The photochromism could stop if:

1) the spirooxazine precipitates into nano crystals. SO is not photochromic in the crystalline form.

2) it could decompose. It depends on your material. Does it contain things that could react eg acid or basic groups. Is the loss of activity occurring even when the material is kept in the dark or is it dissappearing under the action of UV? Does the ring-closed form's absorption spectrum change? Does the material become yellow? These all indicate some decomposition if they occur.

Reading down - the spectrum changed and it became blue. This can be because:

1) it shifts its equilibrium to the open form in the new host matrix. This re-equilibration could take days.

2) acid mediated ring opening.

is the blue color the same as the UV induced coloration? If it is then it supports equilibrium shift. If the blue is different then it is an irreversible reaction.

the blue colour may be possible to photobleach with strong visible light if it is an equilibrium shift.

i suppose that tif the uv induced coloration was initially reversible  on a shorter timescale we can rule out equilibrium shift..

i think therefore that this is a reaction involving acidic species.

Dear Tamizi,

Spirooxazines may be degraded by UV light. However, I think the incorporation of a spirooxazine in an organic-inorganic hybrid silica material (organo-silsesquioxane) should result in improved light stability due to the shielding effect of the matrix.

Anyway, the addition of a light stabilizer may be helpful. Suitable light stabilizers could be sterically hindered amines (HALS), like [1,2,3,4-butanetetracarbonic acid, tetrakis(1,2,2,6,6-penta-methyl-4-piperidinyl)ester, CAS-No.: 91788-83-9].

But it may be difficult to incorporate the light stabilizer together with the spirooxazine into the organo-silsesquioxane matrix?

Best regards.

Karola Schaefer

Dear Dr Hobley, thank you for the guidance. Precursor for the fabricated film used in the study is vinyltriethoxysilane in order to get silsesquioxane with low organic content. It is a sol-gel reaction process & previously acid has been added in order to catalysed the reaction. The photochromic dye is 1,3,3-trimethylindolino naphtho spirooxazine & diluted in ethanol(turn to blue) before mixed with the silsesquioxane sol solution. It true that the blue color changed to yellowish solution & no photochromism changed upon tested with UV light. Color changes (from blue to yellow) can be avoid if sol gel reaction take place without acid or bes-catalysed reactions. I agreed with the dye decomposition lead to yellowish solution. I also agreed that fast shelf time was due to the formation of nano-crystalline form due to the dynamic drying process of silsesquioxane matrix may contributes for loss of -OH groups that needed for dye to actively attached (solvated with -OH groups).

Regarding to the shift of equilibrium, we did experienced it (a few tenths nano compared to dye dissolved in alkohol solution). Still in process to gather & consolidate more results on it. I may need to open textbook on this re-equilibrium & acid mediated ring opening mechanism. Can I check with you on any books or papers regarding to these. Hope that will deepen my understanding. Thanks again. 

Dear Dr Schaefer, In the study vinyl silsesquioxane has been used due to retain the transparent properties of silica materials with a bit of flexibility from organic(vinyl groups). Thanks you for the suggested sterically hinder amines as light stabilizer. I think I have it in my shelf. I dont how hard it could be to harmonize all the compounds together, but I need to do it. Thanks :)