt) and the values of δ1 and δ2 equal to 180.00°.27 The tttt conformers of each subclass were optimized with the AM1 Hamiltonian. The AM1 results show that all three subclasses correspond to minima, both for L-OH trans and cis, with negligible energy differences (∆E = 10-2 kcal/mol). For the Z-OH trans species, we found that the three subclasses correspond to three stable conformers with energy differences of the order of 1 kcal/mol (i.e., larger than for L-OH conformers) with the (- + -) sequence being the most stable. In contrast, for Z-OH cis, two subclasses (- + - and - - +) correspond to unstable structures (due to steric interactions involving the Ph moieties) and must be rejected: the only stable structure corresponds to the (- - -) or equivalently (+ + +) combination. In conclusion, we can state that (i) for both Z and L species, there are stable trans and cis isomers; (ii) within each trans or cis class of isomers, there are, in general, subclasses of conformers, characterized by different sign combinations of the τ1Rτ2 dihedrals; (iii) interestingly, some combinations correspond to enantiomeric forms, as confirmed by the X-ray characterization discussed in section III.C; (iv) tBu conformations are very important to define the lowest energy conformers of DPBT (those characterized by the tttt sequence), because they can alter the energy of a given isomer (L or Z, trans or cis) by as much as 7 kcal/mol (see the Supporting Information); and (v) the energy modulation associated with different conformations of the OH or methyl groups is almost negligible (∆E = 10-2 kcal/mol, see the Supporting Information). B. Equilibrium Structures of DPBT from DFT Calculations. DFT (B3LYP/6-31G**) geometry optimizations were carried out starting from the more stable structures identified by the previous AM1 calculations, those featuring tttt conformations of all tBu groups and in plane OH bonds. Because the main objective of this study is the evaluation of charge transport parameters, we are confident that the choice of the more stable conformers (determined, as discussed above, by the tBu units) is fully representative. Indeed, changes in the conformation of

Structure of a New Class of Diphenyl-bithiophenes

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Figure 3. Trans and cis isomers of L-OH (top) and Z-OH (bottom) DPBT; hydrogen atoms are omitted for clarity.

TABLE 1: B3LYP/6-31G** Optimized Torsion Angles for L-OH DPBT Conformersa L-OH conformer

trans (- - -)

trans (- - +)

cis (- - -)

cis (- + -)

cis (- - +)

τ1 [deg] R [deg] τ2 [deg] βi [deg] δj [deg] energy [hartree] relative energy [kcal/mol]

-25.40 -159.70 -25.40 180.00 0.12 -2346.44619073 0.00

-27.40 -165.50 +27.50 180.00 1.43 -2346.44618082 0.01

-26.55 -24.44 -26.58 180.00 1.43 -2346.44452620 1.04

-28.70 +25.50 -28.70 180.00 2.22 -2346.44495368 0.77

-25.60 -25.70 +26.80 180.00 1.10 -2346.44473159 0.92

a

Absolute and relative energies are also reported.

the phenyl substituent leave practically unaffected the electronic structure of the molecules and, specifically, the shape and energy of the frontier orbitals, because of their localization on the conjugated framework of the molecule. Thus, for each L-OH isomer (cis and trans) and for the Z-OH trans isomer, DFT optimizations were carried out starting from the three stable AM1 conformers (- - -, - + -, - - +), while for the Z-OH cis isomer the only AM1 stable structure (- - -) was considered. The B3LYP/6-31G** calculations on the three L-OH trans conformers lead to only two, practically isoenergetic, stable structures (see Table 1), (- - -) and (- - +), in agreement with AM1 results, whereas the (- + -) relaxes to the (- - -) structure. The absolute and relative energies of the three structures obtained for the L-OH cis isomers are also collected in Table 1, along with the optimized values of the four monitored dihedral angles. It is seen that the most stable L-OH cis conformer is less than 1 kcal/mol above the L-OH trans isomer and that the three cis conformers are very close in energy. Table 2 collects the same information for the Z-OH isomers. In this case, starting from the three AM1 conformers, we obtained only two minima for the Z-OH trans conformer and one for the Z-OH cis conformer. As shown in Table 2, the two Z-OH trans structures are separated by 2.0 kcal/mol. Interestingly, the lowest energy trans structure corresponds exactly to the isomer determined by the X-ray analysis of the Z-OCH3 derivative (see section III.C). Because of the presence of the (trans and cis) conformers, one may consider the possibility that the electrical bistability

