Red Light Applications in Photochemistry

Red Light Applications in Photochemistry

Near-Infrared (NIR) Light Gaining Interest

For most of the previous century, photochemistry invoked images of high-powered mercury lamps, intense UV light and classical reactions such as [2+2]-cycloadditions, cyclizations and radical rearrangements (Ref 1). This changed more than ten years ago with the rediscovery of photoredox chemistry and the increased availability of LEDs (Ref 2). Irradiation of common photocatalysts such as ruthenium and iridium with visible light from blue LEDs (450-470 nm) afforded highly oxidative and reductive photocatalysts able to activate difficult organometallic cross-coupling reactions. Blue LEDs (450 nm) and photoreactors are now common in many synthetic labs with seemingly unlimited applications in organic synthesis (Ref 3). Now, a number of red light applications in photochemistry are expanding the options available to synthetic organic chemists.  This post provides an overview of many of these red light applications.

Osmium Photocatalysts

Several recent papers (see footnotes) further expand the tool kit for synthetic chemists to the Near-IR. The Rovis group at Columbia University and BMS recently published an exciting preprint using red LEDs (Ref 4). Low energy red LED (740 nm) can activate Os(II) photosensitizers to perform several of the same reactions previously demonstrated by blue LEDs but with several potential advantages. The advantages derive from two key differences between the common metal photoredox catalysts – like Ru(II) and Ir(III) – with Os(II) systems (see Figure 1). For Ru(II) and Ir(III) systems, the high extinction coefficients of the photocatalyst prevent light penetration deep into a reaction medium. This limits the amount of light available into the reaction volume and requires modification of the reaction when scaling up photoredox. Often the concentration of the catalyst is lowered for batch reactions (Ref 5) or the reaction is performed in flow (Ref 6). Secondly, in these Ir/Ru systems the ground S0 is excited through a metal-ligand charge transfer band (MLCT) to the excited state S1. It then must inter-system cross to give the T1 species. This decay is both inefficient, the quantum yield for common Ru photocatalyst is ~9%, and results in the loss of ~25% of the light energy thermally necessitating high-energy light.

Figure 1: Comparing traditional metal photoredox with Near-IR Os(II) systems (Figure adapted from Ref 4 and references within)

Comparing traditional metal photoredox with Near-IR Os(II) systems in red light photochemistry

Os(II) photosensitizers can directly access the excited triplet state with NIR radiation (S0⇢T1) converting NIR light into chemical energy with minimal loss. The resulting Os(II) T1 state is of similar energy as found in the Ir and Ru systems (40.8 vs. 46.5 kcal/mol). In the Rovis paper, the group demonstrates photoredox, photopolymerization, and metallaphotoredox reactions as well as a mole scale arene trifluoromethylation in batch. All excellent examples of red light applications in photochemistry.  And because of the lower extinction coefficient, red light can penetrate 10x farther into the reaction medium with an observed advantage in scaling up photoredox chemistry.

Figure 2: Batch Scale 1L trifluoromethylation reaction with Osmium photocatalyst

trifluoromethylation reaction with Osmium photocatalyst in red light photochemistry

In comparing the 1L batch scale up for the Osmium trifluoromethylation scale with the reported ruthenium trifluoromethylation reaction, several key features are observed. At 1L scale, the authors observed a 62% yield after 22 hours with eight 740 nm lights. In this setup, the red light penetrates 23x further into the reaction solution than the blue light. With the osmium, the reaction yield increased by 31.6% with larger scale, while for the ruthenium system the yield lowered 27.5% as scale increased. This demonstration shows advantages that can be achieved in scale up with osmium chemistry.

However, we should point out the success in scaling up iridium photochemistry to large scales reported by Harper, et. al. with predictable experimental control of reaction parameters such as lowering catalyst concentration and light intensity (Ref 5). While not a straight forward translation of the small-scale reactions, photoredox reactions have been performed on an industrial scale leveraging 365-450nm light sources and traditional catalysts. Additionally, osmium itself is problematic due to cost and toxicity concerns, but the method outlined here could be expanded and utilized with the synthesis of other novel photocatalysts.

Triplet Fusion Upconversion

Triplet fusion upconversion is the process where two lower energy photons are converted into one higher energy photon. This process is often utilized in photovoltaics or imaging. Another application of red light in photochemistry is enabling the generation of higher energy photons through this process.

