Weird Science

Light and Color: The Photochromic Properties of Thionine

Polish ver­sion is here

The fol­lo­wing article was ori­gi­nally publi­shed in the jour­nal for edu­ca­tors Che­mia w Szkole (eng. Che­mi­stry in School) (1/2019):

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Świa­tło i barwa. Tio­nina jako barw­nik foto­ch­ro­mowy (eng. Light and Color: The Pho­to­ch­ro­mic Pro­per­ties of Thio­nine), Che­mia w Szkole, 1 (2019), Agen­cja AS Józef Szew­czyk, pp. 15-17

In recent deca­des, so-cal­led smart mate­rials have gai­ned con­si­de­ra­ble atten­tion. These mate­rials com­bine the pro­per­ties of sen­sors and actu­a­tors, mea­ning they change their cha­rac­te­ri­stics in response to an exter­nal sti­mu­lus. They are used in various fields, ran­ging from sur­gery and indu­strial engi­ne­e­ring to modern clo­thing pro­duc­tion and even astro­nau­tics.

While many smart mate­rials result from com­plex manu­fac­tu­ring pro­ces­ses, some are sim­ple eno­ugh — yet still intri­gu­ing — to be repro­du­ced in a labo­ra­tory set­ting. One such exam­ple is a liquid that chan­ges color in response to mecha­ni­cal sti­muli: blue when sha­ken (Fig. 1A) and color­less when left undi­stur­bed (Fig. 1B) [1]. A key com­po­nent in this trans­for­ma­tion is a syn­the­tic dye: methy­lene blue, C16H18ClN3S.

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Fig. 1 – Solu­tion of methy­lene blue and glu­cose in an alka­line envi­ron­ment; A – after sha­king, B – undi­stur­bed

Ano­ther class of these mate­rials inc­lu­des pho­to­ch­ro­mic sub­stan­ces. Pho­to­ch­ro­mism is the phe­no­me­non in which a che­mi­cal com­po­und — or a more com­plex mate­rial — under­goes a rever­si­ble color change when expo­sed to light.

Pho­to­ch­ro­mism is widely used, for exam­ple, in the pro­duc­tion of modern eye­glass len­ses that dar­ken under strong sun­li­ght but remain tran­s­pa­rent in other con­di­tions. Today, such len­ses are pri­ma­rily made from spe­cia­li­zed syn­the­tic poly­mers, the pro­duc­tion of which typi­cally exce­eds the capa­bi­li­ties of ama­teur or school labo­ra­to­ries.

Howe­ver, not all hope is lost. It is pos­si­ble to explore the phe­no­me­non of pho­to­ch­ro­mism using sim­pler methods, one of which invo­lves the use of thio­nine.

Expe­ri­ment

Set­ting up this expe­ri­ment is stra­i­ght­for­ward and sho­uld not pose signi­fi­cant dif­fi­cul­ties. Howe­ver, the fol­lo­wing sub­stan­ces must be obta­i­ned:

Thio­nine, also known as Lauth’s vio­let, is a syn­the­tic dye struc­tu­rally rela­ted to the pre­vio­u­sly men­tio­ned methy­lene blue. The struc­tu­ral for­mula of the thio­nine cation is shown in Fig. 2.

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Fig. 2 – Struc­tu­ral for­mula of thio­nine

Thio­nine is typi­cally used in the form of its chlo­ride or ace­tate salts. In either case, under nor­mal con­di­tions, this sub­stance appe­ars as a dark blue, almost black pow­der (Fig. 3).

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Fig. 3 – Thio­nine

Due to its pro­per­ties, this com­po­und is com­monly used for bio­lo­gi­cal sta­i­ning, such as in micro­sco­pic sam­ple pre­pa­ra­tion [2]. Thio­nine is also used as an anti­dote for expo­sure to high doses of heavy metals [3].

Mohr’s salt is a dou­ble salt of fer­rous sul­fate and ammo­nium sul­fate. It is typi­cally found as a hexa­hy­drate, (NH4)2Fe(SO4)2 · 6H2O, for­ming pale green cry­stals that are highly solu­ble in water (Fig. 4).

