| Porphyrin complexes and metalloporphyrins are developed rapidly inbiochemistry, analytical chemistry, synthetic chemistry, material science, and soon. In these fields porphyrins are used widely for luminescence chemosensorymaterials, especially for optical oxygen sensing materials.Over the past decades, luminescence-based optical oxygen sensors have beengreatly developed because of the determination of molecular oxygen both in gasand liquid phase is very important in many different fields such as analyticalchemistry, medical chemistry and environmental and industrial applications.These sensors are based upon the principle that oxygen is a powerful quencher ofthe luminescent intensity and lifetime of luminescent complexes, and the keyfactors that play a role include the optical properties of luminescent complex andthe solubility and diffusion coefficient of oxygen in the matrix. The mostcommonly used complexes for this application are transition metal complexesespecially ruthenium(II) polypyridyl or phenanthroline complexes andmetalloporphyrins owing to their high quantum yields, large Stokes shifts andlong luminescent lifetimes. The host materials used to encapsulate the luminescentcomplexes are sol-gel and polymer films, but there are some questions for thesematerials. For example, their bad chemical and mechanical stability, oxygen cannot permeate them straightway, and so on. The recent research has shown that theselection of matrix and the amelioration of structure are the key to enhance theoxygen sensing performance.Over the past decade, it has been demonstrated that mesoporous silicas are theexcellent support for developing functional materials. This is due to thatmesoporous silicas have large accessible pore size, high surface areas and periodicnano-scale pores. Recently, we reported new oxygen sensing materials based onplatinum(II) porphyrin complexes assembled in mesoporous silica. This work hasdemonstrated that the assembly between platinum meso-tetrakis-(4-N-methylpyridyl) porphyrin (PtTMPyP4+) and mesoporous silica MCM-41 canlead to the formation of oxygen sensors with high sensitivity. In order to obtainthe oxygen sensors with higher performance, the design and synthesis of newchromophores and optimization of assembly systems should be focused on.We have synthesized two series of star-shaped porphyrins with four and eightalkyl-carbazole arms at their meso position. The NMR, Mass spectra, elementanalyst, UV-VIS and emission spectra are investigated here. Protonatedplatinum/palladium porphyrins could be physically or chemically assembled withmesoporous materials to obtain oxygen sensing materials.Oxygen sensing material is usually composed of two parts, a support matrixand functional molecules of which the luminescent intensity is depended on theoxygen concentration. Oxygen molecule can quench the luminescence of thefunctional molecules through the pore of the matrix. Three platinum porphyrincomplexes with four alkyl-carbazole arms at their meso position (Pt-4Cn-TPP, n =4, 6 and 8), have been synthesized and their protonated complexes have beenassembled with MCM-48 to form sensing materials. The luminescent intensitiesof these materials decrease with increasing the oxygen concentration. Thesensitivity I0/I100 (where I0 and I100 represent the detected luminescent intensitiesof the oxygen sensing materials exposed to 100% nitrogen and 100% oxygen,respectively.) satisfies the requirement of oxygen sensing materials, the quenchingtime is less than 1 s and the recovery time is less than 22 s. Especially ofPt-4C8-TPP4+/MCM-48 (20 mg/g), the sensitivity is 17.74, the quenching time is0.6 s and the recovery time is 6.4 s indicating the potential for the formation ofoxygen sensing material or device. Comparing with conventional oxygen sensingmaterials used polymers or silicones as matrix, the sensing properties usedMCM-48 as matrix have been improved because MCM-48 can enhance theeffective collision between the platinum porphyrin complexes and oxygenquencher. Therefore, oxygen sensing materials with higher performance areobtained.The oxygen sensing performance of platinum porphyrin complexes maybediffers from that of palladium porphyrin complexes for the lifetime of palladiumporphyrins are longer than the homologous platinum porphyrins. In order to studyfurther the effect of porphyrin complexes with different metals assembled withmesoporous molecular sieves on the sensing materials, we have designed andsynthesized three palladium porphyrin complexes with four alkyl-carbazole armsat their meso position (Pd-4Cn-TPP, n = 4, 6 and 8) and assembled theirprotonated complexes with MCM-48 to form sensing materials. The performanceof these materials is better than that of the materials assembled by platinumporphyrins. For Pd-4C8-TPP4+/MCM-48 (20 mg/g), the sensitivity is 57.87, thequenching time is 0.04 s and the recovery time is 73.2 s.For the reported oxygen sensors based on sol-gel, polymer or mesoporous silica,the common composite feature is that luminescent molecules are encapsulated inthe host materials. For these sensing systems, oxygen molecules must diffuse intothe matrix, and then reach to the positions where luminescent molecules located in.On the other hand, when the oxygen molecules depart from the sensors, they mustdiffuse out of the encapsulating matrix. The diffusing into and out processes hasdramatic negative effect on the quenching and recovery times of oxygen sensorsdue to that these processes inevitably take a certain time. Moreover, encapsulatingmediums often have marked impact on the photophysical property ofchromophores and consequently result in the decrease of sensitivity. To overcomethe problems of common optical oxygen sensors, we have designed andsynthesized six "cuttlefish-shaped" platinum/palladium porphyrin complexes witheight alkyl-carbazole arms at their meso position (Pt-8Cn-TPP & Pd-8Cn-TPP, n =4, 6 and 8) and assembled their protonated complexes with MCM-48 to formsensing materials. We found that when the lengths of carbon chains reach to sixand eight, the luminescent molecules have bigger dimensions than MCM-48. Theporphyrins cannot be wholly adsorbed into the pores of MCM-48 and can only beimmobilized onto the exo-surface of MCM-48 by inserting their alky-carbazolearms into the pores of MCM-48 and lifting the porphyrin cores outside. In thiscase, the sensing moieties of chromophore are exposed completely and oxygenmolecules can directly reach to the sensing moieties. Therefore, high sensitivityand fast response time may be achieved. Especially of Pt-8C8-TPP8+/MCM-48(20 mg/g), the sensitivity is 5041.2, the quenching time is 0.04 s and the recoverytime is 32.2 s. Even when the concentration of oxygen is only 0.1%, thesensitivity of Pt-8C8-TPP8+/MCM-48 (20 mg/g) can reach 21.5. It is well knowthat the measurement of oxygen at low concentration is more important, soPt-8C8-TPP8+/MCM-48 has a great potential for the application in oxygen sensors.Comparing with the oxygen sensing materials based homologous platinumporphyrins with eight alkyl-carbazole arms, Pd-8C4-TPP8+/MCM-48 has thebetter performance than Pt-8C4-TPP8+/MCM-48. But the performance ofPd-8C6-TPP8+/MCM-48 and Pd-8C8-TPP8+/MCM-48 are reduced. ForPd-8C8-TPP8+/MCM-48 (20 mg/g), the sensitivity is 923.82, the quenching timeis 0.06 s and the recovery time is 64.8 s.In summary, we have designed and synthesized two series of star-shapedporphyrins with four and eight alkyl-carbazole arms at their meso position. Thenwe selected mesoporous molecular sieves as a new matrix to form assemblymaterials by assembles the luminescent compounds with it. Their protonatedplatinum/palladium porphyrins could be physically or chemically assembled withmesoporous materials to obtain oxygen sensing materials. It was demonstratedthat assembly structural features between chromophore and supporter havedramatic effect on the performance of oxygen sensors. High performance oxygensensing materials can be achieved by arrangement and immobilization ofchromophores on the exo-surface of MCM-48.
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