Photochemistry of earth-abundant metals

Photochemistry of earth-abundant metals

A recurring theme for many of our articles over the last few months is that there just isn’t enough iridium or ruthenium in the earth’s crust to do all of the photochemistry that we would like to perform at scale.

Due to cost and the scarcity of these metals, the development of organic dyes such as benzo[ghi]perylene imides (Ref 1) or acridinium salts (Ref 2) among many others are increasingly common (Also, flower petals!).  But if we are sticking with metals, it would be great if we had access to sustainable alternatives like the use of earth abundant transition metals complexes.  There is an extensive precedent swapping in iron, copper or nickel for traditional organometallic chemistry that previously required palladium, platinum, rhodium, etc.  However, while swapping in an abundant earth metal complex for photocatalysis would also be quite desirable, is it far less common and potentially more challenging.

The use of Ir and Ru photocatalysts is so prevalent, despite their cost and scarcity, because they are fairly perfectly suited for photoredox catalysis.  Ir and Ru afford long excited state lifetimes (microseconds), strong absorbance in the visible region, stable metal-ligand bonds, and high excited reduction and oxidation potentials to readily undergo single-electron transfer (SET) or energy transfer (EnT).   In comparison abundant earth metals often have ultrashort excited lifetimes (pico to nanosecond), labile metal-ligand bonds and less favorable oxidant potentials.  All of which makes bimolecular processes to harvest photochemical energy with 3d transition metals difficult.  But hey, iron is cheap.  Really, really cheap.  And so, with that driving force, is an opportunity to think about different reaction mechanisms and paradigms to take advantage of photochemistry without precious metals.

In a recent review in Angewandte Chemie, Oliver Reiser and coworkers discuss the photochemistry of earth-abundant metals in “Visible-Light-Induced Homolysis of Earth-Abundant Metal-Substrate Complexes: A Complementary Activation Strategy in Photoredox Catalysis” (Ref 3).  The authors are focusing on examples of visible-light induced homolysis (VLIH) of organometallic complexes, so the few examples of abundant earth metals acting as traditional photocatalysts or co-catalysts are not discussed in detail.  Instead, the authors focus on examples of using pre-functionalized organometallic complexes of 3d transition metal complexes that can take advantage of the photo-labile metal-ligand bonds.

An overarching general scheme for the VLIH strategy involves first forming a ground state metal-substrate complex [LnMn(X)-Z] (Figure 1).  This can be achieved through many ways that are familiar in organometallic chemistry such as ligand exchange, oxidative addition, single-electron addition or transmetallation.  Excitation of the metal-substrate complex with visible light affords a generally short-lived excited [LnMn(X)-Z]* species.  Homolysis of the M-Z bond through various inner sphere redox processes affords the substrate radical Z· ready for further modification.  The paper then proceeds to describe examples of the different mechanism aspects of the M-Z bond homolysis and the implications for the desired chemistry based on 3d transition metal complexes including copper, iron, nickel, cobalt, cerium and vanadium. 

Figure 1: General scheme for visible-light induced homolysis (VLIH).  Adapted from Ref 3 – Figure 1

For copper, several interesting examples have recently been described for violet, blue, green and white LEDs.  As shown in Figure 2, Reiser and coworkers recently have described several copper catalyzed functionalization of olefins (Ref 4, 5).  Nickel complexes have shown perhaps the most promised for photocatalysis both in conjunction with iridium catalysts for cross-coupling reactions with blue LEDs (Ref 6) and on their own at lower wavelengths (Ref 7).  As an example of a nickel complex through a VLIH mechanism, the review discusses the recent report by Doyle and coworkers thoroughly investigating the homolysis of Ni intermediates Figure 3 (Ref 8).  Take a look at the full review for in depth discussion of the mechanistic implications of the VLIH mechanism for abundant-earth metals with examples for a variety of 3d metals.

Figure 2:  Recent examples of copper complexes for VLIH reactions (Ref 4, 5)

Figure 3:  VLIH of a Ni(II) species (Ref 8)

Since this review was published, copper shows up again in photoredox catalysis in a big way, in this recent paper in Chem by MacMillan and coworkers using a copper system for alkylation of a wide range of substrates (Ref 9).  While also using iridium, this paper represents the increasing number mechanistic possibilities combining catalysis and light.  We are certainly going to see many more examples of 3d transition metals involved in photocatalysis in the future.

References: 

  • MacKenzie, I. A.; Wang, L.; Onuska, N. P. R.; Williams, O. F.; Begam, K.; Moran, A. M.; Dunietz, B. D.; Nicewicz, D. A. Discovery and Characterization of an Acridine Radical Photoreductant. Nature 2020, 580 (7801), 76–80. https://doi.org/10.1038/s41586-020-2131-1.
  • Cole, A. J. P.; Chen, D.; Kudisch, M.; Pearson, R. M.; Miyake, G. M. Organocatalyzed Birch Reduction Driven by Visible Light. Am. Chem. Soc 2020, 142 (31), 13573–13581. https://dx.doi.org/10.1021/jacs.0c05899
  • Abderrazak, Y.; Bhattacharyya, A.; Reiser, O. Visible‐Light‐Induced Homolysis of Earth‐Abundant Metal‐Substrate Complexes: A Complementary Activation Strategy in Photoredox Catalysis. Chemie Int. Ed. 2021, Early view. https://doi.org/10.1002/anie.202100270.
  • Hossain, A. Vidyasagar, C. Eichinger, C. Lankes, J. Phan, J. Rehbein, O. Reiser, Angew. Chem. Int. Ed. 2018, 57, 8288 – 8292; Angew. Chem. 2018, 130, 8420 – 8424.
  • Engl, O. Reiser, Eur. J. Org. Chem. 2020, 1523 – 1533.
  • Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. Photoredox Catalysis in Organic Chemistry. Org. Chem. 2016, 81 (16), 6898–6926. https://doi.org/10.1021/acs.joc.6b01449.
  • Lim, C. H.; Kudisch, M.; Liu, B.; Miyake, G. M. C-N Cross-Coupling via Photoexcitation of Nickel-Amine Com-Plexes. Am. Chem. Soc. 2018. https://doi.org/10.1021/jacs.8b03744.
  • I. Ting, S. Garakyaraghi, C. M. Taliaferro, B. J. Shields, G. D. Scholes, F. N. Castellano, A. G. Doyle, J. Am. Chem. Soc. 2020, 142, 5800 – 5810
  • Dow, N., Cabre, A. and MacMillan D. W. C. , A general N-alkylation platform via copper metallaphotoredox and silyl radical activation of alkyl halides, Chem (2021), https://doi.org/10.1016/j.chempr.2021.05.005

 

The Attack of the Photocatalytic Microrobots!

