Chinese Chemical Letters  2018, Vol. 29 Issue (1): 99-101   PDF    
β to β Terpyridylene-bridged porphyrin nanorings
Bangshao Yina, Xu Liangb, Weihua Zhub, Ling Xua, Mingbo Zhoua, Jianxin Songa    
a Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research(Ministry of Education of China), Key Laboratory of the Assembly and Application of Organic Functional Molecules, Hunan Normal University, Changsha 410081, China;
b School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
Abstract: 6, 6"-Terpyridylene bridged cyclic porphyrin dimer 2Ni, trimer 3Ni, tetramer 4Ni and pentamer 5Ni were obtained through Suzuki-Miyaura coupling reaction of β, β'-diboryl Ni(Ⅱ) porphyrin with 6, 6"-dibromo-2, 2':6', 2"-terpyridine. Free base porphyrin nanorings 2H-5H were obtained by demetallation of 2Ni-5Ni with sulfuric acid in CHCl3 and then were converted into 2Zn-5Zn upon treatment with Zn(OAc)2 in quantitative yields, respectively. All of these newly synthesized porphyrin nanorings were characterized by high-resolution mass spectrometry and 1H NMR spectroscopy. The photophysical properties of porphyrin nanoring were examined by UV-vis and fluorescence spectra. The electrochemical properties of 2Ni-5Ni were investigated by cyclic voltammetry and differential pulse voltammetry. The UV-vis absorption spectra and fluorescence spectra of these cyclic porphyrin arrays indicate that there exist unique electronic interactions between the constituent porphyrin units in each ring. Electrochemical analysis shows that the trimer 3Ni exhibit different redox behavior, which indicate that the porphyrin units in 3Ni are presumably more coplanar than in other cyclic porphyrin arrays.
Key words: Crossing-coupling     Terpyridine     Nanoring     Porphyrin     Photophysical properties    

Electronically interactive multiporphyrinic systems have attracted considerable attention in recent years, largely because of their potential applications in optoelectronic devices, sensors, photovoltaic devices, nonlinear optical (NLO) materials, photodynamic therapy (PDT) pigments [1] and other applications [2]. The well-elucidated structure of the light-harvesting complex (LH2) [3-5] and its unusual light-harvesting ability have prompted chemists to synthesized cyclic porphyrin arrays to study the excitation energy transfer (EET) and electronic coupling along the wheel [6]. Cyclic porphyrin arrays are often constructed by means of covalent bond, noncovalent bonds or metal coordination bonds [7-11]. Among those, covalently bonded arrays are structurally robust but difficult to synthesize. Covalently bonded cyclic porphyrin oligomers are often linked by 1, 3-butadiyne [12, 13], 2, 5-thienyl [14, 15], 2, 5-pyrrolyl [16, 17], 2, 6-pyridyl [18], azobenzene [19], and other spacers [20]. As rare examples, The Osuka group has reported the synthesis of directly meso to meso linked cyclic porphyrin arrays as photosynthetic models through silver(Ⅰ) promoted coupling reactions of 5, 15-diaryl-substituted zinc(Ⅱ) porphyrin [21]. We reported directly β to β cyclic porphyrin arrays through Suzuki-Miyaura coupling reactions in 2014 [22]. Those cyclic porphyrin arrays display large electronic interactions and energy transfer between the constituent porphyrin units. Song and Osuka have achieved the synthesis of β to β 2, 5-thienylene [14] and 2, 5-pyrrolylene-bridged [17] cyclic porphyrin oligomers through Suzuki-Miyaura coupling reactions. When they examined the Suzuki–Miyaura coupling of β, β'-diborylated Ni(Ⅱ) porphyrin with 2, 6-dibromopyridine, which provided linear oligomers as the main products [18a]. Herein, we wish to report the one-pot efficient synthesis of 6, 6"-2, 2':6', 2"-terpyridylene-bridged porphyrin nanorings through Suzuki-Miyaura coupling of β, β'-diboryl Ni(Ⅱ) porphyrin 1Ni [23] with 6, 6"-dibromo-2, 2':6', 2"-terpyridine, which is effective to construct medium to large porphyrin rings without template.