The Rovis Group previously described a system to use Red LED to initiate photoredox catalysis through a process known as triplet fusion upconversion (Ref 7). In this work, the authors access orange and blue light from low energy infrared light by matching the appropriate sensitizer and an annihilator, generating a highly oxidative/reductive photocatalyst normally accessible only through blue light (Figure 3). In this system, the sensitizer absorbs one photon of light to generate the excited species and decays to give a triple excited sensitizer. One triplet can react with one annihilator to give the excited annihilator. Two annihilators can combine their energy to a higher excited state, with subsequent fluorescence to release of one higher energy photon from one of the annihilators.

This new higher energy photon can activate a photocatalyst (for example Ru or Ir) to initiate a photoredox catalytic cycle. The authors describe this process as generating a magnitude of “light bulbs inside the flask”. This enables the transfer of light/energy for catalysis into biological systems or through photoactive polymers or material) where blue light cannot penetrate but higher energy light than red is needed to activate the reaction.

Figure 3: Electron description of triplet fusion upconversion. Adapted from Ref. 9

Electron description of triplet fusion upconversion in red light photochemistry

Figure 4: Example of a sensitizer/ annihilator pair for emission of 1 photon blue light

Example of a sensitizer/ annihilator pair for emission of 1 photon blue light

Click Chemistry

Yet another example of a red light application in photochemistry is the ability of red LEDs to initiate one of the most commonly used reactions in bioorganic chemistry. Stremhel and coworkers demonstrated an NIR initiated Cu azide-alkyne click reaction (Ref 8). Using a series of cyanine photosensitizers the group was able to catalyze the reduction of Cu(II) to Cu (I) using red LED (790 nm) under ambient temperature (Figure 5). The method accesses the catalytically active Cu(I) species without an additional reducing agent. In this system, the Cu can be used at ppm levels. This work enabled the synthesis of several block polymers and represents a system able to perform click reactions where desired monomers might absorb in UV or visible region or deep in biological tissue.

Figure 5: Click Reaction

reduction of Cu(II) to Cu (I) using red LED (790 nm) under ambient temperature

Red Light in Cancer Therapeutics

Methods for performing NIR chemistry in vivo are of significant interest for photodynamic therapy. The therapeutic window for most photodynamic therapy lies between 600-800 nm, where the energy of each photon is high enough to activate a photosensitizer but low enough to achieve penetration through biological tissue. A demonstration of the use of red LED in vivo is shown in the recent Pt(IV) prodrug study demonstrated by Zhu Guangyu and coworkers (Ref 9). His team developed phorbiplatin, an anti-cancer prodrug activated by red light. Phorbiplatin is inactive in the dark, but upon irradiation with red light (650 nm) is converted to oxaliplatin, an approved anticancer drug and pyropheophorbide, also known to kill tumor cells. This onsite activation of the prodrug minimizes damage to healthy cells. In ten minutes of irradiation with low power 650 nm, 7 mW/cm2, 81% of the phorbiplatin is converted to the oxaliplatin and pyropheophorbide. The selectivity of the prodrug can also lead to significant improvement in activity. For example, phorbiplatin showed improved treatment for in cells and mouse tumors compared to treatment with oxiplatin.

As demonstrated by the phorbiplatin prodrug example, chemistry that can occur within the therapeutic window has great promise for a broad number of applications both in medicine and materials. Whether it is catalysts that can perform chemistry with NIR such as the Cu or Os examples above, or methods for generating higher energy light from red light, expanding this field can have great promise.

Figure 6: Phorbiplatin

phorbiplatin, an anti-cancer prodrug activated by red light

References:

