Design of TTF-Based Phosphazenes Combining a Good Electron

Published on Web 08/10/2006 ... semiconductor16 and one of the best characterized n-type ... side and central groups, on the way to design some new na...
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J. Phys. Chem. B 2006, 110, 16852-16859

Design of TTF-Based Phosphazenes Combining a Good Electron-Donor Capacity and Possible Inclusion Adduct Formation Godefroid Gahungu,†,‡ Bin Zhang,§ and Jingping Zhang*,† Faculty of Chemistry, Northeast Normal UniVersity, Changchun 130024, China, Departement de Chimie, Faculte´ des Sciences, UniVersite´ du Burundi, B.P 2700, Bujumbura, Burundi, and Organic Solid Laboratory, CMS, Institute of Chemistry, Beijing 100080, China ReceiVed: April 28, 2006; In Final Form: June 26, 2006

The physical properties of porous material can be modulated by intercalation of small molecules, whose size, in the case of tris-o-phenylenedioxy)cyclotriphosphazene (TPP)-based materials, vary with the different side groups. Starting from the TPP structure, a series of new derivatives were constructed through the core ring [(NP)3] substitution by [(CNH)3], [(CO)3], and [(CS)3] or/and the side group substitution by tetrathiafulvalene and a series of related fragments including bis(ethylenedithio)tetrathiafulvalene, 2-methylene-1, 3-dithiole, and 2-methylene-5,6-dihydro[1,3]dithiolo[4,5-b][1,4]dithiine. In the side fragment, such a substitution corresponds to the replacement of a ring heteroatom, an addition of substituents, or both. With use of theoretical methodologies based on DFT-B3LYP/6-31G* and HF/6-31G**//B3LYP/6-31G* approaches, molecular geometries and electronic properties including the LUMO and HOMO energies, the HOMO-LUMO gap, as well as the ionization potential (IP) were calculated. In comparison with the commonly used organic superconductors, most of the molecules investigated were predicted to show comparable or better electrondonor strength. Interestingly, a number of cyclophosphazene [(NP)3]-containing compounds were predicted to show the “paddle wheel” shape responsible for inclusion adducts formation, making these compounds to be potential candidates for organic superconductors with the ease of modulating their conducting properties by intercalation of suitable acceptors.

1. Introduction The first two decades after the first synthesis of tetrathiafulvalene (TTF: Chart 1i) in 19701 have witnessed an impressive development of TTF chemistry in the search for improved electrically conducting materials.2 Indeed, shortly after Wudl reported the synthesis of TTF, the first organic metal tetrathiafulvalene-tetracyano-p-quinodimethane (TTF-TCNQ) was discovered.3,4 In 1979, the first molecule-based superconductor based on Bechgaard salts [TMTSF]2X (X ) PF6-, AsF6-)5,6 was prepared by using a selenium-containing TTF analogue, namely tetramethyltetraselenafulvalene (TMTSF, Chart 1i). Since then, a huge amount of work has been carried out to enhance the electron-donating power of TTF analogues to improve the conductivities of salts and charge-transfer (CT) complexes derived from them and an ever growing number of new compounds are being added to this class of metal-like organic solids. Considerable efforts have focused on the preparation of new derivatives of TTF, through the replacement of ring heteroatoms,7 addition of substituents,8 or both.9 These structural modifications cause tremendous changes in the conductivity of their charge transfer salts. From the theoretical investigation point of view, ab initio quantum mechanics was used to examine the structures for BEDT and a number of donors in both the oxidized (M+) and neutral (M) forms.10 It was demonstrated that all donors for superconductors lead to boat conformation * Address correspondence to this author. E-mail: [email protected]. † Northeast Normal University. ‡ Universite ´ du Burundi. § Institute of Chemistry.

CHART 1: Chemical Structures for (i) Tetrathiafulvalene (TTF) and Tetramethyltetraselenafulvalene (TMTSF), (ii) Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF), Bis(ethylenedioxy)tetrathiafulvalene (BETS-TTF), and Bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF), and (iii) Tris(o-phenylenedioxy)cyclotriphosphazene (TPP)a

a The numbering scheme used in the text for the atoms in the TTF region and phenylenedioxyl side of BEDT and TPP (respectively) are also shown.

for M and the planar one for M+. As an electron hops from the former to the latter, the original M distorts from the boat to the planar, while the original M+ distorts from the planar to the boat, leading to a coupling between conduction electrons and vibration that was believed to be the salient coupling for superconductivity. Therefore, it was hypothesized, based on these considerations, that TTF-like molecules, which lead to boat-distorted neutral, will result in superconductivity when combined with suitable acceptors.10 Through the iodine (I2) intercalation to tris(o-phenylenedioxy)cyclotriphosphazene (TPP: Chart 1iii) crystals,11 it has

10.1021/jp062629f CCC: $33.50 © 2006 American Chemical Society Published on Web 08/10/2006

