Syntheses, crystal structures and properties of two novel imino-π-extended TTF derivatives

Solid State Sciences 12 (2010) 391–396

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Syntheses, crystal structures and properties of two novel imino-p-extended TTF derivatives Yulan Zhu a, b, *, Libin Tian a, b, Kuirong Ma a, Yuhe Kan a, Xueling Tang b, Huayou Hu a a b

Jiangsu Key Laboratory for Chemistry of Low-dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huai’an, Jiangsu 223300, PR China Department of Chemistry, Science College, Yanbian University, Yanji, Jilin 133002, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 April 2009 Received in revised form 9 August 2009 Accepted 3 December 2009 Available online 11 December 2009

Two novel conjugated imino-p-extended tetrathiafulvalenes with p-iminobenzene, N,N0 -bis(4,5-bis (methylthio)-1,3-dithiol-2-ylidene)benzene-1,4-diamine (1) and N,N0 -bis(4,5-bis(ethylenedithio)-1,3dithiolo-2-ylidene)benzene-1,4-diamine (2), have been synthesized and characterized by NMR, IR, MS and X-ray single-crystal diffraction. Both the two targets adopt chair-like conformation, and the central rings of p-iminobenzene moieties of the two molecules are severely twisted from the planarity of two dithiole rings, respectively. The UV–vis spectra of 1 and 2 show the lowest-energy absorption bands caused by the HOMO–LUMO one-electron promotion. Cyclic voltammetry (CV) measurements show only one, two-electron irreversible oxidation picks. These experimentally estimated energy levels of the frontier orbital of 1 and 2 (EHOMO 1: ¼ 5.45, 2: 5.47 eV) are in good agreement with those obtained from DFT calculations (EHOMO 1: ¼ 5.5, 2: ¼ 5.3 eV). The high HOMO–LUMO gaps of 1 (4.05 eV) and 2 (4.00 eV) indicate high kinetic stability of the title compounds. Ó 2009 Elsevier Masson SAS. All rights reserved.

Keywords: Imino-p-extended TTFs Crystal structure UV–vis Cyclic voltammetry DFT

1. Introduction Tetrathiafulvalene derivatives (TTFs), due to unique structural, physical and chemical properties and easiest assembly, have been widely explored as building block in both materials and supramolecular chemistry. A promising field is synthesis of varied extended TTF conjugated donor p-systems owing to their novel fascinating architectures and potential applications involved in opto-electronics, photovoltaics, nonlinear optics [1–8]. More recently, the extended TTF derivatives with various kinds of conjugated donor p-systems have become the target of the researchers’ attention. The introduction of p-conjugated spacer can not only provide an energetically narrower HOMO–LUMO gap, but also increase the stability of the oxidized state and facilitate their forming polycation due to the diminution of Coulombic repulsion and mesomeric effects [4]. In addition, large numbers of p–p and/or S/S interactions are involved in supromolecular structures, and further reinforce their framework [9]. Great efforts have been devoted to design of p-extended TTFs with p-quinodimethane structure (exTTF) incorporating an anthracene spacer between the dithiole rings in the preparation of photosynthetic models [10–14]

* Corresponding author. Department of Chemistry, Science College, Yanbian University, Yanji, Jilin 133002, PR China. E-mail address: [email protected] (Y. Zhu). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.12.001

or as materials with marked nonlinear optical properties [15–17]. Only a few, however, focus attention on imino-p-extended TTF derivatives to date, which is important in the field for the molecular design of novel donor p-systems [18–20]. Lorcy et al. first reported p-phenylenediamine-extended TTF derivatives which donor character is more than that of the analogue with a CH bridge instead of an imino group, and later two extended p-redox cage-like compounds are reported by them. Currently, our research is focused on the synthesis of novel imino-p-extended TTF derivatives. Herein, we report the syntheses and crystal structures of the two novel imino-p-extended TTF derivatives. UV–visible spectroscopic, electrochemical measurements and the DFT calculations are also reported, in order to investigate the electron-donor properties and electronic structure. 2. Experimental section 2.1. Materials and method All chemicals purchased were reagent grade and used without further purification. Melting point was uncorrected. IR spectra recorded on an AVATA360 infrared spectrometer with KBr pellets in the 400–4000 cm1 region. NMR spectra were measured on a Bruker AV-400 instrument (CDCl3 as solvent). MS spectra were measured on LCQAdvantatage instrument. Thermal gravity analysis was carried out from room temperature to 700  C using

