Design of Tetrathiafulvalene-Based Phosphazenes Combining a

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J. Phys. Chem. C 2007, 111, 4838-4846

Design of Tetrathiafulvalene-Based Phosphazenes Combining a Good Electron-Donor Capacity and Possible Inclusion Adduct Formation (Part II) Godefroid Gahungu,†,‡ Bin Zhang,§ and Jingping Zhang*,† Faculty of Chemistry, Northeast Normal UniVersity, Changchun 130024, China, Faculte´ des Sciences, UniVersite´ du Burundi, Departement de Chimie, B.P. 2700, Bujumbura, Burundi, and Organic Solid Laboratory, CMS, Institute of Chemistry, Beijing, 100080, P.R. China ReceiVed: October 27, 2006; In Final Form: January 18, 2007

Physical properties of intercalated porous material can be modulated by intercalation of small molecules, as this was demonstrated through the iodine (I2) intercalation into tris(o-phenylenedioxy)cyclotriphosphazene (TPP) crystals. This work describes in depth theoretical considerations of TPP derivatives. The core ring [(NP)3] substitution by [(CO)3], [(CNH)3], and [(CS)3], as well as the side group modification in size and composition (containing tetrathiafulvalene-like fragments), is well described from the most important aspects as their geometry optimization, their highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) consideration together with ionization potentials (IP), and their charged forms. On the basis of PBE0/6-31G(d,p) calculations, the neutral forms of the [(CO)3], [(CNH)3], and [(CS)3] containing derivatives are predicted in a distorted caplike conformation. In the first two series, planar (or nearly planar) conformations are predicted upon oxidation, while IP and HOMO energy calculation revealed an electron-donor strength better or comparable to that of the known tetrathiafulvalene (TTF)-like superconductors. Within the [(NP)3] based derivatives, the results show that the geometry of the central part may be influenced by the structure of the side fragment. Among the most interesting results, a number of derivatives are predicted to show a good electron-donor strength compared to the commonly used TTF-like donors. More interestingly, a larger number of [(NP)3] containing derivatives, especially the O/“NH”-substituted series, are found to combine the good electron-donor capacity and the “paddle wheel” molecular shape, making them good candidates for organic superconductors with the ease of modulating their conducting properties by intercalation of suitable acceptors.

1. Introduction Mainly because of the strategic industrial and environmental applications, such as gas storage, selective gas recognition, and separation, the absorption properties of materials are emerging as a forefront issue of present day research.1 In this field, organic zeolites and molecular assembled materials constitute a competing alternative and are thus still to be extensively explored.2 Originally studied by Allcock,3 tris(o-phenylenedioxy)cyclotriphosphazene (TPP, Chart 1i) became a compound of choice to investigate the structural features of organic zeolites and their potential applications. Studies focused on the stability of the hexagonal modification compared to compact guest-free monoclinic,4 the single-crystal X-ray determination of the crystal structure,5 the investigation of gas storage or aromatic guest insertion by advanced NMR techniques6 and X-ray diffraction,7 the confinement of iodine molecules by several crystallization procedures,8 the insertion of dipolar molecules,9 the obtainment of functional materials by inclusion of electroactive molecules,10 and the formation of nanocomposites containing macromolecules.11 Although a number of theoretical works on phosphazene containing systems can be found in the literature,12 very few were devoted to related organic zeolite. * To whom correspondence [email protected]. † Northeast Normal University. ‡ Universite ´ du Burundi. § Institute of Chemistry.

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Through the iodine (I2) intercalation into TPP crystals,8 it has been shown that physical properties of intercalated porous material can be modulated by intercalation of small molecules. In fact, the trigonal arrangement of phosphazene molecules provides a hexagonal channel structure,13 wherein I2, a twodimensional semiconductor14 and one of the best-characterized molecules for the formation of charge-transfer complexes,15 forms chains by inward diffusion and crystallization. By using this approach, an electrical conductivity of ca. 10-6-10-8 Ω-1 m-1 was then obtained with TPP,8 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.8,9,16 Because of the versatility of phosphazene chemistry, chemical synthesis has allowed variable tunnel diameter by the choice of different side groups.13b-d 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.17 Viewed from the chemical structure point, the latter two correspond to the cyclophosphazene replacement by triazine in TPP in which the phenylenedioxyl side group is substituted by fluorine or both fluorine and bromine. From the above statements, one may reasonably expect an interesting type of material that should result from materials exhibiting the tetrathiafulvalene (TTF)-like systems’ (Chart

10.1021/jp067067e CCC: $37.00 © 2007 American Chemical Society Published on Web 03/07/2007

Tetrathiafulvalene-Based Phosphazenes

J. Phys. Chem. C, Vol. 111, No. 12, 2007 4839

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

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

TABLE 1: Structural Parameters for Neutral BO, BO+, and BO+0.5 from Theory and Experiment (BO,39 ClO3BO,40 and [BO-TTF]6K2[BW12O40]‚11H2O41) B3LYP parameter