TTF-Based Phosphazenes CHART 2: The Modified Core and Side Groups Starting from TPP Structure Where [(CO)3] (1), [(CS)3] (2), and [(CNH)3] (3) Replace [(NP)3] (4), the o-Phenylenedioxyl Side Group Being Systematically Substituted by One of the TTF-like/Modified-TTF-like Donor Fragments (a-r)

been shown that physical properties of intercalated porous material can been modulated by intercalation of small molecules. In fact, the trigonal arrangement of phosphazene molecules provides a hexagonal channel structure,12-15 wherein I2, a 2D semiconductor16 and one of the best characterized n-type molecular donors for the formation of charge-transfer complexes,17 forms chains by inward diffusion and crystallization. By using this approach, an electrical conductivity of ca. 10-610-8 Ω-1 m-1 was then obtained11 with TPP, a phosphazene containing compound, belonging to a growing family of cyclophosphazenes and the simplest member of the class of spirocyclic phosphazene molecules that form tunnel inclusion adducts, also called clathrates.18-20 Due to the versatility of phosphazene chemistry, chemical synthesis has allowed variable tunnel diameter by the choice of different side groups.13-15 Very recently, perfluorinated triazines [2,4,6-tris(p-bromotetrafluorophenoxy)-1,3,5-triazine and 2,4,6-tris(pentafluorophenoxy)-1,3,5-triazine] were also reported to have inclusion character in their crystal structures.21 Viewed from the chemical structure point, the later two correspond to the cyclophosphazene replacement by triazine in TPP, where the phenylenedioxyl side group is substituted by fluorine or both fluorine and bromine. Taking advantage of the possibility to modify both the TPP side and central groups, on the way to design some new native conductive porous materials, which may have interesting properties associated with the special crystal stacking of the molecule parent (TPP), TTF and a series of related fragments including bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF, further abbreviated to BEDT), 2-methylene-5,6-dihydro[1,3]dithiolo[4,5-b][1,4]dithiine (MDDD), 2-methylene-1,3-dithiole (MD), and substituted derivatives are used as parts of the side fragments replacing the phenylenedioxyl in TPP. In these compounds, such a substitution corresponds to the replacement of a ring heteroatom, an addition of substituents, or both (see Charts 1 and 2 for details). For the central ring, however, only the replacement of a ring heteroatom is envisaged. In this way, one may pretend

J. Phys. Chem. B, Vol. 110, No. 34, 2006 16853 to obtain a kind of material having good electron-donor ability and, at the same time, offering the possibility of tunnel formation. Observation of CT transitions when organic electron donors and acceptors are allowed to interact is now a commonplace phenomenon. The relatively high energies for the highest occupied molecular orbitals (HOMO) of the organic donors is known as the origin of the exhibited long wavelength charge transfer (CT) transitions with acceptors.22 This property is manifest in low ionization potentials (IP),22 low oxidation potentials,23 and a very high propensity to oxidative coupling. Theoretically, the electron-donor strength can then be related to the HOMO energy or the IP of a molecule and its relevance to hole injection from electrodes, or from other organic materials, and in applications involving ground-state or photoinduced electron transfer. It should be noted, however, that these secondary properties may also be influenced by their molecular environment, solid-state effects, and intermolecular interactions. All of these factors may affect the IPs of the studied systems differently and thus, gas-phase IPs may not correlate precisely with solid-state IPs or with solution electrochemical data. In this work, we exploited these ideas on the way to suggest a number of new donors as candidates for forming porous superconductors, i.e., combining the capability of inclusion compounds formation and good electron-donor ability. On the other hand, some other electronic properties including the LUMO and HOMO energies and the HOMO-LUMO gap were also predicted. In addition, the correlation between these and the molecular structures is also discussed.The reported results are based on both density functional theory (DFT) and ab initio approaches on the series of models whose, to the best of our knowledge, neither experimental nor theoretical work has been reported so far. 2. Computational Details Starting from the TPP molecular structure (Chart 1iii), the new series of models (M) were constructed by replacing the o-phenylenedioxyl side group by a number of TTF-like fragments as described in Chart 2 (a-r), within C3 symmetry. Representative models on this series are shown in Figures 3 and S1 (given in the Supporting Information). The optimized equilibrium structures for both the neutral (M) and the oxidized (M+) forms, harmonic frequencies, and total energy were calculated based on the DFT method, using the restricted and unrestricted (for the neutral and cationic forms, respectively) hybrid B3LYP that combines Becke’s three-parameter nonlocal exchange functional24,25 with the nonlocal correlation functional of Parr and co-workers26 employing the 6-31G*27-29 Gaussian basis set for each atom. From the calculated 〈S2〉 values, the spin contamination included in the present calculation results was confirmed to be no more than 3.58% (see Table S1 in the Supporting Information for details). For energy calculation, diffuse functions30 needed for cation descriptions were taken into account in some cases. IP of the molecules was calculated as described in the equations below:

IP ) E(M+) - E(M0)

(1)

IP ) -EHOMO

(2)

where E(M0) is the B3LYP/6-31G*(6-31+G*)//B3LYP/6-31G* total energy of the neutral form at the optimized geometry and E(M+) is the total energy of the cationic form, EHOMO being the HOMO energy (according to Koopmans’ theorem31,32) at

16854 J. Phys. Chem. B, Vol. 110, No. 34, 2006

Figure 1. (i) The HOMO of BEDT and (ii) SOMO of BEDT+, both computed with B3LYP/6-31G* (both contoured at 0.03 e au) at the corresponding optimized structures. H, C, and S atoms are shown in white, gray and yellow colors, respectively.

