HETEROCYCLES

Short Paper | Regular issue | Vol. 83, No. 8, 2011, pp. 1859-1866
Received, 14th April, 2011, Accepted, 13th May, 2011, Published online, 25th May, 2011.
DOI: 10.3987/COM-11-12235

■ The Norphthalocyanines Bearing Two TTF Units: Synthesis, Photophysical and Electrochemical Properties

Ruibin Hou, Cuiping Jiang, Tie Chen, Long-Yi Jin, and Bingzhu Yin*

Key Laboratory of Natural Resources of Changbai Mountain & Functional Molecules, Ministry of Education, Yanbian University, Yanji 133002, China

Abstract

A magnesium norphthalocyanine (6a) was prepared by the mixed condensation of phthalonitrile in excess with the dithiomaleonitrile bearing two TTF units (5), using Mg(I1) as a template. Subsequent demetalation of 6a with acetic acid gave 6b in good yield. The structures of target compounds were determined by EA, NMR and TOF MS and their electrochemical and optical properties were also studied by cyclic voltammetry and UV-vis spectroscopy.

Porphyrinic macrocycles, such as porphyrins (Prs), phthalocyanines (Pcs) and porphyrazines (Pzs), are the subject of great interest in areas such as catalysis, photodynamic therapy, and in the fabrication of molecular electronic or magnetic devices.1 Up to now, a great variety of symmetrical porphyrinic macrocycles have been extensively studied.2 However, there have been only a limited number of reports on unsymmetrical porphyrinic macrocycles,3 mainly because they are much more difficult to synthesize and purify. In recent years, there has been a growing interest in the properties of the unsymmetrically Pcs because they display second- and third-order nonlinear optical properties in solution, high thermal stability, mesogenic behavior, Langmuir–Blodgett film formation and semiconducting properties.4 Recently, we reported a series of symmetrical and unsymmetrical Pzs bearing tetrathiafulvalene (TTF) units, including the Pzs annulated directly with four TTF units and linked TTF units with either crown ether or ethylenedithio spacers.5 These Pzs bearing multi-TTF units show good electron-donating properties and give rise to two-, octa- or hexadeca- radical cationic species of TTF moieties.
In the present work, we describe the synthesis, photophysical and electrochemical properties of two new norphthalocyanine derivatives 6a–b, which are composed of two TTF units as electron donor and a norphthalocyanine ring as acceptor moieties covalently linked with two ethylenedithio spacers.

Cross-coupling reaction of 1 and 2 in the triethyl phosphite at 120 °C under Ar gave the red mono-cyanoethyl protected tetrathiafulvalene derivatives 3 in a reasonable yield. Compound 3 was deprotected by treatment with cesium hydroxide in a mixture of DMF and MeOH and the obtained thiolate ion was reacted with excess 1,2-dibromoethane to yield compound 4 which was subsequently converted into key intermediate dithiomaleonitrile 5 bearing two tetrathiafulvalene units linked with two ethylenedithio spacers by reaction with disodium maleonitrile 2,3-dithiolate in DMF. The IR spectrum of 5 shows the typical C≡N stretching vibration at 2212 cm-1, which disappears upon tetramerization to the norphthalocyanine 6a. A mixed condensation of 1 equiv. of dithiomaleonitrile 5 (A) and 30 equiv. of phthalonitrile (B) under classic Linstead macrocyclization conditions (magnesium n-Butoxide/n-Butanol) produced two porphyrinic products, the desired norphthalocyanine 6a (AB3) and the phthalocyanine (B4) (Scheme 1). Fortunately, the large polarity differrences among products allow us to separate easily the desired norphthalocyanine 6a. Demetallation of 6a with acetic acid gave a free base norphthalocyanine 6b in 86% yield as a deep blue powder. Compound 6a-b were soluble in the usual organic solvents except for alcoholic solvents. The MALDI-TOF mass spectra of 6a-b featured peaks at m/z 1876.18 [M + H]+ and 1854.47 [M+H]+, respectively, corresponding to M+ (1875.52) of 6a and M+ (1853.55) of 6b, respectively. The 1H NMR spectra of 6a-b recorded in CDCl3 at 25 °C showed a bit broadened signals except for the terminal methyl groups, which could be explained by considering aggregation in concentrated solutions.5 In the 1H NMR spectrum of 6b, the typical shielding of the inner core NH protons was observed as broad signals at δ = -3.39, which could be exchanged with D2O. Elemental analyses of these compounds were in accord with the proposed molecular formulae.
The optical spectra of all norphthalocyanine exhibit two main bands, a Soret or B band (π→π*, corresponds to a deep π → LUMO transition) between 339-359 nm and another around 650 nm, denoted the Q band (π→π*) (Figure 1a). In general, the 4-fold symmetric porphyrinic macrocycles have similar optical spectra, with single transitions for both the Q and B bands. In contrast, the optical spectra of unsymmetrical norphthalocyanines 6a-b, with three fused benzo rings and two TTF units substituted dithiolene moiety, clearly show a splitting of the Q band. Their optical spectra look quite like those of the norphthalocyanines substituted with 2,3-dialkylthio groups in the literatures.3,6 These differences can be rationalized through Gouterman’s highly simplified four orbital model for the optical spectra of porphyrinic macrocycles.7 For both of 6a and 6b, the Q band strictly followed the Lambert-Beer law (up to 10 µM), indicating that these compounds are essentially free from aggregation in CH2Cl2, THF and DMF. The spectrum of 6a is given as an example (Figure 1b).

