Highly conjugated donor-acceptor dyad based on monotetrathiafulvalene covalently attached to a magnesium norphthalocyanine unit

Journal Pre-proof Highly conjugated donor-acceptor dyad based on monotetrathiafulvalene covalently attached to a magnesium norphthalocyanine unit Ruibin Hou, Li Wang, Fuzhi Wei, Yan Xia, Dongfeng Li PII:

S0022-2860(20)30214-3

DOI:

https://doi.org/10.1016/j.molstruc.2020.127890

Reference:

MOLSTR 127890

To appear in:

Journal of Molecular Structure

Received Date: 23 November 2019 Revised Date:

2 February 2020

Accepted Date: 11 February 2020

Please cite this article as: R. Hou, L. Wang, F. Wei, Y. Xia, D. Li, Highly conjugated donor-acceptor dyad based on monotetrathiafulvalene covalently attached to a magnesium norphthalocyanine unit, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.127890. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Ruibin Hou: Writing-Original draft preparation. Li Wang and Fuzhi Wei: Performing the experiment. Yan Xia and Dongfeng Li: Writing-Reviewing and Editing.

Graphical Abstract

A novel highly conjugated donor–acceptor dyad with tetrathiafulvalene (TTF) covalently attached to a magnesium norphthalocyanine (NPc) unit was synthesized.

Highly

conjugated

donor-acceptor

dyad

based

on

monotetrathiafulvalene covalently attached to a magnesium norphthalocyanine unit Ruibin Hou a,b, Li Wang a,b, Fuzhi Wei a,b, Yan Xia a,b,*, Dongfeng Li a,* a

School of Chemistry and Life Science, Changchun University of Technology, Changchun, 130012, China

b

Advanced Institute of Materials Science, Changchun University of Technology, Changchun 130012, China

Corresponding author: Tel:+86-431-85716671 E-mail: [email protected] (Y. Xia); [email protected] (D. Li).

Abstract: A novel highly conjugated donor–acceptor dyad composed of a tetrathiafulvalene (TTF) moiety covalently attached to a magnesium norphthalocyanine (NPc) unit (1) has been synthesized and full characterized. Ultraviolet–visible spectroscopy and electron paramagnetic resonance data reveal that 1 forms an electron transfer complex with Detailed 2,3,5,6-tetrafluoro-7,7,8,8-tetra-cyanoquinodimethane (F4TCNQ). electrochemical investigations show one irreversible one-electron oxidation wave, two quasi-reversible one-electron oxidation waves, and two irreversible one-electron reduction waves, indicating that 1 is a good π-electron donor. Compound 1 shows clear intramolecular charge transfer (ICT) interactions from the tetrathiafulvalene fragments to the norphthalocyanine core. This phenomenon can be explained by density functional theory (DFT). Key words: tetrathiafulvalene, magnesium norphthalocyanine, intramolecular charge transfer, density functional theory

1. Introduction Combining

porphyrazine

(Pz)

or

phthalocyanine

(Pc)

derivatives

with

tetrathiafulvalene (TTF) is attracting a lot of attention because of the wide range of applications of the resultant conjugates to, for example, organic conductors and photonics materials. In particular, generation of stable charge-separated states after photoexcitation of the Pc and Pz moiety is of great importance because of their pivotal roles in artificial photosynthetic processes. A wide range of symmetrical macrocyclic TTF–Pc and TTF–Pz compounds have been extensively studied. For example, Decurtins and co-workers and Yin and co-workers independently reported preparation of

novel

tetrakis-TTF-annulated

Pc

and

Pz

derivatives,

respectively.

Tetrakis-TTF-annulated Pc potentially has novel electronic and photophysical properties [1-5]. However, to the best of our knowledge, there have been only a few reports concerning unsymmetrical Pc and Pz macrocycles, primarily because they are difficult to synthesize and purify. In the past few years, there has been increasing interest in the properties of unsymmetrical Pcs and Pzs because these compounds show mesogenic behavior, high thermal stability and second- or third-order nonlinear optical properties in solution, and they are also capable of forming LB films exhibiting excellent semiconducting properties. Among the unsymmetrical Pcs, NPc is a Pc with three fused benzo-substituent moieties [6-9]. Not long ago, we reported a series of NPcs bearing the TTF moiety, with the NPcs linked to the TTF unit by either a crown ether or ethylenedithio spacer [10, 11]. These norphthalocyanines bearing the TTF unit show good electron-donating properties and give rise to one or two radical cationic species of TTF moieties. In the present work, we designed and synthesized highly conjugated dyad 1 with a TTF moiety covalently attached to a NPc unit (Scheme 1), and we then investigated the electrochemical and spectral properties and performed theoretical calculations of 1.