the HF/6-31G**//B3LYP/6-31G* level. All calculations were carried out with the aid of the Gaussian 98 package33 except for larger systems (with more than 60 atoms), whose molecular geometry optimizations were calculated with Gaussian 03 suite of programs34 on a PC-based LINUX cluster. The optimized geometries and molecular orbitals were plotted with the Molekel 4.3 version.35 3. Results and Discussion 3.1. Validation of the Chemical Model. Cation-radical salts based on the donor BEDT have been the most fruitful class of materials36 and well over half the organic radical cation-based superconductors are derived from BEDT.37 To evaluate the performance of the chemical models adopted in this work, most of the BEDT related results are compared to its (available) experimental and theoretical results as shown in Tables S2 and S3 (see the Supporting Information). In Chart 1ii, we show the chemical structure of the BEDT molecule, where the definitions of the carbon and sulfur sites as used in the discussion of our results are also given. The corresponding optimized structure is displayed in Figure 1 in both the neutral (Figure 1i) and cationic forms (Figure 1ii), where the HOMO and highest Singly occupied spin unrestricted natural orbital (SOMO) are also plotted, respectively. From these results (Table S1, Supporting Information), a sufficiently good agreement can be observed between our results and available theoretical10 and experimental data,38-40 except for the parameters involving the terminal -CH2CH2- group. So far, such discrepancies were ascribed to the effect of neglecting the anion layer, which is observed in the crystals, and to the ambiguity in the XRD structural measurement since BEDT molecules can take both the staggered and eclipsed conformations.10,41 Also found in excellent agreement with experimental data for BEDT is the B3LYP/6-31G* predicted electronic structure. Indeed, both our results and the scanning tunneling microscopy by Bar et al.42-44 are consistent with the fact that both the HOMO and SOMO, respectively for BEDT and BEDT+, are of π-orbital character largely concentrated on the TTF region. As shown in Figure 1, these orbitals are of π character, dominated by pπ-orbitals of the C and S atoms belonging to the TTF moiety, with some not negligible contributions from the S atoms of the six-membered ring in the case of SOMO. From the results listed in Table S2 (Supporting Information), one can find that compared to available theoretical data, those provided by the current study show ca. 0.1 eV accuracy improvement. Also, it can be found that available experimental gas-phase IPs are between these estimates. This is ascribed to the smaller correlation error for the positive ion than the neutral

Gahungu et al. when based on the total energy differences, usually leading to an underestimated IP value, while through Koopmans’ theorem, one assumes that the orbitals do not relax upon ionization, usually leading to an overestimated value. The average of the IPV and IPKT values which often gives a good estimate for the IP leads to the order BEDT (6.66 eV) > TTF (6.53 eV) > BEDO (6.47 eV). This is consistent with some gas-phase experimental results45 but different from others.46 For the donors taken as reference in our study, IPs are found to be very close to each other. It is then reasonable to think that their electron-donor ability may be affected by their molecular environment. Thus the oxidation potentials in solution are ordered as BEDT > BEDO > TTF, which differs from the order of the gas-phase first IP. Such polarization effects should also be important in crystals. Hence, the structures and packing will influence the charge-transfer properties of the salts of TTFbased donors. This is responsible for the remarkable variety of electronic properties in the molecular crystals based on TTF donors. In this study, all these parameters were not taken into account, and thus relevant results assume the investigated systems in the same molecular environment (gas phase). However, their use is not expected to mitigate against the conclusions presented in this study. In a number of previous works on a series of cyclophosphazenes of the type (NPX2)3 (for a wide set of X substituents),47-49 it has been shown, through the use of the 6-31G*27-29 and DZP50 basis sets, that polarized functions are needed for a good description of their chemistry. In agreement with these, the B3LYP/6-31G* level of theory was successfully used in a comprehensive study of TPP.51 Especially for this, it was demonstrated in the same work that electron correlation is to be taken into account (which is normally done in present day DFT functionals such as B3LYP) for an accurate description of the molecular geometry, while leading to a reliable description of its electronic structure. In combination with the preliminary results obtained with BEDT, reliable results may be reasonably expected for the systems under investigation in this work. Motivated by its successful use in previous works,10 for organic donors with a qualitative atomic composition close to that of most of our systems, a Koopmans’ theory study based on HF/ 6-31G** single calculation at the optimized geometries was also carried out for the sake of comparison, and as a supplement the B3LYP/6-31G* results which seem to underestimate the IP values. 3.2. Molecular Geometries. It is often assumed that the TTF region of the TTF related donors for organic superconductors are flat. Some deviations from planarity have been suggested in crystals containing electron acceptors, (BEDT)nXn.36 Using different levels of theory (HF, MP2, and DFT-B3P86), it had been shown that such organic donors always have a planar TTF region in their cationic form, the neutral being distorted into a boat conformation.52,53 With the aid of B3LYP, the same feature was observed by Liu et al.54 for BEDT. From the molecular geometry point of view, our computational results are in good agreement with available experimental and theoretical studies for BEDT, TMTTSF, BETS, and BEDO, according to which the neutral form has a boat conformation, the corresponding oxidized form always having a planar TTF region (see Figure 1 for BEDT). The origin of the boat distortion was reported to be the behavior of the θ angle (C-X-C bond angle with X ) S, Se) in the pentagon ring and was suggested as the key factor in determining the boat deformation.10,39,53 In Table S4 (Supporting Information), we list its predicted values in all the investigated molecules. According to these,