The Attack of the Photocatalytic Microrobots!

The attack of the photocatalytic microrobots!

We have intended to write a bit about visible-light decomposition of contaminants for a while… so what better entry into the topic than a story about self-propelled autonomous microrobots that can swim through mazes to seek and destroy microplastics?

A group from the University of Chemistry and Technology Prague led by Martin Pumera have accomplished this very feat and describe it in chilling detail in their recent paper “A Maze in Plastic Wastes: Autonomous Motile Photocatalytic Microrobots against Microplastics

At issue for the present work are microplastics, defined as less than 5 mm large pieces of plastic that have broken off from larger plastics from either thermal, mechanical or photochemical stress or as a result of their intentional addition to products as fibers in clothing or additives to personal care products like cosmetics and detergents.  (To note, this is a different scenario than small molecule plastic additives like bisphenols and phthalates.  (Also a major environmental problem)  The authors cite a yearly global production rate of ~380 million metric tons per year of plastics, which is a number so large that it is impossible to be put into any sort of real context.  And it’s increasing. In the environment, the low solubility/hydrophobic nature of microplastics can cause the accumulation of organic pollutants, heavy metals and pathogens on their surface.  They are too small for removal by standard remediation methods for plastic waste (think fishing nets) and small enough that they are easily consumed by aquatic species.  Microplastics maintain most of the stability and low biodegradability we expect from plastics, so they are also with us for a long time.  As a result, microplastics have been found pervasively in pristine environments, tap water and many food products.  So much so, that studies tracking their prevalence can easily be contaminated at each step of the collection and analytical process by microplastics in airborne particles, chemical reagents and filtered lab water among other sources.  As a result, the full extent of their reach is not well-understood. Many thermal, chemical or photocatalytic degradation methods are in use or under development for water purification, degradation of small molecules, persistent pollutants and plastics.  However, light (specifically sun-light) driven methods offer the most energy efficient possibilities.  For mixed plastic wastes, a recent example uses an inexpensive non-toxic carbon nitride/nickel phosphide photocatalyst to reform polyethylene terephthalate (PET) and polylactic acid (PLA) polymer waste into H2 and other chemicals.

(Ref 2) Figure 1:  Example of Photocatalytic degradation of PET polymer from Ref 2

However, most similar technologies work best on larger pieces of plastic.  For the degradation of the microplastics, their small size, relatively chemically inert surface and low solubility make interactions with the photocatalyst difficult to achieve without extensive stirring (energy demanding) or pretreating the surface with deposition of catalyst (not applicable for real world applications).  As we all are familiar, photoredox catalysis offers a wide range of solutions to take advantage of reactive species that can be generated with visible light.  However, we probably don’t want to dump a bucket of most of the photocatalysts that we use on a daily basis in the ocean (for a variety of reasons, toxicity, cost, solubility, lack of recovery).  For that we have photocatalytic microrobots. For the task at hand, the authors turn to their microrobots army (Ref 3).  Previously, the authors have described a visible light-driven particle based on BiVO4 (Ref 4) The irregular star-shaped self-motile photocatalysts (4-8 µm) can under irradiation with sunlight in the presence of dilute 0.1% wt solutions of H2O2 achieve speeds between up to 6 µm/s (Figure 2A).  The robots are able to move, based on what the authors describe as “asymmetrical generation of chemical species on the surface of multifaceted BIVO4 motors induced by light illumination.” (Ref 4) Mainly, the photoexcited electrons react with oxygen or H2Oto generate reactive oxygen species or water.  The electron holes can oxidize water to hydroxyl radical or reduce H2O2.  The asymmetric shape of the particles causes these reactions at different rates, which creates a local electric field and causes charge particles to move forward.  A video of robots in motion can be seen here:   xxxx.   For this work, the particles can be magnetized by addition of Fe3O4 nanoparticles on the surface for recovery or as an additional method to induce motion.  

Figure 2:  Scheme demonstrating the motility of BiVO4/ Fe3O4 microrobots (modified from Ref 1)

Both the motion and the stirring caused by the particles during motion can increase contacts with the microplastics.  Their irregular shapes allow them to attach to polymers of many different shapes and sizes.  This is demonstrated by the adsorption of the microrobots onto microplastic pieces of polylactic acid (PLA) (72% coverage), polycaprolactone (PCL) (44%), polyethylene terephthalate (PET) (66%) and polypropylene PP (27%).  For the experiment, a dish containing microrobots and dilute H2O2 was illuminated with visible light for 3 h. Samples were then dried and washed and the coverage was determined.  Controls with immobile microbots (no illumination) show no deposition.

The recovery of microplastics was demonstrated in an experiment using 5-10 cm long channels containing microplastics of various polymers (Figure 2B).  In the presence of light and H2O2, the microrobots move in a random dispersion through the channel interacting with the microplastic pieces becoming absorbed.  In the presence of a magnetic field, the microplastic-microrobot adducts can be recovered on one side of the channel in similar recoveries as observed for the dispersion experiment discussed above.  The robots even swim a maze to find the plastics. The ability of the microrobots to degrade the microplastics was also investigated.  The plastic pieces gradually lost weight over the course of visible light illumination in the presence of the microrobots.  A 3% weight loss of individual microplastics pieces after 7 days was observed.  The increase of hydrophilicity due to addition of oxygen functional groups to the microplastics was demonstrated by wetting angle and by X-Ray spectroscopy where the surface of the plastics were dramatically changed. So, in this proof of concept, the authors have demonstrated the both the capture, recovery and beginnings of plastic degradation in one system.  There is certainly more work to come from this technology.  Check out the paper for more interesting figures, videos and discussion on the future of using microrobots for remediation.  And for something slightly different, here is video from Pumera’s robot army cleaning up uranium waste from 2019.  All in all, just a really fun work in a field we know very little about and that we hope you enjoy as much as we did.

In more photoredox catalysis related news, a few papers caught our eye this month.  First, is a work by the Molander and Gutierrez labs on a open air, catalyst free trifluoromethylthiolation reaction.

Second, Melchiorre and coworkers shine light on a well-established iridium catalyst (not quite what you are going to expect) to enable enantioselective C-C couplings.