Suzuki-Miyaura cross-coupling reaction is a good mean to construct covalently bonded cyclic porphyrin oligomers. We prepared a series of terpyridine bridged cyclic porphyrin rings 2Ni-5Ni straightforward through one pot Suzuki-Miyaura crosscoupling reaction of 3, 7-diboryl-10, 15, 20-triaryl Ni(Ⅱ) porphyrin (1Ni) with 6, 6-dibromo-2, 2':6', 2-terpyridine. To our surprise, terpyridine-bridged cyclic porphyrin dimer 2Ni (27%), trimer 3Ni (18%), tetramer 4Ni (10%) and pentamer 5Ni (5%) were obtained after separation by GPC and silica-gel chromatography (Scheme 1). Interestingly, the reaction represent rare example of one-pot synthesis of such large cyclic arrays without template molecule. Treatment of 2Ni-5Ni with sulfuric acid in chloroform at room temperature induced demetallation completely to provide 2H-5H, then converted into 2Zn-5Zn upon treatment with Zn (OAc)2 in quantitative yields, respectively (Scheme 1).

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Scheme 1. Synthesis of porphyrin nanorings. Reagents and conditions: a) Pd2(dba)3, PPh3, Cs2CO3, CsF, toluene/DMF, reflux; b) H2SO4, CHCl3, room temperature; c) Zn(OAc)2·2H2O, CHCl3, room temperature, Ar = 3, 5-di-tert-butylphenyl, Bpin = 3, 3, 4, 4-tetramethyl-2, 5-dioxaborolanyl, dba = trans, trans-dibenzylideneacetone.

These newly synthesized porphyrin rings were fully characterized by high-resolution mass spectrometry and 1H NMR, UV–vis absorption, and fluorescence spectroscopy. The 1H NMR spectra of these compounds are quite simple, exhibiting a set of signals for the porphyrins and bridges, indicating their highly symmetric cyclic structures. For example, the 1H NMR spectrum of 3Ni exhibits a singlet peak for the meso protons at 11.96 ppm, one singlet at 9.26 ppm and two doublets at 8.87, 8.84 ppm for the β protons, and two doublet, one multiplet and one triplets at 8.27, 7.82, 8.20, 7.29 ppm for the terpyridine protons. The UV–vis absorption and fluorescence spectra of the Zn (Ⅱ) porphyrins measured in CH2Cl2 are shown in Figs. 1 and 2. The UV–vis absorption spectra of Zn (Ⅱ) porphyrins exhibit broad Soret bands around 430 nm and Q bands around 554 nm, these porphyrin arrays possess large extinction coefficients up to 106 L mol-1 cm-1. A slight redshift of the Soret band was observed as the ring was enlarged. This trend can also be seen in the case of its free base and Ni (Ⅱ) porphyrins (Figs. S13 and S14 in Supporting information). The broadened Soret and Q bands are presumably affected by electronic interactions between the porphyrin units. The fluorescence spectra of Zn (Ⅱ) porphyrins all exhibit typical vibronic structures (Fig. 2). The fluorescence quantum yields in CH2Cl2 at room temperature are 0.0676, 0.0936, 0.0872, and 0.0277 for 2Zn-5Zn, and 0.0665, 0.0953, 0.0918, 0.0837 for 2H-5H. The maximum emission peaks are red shifted in the order 2Zn < 3Zn4Zn5Zn, in line with the absorption peak maximum wavelengths. The peak positions and shapes in the absorption and fluorescence spectra indicate that there exist unique electronic communications between the constituent porphyrin units [21, 22].

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Fig. 1. UV/vis spectra of 2Zn (black line), 3Zn (blue line), 4Zn (red line) and 5Zn (dark cyan line) CH2Cl2.

