Electrochemiluminescence lamp with TEMPO

"Electrochemiluminescence or electrogenerated chemiluminescence (ECL) is a kind of luminescence produced during electrochemical reactions in solutions. ... ... ECL excitation can be caused by energetic electron transfer (redox) reactions of electrogenerated species. Such luminescence excitation is a form of chemiluminescence where one/all reactants are produced electrochemically on the electrodes.[2]" Wiki

I've always been fascinated by this as it provides a possible route to an electrical 'chemiluminescence lamp', so want to explore ways that we may construct a setup that could be made to work in the amateur context.

Just like chemiluminescence, this would work very well if you could generate an otherwise poorly-radiating excited species in solution that upon collision with a fluorescent dye, excites it and then bring about the production of light at reasonable quantum yields.

Consider the hypothetical case where you have a protected/masked form of nitric oxide dissolved in an inert aprotic solvent to which you have dissolved an inert lipophilic electrolyte.

For example, the commercially available compound (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl radical or its ketone analogue (organic chemists will know this as TEMPO) and it's typically used in catalytic amounts in organic oxidations.

TEMPO is a stable radical (bright red in colour) that is soluble in organic solvents. It is stable because of the steric hindrance around the N-O group.

Now if we dissolve TEMPO in an organic solvent and add some of our inert (But)4N PF6 electrolyte, we should be able to readily generate TEMPO+ and TEMPO- at the anode and cathode respectively (much in the same way as one could do it with nitric oxide hypothetically, but you wouldn't be able to dissolve nearly as much NO).



Half reactions are:

TEMPO + e^- <=> TEMPO^-
TEMPO <=> TEMPO⁺ + e^-

Then in solution:

TEMPO⁺ + TEMPO^- => 2TEMPO*
TEMPO* + Dye => Dye* + TEMPO
Dye* => Dye + hv

Then when these meet in solution, charge annihilation possibly results in the generation of excited TEMPO which can then excite a polyaromatic fluorescent dye in solution, for example rubrene to give a yellow light. As one could dissolve TEMPO in high concentrations, one ought to be able to run the electrochemical cell at high rates without too much problem (except for having to dissipate heat of course). These fluorescent dyes are expensive so one want to use them in minimal amounts!

Possibly the 'ultimate' way to make a light source out of such a system would be to dissolve the TEMPO into an inert ionic liquid as both solvent and electrolyte, for example 1-Ethyl-3-methylimidazolium tetrafluoroborate so as to prepare a saturated solution to which are also added your fluorescent dyes, however, this is not easy peasy from the amateur point of view!

EDIT: Pruned and cleaned a lot. Also found that TEMPO substitutes are relatively easy to synthesise from ammonia and acetone, see this thread on TEMPO alternatives. I see no reason why the radical derived from 2,2,6,6-tetramethylpiperidin-4-one could not be used here, as the ketone is nevertheless aprotic and so should be fine to oxidise on the electrodes without gas production, though it might be more susceptible to degradation over time (less inert than non-functionalized TEMPO).

[Edited on 26-10-2013 by deltaH]

UPDATE 4

t = 24h, it's not looking to bad actually. The mixture darkened to a deep orange/red overnight and the flakes are visibly disintegrating. You can see on the bottom the formation of fine white material which I hope is sodium carbonate. I think this reaction is doing ok...

COMMENT: I think there's a much higher chance that the crystals forming is a sodium carbonate hydrate and not sodium bicarbonate, so that means that I am 50% short on sodium hydroxide required in this reaction (I used equimolar NaOH and urea).

This inorganic reaction is thus better described by this equation:

2NaOH + (NH2)2CO => Na2CO3 + 2NH3

Water (from the organic reaction) may be absorbed by the anhydrous sodium carbonate to then form a solid hydrate.

I'm going to let this run for a little longer before adding more sodium hydroxide though, because if my theory is correct, it means that I should see nearly all the NaOH flakes disappear, but plenty of urea prills left. This would confirm it more or less and I'm then happy to add more.

I'm also clipping all four latches now on the container as this reaction will need to run for days and I think I can safely say no gas is evolving.

Furthermore, if you look carefully, you can see the white particles forming on the NaOH flakes only, so I think it's safe to conclude that urea is slowly dissolving into the solution and then reacting on the flakes to form ammonia and insoluble sodium carbonate.



[Edited on 28-10-2013 by deltaH]

Hi Random, yeah I'm thinking this experiment is a dud as well, it's now been 45h, there's even more white granules on the bottom and NaOH flakes slightly smaller, BUT I unfortunately noticed that the urea prills look resolutely the same and unchanged. The solution has also turned brown. I am pretty certain now that the white stuff at the bottom isn't sodium carbonate, but something else. I can only speculate that it's some kind of hemiacetal salt of sodium and acetone , I am not sure of the correct name for that, but basically Na⁺ [CH3O(OH)CCH3]^-. Whatever that may be, the matter remains that my main problem is the urea doesn't appear to be reacting, while the sodium hydroxide does, but very slowly and probably with the acetone.

I will now play a little with the urea hydrolysis with NaOH in aqueous solution to check what those rates are, then try to figure out the best mixed solvent system for acetone. As far as I know, in South Africa, we don't have OTC methanol (guys in the US I believe use a product called Heet). So I will most likely use an acetone/water system.

This would no doubt lead to a two liquid phase system as the dissolving sodium carbonate salt's out as a second liquid phase... perhaps not a bad thing. I've calculated that if I use 200ml water (compare to 220ml acetone I am using), I should have just enough water to fully dissolve the stoichiometric amount of sodium carbonate if it forms correctly.

I'm going to experiment with this now and report back when there's something interesting to report

UPDATE: Well this is shit, as you said, the RT reaction of urea and conc. aqueous NaOH doesn't appear to be doing zilch. I am thoroughly disappointed!

