Weird Science

Red and Blue: Dual-Color Chemiluminescence

Polish ver­sion is here

The fol­lo­wing article was ori­gi­nally publi­shed in the jour­nal for edu­ca­tors Che­mia w Szkole (eng. Che­mi­stry in School) (3/2024):

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Ples M., Czer­wono i nie­bie­sko - dwu­barwna che­mi­lu­mi­ne­scen­cja (eng. Red and Blue – Bi-Color Che­mi­lu­mi­ne­scence), Che­mia w Szkole (eng. Che­mi­stry in School), 3 (2024), Agen­cja AS Józef Szew­czyk, pp. 30-33

Intro­duc­tion

Che­mi­stry is a fasci­na­ting and essen­tial disci­pline in our lives. Unfor­tu­na­tely, it is often per­ce­i­ved as a chal­len­ging sub­ject, where maste­ring even the basics can feel like an insur­mo­un­ta­ble task.

The jour­ney thro­ugh che­mi­stry is fil­led with obstac­les, from deci­phe­ring the cryp­tic lan­gu­age of che­mi­cal equ­a­tions to gra­sping the elu­sive nature of ato­mic and mole­cu­lar struc­tu­res, which is vastly dif­fe­rent from the macro­sco­pic world we inte­ract with daily. Many stu­dents feel disco­u­ra­ged by the immense volume of know­ledge to absorb and the see­min­gly endless sequ­ence of for­mu­las to memo­rize. Howe­ver, hid­den within these dif­fi­cul­ties lies a wealth of cap­ti­va­ting phe­no­mena wai­ting to be explo­red.

Despite these ini­tial chal­len­ges, delving into che­mi­stry can be an incre­di­bly rewar­ding ende­a­vor. It reve­als the mecha­ni­sms gover­ning both the smal­lest par­tic­les of our exi­stence and the uni­verse as a whole, offe­ring insi­ghts into the fun­da­men­tal prin­ci­ples of mat­ter and energy. More­o­ver, under­stan­ding che­mi­stry ena­bles indi­vi­du­als to make infor­med cho­i­ces in various aspects of life, inc­lu­ding reco­gni­zing and selec­ting food ingre­dients.

Che­mi­lu­mi­ne­scence is one of many intri­gu­ing che­mi­cal phe­no­mena, refer­ring to reac­tions that pro­duce light without ther­mal exci­ta­tion. Many such pro­ces­ses exist, but they typi­cally emit light of a sin­gle, spe­ci­fic color. Howe­ver, a reac­tion sys­tem can be desi­gned to emit light in two con­tra­sting colors from oppo­site ends of the spec­trum: red and blue [1].

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Ani­ma­tion: sup­ple­men­tary mate­rial

What Do We Need?

The expe­ri­ment requ­i­res the fol­lo­wing che­mi­cal sub­stan­ces:

For­mal­de­hyde, also known as metha­nal, is the sim­plest alde­hyde (Figure 1). It is a color­less gas with a cha­rac­te­ri­stic pun­gent odor. Its mole­cule con­si­sts of a car­bon atom bon­ded to two hydro­gen atoms and a func­tio­nal -CHO group. It is one of the most impor­tant orga­nic com­po­unds, com­monly used as a pre­cur­sor in the syn­the­sis of many other che­mi­cal sub­stan­ces.

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Figure 1 – Struc­tu­ral for­mula of for­mal­de­hyde

For­mal­de­hyde has a wide range of appli­ca­tions in the che­mi­cal and phar­ma­ceu­ti­cal indu­stries. It is used in the pro­duc­tion of resins, pla­stics, dyes, and as a pre­se­rva­tive in cosme­tics and cle­a­ning pro­ducts (tho­ugh less fre­qu­en­tly today).

Due to its bac­te­ri­ci­dal pro­per­ties, for­mal­de­hyde is used for disin­fec­tion and pre­se­rva­tion of bio­lo­gi­cal spe­ci­mens. It is also employed in fabric fini­shing tre­at­ments and the paper indu­stry.

Howe­ver, it is impor­tant to note that for­mal­de­hyde is a toxic and irri­ta­ting sub­stance. Pro­lon­ged expo­sure can cause respi­ra­tory irri­ta­tion, skin pro­blems, and is con­si­de­red a risk fac­tor for cer­tain dise­a­ses, inc­lu­ding can­cer.

Lumi­nol, or 5-amino-2,3-dihy­dro-1,4-phtha­la­zi­ne­dione (Figure 2), is an orga­nic com­po­und with signi­fi­cant flu­o­re­scence and che­mi­lu­mi­ne­scence pro­per­ties. In a stron­gly alka­line aqu­e­ous solu­tion, when reac­ting with hydro­gen pero­xide in the pre­sence of a cata­lyst, such as spe­ci­fic iron com­po­unds, lumi­nol emits blue light at a wave­length of appro­xi­ma­tely 425 nm.