TABLE 2: B3LYP/6-31G** Optimized Torsion Angles for Z-OH DPBT Conformersa Z-OH conformer

trans (- + -)

trans (- + +)

cis (+ + +)

τ1 [deg] R [deg] τ2 [deg] βi [deg] δj [deg] energy [hartree] relative energy [kcal/mol]

-36.10 +110.70 -36.10 180.00 2.10 -2346.43537989 0.00

-38.60 +87.87 +27.10 180.00 0.70 -2346.43225050 2.00

+39.30 +64.60 +39.30 180.00 0.40 -2346.43469716 0.43

a

Absolute and relative energies are also reported.

displayed by these compounds is connected with an intramolecular isomerization around the central C(2)-C(2′) bond of the bithiophene unit. To explore this possibility, we performed a B3LYP/6-31G** PES scan for the L-OH and Z-OH DPBT compounds, while varying the torsion angle R and optimizing all of the internal degrees of freedom at each step. The resulting energy profile (for details, see the Supporting Information) indicates that, similarly to bithiophene,24,25 the trans isomers are separated from the cis conformers by an energy barrier of =3.5 kcal/mol for L-OH and =5 kcal/mol for Z-OH. From these results, we can state that, at room temperature and in solution, trans-cis isomerization should easily occur for DPBT molecules; in the solid state (both amorphous and crystalline phase), the effect of molecular packing or of the crystalline field should hinder dynamical interconversion between cis and trans

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Fazzi et al.

Figure 4. Arbitrary view of Z-OCH3 DPBT with the labeling of the asymmetric unit. Rotational disorder involving the tert-butyl groups is shown. The less populated tert-butyl conformers are evidenced with unfilled C-C bonds.

species due to the large volume change associated with the isomerization. However, the existence of a variety of symmetry equivalent conformers (i.e., enantiomers obtained by different τ1Rτ2 sequences) separated by relatively low energy barriers suggests that there can be volume-conserving isomerizations that may occur also in the solid state. However, due to their short-lived character, these conformational changes cannot be responsible for the nonvolatile character of the electrical memory. One may also speculate that the applied electric field acting on the molecular permanent dipole moment might promote intramolecular structural relaxation (i.e., change of the R and/or τk torsion angles). However, we expect that effects of this kind should not affect significantly the charge transport, at least at the level of single molecule parameters.28 Indeed, as will be shown below, different DPBT conformers (cis and trans with their τ1Rτ2 combinations) present a very similar electronic structure in terms of frontier orbitals (HOMO and LUMO) and electronic gap; therefore, they are expected to show a very similar behavior in terms of charge transport (see section IV). C. Experimental and Theoretical Structure of Z-OCH3 DPBT. The more stable Z-OH molecular structures were used as initial structures for the theoretical investigations on the Z-OCH3 derivative. Two stable isomeric forms were obtained from DFT optimizations, the cis (+ + +) and the trans (- + -). The trans form is more stable than the cis by 0.37 kcal/mol. As it will be shown, the X-ray results show the presence of the sole trans form, and, interestingly, the isomer detected experimentally is exactly the one computed with the lowest energy. Although calculations suggest the presence of both isomers in the gas phase and, very likely, also in solution, because of their small energy difference, and small interconversion barrier, only the trans form is detected in the crystal, with no evidence of disorder involving the R torsion angle. Given the different shapes and packing requirements of the two conformers (see Figure 3), this result is not surprising. Under the circumstances, crystallization of the more stable trans conformer is favored. This appears indeed a highly plausible and probable outcome. Figure 4 shows the molecular structure of Z-OCH3 DPBT obtained by single crystal X-ray diffraction (for the packing structure of Z-OCH3 DPBT, see the Supporting Information). The essential features of the molecular geometry of the

TABLE 3: Conformational Features of the Z-OCH3 DPBT Molecule in the Crystala selected torsion angles

symbol

C(2)-C(3)-C(4)-C(5) C(2)-S(1)-C(2)-C(3) C(2′)-C(2)-S(1)-C(5) S(1)-C(2)-C(3)-C(6) S(1)-C(2)-C(2′)-S(1′) C(3)-C(2)-C(2′)-C(3′) C(4)-C(3)-C(6)-C(7) C(4)-C(3)-C(6)-C(11) C(2)-C(3)-C(6)-C(7) C(9)-C(8)-C(12)-C(13) C(9)-C(8)-C(12)-C(13′′) C(9)-C(10)-C(17)-C(18) C(9)-C(10)-C(17)-C(18′′) C(16)-O(1)-C(9)-C(10) a