BO

C-C1 C1-S1 S1-C2 C2-C C2-O2 O2-C3 C3-C

1.351 1.787 1.773 1.341 1.368 1.435 1.528

C-C1-S1 C1-S1-C2 S1-C2-C C-C2-O2 C2-O2-C3 O2-C3-C

122.55 93.01 118.19 124.46 110.3 110.36

BO+ 1.386 1.756 1.752 1.360 1.345 1.452 1.523 122.14 94.71 117.43 124.42 111.71 110.39

expa

PBE0 BO+0.5b 1.3685 1.772 1.763 1.351 1.357 1.444 1.526 122.35 93.86 117.81 124.44 111.01 110.38

BO

BO+

bond length (Å) 1.350 1.386 1.769 1.739 1.760 1.739 1.340 1.359 1.359 1.337 1.424 1.439 1.521 1.516 bond angle (°) 122.38 121.99 92.89 94.73 117.98 117.25 124.54 124.5 109.78 111.16 110.32 110.4

BO+0.5 b 1.368 1.754 1.749 1.349 1.348 1.431 1.519 122.18 93.81 117.62 124.52 110.47 110.36

BOc 1.357 1.771 1.752 1.336 1.368 1.441 1.513 122.95 92.3 118.01 124.45 110 111.13

BO+d 1.369 1.743 1.739 1.343 1.349 1.452 1.522 120.97 94.09 116.75 126.03 110.33 110.65

BO+0.5e 1.354 1.750 1.749 1.352 1.359 1.439 1.503 121.98 93.79 117.73 124.93 110.74 110.33

a Exp ) experimental. bAverage of the optimized BO and BO+ structures. cCrystal data from ref 39. dCrystal data from ref 40. eCrystal data of one of the independent molecules (partially oxidized with a charge of +0.5) from ref 41.

1ii,iii) electron-donor strength and inclusion adduct formation. Indeed, it would become easier, by a judicious choice of acceptors to be included into the tunnels, to modulate their physical properties including the electric conductivity. On the basis of quantum chemical calculation, some potential candidates for porous organic superconductors were designed in the first part18 of our continuous investigation on these kind of systems. Taking advantage of the versatility of phosphazene chemistry allowing variable tunnel diameter by the choice of different side groups, these were chosen so as to contain TTF-like fragments through the replacement of ring heteroatoms, addition of substituents, or both (Chart 1ii,iii). This strategy had been used in the preparation of new derivatives of TTF,19-21 leading to structural modifications and tremendous changes in the conductivity of their charge-transfer salts. TPP and some of its derivatives are known to form stable inclusion compounds with a variety of organic molecules7-9,12e,13b-d typically not exceeding the width of parasubstituted benzene derivatives. Particularly, TPP shows a strong affinity to include gaseous I28 and xenon.6d In the case of I2, the Lewis acidity of iodine and the donor capacity of the TPPphenylenedioxy rings lead to the inclusion compound TPP(I2)0.75 stable up to temperatures as high as about 420 K.8 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 TPP and TPP-like zeolites, specific host-guest interactions of the

donor-acceptor type were expected for channels.5 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. So far, O/NH-substituted derivatives of TPP were synthesized through a partial and total O/NH substitution.22 Nothing has been reported, however, about the influence of such a substitution on the electronic properties. To date, no O/S derivative of the same class has been reported. Preliminary to the current work, simple calculations revealed that this kind of substitution may induce some relatively significant changes on TPP electronic properties, including the first ionization potential (IP) and thus the electron-donor strength, without profoundly altering the paddle wheel molecular shape, one of the keys in the spontaneous inclusion adducts formation in the case of TPP and some of its derivatives.23 In this second part, on the same path to design a class of materials that may fulfill such requirements the two strategies are combined, while (compared to the previous work) an effort was made to improve the accuracy through the use of a higher level of theory. Herein, we describe in depth theoretical considerations of a number of cyclotriphophezene derivatives. The core ring substitutions as well as the side group variations are well described from the most important aspects of their geometry optimization, their frontier molecular orbitals (FMO) (i.e., the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO)) considerations, together with their IPs and charged forms.

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Gahungu et al.

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

IPvb

IPac

IPKTd

exp

TTF TTMSF ET BETS BO

6.30 (6.41) 6.12 (6.26) 6.27 (6.36) 6.31 (6.42) 6.08 (6.22)

6.16 (6.27) 5.82 (5.90) 5.97 (6.07) 6.06 (6.08) 5.77 (5.92)

6.66 6.65 7.04 7.05 6.83

6.70e, 6.40f, 6.55g 6.70e, 6.21f, 6.35g 6.46e

a Exp ) experimental. Energies are given in eV. Values in parentheses were computed with the 6-31+G(d,p) basis set. bVertical IP. cAdiabatic IP. dIP from Koopmans’ theorem. eData from ref 43. f Data from ref 44. gAveraged values calculated using experimental data from ref 43 and 44.

Figure 1. PBE0/6-31G(d,p) optimized structures of BO and compound 1b for both the neutral (i and iii, respectively) and cationic (ii and iV, respectively) forms, as well as the corresponding HOMO and SOMO (both contoured at 0.04 e au). For clarity, H, C, S, and O atoms are displayed in white, gray, yellow, and red colors, respectively.