2. Results and Discussion The route for synthesis of target compound 1 is shown in Scheme 1. The

cross-coupling

reaction

of

4,5-dicyano-1,3-dithiol-2-one

2

and

4,5-bis

(alkylthio)-l,3-dithiole-2-thione 3a or 3b in a mixture of triethyl phosphite and toluene under

reflux

and

N2

leads

to

the

key

intermediate

2,3-dicyano-6,7-bis(alkylthio)tetrathiafulvalene 4a or 4b in 36 % or 38 % yield, respectively (see Supporting Information). NC

S

NC

S

O

+

S

SR

S

SR

S

S

S

SR

high dilution, ref lux

NC

S

S

SR

4a R = n-butyl 4b R = n-octyl

3a R = n-butyl 3b R = n-octyl

2

N CN 4a

NC P(OEt)3 / PhCH3

Mg(OBu)2 BuOH

+ CN

N

N Mg

N

N

S

S

S

S

S

N

reflux N

S

N 1

Scheme 1. Synthetic route for tetrathiafulvalene-annulated norphthalocyanine.

Black crystals of 4a and 4b suitable for X-ray diffraction were obtained by slowly evaporating CH2Cl2/CH3OH solutions of 4a and 4b at room temperature. For the packing mode of compound 4a, a layered structure can be found along (011) plane, as shown in Fig. 1A. Inside the layer, two molecules of compound 4a arrange in an alternating head and tail style and their alkyl chains outreach to the border of the layer. In the crystal structure, there are no obvious interactions between the neighboring alkyl chains and the length of every layer is 1.68 nm. Compound 4b has a very similar packing structure as that of compound 4a, as shown in Fig. 1B. It is also a layered structure which the layer distance is 2.36 nm, a little larger than that of compound 4a, due the existing of the long alky chains. The crystallographic data and structural refinement are summarized in Table S1 in the Supporting Information.

A

B

Fig. 1. Packing structures of the key precursors (A) 4a and (B) 4b shown along [100] direction.

Mixed condensation of 20 equiv. of phthalonitrile (A) and 1 equiv of 4 (B) under classic Linstead macrocyclization conditions (magnesium propoxide/n-propanol)

produced two porphyrinic products: the desired mono-TTF-annulated NPc 1 (A3B) and the Pc (A4) (Scheme 1). Fortunately, the large polarity differences among the products allowed the desired NPc 1 to be separated as a deep blue powder (9 % yield). Compound 1 is soluble in usual organic solvents, except for alcoholic solvents. The 1

H NMR spectrum of 1 recorded in CDCl3 at 25 °C shows slightly broadened signals,

except for the terminal methyl groups, which can be explained by aggregation in concentrated solution [12,13]. The MALDI-TOF mass spectrum of 1 has a peak at m/z = 838.10 [M]+, corresponding to M+ (838.08) of 1. The elemental analyses of the compounds are in agreement with the proposed molecular formulae. The optical spectrum of 1 exhibits two main bands (Fig. 2A): a B or Soret band (π→π*, corresponding to a deep π → lowest unoccupied molecular orbital (LUMO) transition) at about 355 nm and a Q band (π → π*) at about 650 nm. In general, four-fold symmetric Pc macrocycles have similar optical spectra, with single transitions for both the Q and B bands. In contrast, the optical spectrum of unsymmetrical NPc 1, which has three benzo-fused pyrrole moieties and one TTF-substituted dithiolene moiety, is similar to those of NPcs substituted with 2,3-dialkylthio groups [7, 9]. These differences can be rationalized through Gouterman’s highly simplified four orbital model for the optical spectra of porphyrinic macrocycles [14]. To further investigate the electron-donating property of the target compound, compound 1 was doped with excess tetra-cyanoquinodimethane (TCNQ) in CH2Cl2, but no charge transfer (CT) band is observed in the 700–1000 nm region, which can be attributed to the presence of the electron-withdrawing NPc ring. However, addition of 1 equiv of F4TCNQ to a CH2Cl2 solution of 1 (5 × 10−4 M) results in appearance of new CT absorption bands centered at λmax = 865 nm in the UV–vis spectrum (Fig. 2A). These new bands correspond to the singly occupied molecular orbital–LUMO transition of the cation radical species of the TTF moiety. Formation of the F4TCNQ−/TTF+ CT complex was confirmed by the EPR spectrum of 1 in CH2Cl2 at 25 °C centered around g = 2.007 and 2.002, which are in the region characteristic of both the TTF radical cation and F4TCNQ radical anion [15, 16]. These results demonstrate that some CT occurs between the TTF unit(s) and F4TCNQ

in solution (Fig. 2B). A

0.6

1 1+1equiv. F4TCNQ

Absorbance

0.5

0.4

0.3

0.2

0.1

0.0 300

400

500

600

700

800

900

1000

Wavelength(nm)

