Financial support by the French National Research Agency (ANR program: DS0304 2016, contract number: ANR-16-CE07-0016) and the French Ministry of Research (doctoral grant of Emeline Placet) are gratefully acknowledged.

1. NHC Photogenerating System: Synthesis and Reactivity
<ol>
	<li>Synthesis of 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-)
	<ol>
		<li>Preparation of the solution of 1,3-dimesitylimidazolium chloride (IMesH+Cl-) in ethanol.
		<ol>
			<li>Add 1.00 g (2.93 mmol) of 1,3-dimesitylimidazolium chloride to a 50 mL round bottom flask equipped with a stir bar.</li>
			<li>Dissolve the 1,3-dimesitylimidazolium chloride in 30 mL of ethanol.</li>
		</ol>
		</li>
	...

<ol>
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W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+Isolable,+Photoswitchable+N-Heterocyclic+Carbene:+On-Demand+Reversible+Ammonia+Activation.">An Isolable, Photoswitchable N-Heterocyclic Carbene: On-Demand Reversible Ammonia Activation.</a> Angewandt Chemie International Edition. 54 (39), 11559-11563 (2015).</li><li>Pinaud, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+Situ+Generated+Ruthenium-Arene+Catalyst+for+Photoactivated+Ring-Opening+Metathesis+Polymerization+through+Photolatent+N-Heterocyclic+Carbene+Ligand.">In Situ Generated Ruthenium-Arene Catalyst for Photoactivated Ring-Opening Metathesis Polymerization through Photolatent N-Heterocyclic Carbene Ligand.</a> Chemistry - A European Journal. 24 (2), 337-341 (2018).</li><li>Konishi, T., Sasaki, Y., Fujitsuka, M., Toba, Y., Moriyama, H., Ito, O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Persistent+C60+anion-radical+formation+via+photoinduced+electron+transfer+from+tetraphenylborate+and+triphenylbutylborate.">Persistent C60 anion-radical formation via photoinduced electron transfer from tetraphenylborate and triphenylbutylborate.</a> Journal of the Chemical Society, Perkin Transactions. 2 (3), 551-556 (1999).</li><li>Ogawa, K. A., Goetz, A. E., Boydston, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Developments+in+Externally+Regulated+Ring-Opening+Metathesis+Polymerization.">Developments in Externally Regulated Ring-Opening Metathesis Polymerization.</a> Synletter. 27 (2), 203-214 (2016).</li><li>Eivgia, O., Lemcoff, N. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Turning+the+Light+On:+Recent+Developments+in+Photoinduced+Olefin+Metathesis.">Turning the Light On: Recent Developments in Photoinduced Olefin Metathesis.</a> Synthesis. 50 (1), 49-63 (2018).</li><li>Monsaert, S., Vila, A. 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Reported here is an easy and versatile protocol for the in-situ generation of NHC upon UV-irradiation at 365 nm. The anion exchange reaction between 1,3-dimesitylimidazolium chloride and sodium tetraphenylborate provides straightforward access to the NHC protected from IMesH+BPh4- in quantitative yield. Nevertheless, if using another starting imidazolium salt, the solvent employed to perform the metathesis reaction should be chosen with care so that it allows the solubilization of both st...