2. Computational Strategy 2.1. Theoretical Methods. All molecular geometry optimizations were carried out with the aid of the PC-based LINUX cluster version of Gaussian 03 package,24 except for smaller systems (with less than 50 atoms) and single point energy whose corresponding calculations were performed using the single PCbased windows version of the Gaussian 03 package.24 During the geometry optimization, the neutral species were constrained within the C3 symmetry. With the aim of accuracy improvement in the molecular geometries, a number of density functional theory (DFT) methods including the PBE1PBE (also called PBE0),25 B3LYP,26 B3P86,27 BP86,28 and the ab initio HartreeFock (HF) methods were used with the 6-31G(d,p)29-31 basis set in the geometry optimization. The equilibrium structures

were located using analytical energy derivatives. The unrestricted formalism was used for the oxidized form as well as the relative neutral-cation energies obtained at the optimized geometries and uniformly estimated from PBE0/6-31+G(d,p)29-32 calculation including diffuse functions32 needed to describe the cation. From the calculated values, the spin contamination included in the present calculation results was confirmed to be in general no more than 2.63% (see Supporting Information, Table S1). For the same reasons that were recently stated,18 the ab initio HF/6-31(d,p) was used to supplement the DFT results through the IP estimation from the Koopman theorem.33,34 The optimized structures and molecular orbitals were manipulated with Molekel 4.335 and GaussView 3.0.24 2.2. The Choice of the Method. From our previous results,18,36 a careful analysis shows that B3LYP/6-31G(d) tends to over estimate both the C-S and P-O bond lengths that are predicted to be ca. 1.786 and 1.645 Å versus ca. 1.754 and 1.613 Å of experimental values in bis(ethylenedithio)tetrathiafulvalene (ET)37 and TPP,38 respectively. Although this cannot be taken as capable of mitigating our previous results as will be discussed in the following sections, a more accurate model is desired. For the sake of accuracy improvement in both the predicted geometries and the electron-donor strength, which are the main key factors of the materials we are interested in, different computational approaches were applied. From the results summarized in Table 1, the 6-31G(d,p) basis set is found to yield an improved accuracy on the 6-31G(d) with both the B3LYP and PBE0 functionals showing good agreement with available experimental data for bisethylenedioxy-tetrathiafulvalene (BO) in both the neutral39 and charged40,41 forms. One can find, however, that for both BO and ET (whose results are not shown here), the PBE0/6-31G(d,p) yields the most accurate structures. These findings agree well with recent reports in which PBE0 appears notably adapted to sulfur-bearing molecules.42 The same conclusion can be drawn from the analysis of the optimized structures for TPP, despite the number of different DFTs used. Indeed, it appears clear that B3P86 and BP86 lead to less accurate geometries than the ones obtained using B3LYP, PBE0, and even HF in combination with the same 6-31G(d,p) basis set. From a careful analysis of the optimization results (see Supporting Information, Table S2), one can find that all the bond angles are well described by all the methods, except for P-N-P whose more than 3° of deviation from the experimental value is predicted by HF. Note however that more

TABLE 3: The Influence of the O/X Substitution (X ) S or NH) on the Optimized Geometry of [(NP)3] Containing Derivatives of TPP (see Charts 1 and 2 for Labeling Scheme)a parameter N-P1 P1-X1 X1-C1 C1-C1′ C1′-X2 X2-P1

TPP

4h1

1.591 1.636 1.378 1.391 1.378 1.636

1.591 1.637 1.376 1.391 1.376 1.637

4h2 bond length (Å) 1.611 2.103 1.768 1.406 1.768 2.103

4h3

4h4

4h5

4h6

1.608 1.689 1.396 1.407 1.396 1.689

1.600 1.638 1.368 1.397 1.765 2.099

1.600 1.644 1.374 1.400 1.391 1.676

1.610 2.112 1.766 1.403 1.389 1.679

N-P-N P-N-P N-P-X1 P-X1-C1 X1-P-X2

117.46 122.55 110.34 109.79 96.00

117.49 122.51 110.33 109.78 95.97

bond angle (°) 116.64 122.36 110.45 94.84 97.48

115.48 124.52 108.97 113.33 91.10

116.95 123.00 109.12 117.58 97.14

116.16 122.76 110.73 112.03 92.95

115.71 122.87 111.30 93.86 94.36

N4-P1-X1-C1 P1-N2-P3-N4

180.00 0.00

179.64 0.00

dihedral angle (°) 149.56 178.70 0.00 0.08

179.49 2.95

179.83 14.57

175.51 17.54

a Only geometrical parameters involved in the common part of all the derivatives of this group are shown. Since the TTF-like moiety of the side group was found to be unaffected by the O/NH (or O/S) substitution, the relevant parameters are not listed.