800

B

600 400

I/a.u

200 0

-200 -400 -600 -800 330

332

334

336

338

340

342

Field/mT

Fig. 2. (A) Electronic absorption spectra of 1 in CH2Cl2 (5 × 10−4 M) and after addition of 1 equiv of F4TCNQ. (B) Electron paramagnetic resonance (EPR) spectrum of 1 in CH2Cl2 (5 × 10−4 M) with 1 equiv of F4TCNQ.

To evaluate the potential of the target compounds to act as electron donors, electrochemical characterization of compound 1 was performed in a mixture of CH3CN/CH2Cl2 (1:4, v/v) by cyclic voltammetry (CV). Fig. 3 shows the cyclic and differential pulse voltammograms (DPV) of 1 within the −1800 to 1800 mV potential window. Compound 1 shows three oxidation couples (E1/2 = 0.452 V, 1.042 V and 1.518 V) and two reduction couples (E1/2 = -1.432 V and -0.895V) within the potential

window of the CH3CN-CH2Cl2 /Bu4PF6 electrolyte system. These six couples are assigned to NPc−4/NPc−5 (I), NPc−3/NPc−4 (II), NPc−2/NPc−3 (III), /TTF+•/TTF (IV), TTF+2/TTF+•(V), and NPc−1/NPc−2 (VI) on the basis of previously reported results [11]. Processes I, II, V and VI are irreversible in terms of the ratio of anodic to cathodic peak currents. In contrast, processes III and IV, which can be assigned to simultaneous first and second oxidation of the TTF unit, are quasi-reversible.

2x10-5 1x10-5

III

II

I

0 -5

-1x10

I/A

1.0x10-5

-5

-2x10

8.0x10

6.0x10-6

I/A

-3x10-5

4.0x10

-4x10-5 -5x10-5

-6

-6

2.0x10-6

0.0

IV

-1.0

-0.5

V

0.0

0.5

1.0

1.5

E/v(vs.SCE)

-5

-6x10

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

E/v(vs.SCE)

Fig. 3. cyclic voltammetry (CV) of 1 in CH3CN-CH2Cl2 (1:4, v/v, 1 × 10−3 M) at a scan rate of 100 mV s−1. The insert shows the differential pulse voltammogram (DPV) of 1 under the same conditions.

top view

side view

Fig. 4. The optimized structure of model 1.

To gain deeper insight into imolecular structure and electronic structures, mono

TTF-NPc 1 is further examined by theoretical calculations. Themolecular geometry is optimized at the B3LYP/6-31G** level of theory. All calculations have been performed with the Gaussian 09 package [17, 18]. The optimized structure of model 1 displays in Fig. 4. The optimized calculations offer a nonplanar structure of 1, where the TTF fragment is banana-shaped, NPc-Mg is plane. Although the crystal structure is not available, the arrangements of banana-shaped TTF and NPc core are rational when compared with the crystal structure of the NPc and TTFs [12, 19].

The electronic density distributions of the highest occupied molecular orbital (HOMO) lowest unoccupied molecular orbital (LUMO) are illustrated in Fig. 5. A particularly interesting feature is the localization and spatial separation of the HOMO and the LUMO. The electron density of the HOMO is mainly located on the sulfur atoms of the dibutyl and TTF moieties, while the LUMO density is mainly concentrated on the NPc-Mg ring. Generally, dye molecules with this type of electron distribution exhibit intramolecular charge transfer (ICT) characteristics.

HOMO

LUMO

Fig. 5. The frontier molecular orbitals of model 1.

3. Conclusions In summary, we have developed a synthetic route to synthesize NPc annulated with TTF bearing two butylthio units (1). Compound 1 is sufficiently stable for purification and further experiments. The ability of compound 1 to act as a donor for F4TCNQ is established, and evidence for formation of a CT complex was obtained by UV–vis and

EPR spectroscopy. Theoretical calculation could be a useful method for improving our understanding of the electronic structure of the interacting molecule. Aggregation of the target compound in a mixed solution and synthesis of long alkyl chain analogs are currently being investigated. 4. Material and methods 4.1. General Information The NMR spectra were recorded with a Bruker AV-400 spectrometer (400 MHz for 1

H and 100 MHz for

13

C). The chemical shifts were referenced to tetramethylsilane

(δH/δC = 0). Mass spectroscopy was performed with a Shimadzu AXIMA-CFR matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer. The Fourier transform infrared (FT-IR) spectra were recorded with a Perkin Elmer 2400 spectrometer (KBr pressed disc method). Elemental analysis was performed

with an

Elementar Vario EL CHN elemental

analyzer.