R τ 1 ) τ2 β1 β1′′ β2 β2′′

crystal [deg]

theory [deg]

0.2(5) -0.6(3) -177.7(2) -179.2(2) 119.2(3) 126.0(3) 138.9(3) -44.1(5) -41.6(4) -175.2(4) -8.5(26) 179.9(5) 14.2(17) 88.0(4)

0.04 -0.70 -176.40 179.54 111.34 121.90 140.20 -41.15 -38.72 -171.47 171.90 91.20

The last column gives the B3LYP/6-31G** optimized values.

TABLE 4: Intermolecular Contacts (If H Atoms Are Involved, Only Contacts That Are 0.25 Å Less than the Sum of van der Waals Radii Are Listed, Taking rC ) 1.70, rH ) 1.20, rS ) 1.80, and rO ) 1.50 Å)a atom 1 H(5) S(1) C(4)

atom 2

sym. transf. for 2

distance [Å]

H(161) C(13′′) C(19′′)

- /2 + x, - /2 + y, z -1/2 + x, -1/2 + y, z -x, -y, -z

2.147 3.342 3.284

1

1

a Atoms of the less populated tert-butyl conformers (occupancy e0.2) are double primed (see Figure 4).

compound are given in Tables 3 and 4 (see the Supporting Information for all of the details in terms of intramolecular contacts, bond lengths, and bond angles); B3LYP/6-31G** results are also reported for comparison. The Z-OCH3 DPBT molecule displays rigorous C2 molecular symmetry, with the 2-fold axis running parallel to the b lattice direction, through the C(2)-C(2′) bond midpoint. In our discussion, atoms that, within the molecule, are related by the intramolecular 2-fold axis carry the same label but are primed. Although there are no tetrahedral chiral centers in the molecule, two enantiomeric conformers, with opposite values of all torsion angles, coexist in the centrosymmetric unit cell. Bond lengths,

Structure of a New Class of Diphenyl-bithiophenes bond angles, and torsion angles largely conform to expectations for crowded and sterically strained dithiophene systems. The only region of the molecule that shows deviations from expected bond length values involves the tert-butyl group centered on C(17), which presents substantial rotational disorder that we were able to model with two conformers, with occupancy factors, respectively, of 0.80 and 0.20. Less important rotational disorder was evidenced in the final stages of refinement also for the tert-butyl group centered on C(12), with an occupancy factor of about 0.10 for the less probable conformer. The disorder model determined experimentally for the tert-butyl groups conforms to the results of the quantum-mechanical analysis (see the Supporting Information). Atoms of the less populated tert-butyl conformers (occupancy e0.2) are double primed in Figure 4 and in Table 3. The values of torsion angles C(3)-C(2)-C(2′)-C(3′) and S(1)-C(2)-C(2′)-S(1′) are unusual for crystalline dithiophene systems; however, this is hardly surprising as we were unable to find in the literature crystal structures of 2,2′-dithienyl systems with phenyl substituents at 3 and 3′. The values of the just-mentioned R torsion angles (126.0(3)° and 119.2(3)°, respectively) are the result of compromises between the shape of the rotational barrier of the bithienyl system, showing the trans minimum at about 158°29,30 and intramolecular steric interactions involving S(1), C(2), C(6), and C(7) with the corresponding primed atoms (see the Supporting Information), which tend to favor the dihedral angles between the thiophene rings close to 100° (see refs 30, 31). The same steric interactions marginally displace torsion angle τ1 and τ2 from the expected minima at ca. 33° (see Figure 4). Indeed, the mentioned contacts confirm that angles τ1, τ2, and R are strongly correlated in Z-OCH3 DPBT. On the contrary, the large number of intermolecular contacts that involve the tert-butyl and the methoxy group do not affect dithienyl atoms intramolecularly, implying that the βi angles are in essence uncorrelated to the τ1, τ2, and R angles, consistent with the conformational analysis of the molecule. Considering the remarkable intermolecular steric constraints, and the discussed rotational disorder of the tert-butyl groups, it is hardly surprising that the density of Z-OCH3 DPBT is among the lowest for substituted oligothiophene systems. In fact, in a CSDF search of related oligothiophenes (i.e., nonmacrocyles, with only C, H, O, and S atoms in the thiophene substituents), only one dithiophene system was found displaying a lower density value, 3,4,3′,4′-tetra-tert-butyl-2,2′-bithiophene,31 which has a density of 1.116 g cm-3, as compared to the value of 1.133 g cm-3, determined crystallographically for Z-OCH3 DPBT. The fact that a large number of intramolecular contacts are apparent in the crystal, whereas few short packing contacts were found, none of them suggesting stacking or other specific intermolecular interactions, is consistent with the small deviations found between the conformation in the crystal and the structure of the trans conformers determined with quantum mechanical calculations for the isolated molecule. Examining the intermolecular contacts (Table 4), it appears very resonable that, whereas the tBu and the OCH3 groups on the phenyl hardly influence the conformation of the molecule, they play a key role in determining the intermolecular packing. IV. Charge Injection and Transport Parameters A. Holes Injection into the Neutral Material. The elucidation of the different electrical behavior of the Z and L DPBT compounds is a complex problem influenced by several factors that can concur to the final result. To discuss the different behavior of these compounds (and their derivatives) with respect to electrical bistability, we must consider both (i) the injection