TTF-Based Phosphazenes

J. Phys. Chem. B, Vol. 110, No. 34, 2006 16855 TABLE 1: The Predicted Energies for the HOMO, LUMO, and HOMO-LUMO (∆E) for the Compounds Considereda B3LYP/6-31G*// B3LYP/6-31G*

LUMO HOMO SOMO ∆E LUMO HOMO

∆E

TTF(p) TTF(d) TMTSF(p) TMTSF(d) BEDT BETS BEDO

-0.94 -0.91 -0.84 -1.17 -1.06 -1.46 -1.10

-4.52 -4.57 -4.21 -4.54 -4.78 -4.86 -4.46

1a 1b 1c 1d 1e 1f

-0.86 -0.38 -0.88 -0.60 -1.00 -0.64

2a 2b 2c 2d 2e 2f

compd

Figure 2. Comparison between the B3LYP/6-31G* optimized structures for 4l in (i) the neutral and (ii) oxidized forms. Hydrogen atoms are not shown for clarity. (O, C, S, P, and N are shown in red, gray, yellow, orange, and blue, respectively).

Figure 3. (i) The HOMO and (ii) SOMO of 4l (for neutral and oxidized forms respectively), both computed at the B3LYP/6-31G* level (both contoured at 0.03 e au).

the angles at S were predicted in the range of 91-96° and that at Se in the range of 88-93°, in agreement with those reported values for the donors of the reference. In the newly designed molecules of interest in this work, the [(CO)3], [(CS)3], and [(CNH)3] cores are predicted in a chair conformation (see Figure S1, Supporting Information), with C-O, C-N, and C-S bond lengths of ca. 1.38, 1.39 and 1.79 Å, the corresponding C-O-C, C-N-C and C-S-C bond angles being predicted at ca. 113((3)°, 124°, and 96((4)°, respectively. For the [(NP)3]-containing ones, the P-N bond was found to be ca. 1.62 Å in a-f based systems and ca. 1.60 Å in the g-o side fragment containing ones. For this series, the P-N-P bond angle was predicted to be 121((2)°, and the P-N-P-N dihedral angle 18((2)° and ca. 0° respectively for the former and the later. From these results, one may figure out the effect of the side fragment on the central part conformation. When the P atom of the cyclophosphazene is covalently linked to the side group through a double bond (PdC), the core lies in a chair conformation. Linking the side group to the core P atom through two single covalent bonds (P-O) leads to a planar conformation for the core ring. As a consequence, no significant geometrical changes are observed in the TPP residue, both the neutral and oxidized forms retaining the neutral TPP residual geometry for 4g-l (g-l fragment based models). Thus, 4g-l molecules display the “paddle wheel” shape that (in combination with the requirements of the crystal packing) was reported to be responsible for the spontaneous inclusion adducts formation in the case of TPP and some of its derivatives.55 In 4m and 4n, however, a remarkable conformational difference was observed in the bridging part, which is predicted in a twisted conformation, the rest of the

HF/6-31G**// B3LYP/6-31G*

-9.30 -9.30 -8.63 -8.60 -8.40 -8.37 -8.44

3.57 3.65 3.36 3.36 3.72 3.41 3.36

3.02 3.05 2.30 2.44 2.81 2.12 2.79

-6.77 -6.83 -6.49 -6.77 -7.12 -7.17 -6.91

9.78 9.88 8.78 9.21 9.93 9.29 9.71

-4.83 -4.56 -4.82 -4.65 -4.89 -4.38

[(CO)3] core -7.59 3.97 -7.82 4.18 -7.71 3.93 -7.31 4.05 -7.35 3.89 -7.28 3.74

2.91 3.73 2.92 3.40 2.71 3.41

-7.33 -7.02 -7.29 -7.13 -7.38 -6.97

10.25 10.75 10.21 10.53 10.09 10.38

-1.06 -0.74 -1.03 -0.77 -1.10 -0.77

-5.33 -5.10 -5.11 -5.04 -4.95 -4.83

[(CS)3] core -7.77 4.27 -8.18 4.36 -8.45 4.08 -7.78 4.27 -7.81 3.85 -7.65 4.06

2.69 3.11 2.61 2.98 2.41 2.95

-7.72 10.41 -7.46 10.57 -7.36 9.98 -7.44 10.42 -7.26 9.66 -6.86 9.80

3a 3b 3c 3d 3e 3f

-1.03 -0.39 -0.70 -0.70 -1.01 -0.47

[(CNH)3] core -4.64 -7.35 3.61 -4.38 -7.46 3.99 -4.76 -7.55 4.06 -4.59 -7.31 3.89 -4.99 -7.42 3.97 -4.39 -7.14 3.92

2.73 3.62 3.14 3.20 2.74 3.51

-7.02 -6.69 -7.13 -6.98 -7.46 -6.87

4a 4b 4c 4d 4e 4f 4g 4h 4i 4j 4k 4l 4m 4n 4o 4p 4q 4r TPP

-2.18 -1.92 -1.87 -2.09 -2.03 -1.90 -1.53 -1.17 -1.67 -1.37 -1.82 -1.32 -1.08 -1.46 -1.26 -1.64 -1.31 -1.32 -0.29