The

ultraviolet–visible (UV–vis) spectra were recorded in CH2Cl2 with an L25 spectrophotometer. Cyclic voltammetry (CV) was performed with a CHI 852C electrochemical workstation with CH2Cl2–CH3CN as the solvent (10−3 M) and 0.1 M Bu4NPF6 as the supporting electrolyte. The counter and working electrodes were made of Pt and glassy carbon, respectively, and the reference electrode was a saturated calomel electrode. The electron spin resonance (EPR) was measured in CH2Cl2 with a JEOL JES FA-200 spectrometer. Starting compounds 2–4 were prepared according to procedures described in the literature [20]. 4.2. General procedure for synthesis of 4a and 4b A solution of 4,5-dicyano-l,3-dithiol-2-one (2) (168 mg, 1.00 mmol) and 4,5-bis(alkylthio)-l,3-dithiole-2-thione (3a, alkyl = n-butyl; 3b, alkyl = n-octyl) (1.00 mmol) in toluene (20 mL) was added dropwise to a solution of triethyl phosphite (4 mL, 23 mmol) in toluene (200 mL) under an argon atmosphere. When the solution was heated at the refluxing temperature, the color of the solution changed to dark purple after 10–15 min. The mixture was stirred for an additional 4–5 h and then allowed to cool to room temperature. The toluene solvent was removed in vacuo. After adding methanol (80 mL), the purple precipitate was collected, washed with

methanol, and dried in vacuo. The product was purified by chromatography on a silica gel column with petroleum ether–dichloromethane (3:1) as the eluent to afford a purple solid (4a or 4b). 4.2.1. 2,3-Dicyano-6,7-bis(butylthio)tetrathiafulvalene 4a Recrystallization from hexane gave 4a as purple flakes. Yield 151 mg (35 %). 1H NMR (400 MHz, CDCl3) δ 0.95 (t, 6H, J = 7.3Hz), 1.37–1.60 (m, 4H), 1.60–1.74 (m, 4H), 2.85 (t, 4H, J = 7.2 Hz);

13

C NMR (100 MHz, CDCl3) δ 13.54, 21.59, 31.71,

36.30, 100.01, 109.07, 118.76, 123.26, 128.26. Anal. calcd for C16H18N2S6: C, 44.62; H, 4.21; N, 6.50. Found: C, 44.46; H, 3.88; N, 6.30. 4.2.2. 2,3-Dicyano-6,7-bis(octylthio)tetrathiafulvalene 4b Recrystallization from hexane gave 4b as purple flakes. Yield 211 mg (38 %). 1H NMR (400 MHz, CDCl3) δ 0.89 (t, 6H, J = 8.0 Hz), 1.20–1.35 (m, 16H), 1.35–1.48 (m, 4H), 1.58–1.69 (m, 4H), 2.82 (t, 4H, J = 8.0 Hz); 13C NMR (100 MHz, CDCl3) δ 14.01, 22.58, 28.41, 28.99, 29.08, 29.67, 31.73, 36.58, 99.94, 109.00, 118.71, 123.86, 128.25. Anal. calcd for C24H34N2S6: C, 53.10; H, 6.31; N, 5.16. Found: C, 53.06; H, 6.58; N, 5.12. 4.3. Synthesis of {2,3-[6,7-bis(6,7-butylthio)tetrathiafulvalene]norphthalocyanine} Mg(II) Magnesium (16.3 mg, 0.68 mmol) metal was dissolved in anhydrous n-BuOH (45 mL) at reflux under N2. To this magnesium butoxide solution, compound 4a (100 mg, 0.23 mmol) and phthalonitrile (589 mg, 4.6 mmol) were added. The mixture was refluxed for 26 h under N2. The solution changed from purplish red to deep blue. The blue mixture was cooled to room temperature. The precipitate was collected by suction and washed with a large amount of CHCl3. The blue solid was purified by chromatography on silica gel with CH2Cl2/CH3OH (200:1–20:1, v/v) to give 1 as a deep blue solid (5.5 mg, yield 2.8 %). Reprecipitation of 1 from CH2Cl2–MeOH gave a deep blue powder (melting point > 250 °C by differential thermal analysis). 1H NMR (400 MHz, CDCl3) δ 0.93 (br, 6H), 1.35 (br, 4H), 1.76 (br , 4H), 2.99 (br , 4H), 7.72 (br, 6H), 8.36 (br, 4H), 9.10 (br, 2H); FT-IR (cm−1): 2956.47, 2925.91, 2854,87,