J. Phys. Chem. C, Vol. 111, No. 12, 2007 4841 3. Results and Discussion

Figure 2. Comparison between the PBE0/6-31G(d,p) optimized structures for 4h3 in its (i) neutral and (ii) oxidized forms. For clarity, C, S, P, and N are shown in gray, yellow, orange, and blue, respectively, while H atoms are not displayed.

accurate bond lengths are provided by HF, followed by PBE0. At this stage, one may prefer using HF/6-31G(d,p) but because UHF needed for open-shell cation was found to yield spin contamination, PBE0//6-31G(d,p) was then preferred. From Table 2 in which both the vertical IP (IPv) and adiabatic (IPa) for the commonly used electron-donors are presented that were predicted based on the same PBE0/6-31G(d,p) chemical model, one can find that, once again, sufficiently accurate IP values are obtained, compared with available experimental data. Indeed, the predicted PBE0/6-31G+(d,p) (and PBE0/6-31G(d,p)) values of 6.30 (6.41) and 6.27 (6.36) eV for IPv values compare well with 6.55 and 6.35 eV of the averaged values calculated from the experimental reports43,44 while showing a little improvement compared to previous theoretical values.45 Also in excellent agreement with experimental data for BO is the PBE0/6-31G(d,p) predicted electronic structure. Indeed, as shown in Figure 1, the HOMO and singly occupied molecualr orbital (SOMO) are of π-character, dominated by pz-orbitals of the C and S atoms belonging to the TTF moiety with some not negligible contributions from the O atoms of the six-membered ring in the case of SOMO. These results agree well with previous reports, both experimental46-48 and theoretical,18,45 for its homologue ET, which seems to be the most widely investigated of the series. In summary, a careful analysis of both the calculated structures (for BO, BO+, TPP, and ET) and IP (for a series of electron donors) presented in this section reveals a good agreement, which gives credit to the PBE0/6-31G(d,p) level of theory. Accordingly, reliable results can be reasonably awaited from the current study on a list of compounds with an intermediate atomic composition between that of TPP and TTFlike donors.

As a first step of our investigation, two groups of derivatives were reinvestigated using the current chosen method. The first is made up of 1a, 1b, 1c, 1d, 1e, and 1f, the second is made up of 1b, 2b, 3b, and 4b. Indeed, by doing this one may expect the conclusion drawn in our previous work based on a different theoretical level about the individual effect of the core and side group on both the molecular shape and electronic properties such as the electron-donor capacity.18 In the following sections, we first compare the current results to those we reported recently (based on a lower quantum chemical level of theory)18 before a detailed analysis of those related to the new series is made. 3.1. Molecular Geometries. It is often assumed that the TTF region of the TTF-related donors for organic superconductors is flat. Some deviations from planarity have been suggested in crystals containing electron acceptors, (ET)nXn.49 Quantum chemical calculations (HF, second-order Møller-Plesset perturbation theory (MP2), and DFT-B3P86),50,51 have shown that such organic donors always have a planar TTF region in their cationic form, the neutral being distorted into a boat conformation. From the molecular geometry point of view, our computational results are in good agreement with available experimental and theoretical studies for ET, tetramethyltetraselenafulvalene (TMTSF), bis(ethylenedioxy)tetrathiafulvalene (BETS), and BO (Figure 1i,ii). The origin of the boat distortion was reported to be the behavior of the C-X-C bond angle (X ) S, Se) in the pentagon ring of the systems and was suggested as the key factor in determining the boat deformation.45 In the newly designed molecules, the [(CO)3], [(CS)3], and [(CNH)3] moieties are predicted in a chair conformation with C-O, C-N, and C-S bond lengths of ca. 1.37, 1.38, and 1.77 Å, and the corresponding C-O-C, C-N-C, and C-S-C bond angles are predicted at ca. 112 ((4), 123, and 100°, respectively. For the [(NP)3]-containing ones, the P-N bond was found to be ca. 1.62 Å in 4b and ca. 1.60 Å in the j-n side fragmentscontaining ones. In this series, the P-N-P and N-P-N bond angles were predicted to be 118 and 119° in 4b, while the angles were predicted to be, respectively, 116 ((1) and 123 ((1)° in the j-n side fragments-containing ones. In 4b and the partial O/S (NH)-substituted derivatives, the P-N-P-N dihedral angles are ca. 20 and more than 14°, respectively, for the former and the latter. 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-X, with X ) O, S, or

TABLE 4: The Individual Influence of the Side Fragment (a-f) and Core Moiety (1-4) on the Electronic Properties (FMO Energies, HOMO-LUMO Gap, IPs) as Provided by PBE0/6-31G(d,p) Calculationa compound

EHOMO

ELUMO

∆E

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

-5.06 (-7.30) -4.75 (-6.94) -5.09 (-7.28) -4.84 (-7.06) -5.12 (-7.22) -4.56 (-6.89)

with [(CO)3] core and a-f side fragments -0.73 (2.94) 4.33 (10.23) -0.14 (3.83) 4.61 (10.77) -0.67 (3.01) 4.42 (10.29) -0.39 (3.47) 4.45 (10.53) -0.78 (2.83) 4.35 (10.05) -0.41 (3.49) 4.14 (10.39)

1b 2b 3b 4b

-4.75 (-6.94) -5.32 (-7.41) -4.63 (-6.68) -4.46 (-6.22)

