oxidative n - demethylation of a thiazine dye is envisioned to occur as illustrated below for the demethylation of tc : in steps a and b , an electron is removed from the tc to produce a radical cation , which is then oxidized again and loses a proton to form an iminium ion . subsequently , the iminium ion undergoes nucleophilic attack by water ( step c ) to produce the carbinolamine intermediate , in which there is a hydroxyl group and an amino group attached to the same carbon atom . the carbinolamine is unstable , and loss of the amine ( step d ) liberates formaldehyde and monodemethylated tc . in the case of demethylation of methylene blue , this would correspond to the formation of azure c from azure a . in photochemical demethylation reactions , tc apparently acts both as the sensitizer and the substrate in photochemical reactions , i . e ., in the absence of free - radical scavengers , it brings about its own photodecomposition . thus , tc absorbs light and becomes a “ photosensitizer ” and brings about a direct reaction of the tc in its singlet excited state with another tc molecule “ substrate .” the substrate molecule reacts with the triplet state of the dye , formed by intersystem crossing ( isc ). singlet oxygen is formed by reaction of ground state oxygen with the triplet state of the dye , followed by reaction of singlet oxygen with the substrate . the “ heavy atom ” effect of a metal ion , such as zinc , sodium , and the like , quenches the triplet state of the dye , returning it to is unreactive ground singlet state , preventing the triplet state from participation in the degradation reactions . the amounts of metal ion required to provide high photochemical stability are not highly critical and can be determined by routine experimentation having regard for this disclosure . in general , i prefer to employ upward of about 4 wt . % zinc or 9 wt . % sodium or both in the dye solution , although greater amounts are not harmful and smaller amounts are at least partially effective . the metal content should be a minimum of upwards of 2 wt % for at least partial effectiveness . the following examples illustrates the practice of the preferred embodiments of the invention and are not intended to serve as limitations on the scope thereof , which is defined only in the appended claims . this example illustrates the light - instability of a tc product with negligible content of metal ions . a sample of the tc product is prepared in accordance with example 1 of my u . s . pat . no . 6 , 086 , 852 , incorporated herein by reference , and further purified by preparative high performance liquid chromatography . the total metal content of this sample is less than 0 . 2 wt . %. an aliquout of this purified tc product sample is deep - blue in color . after standing overnight in the laboratory , exposed to ambient incandescent light , the solution becomes colorless . the sample of example 1 except containing about 4 wt % zn ( as zinc chloride ) and 9 % na ( as sodium chloride ) in the solution is essentially completely stable in the presence of incandescent light . the procedures of examples 1 and 2 are repeated using methylene blue , azure a and azure c instead of tc . similar results are obtained . the procedures of example 1 - 3 are repeated except using only zinc chloride in solution and only sodium chloride in solution . equivalent results are obtained . the procedures of example 14 are repeated except that soluble salts of magnesium , chromium and silicon are used instead of zinc and sodium salts . similar results are obtained .
photochemical demethylation reactions in solutions of thiazine dyes , in which the dye molecules act as both sensitizer and substrate , are reduced by quenching triple - state dye molecules , returning them to the unreactive ground state .