1640.21, 1457.63, 1381.23, 1330.82, 1259.77, 1108.51, 1077.95, 1052.73, 890.79, 723.49. MALDI-TOF MS m/z = 838.10 (M+, 100%, calcd. 838.08). Acknowledgements This work was supported by the National Science Foundation of China (Nos. 21442004 and 21502008), the Department of Science and Technology of Jilin Province (grant No. 00005005031) and the Education Department of Jilin Province (grant No. 2016320). Appendix A. Supplementary data Electronic supplementary information (ESI) available: The complete details on Crystal data and structure refinement of compounds 4a and 4b, mass spectrum of complex 1. References [1] F. Leng, B. Yin, Y. Sheng, Chinese J. Org. Chem. 28 (2008) 1875-1887. [2] J. Sly, P. Kasák, E. Gomar-Nadal, C. Rovira, L. Górriz, P. Thordarson, D.B. Amabilino, A.E. Rowan, R.J. Nolte, Chem. Commun. 10 (2005) 1255-1257. [3] C. Loosli, C. Jia, S.X. Liu, M. Haas, M. Dias, E. Levillain, A. Neels, G. Labat, A. Hauser, S. Decurtins, J. Org. Chem. 70 (2005) 4988-4992. [4] R. Wang, W. Liu, Y. Chen, J.L. Zuo, X.Z. You, Dyes and Pigments 81 (2009) 40-44. [5] R. Hou, B. Li, K. Zhong, H. Li, L.Y. Jin, B.Z. Yin, Eur. J. Org. Chem. 6 (2012) 1138-1146. [6] T. Fukuda, S. Masuda, M. Wahadoszamen, N. Ohta, N. Kobayashi, Dalton T. 31 (2009) 6089-6091. [7] S.J. Lange, J.W. Sibert, A.G.M. Barrett, B.M. Hoffman, Tetrahedron 56 (2000) 7371-7377. [8] L.S. Beall, N.S. Mani, A.J. White, D.J. Williams, A.G. Barrett, B.M. Hoffman, J. Org. Chem. 63 (1998) 5806-5817. [9] T.F. Baumann, M.S. Nasir, J.W. Sibert, A.J.P. White, M.M. Olmstead, D.J. Williams, A.G.M. Barrett, B.M. Hoffman, J. Am. Chem. Soc. 118 (1996) 10479-10486.

[10] J. Guo, Y. Xia, D. Li, R. Hou, Tetrahedron Lett. 57 (2016) 570-573. [11] R. Hou, C. Jiang, T. Chen, L.Y. Jin, B. Yin, Heterocycles 83 (2011) 1859-1866. [12] F. Leng, R. Hou, L. Jin, B. Yin, R. Xiong, J. Porphyr. Phthalocya. 14 (2010) 108-114. [13] R. Hou, L. Jin, B. Yin, Inorg. Chem. Commun. 12 (2009) 739-743. [14] M. Gouterman, The Porphyrins; ed. by Dolphin D, Academic Press: New York, 1978; Vol. III, pp. 1-165. [15] F. Leng, X. Wang, L. Jin, B. Yin, Dyes and Pigments 87 (2010) 89-94. [16] F. Wudl, G.M. Smith, E.J. Hufnagel, Journal of the Chemical Society D: Chemical Communications. 21 (1970) 1453-1454. [17] P.C. Hariharan, J.A. Pople, Mol. Phys. 27 (1974) 209-214. [18] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, et al. Gaussian 09, Gaussian Inc., Wallingford, CT, 2009. [19] T.F. Baumann, J.W. Sibert, M.M. Olmstead, A.G.M. Barrett, B.M. Hoffman, J. Am. Chem. Soc. 116 (1994) 2639-2640. [20] T. Chen, C. Wang, H. Qiu, L.Y. Jin, B. Yin, Heterocycles 71 (2007) 549-555.

Highligts: 1. A novel highly conjugated donor–acceptor dyad with tetrathiafulvalene (TTF) covalently attached to a magnesium norphthalocyanine (NPc) unit was synthesized. 2. The electrochemical and optical properties of the magnesium norphthalocyanine (NPc) unit (1) are fully studied. 3. The results demonstrate that 1 is a good π-electron donor which has clear intramolecular charge transfer (ICT) interactions in molecule.

Declaration of interest statement The authors declared that they have no conflicts of interest to this work.