GUO Guo-Zhe ZHENG Xv-Dong ZHU Ji-Hu ZHANG Yu-Qun LIU Ji-Cheng

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Synthesis and Study of Spectroscopic Properties of 5-(1-Oxido-3-pyridyl)-10,15,20-triphenyl Porphyrin and Its Zn(II) Complex①

GUO Guo-Zhea②ZHENG Xv-DongaZHU Ji-HuaaZHANG Yu-QuanaLIU Jia-Chengb

a(745000)b(730070)

Two novel sensitizers with pyridine-N-oxide zinc porphyrin and its zinc por- phyrin as the anchor group and electron acceptor have been synthesized. The structures have been characterized by UV, elemental analyses and1H NMR. UV and fluorescence spectra show that they have good light absorbing properties in the range of visible light and suggest that they have potential applications in dye-sensitized solar cells.

sensitizer, oxidation, porphyrin, complex, synthesis;

1 INTRODUCTION

Sensitizer is one of the important factors for dye-sensitized solar cells (DSSCs)[1]. Due to the precious metals needed for ruthenium complex dyes, more and more researchers focus on the porphyrin sensitizer[2]. The most immediate reason lies in their remarkably high extinction coefficients compared to ruthenium polypyridyl complexes[3]. What’s more, they have lots of merits, such as cheap, easy tuning LUMO and HOMO energy levels, strong absorption in visible region and stability for long time exposure to natural sunlight. The use of a nanostructured TiO2film together with the SM315 sensitizer was a breakthrough in terms of a commercial application, and an overall conversion efficiency of 13%[4]has been achieved. In particular, efforts have been made to develop better-performing sensitizer. We enhance their absorption in the red region of the solar spectrum. Additionally, we note that binding through anchoring group[5]has previously been shown to improve conversion efficiencies.

Pyridine-N-oxide acts as electron acceptor and anchoring group in sensitizer[6]. As sensitization, the peak associated with the N=O vibration in the FTIR measurement was bathochromically shifted, indica- tive of the formation of a coordinative bond between the oxygen atom in the pyridine-N-oxide moiety and a superficial hydroxyl group of the TiO2substrate. Moreover, pyridine-N-oxide has good stability values.

We designed and synthesized 5-(1-oxido-3-pyridyl)- 10,15,20-triphenyl porphyrin and its Zn(Ⅱ) complex with strong absorption in the range of visible light.

2 EXPERIMENTAL

2. 1 Materials and measurements

The regents and solvents were used as commercial sources without further purification. Column chro- matography was carried out on silica gel. Elemental analyses were performed using a Heraeus CHN analyser. UV-Vis spectra were recorded on a Schimadzu 160 spectrometer. Fluorescence spectra were recorded on a RF-540 spectrometer.1H NMR spectra were recorded on a Bruker DRX-400 Avance spectrometer using CDCl3as solvent and TMS as the internal standard. The chemicals used in this work were purchased from Fluka (Buchs, Switzerland) and used without further purification.

Scheme 1. Synthesis of 5-(1-oxido-3-pyridyl)-10,15,20-triphenyl zinc porphyrin

2. 2 Synthesis of5-(1-oxido-3-pyridyl)- 10,15,20-triphenyl zinc porphyrin

Synthesis of 5-(3-pyridyl)-10,15,20-triphenyl por- phyrin: synthesis of 5-(3-pyridyl)-10,15,20-triphenyl porphyrin was prepared according to literature[7], and was confirmed by means of elemental analyses and1H NMR spectra.

Synthesis of 5-(1-oxido-3-pyridyl)-10,15,20-tri- phenyl porphyrin: (Scheme 1) 5-(3-pyridyl)-10,15,20- triphenyl porphyrin (0.062 g, 0.10 mmol) was dissolved in CH2Cl2(100 mL), and treated with excess meta-chloroperoxybenzoic acid (-CPBA, 60～85%, the rest being benzoic acid and H2O) (3 × 0.03 g aliquots) over 1～2 h while the solution was stirred. Et3N (7 mL) was then added, and the product was preadsorbed and chromatographed on silica, with the solution (Vchloroform:Vpyridine= 10:1) as eluent. The solvent was removed from the main product band to yield 78%. UV-Vis (CHCl3)(nm): 419 (soret band), 516, 549, 591, 648 (Q band) (Fig 1).1H NMR (CDCl3, 400 MHz): 9.11 (s, 1H, 2-pyridyl), 8.94(m, 1H, 6-pyridyl), 8.67 (m, 1H, 4-pyridyl), 7.25 (m, 1H, 5-pyridyl), 8.87(m, 8H,-pyrrole-H), 8.22(m, 6H,-phenyl-H), 7.78 (m, 9H,- and-phenyl-H), –2.83(s, 2H, internal pyrrole). Anal. Calcd. (%) for C43H29N5O: C, 81.54; H, 4.56; N, 11.02. Found (%): C, 81.75; H, 4.63; N, 11.09.

