Chapter 2. Experimental Methods

Transcription

1 Chapter 2 Experimental Methods 2.1. Synthesis of Porphyrin and its Metal Derivatives There are various routes for the synthesis for porphyrin derivatives, but still the old method of Alder and Longo [1, 2, 3, 4] remains in use because of its simplicity and workability. Typically it involves the condensation of pyrrole and aromatic aldehyde for the synthesis of tetraarylporphyrin (TArP) that was the starting material for other derivatives in our course of study tetraphenylporphyrin (TPP). Various groups have reported subtle modification of this method to improve the yield, the maximum of which reported so far is 24 percent. We have further optimized the conditions to achieve a maximum yield of percent in several trial reactions. The preparations of some of the metal derivatives also have been carried out in novel way with a greater yield and a higher purity. The free base porphyrin shows a typical spectrum consisting of a strong Soret band (B-band) and four weak transitions (Qband). The metallated derivative shows the characteristic Soret band and two weak visible bands. The difference between the UV-Vis spectra of the free base porphyrin and its metallated derivative is used to verify whether the metallation has undergone completely or not. The general synthetic scheme adopted for synthesis of porphyrin derivatives is illustrated in Fig

2 Fig 2.1. General Synthetic Scheme for synthesis of porphyrin derivatives 37

3 2.2. Chemicals Pyrrole (Aldrich), Benzaldehyde (Qualigens), Toulaldehyde (Aldrich), Copper acetate (Thomas Baker), Nickel chloride (Thomas Baker), Manganese acetate (Thomas Baker), Cadmium acetate (Thomas Baker), Tin chloride (Thomas Baker), Zinc acetate (Thomas Baker), Fuming nitric acid (freshly prepared) Solvents Propionic acid (Thomas Baker), Acetic anhydride (Merck), Chloroform (Thomas Baker), Petroleum ether (Thomas Baker), Methanol (Merck), Ethanol (Changshu Yangyuan Chemicals), Pyridine (Thomas Baker), DMF (Merck), DMSO (Merck), 1,4-Dioxane (Thomas Baker), THF (Thomas Baker), Diethyl ether (Thomas Baker), Acetone (Thomas Baker), Ethyl acetate (Thomas Baker) Adsorbents 1. Silica gel ( mesh, Merck). 2. Alumina oxide (Thomas Baker) Instrument 1. UV-Vis spectrophotometer ( UV-2450, Shimadzu) 38

4 2.6. Purification of Pyrrole The pyrrole which was slightly coloured was purified by distillation. Typically 50ml of pyrrole and 5 gm of KOH was taken in a 100 ml round bottom flask. The pyrrole was slightly heated and distilled under reduced pressure. The first 2ml was discarded and the last 5ml remaining in the flask was also discarded. The collected colourless pyrrole was used for further synthesis of tetraarylporphyrin Synthesis of 5,10,15,20-arylporphyrin 500ml of propionic acid was taken in 3 necked 1000ml round bottom flask and heated until it just started to boil then 4 ml of pyrrole was added followed by 6 ml of aromatic aldehyde this was refluxed for 40 min. The propionic acid was recovered by distillation and the residue in the round bottom flask was allowed to dry. Then water was added and boiled for few minutes then filtered through Buchner funnel. The residue was washed several times with hot water and methanol till the product was devoid of any smell of propionic acid. The residue was dried and crushed using mortar and pestle then again washed with a jet of hot water several times. The product was then spread in a filter paper and left to dry overnight. The product was then loaded in a silica gel column prepared in chloroform (100%). The free base porphyrin was eluted as a purple band from the column. The solvent was then evaporated and yield of percent was obtained. λmax in nm, (OD) in CHCl3-418 (2.036); (0.100); (0.074); (0.061); (0.041) 39

5 2.8. Synthesis and purification of metal derivatives of prophyrins The metallation reactions and purification for all the porphyrin derivatives are same and representative metallation of H2TPP is given below Synthesis of CuTPP Tetraphenylprophyrin free-base (TPP) and copper acetate was taken in a 1:4 molar ratio. 100mg of TPP was dissolved in minimum amount of chloroform and gm of copper acetate was dissolved in minimum amount of hot methanol. The solutions were mixed in a round bottom flask and refluxed for 30 minutes. The spectral data was taken which showed that metallation was incomplete, and then 0.5 ml of Triethylamine was added to the solution and then again refluxed for 30 minutes. Again spectra was recorded which showed that the metallation was complete Purification The copper porphyrin complex was dried by evaporation and dissolved in minimum amount of chloroform and loaded in a basic aluminum oxide column. It was then eluted with Chloroform and Pet Ether (3:1). The pure metal derivative was obtained as a red band from the column. λmax in nm (OD) in CHCl3-415 (0.263), (0.018). The solvent was then evaporated and the yield obtained was 80mg. The yield in term of the starting material was 80 percent. 40