To evaluate the potential of target compounds to act as a electron donor, the electrochemical characterization of compounds 6a-b were carried out by using cyclic voltammetry (CV) in a mixture of CH2Cl2/CH3CN (9:1, v/v). Figure 2a shows the cyclic and differential pulse voltammograms (DPV) of 6a within a −2000 mV to +2000 mV potential window. Compound 6a shows two reduction couples (E1/2 = -1.435 V and -1.000 V) and three oxidation couples (E1/2 = 0.475 V, 0.863 V and 1.386 V) within the potential window of the CH2Cl2-CH3CN / Bu4PF6 electrolyte system. The five couples observed were assigned to Pz-3/Pz-4 (I), Pz−2/Pz−3 (II), TTF+• /TTF (III), TTF+2/ TTF+• (IV), Pz−1/Pz−2 (V) on the basis of results in the literature.8 Processes I-II and V are irreversible in terms of the ratio of anodic to cathodic

peak currents. Processes III-IV, which can be assigned to the simultaneous first and second oxidations of the TTF unit, are quasi-reversible with anodic to cathodic peak separation (ΔE) of 0.083 V and 0.102 V, respectively, and the unity of the Ipa / Ipc ratios at all scan rates and linear variation of the peak currents.
Metal free norphthalocyanine 6b shows an aggregation tendency in concentrated solution, so the redox processes are complex due to the splitting of original waves, which are also induced by electron transfer of aggregated and monomeric species. Two reduction and two oxidation processes are recorded with the complex 6b at −1.566, −0.93 (−1.212) , 0.476, and 0.875 V vs. SCE, respectively, at 0.100 Vs−1 scan rate. The second reduction couple of the complex are split into two peaks due to aggregation of the species.9 Dilution of the solution of the complex causes the peak current of the waves assigned to the aggregated species decrease to more than that of the monomeric species, supporting the existence of the aggregation-disaggregation equilibrium (Figure 2b).
To further address the donor properties of the newly synthesized compounds, doping studies were conducted using 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) in CH2Cl2. Compounds 6a-b doped with 4 equiv. TCNQ in CH2Cl2, no CT bands were observed in the 600-1000 nm region. But when 6a-b was doped with 1 equiv. F4TCNQ, a new absorption band was produced around λmax 866 nm in the UV-Vis spectrum (Figure 1a). This new band corresponds to the cation radical species of the TTF moieties.10 The formation of the charge-transfer complex between 6a with F4TCNQ in CH2Cl2 was also confirmed by the FT-IR. The FT-IR spectrum of a mixture of 6a and F4TCNQ (1:1) shows the nitrile stretch of the F4TCNQ radical anion at 2196 cm-1 compared to the neutral state of 2218 cm-1.
In summary, we first synthesized two norphthalocyanines (6a-b) bearing two TTF units linked by an ethylenedithio spacer. Compound 6a-b were sufficiently stable for purification and for further experiments. Metal-free norphthalocyanine 6b shows an aggregation tendency in concentrated solution, so a reduction couple is split into two waves due to electron transfer of aggregated and monomeric species. The ability of compounds 6a-b to function as a donor for F4TCNQ was established, and the formation of charge-transfer complexes were confirmed by the UV-Vis and FT-IR spectroscopy. The remetallation of metal-free norphthalocyanine 6a and determination of mesogenic properties of norphthalocyanines are in progress.