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TGA/SDTA851e under N2 atmosphere at a heating rate of 10  C/min. The absorption spectra were recorded on an Australia GBC UV/Vis 916 spectrophotometer in the range of 230–600 nm. The redox properties of the compound were measured by CV in CH2Cl2 at room temperature on a CHI66A electrochemical working station using a glassy carbon as working electrode, the standard Ag/AgCl as reference electrode and tetrabutylammonium perchlorate (0.5 M) as the supporting electrolyte. Cyclic voltammograms were obtained at a scan rate of 50 mV/s. 2.2. Syntheses Compounds 2,4,5-tri(methylthio)-1,3-dithiol perchlorate and 2-methylthio-4,5-bis(ethylenedithio)-1,3-dithiol perchlorate were prepared at 273.15 K under N2 in a three-step synthetic procedure by following the method reported in the literature, as shown in Fig. 1 [18]. Reacting 1 equivalent of 2,4,5-tri(methylthio)-1,3-dithiol perchlorate and 2-methylthio-4,5-bis(ethylenedithio)-1,3-dithiol perchlorate with 0.5 equivalent of p-phenylenediamine, we obtained compounds 1 and 2 in 51% and 48% yield, respectively. Compounds 1 and 2 have been fully characterized by NMR, IR, and MS, as well as by single-crystal X-ray analysis. 2.2.1. Preparation of N,N0 -bis(4,5-bis(methylthio)-1,3-dithiol-2ylidene)benzene-1,4-diamine (1) To a magnetically stirred mixture of p-phenylenediamine (0.648 g, 6 mmol) and pyridine (0.5 mL) was added to a solution of 2,4,5-tri(methylthio)-1,3-dithiol perchlorate (1.02 g, 3 mmol) in methanol (40 mL) at 273.15 K. The mixture was stirred under N2 for 30 min and a yellow precipitate appeared. Stirring was continued at room temperature for 12 h. The solid was filtered, washed with cold methanol. Purification of products was achieved by column chromatography on silica gel using dichloromethane/petroleum ether (5:1) as eluent to give yellow powder (yield 51%). Single-crystals of 1 suitable for X-ray diffraction experiments were grown by CH2Cl2– MeOH. M.p.: 144–145  C. 1H NMR (CDCl3, 400 MHz) d: 2.41 (s, 6H, 2SMe), 2.51 (s, 6H, 2SMe), 7.06 (s, 4H, ArH). 13C NMR (CDCl3, 400 MHz) d: 163.3, 147.5, 127.1, 125.0, 121.3, 19.0. FTIR (KBr): 2974, 2915 (–CH3), 1662 (C]C), 1580 (–C]N–), 1487, 1420 (–C6H4), 1204 (C–N), 972, 927, 889, 810 (C–S). MS (EI) m/z (%): 493.4 (Mþ, 100). 2.2.2. Preparation of N,N0 -bis(4,5-bis(ethylenedithio)-1,3-dithiolo2-ylidene)benzene-1,4-diamine (2) The synthesis was similar to that of 1 except using 2-methylthio-4,5-bis(ethylenedithio)-1,3-dithiol perchlorate instead of 2,4,5-tri(methylthio)-1,3-dithiol perchlorate. The crude product was purified by column chromatography on silica gel using dichloromethane/petroleum ether (3:1) as eluent to give yellow powder (yield 48%). Single-crystals of 2 suitable for X-ray

Fig. 1. Synthesis of compounds 1 and 2.

diffraction experiments were grown by CH2Cl2–MeOH. M.p.: 155– 156  C. 1H NMR (CDCl3, 400 MHz) d: 3.40 (s, 8H, 4CH2), 7.29 (s, 4H, ArH). FTIR (KBr): 2955, 2909 (CH2), 1650 (C]C), 1557 (–C]N–), 1510, 1410 (C6H4), 1206 (C–N), 1105, 1004, 949, 877, 849, 768 (C– S). MS (EI) m/z (%): 489.0 (Mþ, 100). 2.3. Crystallography Diffraction intensity data were collected at 296 K on a Bruker SMART APEX II diffractometer equipped with a CCD area detector and graphite-monochromated MoKa radiation (l ¼ 0.71073 Å). The data sets were corrected for absorption by multi-scan technique. The structures were solved by direct methods and expanded using difference Fourier techniques with SHELX-97 [21]. All nonhydrogen atoms were located with successive difference Fourier maps and refined on F2 by full-matrix least-squares method with anisotropic thermal parameters. The hydrogen atoms were added according to theoretical models. Crystallographic data for the two compounds have been deposited with The Cambridge Crystallographic Data Centre as supplementary publication Nos. 679582 and 691757. Crystallographic data and structural refinements are summarized in Table 1. Selected bond lengths and angles are listed in Table 2. 2.4. Computational method To have a better understanding on the electron-donor properties of compounds, we performed the single-point calculations at DFT/B3LYP/6-31 þ g (d,p) level [22–24] on the neutral 1 and 2, starting from the experimental X-ray structures as input geometries, employing the Gaussian 03 program package [25]. To investigate the nature of electronic transitions, the electronic spectra of 1 and 2 were calculated using the time-dependent DFT (TDDFT) approach at the B3LYP/6-31G* level. 3. Results and discussion 3.1. Description of structure As shown in Fig. 2, both the structural units of 1 and 2 belong to benzene-extended TTF molecule. Two dithiole rings of compounds,