By the way, the urea slaked lime reaction works very well if you melt the urea (m.p = 135C), but I think that's because the urea forms ammonia and cyanuric acid, which then goes on the react with the calcium hydroxide.

Sigh. OK plan C If I can convert my urea to ammonium carbamate, then I think I'll be back in business (similar to your ammonium salt example except carbamate can also be quickly hydrolysed).

[Edited on 29-10-2013 by deltaH]

Quote: Originally posted by deltaH

As far as I know, in South Africa, we don't have OTC methanol (guys in the US I believe use a product called Heet). So I will most likely use an acetone/water system.

Quote: Originally posted by plante1999

Quote: Originally posted by deltaH   As far as I know, in South Africa, we don't have OTC methanol (guys in the US I believe use a product called Heet). So I will most likely use an acetone/water system. Funy to see how regulations are applied in each country. Here, ethanol, even detnature is quite regulated and store prefer not to have the hassles, so they sell 99.9% methanol by the gallon instead (without colorant or anything). It is my standard solvent. It is a shame you can't get it easily. Try to find cooking fuel maybe? They tend to say something like: Contain wood alcohol on them.

Yeah it also amuses me, I mean if things are so bad, then surely all countries should have the same bans, the fact that they don't tells me there is no rationale behind it

We have methylated spirits of course, but I can't even get a little sodium hydroxide to dissolve into it, so think they add some alkanes, as in it probably consists of a mixture of ethanol, methanol and something like hexane.

The problem is South African OTC chemicals are notoriously under labeled. In fact I bought some 'battery acid' the other day with no conc. specification. I even phoned the distributor and he didn't even know lol

I haven't seen any cooking fuel here, people normally just burn methylated spirits for that as far as I know.

EDIT:

Ok here's my plan for making ammonium carbamate from my urea:

I'm going to hydrolyse my urea with a conc. sodium hydroxide solution by heating, eventually it should go with enough time and high enough temperature to form a solution predominantly of carbamates and ammonia:

(NH2)CO(aq) + -OH(aq) => -O2CNH2(aq) + NH3(aq)

The I'm going to add ammonium chloride to this, upon cooling, I might be able to collect out the majority of the sodium as sodium chloride crystals which presumably would be poorly soluble in such concentrate ammonium carbamate solutions.

Na+(aq) + NH4Cl(s) => NaCl(s) + NH4+(aq)

This should also help replenish whatever ammonia I might have lost in the prolonged heating of the urea hydrolysis and also really saturate that solution with ammonia.

Then after decanting/filtering out the sodium chloride, I 'simply' need to crystallise a crop of ammonium carbamate from my pregnant liquor.

Nice plan, except I don't have OTC NH4Cl

If I remember correctly, ammonium carbamate solutions are curious in that they are MORE soluble if you use ammonia solutions than if you used water alone, appearing to defy a 'common ion' effect.

Ok, tried to find an old BASF technical flyer on its ammonium carbamate product where I recall seeing solubility information and the increase in solubility by using ammonia solutions. I think the highest solubility for the ammonium carbamate at RT was something around 60 wt.% using saturated ammonia solutions to dissolve it in, but that's completely off the top of my head.

Sucks, they now make you register with them to get access to their info!

EDIT 2:

I've just also had a thought, I know that for the reaction of polyol's with urea, zinc oxide is used as a catalyst. Perhaps zinc oxide can help here to catalyse the hydrolysis of urea. I'm lucky that one of the few chemicals I have left over from an experiment in my teens (a long time ago) is some zinc chloride, so I could prepare the zinc oxide fairly easily. I might fiddle with that a little as well if I have to time out of interest, but the zinc oxide would form zincate anions in strongly basic solutions.

[Edited on 29-10-2013 by deltaH]

Thanks froot, no I've got plenty of caustic thanks, mine courtesy of Spar

Quote: Originally posted by deltaH

I think I'm going to have a stab at making TEMPOne as per the acetone/ammonia condensation (but with a twist!) followed by oxidation.

It started out because I had a big bag of urea and wanted to use it I was (wrongly) under the impression that it could be hydrolysed easily by strongly caustic media. Agreed, the caustic does seem to be making a lot of shit! But half the fun of experimentation is also trying out new ideas... case in point, this is not working, so shoot me lol

I'm still a little interested in trying ammonium carbamate as I don't think that will make a lot of shit because it's only mildly basic and an excellent source of almost dry ammonia.

Look, I'm probably going to HAVE to hydrolyse my urea to make the ammonia needed for this reaction because I don't want to go buying 20 litres of ammonia from my chem supplier and they don't sell smaller amounts.

I figured if I'm going to have to make this effort (boiling up the urea with a strong base), it might not be a bad idea to convert that into ammonium carbamate.

EDIT:

You know I find it frightening just how many chemicals fall under the 'used in baking' category. Just what the heck are they putting in bread these days lol.

Anyhow, I mention this because I just read that ammonium chloride is also used in commercial baking. Trying to find an OTC source for it here in South Africa, preferably one that I don't need to get ripped off with or required to buy 20kg!

Oh gosh, I just realised I'm missing an obvious route to ammonium chloride, boiling up urea with conc. HCl. I've done a similar hydrolysis to make ammonium phosphate using phosphoric acid! Can't believe I forgot about that!

The reaction would be:

(NH2)2CO(aq) + 2H+(aq) + H2O(l) => 2NH4+(aq) + CO2(g)

This is so classic, boil up urea and sodium hydroxide in one pot to make sodium carbamate, then boil up urea and hydrochloric acid in another to make ammonium chloride and then combine... I like it!

[Edited on 29-10-2013 by deltaH]