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Figure 2 – Struc­tu­ral for­mula of lumi­nol

Lumi­nol has found wide­spread use in foren­sic science as a blood detec­tion agent, due to its abi­lity to reveal even trace amo­unts of hemo­glo­bin. When lumi­nol reacts with hemo­glo­bin, it under­goes "acti­va­tion" (as descri­bed by inve­sti­ga­tors), emit­ting cha­rac­te­ri­stic light that can be recor­ded using visi­ble light ima­ging tech­ni­ques, such as long-expo­sure pho­to­gra­phy. This tech­ni­que, known as the lumi­nol test, is fre­qu­en­tly used by law enfor­ce­ment to unco­ver tra­ces of blood at crime sce­nes.

Bey­ond foren­sics, lumi­nol is also used in bio­lo­gi­cal rese­arch to visu­a­lize the pre­sence of various oxi­di­zing sub­stan­ces or cata­ly­sts that pro­mote its oxi­da­tion. It is no sur­prise that this che­mi­lu­mi­ne­scent com­po­und is also employed in ana­ly­ti­cal che­mi­stry as an indi­ca­tor for detec­ting iron ions.

Lumi­nol, whe­ther in its free form or as a hydro­ch­lo­ride salt, typi­cally appe­ars as a pow­der ran­ging in color from cream, yel­low, to light brown (Photo 1).

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Photo 1 – Lumi­nol syn­the­si­zed by the author

While lumi­nol is not par­ti­cu­larly expen­sive, it is some­ti­mes dif­fi­cult to obtain in school or uni­ver­sity labo­ra­to­ries. Howe­ver, this desi­ra­ble rea­gent can be syn­the­si­zed using rela­ti­vely sim­ple labo­ra­tory methods. In a pre­vious issue of "Che­mia w Szkole," I descri­bed a method I deve­lo­ped for pro­du­cing lumi­nol using widely ava­i­la­ble and ine­xpen­sive rea­gents, with discar­ded labo­ra­tory glo­ves serving as the star­ting mate­rial [2]. As you can see, where there is a will, there is a way. Howe­ver, lumi­nol's toxi­city has not been tho­ro­u­ghly stu­died, and it is suspec­ted to have car­ci­no­ge­nic and aller­ge­nic pro­per­ties. The­re­fore, cau­tion is neces­sary when wor­king with this sub­stance.

Pyro­gal­lol, or 1,2,3-tri­hy­dro­xy­ben­zene, is an orga­nic com­po­und belon­ging to the phe­nol group with an aro­ma­tic cha­rac­ter. Its che­mi­cal struc­ture con­si­sts of three hydro­xyl gro­ups atta­ched to a ben­zene ring (Figure 3). It occurs natu­rally in some plants.

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Figure 3 – Struc­tu­ral for­mula of pyro­gal­lol

In medi­cine, pyro­gal­lol is known for its anti-inflam­ma­tory and antio­xi­dant pro­per­ties. Scien­ti­fic stu­dies have demon­stra­ted its poten­tial in tre­a­ting skin inflam­ma­tions and der­ma­to­lo­gi­cal con­di­tions. Pyro­gal­lol's mecha­nism of action invo­lves its abi­lity to inhi­bit the acti­vity of enzy­mes respon­si­ble for inflam­ma­tory pro­ces­ses and to neu­tra­lize reac­tive oxy­gen spe­cies, the­reby pro­tec­ting cells from oxi­da­tive stress [3].

Addi­tio­nally, pyro­gal­lol is used in the phar­ma­ceu­ti­cal indu­stry for pro­du­cing anti­bac­te­rial drugs and in the syn­the­sis of cer­tain dyes used in the textile indu­stry. It has also been spo­ra­di­cally employed in der­ma­to­logy as an exfo­lia­ting agent (in pso­ria­sis tre­at­ment, pri­ma­rily as oint­ments or alco­hol solu­tions). It is known phar­ma­ceu­ti­cally as Pyro­gal­lo­lum or syno­ny­mo­u­sly as Aci­dum pyro­gal­li­cum.

In the past, pyro­gal­lol was com­monly used as a pho­to­gra­phic deve­lo­per, an absor­bent in gas ana­ly­sis (it stron­gly absorbs oxy­gen in an alka­line solu­tion), and as a hair dye com­po­nent. Howe­ver, due to its poten­tial muta­ge­nic effects, its use in areas invo­lving direct human con­tact has been limi­ted.