-0.14 (3.83) -0.56 (3.11) -0.24 (3.64) -1.77 (1.20)

with b side fragment 4.61 (10.77) 4.76 (10.52) 4.39 (10.32) 2.69 (7.42)

IPv

IPa

IPKT

6.08 (6.20) 5.98 (6.13) 6.31 (6.48) 5.86 (5.98) 6.17 (6.30) 5.67 (5.84)

5.76 (5.90) 5.68 (5.86) 5.81 (5.44) 5.59 (5.75) 5.76 (5.74) 5.42 (5.61)

7.30 6.94 7.28 7.06 7.22 6.89

5.98 6.51 (6.63) 5.82 (5.96) 5.58 (5.72)

5.68 6.14 (6.26) 5.42 (5.57) 4.90 (5.05)

6.94 7.41 6.68 6.22

a Given in parenthesis are PBE0/6-31+G(d,p)//PBE0/6-31G(d,p) values for IP and HF/6-31G(d,p)//PBE0/6-31G(d,p) for other properties. Values given in eV.


Gahungu et al.

TABLE 5: Computed EHOMO, ELUMO, and HOMO-LUMO Gap as a Function of Computational Methoda PBE0/6-31G(d,p)//PBE0/6-31G(d,p) compound

a

EHOMO

HF/6-31G(d,p)//PBE0/6-31G(d,p)

ELUMO

∆E

EHOMO

ELUMO

∆E

ref 3.93 3.73 4.10 3.79 3.73 6.44

-6.66 -6.65 -7.04 -7.05 -6.83 -8.79

3.11 2.55 2.91 2.23 2.87 3.54

9.77 9.20 9.94 9.28 9.70 12.33

TTF TMTSF ET BETS BO TPP

-4.67 -4.70 -4.97 -5.06 -4.63 -6.60

-0.75 -0.97 -0.88 -1.27 -0.91 -0.15

4g1 4h1 4i1 4j1 4k1 4l1

-5.18 -5.05 -5.15 -5.11 -5.19 -4.91

-1.04 -0.95 -1.31 -1.01 -1.38 -1.03

(X1, X2) ) (O, O) 4.14 4.10 3.84 4.10 3.80 3.88

-7.17 -7.02 -7.07 -7.13 -7.16 -7.02

2.64 2.84 2.22 2.75 2.11 2.73

9.81 9.86 9.28 9.88 9.27 9.75

4h2 4i2 4j2 4k2 4l2

-5.10 -5.19 -5.15 -5.21 -4.96

-1.51 -1.53 -1.53 -1.56 -1.49

(X1, X2) ) (S, S) 3.59 3.66 3.62 3.65 3.46

-7.05 -7.09 -7.15 -7.17 -7.05

2.07 2.02 2.03 1.96 2.06

9.12 9.11 9.19 9.13 9.11

4h3 4i3 4j3 4k3 4l3

-4.77 -4.89 -4.87 -4.94 -4.67

-0.72 -1.10 -0.81 -1.20 -0.83

(X1, X2) ) (NH, NH) 4.05 3.79 4.06 3.75 3.86

-6.69 -6.75 -6.82 -6.85 -6.72

3.08 2.44 2.97 2.30 2.95

9.77 9.19 9.77 9.15 9.67

4h4 4i4 4j4 4k4 4l4

-5.06 -5.16 -5.12 -5.19 -4.93

-0.98 -1.33 -1.04 -1.40 -1.05

(X1, X2) ) (O, S) 4.08 3.83 4.08 3.78 3.88

-7.02 -7.06 -7.12 -7.15 -7.02

2.76 2.20 2.69 2.09 2.69

9.78 9.27 9.81 9.24 9.71

4h5 4i5 4j5 4k5 4l5

-4.88 -4.99 -4.96 -5.05 -4.77

-0.80 -1.18 -0.89 -1.26 -0.90

(X1, X2) ) (O, NH) 4.08 3.81 4.08 3.79 3.87

-6.81 -6.88 -6.95 -6.99 -6.85

3.00 2.36 2.90 2.24 2.87

9.81 9.23 9.85 9.23 9.72

4h6 4i6 4j6 4k6 4l6

-4.94 -5.04 -5.02 -5.10 -4.83

-0.89 -1.24 -0.96 -1.32 -0.97

(X1, X2) ) (S, NH) 4.06 3.80 4.06 3.78 3.85

-6.88 -6.92 -6.99 -7.04 -6.89

2.87 2.28 2.78 2.17 2.76

9.74 9.21 9.78 9.21 9.65

Ref refers to the electron donors taken as reference in this work. Values are given in eV.

