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blog post LED Photo-Initiators | IDYLLE

Written by Aurélien Pasturel | 27 avril 2020
 

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: 


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

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 https://doi.org/10.1039/C5PY00258C

 

 

 

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 quote 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:

 

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 react faster and create more crosslinks than Camphor Quinone.

(c) Macromolecules https://doi.org/10.1021/acs.macromol.7b01370

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: 
https://doi.org/10.1039/C5PY00258C

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  https://doi.org/10.1021/acs.macromol.7b01370