Expe­ri­ment

Now that we have all the neces­sary sub­stan­ces, we can pro­ceed with the expe­ri­ment. To do this, we must pre­pare a fresh solu­tion by dis­so­lving, in sequ­ence, the fol­lo­wing sub­stan­ces in 40 cm3 (1.4 fl oz) of distil­led water: 0.8 g (0.03 oz) of sodium hydro­xide, 5 mg (0.0002 oz) of lumi­nol, and 25 g (0.9 oz) of potas­sium car­bo­nate. Once these com­po­nents have dis­so­lved, we add 1 g (0.04 oz) of pyro­gal­lol and 10 cm3 (0.34 fl oz) of a 40% for­mal­de­hyde solu­tion. Achie­ving homo­ge­ne­ity may requ­ire vigo­rous stir­ring. The resul­ting liquid has such an intense red color that it appe­ars almost black (Photo 2). As seen, the solu­tion stron­gly sta­ins even glass.

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Photo 2 – Pre­pa­red solu­tion

The solu­tion sho­uld not be sto­red for long periods or mixed vigo­ro­u­sly, as exces­sive aera­tion may cause pre­ma­ture oxi­da­tion of the pyro­gal­lol.

It is cru­cial to con­duct this expe­ri­ment out­do­ors or under a pro­perly func­tio­ning fume hood, as large amo­unts of gase­ous for­mal­de­hyde are rele­a­sed, whi­ch—as men­tio­ned ear­lie­r—is toxic and irri­ta­ting.

The solu­tion is then pla­ced at the bot­tom of a large, wide-nec­ked ves­se­l—an appro­priate cho­ice would be a bea­ker of at least 1 dm3 (34 fl oz) capa­city. Sepa­ra­tely, 40 cm3 (1.4 fl oz) of per­hy­drol (30% hydro­gen pero­xide solu­tion) is mea­su­red out (Photo 3).

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Photo 3 – Expe­ri­men­tal setup

After pre­pa­ring the mate­rials, quic­kly pour the per­hy­drol into the red solu­tion in the bea­ker and swirl the ves­sel a few times in cir­cu­lar motions. The reac­tion sho­uld be obse­rved in a dar­ke­ned room.

Almost imme­dia­tely, the liquid at the bot­tom of the ves­sel begins to emit a mode­ra­tely strong but cle­arly visi­ble red light (Photo 4A). The reac­tion pro­ce­eds rela­ti­vely calmly; small amo­unts of gas are rele­a­sed, but no signi­fi­cant foa­ming occurs. The red light emis­sion phase lasts for seve­ral seconds (depen­ding on the ini­tial tem­pe­ra­ture), after which the glow gra­du­ally fades. Simul­ta­ne­o­u­sly, the solu­tion begins to foam vigo­ro­u­sly, ini­tia­ting blue light emis­sion. For a brief moment, the solu­tion exhi­bits dual-color che­mi­lu­mi­ne­scence: red at the bot­tom and blue above (Photo 4B). Soon, the red glow disap­pe­ars, the solu­tion bur­sts into bri­ght blue light, foams inten­sely, and heats up almost to boi­ling (Photo 4C). At this stage, par­ti­cu­larly large amo­unts of highly irri­ta­ting gases are rele­a­sed, rein­for­cing the need for pro­per ven­ti­la­tion or con­duc­ting the expe­ri­ment out­do­ors.

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Photo 4 – Che­mi­lu­mi­ne­scence of the descri­bed sys­tem;
A – first stage (red che­mi­lu­mi­ne­scence),
B – tran­s­i­tion stage (bi-color che­mi­lu­mi­ne­scence: red and blue),
C – final stage (blue che­mi­lu­mi­ne­scence, strong foa­ming)

The light emis­sion fades after some time, and the solu­tion cools down. Once the reac­tion is com­plete, the liquid beco­mes nearly color­less.

Expla­na­tion

In this expe­ri­ment, we are dea­ling with two distinct reac­tions: the red light emis­sion is cau­sed by a reac­tion first descri­bed by Trautz and Scho­ri­gin in the early 20th cen­tury, while the blue light is pro­du­ced by the oxi­da­tion of lumi­nol [4] [5].

Regar­ding the Trautz-Scho­ri­gin reac­tion, it invo­lves poly­phe­nols, such as pyro­gal­lol, which con­tain at least two hydro­xyl gro­ups atta­ched direc­tly to car­bon atoms in an aro­ma­tic ring.

This reac­tion can be sim­pli­fied and depic­ted using the reac­tion scheme shown in Figure 4 [6].