References:

  1. Beladi-mousavi, S. M.; Hermanova, S.; Ying, Y.; Plutnar, J.; Pumera, M. A Maze in Plastic Wastes: Autonomous Motile Photocatalytic Microrobots against Microplastics. Mater. Interfaces 2021. https://doi.org/10.1021/acsami.1c04559.
  2. Uekert, T.; Kasap, H.; Reisner, E. Photoreforming of Nonrecyclable Plastic Waste over a Carbon Nitride/Nickel Phosphide Catalyst. Am. Chem. Soc. 2019, 141 (38), 15201–15210. https://doi.org/10.1021/jacs.9b06872.
  3. Wang, H.; Pumera, M. Coordinated behaviors of artificial micro/nanomachines: from mutual interactions to interactions with the environment. Chem. Soc. Rev. 2020, 49, 3211−3230.
  4. Villa, K.; Novotný, F.; Zelenka, J.; Browne, M. P.; Ruml, T.; Pumera, M. Visible-Light-Driven Single-Component BiVO4 Micro- motors with the Autonomous Ability for Capturing Microorganisms. ACS Nano 2019, 13, 8135−8145.

 

The 20 Must Read Photochemistry Papers from 2020

The 20 Must Read Photochemistry Papers from 2020

Year in review 2020.  Let’s all agree to not look back.  20 papers for 2020

sarcastic 2020 logo

Unofficially the Official Logo of 2020…

As the year comes to the close, we thought it was time to have a little fun and look back at the year in photochemistry.  2020 was a big year for us at HepatoChem under some trying circumstances.  Before we move forward to 2021 with the launch of some exciting new equipment and products, we thought that we would look back at some of our favorite photochemistry work in the literature from the past year.  A quick search finds a few thousand (photoredox, photocatalysis, photochemistry, metallaphotoredox) papers so far in 2020 and with(out lifting a finger to verify) very little research effort we’re willing to bet that this is more than in 2019.   Many of the papers on the list break ground and move the field farther into unimaginable areas, while others find photochemical replacements for traditional reactions solving some unmet need.   A few papers are outside the realm of topics that most of us spend any time thinking about but demonstrate the power of photochemistry.  Some of the papers also include nice pictures of EvoluChem photoreactors and equipment and deserve a shout out.  And a few others are just plain weird.  And let’s face it, 2020 was weird for all of us.

If you think we’ve missed your favorite paper or just want to send along an example of your work that you want us to recognize or recognize in the future, send us an email or respond to the tweet for this article (#photoredox20for2020)  If you disagree with the list, let us know how we screwed up by sending us an email here.

To get started, a few FAQ about how we generated this list.

What are the criteria for inclusion?

Each paper probably uses light in some way, either a bulb, LED, laser or sunlight.  We started this exercise without any particular standards or criteria, other than the goal to share the papers we enjoyed (the most) and to make it fun.   It then became readily apparent that there were far more than 20 papers worthy to be included, because there was a ridiculous amount of good work put out this year.  So, at a certain point we just stopped adding more.  Some of the work will likely be familiar to anyone used to the photochemical field; however, in our fake scoring system bonus points were given for open access papers and work from new professors.

Why 20 papers?

The year is 2020, I can’t believe you asked this question.

Are there 20 papers listed?

No, there are probably more or maybe less.  You’ll need to read and count the entire list to find out.

Is there a vote for best photochemistry achievement of 2020?

No.  Awards are stupid.  Do you really think we want to deal with running an internet voting poll and awarding photoredox achievement of the year to this?  https://www.youtube.com/watch?v=vfLZOkn0chc 

Do the links for each article work?

Sometimes, when formatting an email or a links on website, links go to the wrong place.  You can either be confused by this, email us and complain.  Or use google and find the correct article.  Your choice.  Other times, links go to wrong place intentionally and you might find something fun.

Do I win a prize if I click on each article?

You’ll have to click on each article to find out.

How do I get my work on the 21 for 2021 list?

Go here and get to work.   Send us the paper once you’ve submitted.

In no particular order, here are our 20 favorite photochemistry papers for 2020.

1: Organocatalyzed Birch Reduction Driven by Visible Light

Our Take:  We wrote about this one back here.  Just an incredible achievement to find this much reduction potential in a photocatalytic reaction with mild conditions.  Unless you prefer running reactions with liquid ammonia.

Authors:  Justin P. Cole, Dian-Feng Chen, Max Kudisch, Ryan M. Pearson, Chern-Hooi Lim, and Garret M. Miyake
Ref:  J. Am. Chem. Soc. 2020, 142, 31, 13573–13581
Link:  https://pubs.acs.org/doi/abs/10.1021/jacs.0c05899

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2: Development of a Platform for Near-Infrared Photoredox Catalysis

Our Take:  Another paper we wrote about before.  Check this one out to discover all the reasons your favorite photochemical reaction might be better run with a red LED.

Authors:  Benjamin D. Ravetz, Nicholas E. S. Tay, Candice L. Joe, Melda Sezen-Edmonds, Michael A. Schmidt, Yichen Tan, Jacob M. Janey, Martin D. Eastgate, and Tomislav Rovis
Ref:  ACS Cent. Sci. 2020, 6, 11, 2053–2059
Link:  https://pubs.acs.org/doi/abs/10.1021/acscentsci.0c00948

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3: Chemistry glows green with photoredox catalysis 

Our Take:  A quick note on the efficiency and sustainability of photoredox catalysis

Authors:  Giacomo E.M. Crisenza, Paolo Melchiorre
Ref:  Nature Communications, 2020, 11, Article 803
Link:  https://www.nature.com/articles/s41467-019-13887-8

4: Photons as a 21st century reagent

Our Take:  Please read this paper.  Everyone should treat light as a reagent like you would treat any other reagent in a chemical reaction.  To repeat again, everyone should treat light as a reagent like you would treat any other reagent in your reaction.

Authors:  Holly E. Bonfield, Thomas Knauber, François Lévesque, Eric G. Moschetta, Flavien Susanne & Lee J. Edwards
Ref:  Nature Communications, 2020, 11, Article number 804
Link:  https://www.nature.com/articles/s41467-019-13988-4

5: Three-Component Three-Bond Forming Cascade via Palladium Photoredox Catalysis (open access)

Our Take:  Look at that cool photochemical flow cell setup.  I wonder who makes that?

Authors:  Peter Bellotti, Maximilian Koy, Christian Gutheil, Steffen Heuvel and Frank Glorius
Ref:  Chemical Science, 2020, ASAP
Link:  https://pubs.rsc.org/en/content/articlelanding/2021/sc/d0sc05551d#!divAbstract

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6: Metallaphotoredox aryl and alkyl radiomethylation for PET ligand discovery

Our Take:  Amazing work from a collaboration between the Macmillan lab at Princeton with Merck, UPenn and RTI International for last state fast radiolabeling of pharmaceutically relevant compounds.

