Revisiting Noncovalent SO2−Amine Chemistry:  An Indicator−Displacement Assay for Colorimetric Detection of SO2

A supramolecular approach for potential detection of SO2 is presented, which is based on the “old” donor−acceptor chemistry between SO2 and amines and includes an indicator−displacement assay. When amines were added to Zn−tetraphenylporphyrin 1 in CHCl3, the solution changed from red to dark green. A bathochromic shift of Δλ ∼ 10 nm was observed for the Soret band, indicating the formation of 1•amine complexes. After this, SO2 gas was introduced, and the original red color of the solution was restored. The Soret band returned to its position for free porphyrin 1. The 1 •amine complexes dissociated, and new SO2•amine adducts formed. Porphyrin 1 thus served as an indirect colorimetric indicator for SO2. The system discriminates between SO2 and such typical exhaust gases as COX, NOX, and H2O. From the indicator−displacement assay, the K assoc values between 1000 and 30 000 M-1 for SO2•amine complexes were determined, which are comparable to those obtained by direct titration experiments between SO2 and the amines. Spectroscopic features of SO2•amine complexes are also presented.