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18633

Figure 5. B3LYP/6-31G** frontier molecular orbital energies of the trans and cis isomers of Z and L DPBT. Black and orange lines represent the work function of ITO (4.4 eV) and Al (4.3 eV) electrodes,9 respectively. Red and blue lines correspond to computed HOMO and LUMO energy levels.

step from the electrodes and (ii) the charge transport within the organic layer occurring after injection.32,33 Indeed, the different behavior of the two molecules may be due to different injection efficiencies, to different charge transport properties, or to both phenomena that may cooperate to make the Z compound a more efficient electrical resistive memory. Moreover, it is expected that the less efficient of the two processes (i) and (ii) will determine the magnitude of the observed current. We first consider the injection step and compare the frontier molecular orbital energies to the work function of the electrodes used in the experimental studies, ITO and Al, assuming the validity of the Schottky-Mott rule.20 Figure 5 shows in a schematic plot the HOMO (red) and LUMO (blue) energies of the more stable conformers of both cis and trans forms of Z-OH and L-OH. Although the quality of DFT one electron levels may be questioned, we have verified that the same level of theory employed in this work predicts the HOMO level of pentacene very close (within 0.3 eV) to its experimental value in the crystal. Thus, we are confident that the HOMO levels depicted in Figure 5 provide a reasonable estimate for the ionization potential of DPBT in the solid phase. Inspection of Figure 5 shows that the HOMO energies of all of the systems considered are much closer to the work function of ITO (black) and Al (orange) as compared to the LUMO levels. Thus, it is clear that, consistent with experiments,9 the dominant charge transport mechanism must involve holes. In addition, it can be seen that the HOMO level of both cis and trans forms of Z-OH is systematically lower in energy than the HOMO of the two isomeric forms of the L-OH compound. This result indicates that the injection step should be more efficient for the L-OH compound, which may result in a less efficient bistability. Indeed, because hole transfer from the electrode to the molecular material occurs more easily (lower hole injection barrier) than for the Z compound, a relatively large current may characterize the L species already in the OFF state, resulting in a lower ON/OFF ratio. Furthermore, in the hypothesis that the transition to the ON state is associated with the formation of oxidized species, one expects a lower threshold voltage for the ON state in the L case, because the activation barrier is lower. This finding is in nice agreement with experimental evidence for the L species (VTh ) -1.5 V11). B. Structure of Oxidized Species: Effects on the Hole Injection Barrier. At the threshold voltage, we expect that a population of charged species is formed at the electrode-organic layer interface, due to “quasi-resonant” injection of holes. At

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TABLE 5: B3LYP/6-31G** Optimized Torsion Angles and Absolute Energies, for Neutral (0), Singly (1+), and Doubly (2+) Charged Species of Both Trans and Cis DPBT-OH Species 0 trans

1+ trans

2+ trans

0 cis

1+ cis

2+ cis

L-OH τ1 [deg] R [deg] τ2 [deg] βi [deg] δj [deg] energy [hartree]

