Silyl glyoxylates: LED-optimized photoinitiators

Typically, photo-initiators are molecules which, upon UV irradiation, can trigger chemical reactions. Yet, this new class of photo-initiators can be activated by blue light instead . This makes them well suited for LED illumination.

Publication:  Silyl glyoxylates as a New Class of High Performance Photoinitiators: Blue LED Induced Polymerization of Methacrylates in Thin and Thick Films  

Authors:  Mariem Bouzrati-Zerell Julie Kirschner, Christoph P. Fik Maximilian Maier Celine Dietlin Fabrice Morlet-Savary ,  Jean Pierre Fouassier Jean-Michel Becht Joachim E. Klee and  Jacques Lalevée.

#photochemistry #polymerization #LED #Photoinitiators
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Schematic of the photo-activation of the Sylil glyoxilate "DKSi" photo-initiator under Blue Light Excitation (c) Aurélien Pasturel 2021

Why developing Blue-Light photo-initiators in the first place?

From photo-polymerization down to opto-genetics, triggering reactions with light has become an important part of today's research and to work perfectly, the light source and the photo-reagents have to go hand in hand: 

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LED are great light sources

LED have : 

  • Low energy consumption 

  • Low cost 

  • Low maintenance / High life time

  • Low profile / are highly compact

  • Fast response time

  • Control over pulse and intensity

LED are much better than Hg lamps for all those reasons. 

(c) Wikipedia

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LED work best in blue ...

In this graph you have the emission spectra of several LEDs sold by ThorLab's.

Notice how the LED that are emitting in UV (<375 nm) are much less powerfull than the ones emitting in blue (>375nm).

Simply put, UV led's are more difficult to produce and in turn are less powerfull while also being more expensive.

(c) Polymer Chemistry

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 but common photo-iniators don't.

This Jablonski Perrin diagram shows the typical behaviour of benzophenone upon UV (375nm) irradiation. Like many other photo-initiators it works poorly under blue light.

To cite the authors :

"At higher wavelengths (λ > 365 nm) most of the classical/commercial PIs exhibit a low absorption due to their initial development for Hg lamps. "

(c) Aurélien Pasturel 2021

How I met with Silyl glyoxilates

My PhD work was centered around photostructuration of hydrogels for 3D cell culture. Near its end, I was tempted to move away from UV illumination and use a new LED-based illumination system. Unfortunately no photo-initiator seemed to do the trick ...

That's when I stumbled upon this publication: Silyl glyoxylates as a New Class of High Performance Photoinitiators: Blue LED Induced Polymerization of Methacrylates in Thin and Thick Films.

Now let me explain what made me think this was different from the other works I had seen.

What makes them stand out ?

You may argue that there are in fact many other visible (blue) light photo-initiators in use today or under development and you are right;  Camphorquinone is the historical blue light photo-initiator and is commonly used in dental applications. In the litterature, there are extensive reviews listing original, LEd-optimized photo-initiators.

Nonetheless, I think Silyl glyoxylates have distinct features that make them appealing:

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They promote fast kinetics

Photoinitiators are best when they react swiftly. For example when one want to cure a resin in dental applications , the shorter it takes the less disturbing it is for the patient.

When it comes to curing a resin , Silyl glyoxilates reacts faster and create more crosslinks than Camphor Quinone.

(c) Macromolecules

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They have a simple structure

This shows the chemical structure of various LED-optimized photo-initiators including DKSi.

As you can see the glyoxilates have a much simpler structure which oftens translates into : cost efficient synthesis.

(c) Polymer Chemistry:

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They do not leave coloured byproducts

Last but not least, it is often important that the photo-initiator preserves the material optical properties mainly transparency. In those pictures you can see how resins polymerized with camphorquine progressively turn yellow after a few months whereas those polymerized with DkSI kept their transparency.

(c) Macromolecules


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