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Fig. 2. Fluorescence spectra of 2Zn (black line), 3Zn (blue line), 4Zn (red line) and 5Zn (dark cyan line) in CH2Cl2.

The electrochemical properties of 2Ni-5Ni were further investigated by cyclic voltammetry and differential pulse-voltammetry (Fig. 3). The dimer 2Ni has two reversible reduction processes located at E1/2 = -1.67 and -2.15 V in o-DCB, and one quasi-reversible oxidation process located at E1/2 = 0.77 V. Upon increasing the conjugated system from dimer to pentamer, the observed less-to-no difference indicates the expanded molecular structure has a negligible influence on the electrochemical properties. In contrast, the trimer 3Ni reveals significantly different redox behavior. In o-DCB, 3Ni has three reversible reduction processes located at E1/2 = -1.29, -1.68 and -2.15 V, and one quasi-reversible process located at E1/2 = 0.66 V that indicated the strong electronic interactions between coplanar porphyrin moieties of 3Ni [17].

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Fig. 3. CV and DPV measurements of 2Ni, 3Ni, 4Ni and 5Ni in o-dichloro-benzene (o-DCB) containing 0.1 mol/L TBAP.

In summary, we have achieved the synthesis of β to β terpyridine bridged cyclic porphyrin dimer, trimer, tetramer and pentamer through one-pot Suzuki-Miyaura crossing coupling reaction from diborylporphyrin monomer. The 1H NMR of these compounds revealed their highly symmetric structures in solution. The UV–vis absorption and fluorescence spectra of 2Zn-5Zn show that these porphyrin nanorings possess large extinction coefficients and high fluorescence quantum yields. Further exploration into their photophysical properties and interactions with other guest molecules are currently underway in our laboratories.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 21272065), the Opening Fund of Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), Hunan Normal University (No. KLCBTCMR2015-07), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, Scientific Research Fund of Hunan Provincial Education Department (No. 16A125), the National Natural Science Foundation of China (No. 21602058).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.05.003.