6 Synthesis of NiTPP Tetraphenylprophyrin free-base (TPP) and nickel chloride was taken in the ratio of 1:4. 100mg of TPP was dissolved in minimum amount of chloroform and gm of nickel chloride dissolved in minimum amount of methanol was refluxed for half an hour. The spectra were taken which showed that the metallation was incomplete. So, the nickel chloride and TPP was dissolved in minimum amount of DMF and refluxed for 2.5 hrs. Completion of metallation was monitored with UV-vis spectra Purification The dried compound was dissolved in minimum amount of chloroform and then loaded in a silica gel column. It was then eluted with chloroform, fraction eluted showed that the compound had undergone demetallation in the column. Another column was prepared with deactivated silica gel and the compound was eluted using chloroform as an eluant. The spectrum showed that demetallation did not take place. The dried compound was washed several times with Pet Ether. λmax in nm (OD) in CHCl (1.908), (0.164). The yield obtained in terms of the starting material was 90 percent Synthesis of CdTPP TPP and Cadmium acetate was taken in 1:4 molar ratios. 0.1gm of TPP was dissolved in minimum amount of chloroform and gm of acetate was dissolved in DMF. The mixture was refluxed for 30 minutes. The spectrum revealed unsuccessful metallation. The mixture was then refluxed overnight. The spectra recorded showed that the metallation was complete. 41

7 Purification The product was dried completely and was dissolved in minimum volume of CHCl 3 and loaded in a deactivated Silica gel column. The compound was eluted with CHCl3. The dried compound was then repeatedly washed with per ether. λmax in nm (OD) in CHCl (1.998), (0.079); (0.016); 622 (0.010). The yield obtained was 85 percent Synthesis of MnTPP TPP and manganese acetate was taken in 1:4 molar ratio. 0.1 gm of TPP was dissolved in Chloroform and gm of Manganese acetate was dissolved in minimum amount of DMF. Triethylamine was added and then refluxed overnight Purification The compound was dried by evaporating and then was dissolved in minimum amount of chloroform and then loaded in deactivated silica gel column. It was then eluted with chloroform. The dried compound was thoroughly washed with Pet Ether. λmax nm (OD) in CHCl (0.160), (0.035), (0.033). The yield in terms of the starting material was 80 percent Synthesis of SnTPP TPP and Tin chloride was taken in 1:4 molar ratios. 0.1 gm. or TPP was dissolved in Chloroform and gm of tin chloride was dissolved in minimum amount of pyridine. 0.5ml Triethyl amine was added and the mixture refluxed for one hour. Completion of metallation was verified with UV-vis spectra. 42

8 Purification The solution was dried by evaporating. It was then dissolved in minimum amount of Chloroform and was loaded in a silica gel column. The eluant used was Chloroform, the compound was collected and dried by evaporation. The compound was then washed with Pet Ether. λmax in nm, (OD) CHCl (1.251), (0.057), (0.040) in CH2Cl2.The yield in terms of the starting material was 85% Synthesis of ZnTPP TPP and Zinc chloride was taken 1:4 molar ratios. 0.1gm of TPP and gm was dissolved in minimum amount of Chloroform and 0.5ml of Tri-ethylamine was added. This was refluxed for 1hr Purification The compound was dried by evaporation and was dissolved in a minimum amount of Chloroform and was loaded in a column of Silica gel. The pure compound was collected and the spectrum was taken. It was then dried by evaporating and washed with pet ether. λmax in nm (OD) in CH2Cl (2.049), (0.087), (0.014). The yield with respect to the starting material was 90% Synthesis of β-pyrrole substituted mono nitro CuTPP The general strategy adopted for the synthesis of nitro derivatives involves the treatment of CuTPP with freshly prepared fuming nitric acid as illustrated below. 43

9 CuTPP fuming nitric acid Cu(TPP)(NO 2 ) x After several trial reactions it was observed that the following reaction condition gave exclusively mononitro derivative with a small amount of starting material which could be easily separated. Typically 35mg of Cu-TPP was dissolved in 30 ml chloroform. The solution was stirred gently with the addition of one drop of freshly fuming nitric acid for 8 minutes. The reaction mixture was then neutralized by pouring it in a beaker containing cold aqueous sodium hydroxide solution. The solution was thoroughly stirred and washed several times with water. The organic layer was then dried over anhydrous sodium sulphate and the solution was evaporated the product was brown in colour Purification The product was then chromatographed on the deactivated silica gel using chloroform/petroleum ether (2:1, v/v) as the eluant. TLC indicated that the product formed was exclusively mononitro derivative with unreacted CuTPP varying from 2-5%. λmax in nm, (OD) in CHCl3: (0.314); (0.036); 582 (0.027) Demetallation of β-mononitro-cutpp The purified mononitro derivative was dried thoroughly and dissolved in minimum amount of concentrated H2SO4. It was then slowly added to uniformly stir ice-cold solution of ammonia. The compound was extracted with CHCl3 and thoroughly washed with water. The CHCl3 layer \ 44