In chemistry, N-heterocyclic carbenes (NHCs) species fulfill the twofold role of ligand and organocatalyst1. In the former case, the introduction of NHCs has resulted in the design of metal transition catalysts with improved activity and stability2. In the latter case, NHCs have proved to be superior catalysts for manifold organic reactions3,4. Despite this versatility, handling bare NHCs is still a significant challenge5, and producing these highly reactive compounds so they are released in situ and &#34;on demand&#34; is a very attractive goal. Consequently, several strategies have been developed to release NHC in the reaction media which mostly rely on the use of thermolabile progenitors6,7,8. Surprisingly, while this could unleash a novel generation of photoinitiated reactions useful for macromolecular synthesis or preparative organic chemistry6, generation using light as stimulus has been scarcely explored. Recently, a first photo-generating system able to produce NHC has been unveiled9. It consists of 2 components: 2-isopropylthioxanthone (ITX) as photosensitive species and 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-) as the NHC protected form. Consequently, in the following paragraphs, we report a method to generate the NHC&#160;1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm, characterize it, and determine the photochemical mechanism. Then, we describe a protocol to perform ring-opening metathesis polymerization (ROMP) in solution and in miniemulsion using this NHC photogenerating system.
In the first portion, we report a synthesis protocol to produce IMesH+BPh4-. This protocol is based on anion metathesis between the corresponding imidazolium chloride (IMesH+Cl-) and sodium tetraphenylborate (NaBPh4). Then, to demonstrate the in situ formation of NHC, two protocols involving the irradiation at 365 nm of a IMesH+BPh4-/ITX solution in a photoreactor are described. The first consists of monitoring the deprotonation of the imidazolium cation IMesH+ through 1H NMR spectroscopy. Direct evidence for formation of the desired NHC (IMes) is provided in a second method, where the adduct IMes-CS2 is successfully isolated, purified, and characterized.
The second section describes two protocols that shed light on the photochemical mechanism involving the NHC two-component photogenerating system IMesH+BPh4-/ITX. Firstly, an original real-time steady state photolysis experiment reveals that electron transfer is induced by photo-excitation of ITX in the presence of tetraphenylborate. Electron donor properties of this borate anion10 drives a photoreduction of 3ITX* triplet excited-state into ITX&#9679;- radical anion through a so-called photo-sensitized reaction. The formation of NHC confirms that ITX&#9679;- species may further abstract a proton from IMesH+ to produce the desired NHC. Based on acid/base titration using phenol red pH indicator as titrant, a second original protocol is implemented that allows the determination of the yield of released NHC.
In the third section, we describe a protocol in which the above-mentioned photogenerated IMes can be exploited in photopolymerization. Of primary interest is ring-opening metathesis polymerization (ROMP), because this reaction is still at a preliminary stage of development with regard to photoinitiation11,12. Initially limited to ill-defined and highly sensitive tungsten complexes, photoinduced ROMP (photoROMP) has been extended to more stable complexes based on W, Ru, and Os transition metals. Despite the variety of precatalysts, almost all photoROMP processes rely on the direct excitation of a single photoactive precatalyst13. By contrast, we use radiation to create the NHC imidazolidene ligand (IMes), which can react subsequently with a non-photoactive Ru precatalyst [RuCl2(p-cymene)]2 dimer9. In this method, the photogeneration of NHC ligand drives the in situ formation of a highly active ruthenium-arene NHC complex known as RuCl2(p-cymene)(IMes) (Noels&#39; catalyst)14,15. Using this indirect methodology, two distinct photoROMP experiments of norbornene (Nb) are performed: 1) in solution (dichloromethane) and 2) in aqueous dispersed system from a monomer miniemulsion16.

<table>
	<tbody>
		<tr>
			<td>Material</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Dimesitylimidazolium chloride, 97%</td>
			<td>ABCR</td>
			<td>AB130859</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium tetraphenylborate, 99%</td>
			<td>ABCR</td>
			<td>AB118843</td>
			<td></td>
		</tr>
		<tr>
			<td>Dichloro(p-cymene) ruthenium dimer, 98%</td>
			<td>ABCR</td>
			<td>AB113524</td>
			<td></td>
		</tr>
		<tr>
			<td>Norbornene, 99%</td>
			<td>ABCR</td>
			<td>AB171849</td>
			<td></td>
		</tr>
		<tr>
			<td>Isopropythioxanthone, 97%</td>
			<td>Sigma Aldrich</td>
			<td>406317</td>
			<td></td>
		</tr>
		<tr>
			<td>Carbon disulfide, 99.9%</td>
			<td>Sigma Aldrich</td>
			<td>335266</td>
			<td></td>
		</tr>
		<tr>
			<td>Dichloromethane</td>
			<td>Sigma Aldrich</td>
			<td>270997</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>VWR</td>
			<td>20821.31</td>
			<td></td>
		</tr>
		<tr>
			<td>Deuterated DMSO</td>
			<td>Eurisotop</td>
			<td>D010FE</td>
			<td></td>
		</tr>
		<tr>
			<td>Deuterated THF</td>
			<td>Eurisotop</td>
			<td>D149CB</td>
			<td></td>
		</tr>
		<tr>
			<td>1,2-Dichloroethane</td>
			<td>Sigma Aldrich</td>
			<td>284505</td>
			<td></td>
		</tr>
		<tr>
			<td>Brij S 100</td>
			<td>Sigma Aldrich</td>
			<td>466387</td>
			<td></td>
		</tr>
		<tr>
			<td>Hexadecane</td>
			<td>Sigma Aldrich</td>
			<td>H6703</td>
			<td></td>
		</tr>
		<tr>
			<td>Phenol red, 98%</td>
			<td>Sigma Aldrich</td>
			<td>P4633</td>
			<td></td>
		</tr>
		<tr>
			<td>Acetonitrile</td>
			<td>VWR</td>
			<td>83639.290</td>
			<td></td>
		</tr>
		<tr>
			<td>1,3-Bis(mesityl)imidazol-2-ylidene, 97%</td>
			<td>Sigma Aldrich</td>
			<td>696188</td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Rayonet photochemical reactor</td>
			<td>Southern New England Ultraviolet Company</td>
			<td>RPR-200</td>
			<td></td>
		</tr>
		<tr>
			<td>UV lamps for photochemical reactor</td>
			<td>Southern New England Ultraviolet Company</td>
			<td>RPR-3500A</td>
			<td></td>
		</tr>
		<tr>
			<td>1H and 13C NMR spectrometer</td>
			<td>Bruker</td>
			<td colspan="2">Avance III HD spectrometer</td>
		</tr>
		<tr>
			<td>Sonication probe</td>
			<td>BioBlock</td>
			<td>Vibra-cell</td>
			<td></td>
		</tr>
		<tr>
			<td>Gas chromatography</td>
			<td>Varian</td>
			<td>GC3900</td>
			<td></td>
		</tr>
		<tr>
			<td>LED Lamp and Photo-cabinet</td>
			<td>Peschl ultraviolet</td>
			<td>novaLIGHT TLED100-365</td>
			<td></td>
		</tr>
		<tr>
			<td>Dynamic Light Scattering</td>
			<td>Malvern</td>
			<td colspan="2">zetasizer Nano ZS</td>
		</tr>
		<tr>
			<td>365 nm UV-LED light source coupled with a flexible light-guide</td>
			<td>Hamamastu</td>
			<td>LC-L1V3</td>
			<td></td>
		</tr>
		<tr>
			<td>UV/vis spectrometer</td>
			<td>Perkin Elmer</td>
			<td>Lambda 35</td>
			<td></td>
		</tr>
		<tr>
			<td>Hg- Xe lamp with filter centred at 365 nm</td>
			<td>Hamamastu</td>
			<td>LC-9588/01A</td>
			<td></td>
		</tr>
		<tr>
			<td>Radiometer</td>
			<td>Ocean Optics</td>
			<td>USB4000</td>
			<td></td>
		</tr>
	</tbody>
</table>