-25.40 -159.70 -25.40 180.00 0.12 -2346.4462

-2.80 -179.70 -2.80 180.00 1.12 -2346.2380

-1.70 -179.20 -1.70 180.00 0.80 -2345.92044

-28.70 +25.50 -28.70 180.00 2.22 -2346.4450

-0.01 +0.01 -0.02 180.00 0.00 -2346.2372

0.00 0.00 0.00 180.00 0.00 -2345.9202

Z-OH τ1 [deg] R [deg] τ2 [deg] βi [deg] δj [deg] energy [hartree]

-36.10 +110.70 -36.10 180.00 2.10 -2346.4354

-36.00 +134.70 -36.00 180.00 1.50 -2346.2082

-31.00 +148.10 -31.00 180.00 0.90 -2345.8681

+39.30 +64.60 +39.30 180.00 0.40 -2346.4347

+30.10 +47.60 +30.30 180.00 -3.80 -2346.2062

+20.40 +38.00 +20.40 180.00 1.10 -2345.8594

this stage, new species are involved in the hole transport, the oxidized derivatives of DPBT, that is, 1+ and 2+ cations. The presence of such species at the interfaces could affect the injection process in a remarkable way.34,35 To discuss the changes of electronic and molecular structures induced by oxidation of DPBT molecules, we carried out quantum-chemical calculations for the positively charged species. Geometries of the oxidized species were obtained at the same level of theory as for the neutral ones, starting from the B3LYP/6-31G** optimized structural parameters of the corresponding more stable neutral isomers, L-trans (- - -) and L-cis (- + -), Z-trans (- + -) and Z-cis (+ + +), with both OH and OCH3 functional groups. The comparison between neutral (0), singly (1+), and doubly (2+) charged L-OH DPBT, in terms of optimized torsion angles, is reported in Table 5 for both cis and trans conformers. As expected, the charging processes (0f1+ and 1+f2+) force a planar conformation for both conformers (the R torsion angle becomes =180° or =0°) as a result of the change to a more quinoid structure upon charging. Also, trans and cis Z-OH conformers are forced toward a planar geometry during the charging process, as in the L-OH case (see the data in Table 5). However, due to different steric effects, the relaxation of the torsion angles differs substantially: in Z-OH trans the central R torsion angle changes from 110.70° to 148.10° (∆R ) 37.40°); in the Z-OH cis it changes from 64.60° to 38.00° (∆R ) -26.60°). In both cases, a full planarization of the molecular structure is not allowed. The structural relaxation associated with the oxidation processes (1+ and 2+) also affects the BLA (bond length alternation) of CC bonds along a given backbone path (details are reported in the Supporting Information). To underscore the differences in the L and Z structures upon charging, we restricted the attention to CC bonds in the internal 2Th unit and collected the results in Figure 6. The evolution of the BLA (upon charging) on the 2Th unit of L-OH and Z-OH molecules is quite similar (Figure 6). The BLA is completely inverted upon oxidation from the neutral to the doubly charged molecule, except for the Z-OH cis conformer; in this case, an equalization of CC bonds on the 2Th unit occurs for the (1+) charged species, but the inversion of the bond alternation does not occur for the (2+) species. This behavior can be associated with the relatively distorted structure, which hinders a more efficient conjugation of π electrons of the two thiopene rings even in the 2+ charged state.