(1) Hoffman, Norbert, “Photochemical Reactions as Key Steps in Organic Synthesis” Chem. Rev. 2008, 108, 1052-1103. https://pubs.acs.org/doi/10.1021/cr0680336
(2) Jagan M. R. Narayanam, Corey R.J. Stephenson, “Visible Light Photoredox Catalysis: Applications in Organic Synthesis” Chem. Soc. Rev. 2011, 40, 102-113. https://doi.org/10.1039/B913880N
(3) Leyre Marzo, Santhosh K. Pagire, Oliver Reiser and Burkhard König, “Visible-Light Photocatalysis: Does it make a difference in Organic Synthesis, Angew. Chem. Int. Ed., 2018, 57, 10034-10072. https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201709766
(4) Benjamin D. Ravetz, Nicholas E. S. Tay, Candice L. Joe, Melda Sezen-Edmonds, Michael A. Schmidt, Yichen Tan, Jacob M. Janey, Martin D. Eastgate, Tomislav Rovis, “Spin-Forbidden Excitation Enables Infrared Photoredox Catalysis” ChemRxiv, 2020.
(5) Harper, K. C.; Moschetta, E. G.; Bordawekar, S. V.; Wittenberger, S. J. “A laser driven flow chemistry platform for scaling photochemical reactions with visible light.” ACS Cent. Sci. 2019, 5, 109-115. https://pubs.acs.org/doi/10.1021/acscentsci.8b00728
(6) Thomas H. Rehm, “Reactor Technology Concepts for Flow Photochemistry” ChemPhotoChem, 2020, 4, 235-254. https://doi.org/10.1002/cptc.201900247
(7) Ravetz, B. D.; Pun, A. B.; Churchill, E. M; Congreve, D. N.; Rovis, T.; Campos, L. M. “Photoredox catalysis using infrared light via triplet fusion upconversion” Nature 2019, 570, 343-346. https://www.nature.com/articles/s41586-018-0835-2
(8) Kütahya, C.; Yagci, Y.; Strehmel, B. “Near-infrared photoinduced copper-catalyzed azide-alkyne click chemistry with a cyanine comprising a barbiturate group” ChemPhotoChem 2019, 3, 1180-1186. https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/cptc.201900012
(9) Wang Z., Wang N., Cheng S.C., Hirao H., Ko C.C., Zhu G. “Phorbiplatin, a Highly Potent Pt(IV) Antitumor Prodrug That Can Be Controllably Activated by Red Light” Chem, 2019, 5 (12) P3151-3165. https://doi.org/10.1016/j.chempr.2019.08.021

Classic Traffic Light Chemistry Experiment – Photochemistry Style

Classic Traffic Light Chemistry Experiment – Photochemistry Style

At HepatoChem, we’re serious about our chemistry but we don’t take ourselves too seriously.  We wanted to have a little fun re-producing one of the most viral chemistry experiments found on the web… the traffic light chemistry experiment.

You know the one… mix some glucose, sodium hydroxide and indigo carmine solutions together in a flask and get a nice deep yellow solution.  Mix it up once and it will turn to red.  Shake it up again and it turns green!  Then simply let the flask settle and it will revert back to red and finally rest at yellow again.  You can repeat the steps several times until the catalysts start to be eliminated through the reduction and oxidation processes that change the solution’s color.

We hope you enjoy the video and we’d appreciate it if you subscribed to our YouTube Channel or followed us on Twitter (@EvoluChem) so you get the latest updates as we continue to have a little fun with our photoreactors!

Full transcript of the video is below:

“OK, we wanted to have a little fun with this video and participate in Chemistry Twitter’s #FluorescenceFriday and demonstrate a photochemistry version of the well known traffic light experiment.

Now, we’re all familiar with traffic lights.  It starts pretty early… as kids we’re fascinated by the three colors and the meanings behind them.  Many of us spent some portion of our childhood playing the stoplight game.  Then it continues into adulthood – at least for most of us – as we learn the rules of the road on our way to getting our first taste of freedom, our driver’s license.

In fact the green, yellow and red of the basic traffic light is so universal it’s no surprise that YouTube is filled with videos on how you can produce your own traffic light experiment using easily available chemicals.  Just some basic sugar water, sodium hydroxide and indigo carmine in a flask will enable you to go from yellow, to red, to green – then back to red and yellow.  Check out the links in the description if you want to try it at home.

Which got us to thinking… let’s do a version of the traffic light experiment with photochemistry.  And then we realized…(record scratch)… we don’t know how to change a single solution into three different colors with an led light… but then we got an idea…

We have a photoreactor… specifically our very own PhotoRedOx Box… and we have some basic catalysts that will fluoresce the three primary colors of your standard traffic light.

This first catalyst is really just a control.  It’s simply soapy water.

Our second catalyst is going to be our green.  What is it?  Good question… we forgot to write it down.  But if you’re interested, let us know in the comments and we’ll figure it out.

The third catalyst is going to be our yellow.  And the fourth and final catalyst will be our red.