Table 1 Crystallographic data and refinement details for 1 and 2.

Empirical formula Formula weight Temperature (K) Crystal system Space group a (Å) b (Å) c (Å) a ( ) b ( ) g ( ) V (Å3) Z Dcalc (g cm3) m (mm1) F (000) Reflections/unique Parameters Rint Goodness of fit R indices [I > 2s(I)] R (all data)

1

2

C16H16N2S8 492.87 296(2) Monoclinic P21/c 7.2393(9) 11.3918(2) 13.2705(2) 90.00 105.83 90.00 1052.9(2) 2 1.555 0.853 508 10568/3119 120 0.025 1.074 0.0369, 0.0859 0.0441, 0.0924

C16H12N2S8 488.84 296(2) Triclinic Pı 6.7208(7) 8.1686(9) 9.7380(1) 105.38(3) 91.53(3) 102.65(3) 500.9(9) 1 1.621 0.896 250 6323/1970 118 0.047 1.021 0.0445, 0.1202 0.0681, 0.1447

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3.2. Thermal gravimetric analysis (TGA)

Table 2 Selected bond lengths (Å) and angles ( ) for 1 and 2. Compound 1 C1–C3 C2–N1 C3–S2 C5–N1 C3–C1–S3 N1–C2–S2 C1–C3–S2 C6–C5–N1 C1–S1–C2 Compound 2 C1–C3 C2–N1 C3–S3 C4–C5 C3–C1–S4 N1–C2–S1 C1–C3–S3 C7–C6–N1 C1–S1–C2

1.340(3) 1.269(2) 1.735(2) 1.414(3) 126.93(2) 118.33(2) 116.70(2) 125.40(2) 96.44(1) 1.325(6) 1.267(4) 1.747(4) 1.445(8) 129.63(3) 119.0(3) 127.1(3) 123.5(3) 95.99(2)

C1–S3 C2–S2 C3–S4 C8–S4 C3–C1–S1 N1–C2–S1 C1–C3–S4 C7–C5–N1 C3–S2–C2

1.751(2) 1.761(2) 1.760(2) 1.780(3) 117.27(2) 129.64(2) 127.52(2) 116.63(2) 97.61(1)

393

C1–S1 C2–S1 C4–S3 S3–C1–S1 S2–C2–S1 S2–C3–S4 C2–N1–C5 C1–S3–C4

C1–S4 C2–S1 C3–S2 C6–N1

1.747(3) 1.753(3) 1.751(4) 1.406(4)

C1–S1 C2–S2 C4–S3

C3–C1–S1 N1–C2–S2 C1–C3–S2 C8–C6–N1 C3–S2–C2

117.0(3) 128.2(3) 117.8(3) 118.1(3) 95.11(2)

S4–C1–S1 S1–C2–S2 S3–C3–S2 C2–N1–C6 C3–S3–C4

1.757(2) 1.767(2) 1.792(3) 115.78(1) 111.95(1) 115.76(1) 124.25(2) 100.15(1) 1.747(4) 1.771(4) 1.781(6) 113.5(2) 112.81(2) 115.0(2) 121.3(3) 98.6(2)

separated by p-phenylenediamine, reveal equatorial planarity with torsion angles S(1)–S(2)–S(1A)–S(2A) of 0 , different from the boatshaped structure usually found in neutral derivatives of p-extended TTF [26–28]. The planes of the bis(imino)benzene, twisted from the planarity of the two dithiole rings, form a dihedral angle of 37.97 for 1, and 60.2 for 2, respectively. In Fig. 2, the substituent groups of the dithiole core are located in the opposite direction, resulting in a chair-like molecular conformation. We may assume this conformation is apparently forced by steric repulsion between sulfur atoms of the dithiole ring and the peri-hydrogen atoms. Because the S–H distances are 2.637 Å (1) and 2.867 Å (2), respectively, shorter enough than the sum of Van der Waals radii between H atom (1.17 Å) and S atom (1.8 Å). The packing diagram of 1 (Fig. 3) shows a mixed donor-iminobenzene stacks with the short S/S contacts in the bc plane. The intermolecular S(1)/S(2) and S(2)/S(4) distances are 3.813 Å and 3.750 Å, respectively. Interestingly, the packing diagram of 2 shows a complete segregation of donor and iminobenzene fragments, which organize in homostacks of dithiole ring and iminobenzene in head-to-head alignment mode along the a-axis, with short interstack S(1)/S(3) contacts (3.964 Å).