Authors: Robert W. Pipal, Kenneth T. Stout, Patricia Z. Musacchio, Sumei Ren, Thomas J. A. Graham, Stefan Verhoog, Liza Gantert, Talakad G. Lohith, Alexander Schmitz, Hsiaoju S. Lee, David Hesk, Eric D. Hostetler, Ian W. Davies & David W. C. MacMillan
Ref:  Nature, 2020, 6797.
Link:  https://www.nature.com/articles/s41586-020-3015-0

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7: Synthesis of azetidines via visible-light-mediated intermolecular [2+2] photocycloadditions

Our Take:   Very interesting work making highly strained four-membered ring building blocks photochemically.

Author:  Marc R. Becker, Emily R. Wearing & Corinna S. Schindler
Ref:  Nature Chemistry, 12, 898-905 (2020)
Link:  https://www.nature.com/articles/s41557-020-0541-1

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8: U.S. Food and Drug Administration-Certified Food Dyes as Organocatalysts in the Visible Light-Promoted Chlorination of Aromatics and Heteroaromatics

Our Take:  While it was the weirdness of the title that caught our eye here, being able to use non-toxic catalysts that are produced cheaply in metric ton quantities to do important chemistry is certainly valuable.  There is only so much iridium out there for everyone else to use.

Authors:  David A. Rogers, Megan D. Hopkins, Nitya Rajagopal, Dhruv Varshney, Haley A. Howard, Gabriel LeBlanc*, and Angus A. Lamar*
Ref:  ACS Omega, 2020, 5, 13, 7693-7704
Link: https://pubs.acs.org/doi/10.1021/acsomega.0c00631

9: LED‐NMR Monitoring of an Enantioselective Catalytic [2+2] Photocycloaddition 

Our Take:  Oh, how far we’ve come.  So, we’re using LED’s inside of NMRs now to study enantioselective photochemical reaction mechanisms for rapid reactions that are complete in under 5 minutes?  WTF.  It’s a shame that everyone says all this photochemistry stuff won’t scale up or else this field might really catch on….

Authors:  Kazimer L. Skubi, Wesley B. Swords, Heike Hofstetter, and Tehshik P. Yoon
Ref:  ChemPhotoChem 2020, (4), 685-690.
Link:  https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/cptc.202000094

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10: Upscaling Photoredox Cross-Coupling Reactions in Batch Using Immersion-Well Reactors

Our Take:  Ok, fine.  You’ve made a few tens of grams of your product with LED’s.  Are you sure you can really scale up this thing for something useful?

Authors:  Isabelle Grimm, Simone T. Hauer, Tim Schulte, Gina Wycich, Karl D. Collins, Kai Lovis, and Lisa Candish
Ref:  Org. Process Res. Dev. 2020, 24, 6, 1185-1193.
Link:  https://dx.doi.org/10.1021/acs.oprd.0c00070

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11: Development and Execution of a Production-Scale Continuous [2 + 2] Photocycloaddition

Our Take:  5 kg/day.  We good now?  Have we settled this yet?  If not convinced, you can find many more scale up success stories.

Authors:  Matthew G. Beaver, En-xuan Zhang, Zhi-qing Liu, Song-yuan Zheng, Bin Wang, Jiang-ping Lu, Jian Tao, Miguel Gonzalez, Siân Jones, and Jason S. Tedrow
Ref: Org. Process Res. Dev. 2020, 24, 10, 2139–2146
Link:  https://pubs.acs.org/doi/10.1021/acs.oprd.0c00185

12: C(sp3)–H functionalizations of light hydrocarbons using decatungstate photocatalysis in flow

Our Take:  Very cool work using a cheap photocatalyst for activation of light alkanes including methane.

Authors:  Gabriele Laudadio, Yuchao Deng, Klaas van der Wal, Davide Ravelli, Manuel Nuño, Maurizio Fagnoni, Duncan Guthrie, Yuhan Sun, Timothy Noël
Ref:  Science, 2020, (369), 92-96.
Link:  https://science.sciencemag.org/content/369/6499/92/tab-pdf

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13: Organocatalytic Approach to Photochemical Lignin Fragmentation

Our Take:  Metal-free reductive cleavage of lignin using an organic photocatalyst.

Authors Cheng Yang, Markus D. Kärkäs, Gabriel Magallanes, Kimberly Chan, and Corey R. J. Stephenson
Ref:  Org. Lett.  2020, 22, 20, 8082–8085
Link:  https://pubs.acs.org/doi/10.1021/acs.orglett.0c03029

14: Discovery of Key TIPS‐Naphthalene for Efficient Visible‐to‐UV Photon Upconversion under Sunlight and Room Light

Our Take:  Harvesting low energy sunlight and turning it into localized higher energy UV light.  I’m sure we can find a few applications for this technology.  Just a guess.

Authors:  Naoyuki Harada, Yoichi Sasak, Masanori Hosoyamada, Nobuo Kimizuka and Nobuhiro Yanai
Ref:  Angewandte Int. Ed., 2020, ASAP.
Link:  https://onlinelibrary.wiley.com/doi/10.1002/anie.202012419

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15: Potent Reductants via Electron-Primed Photoredox Catalysis: Unlocking Aryl Chlorides for Radical Coupling

Our Take:  Electrochemistry and Photochemistry, together?  That’s two things in this paper for old-school chemists to complain about.

Authors:  Nicholas G. W. Cowper, Colleen P. Chernowsky, Oliver P. Williams, and Zachary K. Wickens
Ref:  J. Am. Chem. Soc. 2020, 142, 5, 2093–2099
Link:  https://pubs.acs.org/doi/10.1021/jacs.9b12328

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16: Efficient Photoredox-Mediated C–C Coupling Organic Synthesis and Hydrogen Production over Engineered Semiconductor Quantum Dots

Our Take:  The title could use more buzzwords, but the technology is very cool.

Authors:  Ming-Yu Qi, Yue-Hua Li, Masakazu Anpo, Zi-Rong Tang, and Yi-Jun Xu
Ref: ACS Catal. 2020, 10, 23, 14327–14335
Link:  https://pubs.acs.org/doi/10.1021/acscatal.0c04237

17: Hydroarylation of Arenes via Reductive Radical-Polar Crossover

Our Take:  Very useful reaction for dearomatization without H2, metals or low temperatures.

Authors:  Autumn R. Flynn, Kelly A. McDaniel, Meredith E. Hughes, David B. Vogt, and Nathan T. Jui
Ref: J. Am. Chem. Soc. 2020, 142, 20, 9163–9168
Link:  https://pubs.acs.org/doi/10.1021/jacs.0c03926

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18: Development of a Large-Enrollment Course-Based Research Experience in an Undergraduate Organic Chemistry Laboratory: Structure–Function Relationships in Pyrylium Photoredox Catalysts

Comment:  Adding photochemistry to the undergraduate curriculum is incredibly important, specifically experiments for undergraduate lab.

