A sensitive and easily distinguishable cobalt-free humidity indicator of porphyrin-silica gel-MgCl(2) composite was prepared from pH-induced spectra changeable tetraarylporphyrin, silica gel (SiO(2)), and MgCl(2). The pH change arose from proton release under dry conditions, and proton capture under humid conditions by a reversible reaction between MgCl(2) and a silanol group of SiO(2). A pink-orange porphyrin-Si(OH)(2)-MgCl(2) composite was dried to give a green protonated porphyrin-SiO(2)Mg composite. The optimized concentrations of MgCl(2) to make the concentrations of protonated porphyrin maximum under dry conditions were determined by absorption spectrometry of the green composite using a confocal laser scanning microscope as a microscopic spectrometer. Moreover, the green composite was prepared by heating dichloro(tetraarylporphyrinato)phosphorus chloride with MgCl(2) and SiO(2). The humidity-sensitivity of the green composite was evaluated by the absorption spectra under controlled humidity. A distinguishable color change of the green composite took place below 30% of relative humidity.
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