NH) leads to a planar conformation for the core ring when the two P-X bonds are identical (i.e., in the totally O/S (or NH) substituted derivatives). A very interesting observation is the distortion of the caplike neutral conformation of the neutral form into the planar conformation upon oxidization. It was demonstrated that all TTF-like superconductors lead to the boat conformation for neutral forms and to the planar conformation for cationic forms, a behavior, which compares well to the one exhibited by 1b (Figure 1iii, iV) and 3b (see Supporting Information, Figure S1). As an electron hops from the neutral to the cation, the former distorts to the planar, while the original distorts from the planar to the caplike conformation leading to a coupling between conduction electrons and vibration that, in TTF-like donors, is believed to be the salient coupling for superconductivity.45 On the basis of the current predictions, the [(CO)3] and [(CNH)3] based systems would lead to totally planar cationic conformations, while in the other cases, the side fragments and the core are planar but not coplanar. On the basis of these considerations,

one can therefore hypothesize that such derivatives, which lead to cap-distorted neutral, may result in superconductivity when combined with suitable acceptors. As previously shown,18 [(NP)3] containing derivatives with TTF-like side fragment of the type h-l, may lead to potential candidates for superconductors, which may combine a good electron-donor ability and a possible inclusion adduct formation. Because the paddle wheel is known as one of the important key factors,23 we found it interesting to investigate the effect of the O/S, O/NH and S/NH substitution on the molecular shape. For the sake of comparison and to evaluate the effect of the O/S and O/NH heterosubstitution on the molecular shape, selected geometrical parameters for a group of derivatives resulting from such a substitution are listed in Table 3. From these results, one may find that the P-N bond lengths remain practically unaffected, the main deviation being found to be less than 0.02 Å. The same observation can be made for N-P-N and P-N-P bond angles whose predicted values only deviate by less than 2° from those in TPP. The half substitution affects



Figure 3. (i) The HOMO and (ii) SOMO of 4h3 (for neutral and oxidized forms, respectively), both computed at the PBE0/6-31G(d,p) level (both contoured at 0.03 e au).

the planarity of the central core ring, which is converted into the chair conformation with a P-N-P-N dihedral angle of ca. 3° for (X1, X2) ) (O, S), ca. 15° for (X1, X2) ) (O, NH), and ca. 18° in the case of (X1, X2) ) (S, NH). Figure 2 shows the optimized structures of 4h3 in both the neutral and oxidized forms (see Supporting Information, Figures S2 and S3). In general, most of the new derivatives are predicted to preserve the paddle wheel molecular shape, the TTF-extended side group retaining the TTF-like donors behavior (TTF-like moiety distortion into the planar) during the oxidization process as summarized in Figure 3. 3.2. Electronic Properties. From the theoretical standpoint, the electron-donor strength can be related to the EHOMO or the IP of a molecule and its relevance to the hole injection from electrodes, or from other organic materials, and in applications involving the ground-state or photoinduced electron transfer. These secondary properties may also be influenced by the solidstate effects, intermolecular interactions, and molecular environment.52 All of these factors may differently affect the IPs of the studied systems and thus gas-phase IPs may not correlate precisely with solid-state IPs or with solution electrochemical data. 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 we can draw from this study. In this section, a detailed analysis of the electronic properties including the FMO consideration together with the IPs of the systems under investigations is provided. In addition, an eventual correlation between the IPs and the EHOMO is attempted and the influence of the substitution of the side fragment or the core on these parameters is also discussed. In Tables 4 and 5, we present the predicted EHOMO, ELUMO, HOMO-LUMO gap (at different levels of theory), as well as the IPs. From the results presented in Tables 4 and 5 (see also Supporting Information, Table S4), it can be seen that the magnitudes of the eigenvalues and their relative differences are different within DFT and HF approaches with 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 5 display the correlation between the IPs and the EHOMO for systems starting from different side fragments. 3.2.1. Substitution Effect on the Electronic Structures and Properties. 3.2.1.1. Effect of the Core Part. To get some insight into the influence of the central part on the FMO energies and IP (and thus the electron-donor strength), one can consider a series of any group of systems built up with a same side fragment (a-f) and the four core parts. Taking those containing the b side fragment (1b, 2b, 3b, and 4b) as an example, the EHOMO is predicted in the increasing order of 2b (-5.32 eV)
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) confirming our recent results.18 3.2.1.2. Effect of Substitution of Side Group. The same results listed in Table 4 show that within the [(CO)3] core based systems with the side fragments a-f (see Chart 2), the ELUMO prediction shows the increasing order of 1e < 1a < 1c < 1f < 1d < 1b


Gahungu et al.

CHART 2: The Structures of the Core Rings (1-4) and Modified Side Group by a Systematic Substitution with Modified (Half) TTF-Like Donors Fragments (a-l) and a Total (or Partial) Substitution of the O Atom (by S or NH) at the Bridging Part between the Core and Side Fragment in TPPa

a In our discussion, those are labeled using the corresponding substituting scheme in the bridging part 1-6 side fragment a-l (for example, 4h3 refers to the TPP derivative containing the central core 4 and the side fragment h with X1 ) X2 ) NH).