EXPERIMENTAL
The 1H NMR spectra were recorded with a Bruker AV-300 spectrometer, and chemical shifts were referenced relative to tetramethylsilane. The UV-vis spectra were taken on a Hitachi U-3010 spectrophotometer. MALDI-TOF-MS data were obtained by a Shimadzu AXIMA-CFRTM plus spectrometer, using a 1,8,9-anthracenetriol (DITH) matrix. Cyclic voltammetry was carried out on a Potentiostat/Galvanostat 273A instrucment employing 0.1M Bu4NPF6 as the supporting electrolyte in chloroform, with sweep speed of 100 mv/s. Counter and working electrdes were made of platinum and Glass-Carbon (GCE, 4.00 mm diameter), respectively, and the reference electrode was calomel electrode (SCE). Starting materials 1 and 2 were prepared according to reference.11
6,7-Bis(dodecylthio)-2-(2-cyanoethyl)thiotetrathiafulvalene (3): A suspension of compound 1 (9.48 g，17.7 mmol) and 2 (3.0 g, 14.8 mmol) in freshly distilled P(OEt)3 (60 mL) under Ar was stirred at 120 °C for 4 h and then the reaction mixture cooled to rt. The P(OEt)3 was removed in vacuum and the residue was purified by column chromatography on silica gel with CH2Cl2/pet. ether (1:1, v/v) as an eluent to afford a yellow powder. The solid was recrystallized from MeCN-EtOH to gave 3 as orange needles 4.0 g, yield 53%, mp 73-74 °C. 1H NMR (CDCl3): δ 0.88 (m, 6H), 1.20-1.50 (m, 36H), 1.59-1.67 (m, 4H), 2.70 (t, J = 7.2 Hz, 2H), 2.83 (t, J = 7.2 Hz, 4H), 3.00 (t, J = 7.2 Hz, 2H), 6.56 (s, 1H); TOF-MS, m/z (%) = 689.24 (M+ , 100). Anal. Calcd for C33H55NS7: C, 57.42; H, 8.03; N, 2.03. Found: C, 57.60; H, 8.09; N, 2.12.
2-(2-Bromoethylthio)-6,7-bis(dodecylthio)tetrathiafulvalene (4): To a solution of 3 (1.0 g, 1.45 mmol) in dry DMF (28 mL) was added a solution of CsOH·H2O (292 mg, 1.74 mmol) in anhydrous MeOH (5 mL) over a period of 45 min. The mixture was stirred for an additional 30 min and 1,2-dibromoethane (2.72 g, 14.5 mmol) was injected to the mixture in 45 min. The solution was stirred overnight. After seperation by column chromatography on silica gel with CH2Cl2/pet. ether (60-90 °C) (1:3, v/v) as eluant, 4 was obtained as a reddish yellow solid. The solid was recrystallized from CH2Cl2-EtOH to gave an orange needles 0.81g, yield 75%, mp 41-42 °C. 1H NMR (CDCl3): δ 0.88 (m, 6H), 1.26 (br, 32H), 1.40 (br, 4H), 1.63 (br, 4H), 2.82 (br, 4H), 3.13 (t, J = 7.4 Hz, 2H), 3.50 ( t, J = 7.4 Hz, 2H), 6.47 (s, 1H); TOF-MS, m/z (%) = 744.15 (M+, 100); Anal. Calcd for C32H55BrS7: C, 51.65; H, 7.45. Found: C, 51.41; H, 7.49.
1,2-Bis[6,7-bis(dodecylthio)tetrathiafulvalen-2-ylthioethylthio]-1,2-dicyanoethene (5): A solution of 4 (500 mg, 0.672 mmol) and disodium maleonitrile-2,3-dithiolate (63 mg，0.336 mmol) in anhydrous degassed DMF (20 mL) were stirred at 90 °C for overnight under Ar. The solvent was removed in vacuum and the residue was purified by column chromatography on silica gel with CH2Cl2/pet. ether (1:1, v/v) as an eluent to afford 5 as a brown powder 231 mg, yield 43%, mp 47-48 °C. 1H NMR (CDCl3): δ 0.88 (t, J = 7.61 Hz, 12H), 1.26 (m, 64H), 1.40 (br, 8H), 1.61-1.64 (m, 8H), 2.82(t, J = 6.7 Hz, 4H), 3.05 (t, J = 7.2 Hz, 4H), 3.39 (t, J = 7.2 Hz, 2H), 6.52 (s, 2H); MALDI-TOF MS m/z (%) = 1467.88(M+ + 1, 100).; Anal. Calcd for C68H110N2S16: C, 55.61; H, 7.55; N, 1.91. Found: C, 55.15; H, 7.50; N, 1.88.