Authors:  Cole L. Cruz, Natalie Holmberg-Douglas, Nicholas P. R. Onuska, Joshua B. McManus, Ian A. MacKenzie, Bryant L. Hutson, Nita A. Eskew, and David A. Nicewicz
Ref:  J. Chem. Educ. 2020, 97, 6, 1572–1578
Link:  https://pubs.acs.org/doi/10.1021/acs.jchemed.9b00786

19: High-throughput Synthesis and Screening of Iridium(III) Photocatalysts for the Fast and Chemoselective Dehalogenation of Aryl Bromides

Our Take:  A rapid assay for screening over 1,000 unique iridium catalysts from the lab that first synthesized several of the most commonly used Iridium catalysts used today over a decade ago.

Authors:  Velabo Mdluli, Stephen Diluzio, Jacqueline Lewis, Jakub F. Kowalewski, Timothy U. Connell, David Yaron, Tomasz Kowalewski, and Stefan Bernhard
Ref:  ACS Catal. 2020, 10, 13, 6977–6987
Link:  https://pubs.acs.org/doi/10.1021/acscatal.0c02247

20: UV Light Generation and Challenging Photoreactions Enabled by Upconversion in Water

Our Take: Blue to UV-upconversion in water.  Water-soluble iridium catalysts.  Degradation of some pretty nasty contaminants.    Much more to come from this technology.

Author:  Björn Pfund, Debora M. Steffen, Mirjam R. Schreier, Maria-Sophie Bertrams, Chen Ye, Karl Börjesson, Oliver S. Wenger, and Christoph Kerzig
Ref:  J. Am. Chem. Soc. 2020, 142, 23, 10468–10476
Link:  https://pubs.acs.org/doi/10.1021/jacs.0c02835

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So many great papers like we said earlier, so we thought we’d be like 2020 and have this article seemingly never end…

21: Rapid High-Resolution Visible Light 3D Printing

Our Take:  Some of us use 3D printers to make photoreactors to run photocatalysis.  Some people using photoredox to 3D print.  The circle of life.

Authors:  Dowon Ahn, Lynn M. Stevens, Kevin Zhou, and Zachariah A. Page
Ref:  ACS Cent. Sci. 2020, 6, 9, 1555–1563
Link:  https://pubs.acs.org/doi/10.1021/acscentsci.0c00929

22: Photocatalytic α-Tertiary Amine Synthesis via C–H Alkylation of Unmasked Primary Amines

Our Take: First paper from the Cresswell group early this year.  Congrats.  Check out their photo page (https://cresswell-lab.wixsite.com/cresswellgroup/photos).  Also, we retroactively assign Photochemistry Achievement of the Year for 2019 to this… incredible, edible lab diorama.

Authors:  Alexander James Cresswell, Alison Ryder, William Cunningham, George Ballantyne, Tom Mules, Anna Kinsella, Jacob Turner-Dore, Catherine Alder, Lee Edwards, Blandine McKay, and Matthew Grayson
Ref:  Angew. Chem. Int. Ed., 2020, 59, 14986-14991.
Link:  https://doi.org/10.1002/anie.202005294

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And to close, a few reviews for the year…

Visible-Light Photocatalysis as an Enabling Technology for Drug Discovery: A Paradigm Shift for Chemical Reactivity

Our Take:  Nice review with a summary of photocatalysis platforms within drug discovery programs.

Authors:  Peijun LiPeijun Li, Jack A. Terrett, and Jason R. Zbieg
Ref:  ACS Med. Chem. Lett. 2020, 11, 11, 2120–2130
Link:  https://pubs.acs.org/doi/10.1021/acsmedchemlett.0c00436

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Stereoinduction in Metallaphotoredox Catalysis

Our Take:  Summary of the state of art in chiral photoredox reactions.

Authors:  Alexander Lipp, Shorouk O. Badir, and Gary A. Molander
Ref:  Angewandte Chemie Int. Ed. 2020, ASAP
Link:  https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.202007668

Light-Triggered Click Chemistry

Comment:  A good summary of the current state of the art in light activated click chemistry

Authors:  Gangam Srikanth Kumar and Qing Lin
Ref:  Chem. Rev. 2020, ASAP
Link:  https://dx.doi.org/10.1021/acs.chemrev.0c00799

Light-Promoted Organic Transformations Utilizing Carbon-Based Gas Molecules as Feedstocks

Our Take:  Summary of the state of the art in photochemical reactions using feedstocks.

Authors:  Bin Cai, Han Wen Cheo, Tao Liu, and Jie Wu
Ref:  Angewandte Chemie Int. Ed., 2020, ASAP
Link:  https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.202010710

New Redox Strategies in Organic Synthesis by Means of Electrochemistry and Photochemistry

Our Take:  A good review combining the topics of electrochemistry and photochemistry.

Authors:  Jinjian Liu, Lingxiang Lu, Devin Wood, and Song Lin
Ref:  ACS Cent. Sci. 2020, 6, 8, 1317–1340
Link:  https://pubs.acs.org/doi/10.1021/acscentsci.0c00549

If you made it this far, thanks for reading and hope you enjoyed! 

Hope everyone out there has a great end of 2020 and we look forward to hearing from you in 2021.

 

 

 

 

 

Photochemistry 101, Part III: Setting Up Your Initial Photochemistry Reactions

Photochemistry 101, Part III: Setting Up Your Initial Photochemistry Reactions

Setting Up Your Initial Photochemistry Reactions

This is the third and final part of a three part series designed to help you get started by understanding light sources in photochemistry.  Missed the start of the series where we cover the basics and core principles?  No worries, you can read it here

Visible light photoredox catalysis uses the excited states of metal complexes and organic dyes to perform energy transfer and single-electron transfer (SET) processes for an ever-increasing number of useful synthetic transformations.  As should be apparent from reading Part 1 (/photochemistry-101-everything-you-need-to-know-to-get-started-part-i/) and Part 2 (/photochemistry-101-part-ii-understanding-and-measuring-light-sources/), we think photochemistry is pretty cool.  But is it useful?  The possibilities of where to start first can seem daunting.  A standard reaction setup is imperative for reproducible chemistry from lab to lab enabling a low barrier to entry for the field.  As you just read, step one to standardizing reactions is a better understanding of the light source.  The second step is the reactor.  Let’s do a quick review before we start providing details on your initial photochemistry reactions.