10 was dried over anhydrous sodium sulphate. The product was purified by column chromatography with basic alumina as the adsorbent. The compound was eluted using CHCl3: Pet Ether (1:10). The purity of the compound was checked by TLC. λmax in nm (OD) (0.470), (0.041), 563 (sh); (0.016); 664 (0.027) in CH2Cl Synthesis of β-mono bromo-5, 10, 15, 20-tetraphenylporphyrin To a three neck round-bottom flask charged with 5, 10, 15, 20-tetraphenylporphyrin (279 mg, 0.45 mmol) and chloroform (60 ml) was added 2 ml pyridine. After refluxing for 5 min, a solution of N-bromosuccinamide (242mg, 1.35 mmol, dissolved in 60 ml chloroform) was added dropwise over 3-4 h into the flask. The solution was refluxed for an additional 30 min, quenched with acetone (50 ml), and concentrated by rotary evaporator to dryness Purification The residue was purified column chromatography (silica gel, CHCl3:Pet Ether (v/v) - 1:1). The title compound was obtained as purple solid (200 mg, 64% yield). 1 H NMR (300 MHz, CDCl3): (m, 5H), 8.76 (m, 2H), 8.20 (m, 6H), 8.09(d, J) 6.6 Hz, 2H), (m, 12H), (s, 2H). 13 C NMR (75 MHz, CDCl3): ä 142.0, 141.7, 140.8, 138.0, 134.6, 134.5, 134.4, 134.3, 133.8, 129.6, 128.8, 128.2, 127.9, 127.5, 126.9, 126.8, 126.7, 126.6, 120.7, 120.3, 119.9, UV-vis, CH2Cl2, λmax in nm, (OD): (1.00), 518 (0.089), 550 (0.043), 594 (0.041), 649 (0,038). LCMS-EI ([M] + ): Calcd for C44H29BrN4, ; found, , with an isotope distribution pattern that is the same as the calculated one. 45

11 Synthesis of β monocyano-5,10,15,20- tetraphenylporphyrin: To a 100ml RB flask 100mg (0.14mmol) of β-monobromo-5,10,15,20-tetraphenylporphyrin was taken and dissolved in DMF. To it mg (2.1mmol) of CuCN in 1:15 ratio (dissolved in DMF) was added and the mixture was refluxed for about 3 hrs. Reaction was monitored by TLC and UV-Visible spectrophotometer. After completion, the reaction was quenched with 20% HCl, washed with water and then concentrated to dryness in an evaporator Purification The compound was purified using silica gel column eluting with 1:1(v/v) CHCl3:pet ether mixture. The title compound was obtained as reddish purple solid (93 mg, 82 % yield). UV-vis, CH2Cl2, λmax in nm, (OD): (1.09), (0.08), (0.05). LCMS-EI ([M] + ): Calcd for C45H27N5Cu, 700.5; found, , with an isotope distribution pattern that is the same as the calculated one. IR (cm -1 ): (CN stretching). It was then demetallated with H2SO4 and ammonia solution, purified with alumina column. UV-vis, CH2Cl2, λmax in nm, (log ϵ): (1.59), (0.123), (0.60), (0.057), (0.078). LCMS-EI ([M]+): Calcd for C45H29N5, 639.0; found IR (cm -1 ): (CN stretching). It was then refluxed with Ni acetate in DMF and purified from basic alumina column. UV-vis, CH2Cl2, λmax in nm, (OD): (1.123), (0.073), (0.046). LCMS-EI ([M] + ): Calcd for C45H27N5Ni, ; found IR (cm -1 ): (CN stretching) Metallation of other Derivatives of TPP &TTP The metallation of bromo, cyano and nitro porphyrin derivatives were carried out similar to the unsubstituted porphyrins as described above. 46

12 2.9. Construction of the Sensor Array Initially spin coating method was used to generate uniform thin film spots of sensor element. However, since the size of single spot was large it was not possible to construct a colorimetric sensor array of large number of spots within a small defined size. This led us to try out and generate uniform thin film spots with capillary tubes. Through trial and error we arrived at suitable solvents and concentration for different porphyrins that ensured thin film spot of the porphyrins on the glass side though not entirely uniform. To check the validity of the method, we compared the sensitivity of the sensor element generated both by spin coating technique and capillary method towards various organic solvents like ethanol, nitrobenzene, toluene etc. The RGB analysis of the pre- and post-exposure for thin film spots generated by the above mentioned capillary method was done for section that was uniform. These RGB values were compared with the result obtained from the spots generated by spin coating method. It was observed that the results were comparable. This exercise was repeated for all the porphyrins 5 times each and a good consistency was observed in all the case. All the colorimetric sensor array was thus generated using capillary method. Pre-exposure Post exposure Figure 2.2.: Prototype of sensor array structure before and after exposure 47