We placed all of the catalysts into one of our vial holders and… look, I know what you’re thinking…(DUH)  it’s pretty easy to tell which colors are going to be which… and you’re right… but we’re doing this for the lighting effect…

With the vial holder placed in the photoreactor we can turn the led on…

We’ll go through each one individually…

You can see the 4th catalyst is already in there and is kind of a purply red…

Our third catalyst is a strong yellow and you may recognize the glow from some of our other videos…

Finally we place the green in the vial and whoa… that is bright… sort of like those neon glow sticks you played with as a kid or at your last music festival… that green is eerily reminiscent of a movie from my childhood… ah, that’s right… careful Superman!

Anyway, thanks for watching our first contribution to #FluorescenceFriday.  If you have any questions about what we did here today or have any other ideas for things you’d like to see in the future, please let us know in the comments or follow us on Twitter.”

[end transcript]

Photocatalytic α‐Tertiary Amine Synthesis via C−H Alkylation of Unmasked Primary Amines

Photocatalytic α‐Tertiary Amine Synthesis via C−H Alkylation of Unmasked Primary Amines

Abtract

A) Prior art for catalytic α‐C−H alkylation of primary amines; B) This work. EWG=electron‐withdrawing group.

A practical, catalytic entry to α,α,α‐trisubstituted (α‐tertiary) primary amines by C−H functionalisation has long been recognised as a critical gap in the synthetic toolbox. We report a simple and scalable solution to this problem that does not require any in situ protection of the amino group and proceeds with 100 % atom‐economy. Our strategy, which uses an organic photocatalyst in combination with azide ion as a hydrogen atom transfer (HAT) catalyst, provides a direct synthesis of α‐tertiary amines, or their corresponding γ‐lactams. We anticipate that this methodology will inspire new retrosynthetic disconnections for substituted amine derivatives in organic synthesis, and particularly for challenging α‐tertiary primary amines.

Authors: Alison S. H. Ryder Dr. William B. Cunningham George Ballantyne Tom Mules Anna G. Kinsella Jacob Turner‐Dore Dr. Catherine M. Alder Lee J. Edwards Dr. Blandine S. J. McKay Dr. Matthew N. Grayson Dr. Alexander J. Cresswell

Link
https://onlinelibrary.wiley.com/doi/full/10.1002/anie.202005294#.Xr2aciKIkvU.twitter

 

Electron Donor-Acceptor (EDA) Complexes in Photochemistry

Electron Donor-Acceptor (EDA) Complexes in Photochemistry

Electron Donor-Acceptor (EDA) Complexes in Photochemistry

The photochemistry of Electron Donor-Acceptor (EDA) complexes provides a new option to create complex chemical structures (scaffolds) without the need for expensive photocatalysts like iridium or ruthenium. With the recent publication by Paolo Melchiorre and co-authors of an in-depth open-access review of “Synthetic Methods Driven by the Photoactivity of Electron Donor–Acceptor Complexes” in JACS  J. Am. Chem. Soc. (2020) 142(12):5461-5476 (Ref 1), we thought it was a great opportunity to discuss modern photochemical approaches using Electron Donor-Acceptor chemistry in synthetic schemes with comparison to the more prevalent photoredox or metalloredox chemistry (Ref 2).

The recent surge of interest in synthetic methods in photochemistry has been driven by photoredox catalysis, a process in which a photocatalyst utilizes the energy of visible light to drive a reaction between two substrates which would not proceed otherwise (Ref 3,4). In many cases, the excited state of the photocatalyst can act as both an oxidant and reductant as needed, transferring or receiving an electron at the appropriate time (Figure 1). This is the general reaction scheme enabling the multitude of photoredox or metallophotoredox catalytic reactions driven by the ruthenium and iridium photocatalysts that have become prevalent over the last decade .

Figure 1: General roles of an excited photocatalyst as both an oxidant and reductant in photoredox catalysis

Electron Donor-Acceptor EDA complexes figure 1

Differing from photoredox catalysis, photoactive electron donor-acceptor complexes do not require a photocatalyst  (Figure 2). An electron donor (D) and an acceptor (A), which often do not absorb light individually, upon complexation can absorb visible light to undergo single-electron-transfer generating radical intermediates. This approach affords the opportunity to generate radicals (D+., A-.) from substrates which typically would not absorb in the visible spectrum. The challenge from the synthetic perspective is avoiding the back electron transfer reaction, generating unproductively the two monomers in their ground state. The resurgence of electron-donor-acceptor synthetic methods as a field of photochemistry drew from two independent observations in 2013 by the Chatani group with photoredox (Ref 5) and the Melchiorre group studying organocatalysis (Ref 6). Control experiments for specific substrate combinations in the presence of light proceeded in the absence of catalyst.