{2,3-Bis[6,7-bis(dodecylthio)tetrafulvalen-2-ylthioethylthio]norphthalocyanine}magnesium(II) (6a)
Magnesium (16.3 mg, 0.68 mmol) metal was dissolved in anhydrous n-BuOH (45 mL) at reflux under Ar. To this magnesium butoxide solution was added the compound 5 (100 mg，0.068 mmol) and phthalonitrile (174 mg, 1.36 mmol). The mixture was refluxed for 26 h under Ar. The solution color changed from purplish red to deep blue. The blue mixture was cooled to rt. The precipitate was collected by suction and washed with large amounts of CHCl3. The blue solid was purified by chromatography on silica gel with CH2Cl2/pet. ether (200:1~100:1, v/v) to give 6a as a deep blue solid 15.2 mg, yield 12.5%. Reprecipitation of 6a from CH2Cl2-MeOH gave a deep blue powder, mp > 250 °C (by DTA). 1H- NMR (CDCl3)，δ：0.85 (t, J = 5.85 Hz, 12H), 1.15 (br, 64H), 1.40 (br , 8H), 2.42 (br , 8H), 2.57 (br, 8H), 3.05 (br, 4H), 3.84 (br, 4H), 6.07 (s, 2H) 7.81 (br, 2H), 7.99 (br, 8H), 8.23 (br, 2H), 8.85 (br, 8H); UV (CH2Cl2) λmax: 697 (ε = 69100), 650 (ε = 47800), 635 (sh, ε =44700), 331 (sh, ε = 20900), 357 (ε = 70000); MALDI-TOF MS m/z (%) = 1876.18 (M+ + 1, 100); Anal. Calcd for C92H122MgN8S16: C, 58.86; H, 6.55; N, 5.97. Found: C, 57.64; H, 7.06; N, 5.80.
2,3-Bis[6,7-bis(dodecylthio)tetrafulvalen-2-ylthioethylthio]norphthalocyanine (6b)
A solution of magnesium norphthalocyanine 6a (20.0 mg, 0.011 mmol) in a mixture of acetic acid (0.5 mL) and CH2Cl2 (0.5 mL) was stirred at room temperature for 72 h, then Et2O was added and the resulting suspension brought to pH 7 with 1M NaOH. The organic phase was washed with a saturated NH4Cl aq. and water, dried over MgSO4. The solution was concentrated in vacuo and the residue was purified by column chromatography on silica gel (CH2Cl2/pet. ether, 3:1) to give a blue solid. Reprecipitation of the solid from CH2Cl2-MeOH gave 6b as a deep blue powder. Yield: 17 mg (86%), mp 106 °C (to be soften). 1H NMR (CDCl3): δ -3.39 (br, 2H), 0.86 (t, J = 3 Hz, 12H), 1.00-1.80 (m, 78H), 2.42 (t, J = 6.6 Hz, 4H), 2.57 (t, J = 6.6 Hz, 4H), 3.26 (t, J = 6.7 Hz, 4H), 4.06 (t, J = 6.6 Hz, 4H), 6.28 (s, 2H), 7.26-7.72 (m, 8H), 8.14 (d, J = 6.7 Hz, 4H), 8.27 (d, J = 6.7 Hz, 2H); UV (CH2Cl2) λmax: 704 (ε = 39900), 670 (ε = 9700), 610 (ε = 29000), 566 (sh, ε =15600), 338 (ε = 57200); MALDI-TOP MS m/z: 1854.47 (M++1, 100); Anal. Calcd. for C92H124N8S16: C, 59.57; H, 6.74; N, 6.04. Found: C, 59.61 ; H, 6.70 ; N, 6.05.

ACKNOWLEDGEMENTS
The authors acknowledge financial support from the National Natural Science Foundation of China (grant No.21062022), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20102201110001) and the Open Project of the State Key Laboratory of Supramolecular Structure and Materials, Jilin University.

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