Synthesis of 5-(1-oxido-3-pyridyl)-10,15,20-tri- phenyl zinc porphyrin: (Scheme 1) 5-(1-oxido-3- pyridyl)-10,15,20-triphenyl porphyrin (0.063 g, 0.10 mmol) and Zn(OAc)2·2H2O (0.11 g, 0.50 mmol) were added to the solution mixture of CHCl3(50 mL) and MeOH (10 mL). The mixture was refluxed for 2 h, then the solution was removed and washed with sodium sulfate solution. The zinc porphyrin was purified using a column chromatography (basic silica gel, CHCl3as eluent), then concentrated and dried. Yield: (98%). UV-Vis (CHCl3)(nm): 421 (soret band), 563, 607 (Q band) (Fig. 1).1H NMR (CDCl3, 400 MHz): 8.99 (s, 1H, 2-pyridyl), 8.92(m, 1H, 6-pyridyl), 8.73 (m, 1H, 4-pyridyl), 7.24 (m, 1H, 5-pyridyl), 8.91(m, 8H,-pyrrole-H), 8.24(m, 6H,-phenyl-H), 7.76 (m, 9H,- and-phenyl-H). Anal. Calcd. (%) for C43H27N5OZn: C, 74.25; H, 3.87; N, 10.03. Found (%): C, 74.30; H, 3.92; N, 10.08.

Fig. 1. UV-Vis spectra of 5-(1-oxido-3-pyridyl)-10,15,20-triphenyl porphyrin(blank) and its zinc porphyrin (red)

Fig. 2. Fluorescence spectra of 5-(1-oxido-3-pyridyl)-10,15,20-triphenylporphyrin(blank) and its zinc porphyrin(red)

3 RESULTS AND DISCUSSION

N-Oxidation of the pyridyl substitutent was accomplished using-CPBA; the oxidation was sluggish at 0 °C, but proceeded readily at r.t. When the peroxyacid was added in small aliquots, only the (oxidopyridyl)porphyrin was obtained. As an oxido- pyridyl group is more electron-withdrawing than a phenyl group, the (oxidopyridyl) porphyrin is more resistant to oxidative degradation, and thus the (oxidopyridyl)porphyrin-N-oxides could be isolated.

The porphyrin-N-oxides are much more soluble in weakly polar solvents (CH2Cl2, CHCl3) than its parent pyridyl compound, and are soluble in MeOH. The difference in solubility may be related to less aggregation in the porphyrin-N-oxides because of steric effects of the macrocyclic oxygens; also, these oxygen atoms may be well involved in hydrogen- bonding.

5-(1-Oxido-3-pyridyl)-10,15,20-triphenyl porphy- rin has an intense soret at 419 and four moderate Q bands at 516, 549, 591, and 648 nm. 5-(1-Oxido-3- pyridyl)-10,15,20-triphenyl zinc porphyrin shows an intense soret at 421 and moderate Q bands at 563 and 607 nm. Compared with the former, firstly, the number of Q bands is reduced. The main reason is that zinc porphyrin complex is formed. Secondly, soret and Q bands of zinc porphyrin are red-shifted and broadened and the intensity of the Q band relative to that of the soret band is enhanced. Soret and Q bands arise from* transitions and can be explained in terms of a linear combination of transitions from the slightly splitted highest occupied molecular orbital (HOMO) and HOMO-1to a degenerated pair of the lowest unoccupied molecular orbital (LUMO) and LUMO-1. The configuration interaction leads to the intense soret band at the short wavelength and the moderate Q bands at the long wavelength. Elongation of theconjugation and loss of symmetry in porphyrins cause splitting in theand* levels and a decrease in the HOMO-LUMO gap, resulting in broadening and a red shift of the absorption bands together with an increasing intensity of the Q bands relative to that of the soret band. In such a case, the cell performance of the porphyrin-sensitized solar cells would be improved by the enhanced light absorption. Porphyrin possesses an intense soret band at 400～450 nm and moderate Q bands at 500～650 nm as a result ofelongation with low symmetry, and they have beenregarded as potential photosensitizers in dye-sensi-tized solar cells.

The room-temperature fluorescence spectra of the 5-(1-oxido-3-pyridyl)-10,15,20-triphenyl porphyrin and its zinc porphyrin in CHCl3solution are shown in Fig. 2. The emission spectra consist of two bands. The weak band at≈ 646 nm is assigned as Q(0,1) and the stronger band at≈ 616 nm as Q(0,0) in the zinc porphyrin. The weak band at≈ 715 nm is assigned as Q(0,1) and the stronger band at≈ 652 nm as Q(0,0) in free porphyrin. The major dif- ferences between the zinc porphyrin compared with the corresponding 5-(1-oxido-3-pyridyl)-10,15,20- triphenyl porphyrin are the remarkable hypsochromic shifts of ca. 69 nm for the strongest emission band Q(0,1) and ca. 36 nm for the second band Q(0,0) due to the metal coordination.

4 CONCLUSION

In conclusion, we have successfully synthesized 5-(1-oxido-3-pyridyl)-10,15,20-triphenyl porphyrin and its Zn(Ⅱ) complex with-CPBA. UV and fluorescence spectra show that they have good light absorbing properties in the range of visible light and suggest that they have potential applications in dye-sensitized solar cells.

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11 October 2017;

17 November 2017

① This work was supported by the University Project of Gansu Province (2017A-095) and the 13th Five-Year Period Education Plan of Gansu Province (GS[2017]GHB0360)

. Born in 1988, majoring in functional coordination chemistry. E-mail: 2660859870@qq.com

10.14102/j.cnki.0254-5861.2011-1848