TABLE 6: Ionization Potentials (eV) as a Function of Computational Approach and (O, O)/(X1, X2) Substitutiona totally substituted derivatives compound 4g1 4h1 4i1 4j1 4k1 4l1 4h2 4i2 4j2 4k2 4l2 4h3 4i3 4j3 4k3 4l3 a

IPv

IPa

(X1, X2) ) (O, O) 5.85 (5.96) 5.71 (5.83) 5.79 (5.92) 5.64 (5.78) 5.91 (6.02) 5.70 (5.72) 5.78 (5.88) 5.65 (5.77) 5.84 5.73 5.60 (5.75) 5.46 (5.61) (X1, X2) ) (S, S) 5.83 (5.94) 5.68 (5.82) 5.89 5.74 5.80 5.69 5.86 na 5.65 5.50 (X1, X2) ) (NH, NH) 5.50 (5.64) 5.35 (5.50) 5.59 5.42 5.53 5.41 5.59 5.49 5.38 5.23

partially substituted derivatives IPKT

compound

IPv

IPa

IPKT

(X1, X2) ) (O, S) 7.17 7.02 7.07 7.13 7.16 7.02

4h4 4i4 4j4 4k4 4l4

7.05 7.09 7.15 7.17 7.05

4h5 4i5 4j5 4k5 4l5

6.69 6.75 6.82 6.85 6.72

4h6 4i6 4j6 4k6 4l6

5.79 (5.91) 5.64 (5.77) 5.87 5.71 5.77 5.66 5.85 na 5.62 5.47 (X1, X2) ) (O, NH) 5.61 (5.75) 5.46 (5.61) 5.70 5.53 5.63 5.50 5.69 5.58 5.47 5.31 (X1, X2) ) (S, NH) 5.67 (5.79) 5.52 (5.65) 5.74 5.59 5.68 5.56 5.74 na 5.51 5.36

7.02 7.06 7.12 7.15 7.02 6.81 6.88 6.95 6.99 6.85 6.88 6.92 6.99 7.04 6.89

Given in parenthesis are PBE0/6-31+G(d,p)//PBE0/6-31G(d,p) values; na ) calculation not achieved. See Chart 2 for labeling scheme.

and the prediction for EHOMO being 1e < 1c < 1a < 1d < 1b < 1f. As a result, our prediction shows an increasing ∆E in the order of 1f < 1e < 1a < 1c < 1d < 1b. The ESOMO increases in the order of 1b < 1c < 1a < 1e < 1d < 1f. These results also support our previous reports.18 Within the predicted IP values, a relatively remarkable IP and EHOMO dependence on the side fragment is found within the increasing order of 1f < 1d < 1b < 1a < 1e < 1c. In general, all the results are consistent with an EHOMO increase when the O atom substitutes S or when the latter substitutes Se. The same conclusion can be drawn in the case of ELUMO with a negligible difference being observed between 1d and 1f. 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 (a versus b) lower both the ELUMO and EHOMO, which is the same conclusion being also drawn for the S/O substitution in the six-membered ring. 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 part and different side fragments. Figure 5A shows the effect of the -H/-SCH3 and the heterosubstitution, and 5B shows the effect of (O, O)/(X1, X2) substitution. Within the g-l containing systems, the IP is found in the increasing order of l

< h < j < i < g < k and h3 < h5 < h6 < h1 < h4 < h2 within those built up with h1-h6 fragments. From a careful analysis of these results and those listed in Tables 5 and 6, we may conclude that going from the O heteroatom in the sixmembered ring to the five-membered ring of the half-TTF increases the electron-donor ability, which agrees with previous experimental and theoretical findings. According to these findings, BO (Chart 1ii, a S/O substituted derivative of ET) shows a lower first ionization energy45 thereby making substituted TTF a better electron donor than TTF itself, leading to donor-acceptor solids with superior conducting and superconducting properties. The current results also show that the O/NH substitution induces a relatively significant decrease in the IP therefore increasing the electron-donor strength with the reverse being observed with the O/S substitution. Finally, the same results show that partial substitution leads to TPP derivatives whose IP values (electron-donor strength) may be in between those of the corresponding derivatives from the total O/NH and O/S substitutions. 3.3. Comparison with the Commonly Used Electron Donors. As the electron-transfer donor strength can be related to the EHOMO or the IP of a molecule, these properties were calculated to estimate the electron-donor ability of the systems under investigation. From the electron-donor ability point of



TABLE 7: A Comparative Study of TPP and Compound 4h1: Molecular Structures (of Both the Neutral and Charged Forms) and Some of Their Electronic Properties TPP parameter P-N P-O O-C1 C1-C1′ C1-C2 C2-C3 C3-C3′

cationa

4h1 neutral

bond length (Å) 1.589 -0.002 1.640 0.004 1.365 -0.013 1.407 0.016 1.381 0.002 1.393 -0.006 1.408 0.015

P-N-P N-P-N N-P-O O-P-O P-O-C1 O-C1-C1′ O-C1-C2 C1′-C1-C2 C1-C2-C3

122.03 117.97 110.28 95.43 110.34 111.95 126.07 121.98 116.36

ELUMO EHOMO ∆E IPv IPa

-0.15 -6.60 6.44 7.70 7.63

bond angle (°) -0.52 0.52 -0.06 -0.57 0.55 -0.26 0.05 0.21 -0.65 other propertiesb

cationa

neutral

1.590 1.638 1.372 1.395 1.376 1.400 1.402

-0.001 0.001 -0.004 0.004 -0.002 0.002 0.001

122.42 117.58 110.37 95.79 109.92 112.18 125.68 122.18 116.41

-0.09 0.09 0.05 -0.18 0.14 -0.06 0.04 0.10 -0.17

-0.95 -5.01 4.10 5.79 5.64

a Negative and positive values (for the geometrical parameters) express, respectively, the increasing and decreasing of the parameters of interest. bEnergies are given in eV. See Chart 1 for the definition of the parameters.