The TGA curves of 1–2 are shown in Fig. 4. The TGA curve of 1 consists of one weight loss from 250  C to 380  C, owing to the release of 1,3-dithiole ring. There is also one step of weight loss for 2 occurring in the region 150–320  C, which corresponds to the loss of 1,3-dithiole ring. The total weight losses are 78.82% for 1 and 79.02% for 2, consistent with the calculated value of 78.69% and 78.86%, respectively. The thermal stability of 1 is superior to that of 2, which could be a good evidence of the presence of different substituent group within two structures, and the strength of such CH3 group is larger than the attraction of –CH2–CH2– group. For the thermal stability of the framework of 1, we can conjecture that the –CH3 groups are helpful for the stability of framework.

3.3. Photophysical and electrochemical properties The photophysical and electrochemical data of 1 and 2 are collected in Table 3. The UV–visible absorption spectra of 1 and 2 in CH2Cl2 (1 105 M) are shown in Fig. 5. The bands centered at 306 nm (32700 cm1) and 347 nm (28800 cm1) for 1, but 304 nm (32900 cm1) and 364 nm (27500 cm1) for 2, due to the electronreleasing character of the substituents attached to the dithiole rings (SCH3, (SCH2)2). Result reveals that the lowest-energy absorption band above 340 nm is red-shifted, in comparison to that of the parent TTF (317 nm) [29]. The result of UV–vis research will be further discussed in the theoretical calculation section. CV experiments in the scan range of 0–1.5 V show one irreversible oxidation wave (E1ox ¼1.00 V for 1, E1ox ¼1.02 V for 2), as shown in Fig. 6. The HOMO energy levels are 5.45 for 1 and 5.47 eV for 2, respectively, below the vacuum level. Both the HOMO energy levels of compounds 1 and 2 are similar, indicating the similar sensitive to oxidative degradation. According to the energy gap Eg between HOMO and LUMO in Uv/vis and CV spectra, the LUMO energy levels can be obtained, 2.50 eV for 1 and 2.55 eV for 2. These experimentally estimated energy levels of the frontier orbital of 1 and 2 are approximately in consistent with those obtained from DFT calculations (see quantum chemistry calculations). The substituents attached to the dithiole rings (SCH3, (SCH2)2) have little effect on the photophysical and electrochemical properties of the molecules. As a sequential work, we now turn to consider the new studies with other substituents and investigate effect on the photophysical properties.

Fig. 2. Molecular structures of 1 (a) and 2 (b) showing the atom-labeling scheme (30% thermal ellipsoids).

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Fig. 4. TGA curves of 1 and 2 in N2 at a heating rate of 10  C min1.

Fig. 3. Packing diagrams of 1 (top) and 2 (bottom) along the a-axis, with an emphasis on short inter-stack S/S contacts (dashed lines).

for 1 and 4.00 eV for 2) implies high kinetic stability and low chemical reactivity. Because they are energetically unfavorable to extract electron from a low-lying HOMO, then to add electron to a high-lying LUMO [31]. A highly energetic HOMO–LUMO gap is indicative of the absence of a certain degree of conjugation, which is caused by the unplanar structures for two compounds with central rings of p-iminobenzene moieties twisted from two 1,3dithiole rings. The HOMO is mainly situated in the extended TTF donor moiety, while localized on the middle of phenylenediamine segment for the LUMO. Single-point energy calculations on 1 and 2 (all in the neutral conformation) provide the first and second vertical ionization energies (IE) (Table 4). The vertical IEs of the parent TTF (devoid of the substituents) were previously calculated to be 6.49 and 11.10 eV, respectively [32]. The results show that the donor ability of compounds 1 and 2 are slightly less (gas phase) than TTF. To investigate the nature of electronic transitions, the electronic spectra of 1 and 2 were calculated using the time-dependent DFT (TDDFT) approach at the B3LYP/6-31G* starting from the experimental X-ray structures as input geometries. The strong absorption wavelength l, oscillator strength f, and main configuration are listed in Table 5. The results of calculations reveal that the low-energy absorption bands center at 345.9 nm (3.58 eV) for 1 and 365.7 nm (3.39 eV) for 2, respectively, based on the first electronic excited state. For this kind of electronic transition, we calculated the oscillator strength f, 1.029 and 0.341. The results indicate the transition is mainly attributed to the promotion of one-electron from HOMO to LUMO. As can be seen in Fig. 7, the HOMO–LUMO promotion for compounds 1 and 2 show some electron-density transfer from the dithiole rings to the phenylenediamine units. The

