Substituents para-to-ortho positioning effect driving the photoreactivity of a dibenzothiophene-based oxalate series used as LEDs excitable free radical photoinitiators $(document).ready(function() { if (!$("html").hasClass("ie8")) { $.ajax({ url: "https://crossmark-cdn.crossref.org/widget/v2.0/widget.js", dataType: "script", cache: true }).done(function() { $(".crossmark-button").addClass("visible"); }); } });

Abstract

Four dibenzothiophene-based (DBT) methyl oxalates, synthesized through a one-step Friedel-Crafts acylation reaction, were developed as high-performance photoinitiators (PIs) for free radical polymerization of acrylate resins excitable using LED sources which emit in the near-UV and visible region (i.e. 365-425 nm). These type I PIs show good optical absorption properties, (ε365nm=1000~5300 M-1•cm-1). Their respective photophysical properties as well as the sequential photocleavage mechanism were investigated using a large set of theoretical and experimental methods. A relevant structure/reactivity relationship opening the way to enhance the PIs performances is highlighted. Indeed, a para-to-ortho positioning effect of the oxalate-based substituent both regulates the absorption and photocleavage abilities within this DBT series. Para-isomers consequently exhibit a photoinitiating reactivity whose efficiency is more than one order of magnitude higher than their ortho homologues. Associated to a H-donor alkylamine co-reactant, these photoinitiating efficiency can be even more amplified due to an additional reactional pathway that judiciously assigns a new role to the aryloyl radicals by-products. These whole results correlate the PIs photopolymerization performances with the photoinduced processes occurring in the excited states.