One-Dimensional Bis(dipyrrinato)zinc(II)-Linked Porphyrinatozinc(II) Polymer: Synthesis, Exfoliation Into Single Wires, and Photofunctionality


 One-dimensional bis(dipyrrinato)zinc(II)-linked porphyrinatozinc(II) polymer, 2 were synthesized by facile metal complexation reaction between 5,15-bis(3,5-dioctyloxyphenyl)-10,20-bis(dipyrrinato)porphyrinatozinc(II),1 and zinc(II) acetate. The bulky substituents on the porphyrin units allows 2 to be exfoliated into single molecular wires with a 2.8 nm height and 1.4 Om length. 2 exhibited promising photofunctionality derived from electronic interaction between bis(dipyrrinato)zinc and porphyrinatozinc(II) moieties, which can be engaged in energy transfer system such as photonic molecular wires.


Introduciton
Coordination polymers, whether one-dimension (1D), two-dimension (2D) or three-dimension (3D) based on their structures, can be synthesized basically in a very simple manner by a reaction of metal ions with bridging ligands in suitable solvents [1][2]. For years, 1D coordination polymers such as nanowires, [3][4] nanoribbons [5], nanofibers [6], nanorods [7] and so forth [8] play important roles in fabricating various nanoscale functional materials applicable to sensing [9], energy storage [10], and others [11][12][13]. In light harvesting application, porphyrin-based arrays are usually adopted due to their strong light absorption properties [14][15]. However, intense light absorption of porphyrins is generally limited only to the Soret band in the blue region. Subsequently, this became a limitation for porphyrins to be fully utilized in sun light harvesting. Therefore, hybridization between porphyrin and another pigment moiety is an efficient way to absorb the light over the whole visible region. Combination of porphyrin with dipyrrin has been studied [16][17][18]. Dipyrrin derivatives are bidentate ligands, easily coordinate with numerous divalent metal ions such as nickel(II), copper(II) and zinc(II) to afford bis(dipyrrinato)metal(II) which absorb and emit light intensely wherein highly useful in fabricating light-harvesting arrays as well as can act as emitting subunit in energy transfer system. Therefore, the integration of bis(dipyrrinato)zinc(II) with porphyrin building blocks are expected to display various photofunctions.

Chemicals
All chemicals were purchased from Kanto Chemical Co., Tokyo Chemical Industry Co. Ltd or Wako Pure Chemical Industries Ltd, were used as received, unless otherwise stated. Pyrrol was purchased from Sigma-Aldrich and distilled under reduced pressure prior to use. Water was purified with a Milli-Q water system (Milipore Co.). 1 was synthesized according to previous literature studies with a few modifications.

Instrumentations
All 1 H NMR data were recorded on a DRX500 (Bruker) using CDCl3 as the solvent and tetramethylsilane (δH = 0.00) as an internal standard. High-resolution fast atom bombardment mass spectrometry (HR-FAB-MS) was conducted using a JMS-700 MStation mass spectrometer (JEOL Ltd.). UV/vis spectra were obtained with a V-570 spectrometer (JASCO). Raman spectra were recorded using a NRS-5100 (JASCO). AFM measurements were carried out using an Agilent Technologies 5500 scanning probe microscope in the high-amplitude mode (tapping mode) with a silicon cantilever Nano World PPP-NCL probe. Photofunctionality of 2-modified SnO2 was measured using a xenon lamp (MAX-302, Asahi Spectra Co., Ltd.) as photon flux source and a monochromator (CT-10, JASCO Corporation) to monochromate the photon flux. An electrochemical analyzer (ALS 750A, BAS Inc.) was used to control the electrode potential and photocurrent acquisition of the photoelectric conversion system. A photon counter (8230E and 82311B, ADC Corporation) was engaged in quantifying the photon flux of the incident light. All experiments were carried out under an ambient condition unless otherwise stated. The molecular size of 1 was estimated by DFT calculation. The DFT calculation was carried out using a Gaussian09 Revision D.01 program package. The geometry optimization was performed using B3LYP functional with the LANL2DZ basis set for Zn and 6-31g(d) basis set for the other atoms and the result was visualized using GaussView 5.0.8 software.