-4.61 -4.25 -4.41 -4.46 -4.72 -4.13 -4.96 -4.63 -4.72 -4.67 -4.69 -4.57 -4.83 -4.91 -4.72 -4.75 -4.76 -4.66 -6.33

[(PN)3] core -7.09 2.43 -7.42 2.32 -7.79 2.53 -7.06 2.37 -7.09 2.69 -6.94 2.23 -6.92 3.43 -6.81 3.46 -6.74 3.05 -6.66 3.31 -6.61 2.87 -6.64 3.24 -6.74 3.75 na 3.46 -6.89 3.46 -6.93 3.11 -6.77 3.45 -6.64 3.34 -9.69 6.04

0.76 1.10 1.12 0.84 0.98 0.98 2.52 2.69 1.94 2.50 2.59 2.56 2.85 2.20 2.65 1.98 2.56 2.55 3.50

-6.75 7.50 -6.24 7.35 -6.49 7.60 -6.59 7.43 -6.99 7.97 -6.30 7.28 -7.02 9.55 -6.95 9.64 -6.91 8.86 -6.95 9.45 -6.98 9.58 -6.92 9.47 -7.04 9.89 -7.08 9.28 -7.04 9.70 -7.06 9.04 -7.12 9.68 -7.05 9.60 -8.81 12.31

9.75 10.31 10.26 10.18 10.20 10.38

a Energies are given in eV; (p) and (d) refer to the planar and distorted conformations (respectively) in the neutral form; na ) calculation not achieved.

side fragment lying in a planar conformation not perpendicular to the cyclophosphazene ring (see Figure S2, Supporting Information). 3.3. Electronic Properties. In Tables 1 and 2 (also Tables S5 and S6, Suppoting Information), we present the predicted electronic properties for the series of compounds considered, at different levels of theory (HOMO, LUMO, LUMO-HOMO (∆E)) and energies (Table 1) as well as the IPs (Table 2). From the results presented in Table 1, it can be seen that the magnitudes of the eigenvalues and their relative differences are different within DFT and HF approaches, the main feature being not their absolute eigenvalues, but the relative tendency of the values. For a good interpretation of these results, Figures 4 and

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Gahungu et al.

TABLE 2: Computed Ionization Potentials (IPv, IPa, and IPKT) as a Function of Computational Approacha compd

IPvb

IPac

IPKT d

exptl

TTF(p) TTF(d) TMTSF(p) TMTSF(d) BEDT BETS BEDO

6.29 (6.44) 6.34 (6.49) 5.82 (6.01) 6.10 (6.27) 6.20 (6.33) 6.24 (6.39) 6.03 (6.22)

6.15 (6.30) 6.15 (6.31) 5.68 (5.87) 5.81 (5.94) 5.92 (6.07) 6.01 (6.39) 5.74 (5.93)

6.77 6.83 6.49 6.79 7.12 7.18 6.91

6.7;e 6.4f

1a 1b 1c 1d 1e 1f

5.96 5.90 6.13 5.77 6.02 5.59

[(CO)3] core 5.67 5.63 5.71 5.52 5.93 5.35

7.33 7.02 7.29 7.13 7.38 6.97

2a 2b 2c 2d 2e 2f

6.44 6.57 6.46 6.12 6.14 6.00

[(CS)3] core 6.04 6.32 6.37 5.89 5.93 5.80

7.72 7.46 7.36 7.44 7.26 6.86

3a 3b 3c 3d 3e 3f

5.75 5.69 6.03 5.69 6.05 5.59

[(CNH)3] core 5.43 5.25 5.57 5.43 5.70 5.25

7.02 6.69 7.13 6.98 7.46 6.87

4a 4b 4c 4d 4e 4f 4g 4h 4i 4j 4k 4l 4m 4n 4o 4p 4q 4r TPP

[(NP)3] core 5.66 5.27 5.49 5.22 5.60 5.29 5.52 5.21 5.72 5.41 5.27 4.95 5.52 5.47 5.51 (5.69) 5.46 (5.64) 5.48 5.45 5.39 (5.56) 5.35 (5.48) 5.39 5.36 5.32 (5.51) 5.26 (5.45) 5.59 5.46 5.76 nag 5.61 5.52 5.54 5.53 5.57 5.48 5.48 5.38 7.52 7.45

6.7;e 6.21f 6.46e

6.75 6.24 6.49 6.59 6.99 6.30 7.02 6.95 6.91 6.95 6.98 6.92 7.04 7.08 7.04 7.06 7.12 7.05 8.81

Figure 4. Plots of IPKT, IPv, and IPa versus -EHOMO for systems built up with different core parts (from 1-4) and the same side fragment a for A and b for B (see Chart 2 for labeling scheme).

a Energies are given in eV; (p) and (d) refer to the planar and distorted conformations (respectively) in the neutral form; values in parentheses were computed with the 6-31+G* basis set. b Vertical ionization potential. c Adiabatic ionization potential. d Ionization potential from Koopmans’ theorem. e Data from ref 45. f Data from ref 46. g na ) calculation not achieved.