The structural changes associated with charging are reflected in a shift of the frontier orbitals that can be monitored by computing the one electron energy levels in the neutral configuration at the geometries of the charged derivatives,36,37 as shown in Figure 7. As expected, the HOMO orbital shifts upward for all species considered. Accordingly, we can infer that the injection process from the electrode into charged species will be characterized by a much lower activation barrier, and hence it will be strongly favored as compared to the injection into the neutral species. Comparison between L and Z DPBT compounds (see Figure 7) shows that the hole injection barrier for the L is lower than that for the Z species, also in the presence of charged molecules. However, we expect a qualitatively most relevant change upon charging the Z compound, whose hole injection efficiency should be remarkably enhanced with respect to the neutral case. This result implies that, in the presence of charged species, hole injection may not be the dominant process determining and limiting the electrical behavior (the appearance and the magnitude of the ON current). Rather, the efficiency of bulk charge transport across the organic material may become of primary importance. For this reason, we will discuss the intermolecular charge transport (and relative molecular parameters) in the next section. C. Reorganization Energies and Bulk Charge Transport. The above discussion implies that the low OFF current is determined by a relatively large hole injection barrier for both L and Z species, which limits (and determines) the conductivity, regardless of the efficiency of the charge transport processes whithin the organic layer. In contrast, the larger ON current is driven by a small injection barrier due to the formation of a layer of oxidized organic molecules at the electrodesemiconductor interface. In this regime, the relevant process that mainly affects the conductivity should be the charge transport mechanism across the bulk of the organic material. In other words, if the hole injection is not the rate-determining step (low injection barrier), the efficiency of the charge transport mechanism across the organic material will determine the charge mobility and, ultimately, the magnitude of electric current. Under these assumptions, we have to take into account the fact that, after the transition to the ON state, new charge carriers can contribute to the transport process, oxidized molecules that can be present also in the bulk of the organic material. We assume that the mechanism for the hole transport is mainly due to intermolecular hopping. According to the above discussion, we can conceive several hopping events, involving neutral,

Structure of a New Class of Diphenyl-bithiophenes

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18635

singly charged, or doubly charged species. Among others, we can preliminarily restrict our attention to processes characterized by identical products and reactants such as:

M0 + M1+ T M1+ + M0 M +M 0

1+

M

2+

+M

2+

(I)

+M

(II)

TM + M

(III)

2+

TM

2+

0

1+

where Mn is a given DPBT molecule in the nth electronic state (0, 1+, and 2+). These processes can be described in the framework of the Marcus semiclassical formulation of the charge transport rate constant.38,39 Because of the fact that ∆G0 ) 0 for the processes considered, the rate constant (kET) simplifies to:

kET )

[ ]

1 -λ 4π V2 exp h 4πλk T ij 4kBT √ B

(1)

where λ is the reorganization energy for the intermolecular charge transfer, Vij is the electronic coupling integral between neighboring molecules, h is Planck’s constant, kB is the Boltzmann constant, and T is the absolute temperature. In this section, we wish to discuss the differences among the above three hopping events by focusing the attention to the reorganization energy. The reorganization energy λ is determined by two contributions: the first represents the effect of the sourrounding media (e.g., outer sphere reorganization energy λ0), while the second term provides a measure of the intramolecular electron-

vibration interaction occurring upon charging. In this work, we considered the intramolecular contribution and computed this term for all of the stable species (Z and L, cis and trans isomers) discussed in the previous sections, by using the adiabatic potential (AP) method,22,40,41 according to the expression: 1 2 λ ) λrel + λrel ) (Eji - Ejj) + (Eij - Eii)

(2)

where Eij represent the value of the SCF energy calculated by using the optimized geometry of the species with charge i evaluated on its jth charge electronic state (in our case, i and j can be 0, 1+, and 2+, respectively). The λ parameters associated with the charge transfer (CT) reactions listed above are reported in Table 6. The changes of torsion angle R upon charging are also reported in Table 6 to compare the extent of this structural effect upon charging. As expected, the λ associated with reaction II is quite large for both DPBT species (Z-OH and L-OH) and in particular for the Z-OH trans molecule. This CT process corresponds to the creation of a doubly charged cation starting from the neutral molecule, thus implying a high reorganization energy. The large geometry rearrangement is also confirmed by the geometrical parameters ∆R and by the BLA reported in Figure 6. Conversely, in the (I) and (III) CT reactions, the λ values are slightly smaller, although still quite large. In particular, for the Z species the reorganization energies are always greater than for the L species, indicating a remarkable charge trapping effect. L species

Figure 6. BLA (for neutral and charged states) along the CC bonds sequence in the 2Th unit as defined in Figure 1. (Top) L-OH DPBT. (Bottom) Z-OH DPBT.