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Fig. 4 – Mohr’s salt

Mohr’s salt serves as a donor of iron(II) ions (Fe2+) in this expe­ri­ment. While it might seem pos­si­ble to replace it with any iron(II) salt, this is not an easy sub­sti­tu­tion. Iron(II) ions are highly unsta­ble in solu­tion and rea­dily oxi­dize to iron(III) ions (Fe3+). Howe­ver, Mohr’s salt, both in its hydra­ted and solu­tion forms, is rela­ti­vely resi­stant to oxi­da­tion. This sta­bi­lity may be due to the sli­gh­tly aci­dic con­di­tions pro­vi­ded by ammo­nium ion hydro­ly­sis. Never­the­less, solu­tions of this salt sho­uld always be fre­shly pre­pa­red. Altho­ugh not highly toxic, Mohr’s salt is an irri­tant and sho­uld not come into con­tact with skin.

Sul­fu­ric acid(VI) is highly cor­ro­sive and sho­uld be han­dled with extreme cau­tion.

As always, appro­priate per­so­nal pro­tec­tive equ­ip­ment sho­uld be used when wor­king with che­mi­cals.

From this point onward, all pro­ce­du­res (unless other­wise spe­ci­fied) sho­uld be per­for­med under low-inten­sity ligh­ting.

To begin, a solu­tion of the dye must be pre­pa­red. You don’t need high pre­ci­sion; just dis­so­lve a small amo­unt of thio­nine in distil­led water until you obtain a dark blue solu­tion (Fig. 5).

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Fig. 5 – Dye solu­tion

Next, pre­pare a solu­tion of 2.5 g (0.088 oz) of Mohr’s salt in 50 cm3 (1.69 fl oz) of distil­led water, sli­gh­tly aci­di­fied with sul­fu­ric acid(VI). This solu­tion is com­ple­tely color­less (Fig. 6) [4].

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Fig. 6 – Aci­di­fied iron(II) solu­tion

Then, add a suf­fi­cient amo­unt of dye solu­tion to the iron(II) solu­tion to obtain a blue but still tran­s­pa­rent liquid (Fig. 7). Achie­ving the desi­red result may requ­ire adju­sting the dye con­cen­tra­tion.

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Fig. 7 – Pre­pa­red solu­tion

The pre­pa­red blue solu­tion (Fig. 8A) sho­uld be expo­sed to intense visi­ble light, such as from an incan­de­scent, com­pact flu­o­re­scent, or high-power LED lamp. After expo­sure, the solu­tion under­goes rapid (within seconds) deco­lo­ri­za­tion (Fig. 8B). Once the light source is remo­ved, the solu­tion almost imme­dia­tely begins to regain its color (Fig. 8C), retur­ning to its ori­gi­nal deep blue shade after about 60 seconds (Fig. 8D).

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Fig. 8 – Obse­rved chan­ges; A – ini­tial state, B – illu­mi­na­ted solu­tion, C – solu­tion imme­dia­tely after light remo­val, D – solu­tion after 60 seconds

This color change cycle can be repe­a­ted mul­ti­ple times by swit­ching the light source on and off. By appro­pria­tely sca­ling the expe­ri­ment, it can be demon­stra­ted even before a large audience.

Expla­na­tion

The pho­to­ch­ro­mic beha­vior of this solu­tion results from a fasci­na­ting redox pro­cess. Iron(II) ions (Fe2+) rea­dily oxi­dize to iron(III) ions (Fe3+), making them effec­tive redu­cing agents. Howe­ver, oxi­da­tion by dis­so­lved oxy­gen is hin­de­red due to its rela­ti­vely low con­cen­tra­tion and the pH of the reac­tion medium. Upon expo­sure to light, Fe2+ ions begin to oxi­dize to Fe3+ by redu­cing dye mole­cu­les, which in turn convert into their color­less leuco form. This pro­cess mani­fe­sts as the deco­lo­ri­za­tion of the solu­tion. Under low-light con­di­tions, the reverse reac­tion beco­mes ther­mo­dy­na­mi­cally favo­ra­ble, cau­sing the leuco-thio­nine to be reo­xi­di­zed into its colo­red form.

Altho­ugh this pro­cess is rever­si­ble, it is not enti­rely so — each cycle sli­gh­tly deple­tes the ava­i­la­ble Fe2+ ions. Howe­ver, at the given con­cen­tra­tions, the color chan­ges can be obse­rved nume­rous times, making this expe­ri­ment highly sui­ta­ble for edu­ca­tio­nal demon­stra­tions.

Refe­ren­ces:

All pho­to­gra­phs and illu­stra­tions were cre­a­ted by the author.

The above text inc­lu­des minor edi­to­rial modi­fi­ca­tions com­pa­red to the ver­sion publi­shed in the jour­nal, aimed at sup­ple­men­ting and adap­ting it for online pre­sen­ta­tion.

Adden­dum

The effect of this expe­ri­ment can be seen in the fol­lo­wing video:

Marek Ples

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