Why do you use triethylene glycol as your solvent for the first step rather than high boiling mineral oil?

Triethylene glycol is used as your solvent for the first step because there is something in the high boiling mineral oil which drops the activated state of the chemiluminescent molecule back to the ground state before it can emit.

Explanation:

There are some reasons why triethylene glycol is used as the first solvent rather than using the high boiling mineral oil as the first solvent. The reasons for this are that you need to dissolve a polar organic compound. But the polar organic compound will not be able to dissolve properly in a non polar solvent like mineral oil but it will dissolve in triethylene glycol.

Moreover the high boiling mineral oil will drop the activated state of the chemiluminescent molecule back to the ground state before it can emit because of the aromatic components in it like benzenoid while there is nothing in glycol like this.