5 display the correlation between the IPs and the HOMO energy for systems starting from different donor side fragments and the same core part or vice versa. A global view of the FMO energy evolution vs the chemical structure of the systems under investigation is provided in Figure S3 (see the Supporting Information). 3.3.1. Substitution Effect on the Electronic Structures and Properties. Starting from the commonly known electron donors taken as reference in this work (the first five on the list), one may figure out, from the results given in Tables 1 and 2, that both DFT-B3LYP and HF are consistent with the ELUMO increasing order of BETS < TMTSF < BEDO < BEDT < TTF, the EHOMO increasing one of BETS < BEDT < TMTSF

Figure 5. Plots of IPKT, IPv, and IPa versus -EHOMO for systems built up with different donor side fragments (from a-f in A/g-l in B) and the same core part [(NP)3] (see Chart 2 for labeling scheme).

< TTF < BEDO, and that on the energy gap of TMTSF e BETS e BEDO < TTF < BEDT. Within DFT results, the

TTF-Based Phosphazenes ESOMO was predicted in the increasing order of TTF < TMTSF < BEDO < BEDT < BETS. Note that a relatively significant difference between IP values calculated at HF and B3LYP levels is observed (see comments in the Supporting Information), which does not affect the general trend of our results and thus the expected conclusions from them. From the predicted IP values (independently on the calculation method), the electron-donor ability is found in the increasing order of TTF < BETS < BEDT < TMTSF < BEDO (from both IPv and IPa) and BETS < BEDT < BEDO < TTF < TMTSF with DFT and HF, respectively. From these results, one may figure out that among these electron-donor molecules taken as reference in this work, BEDO, the S/O substituted derivatives of BEDT, show the strongest tendency to be oxidized, in good agreement with a number of previous works, both theoretical10 and experimental.45 3.3.1.1. Effect of Substitution of Side Group. Within the [(CO)3] core based systems, with the side fragments ranging from a to f (see Chart 2), the ELUMO prediction shows the increasing order of 1e < 1c < 1a < 1f < 1d < 1b, while the one for EHOMO being 1e < 1a < 1c < 1d < 1b < 1f with the later being supported by both DFT and HF results. As a result, the ∆E prediction shows the increasing order of 1f < 1e < 1c < 1a < 1d < 1b. The ESOMO increases in the order of 1b < 1c < 1a < 1e < 1d < 1f. Within the [(CS)3], [(CNH)3], and [(NP)3] core parts based systems, with the same side fragments ranging from a to f, almost the same trends can be observed, with some little changes in order being observed especially for EHOMO in [(CS)3] based derivatives. In general, all the results are consistent with an EHOMO increase when the O atom substitutes S, or when the later substitutes Se. The same conclusion can be drawn in the case of ELUMO with a negligible difference (no more than 0.04 eV) being observed between 1d and 1f [(CO)3] derivatives or between 2d and 2f [(CS)3] derivatives. From these results, one may figure out that in general, both S/Se substitution in the five-member ring (compare b to c or d to e based systems) and the H/SCH3 one (compare a to b) lower both the LUMO and HOMO energies. On the other hand, the S/O one in the six-membered ring was predicted to stabilize them. All the trends described above are summarized in Figure 4. In Figure 5, we show the correlation between the predicted IPs and the EHOMO for two groups of systems built up with the same core parts but different side fragments. From this, one may clearly find that the lowest IP values among the a-f containing systems correspond to the f fragment based one, while in the second group, made of g-l fragments, the smallest IP was obtained for the l fragment based system. We may conclude that going from the O heteroatom in the six-membered ring to the five-membered ring of the half-TTF increases the electron-donor ability, in good agreement with previous experimental and theoretical findings according to which BEDO (Chart 1ii: a S/O substituted derivative of BEDT) shows a lower first ionization energy, 10,45 thereby making substituted TTF a better electron donor than TTF itself, leading to donor/acceptor solids with superior conducting and superconducting properties. 3.3.1.2. Effect of the Core Part. To get some insight into the influence of the central part on the FMO energies, one can consider a series of any group of systems built up with the same fragment (a-f) and the four core parts. Taking those starting from the b side fragment (1b, 2b, 3b, and 4b) as an example, the EHOMO is predicted in the increasing order of 2b (-5.10 eV) < 1b (-4.56 eV) < 3b (-4.38 eV) < 4b (-4.25 eV) and the ELUMO in the order of 4b (-1.92 eV) < 2b (-0.74 eV) < 3b (-0.39 eV) < 1b (-0.38 eV) within DFT-B3LYP. In Figure