First, it is a simple concept at first but needs to be stated.  The only light that is useful to running your reaction is the light that actually makes its way into the flask.  The bright blue light shining on the back of your hood isn’t doing anything.  For this reason, we have developed a series of photoreactors to maximize light intensity, control temperature and standardize reaction conditions that are currently in use in both industrial and academic settings (Ref 20).  For use with a standardized setup, (the EvoluChem Photoredox Box), we have selected and adapted four reactions from the literature as a convenient starting point for those new to photochemistry.  Each reaction has been tested and validated in our equipment.  Each is available as a part of bundle including photoreactor, sample holders, LED’s and premixed reactions to run test reaction, as well as three substrate combinations of your choice (see https://www.hepatochem.com/photoreactors-leds-accessories/photochemistry-starter-bundle/).

Experimental Details

Each reaction is performed in the Evoluchem PhotoRedOx box, equipped with either an 18W 450 nm or 365 nm Evoluchem LED.  Reactions are performed in 4 mL vials equipped with a Teflon septa cap containing pre-weighed photocatalyst, co-catalyst base and reagents.  Substrate solutions are added via syringe and the reaction is sparged with a N2 line via needle for 5 minutes prior to turning on the LED’s.  The reactor is equipped with a fan that holds the reaction temperature stable at ~30 °C.  Reactions are run for 18-24 hr.  Product analysis is performed by LC-MS.

Initial Photochemistry Reactions: C-C cross-coupling with amino acid decarboxylation

Adapted from Ref 14
The first reaction that we want to highlight is an Iridium/nickel catalyzed carbon-carbon bond formation as described by MacMillan and coworkers (Ref 14).  This approach uses a commercially available iridium catalyst (Ir[dF(CF3)ppy]2(dtbbpy)PF6 (structure in Figure 2) with a CFL bulb.  The photoredox cycle activates a nickel catalyzed organometallic cycle for the coupling of α-carboxyl sp3-carbons with aryl halides.  The nickel catalyst is formed in situ between NiCl2 with dtbbpy as a ligand.  The reaction requires a base (Cs2CO3) and sparging with nitrogen to remove oxygen.   The reaction demonstrated coupling for a wide range of carboxylic acids with aryl bromides, iodides and select chlorides.  The test reaction we have selected is the coupling of N-Boc-Valine and 4-bromoacetophenone which we have adapted from the reported procedure to use 450 nm LED instead of the CFL (Figure 8).  

initial photochemistry reactions: Carbon-carbon formation between sp3 carbons and aryl halides

Figure 8: Carbon-carbon formation between sp3 carbons and aryl halides (adapted from Ref 14)

Initial Photochemistry Reactions: C-O bond formation

The second reaction that we have selected is also by MacMillan and coworkers using Ir/Ni catalysis, however this time for C-O bond formation (Ref 21).  This reaction also uses (Ir[dF(CF3)ppy]2(dtbbpy)PF6, NiCl2 and dtbbpy as a ligand.  This reaction requires quinuclidine as an electron donor/acceptor and an additional base.  The reaction uses primary and secondary alcohol for C-O bond formation with aryl bromides.  The reaction requires a base (K2CO3) and sparging with nitrogen to remove oxygen.   Nickel itself is unable to perform C-O couplings without the use of high temperatures due to the stability of nickel-alkoxide complexes.  The photoredox cycle using a 450 nm LED and the iridium catalyst can activate this cycle at room temperature for a wide variety of alcohols and aryl bromides.  The test reaction that we have selected is the coupling of cyclohexanol and 4-bromoacetophenone with 450 nm (Figure 9).

initial photochemistry reactions: C-O bond formation

Figure 9: C-O bond formation (Adapted from Ref 21)

Initial Photochemistry Reactions: C-C Cross coupling with BF3K reagents

The third reaction that we want to highlight is also a C-C bond formation, however this time using an alkyl BF3K reagent as the coupling partner.  Molander and coworkers have performed a substantial amount of work using BF3K reagents for photoredox catalyzed transformations.  The reaction we are focusing on is the Iridium/nickel catalyzed cross coupling of aryl bromides with secondary alkyl BF3K reagents (Ref 22).  BF3K reagents are easy to handle, bench stable solid reagents useful for many cross-coupling reactions.  This reaction works extremely well for a wide range of alkyl-BF3K with aryl bromides and can very quickly be used to generate a large series of analogues. The reaction requires a base (Cs2CO3) and sparging with nitrogen to remove oxygen.    The test reaction we have selected uses cyclohexyl-BF3K and 4-bromoacetophenone (Figure 10).  We have modified the reaction condition to use Ir[dF(CF3)ppy]2(dtbbpy)PF6 instead of Ir[dF(CF3)ppy]2(bpy)PF6 (slightly different ligand), lowered the catalyst loading and use 450 nm LEDs instead of CFL.

initial photochemistry reactions: C-C cross-coupling with BF3K reagents

Figure 10:  C-C cross-coupling with BF3K reagents (adapted from Ref 22)

Initial Photochemistry Reactions: C−N Cross-Coupling via Photoexcitation of Nickel−Amine Complexes

The final reaction we have selected is a carbon-nitrogen cross coupling reaction as described by Miyake and coworkers (Ref 23).  This is the first reaction that we have selected that does not include an iridium catalyst or 450 nm LED.  Here, the inexpensive NiBr2 salt forms a photoactive complex with primary and secondary amines that can be excited by 365 nm LED’s to give C-N bond formation products using aryl bromides.  When excess amine is used, the addition of quinuclidine is not required. The reaction should be sparged with nitrogen to remove oxygen.    Of note, this reaction is run at higher concentration than the previously selected reactions and the Ni-amine complex could be described as the photocatalyst for the reaction.  We have selected the coupling of morpholine with 4-bromoacetophenone without quinuclidine.  

initial photochemistry reactions: C-N Cross-coupling reaction

Figure 11:  C-N Cross-coupling reaction (Ref 23)

 

We hope you have enjoyed this series on Photochemistry 101 and invite you to email us (info@hepatochem.com) or follow us on Twitter (@EvoluChem) to suggest more content and subject areas you would like us to cover.  If you have any questions about the experiments above, or just find yourself stuck and looking for a good listener… drop us a line!  Check out our starter bundle and give photoredox catalysis a try!