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Fazzi et al.

are characterized by lower λ for the (III) CT process than for the (I) CT reaction, whereas for the Z species λ remains almost constant. The magnitude of the reorganization parameters affects the kET hole transfer rate. Although the influence of charge transfer integrals has to be considered, assuming similar Vij values for the Z and L compounds, one is led to conclude that the rate of charge transfer (and hence the mobility) should be slightly higher for the L compound than for the Z, especially for processes involving only charged species (like III). Focusing on the Z compound, we can conclude that the rate constants associated with hopping (I) and (III) should be quite similar, while process II should be slower. Because process I is likely to contribute to the current both in the OFF and in the ON state, and process III can take place only in the presence of a population of oxidized species (ON state), the calculated reorganization parameters do not suggest relevant differences between hole mobilities in the OFF and ON state. At the present stage, this result supports the conclusion inferred on the basis of computed orbital energies, that the resistivity in the OFF state is determined by the activated hole injection process, whereas the ON current is likely to be determined by bulk charge transport and the ON state is characterized by charge trapping mechanisms (high λ values). Reorganization parameters were also computed for the OCH3 derivatives of the L and Z compounds, and the results are collected in Table 7. We can notice that the reorganization energies of Z-OCH3 are in general larger than those of the Z-OH molecules, especially those involving only charged species. The same trend in the λ values is found also for L-OCH3. This suggests that, in the hypothesis that the transfer integrals are scarcely sensitive to the different chemical substituents, the presence of the OCH3 functional group may result in a lowering of the charge mobility. Finally, we remark that the computed reorganization parameters of DPBTs are quite large as compared to those of high mobility organic semiconductors like tetracene, pentacene, or rubrene.21,22,42 This fact indicates that the injected charges are more strongly trapped in DPBT, a characteristic which is expected to be valuable to obtain materials with a bistable behavior35,43-45 (notice that oxidized species with a good stability are required to lower the barrier to hole injection and the present study quantifies the effect in terms of computed molecular parameters). According to the information collected from the calculations, we can summarize the difference between Z and L compounds as follows: DPBT molecules show slightly different reorganization parameters, pointing to a possibly larger mobility of the L species as compared to the Z molecules. It is clear, however, that besides bulk charge transport, also the efficiency of the TABLE 6: Reorganization Energies for the Three CT Reactions Involving DPBT-OH DPBT

1 λrel (eV)

2 λrel (eV)

λ (eV)

∆R

CT reac.

Z trans

0.25 1.02 0.25 0.20 0.88 0.23 0.20 0.70 0.17 0.21 0.71 0.17

0.24 0.98 0.23 0.20 0.74 0.20 0.22 0.77 0.16 0.25 0.84 0.16

0.49 2.00 0.48 0.40 1.62 0.43 0.42 1.47 0.33 0.46 1.55 0.33

24.00° 37.40° 13.40° -17.00° -26.60° -9.60° 20.00° 20.00° 0.00° -25.50° -25.50° 0.00°

I II III I II III I II III I II III

Z cis L trans L cis

Figure 7. Frontier orbital energies (HOMO (red) and LUMO (blue)) computed for the neutral configuration at the geometries of both neutral (geom: 0) and singly charged (geom: 1+) species of the more stable trans conformers of both L-OH and Z-OH DPBT.

TABLE 7: Reorganization Energies for the Three CT Reactions Involving DPBT-OCH3 DPBT

1 λrel (eV)

2 λrel (eV)

λ (eV)

∆R

CT reac.

Z-OCH3

0.30 1.30 0.35 0.19 0.74 0.20

0.28 1.15 0.40 0.22 0.80 0.19

0.58 2.45 0.75 0.41 1.54 0.39

29.00° 33.80° 5.00° 17.00° 17.00° 0.00°

I II III I II III

L-OCH3

injection process has to be considered, to understand the bistability behavior shown by DPBT materials. In particular, our calculations indicate that the hole injection is an activated process for the OFF state, with an energy barrier higher for the Z molecule than for the L one. Once the molecules become charged, one electron levels shift upward, and their energy becomes better aligned with the work function of the electrodes, enhancing the carrier injection. At this stage, the injection becomes a fast process, and the efficiency of charge transport within the organic semiconductor may become the relevant process determining the ON current. The magnitude of the computed intramolecular reorganization parameters suggests that a remarkable trapping of the charge carriers is likely to occur, especially for the Z species. V. Conclusions We have presented a quantum-chemical study on two isomers (L and Z) of DPBT: these organic molecules have been recently synthesized and tested as active components in nonvolatile organic memory cells. Several conformers have been considered, and the most stable isomers for each species have been determined. The structural determination has been successfully validated by the experimental crystal structure of the Z-OCH3 derivative, showing that the only conformer present in the crystal is that predicted as the most stable from quantum-chemical calculations. Although not foreseeable with certainty, this fact is a very reasonable result, considering the modest intermolecular interactions of the sterically highly strained Z-OCH3 molecules in crystals. To investigate the nature of the bistability behavior of DPBTs, quantum-chemical calculations have also been carried out on the positively charged species of the Z and L systems. Upward shifts of the one electron HOMO levels have been found for the charged species as compared to the neutral molecules and employed to estimate the efficiency of the hole injection process. The results indicate that the injection process at the interface is certainly activated for the neutral species, with a lower activation for L DPBT as compared to the Z species.