J. Phys. Chem. B, Vol. 110, No. 34, 2006 16857 4 is shown the correlation between the computed IPs and EHOMO in two groups of systems built up with the same side donor fragment (a in Figure 4A and b in Figure 4B) with different core parts (from 1-4). Also provided by the same figure is the good correlation between the predicted IP value (both IPKT, IPa, and IPv) and the HOMO energy. From this figure and the results summarized in Table 2, the general trend that can be observed is the relatively remarkable IP and EHOMO dependence on the core part within the increasing order of 4 > 3 >1 > 2 for IP and the reverse for EHOMO. These results indicate that with the same side fragment, the best electron-donor ability may be attained with 4, i.e., the [(NP)3] core part. 3.3.2. Comparison with the Commonly Known Electron Donor. Since the electron-transfer (ET) donor strength can be related to the HOMO energy or the IP of a molecule, these properties were calculated to estimate the ET-donor ability of these systems under investigation. From the ET-donor ability point of view, one can figure out that compared to the commonly used electron donors whose predicted IPv (IPKT) ranges from 5.82 to 6.29 eV (6.49-7.18 eV), a comparable ET-donor ability can be reached by adopting the building approach we developed in this work: i.e., 5.59-6.02 eV (6.97-7.38 eV), 5.80-6.37 eV (6.86-7.72 eV), 5.69-6.05 eV (6.69-7.46 eV) respectively for the [(CO)3], [(CS)3], [(CNH)3], and [(NP)3] based systems, with the side fragments ranging from a to f. Especially interesting is the ET-donor strength that is predicted within [(NP)3] based systems. Indeed, the smallest IP value, ca. 5 eV (6.3 eV), i.e., ca. 1 eV (0.53 eV) lower than the one for TTF, was predicted for 4f implying a better ET-donor strength. Note that within the same group, all the systems are predicted to be stronger ET-donors than the commonly used ones taken as reference in this work. It is also noteworthy that all the new systems are predicted to show a better electron-donor strength than the one predicted for TPP. Indeed, both the adiabatic and vertical IP values are predicted to be ca. 1-2.5 eV higher than those predicted for the new compounds. 3.4. Comparison with TPP. TPP and some of its derivatives are known to form stable inclusion compounds with a variety of organic molecules13-15,18,19,56,57 typically not exceeding the width of para-substituted benzene derivatives. Particularly, TPP shows a strong affinity to include gaseous iodine11 and xenon.58 In the case of I2, the Lewis acidity of iodine and the donor capacity of the TPP-phenylenedioxy rings lead to the inclusion compound TPP(I2)0.75 stable up to temperatures as high as about 420 K.11 With TPP and some of its derivatives, the possibility to vary the shape, the diameter, and the chemical environment of adsorption sites is an advantage compared to purely inorganic zeolites. In these, specific host-guest interactions of the donoracceptor type are expected for channels.59 Beside the TPP “paddle wheel” molecular shape and the corresponding crystal requirements, the donor capacity of the side group appears to be another important parameter in the tuning of TPP physical properties. Compared to the TPP one, the molecular geometries of the [(CO)3], [(CS)3], and [(CNH)3] based systems (from 1a to 3f) are predicted to be very far from the former. The [(NP)3] containing systems other than 4g-n can also be added to the same list. Interestingly, 4g-l molecules show a “paddle wheel” shape and may then be anticipated to possibility form channeltype inclusion compounds. In Table 3, we summarize the predicted electronic properties of TPP in comparison with compound 4h. Those include the FMO energies, the gap energy, and the electron-donor strength

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Gahungu et al.

TABLE 3: A Comparative Study of TPP and Compound 4h: Molecular Structure and Some of Their Electronic Properties (see Chart 1iii for the Definition of the parameters) TPP parameter P-N P-O O-C1 C1-C1′ C1-C2 C2-C3 C3-C3′ N-P-N P-N-P N-P-O O-P-O P-O-C1 O-C1-C1′ C1-C1′-C2 C1′-C2-C3 C2-C3-C3′ ELUMO EHOMO ∆E IPv IPa

4h neutral

cationa

bond distance (Å) 1.597 0.002 1.645 -0.005 1.388 0.014 1.394 -0.016 1.382 -0.003 1.403 0.006 1.397 -0.015

1.596 1.646 1.386 1.394 1.382 1.402 1.405

0.000 0.001 -0.004 0.004 -0.002 0.002 0.000

bond angle (deg) 117.263 -0.499 122.737 0.499 110.424 0.040 95.808 0.556 109.921 -0.498 112.175 0.221 121.779 -0.164 117.033 0.564 121.188 -0.400

117.305 122.695 110.415 95.776 109.904 112.209 122.070 116.637 121.291

neutral

-0.29 -6.33 6.04 7.52 7.45

cationa

other propertiesb

0.088 -0.088 0.0370 -0.190 0.150 -0.056 0.097 -0.177 0.082

-1.17 -4.96 3.46 5.51 5.46

a Negative and positive values (for the geometrical parameters) express respectively the increasing and decreasing of the parameters of interest. b Energies are given in eV.

through both the vertical and the adiabatic IPs. In addition, geometrical parameters involving the common part are also provided for both the neutral and the cationic forms of the two molecules. Although based on a quite modest quantum chemical level of calculation (due to the limitation of the computational resources for large systems such as 4g-r), some interesting insights were gained. From the results listed in Table 3, which also resemble those predicted for compounds 4g and 4i-l, it appears clear that TPP geometry is conserved in both the neutral and cationic forms of the new derivatives. In these, the TTF or TTF-like regions (whose corresponding geometrical parameters are not shown) were found to behave as described for BEDT upon the injection of a positive charge. The EHOMO is lifted by about ca. 1.4 eV and the ELUMO lowered by ca. 0.9 eV when comparing TPP to 4h. As a consequence, the gap energy (∆E) decreases by 2.6 eV while the first IP (both IPv and IPa) decreases by ca. 2 eV, implying a net difference in electrondonor strength between TPP and the TTF-like fragment based molecules. Such an important property, combined with their good electron-donor ability as provided by the current study, may make these systems potential candidates as organic superconductors, with the ease of modulation of the conducting properties by the possible intercalation (in the tunnels) of judiciously chosen acceptors. 4. Conclusion Molecular structures and electronic properties were investigated for a series of TTF-like fragments containing systems using theoretical methodologies based on DFT-B3LYP/6-31G* and HF/6-31G**//B3LYP/6-31G* approaches. Judged from the calculated IPs and HOMO energy point of view, a series of potential systems, showing an ET-donor strength comparable