References
[1]
(a) M. G. H. Vicente, L. Jaquinod, K. M. Smith, Chem. Commun. (1999) 1771-1782;
(b) A. Tsuda, A. Osuka, Science 293(2001) 79-82;
(c) M. Pawlicki, H. A. Collins, R. G. Denning, H. L. Anderson, Angew. Chem Int. Ed. 48(2009) 3244-3266;
(d) N. Aratani, D. Kim, A. Osuka, Chem. Asian J. 4(2009) 1172-1182;
(e) G. de la Torre, P. Vázquez, F. Agulló-López, T. Torres, Chem. Rev. 104(2004) 3723-3750;
(f) J. E. Raymond, A. Bhaskar, T. Goodson Ⅲ, et al., J. Am Chem. Soc. 130(2008) 17212-17213.
[2]
(a) X. Peng, N. Aratani, A. Takagi, et al., J. Am. Chem. Soc. 126(2004) 4468-4469;
(b) F. Hajjaj, Z. S. Yoon, M. C. Yoon, et al., J. Am Chem. Soc. 128(2006) 4612-4623;
(c) Y. Xie, J. P. Hill, M. Akada, et al., Chem. Commun. 47(2011) 2285-2287.
[3]
G. McDermott, S.M. Prince, A.A. Freer, et al., Nature 374(1995) 517-521. DOI:10.1038/374517a0
[4]
J. Koepke, X. Hu, C. Muenke, K. Schulten, H. Michel, Structure 4(1996) 581-597. DOI:10.1016/S0969-2126(96)00063-9
[5]
A.W. Roszak, T.D. Howard, J. Southall, et al., Science 302(2003) 1969-1972. DOI:10.1126/science.1088892
[6]
Y. Nakamura, N. Aratani, A. Osuka, Chem. Soc. Rev. 36(2007) 831-845. DOI:10.1039/b618854k
[7]
X. Chi, A.J. Guerin, R.A. Haycock, C.A. Hunter, L.D. Sarson, J. Chem. Soc. Chem. Commun.(1995), 567-2569.
[8]
(a) J. Li, A. Ambroise, S. I. Yang, et al., J. Am. Chem. Soc. 121(1999) 8927-8940;
(b) O. Mongin, A. Schuwey, M. A. Vallot, A. Gossauer, Tetrahedron Lett. 40(1999) 8347-8350.
[9]
Y. Kuramochi, A. Satake, Y. Kobuke, J. Am. Chem. Soc. 126(2004) 8668-8669. DOI:10.1021/ja048118t
[10]
(a) T. Hori, N. Aratani, A. Takagi, et al., Chem. Eur. J. 12(2006) 1319-1327;
(b) Y. Nakamura, N. Aratani, H. Shinokubo, et al., J. Am Chem. Soc. 128(2006) 4119-4127.
[11]
(a) K. I. Sugiura, Y. Fujimoto, Y. Sakata, Chem. Commun. (2000) 1105-1106;
(b) A. Kato, K. I. Sugiura, H. Miyasaka, et al., Chem. Lett. 33(2004) 578-579.
[12]
(a) I. Hisaki, S. Hiroto, K. S. Kim, et al., Angew. Chem. Int. Ed. 46(2007) 5125-5128;
(b) S. Tokuji, H. Yorimitsu, A. Osuka, Angew. Chem. Int. Ed. 51(2012) 12357-12361.
[13]
(a) H. J. Hogben, J. K. Sprafke, M. Hoffmann, M. Pawlicki, H. L. Anderson, J. Am. Chem. Soc. 133(2011) 20962-20969;
(b) D. V. Kondratuk, L. M. A. Perdigao, M. C. O'Sullivan, et al., Angew. Chem. Int. Ed. 51(2012) 6696-6699;
(c) P. Neuhaus, A. Cnossen, J. Q. Gong, L. M. Herz, H. L. Anderson, Angew. Chem. Int. Ed. 54(2015) 7344-7348.
[14]
J. Song, S.Y. Jang, S. Yamaguchi, et al., Angew. Chem. Int. Ed. 47(2008) 6004-6007. DOI:10.1002/anie.v47:32
[15]
J. Song, N. Aratani, H. Shinokubo, A. Osuka, Chem. Sci. 2(2011) 748-751. DOI:10.1039/c0sc00605j
[16]
J. Song, N. Aratani, H. Shinokubo, A. Osuka, J. Chem. Eur. 16(2010) 13320-13324. DOI:10.1002/chem.v16.45
[17]
(a) Y. Rao, J. O. Kim, W. Kim, et al., Chem. Eur. J. 22(2016) 8801-8804.
[18]
(a) J. Song, P. Kim, N. Aratani, et al., Chem. Eur. J. 16(2010) 3009-3012;
(b) J. Song, N. Aratani, J. H. Heo, et al., J. Am Chem. Soc. 132(2010) 11868-11869;
(c) J. Song, N. Aratani, H. Shinokubo, A. Osuka, J. Am. Chem. Soc. 132(2010) 16356-16357.
[19]
W. Huang, S.K. Lee, Y.M. Sung, et al., Chem. Eur. J. 21(2015) 15328-15338. DOI:10.1002/chem.201502296
[20]
H.W. Jiang, T. Tanaka, T. Kim, et al., Angew. Chem. Int. Ed. 54(2015) 15197-15201. DOI:10.1002/anie.201507822
[21]
Y. Nakamura, I.W. Hwang, N. Aratani, et al., J. Am. Chem. Soc. 127(2005) 236-246. DOI:10.1021/ja045254p
[22]
H. Cai, K. Fujimoto, J.M. Lim, et al., Angew. Chem. Int. Ed. 53(2014) 11088-11091. DOI:10.1002/anie.201407032
[23]
H. Hata, H. Shinokubo, A. Osuka, J. Am. Chem. Soc. 127(2005) 8264-8265. DOI:10.1021/ja051073r