Figure 2: General Strategy for Photoactivity of Electron Donor Complexes
Electron Donor-Acceptor EDA complexes figure 2

Direct Coupling Between Electron Donor-Acceptor EDA Complexes

The most straightforward approach to EDA complexes is the direct coupling of the two components upon light activation. In this approach, the viability of the reaction is intrinsic in the electron properties of the two partners and the resulting radicals. One common strategy involves a suitable leaving group within the electron donor-acceptor complex to initiate an irreversible fragmentation outcompeting back electron transfer (Figure 3A). The majority of examples in this class involve C-C bond formation with a few specialized examples for C-S bond formation (see table below).

A limitation of this approach is the requirement of highly polar donor and acceptor molecules which ultimately end up in the product. Alternately, a sacrificial donor can be used to form an EDA complex which upon excitation generates a radical intermediate suitable for reaction with a radical trap (Figure 3B). If the leaving group also can act as a redox auxiliary, then the synthetic scope of potential reactants can expand greatly (Figure 3C). Using the redox auxiliary allows for a suitably polar acceptor to form the EDA complex but generates a radical upon loss of the redox auxiliary that is not biased by internal stabilization or activation in molecule. Versatile redox auxiliaries that can be easily added and removed from desired reactants have the potential to allow synthetic platforms for broad use of the electron donor-acceptor EDA complexes.

Figure 3: Strategies to utilizing Photoactivation of Election Donor-Acceptor Complexes

Electron Donor-Acceptor EDA complexes figure 3

 

A few Examples of common components for Electron donor-acceptor complexes are shown in Figure 4.

Figure 4: Representative Photoactive Electron Donor-Acceptor complexes

Type Donor Acceptor Radical Trap Product
EDA with built in leaving

(ref 7)

EDA_figure 4-D1 EDA_figure 4-A1
EDA with built in leaving

(ref 5)

EDA_figure 4-D2 EDA_figure 4-A2 EDA_figure 4-P2
Sacrificial Donor

(ref 8)

EDA_figure 4-D3 EDA_figure 4-A3 EDA_figure 4-R3 EDA_figure 4-P3
Sacrificial Donor Redox auxiliary

(ref 9)

EDA_figure 4-D4 EDA_figure 4-A4 EDA_figure 4-P4
Sacrificial Donor, Redox auxiliary radical trap

(ref 10)

EDA_figure 4-D5 EDA_figure 4-A5 EDA_figure 4-R5 EDA_figure 4-P5

 

Photoactivity of Catalytical EDA complexes

To avoid the use of stoichiometric reagents, an important approach in this field incorporates the formation of the EDA complex into a catalytic cycle. In this approach, a catalyst activates one of the substrates (weakly polar) into a more polar form triggering the formation of the EDA complex which can then be photoactive. This mode of reactivity affords the possibility of asymmetric catalysis with a chiral catalyst. In this scheme a weak donor interacts with an organocatalyst.

Figure 5: Catalyst-donor complex drives EDA formation and initiates catalytic cycle

Electron Donor-Acceptor EDA complexes figure 5

Outlook

Whether the EDA complex derives from a stoichiometric complex of a donor or acceptor or a more complicated catalytic cycle involving the formation of a pre-catalyst, this review demonstrates the synthetic utility of this exciting field.

Therefore, new redox auxiliaries, chiral ligands, and novel reactions could potentially enable a new approach for building medicinal chemistry libraries generated through photochemistry.

Photochemical reactions proceed in mild conditions with high functional group tolerance and can be applicable to an expansive array of synthetic chemistry. With recent advances and discussions on the process of “photon equivalents” (treating light as a controlled reagent in synthesis) there should be no limit to the scope and scale of photochemical strategies discussed herein and their use in chemical production (Ref 11). We hope that you check out the review for the historical perspective, utility and future challenges of this highly valuable synthetic approach to manipulating radical chemistry.