Structure of a New Class of Diphenyl-bithiophenes Within the framework of the Marcus formalism, intramolecular reorganization parameters have been evaluated. These provide some indication on the efficiency of the intermolecular charge transport across the organic material and on the stability of the trapped charges. The results of the calculations suggest that both processes (injection and charge transport) have to be considered to rationalize the observed electrical bistability and the different behavior of Z and L molecules. In particular, the calculated molecular parameters suggest that the conductivity in the OFF state is limited by an activated hole injection process, while the ON current is connected with the formation of charged species and is likely to be limited by an activated charge transport mechanism. Acknowledgment. We gratefully thank Dr. A. Bianco and Dr. E. Canesi for helpful scientific discussions and for the preparation of Z-OCH3 single crystal sample. We express our gratitude to Professor M. Sampietro and his research group for the experimental electrical characterization of DPBT molecules. This work has been partly supported by grants from the Italian Ministry of Education, University and Research through FIRB projects “Molecular compounds and hybrid nanostructured materials with resonant and non resonant optical properties for photonic devices” (RBNE033KMA), and by PRIN projects “Molecular materials and nanostructures for photonics and nanophotonics” (2004033197) and “Organic functionalized organized materials: characterization, organization processes and advanced technological applications” (2006034121). Supporting Information Available: PES scans, relative energies, X-ray structures, and structural reorganization of molecules and fragments. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Scott, C.; Bozano, L. D. AdV. Mater. 2007, 19, 1452. (2) Ling, Q. D.; Liaw, D. J.; Teo, E. Y. H.; Zhu, C.; Chan, D. S. H.; Kang, E. T.; Neoh, K. G. Polymer 2007, 48, 5182. (3) Mukherjee, B.; Pal, A. J. J. Phys. Chem. B 2003, 107, 2531. (4) Mukherjee, B.; Batabyal, S. K.; Pal, A. J. AdV. Mater. 2007, 19, 717. (5) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnel, J. D.; Stapleton, J. J.; Price, D. W., Jr.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303. (6) Solak, A. O.; Ranganathan, S.; Itoh, T.; McCreery, R. L. Electrochem. Solid State Lett. 2002, 5, E43. (7) Hefczyc, A.; Beckmann, L.; Becker, E.; Johannes, H. H.; Kowalsky, W. Phys. Status Solidi A 2008, 205, 647. (8) Canesi, E. V.; Dassa, G.; Botta, C.; Bianco, A.; Bertarelli, C.; Zerbi, G. The Open Chem. Phys. J. 2008, 1, 23. (9) Caironi, M.; Natali, D.; Sampietro, M.; Bertarelli, C.; Bianco, A.; Dundulachi, A.; Canesi, E.; Zerbi, G. Appl. Phys. Lett. 2006, 89, 243519. (10) Caironi, M.; Natali, D.; Canesi, E.; Bianco, A.; Bertarelli, C.; Zerbi, G.; Sampietro, M. Thin Solid Films; 2008, doi: 10.1016/j.tsf.2008.03.013. (11) Canesi, E.; Natali, D. Private communication. (12) Das, B. C.; Pal, A. J. Org. Electron. 2008, 9, 39. (13) Bandyopadhyay, A.; Chowdhury, A.; Pal, A. J. Opt. Mater. 2006, 28, 1432. (14) Hu, J.; Li, Y.; Ji, Z.; Jiang, G.; Yang, L.; Hu, W.; Gao, H.; Jiang, L.; Wen, Y.; Song, Y.; Zhu, D. J. Mater. Chem. 2007, 17, 3530. (15) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliteni, A. G. G.; Polidori, G. P.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115. (16) Sheldrick, G. M. “SHELXL97”. Release 97-2. Program for the Refinement of Crystal Structures; University of Go¨ttingen: Germany, 1997. (17) Dewar, M.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 2338.

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