or better than the one for the commonly known ET-donors, were designed. Among these, a special group, that of cyclophosphazene-containing systems, was designed to combine a good ET-donor strength and the “paddle wheel” molecular shape responsible for inclusion adducts formation. From these, tunnels of variable diameter can then be awaited depending on the side group used. This is important since it can provide ease in modulating the conducting properties by intercalation of judiciously chosen acceptors. Acknowledgment. Financial support from the NNSFC (Nos. 50473032 and 20473095), SRF for ROCS, SEM, and Outstanding Youth Project of Jilin Province is gratefully acknowledged. Supporting Information Available: Cartesian coordinates of the relevant structures, summary of the computational results not given in this text, and the 3D geometries of representative models for each TPP substituted derivative. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wudl, F.; Smith G. M.; Hufnagel, E. J. J. Chem. Soc., Chem. Commun. 1970, 1453. (2) Segura, J. L.; Martin, N. Angew. Chem., Int. Ed. 2001, 40, 1372. (3) Ferraris, J.; Cowan, D. O.; Walatka, V. V.; Perlstein, J. H. J. Am. Chem. Soc. 1973, 95, 948. (4) Coleman, L. B.; Cohen, M. J.; Sandman, D. J.; Yamagishi, F. G.; Garito, A. F.; Heeger, A. J. Solid State Commun. 1973, 12, 1125. (5) Andrieux, A.; Duroure, C.; Jerome, D.; Bechgaard, K. J. Phys. Lett. 1979, 40, 381. (6) Jerome, D.; Mazaud, A.; Ribault, M.; Bechgaard, K. J. Phys. Lett. 1980, 41, L95. (7) Lakshmikantham, M. V.; Cava, M. P.; Garito, A. F. Chem. Commun. 1975, 383. (8) Moses, P. R.; Chambers, J. Q. J. Am. Chem. Soc. 1974, 96, 945. (9) Spencer, H. K.; Lakshmikantham, M. V.; Cava, M. P.; Garito, A. F. Chem. Commun. 1975, 867. (10) (a) Demiralp, E.; Goddard, W. A., III J. Phys. Chem. 1994, 98, 9781. (b) Demiralp, E.; Dasgupta, S.; Goddard, W. A., III J. Am. Chem. Soc. 1995, 117, 8154. (c) Demiralp E.; Goddard, W. A., III Synth. Met. 1995, 72, 297. (d) Demiralp E.; Goddard, W. A., III J. Phys. Chem. A. 1997, 101, 8128. (11) Hertzsch, T.; Budde, F.; Weber, E.; Hulliger, J. Angew. Chem., Int. Ed. 2002, 41, 2281. (12) Allcock, H. R.; Allen, R. W.; Bissel, E. C.; Smeltz, L. A.; Teeter, M. J. Am. Chem. Soc. 1976, 98, 17, 5120. (13) Allcock, H R.; Siegel, L A. J. Am. Chem. Soc. 1964, 86, 5140. (14) Allcock, H. R.; Stein M T. J. Am. Chem. Soc. 1974, 96, 49. (15) Allcock, H. R.; Stein, M. T.; Bissell, E. C. J. Am. Chem. Soc. 1974, 96, 4795. (16) Balchin, A. S.; Drickamer, H. G. J. Chem. Phys. 1961, 34, 1948. (17) DeBoer, G.; Burnett, J. W.; Young, M. A. Chem. Phys. Lett. 1996, 259, 368. (18) Hertzsch, T.; Kluge, S.; Weber, E.; Budde, F.; Hulliger J. AdV. Mater. 2001, 13, 1864. (19) Hertzsch, T.; Budde, F.; Weber E.; Hulliger J. Angew. Chem., Int. Ed. 2002, 41, 2282. (20) Maas, H.; Calzaferri, G. Angew. Chem., Int. Ed. 2002, 41, 2284. (21) Reichenbacher, K.; Suss, Heike I.; Stoeckli-Evans, H.; Bracco, S.; Sozzani, P.; Weberd, E.; Hulliger, J. New J. Chem. 2004, 28, 393. (22) Haink, H. J.; Adams, J. E.; Huher, R. J. Ber. Bunsen-Ges. Phys. Chem. 1974, 78, 436. (23) Ambrose J. F.; Nelson, R. F. J. Electrochem. Soc. 1968, 115, 1159. (24) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (25) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (26) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (27) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209. (28) Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. (29) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265. (30) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (31) Koopmans, T. Physica 1934, 1, 104. (32) Richards, W. G. Int. J. Mass Spectrom. Ion Phys. 1969, 2, 419. (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.

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