References:

(1) Giacomo E. M. Crisenza, Daniele Mazzarella, Paolo Melchiorre, Synthetic Methods Driven by the Photoactivity of Electron Donor–Acceptor Complexes, J. Am. Chem. Soc. 2020, 142, 12, 5461-5476. (https://pubs.acs.org/doi/10.1021/jacs.0c01416)
(2) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. Photoredox Catalysis in Organic Chemistry. J. Org. Chem. 2016, 81, 6898−6926. https://pubs.acs.org/doi/abs/10.1021/acs.joc.6b01449
(3) Marzo, L.; Pagire, S. K.; Reiser, O.; König, B. Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis? Angew. Chem., Int. Ed. 2018, 57, 10034−10072. https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201709766
(4) Rory C. McAtee, Edward J McClain, R.J. Stephenson, Illuminating Photoredox Catalysis, Trends in Chemistry, 1, 1 P111-125. https://www.cell.com/trends/chemistry/fulltext/S2589-5974(19)30009-7

(5) Tobisu, M.; Furukawa, T.; Chatani, N. Visible Light-mediated Direct Arylation of Arenes and Heteroarenes Using Diaryliodonium Salts in the Presence and Absence of a Photocatalyst. Chem. Lett. 2013, 42, 1203−1205. https://www.journal.csj.jp/doi/abs/10.1246/cl.130547
(6) Arceo, E.; Jurberg, I. D.; Álvarez-Fernández, A.; Melchiorre, P. Photochemical activity of a key donor-acceptor complex can drive stereoselective catalytic α-alkylation of aldehydes. Nat. Chem. 2013, 5, 750−756. https://www.nature.com/articles/nchem.1727

(7) Kandukuri, S. R.; Bahamonde, A.; Chatterjee, I.; Jurberg, I. D.; Escudero-Adán, E. C.; Melchiorre, P. X-Ray Characterization of an Electron Donor-Acceptor Complex Drives the Photochemical Alkylation of Indoles. Angew. Chem., Int. Ed. 2015, 54, 1485−1489. https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201409529

(8) Sun, X.; Wang, W.; Li, Y.; Ma, J.; Yu, S. Halogen-Bond- Promoted Double Radical Isocyanide Insertion under Visible-Light Irradiation: Synthesis of 2-Fluoroalkylated Quinoxalines. Org. Lett. 2016, 18, 4638−4641. https://pubs.acs.org/doi/10.1021/acs.orglett.6b02271

(9) Davies, J.; Booth, S. G.; Essafi, S.; Dryfe, R. A. W.; Leonori, D. Visible-Light-Mediated Generation of Nitrogen-Centered Radicals: Metal-Free Hydroamination and Aminohydroxylation Cyclization Reactions. Angew. Chem., Int. Ed. 2015, 54, 14017−14021 https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201507641

(10) Zhang, J.; Li, Y.; Xu, R.; Chen, Y. Donor-Acceptor Complex Enables Alkoxyl Radical Generation for Metal-Free C(sp3)- C(sp3) Cleavage and Allylation/Alkenylation. Angew. Chem., Int. Ed. 2017, 56, 12619−12623. https://pubmed.ncbi.nlm.nih.gov/28809077/

(11) Emily B. Corcoran, Jonathan P. McMullen, François Lévesque, Michael K. Wismer and John R. Naber Photon Equivalents as a Parameter for Scaling Photoredox Reactions in Flow: Translation of Photocatalytic C−N Cross‐Coupling from Lab Scale to Multikilogram Scale Angew. Chem Int. Ed. 2020, Online. https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201915412

A Standard Ferrioxalate Actinometer Protocol

A Standard Ferrioxalate Actinometer Protocol

Before We Begin…

This post is an overview of how we perform a ferrioxalate actinometer protocol to determine photon flux in our various photoreactors. We try to go into as much detail as possible so you can replicate these steps in your own lab and with your own equipment. Please check out some of our earlier posts if you’re interested in the foundations of actinometry or want some general background on how to measure light in photochemical reactions. And if this post helps you successfully measure your own photochemical reactions or equipment, we would love to hear about it!  Reach out to us using the contact form or message us on Twitter @EvoluChem.

A Quick Description of The Ferrioxalate Protocol

This ferrioxalate actinometer protocol consists of irradiating a solution of ferrioxalate (Fe3+) to measure the production rate of Fe2+. The solution of ferrioxalate is made in diluted H2SO4.  The resulting solution is light sensitive, so care should be taken to keep the room as dark as possible while running the protocol.