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Figure 4 – Sim­pli­fied scheme of the Trautz-Scho­ri­gin reac­tion, descri­bed in the text

Pyro­gal­lol 1, under alka­line con­di­tions in the pre­sence of for­mal­de­hyde, is oxi­di­zed to ortho­qu­i­none 2. From here, the reac­tion can pro­ceed along two dif­fe­rent path­ways: if oxy­gen is ava­i­la­ble, ortho­qu­i­none rapi­dly poly­me­ri­zes or reacts with ano­ther mole­cule of unre­ac­ted pyro­gal­lol, for­ming an unsta­ble pur­pu­ro­gal­lin 3 under the descri­bed con­di­tions. In both reac­tion path­ways, one of the final pro­ducts is sin­glet oxy­gen 1O2.

The gro­und state of oxy­gen O2 is the tri­plet state 3O2, cha­rac­te­ri­zed by two unpa­i­red elec­trons, effec­ti­vely making it a radi­cal. Sin­glet oxy­gen 1O2, on the other hand, is in a higher energy state, making it unsta­ble and prone to spon­ta­ne­o­u­sly retur­ning to the tri­plet oxy­gen state 3O2. The energy dif­fe­rence, appro­xi­ma­tely 94.3 kJ/mol (22.6 kcal/mol), must be rele­a­sed into the sur­ro­un­dings. Here, this occurs thro­ugh the emis­sion of elec­tro­ma­gne­tic radia­tion at a wave­length of λ = 1270 nm. This might seem sur­pri­sing, as this wave­length falls within the infra­red range and is invi­si­ble to the human eye. Howe­ver, we do obse­rve visi­ble light in this expe­ri­ment. It is belie­ved that the con­cen­tra­tion of sin­glet oxy­gen gene­ra­ted during the reac­tion is high eno­ugh that when two of its mole­cu­les col­lide, they emit elec­tro­ma­gne­tic radia­tion with twice the energy (λ = 634 nm). This radia­tion appe­ars as beau­ti­ful red light [7].

Inte­re­stin­gly, this reac­tion can occur with many poly­phe­nols and their deri­va­ti­ves, inc­lu­ding cate­chin deri­va­ti­ves found in tea (espe­cially green tea), such as epi­gal­lo­ca­te­chin gal­late C22H18O11. A reso­ur­ce­ful che­mist, as it turns out, can even make green tea glow [8].

Regar­ding lumi­nol, it is impor­tant to remem­ber that in a stron­gly alka­line solu­tion, it under­goes keto-enol tau­to­me­ri­za­tion, mea­ning that within the reac­tion sys­tem, it exi­sts in both its ketone form (where the nega­tive charge is loca­li­zed on nitro­gen atoms) and its enol form (where the charge is loca­li­zed on oxy­gen atoms). Both forms exist in dyna­mic equ­i­li­brium, con­stan­tly inter­co­nver­ting. The more reac­tive enol form under­goes oxi­da­tion by hydro­gen pero­xide H2O2, pro­du­cing a cyc­lic pero­xide (which typi­cally requ­i­res an addi­tio­nal cata­lyst, tho­ugh under the descri­bed con­di­tions, one is not neces­sary). Due to the pre­sence of a pero­xide bridge in its struc­ture, this com­po­und is highly unsta­ble. It spon­ta­ne­o­u­sly decom­po­ses into a nitro­gen mole­cule N2 and an exci­ted-state 3-ami­no­ph­tha­late, which returns to the gro­und state by emit­ting light at a wave­length cor­re­spon­ding to the obse­rved blue color.

Sum­mary

This expe­ri­ment demon­stra­tes a uni­que case of dual-color che­mi­lu­mi­ne­scence, resul­ting from the com­bi­na­tion of two sepa­rate che­mi­cal reac­tions. The first, the Trautz-Scho­ri­gin reac­tion, gene­ra­tes sin­glet oxy­gen, which, upon col­li­sion of two exci­ted mole­cu­les, emits red light. The second, the oxi­da­tion of lumi­nol, pro­du­ces an exci­ted ami­no­ph­tha­late spe­cies, which emits blue light as it returns to its gro­und state.

By care­fully con­trol­ling reac­tion con­di­tions, it is pos­si­ble to brie­fly obse­rve simul­ta­ne­ous red and blue che­mi­lu­mi­ne­scence in the same solu­tion, making this an excel­lent demon­stra­tion of pho­to­che­mi­cal and oxi­da­tion pro­ces­ses. Addi­tio­nally, this reac­tion can serve as a star­ting point for fur­ther expe­ri­ments, such as testing how dif­fe­rent poly­phe­nols affect the red emis­sion or how cata­ly­sts can enhance the lumi­nol reac­tion.

Impor­tan­tly, this expe­ri­ment high­li­ghts how diverse che­mi­cal reac­tions can inte­ract in the same sys­tem, cre­a­ting visu­ally spec­ta­cu­lar effects while also demon­stra­ting fun­da­men­tal con­cepts of reac­tion kine­tics, energy trans­fer, and oxi­da­tion-reduc­tion pro­ces­ses.

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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