L2 L3
A procedure for the synthesis of 3,5-dioctyloxybenzaldehyde (L4) Alcohol, L3 (8.14 g, 22 mmol) was dissolved in dry CH2Cl2 (60 mL). Sodium acetate (3.7 g, 44.6 mmol) was suspended in this solution, and the resultant suspension was cooled to 0 °C. Pyridinium chlorochromate (PCC) (10.6 g, 49 mmol) was carefully added to the suspension at 0 °C. After stirring for 2 hours, diethyl ether was added to the reaction mixture, and the liquid phase was separated by decantation. The residual gummy solid was washed with diethyl ether several times. The combined liquid phase was passed through a florisil short column. Diethyl ether was evaporated. The product was crystallized during evaporation. Colorless oil. Yield

A procedure for the synthesis of synthesis of 2-methylpyrrole
In a 100-mL round flask, KOH (8.9 g, 158 mmol) was dissolved in ethylene glycol. 2formylpyrrol (3.0 g, 32 mmol) and hydrazine monohydrate (5 mL, 158 mmol) were added. The solution was stirred at 150 o C for 20 hours. After cooled to room temperature, the reaction mixture was diluted with 100 mL of water and then extracted with diethyl ether (10 mL × 6). The organic layer was washed with water (50 mL × 3) and dried over Na2SO4, filtered, and evaporated under reduced pressure.
A mixture of 2-methylpyrrole (3 mL, 35 mmol) and pinacolborane (3.7 mg, 16 mmol) was flushed with argon for 10 minutes and treated with trifluoroacetic acid (0.05 mL, 0.64 mmol). The mixture was stirred for 3 hours at room temperature and then p-chloranil (4.7 mg, 19 mmol) was added and the mixture was stirred for additional 5 minutes. Removal of solvent and excess pyrrole followed by chromatography (alumina, 100% CH2Cl2) afforded orange solid.

A procedure for the synthesis of 5-(4-bromophenyl)dipyrromethane
Freshly distilled pyrrole (50 mL, 720 mmol) and 4-bromobenzaldehyde (5.33 g, 29 mmol) were added to a dry, round-bottomed flask and degassed with a stream of nitrogen for 5 minutes. Trifluoroacetic acid (0.22 mL, 2.8 mmol) was then added, and the solution was stirred under nitrogen at room temperature for 5 minutes before being quenched with 0.1 M NaOH. Ethyl acetate was then added. The organic phase was washed with water and dried over anhydrous Na2SO4, and the solvent was removed under vacuum to afford an orange-brown oil. The excess of pyrrole was recovered by vacuum distillation and the crude material was purified by flash column chromatography (eluent: SiO2 hexane/ethyl acetate 90/10  hexane/ethyl acetate 30/70) to afford yield: 3.9 g (13 mmol, 45%).

A procedure for the synthesis of 5,15-bis(3,5-dioctyloxyphenyl)-10,20-di(4bromophenyl)porphyrinatozinc(II), P1
A solution of 3,5-dioctyloxybenzaldehyde (1.83 g, 5 mmol) and 5-(4-bromophenyl) dipyrromethane (1.5 g, 5 mmol) in dry CH2Cl2 (1.5 L) was stirred under Ar and the vessel was protected from light by aluminum foil. Trifluoroacetic acid (0.23 mL, 3.0 mmol) was added via syringe, and the resulting solution was stirred for 3 hours at room temperature. 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (1.70 g, 8 mmol) was added to the solution, and the resulting solution was stirred for an additional 2 hours. The reaction mixture was then neutralized by triethylamine and passed over an alumina column to remove polymeric materials. The solvent was removed by a rotary evaporator and a solution of Zn(OAc)2 in methanol was added to a