Samples are taken at specific points in time and mixed with a AcONa (sodium acetate) buffer solution and phenanthroline solution to generate a phenanthroline Fe2+ complex.

The subsequent Fe2+concentration is then determined using spectrometry measurement of the phenanthroline Fe2+ complex absorption at 510 nm. A calibration curve is made using FeSO4 solution and phenanthroline.

Synthesis of Ferrioxalate K3Fe(C2O4)3.3H2O

In the dark (under a red light) and at room temperature, combine 50 mL of a 1.5 M aqueous solution of FeCl3 (12.16 g) with 150 mL of a 1.5 M aqueous solution of K2C2O4.H2O (41.45 g). After 30 minutes, filter the solid off and recrystallize it 3 times in 50 ml of water. Store in an amber vial and place the solution in a desiccator overnight. Typically, 7-10 g are obtained. The resulting solid can be stored for months.

Reagents:

Actinometric Solution

This solution must be made in the dark as once the complex is in solution it reacts to all light. The solution is made from 9.03 g of K3Fe(C2O4)3, 12.2 mL 1N H2SO4 and 110 mL H2O.  It should be stored at room temperature in a dark bottle wrapped in aluminum foil.

AcONa Buffer Solution

Mix 12 mL of AcONa solution (8.2g/100 mL), 7.2 mL 1N H2SO4 and 0.8 mL of H2O.

Spectrometric Solution

This solution is prepared for each sample by mixing 2.5 µL AcONa buffer, 100 µL of phenanthroline solution (0.1 g into 100 mL of H2O), and 892 µL H2O in a standard 96 deep well plate.

Actinometric Measurement

In a dark room under red light, prepare your setup with a light source and set your vial. NOTE: For a quick discussion of how best to evaluate light sources, please click here.  Pipette the actinometric solution (in the tin foil bottle) into the vial. Irradiate the vials for exactly 5 or 10 second intervals taking 5 µL samples and transferring them to the clear 96 well plate containing the spectrometric solution. NOTE: You should be wearing protective orange goggles when the light is on. Once irradiation is over, transfer 200 µL of each sample to a reading 96 well plate. Wrap the plate in aluminum foil as it is still light sensitive. Measure the sample absorption with a plate reader at 510 nm.

Example of Fe-Phenanthroline complexe absorbance measurement of experiment timepoints. Six different vials (A, B, C, D, E, and F) at different position from a light source.

Calibration Curve Preparation

Prepare a 0.4 mM ferrous solution, by adding 278.01 mg of FeSO4, 7 H2O (278.01 g/mol, 1 mmol) in 1 mL of H2SO4 (1 N) and dilute to 10 mL with H2O (0.1 M). Then transfer 80 µL into 2 ml of H2SO4 (1 N) diluted into 20 mL (4 mM).

Prepare buffer-phenanthroline solution, by adding 477.15 mg AcONa and 10.99 mg phenanthroline together, and then dissolve in 6 mL H2O. Add 3.6 mL 1N H2SO4 and 0.4 mL H2O.

Prepare the 9-point calibration in a 96 deep well plate by mixing the FeSO4 solution, buffer-phenanthroline solution and H2O with the volumes described below.  Let the solution sit for 30 minutes.  Transfer 200 µL in a flat bottom transparent well plate. Measure the sample’s absorption on a plate reader at 510 nm.

Fe2+  Concentration (mM) Volume buffer-phenanthroline solution Volume 0.4mM FeSO4 Volume H2O
0 350 µL 0 650 µL
0.02 350 µL 50 µL 600 µL
0.04 350 µL 100 µL 550 µL
0.06 350 µL 150 µL 500 µL
0.08 350 µL 200 µL 450 µL
0.1 350 µL 250 µL 400 µL
0.12 350 µL 300 µL 350 µL
0.14 350 µL 350 µL 300 µL
0.16 350 µL 400 µL 250 µL

 

Chart of typical calibration curve

Typical calibration curve

 

Determine Photon Flux In Your Photochemical Experiments

To find out how to calculate the photon flux using your results, check out our post Determining Photon Flux Using Actinometry.

Did this post help you successfully measure your own photochemical reactions or equipment?  We would love to hear about it!  Reach out to us using the contact form or message us on Twitter @EvoluChem.