You just read the third and final part of a three part series designed to help you get started in photochemistry.  Below are links to all three parts of the series.  Any questions?  Send them to info@hepatochem.com, we’d love to hear from you!
Here’s the entire series:

Photochemistry 101, Part I: Everything You Need To Know To Get Started

Photochemistry 101, Part II: Understanding and Measuring Light Sources

Photochemistry 101, Part III: Setting Up Your Initial Photochemistry Reactions

 

References

  1. Yes, this is a simplified explanation, there are entire textbooks written about this stuff. 
  2. https://www.hepatochem.com/red-light-applications-in-photochemistry/
  3. Don’t worry, there’s still room for you to synthesize 50 nearly identical derivatives of your favorite chromophore.
  4. https://www.hepatochem.com/electron-donor-acceptor-eda-complexes-in-photochemistry/
  5. Tucker, J. and Stephenson, C. R. J.  “Shining Light on Photoredox Catalysis:  Theory and Synthetic Applications”, Journal of Organic Chemistry, 2012, 77, 1617-1622.
  6. Ischay, M. A.; Anzovino, M. E.; Du, J.; Yoon, T. P. J. Am. Chem. Soc. 2008, 130, 12886. 
  7. Narayanam, J. M. R.; Tucker, J. W.; Stephenson, C. R. J. J. Am. Chem. Soc. 2009, 131, 8756.
  8. Nicewicz, D. A.; MacMillan, D. W. C. Science 2008, 322, 77. 
  9. 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
  10.  Romero, N., Nicewicz, “Organic Photoredox Catalysis”, Chemical Reviews, 2016 (116), 10075-10166. 
  11.  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.  
  12.   Harper, K.  Moschetta, E., Bordawekar, S., Wittenberger, S.  “A Laser Driven Flow Chemistry Platform for Scaling Photochemical Reactions with Visible light., ACS Central Science, 2019 (5), 109-115.
  13.  Justin P. Cole, Dian-Feng Chen, Max Kudisch, Ryan M. Pearson, Chern-Hooi Lim, and Garret M. Miyake, “Organocatalyzed Birch Reduction Driven by Visible light, J. Am. Chem. Soc, 2020, 142, 13573-13581.  https://pubs.acs.org/doi/abs/10.1021/jacs.0c05899
  14.  Zuo, Z., Ahneman, D., Chu, L., Terrett, J., Doyle, A., Macmillan, D.  “Merging photoredox with nickel catalysis:  Coupling of α-carboxyl sp3-carbons with aryl halides” Science, 2014 (345), 437-440.
  15.  Bonfield, H.E., Knauber, T., Lévesque, F. et al. Photons as a 21st century reagent. Nat Commun 11, 804 (2020) https://doi.org/10.1038/s41467-019-13988-4
  16.  https://www.hepatochem.com/evaluating-light-sources-in-photochemistry/
  17.  https://www.hepatochem.com/determine-photon-flux-using-actinometry/
  18.   Hatchard C.G.; Parker C.A. “A new sensitive chemical actinometer. 2. Potassium ferrioxalate as a standard chemical actinometer.” Proc. R. Soc. London, Ser. A. 1956, 235, 518-536.
  19.  https://www.hepatochem.com/standard-ferrioxalate-actinometer-protocol/
  20.  https://www.hepatochem.com/photoreactors-leds-accessories/#photoreactors
  21.  Terret, J. Cuthbertson, J.  Shurtleff, V. MacMillan, D. “Switching on elusive organometallic mechanisms with photoredox catalysis”.  Nature, 2015, 524, 330-334.
  22.  Primer, D., Karakaya, I.  Tellis, J.  Molander, G.  “Single-Electron Transmetallation:  An Enabling Technology for Secondary Alkylboron Cross-Coupling”.  J. Am.Chem. Soc.  2015, 137, 2195.
  23.  Lim, C.H., Kudisch, M., Liu, B., Miyake, G. “C-N Cross-Coupling via Photoexcitation of Nickel-Amine Complexes” J. Am. Chem. Soc. 2018, 140, 24, 7667-7673.
Photochemistry 101, Part II: Understanding and Measuring Light Sources

Photochemistry 101, Part II: Understanding and Measuring Light Sources

Part II: Understanding and Measuring Light Sources 

This is the second part of a three part series designed to help you get started by understanding light sources in photochemistry.  Missed the first part of the series where we cover the basics and core principles?  No worries, you can read it here

The ease in setting up photochemical reactions led to a rapid adoption of photoredox chemistry.  Much of the early photoredox research discussed in Part 1 was performed using readily available, commercial CFLs, flood lights, household light bulbs or low energy LED strips.  Often reactions were cooled (or not) with an external fan in an attempt to keep the temperature low from the heat of the light source or generated from the reaction.  Unfortunately, little was reported or understood at the time for the wavelength and intensity of light in the reaction flask.  Eventually, higher energy, single wavelength LEDs became the light source of choice for most chemists but details on the light used for reactions remained sparse.  Often, the prevailing criticism of photochemistry is that a small-scale reaction works but scaling up is impossible.   This can be directly attributed to two factors, not knowing how much light is available from your light source and the light being absorbed by your catalyst (more on this at a later date) (Ref 12). 

Recently, there has been a concerted effort to treat the light used in the reaction with the same care and focus as you would any stochiometric reagent in a reaction (Ref 15)  Not reporting the details of the light being used in your reaction is the equivalent of saying you “heated the reaction” without reporting a temperature.   However, determining the intensity and type of light that makes its way into a reaction vial is more complicated than you might think.  Part of this is due to the difficulty in how we historically discuss the brightness and intensity of light for commercially available light sources.   The second problem derives from making 2-dimensional measurements of light to mimic a 3-dimensional reaction.  (fine for the light on a solar cell, not as great for a reaction flask) (Ref 16).  

Light is generally divided into three classifications, ultraviolet wavelength (100 to 380 nm), visible light (380-700 nm) and infrared (greater than 700 nm).  Sunlight itself is a combination of all of these.  The radiation that reaches earth from the sun is a wide collection of wavelengths, ranging from 100 nm to 1 mm.  Almost everything below 280 nm is blocked by the earth’s atmosphere (for now), while the collection of wavelengths in the visible region we perceive as white light.   The longer infrared wavelengths (heat radiation) make up about 50% of the radiation that we receive from the sun.  Similarly, commercial household light bulbs have sought to mimic the white light that we perceive in nature.

The units that we commonly use to describe the brightness (or intensity of light) – whether sunlight or household light sources – find their origin in sunlight and how we perceive it with the human eye.  When looking to determine “brightness” of sunlight, we want to know the sum of the all the wavelengths and total energy that is dispersed over a large area.  Luminous flux is the measure of the total quantity of visible light emitted by a source weighted according to human eyes sensitivity to various wavelengths (measured in lumens).  The unit lumen is the amount of light emitted by a source per unit time.  In other words, lumens represent the amount of visible light generated by the bulb or the sun or whatever you are measuring.  A lux meter is then used to measure the amount of light in a specific position over a certain area (lux = lumen/m2).  