Preparation and Characterization of 1
In the present study, we fabricated new bis(dipyrrinato)zinc(II)-linked porphyrinic wires, 2 by simply mixing the 5,15-bis (3,5- temperature for 30 hours and the resulting precipitate was filtered, washed with dichloromethane and methanol, then dried under reduced pressure to give bis(dipyrrinato)zinc(II) linked porphyrinatozinc(II) polymer wires,2 as powdery solid. Upon ultrasonication 2 in toluene, the suspension exhibited Tyndall scattering once illuminated with green luminous flux, indicating the presence of nanoarchitecture in solution. The dispersibility of 2 was excellent in most organic solvents due to the presence of long alkoxy chain. Thus, the completion of metal complexation could be monitored by UV-vis spectroscopy. After ultrasonication, a solution of 2 in dichloromethane was dispersed on a highly ordered pyrolytic graphite (HOPG), and the atomic force microscopy (AFM) measurement was conducted. A topographic image in Fig. 1a displays several white lines with a length of 1.4 mm, of which height is 2.8 nm. This height is consistent with the optimized molecular structure of 1 calculated using density-functional theory (DFT) [21] shown in Fig. 1b. Absorption bands for porphyrin; a strong Soret band at 428 nm and a Q band at 552 nm. In the spectrum of 2, one additional absorption peak appears at 493 nm, which is ascribed to the bis(dipyrrinato)zinc(II) moiety [19], confirming that the complexation between dipyrrin ligands and zinc(II) ions occurred successfully. For bis(dipyrrinato)zinc(II)-linked porphyrinatozinc(II) wires, energy transfer from the bis(dipyrrinato)zinc moiety to the porphyrinatozinc(II) moiety is expected to occur when the bis(dipyrrinato)zinc moiety is excited at 493 nm because the Q band locates in the lower energy at 552 nm as shown in Fig. 2a. The exciton will transfer. This unique feature makes these wires as one of the possible candidates to be engaged in energy transfer system such as photonic molecular wires. Upon light irradiation, 1 exhibits fluorescence emission in the range of 500-700 nm as shown in Figure 2b. Two emission peaks appear at 601 nm and 648 nm owing to the porphyrin moiety. It should be noted that free dipyrrins generally emit very weak fluorescence [20]. The emission of 2 depends on the excitation wavelength. The excitation at 426 nm for the Soret band of the porphyrin moiety gives two emission peaks at 601 nm and 648 nm similar to 1. When irradiated at 493 nm for the excitation of the bis(dipyrrinato)zinc moiety, these two peaks appear, indicating the occurrence of energy transfer from the excited bis(dipyrrinato)zinc(II) moiety to the porphynatozinc(II) moiety. An additional peak is observed at 519 nm which is ascribed to the * emission of the plain dipyrrinato ligands [18]. Photoluminescence quantum yields, PL of 1 and 2 when irradiated at 428 nm were 1.0% and 2.0%, respectively. Higher PL value of 2 than 1 would be the contribution of the luminescence from the porphynatozinc(II) moiety.

Results and Discussions
A photoelectron conversion system was fabricated using 2 -modified SnO2 as photoanode in an argon saturated 0.1 M Na2SO4 aq. containing 50 mM triethanolamine (TEAO) as an electron donating sacrificial reagent. The photoanode was prepared by casting the dispersion of 2 in dichloromethane on SnO2. The photocurrent generation was observed when the bias potential more positive than 0.12 V vs. Ag/AgCl was applied to the working electrode. Figure 4 shows an action spectrum for the photocurrent at 0.12 V vs. Ag/AgCl, where the photo-response occurs over the whole range of 400-520 nm. This relatively wide range came from the hybridized structure of 2 composed of bis(dipyrrinato)zinc(II) and porphyrinatozinc(II). The quantum yield of photo-electron conversion was 0.19%, which is moderate but lower than the 2D polymer of bis(dipyrrinato)zinc(II) and porphyrinatozinc(II) hybrids [16] probably because of less efficient energy transfer pathways. The relationship between internal quantum efficiency (IQE) value and absorbance of 2 at 430 nm was studied by preparing several numbers of modified SnO2 with 2. At lower absorbance value (0.05 and 0.06), IQE was recorded around 0.01% to 0.02%. Lower IQE values might cause by the smaller number of photons being absorbed by the sensitizer. As shown in Fig. 4, the quantum yield reached the maximal value with the absorbance at 0.098, at 430 nm. However, when the optical density reached beyond >0.1, the IQE decreased significantly. One possible reason is when the optical density of 2 is increased, the interaction with triethanolamine (TEOA)becomes more difficult and leads to restriction of electron donating from TEOA into the hole in the highest occupied molecular orbital of 2.

Conclusion
In conclusion, we successfully synthesized one-dimensional bis(dipyrrinato)zinc(II)linked porphyrinatozinc(II) polymer wires by simple coordination reaction. The wires can be exfoliated into single molecular wires and exhibit photofunctionality based on the electronic interaction between the two complex units, which is utilizable for photoelectron conversion system.

Conflicts of interest
There are no conflicts to declare.