view, the current study shows clearly that compared to the commonly used electron donors whose predicted IPv (IP from Koopmans’ theorem, IPKT) ranges from 6.22 to 6.41 eV (6.657.05 eV), a comparable electron-donor ability can be reached by adopting the building approach developed in this work (i.e., 5.67-6.17 eV (6.89-7.30 eV) and 5.38-5.91 eV (6.69-7.17 eV), respectively, for the [(CO)3] and [(NP)3] based systems with the side fragments ranging from a to n). Especially interesting is the electron-donor strength that is predicted within [(NP)3] based systems. Indeed, for most of the new derivatives, the FMO symmetry on the TTF (TTF-like) extended side fragment is similar to the FMO of the TTF-like molecules as shown in Figure 5 for a representative of them. In addition, the smallest IP value of ca. 5.38 eV (6.22 eV) (i.e., ca. 1 eV (0.44 eV) lower than the one for TTF) was attained, implying that a better electron-donor strength may be obtained by using the strategy proposed in this work. Within the group [(NP)3]-based compounds, most of them are predicted to be stronger electron donors than most of the commonly used ones taken as reference in this work. 3.4. Comparison with TPP. Compared to the TPP geometry, the molecular geometries of the [(CO)3], [(CS)3], and [(CNH)3]based systems are predicted to be very far from the former. Interestingly, most of the rest of the other derivatives show a paddle wheel shape and may then be anticipated to form channel type inclusion compounds. In Table 7, we summarize the predicted electronic properties of TPP in comparison with compound 4h1. Those include the FMO energies, the gap energy (∆E), and the electron-donor strength through both IPvs and IPas. In addition, geometrical parameters involving the common part are also provided for both the neutral and the cationic forms of the two molecules. On the basis of a higher quantum chemical level of calculation compared to our previous results, we believe that some interestingly reliable insights can be gained from them.

In the new derivatives, the TTF-like regions (whose corresponding geometrical parameters are not shown) were found to behave as described for BO and ET18,45 upon injection of a positive charge. The EHOMO is lifted by about ca. 1.6 eV and the ELUMO lowered by ca. 0.8 eV when comparing TPP to 4h1. As a consequence, the ∆E decreases by 2.34 eV while the first IP (both IPv and IPa) decreases by ca. 2 eV, implying a net difference in electron-donor strength between TPP and the TTFlike fragment-based molecules. All these results compare well with those of 1.4 and 0.9 eV previously reported.18 To reference some works,6a,16a channels within TPP-like crystals are less common with larger paddles. However, no limitation in size/ composition for the side group to yield common tunnels seems to be reported yet. Therefore, it may be reasonable to hope that such a large number of systems (within a wide range in size and composition) that we suggested in this work may include some exploitable for inclusion adduct formation issue. Combined with their good electron-donor ability as provided by the current study, such an important property 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 TPP derivatives containing TTF-like fragments using theoretical methodologies based on DFTPBE0/6-31G(d,p) and HF/6-31G(d,p)//DFT-PBE0/6-31G(d,p) approaches. Reinforcing the conclusion obtained from an earlier study based on lower level of theory, substituting [(PN)3] in TPP with [(CO)3], [(CS)3], and [(CNH)3] ring and the side fragment with half TTF-like fragment may lead to a series of derivatives showing an electron-donor strength comparable or better than the one for the commonly known electron donors. In addition, a new series of candidates for organic superconductors were designed based on the TTF-like extension of TPP side fragment and both partial and total O/S (O/NH or S/NH) substitution. More interestingly, the last group was predicted to combine a good ET-donor strength and the paddle wheel molecular shape responsible for inclusion adducts formation with the O/NH substituted derivatives showing a better electron-donor ability. From this series of derivatives, tunnels of variable diameter can then be awaited depending on the side group used. This is important as it can provide ease in the modulation of the conducting properties by intercalation of judiciously chosen acceptors. Acknowledgment. Financial supports from the NSFC (No. 50473032; 20473095) and Excellent Personnel of Excellent Youth Fund of Ministry of Education are gratefully acknowledged. Supporting Information Available: Cartesian coordinates of representatives for the relevant structures, details of the computational results, and figures not given in the main text are provided. The Cartesian coordinates for the rest of the optimized structures can be given by the authors upon request. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Chae, H. K.; Siberio-Perez, D. Y.; Kim, J.; Go, Y.; Eddaoudi, M.; Matzger, A. J.; Keeffe, M. O.; Yaghi, O. M. Nature. 2004, 427, 523. (2) Hertzsch, T.; Hulliger, J.; Weber, E.; Sozzani, P. In Encyclopedia of Supramolecular Chemistry; Marcel Dekker: New York, 2004; pp 996.

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