photogeneration of n-heterocyclic carbenes: application in photoinduced ring-opening metathesis polymerization

We report a method to generate the N-heterocyclic carbene (NHC) 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm to characterize IMes and determine the corresponding photochemical mechanism. Then, we describe a protocol to perform ring-opening metathesis polymerization (ROMP) in solution and in miniemulsion using this NHC-photogenerating system. To photogenerate IMes, a system comprising 2-isopropylthioxanthone (ITX) as the sensitizer and 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-) as the protected form of NHC is employed. IMesH+BPh4- can be obtained in a single step by anion exchange between 1,3-dimesitylimidazolium chloride and sodium tetraphenylborate. A real-time steady-state photolysis setup is described, which hints that the photochemical reaction proceeds in two consecutive steps: 1) ITX triplet is photo-reduced by the borate anion and 2) subsequent proton transfer takes place from the imidazolium cation to produce the expected NHC IMes. Two separate characterization protocols are implemented. Firstly, CS2 is added to the reaction media to evidence the photogeneration of NHC through formation of the IMes-CS2 adduct. Secondly, the amount of NHC released in situ is quantified using acid-base titration. The use of this NHC photo-generating system for the ROMP of norbornene is also discussed. In solution, a photopolymerization experiment is conducted by mixing ITX, IMesH+BPh4-, [RuCl2(p-cymene)]2 and norbornene in CH2Cl2, then irradiating the solution in a UV reactor. In a dispersed medium, a monomer miniemulsion is first formed then irradiated inside an annular reactor to produce a stable poly(norbornene) latex.

Step 1.1 describes the efficient anion metathesis between 1,3-dimesitylimidazolium chloride (IMesH+Cl-) and sodium tetraphenylborate (NaBPh4) to yield 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-). The desired photolatent NHC is obtained in excellent yield (98%). Figure 1 shows 1H and 13C NMR spectra, both testifying that a pure product exhibiting the correct st...

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Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

We describe a protocol to photogenerate N-heterocyclic carbenes (NHCs) by UV irradiation of a 2-isopropylthioxanthone/imidazolium tetraphenylborate salt system. Methods to characterize the photoreleased NHC and elucidate the photochemical mechanism are proposed. The protocols for ring-opening metathesis photopolymerization in solution and miniemulsion illustrate the potential of this 2-component NHC photogenerating system.

We report a method to generate the N-heterocyclic carbene (NHC) 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm to characterize IMes ...