3.4. Quantum chemistry calculations Frontier orbital (Fig. 7) was obtained by density functional theory (DFT) single-point calculation at the B3LYP/6-31 þ g (d,p) level. The orbital energy analysis for compounds show EHOMO and ELUMO values are 5.5 and 1.45 eV for 1, 5.35 and 1.35 eV for 2, respectively, corresponding to HOMO–LUMO energy gaps 4.05 eV for 1 and 4.00 eV for 2. There is the difference between the value of the HOMO–LUMO gap determined experimentally and the one calculated by DFT. The fundamental reason for this is that the solvent effect and the calculation error are not considered. The HOMO–LUMO energy gap has been used as a simple indicator of kinetic stability [30,31]. A large HOMO–LUMO energy gap (4.05 eV

Table 3 Photophysical and electrochemical data of 1 and 2. Compound

ledge/nma

Eg/eVb

peak Eox =V

onset =Vc Eox

EHOMO/eVd

ELUMO/eVe

1 2

420 425

2.95 2.92

1.00 1.02

0.85 0.87

5.45 5.47

2.50 2.55

a

Absorption edge. Determined from the absorption edge. c Onset potentials of first oxidation wave determined by cv: Ag/Agþ as a reference electrode, GCE as the working electrode with 0.5 mol dm3 BuNClO4, 50 mVs1 in CH2Cl2. d onset þ 4:58Þ. Calculated according to EHOMO ¼ eðEox e All values were estimated from the optical band gaps and EHOMO. b

Y. Zhu et al. / Solid State Sciences 12 (2010) 391–396

395

Fig. 7. Calculated HOMO (bottom) and LUMO (top) orbitals and HOMO–LUMO gaps (B3LYP/6-31 þ g(d,p)) for the compounds 1 and 2.

Fig. 5. UV–vis absorption spectra in dichloromethane at room temperature.

absorption band centered at 275.0 nm (4.51 eV) for 1 and 301.4 nm (4.11 eV) for 2 are mainly resulted from the HOMO  1 / LUMO þ 2 and HOMO  2 / LUMO one-electron promotion, respectively. The calculated vertical excitation energies are in good agreement with the experimental values.

Table 4 Single-point energy calculations provide first and second vertical ionization energies (IE/eV).

þ 2þ

TTF

1

2

6.49 11.10

6.56 9.77

6.47 9.23

Table 5 Electronic spectral data of 1 and 2 calculated with TDDFT at the B3LYP/6-31G* level.

lcal/nm

f

Transition nature

Coefficient

Transition energy/eV

lexp/nm

1 345.9

1.029

0.65538

3.58

347

275.0

0.071

HOMO / LUMO(86%) HOMO  1/ LUMO þ 2 (55%) HOMO  1/ LUMO þ 1 (17%)

0.52398

4.51

306

HOMO / LUMO (80%) HOMO  2 / LUMO (57%) HOMO  1 / LUMO þ 2 (13%) HOMO / LUMO þ 1 (12%)

0.63177

3.39

364

0.53581

4.11

304

2 365.7

0.341

301.4

0.183

0.29347

0.25807 0.24787

4. Conclusion In conclusion, we have synthesized two novel conjugated imino-p-extended tetrathiafulvalenes with p-iminobenzene. The single-crystal X-ray analyses revealed nonplanar structures, with mixed donor-iminobenzene stacks in the packing of 1, and complete segregation of donor and iminobenzene fragments of 2. Photophysical and electrochemical experiments are investigated and further confirmed by DFT calculations. The high HOMO–LUMO gaps indicate high kinetic stability of the two compounds. Further studies on the synthesis of a series of novel imino-p-extended tetrathiafulvalenes as well as their applications are in progress. Acknowledgements

Fig. 6. Cyclic voltammograms of 1 and 2.

We are thankful for the financial support from National Nature Science Foundation of China (Nos.20571029, 20671038) and Jiangsu

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