Most light sources report color (wavelength) and electrical power (wattage).  The electrical power rating is the indication of that light’s power.  Nearly all commercial bulbs are rated in lumens, a unit that averages the full spectrum of light.  Monochromatic LED’s make these measurements irrelevant.  A CFL and LED with the same power rating will not have the same luminous efficacy and will not deliver the same amount of energy to a reaction.  Additionally, household light bulbs diffuse light in all directions while focused light sources such as LEDs focus light in one direction.  Variance in beam angle between different types of LEDs further complicate the amount light that ends up in your reaction vial (see Figure 7 below).  

For LEDs, radiant flux (measured in watts which are J/s) and light intensity (irradiance, mW/cm2) give us a more accurate measurement.   We can use a radiospectrometer to measure an LED’s power (in watt) and light intensity (irradiance in watt/cm2) at a specified distance from the light source, as well as determine the wavelength.   Irradiance is measured at a single point in one direction so it can be used to directly compare different light sources.   Irradiance and lux are not equivalent as irradiance is not based on the human eye sensitivity.  

Chart showing irradiance impacted by light beam angle

The y-axis represents the light intensity (irradiance) while the x-axis represents the beam’s angle. The chart above demonstrates that a 20 W LED light with 20 degrees of beam angle is as efficient as an 80W LED light with 40 degrees of angle

 

With irradiance, we are getting closer to the answer, but we are still looking at a 2-D measurement.  What we would really like to know is the number of photons being absorbed by the whole reaction (photon flux).  Photon flux depends on a number of factors, including the light source (power, spectrum), the position and shape of the reaction vial and the reaction volume.  To solve this problem, we need actinominetry.  Actinometry is any chemical method for directly measuring the amount of light penetrating your reaction (photon flux). The actinomer is the chemical used to quantify the light.  We recently described the actinometric method that we use for determining the light in our photoreactors (Ref 17) using a well established ferrioxalate actinometer (Ref 18).

Ferrioxalate is a versatile actinomer with a range between 250 nm to 500 nm.  The Fe(III) compound becomes light sensitive in solution, but stable when kept in the dark.  A solution of ferrioxalate can be used in your reaction vial and flask in your reactor as you would set up a standard reaction (albeit in a dark room).  Upon irradiation of the sample, the Fe(III) is reduced to Fe(II). Treatment of the Fe(II) species with a phenanthroline solution generates a Fe(II)phenanthroline complex, which can be quantified in comparison to a calibration curve.  The amount of Fe(II)phen is proportional to the photons absorbed.  Monitoring the time course of irradiation allows you to determine the rate of Fe(II) formation which can be calculated to the photon radian flux in Einsteins/s (energy in one mole of photon).  This can be converted for a specific wavelength to determine the number of watts absorbed by the reaction.  (a detailed protocol for the synthesis of reagents and description of the math involved can be found in Ref 19). 

While it may at first seem complicated, the tools exist to determine directly the photon flux in any reaction setup.  With this information, the photon flux from one experiment can be directly compared to any reaction setup as the method is scaled or transferred from lab to lab.  Having good familiarity with the above concepts around light, actinometry and photon flux provides a great foundation for the final part of our Getting Started in Photochemistry series where we’ll walk you through setting up your first reactions.

You just read the second part of a three part series designed to help you get started in photochemistry.  Below are links to all three parts of the series.  Any questions?  Send them to info@hepatochem.com, we’d love to hear from you!
Here’s the entire series:

Photochemistry 101, Part I: Everything You Need To Know To Get Started

Photochemistry 101, Part II: Understanding and Measuring Light Sources

Photochemistry 101, Part III: Setting Up Your Initial Photochemistry Reactions

 

 

References

  1. Yes, this is a simplified explanation, there are entire textbooks written about this stuff. 
  2. https://www.hepatochem.com/red-light-applications-in-photochemistry/
  3. Don’t worry, there’s still room for you to synthesize 50 nearly identical derivatives of your favorite chromophore.
  4. https://www.hepatochem.com/electron-donor-acceptor-eda-complexes-in-photochemistry/
  5. Tucker, J. and Stephenson, C. R. J.  “Shining Light on Photoredox Catalysis:  Theory and Synthetic Applications”, Journal of Organic Chemistry, 2012, 77, 1617-1622.
  6. Ischay, M. A.; Anzovino, M. E.; Du, J.; Yoon, T. P. J. Am. Chem. Soc. 2008, 130, 12886. 
  7. Narayanam, J. M. R.; Tucker, J. W.; Stephenson, C. R. J. J. Am. Chem. Soc. 2009, 131, 8756.
  8. Nicewicz, D. A.; MacMillan, D. W. C. Science 2008, 322, 77. 
  9. 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
  10.  Romero, N., Nicewicz, “Organic Photoredox Catalysis”, Chemical Reviews, 2016 (116), 10075-10166. 
  11.  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. 
  12. Harper, K.  Moschetta, E., Bordawekar, S., Wittenberger, S.  “A Laser Driven Flow Chemistry Platform for Scaling Photochemical Reactions with Visible light., ACS Central Science, 2019 (5), 109-115.
  13. Justin P. Cole, Dian-Feng Chen, Max Kudisch, Ryan M. Pearson, Chern-Hooi Lim, and Garret M. Miyake, “Organocatalyzed Birch Reduction Driven by Visible light, J. Am. Chem. Soc, 2020, 142, 13573-13581.  https://pubs.acs.org/doi/abs/10.1021/jacs.0c05899
  14. Zuo, Z., Ahneman, D., Chu, L., Terrett, J., Doyle, A., Macmillan, D.  “Merging photoredox with nickel catalysis:  Coupling of α-carboxyl sp3-carbons with aryl halides” Science, 2014 (345), 437-440.
  15. Bonfield, H.E., Knauber, T., Lévesque, F. et al. Photons as a 21st century reagent. Nat Commun 11, 804 (2020) https://doi.org/10.1038/s41467-019-13988-4
  16.  https://www.hepatochem.com/evaluating-light-sources-in-photochemistry/
  17.  https://www.hepatochem.com/determine-photon-flux-using-actinometry/
  18.   Hatchard C.G.; Parker C.A. “A new sensitive chemical actinometer. 2. Potassium ferrioxalate as a standard chemical actinometer.” Proc. R. Soc. London, Ser. A. 1956, 235, 518-536.
  19.  https://www.hepatochem.com/standard-ferrioxalate-actinometer-protocol/