Synthesis of Novel Porphyrin and its Complexes Covalently Linked to Multi-Walled Carbon Nanotubes and Study of their Spectroscopy
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Novel covalent porphyrin and its complexes (Co2+, Zn2+) functionalized multi-walled carbon nanotubes (MWNTs) have been successfully synthesized by the reaction of the carboxyl on the surface of MWNTs which was synthesized to use carbon radicals generated by the thermal decomposition of azodiisobutyronitrile (AIBN) with 5-p-hydroxyphenyl-10,15,20-triphenyl-porphyrin and its complexes (Co2+, Zn2+). Three resulting nanohybrids were characterized by spectroscopy (FT-IR, Raman, and UV-vis), TGA, and TEM. The quality of porphyrin attached to the MWNTs was determined from thermogravimeric analysis (TGA) of the MWNTs, which showed a weight loss of about 60%. The Raman and absorption spectroscopy data showed that the electronic properties of modified MWNTs were mostly retained, without damaging their one-dimensional electronic properties. From fluorescence measurements, it was observed that the porphyrin and its complexes (Co2+, Zn2+) were nearly quenched by MWNTs, indicating that this covalently modified mode facilitated the effective energy or electron transfer between the excited porphyrin moiety and the extended π-system of MWNTs.
KeywordsSynthesis Characterization Multi-walled carbon nanotubes Porphyrin Complexes Covalent functionalization Spectroscopy
The resulting nanohybrids were characterized by transmission electron microscopy (TEM, Hitachi H-600, 10 kV), Fourier transform infrared (FTIR) spectrometry (FTS 7000 Series), Raman spectrometry (Raman, Bruker RFS100/s), UV-Vis spectrometry (Varian CARY50), Thermo gravimetric analysis (TGA, PE-7), and fluorescent spectrometry (Hitachi F-4500). Samples for TEM were prepared by the dispersion of material in methanol with the help of an ultrasonic bath. TGA analyses were preformed in air flow with a heating rate of 10 °C/min. Samples for characterization with the UV-visible spectrometer were dispersed in chloroform with the help of an ultrasonic bath.
MWNTs generated via chemical vapor deposition (CVD) method were purchased from Shenzhen nanoport (Shenzhen, China, purity >90%). Other reagents including Fe(NO3)3 · 9H2O, AIBN, toluene, sodium hydroxide, methanol, hydrochloric acid, toluene, pyrrole, propanonic acid,p-hydroxybenzaldehyde, benzaldehyde, chloroform, thionyl chloride, acetic acid, triethylamine, tetrahydrofuran, cobalt acetate, and zinc acetate were AR grade and were used as supplied.
Synthesis of MWNTs-CH2COOH
According to the literature , 180 mg MWNTs were dispersed in 180 mL toluene by stirring for 30 min. Then, under continuous stirring, 36 mL toluene solution containing 11.52 g AIBN was added to the dispersion. Subsequently, in an Ar atmosphere, the resulting dispersion was heated at 75 °C for 4 h. The product was washed with toluene repeatedly and dried in vacuum at 40 °C overnight, thus, MWNTs-cyano (MWNTs-CH2CN) were produced. A total of 100 mg MWNTs-CH2CN were dispersed in a mixture of methanol and 10 M sodium hydroxide; and the resulting materials were subjected to refluxing at 60 °C for 48 h. The product was washed with distilled water for several times. Finally, the purified MWNTs-CH2COONa was dried in vacuum at 40 °C overnight. The materials were washed with 2 M hydrochloric acid, resulting in MWNTs-CH2COOH.
Synthesis of 5-p-Hydroxylphenyl-10,15,20-Triphenyl Porphyrin (MHTPP) and its Complexes CoMTPP and ZnMTPP
Thep-hydroxybenzaldehyde (3.0744 g, 0.0252 mol) and benzaldehyde (8.0242 g, 0.0757 mol) were added to 300 mL of propionic acid. The temperature was maintained between 124 and 126 °C, and pyrrole (6.78 g, 0.101 mol) was slowly added. After the reaction mixture was refluxed for 40 min and cooled, 200 mL of propionic acid was removed by distillation under vacuum (10 mmHg). Then, 100 mL absolute ethanol was added to the reaction mixture, and this was stored overnight in a freezer at 5 °C, and then the purple precipitation was filtered off and washed with a minimal amount of methanol and dried. Further, purification was carried out by dissolving this material in a minimal amount of chloroform, and this solution was separated on a chromatographic column of neutral alumina using chloroform as the eluent. The first pink band contains 5,10,15,20-tetra(phenyl)porphyrin. The third band, which moved very slowly, contained muti-hydroxyporphyrin. The second band which contained the target material (MHTPP) was collected and was dried under vacuum on a rotary evaporator. Then, this material was redissolved in reagent grade chloroform and separated on a chromatographic column of silica gel using chloroform as the eluent. Finally, the second purple band was collected and dried under vaccum on rotary evaporator and later dried in a vacuum, to obtain a yield of 1.73 g (8.3%). To obtain the CoMTPP, MHTPP (0.126 g, 0.2 mmol) was added to DMF (50 mL) and the mixture was stirred until the porphyrin dissolved before adding Co(OAc)2(0.107 g, 0.6 mmol). The resulting reaction mixture was refluxed for half an hour and cooled, and 100 mL of chilled, distilled water was added. Finally, the purple solid was filtered, washed with water, and dried under vacuum to obtain the CoMTPP. ZnMTPP was prepared by following the similar procedures.
Synthesis of the Composite I, the Composite II, and the Composite III
To prepare covalently grafted porphyrins and its complexes to MWNTs , MWNTs-CH2COOH was used to attach a functional MHTPP. First, MWNTs-CH2COOH treated with thionyl chloride was reacted with excess MHTPP in toluene in the presence of triethylamine at 100 °C for 24 h under a pure nitrogen atmosphere. To remove the unreacted MHTPP, the tubes were washed thoroughly with a plenty of methanol, followed by a small amount of acetic acid and triethylamine, and finally with tetrahydrofuran (THF), resulting in the composite I. The composite I was then dried at 40 °C for 5 h under vacuum. The composite II and the composite III were prepared by following the similar procedures.
Results and Discussion
The degree of composites (I–III) quenching of emission
In conclusion, unlike those produce with strong oxidative acid treatment, carboxyl groups on MWNT surface prepared by the thermal decomposition of azodiisobutyronitrile (AIBN) retained most of their electronic properties, without damaging their one-dimensional electronic properties and all of carboxyl groups were on the MWNT sidewall. The quantity of porphyrin with covalently linked MWNT surface synthesized by this method reaches about 60 wt%. The absorption and fluorescence of these nanohybrids show that the carbon nanotubes serve as an efficient electron or energy acceptor and pave the way to construct novel photovoltaic devices and light-harvesting systems using various porphyrin-functionalized carbon nanotubes.
We thank Dr. B. D. Wang of College of Chemistry and Chemical Engineering at Lanzhou University for his hearty assistance in acquiring fluorescent data.
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