Abstract
A sol-gel methodology has been duly developed in order to perform a controlled covalent coupling of tetrapyrrole macrocycles (e.g., porphyrins, phthalocyanines, naphthalocyanines, chlorophyll, etc.) to the pores of metal oxide networks. The resulting absorption and emission spectra intensities in the UV-VIS-NIR range have been found to depend on the polarity existing inside the pores of the network; in turn, this polarization can be tuned through the attachment of organic substituents to the tetrapyrrrole macrocycles before bonding them to the pore network. The paper shows clear evidence of the real possibility of maximizing fluorescence emissions from metal-free bases of substituted tetraphenylporphyrins, especially when these molecules are bonded to the walls of functionalized silica surfaces via the attachment of alkyl or aryl groups arising from the addition of organo-modified alkoxides.
Highlights
We present the first results of a mixed strategy that consists of covalently bonding the H2T(p-COOH)pp and H2T(o-NH2)PP species inside the pores of organomodified silica, as a way to reduce the interactions with the pore walls and tune the polarity inside the pores of the network optimizing the fluorescence of the trapped species
As stated in previous paragraphs of the introduction section, the stability and luminescent properties of tetrapyrrole macrocycles trapped inside metal oxide networks, synthesized by the sol-gel method, were better preserved when these species were covalently bonded to the pore network
All the above results and analyses prove that the postulated synthetic methodologies make possible the covalent bonding of free bases of porphyrins, in stable and monomeric form inside metal oxide pore networks
Summary
The physical trapping of H2T(S)PP species inside the pores of translucent silica xerogels was performed by means of tetraphenylporphyrins including –OH, NH2, and -COOH substituent groups at the ortho- or para-positions Through this option, porphyrins were trapped in stable and monomeric form, the fluorescence was only partially preserved in the case of ortho-amine-substituted porphyrins, H2T(o-NH2)PP [36]. It was found that the absorbance of UV-Vis bands, in the respective spectra of these samples, depended on the length of the alkyl chain attached to the silica pore walls This result hinted at the possibility of decreasing the population of the Si-OH groups and tuning the polarity inside the pores as a means for optimizing the display of the luminescent properties of the species trapped inside the network [40]. We present the first results of a mixed strategy that consists of covalently bonding the H2T(p-COOH)pp and H2T(o-NH2)PP species (i.e., those that showed the better results in the previous experiments) inside the pores of organomodified silica, as a way to reduce the interactions with the pore walls and tune the polarity inside the pores of the network optimizing the fluorescence of the trapped species
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